Java How To Program, 10th Edition | Andy Farela

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www.allitebooks.com ONLINE ACCESS Thank you for purchasing a new copy of Java™ How to Program, Tenth Edition, Early Objects. Your textbook includes 12 months of prepaid access to the book’s Companion Website. This prepaid subscription provides you with full access to the following student support areas: • VideoNotes (step-by-step video tutorials specifically designed to enhance the programming concepts presented in this textbook) • Source code • Premium web chapters and appendices Additional Comments from Recent Editions Reviewers ❝ Updated to reflect the state of the art in Java technologies; deep and crystal clear explanations. The social-consciousness [Making a Difference] exercises are something new and refreshing. Nice introduction to Java networking.~—José Antonio González Seco, Parliament of Andalusia ❝An easy-to-read conversational style. Clear code examples propel readers to become proficient in Java.~—Patty Kraft, San Diego State U. ❝ The [early] introduction of the class concept is clearly presented. A comprehensive overview of control structures and the pitfalls that befall new programmers. I applaud the authors for their topical research and illustrative examples. The arrays exercises are sophisticated and interesting. The clearest explanation of pass-by-value and pass-by-reference that I’ve encountered. A logical progression of inheritance and the rationale for properly implementing encapsulation in a system involving an inheritance hierarchy. The polymorphism and exception handling discussions are the best I’ve seen. An excellent strings chapter. I like the [recursion] discussions of the ‘Lo Fractal’ and backtracking (which is useful in computer vision applications). A good segue into a data structures course.~—Ric Heishman, George Mason University ❝ Practical top-down, solution approach to teaching programming basics, covering pseudocode, algorithm development and activity diagrams. Of immense value to practitioners and students of the object-oriented approach. Demystifies inheritance and polymorphism, and illustrates their use in getting elegant, simple and maintainable code. The [optional] OO design case study presents the object-oriented approach in a simple manner, from requirements to Java code.~—Vinod Varma, Astro Infotech Private Limited ❝ Easy-to-follow examples provide great teaching opportunities! I like the [optional] graphics track early in the book—the exercises will be fun Use a coin to scratch off the coating and reveal your student access code. Do not use a knife or other sharp object as it may damage the code. To access the Java How to Program, Tenth Edition, Early Objects Companion Website for the first time, you will need to register online using a computer with an Internet connection and a web browser. The process takes just a couple of minutes and only needs to be completed once. 1. Go to http://www.pearsonhighered.com/deitel/ 2. Click on Companion Website. 3. Click on the Register button. 4. On the registration page, enter your student access code* found beneath the scratchoff panel. Do not type the dashes. You can use lower- or uppercase. 5. Follow the on-screen instructions. If you need help at any time during the online registration process, simply click the Need Help? icon. for the students. OO design techniques are incorporated throughout. The concept of inheritance is built through examples and is very understandable. Great examples of polymorphism and interfaces. Great comparison of recursion and iteration. The searching and sorting chapter is just right. A simplified explanation of Big O—the best I’ve read! I appreciate the coverage of GUI threading issues. Great approach to Java web technologies.~ —Sue McFarland Metzger, Villanova University ❝ The Making a Difference exercises are inspired—they have a real contemporary feeling, both in their topics and in the way they encourage the student to gather data from the Internet and bring it back to the question at hand.~—Vince O’Brien, Pearson Education (our publisher) ❝ Most major concepts are illustrated by complete, annotated programs. Abundant exercises hone your understanding of the material. JDBC is explained well.~—Shyamal Mitra, University of Texas at Austin ❝ The best introductory textbook that I’ve encountered. I wish I had this book when I was learning how to program! Good introduction to the software engineering process.~—Lance Andersen, Oracle Corporation ❝ You’ll be well on your way to becoming a great Java programmer with this book.~—Peter Pilgrim, Java Champion, Consultant ❝ A good objects-early introduction to Java. Exceptionally well-written recursion chapter. Excellent descriptions of the search and sort algorithms and a gentle introduction to Big-O notation—the examples give the code for the algorithms, and output that creates a picture of how the algorithms work.~—Diana Franklin, University of California, Santa Barbara ❝ Suitable for new programmers, intermediate-level programmers who want to hone their skills, and expert programmers who need a well-organized reference. Event handling and layouts are well explained.~—Manjeet Rege, Rochester Institute of Technology ❝Beautiful collections of exercises—a nice illustration of how to use Java to generate impressive graphics.~—Amr Sabry, Indiana University 6. Once your personal Login Name and Password are confirmed, you can begin using the Java How to Program, Tenth Edition, Early Objects Companion Website! ❝ The [optional] OOD ATM case study puts many concepts from previous chapters together in a plan for a large program, showing the object-oriented design process—the discussion of inheritance and polymorphism is especially good as the authors integrate these into the design.~ To log in after you have registered: ❝ The transition from design to implementation is explained powerfully—the reader can easily understand the design issues and how to implement them in Java.~—S. Sivakumar, Astro Infotech Private Limited You only need to register for this Companion Website once. After that, you can log in any time at http://www.pearsonhighered.com/deitel/ by providing your Login Name and Password when prompted. *Important: The access code can only be used once. This subscription is valid for 12 months upon activation and is not transferable. If this access code has already been revealed, it may no longer be valid. If this is the case, you can purchase a subscription at http://www.pearsonhighered.com/deitel/, by going to the Java How to Program, Tenth Edition, Early Objects book and following the on-screen instructions. www.allitebooks.com —Susan Rodger, Duke University ❝ Comprehensive introduction to Java, now in its eighth major iteration. With clear descriptions, useful tips and hints, and well thought out exercises, this is a great book for studying the world’s most popular programming language.~—Simon Ritter, Oracle Corporation ❝ Comprehensive treatment of Java programming, covering both the latest version of the language and Java SE APIs, with its concepts and techniques reinforced by a plethora of well-thought-through exercises.~—Dr. Danny Coward, Oracle Corporation ❝ There are many Java programming books in the world. This textbook is the best one. If you like to introduce object-oriented programming early and smoothly, then this is the right one for you!~—Dr. Huiwei Guan, North Shore Community College More Comments on Facing Page www.allitebooks.com Deitel® Series Page How To Program Series Android How to Program, 2/E C++ How to Program, 9/E C How to Program, 7/E Java™ How to Program, 10/E Java™ How to Program, Late Objects Version, 10/E Internet & World Wide Web How to Program, 5/E Visual C++® 2008 How to Program, 2/E Visual Basic® 2012 How to Program, 6/E Visual C#® 2012 How to Program, 5/E Simply Series Simply C++: An App-Driven Tutorial Approach Simply Java™ Programming: An App-Driven Tutorial Approach Simply C#: An App-Driven Tutorial Approach Simply Visual Basic® 2010: An App-Driven Approach, 4/E CourseSmart Web Books www.deitel.com/books/CourseSmart/ C++ How to Program, 8/E and 9/E Simply C++: An App-Driven Tutorial Approach Java™ How to Program, 9/E and 10/E Simply Visual Basic® 2010: An App-Driven Approach, 4/E (continued from previous column) Visual Basic® 2012 How to Program, 6/E Visual Basic® 2010 How to Program, 5/E Visual C#® 2012 How to Program, 5/E Visual C#® 2010 How to Program, 4/E Deitel® Developer Series Android for Programmers: An App-Driven Approach, 2/E, Volume 1 C for Programmers with an Introduction to C11 C++11 for Programmers C# 2012 for Programmers Dive Into® iOS 6 for Programmers: An App-Driven Approach Java™ for Programmers, 3/E JavaScript for Programmers LiveLessons Video Learning Products www.deitel.com/books/LiveLessons/ Android App Development Fundamentals C++ Fundamentals Java™ Fundamentals C# 2012 Fundamentals C# 2010 Fundamentals iOS® 6 App Development Fundamentals JavaScript Fundamentals Visual Basic® Fundamentals To receive updates on Deitel publications, Resource Centers, training courses, partner offers and more, please join the Deitel communities on • Facebook®—facebook.com/DeitelFan • Twitter®—@deitel • Google+™—google.com/+DeitelFan • YouTube™—youtube.com/DeitelTV • LinkedIn®—linkedin.com/company/deitel-&-associates and register for the free Deitel ® Buzz Online e-mail newsletter at: www.deitel.com/newsletter/subscribe.html To communicate with the authors, send e-mail to: [email protected] For information on Dive-Into® Series on-site seminars offered by Deitel & Associates, Inc. worldwide, write to us at [email protected] or visit: www.deitel.com/training/ For continuing updates on Pearson/Deitel publications visit: www.deitel.com www.pearsonhighered.com/deitel/ Visit the Deitel Resource Centers that will help you master programming languages, software development, Android and iOS app development, and Internet- and web-related topics: www.deitel.com/ResourceCenters.html www.allitebooks.com Paul Deitel Deitel & Associates, Inc. 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Rather than put a trademark symbol in every occurrence of a trademarked name, we state that we are using the names in an editorial fashion only and to the benefit of the trademark owner, with no intention of infringement of the trademark. www.allitebooks.com Contents Chapters 26–34 and Appendices F–N are PDF documents posted online at the book’s Companion Website (located at www.pearsonhighered.com/deitel/). See the inside front cover for information on accessing the Companion Website. Foreword xxiii Preface xxv Before You Begin xxxix 1 Introduction to Computers, the Internet and Java 1 1.1 1.2 Introduction Hardware and Software 1.2.1 Moore’s Law 1.2.2 Computer Organization Data Hierarchy Machine Languages, Assembly Languages and High-Level Languages Introduction to Object Technology 1.5.1 The Automobile as an Object 1.5.2 Methods and Classes 1.5.3 Instantiation 1.5.4 Reuse 1.5.5 Messages and Method Calls 1.5.6 Attributes and Instance Variables 1.5.7 Encapsulation and Information Hiding 1.5.8 Inheritance 1.5.9 Interfaces 1.5.10 Object-Oriented Analysis and Design (OOAD) 1.5.11 The UML (Unified Modeling Language) Operating Systems 1.6.1 Windows—A Proprietary Operating System 1.6.2 Linux—An Open-Source Operating System 1.6.3 Android Programming Languages Java A Typical Java Development Environment Test-Driving a Java Application 2 4 4 5 6 9 10 10 11 11 11 11 11 12 12 12 12 13 13 13 14 14 15 17 17 21 1.3 1.4 1.5 1.6 1.7 1.8 1.9 1.10 www.allitebooks.com viii 1.11 1.12 1.13 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3 3.1 3.2 Contents Internet and World Wide Web 1.11.1 The Internet: A Network of Networks 1.11.2 The World Wide Web: Making the Internet User-Friendly 1.11.3 Web Services and Mashups 1.11.4 Ajax 1.11.5 The Internet of Things Software Technologies Keeping Up-to-Date with Information Technologies Introduction to Java Applications; Input/Output and Operators Introduction Your First Program in Java: Printing a Line of Text Modifying Your First Java Program Displaying Text with printf Another Application: Adding Integers 2.5.1 import Declarations 2.5.2 Declaring Class Addition 2.5.3 Declaring and Creating a Scanner to Obtain User Input from the Keyboard 2.5.4 Declaring Variables to Store Integers 2.5.5 Prompting the User for Input 2.5.6 Obtaining an int as Input from the User 2.5.7 Prompting for and Inputting a Second int 2.5.8 Using Variables in a Calculation 2.5.9 Displaying the Result of the Calculation 2.5.10 Java API Documentation Memory Concepts Arithmetic Decision Making: Equality and Relational Operators Wrap-Up Introduction to Classes, Objects, Methods and Strings Introduction Instance Variables, set Methods and get Methods 3.2.1 Account Class with an Instance Variable, a set Method and a get Method 3.2.2 AccountTest Class That Creates and Uses an Object of Class Account 3.2.3 Compiling and Executing an App with Multiple Classes 3.2.4 Account UML Class Diagram with an Instance Variable and set and get Methods 3.2.5 Additional Notes on Class AccountTest www.allitebooks.com 25 26 26 26 27 27 28 30 34 35 35 41 43 45 45 46 46 47 48 48 49 49 49 49 50 51 54 58 69 70 71 71 74 77 77 78 Contents 3.2.6 3.3 3.4 3.5 3.6 3.7 4 Software Engineering with private Instance Variables and public set and get Methods Primitive Types vs. Reference Types Account Class: Initializing Objects with Constructors 3.4.1 Declaring an Account Constructor for Custom Object Initialization 3.4.2 Class AccountTest: Initializing Account Objects When They’re Created Account Class with a Balance; Floating-Point Numbers 3.5.1 Account Class with a balance Instance Variable of Type double 3.5.2 AccountTest Class to Use Class Account (Optional) GUI and Graphics Case Study: Using Dialog Boxes Wrap-Up ix Control Statements: Part 1; Assignment, ++ and -- Operators 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 4.11 4.12 4.13 4.14 4.15 4.16 Introduction Algorithms Pseudocode Control Structures if Single-Selection Statement if…else Double-Selection Statement Student Class: Nested if…else Statements while Repetition Statement Formulating Algorithms: Counter-Controlled Repetition Formulating Algorithms: Sentinel-Controlled Repetition Formulating Algorithms: Nested Control Statements Compound Assignment Operators Increment and Decrement Operators Primitive Types (Optional) GUI and Graphics Case Study: Creating Simple Drawings Wrap-Up 5 Control Statements: Part 2; Logical Operators 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 5.10 5.11 5.12 Introduction Essentials of Counter-Controlled Repetition for Repetition Statement Examples Using the for Statement do…while Repetition Statement switch Multiple-Selection Statement Class AutoPolicy Case Study: Strings in switch Statements break and continue Statements Logical Operators Structured Programming Summary (Optional) GUI and Graphics Case Study: Drawing Rectangles and Ovals Wrap-Up 79 80 81 81 82 84 85 86 90 93 101 102 102 103 103 105 106 111 113 115 119 126 131 131 134 135 139 152 153 153 155 159 163 165 171 174 176 182 187 190 x Contents 6 Methods: A Deeper Look 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 6.10 6.11 6.12 6.13 6.14 Introduction Program Modules in Java static Methods, static Fields and Class Math Declaring Methods with Multiple Parameters Notes on Declaring and Using Methods Method-Call Stack and Stack Frames Argument Promotion and Casting Java API Packages Case Study: Secure Random-Number Generation Case Study: A Game of Chance; Introducing enum Types Scope of Declarations Method Overloading (Optional) GUI and Graphics Case Study: Colors and Filled Shapes Wrap-Up 7 Arrays and ArrayLists 7.1 7.2 7.3 7.4 Introduction Arrays Declaring and Creating Arrays Examples Using Arrays 7.4.1 Creating and Initializing an Array 7.4.2 Using an Array Initializer 7.4.3 Calculating the Values to Store in an Array 7.4.4 Summing the Elements of an Array 7.4.5 Using Bar Charts to Display Array Data Graphically 7.4.6 Using the Elements of an Array as Counters 7.4.7 Using Arrays to Analyze Survey Results Exception Handling: Processing the Incorrect Response 7.5.1 The try Statement 7.5.2 Executing the catch Block 7.5.3 toString Method of the Exception Parameter Case Study: Card Shuffling and Dealing Simulation Enhanced for Statement Passing Arrays to Methods Pass-By-Value vs. Pass-By-Reference Case Study: Class GradeBook Using an Array to Store Grades Multidimensional Arrays Case Study: Class GradeBook Using a Two-Dimensional Array Variable-Length Argument Lists Using Command-Line Arguments Class Arrays Introduction to Collections and Class ArrayList (Optional) GUI and Graphics Case Study: Drawing Arcs Wrap-Up 7.5 7.6 7.7 7.8 7.9 7.10 7.11 7.12 7.13 7.14 7.15 7.16 7.17 7.18 200 201 201 203 205 208 209 210 211 213 218 222 225 227 230 243 244 245 246 247 247 248 249 251 251 253 254 256 256 256 257 257 262 263 265 266 272 275 281 283 285 287 291 294 Contents 8 Classes and Objects: A Deeper Look 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9 8.10 8.11 8.12 8.13 8.14 8.15 8.16 8.17 Introduction Class Case Study Controlling Access to Members Referring to the Current Object’s Members with the this Reference Time Class Case Study: Overloaded Constructors Default and No-Argument Constructors Notes on Set and Get Methods Composition enum Types Garbage Collection static Class Members static Import final Instance Variables Package Access Using BigDecimal for Precise Monetary Calculations (Optional) GUI and Graphics Case Study: Using Objects with Graphics Wrap-Up 9 Object-Oriented Programming: Inheritance 9.1 9.2 9.3 9.4 Introduction Superclasses and Subclasses protected Members Relationship Between Superclasses and Subclasses 9.4.1 Creating and Using a CommissionEmployee Class 9.4.2 Creating and Using a BasePlusCommissionEmployee Class 9.4.3 Creating a CommissionEmployee–BasePlusCommissionEmployee Inheritance Hierarchy 9.4.4 CommissionEmployee–BasePlusCommissionEmployee Inheritance Hierarchy Using protected Instance Variables 9.4.5 CommissionEmployee–BasePlusCommissionEmployee Inheritance Hierarchy Using private Instance Variables Constructors in Subclasses Class Object (Optional) GUI and Graphics Case Study: Displaying Text and Images Using Labels Wrap-Up 9.5 9.6 9.7 9.8 10 10.1 10.2 10.3 10.4 Time Object-Oriented Programming: Polymorphism and Interfaces Introduction Polymorphism Examples Demonstrating Polymorphic Behavior Abstract Classes and Methods xi 315 316 316 321 322 324 330 330 332 335 337 338 342 343 344 345 348 352 360 361 362 364 365 365 371 376 379 382 387 387 388 391 395 396 398 399 401 xii Contents 10.5 Case Study: Payroll System Using Polymorphism 10.5.1 Abstract Superclass Employee 10.5.2 Concrete Subclass SalariedEmployee 10.5.3 Concrete Subclass HourlyEmployee 10.5.4 Concrete Subclass CommissionEmployee 10.5.5 Indirect Concrete Subclass BasePlusCommissionEmployee 10.5.6 Polymorphic Processing, Operator instanceof and Downcasting 10.6 Allowed Assignments Between Superclass and Subclass Variables 10.7 final Methods and Classes 10.8 A Deeper Explanation of Issues with Calling Methods from Constructors 10.9 Creating and Using Interfaces 10.9.1 Developing a Payable Hierarchy 10.9.2 Interface Payable 10.9.3 Class Invoice 10.9.4 Modifying Class Employee to Implement Interface Payable 10.9.5 Modifying Class SalariedEmployee for Use in the Payable Hierarchy 10.9.6 Using Interface Payable to Process Invoices and Employees Polymorphically 10.9.7 Some Common Interfaces of the Java API 10.10 Java SE 8 Interface Enhancements 10.10.1 default Interface Methods 10.10.2 static Interface Methods 10.10.3 Functional Interfaces 10.11 (Optional) GUI and Graphics Case Study: Drawing with Polymorphism 10.12 Wrap-Up 11 Exception Handling: A Deeper Look 11.1 11.2 11.3 11.4 11.5 11.6 11.7 11.8 11.9 11.10 11.11 11.12 11.13 Introduction Example: Divide by Zero without Exception Handling Example: Handling ArithmeticExceptions and InputMismatchExceptions When to Use Exception Handling Java Exception Hierarchy finally Block Stack Unwinding and Obtaining Information from an Exception Object Chained Exceptions Declaring New Exception Types Preconditions and Postconditions Assertions try-with-Resources: Automatic Resource Deallocation Wrap-Up 12 GUI Components: Part 1 12.1 Introduction 404 405 407 409 411 413 414 419 419 420 421 422 423 424 426 428 430 431 432 432 433 433 433 436 441 442 443 445 451 451 454 459 461 464 465 465 467 467 473 474 Contents xiii 12.2 12.3 12.4 12.5 12.6 12.7 12.8 12.9 12.10 Java’s Nimbus Look-and-Feel Simple GUI-Based Input/Output with JOptionPane Overview of Swing Components Displaying Text and Images in a Window Text Fields and an Introduction to Event Handling with Nested Classes Common GUI Event Types and Listener Interfaces How Event Handling Works 475 476 479 481 485 491 493 495 498 499 501 504 508 511 513 518 522 525 528 530 532 536 538 539 542 13 Graphics and Java 2D 13.1 13.2 13.3 13.4 13.5 13.6 13.7 13.8 13.9 Introduction Graphics Contexts and Graphics Objects Color Control Manipulating Fonts Drawing Lines, Rectangles and Ovals Drawing Arcs Drawing Polygons and Polylines Java 2D API Wrap-Up 14 Strings, Characters and Regular Expressions 14.1 14.2 14.3 Introduction Fundamentals of Characters and Strings Class String 14.3.1 String Constructors 14.3.2 String Methods length, charAt and getChars 14.3.3 Comparing Strings JButton Buttons That Maintain State 12.10.1 JCheckBox 12.10.2 JRadioButton 12.11 JComboBox; Using an Anonymous Inner Class for Event Handling 12.12 JList 12.13 Multiple-Selection Lists 12.14 Mouse Event Handling 12.15 Adapter Classes 12.16 JPanel Subclass for Drawing with the Mouse 12.17 Key Event Handling 12.18 Introduction to Layout Managers 12.18.1 FlowLayout 12.18.2 BorderLayout 12.18.3 GridLayout 12.19 Using Panels to Manage More Complex Layouts 12.20 JTextArea 12.21 Wrap-Up 555 556 558 559 566 571 575 578 581 588 596 597 597 598 598 599 600 xiv Contents 14.5 14.6 14.7 14.8 14.3.4 Locating Characters and Substrings in Strings 14.3.5 Extracting Substrings from Strings 14.3.6 Concatenating Strings 14.3.7 Miscellaneous String Methods 14.3.8 String Method valueOf Class StringBuilder 14.4.1 StringBuilder Constructors 14.4.2 StringBuilder Methods length, capacity, setLength and ensureCapacity 14.4.3 StringBuilder Methods charAt, setCharAt, getChars and reverse 14.4.4 StringBuilder append Methods 14.4.5 StringBuilder Insertion and Deletion Methods Class Character Tokenizing Strings Regular Expressions, Class Pattern and Class Matcher Wrap-Up 15 Files, Streams and Object Serialization 15.1 15.2 15.3 15.4 15.8 Introduction Files and Streams Using NIO Classes and Interfaces to Get File and Directory Information Sequential-Access Text Files 15.4.1 Creating a Sequential-Access Text File 15.4.2 Reading Data from a Sequential-Access Text File 15.4.3 Case Study: A Credit-Inquiry Program 15.4.4 Updating Sequential-Access Files Object Serialization 15.5.1 Creating a Sequential-Access File Using Object Serialization 15.5.2 Reading and Deserializing Data from a Sequential-Access File Opening Files with JFileChooser (Optional) Additional java.io Classes 15.7.1 Interfaces and Classes for Byte-Based Input and Output 15.7.2 Interfaces and Classes for Character-Based Input and Output Wrap-Up 16 Generic Collections 16.1 16.2 16.3 16.4 16.5 16.6 Introduction Collections Overview Type-Wrapper Classes Autoboxing and Auto-Unboxing Interface Collection and Class Collections Lists 16.6.1 ArrayList and Iterator 16.6.2 LinkedList 14.4 15.5 15.6 15.7 605 607 608 608 610 611 612 612 614 615 617 618 623 624 633 644 645 645 647 651 651 655 657 661 662 663 668 670 673 673 675 676 684 685 685 687 687 687 688 689 691 Contents 16.7 16.8 16.9 16.10 16.11 16.12 16.13 16.14 16.15 16.16 Collections Methods 16.7.1 Method sort 16.7.2 Method shuffle 16.7.3 Methods reverse, fill, copy, max and min 16.7.4 Method binarySearch 16.7.5 Methods addAll, frequency and disjoint Stack Class of Package java.util Class PriorityQueue and Interface Queue Sets Maps Properties Class Synchronized Collections Unmodifiable Collections Abstract Implementations Wrap-Up 17 Java SE 8 Lambdas and Streams 17.1 17.2 Introduction Functional Programming Technologies Overview 17.2.1 Functional Interfaces 17.2.2 Lambda Expressions 17.2.3 Streams IntStream Operations 17.3.1 Creating an IntStream and Displaying Its Values with the forEach Terminal Operation 17.3.2 Terminal Operations count, min, max, sum and average 17.3.3 Terminal Operation reduce 17.3.4 Intermediate Operations: Filtering and Sorting IntStream Values 17.3.5 Intermediate Operation: Mapping 17.3.6 Creating Streams of ints with IntStream Methods range and 17.3 rangeClosed 17.4 17.5 17.6 Manipulations 17.4.1 Creating a Stream 17.4.2 Sorting a Stream and Collecting the Results 17.4.3 Filtering a Stream and Storing the Results for Later Use 17.4.4 Filtering and Sorting a Stream and Collecting the Results 17.4.5 Sorting Previously Collected Results Stream Manipulations 17.5.1 Mapping Strings to Uppercase Using a Method Reference 17.5.2 Filtering Strings Then Sorting Them in Case-Insensitive Ascending Order 17.5.3 Filtering Strings Then Sorting Them in Case-Insensitive Descending Order Stream Manipulations 17.6.1 Creating and Displaying a List Stream xv 696 697 700 702 704 706 708 710 711 714 718 721 721 722 722 729 730 731 732 733 734 736 738 739 739 741 742 743 743 744 745 745 745 745 746 747 748 748 748 750 xvi Contents 17.6.2 Filtering Employees with Salaries in a Specified Range 17.6.3 Sorting Employees By Multiple Fields 17.6.4 Mapping Employees to Unique Last Name Strings 17.6.5 Grouping Employees By Department 17.6.6 Counting the Number of Employees in Each Department 17.6.7 Summing and Averaging Employee Salaries 17.7 Creating a Stream from a File 17.8 Generating Streams of Random Values 17.9 Lambda Event Handlers 17.10 Additional Notes on Java SE 8 Interfaces 17.11 Java SE 8 and Functional Programming Resources 17.12 Wrap-Up 18 Recursion 18.1 18.2 18.3 18.4 18.5 18.6 18.7 18.8 18.9 Introduction Recursion Concepts Example Using Recursion: Factorials Reimplementing Class FactorialCalculator Using Class BigInteger Example Using Recursion: Fibonacci Series Recursion and the Method-Call Stack Recursion vs. Iteration Towers of Hanoi Fractals 18.9.1 Koch Curve Fractal 18.9.2 (Optional) Case Study: Lo Feather Fractal 18.10 Recursive Backtracking 18.11 Wrap-Up 19 Searching, Sorting and Big O 19.1 19.2 19.3 Introduction Linear Search Big O Notation 19.3.1 O(1) Algorithms 19.3.2 O(n) Algorithms 19.3.3 O(n2) Algorithms 19.3.4 Big O of the Linear Search Binary Search 19.4.1 Binary Search Implementation 19.4.2 Efficiency of the Binary Search Sorting Algorithms Selection Sort 19.6.1 Selection Sort Implementation 19.6.2 Efficiency of the Selection Sort Insertion Sort 19.4 19.5 19.6 19.7 751 752 754 755 756 756 758 761 763 763 764 764 776 777 778 779 781 783 786 787 789 791 791 792 801 802 810 811 812 814 814 815 815 816 816 817 820 820 821 821 824 824 Contents 19.7.1 Insertion Sort Implementation 19.7.2 Efficiency of the Insertion Sort 19.8 Merge Sort 19.8.1 Merge Sort Implementation 19.8.2 Efficiency of the Merge Sort 19.9 Big O Summary for This Chapter’s Searching and Sorting Algorithms 19.10 Wrap-Up 20 Generic Classes and Methods 20.1 20.2 20.3 20.4 20.5 20.6 20.7 20.8 20.9 Introduction Motivation for Generic Methods Generic Methods: Implementation and Compile-Time Translation Additional Compile-Time Translation Issues: Methods That Use a Type Parameter as the Return Type Overloading Generic Methods Generic Classes Raw Types Wildcards in Methods That Accept Type Parameters Wrap-Up 21 Custom Generic Data Structures 21.1 21.2 21.3 21.4 21.5 21.6 21.7 21.8 Introduction Self-Referential Classes Dynamic Memory Allocation Linked Lists 21.4.1 Singly Linked Lists 21.4.2 Implementing a Generic List Class 21.4.3 Generic Classes ListNode and List 21.4.4 Class ListTest 21.4.5 List Method insertAtFront 21.4.6 List Method insertAtBack 21.4.7 List Method removeFromFront 21.4.8 List Method removeFromBack 21.4.9 List Method print 21.4.10 Creating Your Own Packages Stacks Queues Trees Wrap-Up 22 GUI Components: Part 2 22.1 22.2 Introduction JSlider xvii 825 827 827 828 832 833 834 839 840 840 842 845 848 849 856 860 864 869 870 871 871 872 872 873 878 878 878 879 880 881 882 882 886 890 893 900 911 912 912 xviii Contents 22.3 22.4 22.5 22.6 22.7 22.8 22.9 22.10 22.11 Understanding Windows in Java Using Menus with Frames 23 Concurrency 23.1 23.2 Introduction Thread States and Life Cycle 23.2.1 New and Runnable States 23.2.2 Waiting State 23.2.3 Timed Waiting State 23.2.4 Blocked State 23.2.5 Terminated State 23.2.6 Operating-System View of the Runnable State 23.2.7 Thread Priorities and Thread Scheduling 23.2.8 Indefinite Postponement and Deadlock Creating and Executing Threads with the Executor Framework Thread Synchronization 23.4.1 Immutable Data 23.4.2 Monitors 23.4.3 Unsynchronized Mutable Data Sharing 23.4.4 Synchronized Mutable Data Sharing—Making Operations Atomic Producer/Consumer Relationship without Synchronization Producer/Consumer Relationship: ArrayBlockingQueue (Advanced) Producer/Consumer Relationship with synchronized, wait, notify and notifyAll (Advanced) Producer/Consumer Relationship: Bounded Buffers (Advanced) Producer/Consumer Relationship: The Lock and Condition Interfaces Concurrent Collections Multithreading with GUI: SwingWorker 23.11.1 Performing Computations in a Worker Thread: Fibonacci Numbers 23.11.2 Processing Intermediate Results: Sieve of Eratosthenes sort and parallelSort Timings with the Java SE 8 Date/Time API Java SE 8: Sequential vs. Parallel Streams (Advanced) Interfaces Callable and Future (Advanced) Fork/Join Framework Wrap-Up 23.3 23.4 23.5 23.6 23.7 23.8 23.9 23.10 23.11 23.12 23.13 23.14 23.15 23.16 JPopupMenu Pluggable Look-and-Feel JDesktopPane and JInternalFrame JTabbedPane BoxLayout Layout Manager Layout Manager GridBagLayout Wrap-Up 916 917 925 928 933 936 938 942 952 957 www.allitebooks.com 958 960 961 961 961 961 961 962 962 963 963 967 968 968 969 974 976 984 987 994 1002 1009 1011 1012 1018 1025 1027 1030 1034 1034 Contents 24 Accessing Databases with JDBC 24.1 24.2 24.3 24.4 Introduction Relational Databases A books Database SQL 24.4.1 Basic SELECT Query 24.4.2 WHERE Clause 24.4.3 ORDER BY Clause 24.4.4 Merging Data from Multiple Tables: INNER JOIN 24.4.5 INSERT Statement 24.4.6 UPDATE Statement 24.4.7 DELETE Statement 24.5 Setting up a Java DB Database 24.5.1 Creating the Chapter’s Databases on Windows 24.5.2 Creating the Chapter’s Databases on Mac OS X 24.5.3 Creating the Chapter’s Databases on Linux 24.6 Manipulating Databases with JDBC 24.6.1 Connecting to and Querying a Database 24.6.2 Querying the books Database 24.7 RowSet Interface 24.8 PreparedStatements 24.9 Stored Procedures 24.10 Transaction Processing 24.11 Wrap-Up 25 JavaFX GUI: Part 1 25.1 25.2 25.3 25.4 Introduction JavaFX Scene Builder and the NetBeans IDE JavaFX App Window Structure Welcome App—Displaying Text and an Image 25.4.1 Creating the App’s Project 25.4.2 NetBeans Projects Window—Viewing the Project Contents 25.4.3 Adding an Image to the Project 25.4.4 Opening JavaFX Scene Builder from NetBeans 25.4.5 Changing to a VBox Layout Container 25.4.6 Configuring the VBox Layout Container 25.4.7 Adding and Configuring a Label 25.4.8 Adding and Configuring an ImageView 25.4.9 Running the Welcome App Tip Calculator App—Introduction to Event Handling 25.5.1 Test-Driving the Tip Calculator App 25.5.2 Technologies Overview 25.5.3 Building the App’s GUI 25.5.4 TipCalculator Class 25.5.5 TipCalculatorController Class 25.5 xix 1045 1046 1047 1048 1052 1052 1053 1055 1056 1058 1059 1060 1060 1061 1062 1063 1063 1063 1067 1080 1082 1098 1098 1099 1107 1108 1109 1110 1111 1111 1113 1114 1114 1115 1116 1116 1116 1117 1118 1119 1119 1122 1126 1128 xx 25.6 25.7 Contents Features Covered in the Online JavaFX Chapters Wrap-Up 1133 1134 Chapters on the Web 1141 A Operator Precedence Chart 1143 B ASCII Character Set 1145 C Keywords and Reserved Words 1146 D Primitive Types 1147 E Using the Debugger 1148 E.1 E.2 E.3 E.4 E.5 E.6 E.7 Introduction Breakpoints and the run, stop, cont and print Commands The print and set Commands Controlling Execution Using the step, step up and next Commands The watch Command The clear Command Wrap-Up 1149 1149 1153 1155 1158 1160 1162 Appendices on the Web 1165 Index 1167 Online Chapters and Appendices Chapters 26–34 and Appendices F–N are PDF documents posted online at the book’s Companion Website (located at www.pearsonhighered.com/deitel/). See the inside front cover for information on accessing the Companion Website. 26 27 28 29 30 JavaFX GUI: Part 2 JavaFX Graphics and Multimedia Networking Java Persistence API (JPA) JavaServer™ Faces Web Apps: Part 1 Contents 31 32 33 JavaServer™ Faces Web Apps: Part 2 34 (Optional) ATM Case Study, Part 2: Implementing an Object-Oriented Design F G H I J K L M N Using the Java API Documentation REST-Based Web Services (Optional) ATM Case Study, Part 1: Object-Oriented Design with the UML Creating Documentation with javadoc Unicode® Formatted Output Number Systems Bit Manipulation Labeled break and continue Statements UML 2: Additional Diagram Types Design Patterns xxi This page intentionally left blank Foreword I’ve been enamored with Java even prior to its 1.0 release in 1995, and have subsequently been a Java developer, author, speaker, teacher and Oracle Java Technology Ambassador. In this journey, it has been my privilege to call Paul Deitel a colleague, and to often leverage and recommend his Java How To Program book. In its many editions, this book has proven to be a great text for college and professional courses that I and others have developed to teach the Java programming language. One of the qualities that makes this book a great resource is its thorough and insightful coverage of Java concepts, including those introduced recently in Java SE 8. Another useful quality is its treatment of concepts and practices essential to effective software development. As a long-time fan of this book, I’d like to point out some of the features of this tenth edition about which I’m most excited: • An ambitious new chapter on Java lambda expressions and streams. This chapter starts out with a primer on functional programming, introducing Java lambda expressions and how to use streams to perform functional programming tasks on collections. • Although concurrency has been addressed since the first edition of the book, it is increasingly important because of multi-core architectures. There are timing examples—using the new Date/Time API classes introduced in Java SE 8—in the concurrency chapter that show the performance improvements with multi-core over single-core. • JavaFX is Java’s GUI/graphics/multimedia technology moving forward, so it is nice to see a three-chapter treatment of JavaFX in the Deitel live-code pedagogic style. One of these chapters is in the printed book and the other two are online. Please join me in congratulating Paul and Harvey Deitel on their latest edition of a wonderful resource for computer science students and software developers alike! James L. Weaver Java Technology Ambassador Oracle Corporation This page intentionally left blank Preface “The chief merit of language is clearness…” —Galen Welcome to the Java programming language and Java How to Program, Tenth Edition! This book presents leading-edge computing technologies for students, instructors and software developers. It’s appropriate for introductory academic and professional course sequences based on the curriculum recommendations of the ACM and the IEEE, and for AP Computer Science exam preparation. If you haven’t already done so, please read the back cover and inside back cover—these concisely capture the essence of the book. In this Preface we provide more detail. We focus on software engineering best practices. At the heart of the book is the Deitel signature “live-code approach”—rather than using code snippets, we present concepts in the context of complete working programs that run on recent versions of Windows®, OS X® and Linux®. Each complete code example is accompanied by live sample executions. Keeping in Touch with the Authors As you read the book, if you have questions, send an e-mail to us at [email protected] and we’ll respond promptly. For updates on this book, visit http://www.deitel.com/books/jhtp10 subscribe to the Deitel ® Buzz Online newsletter at http://www.deitel.com/newsletter/subscribe.html and join the Deitel social networking communities on • Facebook® (http://www.deitel.com/deitelfan) • Twitter® (@deitel) • Google+™ (http://google.com/+DeitelFan) • YouTube® (http://youtube.com/DeitelTV) • LinkedIn® (http://linkedin.com/company/deitel-&-associates) Source Code and VideoNotes All the source code is available at: http://www.deitel.com/books/jhtp10 and at the book’s Companion Website (which also contains extensive VideoNotes): http://www.pearsonhighered.com/deitel xxvi Preface Modular Organization1 Java How to Program, 10/e, is appropriate for programming courses at various levels, most notably CS 1 and CS 2 courses and introductory course sequences in related disciplines. The book’s modular organization helps instructors plan their syllabi: Introduction • Chapter 1, Introduction to Computers, the Internet and Java • Chapter 2, Introduction to Java Applications; Input/Output and Operators • Chapter 3, Introduction to Classes, Objects, Methods and Strings Additional Programming Fundamentals • Chapter 4, Control Statements: Part 1; Assignment, ++ and -- Operators • Chapter 5, Control Statements: Part 2; Logical Operators • Chapter 6, Methods: A Deeper Look • Chapter 7, Arrays and ArrayLists • Chapter 14, Strings, Characters and Regular Expressions • Chapter 15, Files, Streams and Object Serialization Object-Oriented Programming and Object-Oriented Design • Chapter 8, Classes and Objects: A Deeper Look • Chapter 9, Object-Oriented Programming: Inheritance • Chapter 10, Object-Oriented Programming: Polymorphism and Interfaces • Chapter 11, Exception Handling: A Deeper Look • (Online) Chapter 33, ATM Case Study, Part 1: Object-Oriented Design with the UML • (Online) Chapter 34, ATM Case Study Part 2: Implementing an Object-Oriented Design Swing Graphical User Interfaces and Java 2D Graphics • Chapter 12, GUI Components: Part 1 • Chapter 13, Graphics and Java 2D • Chapter 22, GUI Components: Part 2 Data Structures, Collections, Lambdas and Streams • Chapter 16, Generic Collections • Chapter 17, Java SE 8 Lambdas and Streams • Chapter 18, Recursion • Chapter 19, Searching, Sorting and Big O • Chapter 20, Generic Classes and Methods • Chapter 21, Custom Generic Data Structures 1. The online chapters will be available on the book’s Companion Website for Fall 2014 classes. New and Updated Features xxvii Concurrency; Networking • Chapter 23, Concurrency • (Online) Chapter 28, Networking JavaFX Graphical User Interfaces, Graphics and Multimedia • Chapter 25, JavaFX GUI: Part 1 • (Online) Chapter 26, JavaFX GUI: Part 2 • (Online) Chapter 27, JavaFX Graphics and Multimedia Database-Driven Desktop and Web Development • Chapter 24, Accessing Databases with JDBC • (Online) Chapter 29, Java Persistence API (JPA) • (Online) Chapter 30, JavaServer™ Faces Web Apps: Part 1 • (Online) Chapter 31, JavaServer™ Faces Web Apps: Part 2 • (Online) Chapter 32, REST-Based Web Services New and Updated Features Here are the updates we’ve made for Java How to Program, 10/e: Java Standard Edition: Java SE 7 and the New Java SE 8 • Easy to use with Java SE 7 or Java SE 8. To meet the needs of our audiences, we designed the book for college and professional courses based on Java SE 7, Java SE 8 or a mixture of both. The Java SE 8 features are covered in optional, easy-toinclude-or-omit sections. The new Java SE 8 capabilities can dramatically improve the programming process. Figure 1 lists some new Java SE 8 features that we cover. Java SE 8 features Lambda expressions Type-inference improvements @FunctionalInterface annotation Parallel array sorting Bulk data operations for Java Collections—filter, map and reduce Library enhancements to support lambdas (e.g., java.util.stream, java.util.function) Date & Time API (java.time) Java concurrency API improvements static and default methods in interfaces Functional interfaces—interfaces that define only one abstract method and can include static and default methods JavaFX enhancements Fig. 1 | Some new Java SE 8 features. xxviii • Preface Java SE 8 lambdas, streams, and interfaces with default and static methods. The most significant new features in JavaSE 8 are lambdas and complementary technologies, which we cover in detail in the optional Chapter 17 and optional sections marked “Java SE 8” in later chapters. In Chapter 17, you’ll see that functional programming with lambdas and streams can help you write programs faster, more concisely, more simply, with fewer bugs and that are easier to parallelize (to get performance improvements on multi-core systems) than programs written with previous techniques. You’ll see that functional programming complements objectoriented programming. After you read Chapter 17, you’ll be able to cleverly reimplement many of the Java SE 7 examples throughout the book (Fig. 2). Pre-Java-SE-8 topics Corresponding Java SE 8 discussions and examples Chapter 7, Arrays and ArrayLists Sections 17.3–17.4 introduce basic lambda and streams capabilities that process one-dimensional arrays. Section 10.10 introduces the new Java SE 8 interface features (default methods, static methods and the concept of functional interfaces) that support functional programming with lambdas and streams. Section 17.9 shows how to use a lambda to implement a Swing event-listener functional interface. Section 17.5 shows how to use lambdas and streams to process collections of String objects. Section 17.7 shows how to use lambdas and streams to process lines of text from a file. Shows that functional programs are easier to parallelize so that they can take advantage of multi-core architectures to enhance performance. Demonstrates parallel stream processing. Shows that Arrays method parallelSort improves performance on multi-core architectures when sorting large arrays. Section 25.5.5 shows how to use a lambda to implement a JavaFX event-listener functional interface. Chapter 10, Object-Oriented Programming: Polymorphism and Interfaces Chapters 12 and 22, GUI Components: Part 1 and 2, respectively Chapter 14, Strings, Characters and Regular Expressions Chapter 15, Files, Streams and Object Serialization Chapter 23, Concurrency Chapter 25, JavaFX GUI: Part 1 Fig. 2 | Java SE 8 lambdas and streams discussions and examples. • Java SE 7’s try-with-resources statement and the AutoClosable Interface. AutoClosable objects reduce the likelihood of resource leaks when you use them with the try-with-resources statement, which automatically closes the AutoClosable objects. In this edition, we use try-with-resources and AutoClosable objects as appropriate starting in Chapter 15, Files, Streams and Object Serialization. • Java security. We audited our book against the CERT Oracle Secure Coding Standard for Java as appropriate for an introductory textbook. http://bit.ly/CERTOracleSecureJava See the Secure Java Programming section of this Preface for more information about CERT. www.allitebooks.com New and Updated Features xxix • Java NIO API. We updated the file-processing examples in Chapter 15 to use features from the Java NIO (new IO) API. • Java Documentation. Throughout the book, we provide links to Java documentation where you can learn more about various topics that we present. For Java SE 7 documentation, the links begin with http://docs.oracle.com/javase/7/ and for Java SE 8 documentation, the links begin with http://download.java.net/jdk8/ These links could change when Oracle releases Java SE 8—possibly to links beginning with http://docs.oracle.com/javase/8/ For any links that change after publication, we’ll post updates at http://www.deitel.com/books/jhtp10 Swing and JavaFX GUI, Graphics and Multimedia • Swing GUI and Java 2D graphics. Java’s Swing GUI is discussed in the optional GUI and graphics sections in Chapters 3–10 and in Chapters 12 and 22. Swing is now in maintenance mode—Oracle has stopped development and will provide only bug fixes going forward, however it will remain part of Java and is still widely used. Chapter 13 discusses Java 2D graphics. • JavaFX GUI, graphics and multimedia. Java’s GUI, graphics and multimedia API going forward is JavaFX. In Chapter 25, we use JavaFX 2.2 (released in 2012) with Java SE 7. Our online Chapters 26 and 27—located on the book’s companion website (see the inside front cover of this book)—present additional JavaFX GUI features and introduce JavaFX graphics and multimedia in the context of Java FX 8 and Java SE 8. In Chapters 25–27 we use Scene Builder—a drag-and-drop tool for creating JavaFX GUIs quickly and conveniently. It’s a standalone tool that you can use separately or with any of the Java IDEs. • Scalable GUI and graphics presentation. Instructors teaching introductory courses have a broad choice of the amount of GUI, graphics and multimedia to cover— from none at all, to optional introductory sections in the early chapters, to a deep treatment of Swing GUI and Java 2D graphics in Chapters 12, 13 and 22, and a deep treatment of JavaFX GUI, graphics and multimedia in Chapter 25 and online Chapters 26–27. Concurrency • Concurrency for optimal multi-core performance. In this edition, we were privileged to have as a reviewer Brian Goetz, co-author of Java Concurrency in Practice (Addison-Wesley). We updated Chapter 23, with Java SE 8 technology and idiom. We added a parallelSort vs. sort example that uses the Java SE 8 Date/Time API to time each operation and demonstrate parallelSort’s better performance on a multi-core system. We include a Java SE 8 parallel vs. sequential stream processing example, again using the Date/Time API to show performance improvements. Fi- xxx Preface nally, we added a Java SE 8 CompletableFuture example that demonstrates sequential and parallel execution of long-running calculations. class. We use class SwingWorker to create multithreaded user interfaces. In online Chapter 26, we show how JavaFX handles concurrency. • SwingWorker • Concurrency is challenging. Programming concurrent applications is difficult and error-prone. There’s a great variety of concurrency features. We point out the ones that most people should use and mention those that should be left to the experts. Getting Monetary Amounts Right • Monetary amounts. In the early chapters, for convenience, we use type double to represent monetary amounts. Due to the potential for incorrect monetary calculations with type double, class BigDecimal (which is a bit more complex) should be used to represent monetary amounts. We demonstrate BigDecimal in Chapters 8 and 25. Object Technology • Object-oriented programming and design. We use an early objects approach, introducing the basic concepts and terminology of object technology in Chapter 1. Students develop their first customized classes and objects in Chapter 3. Presenting objects and classes early gets students “thinking about objects” immediately and mastering these concepts more thoroughly. [For courses that require a lateobjects approach, consider Java How to Program, 10/e, Late Objects Version.] • Early objects real-world case studies. The early classes and objects presentation features Account, Student, AutoPolicy, Time, Employee, GradeBook and Card shuffling-and-dealing case studies, gradually introducing deeper OO concepts. • Inheritance, Interfaces, Polymorphism and Composition. We use a series of realworld case studies to illustrate each of these OO concepts and explain situations in which each is preferred in building industrial-strength applications. • Exception handling. We integrate basic exception handling early in the book then present a deeper treatment in Chapter 11. Exception handling is important for building “mission-critical” and “business-critical” applications. Programmers need to be concerned with, “What happens when the component I call on to do a job experiences difficulty? How will that component signal that it had a problem?” To use a Java component, you need to know not only how that component behaves when “things go well,” but also what exceptions that component “throws” when “things go poorly.” • Class Arrays and ArrayList. Chapter 7 covers class Arrays—which contains methods for performing common array manipulations—and class ArrayList— which implements a dynamically resizable array-like data structure. This follows our philosophy of getting lots of practice using existing classes while learning how to define your own classes. The chapter’s rich selection of exercises includes a substantial project on building your own computer through the technique of software simulation. Chapter 21 includes a follow-on project on building your own compiler that can compile high-level language programs into machine language code that will execute on your computer simulator. New and Updated Features • xxxi Optional Online Case Study: Developing an Object-Oriented Design and Java Implementation of an ATM. Online Chapters 33–34 include an optional case study on object-oriented design using the UML (Unified Modeling Language™)—the industry-standard graphical language for modeling object-oriented systems. We design and implement the software for a simple automated teller machine (ATM). We analyze a typical requirements document that specifies the system to be built. We determine the classes needed to implement that system, the attributes the classes need to have, the behaviors the classes need to exhibit and specify how the classes must interact with one another to meet the system requirements. From the design we produce a complete Java implementation. Students often report having a “light-bulb moment”—the case study helps them “tie it all together” and really understand object orientation. Data Structures and Generic Collections • Data structures presentation. We begin with generic class ArrayList in Chapter 7. Our later data structures discussions (Chapters 16–21) provide a deeper treatment of generic collections—showing how to use the built-in collections of the Java API. We discuss recursion, which is important for implementing tree-like, data-structure classes. We discuss popular searching and sorting algorithms for manipulating the contents of collections, and provide a friendly introduction to Big O—a means of describing how hard an algorithm might have to work to solve a problem. We then show how to implement generic methods and classes, and custom generic data structures (this is intended for computer-science majors—most programmers should use the pre-built generic collections). Lambdas and streams (introduced in Chapter 17) are especially useful for working with generic collections. Database • JDBC. Chapter 24 covers JDBC and uses the Java DB database management system. The chapter introduces Structured Query Language (SQL) and features an OO case study on developing a database-driven address book that demonstrates prepared statements. • Java Persistence API. The new online Chapter 29 covers the Java Persistence API (JPA)—a standard for object relational mapping (ORM) that uses JDBC “under the hood.” ORM tools can look at a database’s schema and generate a set of classes that enabled you to interact with a database without having to use JDBC and SQL directly. This speeds database-application development, reduces errors and produces more portable code. Web Application Development • Java Server Faces (JSF). Online Chapters 30–31 have been updated to introduce the latest JavaServer Faces (JSF) technology, which facilitates building JSF webbased applications. Chapter 30 includes examples on building web application GUIs, validating forms and session tracking. Chapter 31 discusses data-driven, Ajax-enabled JSF applications—the chapter features a database-driven multitier web address book that allows users to add and search for contacts. • Web services. Chapter 32 now concentrates on creating and consuming RESTbased web services. The vast majority of today’s web services now use REST. xxxii Preface Secure Java Programming It’s difficult to build industrial-strength systems that stand up to attacks from viruses, worms, and other forms of “malware.” Today, via the Internet, such attacks can be instantaneous and global in scope. Building security into software from the beginning of the development cycle can greatly reduce vulnerabilities. We incorporate various secure Java coding practices (as appropriate for an introductory textbook) into our discussions and code examples. The CERT® Coordination Center (www.cert.org) was created to analyze and respond promptly to attacks. CERT—the Computer Emergency Response Team—is a government-funded organization within the Carnegie Mellon University Software Engineering Institute™. CERT publishes and promotes secure coding standards for various popular programming languages to help software developers implement industrialstrength systems that avoid the programming practices which leave systems open to attack. We’d like to thank Robert C. Seacord, Secure Coding Manager at CERT and an adjunct professor in the Carnegie Mellon University School of Computer Science. Mr. Seacord was a technical reviewer for our book, C How to Program, 7/e, where he scrutinized our C programs from a security standpoint, recommending that we adhere to the CERT C Secure Coding Standard. This experience influenced our coding practices in C++ How to Program, 9/e and Java How to Program, 10/e as well. Optional GUI and Graphics Case Study Students enjoy building GUI and graphics applications. For courses that introduce GUI and graphics early, we’ve integrated an optional 10-segment introduction to creating graphics and Swing-based graphical user interfaces (GUIs). The goal of this case study is to create a simple polymorphic drawing application in which the user can select a shape to draw, select the characteristics of the shape (such as its color) and use the mouse to draw the shape. The case study builds gradually toward that goal, with the reader implementing polymorphic drawing in Chapter 10, adding an event-driven GUI in Chapter 12 and enhancing the drawing capabilities in Chapter 13 with Java 2D. • Section 3.6—Using Dialog Boxes • Section 4.15—Creating Simple Drawings • Section 5.11—Drawing Rectangles and Ovals • Section 6.13—Colors and Filled Shapes • Section 7.17—Drawing Arcs • Section 8.16—Using Objects with Graphics • Section 9.7—Displaying Text and Images Using Labels • Section 10.11—Drawing with Polymorphism • Exercise 12.17—Expanding the Interface • Exercise 13.31—Adding Java2D Teaching Approach Java How to Program, 10/e, contains hundreds of complete working examples. We stress program clarity and concentrate on building well-engineered software. Teaching Approach xxxiii VideoNotes. The Companion Website includes extensive VideoNotes in which co-author Paul Deitel explains in detail most of the programs in the book’s core chapters. Students like viewing the VideoNotes for reinforcement of core concepts and for additional insights. Syntax Coloring. For readability, we syntax color all the Java code, similar to the way most Java integrated-development environments and code editors syntax color code. Our syntax-coloring conventions are as follows: comments appear in green keywords appear in dark blue errors appear in red constants and literal values appear in light blue all other code appears in black Code Highlighting. We place yellow rectangles around key code segments. Using Fonts for Emphasis. We place the key terms and the index’s page reference for each defining occurrence in bold maroon text for easier reference. We emphasize on-screen components in the bold Helvetica font (e.g., the File menu) and emphasize Java program text in the Lucida font (for example, int x = 5;). Web Access. All of the source-code examples can be downloaded from: http://www.deitel.com/books/jhtp10 http://www.pearsonhighered.com/deitel Objectives. The opening quotes are followed by a list of chapter objectives. Illustrations/Figures. Abundant tables, line drawings, UML diagrams, programs and program outputs are included. Programming Tips. We include programming tips to help you focus on important aspects of program development. These tips and practices represent the best we’ve gleaned from a combined seven decades of programming and teaching experience. Good Programming Practice The Good Programming Practices call attention to techniques that will help you produce programs that are clearer, more understandable and more maintainable. Common Programming Error Pointing out these Common Programming Errors reduces the likelihood that you’ll make them. Error-Prevention Tip These tips contain suggestions for exposing bugs and removing them from your programs; many describe aspects of Java that prevent bugs from getting into programs in the first place. Performance Tip These tips highlight opportunities for making your programs run faster or minimizing the amount of memory that they occupy. Portability Tip The Portability Tips help you write code that will run on a variety of platforms. xxxiv Preface Software Engineering Observation The Software Engineering Observations highlight architectural and design issues that affect the construction of software systems, especially large-scale systems. Look-and-Feel Observation The Look-and-Feel Observations highlight graphical-user-interface conventions. These observations help you design attractive, user-friendly graphical user interfaces that conform to industry norms. Summary Bullets. We present a section-by-section bullet-list summary of the chapter. For ease of reference, we include the page number of each key term’s defining occurrence in the text. Self-Review Exercises and Answers. Extensive self-review exercises and answers are included for self study. All of the exercises in the optional ATM case study are fully solved. Exercises. The chapter exercises include: • simple recall of important terminology and concepts • What’s wrong with this code? • What does this code do? • writing individual statements and small portions of methods and classes • writing complete methods, classes and programs • major projects • in many chapters, Making a Difference exercises that encourage you to use computers and the Internet to research and solve significant social problems. Exercises that are purely SE 8 are marked as such. Check out our Programming Projects Resource Center for lots of additional exercise and project possibilities (www.deitel.com/ ProgrammingProjects/). Index. We’ve included an extensive index. Defining occurrences of key terms are highlighted with a bold maroon page number. The print book index mentions only those terms used in the print book. The online chapters index includes all the print book terms and the online chapter terms. Software Used in Java How to Program, 10/e All the software you’ll need for this book is available free for download from the Internet. See the Before You Begin section that follows this Preface for links to each download. We wrote most of the examples in Java How to Program, 10/e, using the free Java Standard Edition Development Kit (JDK) 7. For the optional Java SE 8 modules, we used the OpenJDK’s early access version of JDK 8. In Chapter 25 and several online chapters, we also used the Netbeans IDE. See the Before You Begin section that follows this Preface for more information. You can find additional resources and software downloads in our Java Resource Centers at: www.deitel.com/ResourceCenters.html Instructor Supplements xxxv Instructor Supplements The following supplements are available to qualified instructors only through Pearson Education’s Instructor Resource Center (www.pearsonhighered.com/irc): • PowerPoint® slides containing all the code and figures in the text, plus bulleted items that summarize key points. • Test Item File of multiple-choice questions (approximately two per book section). • Solutions Manual with solutions to the vast majority of the end-of-chapter exercises. Before assigning an exercise for homework, instructors should check the IRC to be sure it includes the solution. Please do not write to us requesting access to the Pearson Instructor’s Resource Center which contains the book’s instructor supplements, including the exercise solutions. Access is limited strictly to college instructors teaching from the book. Instructors may obtain access only through their Pearson representatives. Solutions are not provided for “project” exercises. If you’re not a registered faculty member, contact your Pearson representative or visit www.pearsonhighered.com/educator/replocator/. Acknowledgments We’d like to thank Abbey Deitel and Barbara Deitel for long hours devoted to this project. We’re fortunate to have worked on this project with the dedicated team of publishing professionals at Pearson. We appreciate the guidance, wisdom and energy of Tracy Johnson, Executive Editor, Computer Science. Tracy and her team handle all of our academic textbooks. Carole Snyder recruited the book’s academic reviewers and managed the review process. Bob Engelhardt managed the book’s publication. We selected the cover art and Laura Gardner designed the cover. Reviewers We wish to acknowledge the efforts of our recent editions reviewers—a distinguished group of academics, Oracle Java team members, Oracle Java Champions and other industry professionals. They scrutinized the text and the programs and provided countless suggestions for improving the presentation. We appreciate the guidance of Jim Weaver and Johan Vos (co-authors of Pro JavaFX 2), and Simon Ritter on the three JavaFX chapters. Tenth Edition reviewers: Lance Andersen (Oracle Corporation), Dr. Danny Coward (Oracle Corporation), Brian Goetz (Oracle Corporation), Evan Golub (University of Maryland), Dr. Huiwei Guan (Professor, Department of Computer & Information Science, North Shore Community College), Manfred Riem (Java Champion), Simon Ritter (Oracle Corporation), Robert C. Seacord (CERT, Software Engineering Institute, Carnegie Mellon University), Khallai Taylor (Assistant Professor, Triton College and Adjunct Professor, Lonestar College—Kingwood), Jorge Vargas (Yumbling and a Java Champion), Johan Vos (LodgON and Oracle Java Champion) and James L. Weaver (Oracle Corporation and author of Pro JavaFX 2). Previous editions reviewers: Soundararajan Angusamy (Sun Microsystems), Joseph Bowbeer (Consultant), William E. Duncan (Louisiana State University), Diana Franklin xxxvi Preface (University of California, Santa Barbara), Edward F. Gehringer (North Carolina State University), Ric Heishman (George Mason University), Dr. Heinz Kabutz (JavaSpecialists.eu), Patty Kraft (San Diego State University), Lawrence Premkumar (Sun Microsystems), Tim Margush (University of Akron), Sue McFarland Metzger (Villanova University), Shyamal Mitra (The University of Texas at Austin), Peter Pilgrim (Consultant), Manjeet Rege, Ph.D. (Rochester Institute of Technology), Susan Rodger (Duke University), Amr Sabry (Indiana University), José Antonio González Seco (Parliament of Andalusia), Sang Shin (Sun Microsystems), S. Sivakumar (Astra Infotech Private Limited), Raghavan “Rags” Srinivas (Intuit), Monica Sweat (Georgia Tech), Vinod Varma (Astra Infotech Private Limited) and Alexander Zuev (Sun Microsystems). A Special Thank You to Brian Goetz We were privileged to have Brian Goetz, Oracle’s Java Language Architect and Specification Lead for Java SE 8’s Project Lambda, and co-author of Java Concurrency in Practice, do a detailed full-book review. He thoroughly scrutinized every chapter, providing extremely helpful insights and constructive comments. Any remaining faults in the book are our own. Well, there you have it! As you read the book, we’d appreciate your comments, criticisms, corrections and suggestions for improvement. Please address all correspondence to: [email protected] We’ll respond promptly. We hope you enjoy working with Java How to Program, 10/e, as much as we enjoyed writing it! Paul and Harvey Deitel About the Authors Paul Deitel, CEO and Chief Technical Officer of Deitel & Associates, Inc., is a graduate of MIT, where he studied Information Technology. He holds the Java Certified Programmer and Java Certified Developer designations, and is an Oracle Java Champion. Through Deitel & Associates, Inc., he has delivered hundreds of programming courses worldwide to clients, including Cisco, IBM, Siemens, Sun Microsystems, Dell, Fidelity, NASA at the Kennedy Space Center, the National Severe Storm Laboratory, White Sands Missile Range, Rogue Wave Software, Boeing, SunGard Higher Education, Nortel Networks, Puma, iRobot, Invensys and many more. He and his co-author, Dr. Harvey M. Deitel, are the world’s best-selling programming-language textbook/professional book/video authors. Dr. Harvey Deitel, Chairman and Chief Strategy Officer of Deitel & Associates, Inc., has over 50 years of experience in the computer field. Dr. Deitel earned B.S. and M.S. degrees in Electrical Engineering from MIT and a Ph.D. in Mathematics from Boston University. He has extensive college teaching experience, including earning tenure and serving as the Chairman of the Computer Science Department at Boston College before founding Deitel & Associates, Inc., in 1991 with his son, Paul. The Deitels’ publications About Deitel® & Associates, Inc. xxxvii have earned international recognition, with translations published in Japanese, German, Russian, Spanish, French, Polish, Italian, Simplified Chinese, Traditional Chinese, Korean, Portuguese, Greek, Urdu and Turkish. Dr. Deitel has delivered hundreds of programming courses to corporate, academic, government and military clients. About Deitel® & Associates, Inc. Deitel & Associates, Inc., founded by Paul Deitel and Harvey Deitel, is an internationally recognized authoring and corporate training organization, specializing in computer programming languages, object technology, mobile app development and Internet and web software technology. The company’s training clients include many of the world’s largest companies, government agencies, branches of the military, and academic institutions. The company offers instructor-led training courses delivered at client sites worldwide on major programming languages and platforms, including Java™, Android app development, Objective-C and iOS app development, C++, C, Visual C#®, Visual Basic®, Visual C++®, Python®, object technology, Internet and web programming and a growing list of additional programming and software development courses. Through its 39-year publishing partnership with Pearson/Prentice Hall, Deitel & Associates, Inc., publishes leading-edge programming textbooks and professional books in print and a wide range of e-book formats, and LiveLessons video courses. Deitel & Associates, Inc. and the authors can be reached at: [email protected] To learn more about Deitel’s Dive-Into® Series Corporate Training curriculum, visit: http://www.deitel.com/training To request a proposal for worldwide on-site, instructor-led training at your organization, e-mail [email protected]. Individuals wishing to purchase Deitel books and LiveLessons video training can do so through www.deitel.com. Bulk orders by corporations, the government, the military and academic institutions should be placed directly with Pearson. For more information, visit http://www.informit.com/store/sales.aspx This page intentionally left blank www.allitebooks.com Before You Begin This section contains information you should review before using this book. Any updates to the information presented here will be posted at: http://www.deitel.com/books/jhtp10 In addition, we provide Dive-Into® videos (which will be available in time for Fall 2014 classes) that demonstrate the instructions in this Before You Begin section. Font and Naming Conventions We use fonts to distinguish between on-screen components (such as menu names and menu items) and Java code or commands. Our convention is to emphasize on-screen components in a sans-serif bold Helvetica font (for example, File menu) and to emphasize Java code and commands in a sans-serif Lucida font (for example, System.out.println()). Software Used in the Book All the software you’ll need for this book is available free for download from the web. With the exception of the examples that are specific to Java SE 8, all of the examples were tested with the Java SE 7 and Java SE 8 Java Standard Edition Development Kits (JDKs). Java Standard Edition Development Kit 7 (JDK 7) JDK 7 for Windows, OS X and Linux platforms is available from: http://www.oracle.com/technetwork/java/javase/downloads/index.html Java Standard Edition Development Kit (JDK) 8 At the time of this publication, the near-final version of JDK 8 for Windows, OS X and Linux platforms was available from: https://jdk8.java.net/download.html Once JDK 8 is released as final, it will be available from: http://www.oracle.com/technetwork/java/javase/downloads/index.html JDK Installation Instructions After downloading the JDK installer, be sure to carefully follow the JDK installation instructions for your platform at: http://docs.oracle.com/javase/7/docs/webnotes/install/index.html Though these instructions are for JDK 7, they also apply to JDK 8—you’ll need to update the JDK version number in any version-specific instructions. xl Before You Begin Setting the PATH Environment Variable The PATH environment variable on your computer designates which directories the computer searches when looking for applications, such as the applications that enable you to compile and run your Java applications (called javac and java, respectively). Carefully follow the installation instructions for Java on your platform to ensure that you set the PATH environment variable correctly. The steps for setting environment variables differ by operating system and sometimes by operating system version (e.g., Windows 7 vs. Windows 8). Instructions for various platforms are listed at: http://www.java.com/en/download/help/path.xml If you do not set the PATH variable correctly on Windows and some Linux installations, when you use the JDK’s tools, you’ll receive a message like: 'java' is not recognized as an internal or external command, operable program or batch file. In this case, go back to the installation instructions for setting the PATH and recheck your steps. If you’ve downloaded a newer version of the JDK, you may need to change the name of the JDK’s installation directory in the PATH variable. JDK Installation Directory and the bin Subdirectory The JDK’s installation directory varies by platform. The directories listed below are for Oracle’s JDK 7 update 51: • 32-bit JDK on Windows: C:\Program Files (x86)\Java\jdk1.7.0_51 • 64-bit JDK on Windows: C:\Program Files\Java\jdk1.7.0_51 • Mac OS X: /Library/Java/JavaVirtualMachines/jdk1.7.0_51.jdk/Contents/Home • Ubuntu Linux: /usr/lib/jvm/java-7-oracle Depending on your platform, the JDK installation folder’s name might differ if you’re using a different update of JDK 7 or using JDK 8. For Linux, the install location depends on the installer you use and possibly the version of Linux that you use. We used Ubuntu Linux. The PATH environment variable must point to the JDK installation directory’s bin subdirectory. When setting the PATH, be sure to use the proper JDK-installation-directory name for the specific version of the JDK you installed—as newer JDK releases become available, the JDK-installation-directory name changes to include an update version number. For example, at the time of this writing, the most recent JDK 7 release was update 51. For this version, the JDK-installation-directory name ends with "_51". Setting the CLASSPATH Environment Variable If you attempt to run a Java program and receive a message like Exception in thread "main" java.lang.NoClassDefFoundError: YourClass Setting the JAVA_HOME Environment Variable xli then your system has a CLASSPATH environment variable that must be modified. To fix the preceding error, follow the steps in setting the PATH environment variable, to locate the CLASSPATH variable, then edit the variable’s value to include the local directory—typically represented as a dot (.). On Windows add .; at the beginning of the CLASSPATH’s value (with no spaces before or after these characters). On other platforms, replace the semicolon with the appropriate path separator characters—typically a colon (:). Setting the JAVA_HOME Environment Variable The Java DB database software that you’ll use in Chapter 24 and several online chapters requires you to set the JAVA_HOME environment variable to your JDK’s installation directory. The same steps you used to set the PATH may also be used to set other environment variables, such as JAVA_HOME. Java Integrated Development Environments (IDEs) There are many Java integrated development environments that you can use for Java programming. For this reason, we used only the JDK command-line tools for most of the book’s examples. We provide Dive-Into® videos (which will be available in time for Fall 2014 classes) that show how to download, install and use three popular IDEs—NetBeans, Eclipse and IntelliJ IDEA. We use NetBeans in Chapter 25 and several of the book’s online chapters. NetBeans Downloads You can download the JDK/NetBeans bundle from: http://www.oracle.com/technetwork/java/javase/downloads/index.html The NetBeans version that’s bundled with the JDK is for Java SE development. The online JavaServer Faces (JSF) chapters and web services chapter use the Java Enterprise Edition (Java EE) version of NetBeans, which you can download from: https://netbeans.org/downloads/ This version supports both Java SE and Java EE development. Eclipse Downloads You can download the Eclipse IDE from: https://www.eclipse.org/downloads/ For Java SE development choose the Eclipse IDE for Java Developers. For Java Enterprise Edition (Java EE) development (such as JSF and web services), choose the Eclipse IDE for Java EE Developers—this version supports both Java SE and Java EE development. IntelliJ IDEA Community Edition Downloads You can download the free IntelliJ IDEA Community Edition from: http://www.jetbrains.com/idea/download/index.html The free version supports only Java SE development. xlii Before You Begin Obtaining the Code Examples The examples for Java How to Program, 10/e are available for download at http://www.deitel.com/books/jhtp10/ under the heading Download Code Examples and Other Premium Content. The examples are also available from http://www.pearsonhighered.com/deitel When you download the ZIP archive file, write down the location where you choose to save it on your computer. Extract the contents of examples.zip using a ZIP extraction tool such as 7-Zip (www.7-zip.org), WinZip (www.winzip.com) or the built-in capabilities of your operating system. Instructions throughout the book assume that the examples are located at: on Windows • C:\examples • your user account home folder’s examples subfolder on Linux • your Documents folders examples subfolder on Mac OS X Java’s Nimbus Look-and-Feel Java comes bundled with a cross-platform look-and-feel known as Nimbus. For programs with Swing graphical user interfaces (e.g., Chapters 12 and 22), we configured our test computers to use Nimbus as the default look-and-feel. To set Nimbus as the default for all Java applications, you must create a text file named swing.properties in the lib folder of both your JDK installation folder and your JRE installation folder. Place the following line of code in the file: swing.defaultlaf=com.sun.java.swing.plaf.nimbus.NimbusLookAndFeel For more information on locating these folders visit http://docs.oracle.com/javase/ 7/docs/webnotes/install/index.html. [Note: In addition to the standalone JRE, there’s a JRE nested in your JDK’s installation folder. If you’re using an IDE that depends on the JDK (e.g., NetBeans), you may also need to place the swing.properties file in the nested jre folder’s lib folder.] You’re now ready to begin your Java studies with Java How to Program, 10/e. We hope you enjoy the book! 1 Introduction to Computers, the Internet and Java Man is still the most extraordinary computer of all. —John F. Kennedy Good design is good business. —Thomas J. Watson, Founder of IBM Objectives In this chapter you’ll: ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Learn about exciting recent developments in the computer field. Learn computer hardware, software and networking basics. Understand the data hierarchy. Understand the different types of programming languages. Understand the importance of Java and other leading programming languages. Understand object-oriented programming basics. Learn the importance of the Internet and the web. Learn a typical Java programdevelopment environment. Test-drive a Java application. Learn some key recent software technologies. See how to keep up-to-date with information technologies. 2 Chapter 1 Introduction to Computers, the Internet and Java 1.1 Introduction 1.2 Hardware and Software 1.2.1 Moore’s Law 1.2.2 Computer Organization 1.3 Data Hierarchy 1.4 Machine Languages, Assembly Languages and High-Level Languages 1.5 Introduction to Object Technology 1.5.1 1.5.2 1.5.3 1.5.4 1.5.5 1.5.6 1.5.7 1.5.8 1.5.9 1.5.10 The Automobile as an Object Methods and Classes Instantiation Reuse Messages and Method Calls Attributes and Instance Variables Encapsulation and Information Hiding Inheritance Interfaces Object-Oriented Analysis and Design (OOAD) 1.5.11 The UML (Unified Modeling Language) 1.6 Operating Systems 1.6.1 Windows—A Proprietary Operating System 1.6.2 Linux—An Open-Source Operating System 1.6.3 Android 1.7 Programming Languages 1.8 Java 1.9 A Typical Java Development Environment 1.10 Test-Driving a Java Application 1.11 Internet and World Wide Web 1.11.1 The Internet: A Network of Networks 1.11.2 The World Wide Web: Making the Internet User-Friendly 1.11.3 Web Services and Mashups 1.11.4 Ajax 1.11.5 The Internet of Things 1.12 Software Technologies 1.13 Keeping Up-to-Date with Information Technologies Self-Review Exercises | Answers to Self-Review Exercises | Exercises | Making a Difference 1.1 Introduction Welcome to Java—one of the world’s most widely used computer programming languages. You’re already familiar with the powerful tasks computers perform. Using this textbook, you’ll write instructions commanding computers to perform those tasks. Software (i.e., the instructions you write) controls hardware (i.e., computers). You’ll learn object-oriented programming—today’s key programming methodology. You’ll create and work with many software objects. For many organizations, the preferred language for meeting their enterprise programming needs is Java. Java is also widely used for implementing Internet-based applications and software for devices that communicate over a network. Forrester Research predicts more than two billion PCs will be in use by 2015.1 According to Oracle, 97% of enterprise desktops, 89% of PC desktops, three billion devices (Fig. 1.1) and 100% of all Blu-ray Disc™ players run Java, and there are over 9 million Java developers.2 According to a study by Gartner, mobile devices will continue to outpace PCs as users’ primary computing devices; an estimated 1.96 billion smartphones and 388 million tablets will be shipped in 2015—8.7 times the number of PCs.3 By 2018, the mobile applications 1. 2. 3. http://www.worldometers.info/computers. http://www.oracle.com/technetwork/articles/java/javaone12review-1863742.html. http://www.gartner.com/newsroom/id/2645115. 1.1 Introduction 3 Devices Airplane systems Blu-ray Disc™ players Credit cards e-Readers Home appliances Lottery terminals MRIs Transportation passes Smart cards Smartphones TV set-top boxes ATMs Cable boxes CT scanners Game consoles Home security systems Medical devices Parking payment stations Robots Smart meters Tablets Thermostats Automobile infotainment systems Copiers Desktop computers GPS navigation systems Light switches Mobile phones Printers Routers Smartpens Televisions Vehicle diagnostic systems Fig. 1.1 | Some devices that use Java. (apps) market is expected to reach $92 billion.4 This is creating significant career opportunities for people who program mobile applications, many of which are programmed in Java (see Section 1.6.3). Java Standard Edition Java has evolved so rapidly that this tenth edition of Java How to Program—based on Java Standard Edition 7 (Java SE 7) and Java Standard Edition 8 (Java SE 8)—was published just 17 years after the first edition. Java Standard Edition contains the capabilities needed to develop desktop and server applications. The book can be used with either Java SE 7 or Java SE 8 (released just after this book was published). All of the Java SE 8 features are discussed in modular, easy-to-include-or-omit sections throughout the book. Prior to Java SE 8, Java supported three programming paradigms—procedural programming, object-oriented programming and generic programming. Java SE 8 adds functional programming. In Chapter 17, we’ll show how to use functional programming to write programs faster, more concisely, with fewer bugs and that are easier to parallelize (i.e., perform multiple calculations simultaneously) to take advantage of today’s multi-core hardware architectures to enhance application performance. Java Enterprise Edition Java is used in such a broad spectrum of applications that it has two other editions. The Java Enterprise Edition (Java EE) is geared toward developing large-scale, distributed networking applications and web-based applications. In the past, most computer applications ran on “standalone” computers (computers that were not networked together). Today’s applications can be written with the aim of communicating among the world’s computers via the Internet and the web. Later in this book we discuss how to build such web-based applications with Java. 4. https://www.abiresearch.com/press/tablets-will-generate-35-of-this-years-25billion-. 4 Chapter 1 Introduction to Computers, the Internet and Java Java Micro Edition The Java Micro Edition (Java ME)—a subset of Java SE—is geared toward developing applications for resource-constrained embedded devices, such as smartwatches, MP3 players, television set-top boxes, smart meters (for monitoring electric energy usage) and more. 1.2 Hardware and Software Computers can perform calculations and make logical decisions phenomenally faster than human beings can. Many of today’s personal computers can perform billions of calculations in one second—more than a human can perform in a lifetime. Supercomputers are already performing thousands of trillions (quadrillions) of instructions per second! China’s National University of Defense Technology’s Tianhe-2 supercomputer can perform over 33 quadrillion calculations per second (33.86 petaflops)!5 To put that in perspective, the Tianhe-2 supercomputer can perform in one second about 3 million calculations for every person on the planet! And—these supercomputing “upper limits” are growing quickly. Computers process data under the control of sequences of instructions called computer programs. These software programs guide the computer through ordered actions specified by people called computer programmers. In this book, you’ll learn a key programming methodology that’s enhancing programmer productivity, thereby reducing software development costs—object-oriented programming. A computer consists of various devices referred to as hardware (e.g., the keyboard, screen, mouse, hard disks, memory, DVD drives and processing units). Computing costs are dropping dramatically, owing to rapid developments in hardware and software technologies. Computers that might have filled large rooms and cost millions of dollars decades ago are now inscribed on silicon chips smaller than a fingernail, costing perhaps a few dollars each. Ironically, silicon is one of the most abundant materials on Earth—it’s an ingredient in common sand. Silicon-chip technology has made computing so economical that computers have become a commodity. 1.2.1 Moore’s Law Every year, you probably expect to pay at least a little more for most products and services. The opposite has been the case in the computer and communications fields, especially with regard to the hardware supporting these technologies. For many decades, hardware costs have fallen rapidly. Every year or two, the capacities of computers have approximately doubled inexpensively. This remarkable trend often is called Moore’s Law, named for the person who identified it in the 1960s, Gordon Moore, co-founder of Intel—the leading manufacturer of the processors in today’s computers and embedded systems. Moore’s Law and related observations apply especially to the amount of memory that computers have for programs, the amount of secondary storage (such as disk storage) they have to hold programs and data over longer periods of time, and their processor speeds—the speeds at which they execute their programs (i.e., do their work). 5. http://www.top500.org/. 1.2 Hardware and Software 5 Similar growth has occurred in the communications field—costs have plummeted as enormous demand for communications bandwidth (i.e., information-carrying capacity) has attracted intense competition. We know of no other fields in which technology improves so quickly and costs fall so rapidly. Such phenomenal improvement is truly fostering the Information Revolution. 1.2.2 Computer Organization Regardless of differences in physical appearance, computers can be envisioned as divided into various logical units or sections (Fig. 1.2). Logical unit Description Input unit This “receiving” section obtains information (data and computer programs) from input devices and places it at the disposal of the other units for processing. Most user input is entered into computers through keyboards, touch screens and mouse devices. Other forms of input include receiving voice commands, scanning images and barcodes, reading from secondary storage devices (such as hard drives, DVD drives, Blu-ray Disc™ drives and USB flash drives—also called “thumb drives” or “memory sticks”), receiving video from a webcam and having your computer receive information from the Internet (such as when you stream videos from YouTube® or download e-books from Amazon). Newer forms of input include position data from a GPS device, and motion and orientation information from an accelerometer (a device that responds to up/down, left/right and forward/backward acceleration) in a smartphone or game controller (such as Microsoft® Kinect® and Xbox®, Wii™ Remote and Sony® PlayStation® Move). This “shipping” section takes information the computer has processed and places it on various output devices to make it available for use outside the computer. Most information that’s output from computers today is displayed on screens (including touch screens), printed on paper (“going green” discourages this), played as audio or video on PCs and media players (such as Apple’s iPods) and giant screens in sports stadiums, transmitted over the Internet or used to control other devices, such as robots and “intelligent” appliances. Information is also commonly output to secondary storage devices, such as hard drives, DVD drives and USB flash drives. A popular recent form of output is smartphone vibration. This rapid-access, relatively low-capacity “warehouse” section retains information that has been entered through the input unit, making it immediately available for processing when needed. The memory unit also retains processed information until it can be placed on output devices by the output unit. Information in the memory unit is volatile—it’s typically lost when the computer’s power is turned off. The memory unit is often called either memory, primary memory or RAM (Random Access Memory). Main memories on desktop and notebook computers contain as much as 128 GB of RAM. GB stands for gigabytes; a gigabyte is approximately one billion bytes. A byte is eight bits. A bit is either a 0 or a 1. Output unit Memory unit Fig. 1.2 | Logical units of a computer. (Part 1 of 2.) 6 Chapter 1 Introduction to Computers, the Internet and Java Logical unit Description Arithmetic and logic unit (ALU) This “manufacturing” section performs calculations, such as addition, subtraction, multiplication and division. It also contains the decision mechanisms that allow the computer, for example, to compare two items from the memory unit to determine whether they’re equal. In today’s systems, the ALU is implemented as part of the next logical unit, the CPU. This “administrative” section coordinates and supervises the operation of the other sections. The CPU tells the input unit when information should be read into the memory unit, tells the ALU when information from the memory unit should be used in calculations and tells the output unit when to send information from the memory unit to certain output devices. Many of today’s computers have multiple CPUs and, hence, can perform many operations simultaneously. A multi-core processor implements multiple processors on a single integrated-circuit chip—a dual-core processor has two CPUs and a quad-core processor has four CPUs. Today’s desktop computers have processors that can execute billions of instructions per second. This is the long-term, high-capacity “warehousing” section. Programs or data not actively being used by the other units normally are placed on secondary storage devices (e.g., your hard drive) until they’re again needed, possibly hours, days, months or even years later. Information on secondary storage devices is persistent—it’s preserved even when the computer’s power is turned off. Secondary storage information takes much longer to access than information in primary memory, but its cost per unit is much less. Examples of secondary storage devices include hard drives, DVD drives and USB flash drives, some of which can hold over 2 TB (TB stands for terabytes; a terabyte is approximately one trillion bytes). Typical hard drives on desktop and notebook computers hold up to 2 TB, and some desktop hard drives can hold up to 4 TB. Central processing unit (CPU) Secondary storage unit Fig. 1.2 | Logical units of a computer. (Part 2 of 2.) 1.3 Data Hierarchy Data items processed by computers form a data hierarchy that becomes larger and more complex in structure as we progress from the simplest data items (called “bits”) to richer ones, such as characters and fields. Figure 1.3 illustrates a portion of the data hierarchy. Bits The smallest data item in a computer can assume the value 0 or the value 1. It’s called a bit (short for “binary digit”—a digit that can assume one of two values). Remarkably, the impressive functions performed by computers involve only the simplest manipulations of 0s and 1s—examining a bit’s value, setting a bit’s value and reversing a bit’s value (from 1 to 0 or from 0 to 1). Characters It’s tedious for people to work with data in the low-level form of bits. Instead, they prefer to work with decimal digits (0–9), letters (A–Z and a–z), and special symbols (e.g., $, @, %, www.allitebooks.com 1.3 Data Hierarchy Judy J u d y 00000000 01001010 1 Sally Black Tom Blue Judy Green Iris Orange Randy Red Green 7 File Record Field Unicode character J Bit Fig. 1.3 | Data hierarchy. &, *, (, ), –, +, ", :, ? and /). Digits, letters and special symbols are known as characters. The computer’s character set is the set of all the characters used to write programs and represent data items. Computers process only 1s and 0s, so a computer’s character set represents every character as a pattern of 1s and 0s. Java uses Unicode® characters that are composed of one, two or four bytes (8, 16 or 32 bits). Unicode contains characters for many of the world’s languages. See Appendix H for more information on Unicode. See Appendix B for more information on the ASCII (American Standard Code for Information Interchange) character set—the popular subset of Unicode that represents uppercase and lowercase letters, digits and some common special characters. Fields Just as characters are composed of bits, fields are composed of characters or bytes. A field is a group of characters or bytes that conveys meaning. For example, a field consisting of uppercase and lowercase letters can be used to represent a person’s name, and a field consisting of decimal digits could represent a person’s age. Records Several related fields can be used to compose a record (implemented as a class in Java). In a payroll system, for example, the record for an employee might consist of the following fields (possible types for these fields are shown in parentheses): 8 Chapter 1 Introduction to Computers, the Internet and Java • Employee identification number (a whole number) • Name (a string of characters) • Address (a string of characters) • Hourly pay rate (a number with a decimal point) • Year-to-date earnings (a number with a decimal point) • Amount of taxes withheld (a number with a decimal point) Thus, a record is a group of related fields. In the preceding example, all the fields belong to the same employee. A company might have many employees and a payroll record for each. Files A file is a group of related records. [Note: More generally, a file contains arbitrary data in arbitrary formats. In some operating systems, a file is viewed simply as a sequence of bytes— any organization of the bytes in a file, such as organizing the data into records, is a view created by the application programmer. You’ll see how to do that in Chapter 15.] It’s not unusual for an organization to have many files, some containing billions, or even trillions, of characters of information. Database A database is a collection of data organized for easy access and manipulation. The most popular model is the relational database, in which data is stored in simple tables. A table includes records and fields. For example, a table of students might include first name, last name, major, year, student ID number and grade point average fields. The data for each student is a record, and the individual pieces of information in each record are the fields. You can search, sort and otherwise manipulate the data based on its relationship to multiple tables or databases. For example, a university might use data from the student database in combination with data from databases of courses, on-campus housing, meal plans, etc. We discuss databases in Chapter 24. Big Data The amount of data being produced worldwide is enormous and growing quickly. According to IBM, approximately 2.5 quintillion bytes (2.5 exabytes) of data are created daily and 90% of the world’s data was created in just the past two years!6 According to a Digital Universe study, the global data supply reached 2.8 zettabytes (equal to 2.8 trillion gigabytes) in 2012.7 Figure 1.4 shows some common byte measurements. Big data applications deal with such massive amounts of data and this field is growing quickly, creating lots of opportunity for software developers. According to a study by Gartner Group, over 4 million IT jobs globally will support big data by by 2015.8 6. 7. 8. http://www-01.ibm.com/software/data/bigdata/. http://www.guardian.co.uk/news/datablog/2012/dec/19/big-data-study-digitaluniverse-global-volume. http://tech.fortune.cnn.com/2013/09/04/big-data-employment-boom/. 1.4 Machine Languages, Assembly Languages and High-Level Languages Unit Bytes Which is approximately 1 kilobyte (KB) 1 megabyte (MB) 1 gigabyte (GB) 1 terabyte (TB) 1 petabyte (PB) 1 exabyte (EB) 1 zettabyte (ZB) 1024 bytes 1024 kilobytes 1024 megabytes 1024 gigabytes 1024 terabytes 1024 petabytes 1024 exabytes 103 (1024 bytes exactly) 106 (1,000,000 bytes) 109 (1,000,000,000 bytes) 1012 (1,000,000,000,000 bytes) 1015 (1,000,000,000,000,000 bytes) 1018 (1,000,000,000,000,000,000 bytes) 1021 (1,000,000,000,000,000,000,000 bytes) 9 Fig. 1.4 | Byte measurements. 1.4 Machine Languages, Assembly Languages and HighLevel Languages Programmers write instructions in various programming languages, some directly understandable by computers and others requiring intermediate translation steps. Hundreds of such languages are in use today. These may be divided into three general types: 1. Machine languages 2. Assembly languages 3. High-level languages Machine Languages Any computer can directly understand only its own machine language, defined by its hardware design. Machine languages generally consist of strings of numbers (ultimately reduced to 1s and 0s) that instruct computers to perform their most elementary operations one at a time. Machine languages are machine dependent (a particular machine language can be used on only one type of computer). Such languages are cumbersome for humans. For example, here’s a section of an early machine-language payroll program that adds overtime pay to base pay and stores the result in gross pay: +1300042774 +1400593419 +1200274027 Assembly Languages and Assemblers Programming in machine language was simply too slow and tedious for most programmers. Instead of using the strings of numbers that computers could directly understand, programmers began using English-like abbreviations to represent elementary operations. These abbreviations formed the basis of assembly languages. Translator programs called assemblers were developed to convert early assembly-language programs to machine language at computer speeds. The following section of an assembly-language payroll program also adds overtime pay to base pay and stores the result in gross pay: load add store basepay overpay grosspay 10 Chapter 1 Introduction to Computers, the Internet and Java Although such code is clearer to humans, it’s incomprehensible to computers until translated to machine language. High-Level Languages and Compilers With the advent of assembly languages, computer usage increased rapidly, but programmers still had to use numerous instructions to accomplish even the simplest tasks. To speed the programming process, high-level languages were developed in which single statements could be written to accomplish substantial tasks. Translator programs called compilers convert high-level language programs into machine language. High-level languages allow you to write instructions that look almost like everyday English and contain commonly used mathematical notations. A payroll program written in a high-level language might contain a single statement such as grossPay = basePay + overTimePay From the programmer’s standpoint, high-level languages are preferable to machine and assembly languages. Java is one of the most widely used high-level programming languages. Interpreters Compiling a large high-level language program into machine language can take considerable computer time. Interpreter programs, developed to execute high-level language programs directly, avoid the delay of compilation, although they run slower than compiled programs. We’ll say more about interpreters in Section 1.9, where you’ll learn that Java uses a clever performance-tuned mixture of compilation and interpretation to run programs. 1.5 Introduction to Object Technology Today, as demands for new and more powerful software are soaring, building software quickly, correctly and economically remains an elusive goal. Objects, or more precisely, the classes objects come from, are essentially reusable software components. There are date objects, time objects, audio objects, video objects, automobile objects, people objects, etc. Almost any noun can be reasonably represented as a software object in terms of attributes (e.g., name, color and size) and behaviors (e.g., calculating, moving and communicating). Software-development groups can use a modular, object-oriented design-and-implementation approach to be much more productive than with earlier popular techniques like “structured programming”—object-oriented programs are often easier to understand, correct and modify. 1.5.1 The Automobile as an Object To help you understand objects and their contents, let’s begin with a simple analogy. Suppose you want to drive a car and make it go faster by pressing its accelerator pedal. What must happen before you can do this? Well, before you can drive a car, someone has to design it. A car typically begins as engineering drawings, similar to the blueprints that describe the design of a house. These drawings include the design for an accelerator pedal. The pedal hides from the driver the complex mechanisms that actually make the car go faster, just as the brake pedal “hides” the mechanisms that slow the car, and the steering wheel “hides” the mechanisms that turn the car. This enables people with little or no knowledge of how engines, braking and steering mechanisms work to drive a car easily. 1.5 Introduction to Object Technology 11 Just as you cannot cook meals in the kitchen of a blueprint, you cannot drive a car’s engineering drawings. Before you can drive a car, it must be built from the engineering drawings that describe it. A completed car has an actual accelerator pedal to make it go faster, but even that’s not enough—the car won’t accelerate on its own (hopefully!), so the driver must press the pedal to accelerate the car. 1.5.2 Methods and Classes Let’s use our car example to introduce some key object-oriented programming concepts. Performing a task in a program requires a method. The method houses the program statements that actually perform its tasks. The method hides these statements from its user, just as the accelerator pedal of a car hides from the driver the mechanisms of making the car go faster. In Java, we create a program unit called a class to house the set of methods that perform the class’s tasks. For example, a class that represents a bank account might contain one method to deposit money to an account, another to withdraw money from an account and a third to inquire what the account’s current balance is. A class is similar in concept to a car’s engineering drawings, which house the design of an accelerator pedal, steering wheel, and so on. 1.5.3 Instantiation Just as someone has to build a car from its engineering drawings before you can actually drive a car, you must build an object of a class before a program can perform the tasks that the class’s methods define. The process of doing this is called instantiation. An object is then referred to as an instance of its class. 1.5.4 Reuse Just as a car’s engineering drawings can be reused many times to build many cars, you can reuse a class many times to build many objects. Reuse of existing classes when building new classes and programs saves time and effort. Reuse also helps you build more reliable and effective systems, because existing classes and components often have undergone extensive testing, debugging and performance tuning. Just as the notion of interchangeable parts was crucial to the Industrial Revolution, reusable classes are crucial to the software revolution that has been spurred by object technology. Software Engineering Observation 1.1 Use a building-block approach to creating your programs. Avoid reinventing the wheel— use existing high-quality pieces wherever possible. This software reuse is a key benefit of object-oriented programming. 1.5.5 Messages and Method Calls When you drive a car, pressing its gas pedal sends a message to the car to perform a task— that is, to go faster. Similarly, you send messages to an object. Each message is implemented as a method call that tells a method of the object to perform its task. For example, a program might call a bank-account object’s deposit method to increase the account’s balance. 1.5.6 Attributes and Instance Variables A car, besides having capabilities to accomplish tasks, also has attributes, such as its color, its number of doors, the amount of gas in its tank, its current speed and its record of total 12 Chapter 1 Introduction to Computers, the Internet and Java miles driven (i.e., its odometer reading). Like its capabilities, the car’s attributes are represented as part of its design in its engineering diagrams (which, for example, include an odometer and a fuel gauge). As you drive an actual car, these attributes are carried along with the car. Every car maintains its own attributes. For example, each car knows how much gas is in its own gas tank, but not how much is in the tanks of other cars. An object, similarly, has attributes that it carries along as it’s used in a program. These attributes are specified as part of the object’s class. For example, a bank-account object has a balance attribute that represents the amount of money in the account. Each bankaccount object knows the balance in the account it represents, but not the balances of the other accounts in the bank. Attributes are specified by the class’s instance variables. 1.5.7 Encapsulation and Information Hiding Classes (and their objects) encapsulate, i.e., encase, their attributes and methods. A class’s (and its object’s) attributes and methods are intimately related. Objects may communicate with one another, but they’re normally not allowed to know how other objects are implemented—implementation details are hidden within the objects themselves. This information hiding, as we’ll see, is crucial to good software engineering. 1.5.8 Inheritance A new class of objects can be created conveniently by inheritance—the new class (called the subclass) starts with the characteristics of an existing class (called the superclass), possibly customizing them and adding unique characteristics of its own. In our car analogy, an object of class “convertible” certainly is an object of the more general class “automobile,” but more specifically, the roof can be raised or lowered. 1.5.9 Interfaces Java also supports interfaces—collections of related methods that typically enable you to tell objects what to do, but not how to do it (we’ll see an exception to this in Java SE 8). In the car analogy, a “basic-driving-capabilities” interface consisting of a steering wheel, an accelerator pedal and a brake pedal would enable a driver to tell the car what to do. Once you know how to use this interface for turning, accelerating and braking, you can drive many types of cars, even though manufacturers may implement these systems differently. A class implements zero or more interfaces, each of which can have one or more methods, just as a car implements separate interfaces for basic driving functions, controlling the radio, controlling the heating and air conditioning systems, and the like. Just as car manufacturers implement capabilities differently, classes may implement an interface’s methods differently. For example a software system may include a “backup” interface that offers the methods save and restore. Classes may implement those methods differently, depending on the types of things being backed up, such as programs, text, audios, videos, etc., and the types of devices where these items will be stored. 1.5.10 Object-Oriented Analysis and Design (OOAD) Soon you’ll be writing programs in Java. How will you create the code (i.e., the program instructions) for your programs? Perhaps, like many programmers, you’ll simply turn on 1.6 Operating Systems 13 your computer and start typing. This approach may work for small programs (like the ones we present in the early chapters of the book), but what if you were asked to create a software system to control thousands of automated teller machines for a major bank? Or suppose you were asked to work on a team of 1,000 software developers building the next generation of the U.S. air traffic control system? For projects so large and complex, you should not simply sit down and start writing programs. To create the best solutions, you should follow a detailed analysis process for determining your project’s requirements (i.e., defining what the system is supposed to do) and developing a design that satisfies them (i.e., specifying how the system should do it). Ideally, you’d go through this process and carefully review the design (and have your design reviewed by other software professionals) before writing any code. If this process involves analyzing and designing your system from an object-oriented point of view, it’s called an object-oriented analysis-and-design (OOAD) process. Languages like Java are object oriented. Programming in such a language, called object-oriented programming (OOP), allows you to implement an object-oriented design as a working system. 1.5.11 The UML (Unified Modeling Language) Although many different OOAD processes exist, a single graphical language for communicating the results of any OOAD process has come into wide use. The Unified Modeling Language (UML) is now the most widely used graphical scheme for modeling object-oriented systems. We present our first UML diagrams in Chapters 3 and 4, then use them in our deeper treatment of object-oriented programming through Chapter 11. In our optional online ATM Software Engineering Case Study in Chapters 33–34 we present a simple subset of the UML’s features as we guide you through an object-oriented design experience. 1.6 Operating Systems Operating systems are software systems that make using computers more convenient for users, application developers and system administrators. They provide services that allow each application to execute safely, efficiently and concurrently (i.e., in parallel) with other applications. The software that contains the core components of the operating system is the kernel. Popular desktop operating systems include Linux, Windows and Mac OS X. Popular mobile operating systems used in smartphones and tablets include Google’s Android, Apple’s iOS (for its iPhone, iPad and iPod Touch devices), Windows Phone 8 and BlackBerry OS. 1.6.1 Windows—A Proprietary Operating System In the mid-1980s, Microsoft developed the Windows operating system, consisting of a graphical user interface built on top of DOS—an enormously popular personal-computer operating system that users interacted with by typing commands. Windows borrowed many concepts (such as icons, menus and windows) popularized by early Apple Macintosh operating systems and originally developed by Xerox PARC. Windows 8 is Microsoft’s latest operating system—its features include PC and tablet support, a tiles-based user interface, security enhancements, touch-screen and multi-touch support, and more. Windows is a proprietary operating system—it’s controlled by Microsoft exclusively. It’s by far the world’s most widely used operating system. 14 Chapter 1 Introduction to Computers, the Internet and Java 1.6.2 Linux—An Open-Source Operating System The Linux operating system—which is popular in servers, personal computers and embedded systems—is perhaps the greatest success of the open-source movement. The opensource software development style departs from the proprietary development style (used, for example, with Microsoft’s Windows and Apple’s Mac OS X). With open-source development, individuals and companies—often worldwide—contribute their efforts in developing, maintaining and evolving software. Anyone can use and customize it for their own purposes, typically at no charge. The Java Development Kit and many related Java technologies are now open source. Some organizations in the open-source community are the Eclipse Foundation (the Eclipse Integrated Development Environment helps Java programmers conveniently develop software), the Mozilla Foundation (creators of the Firefox web browser), the Apache Software Foundation (creators of the Apache web server that delivers web pages over the Internet in response to web-browser requests) and GitHub and SourceForge (which provide the tools for managing open-source projects). Rapid improvements to computing and communications, decreasing costs and opensource software have made it easier and more economical to create software-based businesses now than just a few decades ago. Facebook, which was launched from a college dorm room, was built with open-source software.9 A variety of issues—such as Microsoft’s market power, the relatively small number of user-friendly Linux applications and the diversity of Linux distributions (Red Hat Linux, Ubuntu Linux and many others)—have prevented widespread Linux use on desktop computers. But Linux has become extremely popular on servers and in embedded systems, such as Google’s Android-based smartphones. 1.6.3 Android Android—the fastest-growing mobile and smartphone operating system—is based on the Linux kernel and uses Java. Experienced Java programmers can quickly dive into Android development. One benefit of developing Android apps is the openness of the platform. The operating system is open source and free. The Android operating system was developed by Android, Inc., which was acquired by Google in 2005. In 2007, the Open Handset Alliance™—which now has 87 company members worldwide (http://www.openhandsetalliance.com/oha_members.html)— was formed to develop, maintain and evolve Android, driving innovation in mobile technology and improving the user experience while reducing costs. As of April 2013, more than 1.5 million Android devices (smartphones, tablets, etc.) were being activated daily.10 By October 2013, a Strategy Analytics report showed that Android had 81.3% of the global smartphone market share, compared to 13.4% for Apple, 4.1% for Microsoft and 1% for Blackberry.11 Android devices now include smartphones, tablets, e-readers, robots, jet engines, NASA satellites, game consoles, refrigerators, televisions, cameras, health-care 9. 10. 11. http://developers.facebook.com/opensource. http://www.technobuffalo.com/2013/04/16/google-daily-android-activations-1-5million/. http://blogs.strategyanalytics.com/WSS/post/2013/10/31/Android-Captures-Record81-Percent-Share-of-Global-Smartphone-Shipments-in-Q3-2013.aspx. 1.7 Programming Languages 15 devices, smartwatches, automobile in-vehicle infotainment systems (for controlling the radio, GPS, phone calls, thermostat, etc.) and more.12 Android smartphones include the functionality of a mobile phone, Internet client (for web browsing and Internet communication), MP3 player, gaming console, digital camera and more. These handheld devices feature full-color multitouch screens which allow you to control the device with gestures involving one touch or multiple simultaneous touches. You can download apps directly onto your Android device through Google Play and other app marketplaces. At the time of this writing, there were over one million apps in Google Play, and the number is growing quickly.13 We present an introduction to Android app development in our textbook, Android How to Program, Second Edition, and in our professional book, Android for Programmers: An AppDriven Approach, Second Edition. After you learn Java, you’ll find it straightforward to begin developing and running Android apps. You can place your apps on Google Play (play.google.com), and if they’re successful, you may even be able to launch a business. Just remember—Facebook, Microsoft and Dell were all launched from college dorm rooms. 1.7 Programming Languages In this section, we comment briefly on several popular programming languages (Fig. 1.5). In the next section, we introduce Java. Programming language Fortran COBOL Pascal Description Fortran (FORmula TRANslator) was developed by IBM Corporation in the mid1950s for scientific and engineering applications that require complex mathematical computations. It’s still widely used, and its latest versions support object-oriented programming. COBOL (COmmon Business Oriented Language) was developed in the late 1950s by computer manufacturers, the U.S. government and industrial computer users based on a language developed by Grace Hopper, a U.S. Navy Rear Admiral and computer scientist who also advocated for the international standardization of programming languages. COBOL is still widely used for commercial applications that require precise and efficient manipulation of large amounts of data. Its latest version supports object-oriented programming. Research in the 1960s resulted in structured programming—a disciplined approach to writing programs that are clearer, easier to test and debug and easier to modify than large programs produced with previous techniques. One result of this research was the development in 1971 of the Pascal programming language, which was designed for teaching structured programming and was popular in college courses for several decades. Fig. 1.5 | Some other programming languages. (Part 1 of 3.) 12. 13. http://www.businessweek.com/articles/2013-05-29/behind-the-internet-of-thingsis-android-and-its-everywhere. http://en.wikipedia.org/wiki/Google_Play. 16 Chapter 1 Programming language Ada Basic C C++ Objective-C Visual Basic Visual C# PHP Perl Python JavaScript Introduction to Computers, the Internet and Java Description Ada, based on Pascal, was developed under the sponsorship of the U.S. Department of Defense (DOD) during the 1970s and early 1980s. The DOD wanted a single language that would fill most of its needs. The Ada language was named after Lady Ada Lovelace, daughter of the poet Lord Byron. She’s credited with writing the world’s first computer program in the early 1800s (for the Analytical Engine mechanical computing device designed by Charles Babbage). Ada also supports object-oriented programming. Basic was developed in the 1960s at Dartmouth College to familiarize novices with programming techniques. Many of its latest versions are object oriented. C was developed in the early 1970s by Dennis Ritchie at Bell Laboratories. It initially became widely known as the UNIX operating system’s development language. Today, most of the code for general-purpose operating systems is written in C or C++. C++, which is based on C, was developed by Bjarne Stroustrup in the early 1980s at Bell Laboratories. C++ provides several features that “spruce up” the C language, but more important, it provides capabilities for object-oriented programming. Objective-C is another object-oriented language based on C. It was developed in the early 1980s and later acquired by NeXT, which in turn was acquired by Apple. It has become the key programming language for the OS X operating system and all iOSpowered devices (such as iPods, iPhones and iPads). Microsoft’s Visual Basic language was introduced in the early 1990s to simplify the development of Microsoft Windows applications. Its latest versions support objectoriented programming. Microsoft’s three object-oriented primary programming languages are Visual Basic (based on the original Basic), Visual C++ (based on C++) and Visual C# (based on C++ and Java, and developed for integrating the Internet and the web into computer applications). PHP, an object-oriented, open-source scripting language supported by a community of users and developers, is used by millions of websites. PHP is platform independent—implementations exist for all major UNIX, Linux, Mac and Windows operating systems. PHP also supports many databases, including the popular open-source MySQL. Perl (Practical Extraction and Report Language), one of the most widely used objectoriented scripting languages for web programming, was developed in 1987 by Larry Wall. It features rich text-processing capabilities. Python, another object-oriented scripting language, was released publicly in 1991. Developed by Guido van Rossum of the National Research Institute for Mathematics and Computer Science in Amsterdam (CWI), Python draws heavily from Modula-3—a systems programming language. Python is “extensible”—it can be extended through classes and programming interfaces. JavaScript is the most widely used scripting language. It’s primarily used to add dynamic behavior to web pages—for example, animations and improved interactivity with the user. It’s provided with all major web browsers. Fig. 1.5 | Some other programming languages. (Part 2 of 3.) www.allitebooks.com 1.8 Java Programming language Ruby on Rails 17 Description Ruby, created in the mid-1990s, is an open-source, object-oriented programming language with a simple syntax that’s similar to Python. Ruby on Rails combines the scripting language Ruby with the Rails web application framework developed by 37Signals. Their book, Getting Real (gettingreal.37signals.com/toc.php), is a must read for web developers. Many Ruby on Rails developers have reported productivity gains over other languages when developing database-intensive web applications. Fig. 1.5 | Some other programming languages. (Part 3 of 3.) 1.8 Java The microprocessor revolution’s most important contribution to date is that it enabled the development of personal computers. Microprocessors have had a profound impact in intelligent consumer-electronic devices. Recognizing this, Sun Microsystems in 1991 funded an internal corporate research project led by James Gosling, which resulted in a C++based object-oriented programming language that Sun called Java. A key goal of Java is to be able to write programs that will run on a great variety of computer systems and computer-controlled devices. This is sometimes called “write once, run anywhere.” The web exploded in popularity in 1993, and Sun saw the potential of using Java to add dynamic content, such as interactivity and animations, to web pages. Java drew the attention of the business community because of the phenomenal interest in the web. Java is now used to develop large-scale enterprise applications, to enhance the functionality of web servers (the computers that provide the content we see in our web browsers), to provide applications for consumer devices (cell phones, smartphones, television set-top boxes and more) and for many other purposes. Java is also the key language for developing Android smartphone and tablet apps. Sun Microsystems was acquired by Oracle in 2010. Java Class Libraries You can create each class and method you need to form your Java programs. However, most Java programmers take advantage of the rich collections of existing classes and methods in the Java class libraries, also known as the Java APIs (Application Programming Interfaces). Performance Tip 1.1 Using Java API classes and methods instead of writing your own versions can improve program performance, because they’re carefully written to perform efficiently. This also shortens program development time. 1.9 A Typical Java Development Environment We now explain the steps to create and execute a Java application. Normally there are five phases—edit, compile, load, verify and execute. We discuss them in the context of the Java 18 Chapter 1 Introduction to Computers, the Internet and Java SE 8 Development Kit (JDK). See the Before You Begin section for information on downloading and installing the JDK on Windows, Linux and OS X. Phase 1: Creating a Program Phase 1 consists of editing a file with an editor program, normally known simply as an editor (Fig. 1.6). Using the editor, you type a Java program (typically referred to as source code), make any necessary corrections and save it on a secondary storage device, such as your hard drive. Java source code files are given a name ending with the .java extension, indicating that the file contains Java source code. Phase 1: Edit Editor Disk Program is created in an editor and stored on disk in a file whose name ends with .java Fig. 1.6 | Typical Java development environment—editing phase. Two editors widely used on Linux systems are vi and emacs. Windows provides OS X provides TextEdit. Many freeware and shareware editors are also available online, including Notepad++ (notepad-plus-plus.org), EditPlus (www.editplus.com), TextPad (www.textpad.com) and jEdit (www.jedit.org). Integrated development environments (IDEs) provide tools that support the software development process, such as editors, debuggers for locating logic errors (errors that cause programs to execute incorrectly) and more. There are many popular Java IDEs, including: Notepad. • Eclipse (www.eclipse.org) • NetBeans (www.netbeans.org) • IntelliJ IDEA (www.jetbrains.com) On the book’s website at www.deitel.com/books/jhtp10 we provide Dive-Into® videos that show you how to executes this book’s Java applications and how to develop new Java applications with Eclipse, NetBeans and IntelliJ IDEA. Phase 2: Compiling a Java Program into Bytecodes In Phase 2, you use the command javac (the Java compiler) to compile a program (Fig. 1.7). For example, to compile a program called Welcome.java, you’d type javac Welcome.java in your system’s command window (i.e., the Command Prompt in Windows, the Terminal application in OS X) or a Linux shell (also called Terminal in some versions of Linux). If the program compiles, the compiler produces a .class file called Welcome.class that contains the compiled version. IDEs typically provide a menu item, such as Build or Make, that invokes the javac command for you. If the compiler detects errors, you’ll need to go back to Phase 1 and correct them. In Chapter 2, we’ll say more about the kinds of errors the compiler can detect. 1.9 A Typical Java Development Environment Phase 2: Compile Compiler Disk 19 Compiler creates bytecodes and stores them on disk in a file whose name ends with .class Fig. 1.7 | Typical Java development environment—compilation phase. The Java compiler translates Java source code into bytecodes that represent the tasks to execute in the execution phase (Phase 5). The Java Virtual Machine (JVM)—a part of the JDK and the foundation of the Java platform—executes bytecodes. A virtual machine (VM) is a software application that simulates a computer but hides the underlying operating system and hardware from the programs that interact with it. If the same VM is implemented on many computer platforms, applications written for that type of VM can be used on all those platforms. The JVM is one of the most widely used virtual machines. Microsoft’s .NET uses a similar virtual-machine architecture. Unlike machine-language instructions, which are platform dependent (that is, dependent on specific computer hardware), bytecode instructions are platform independent. So, Java’s bytecodes are portable—without recompiling the source code, the same bytecode instructions can execute on any platform containing a JVM that understands the version of Java in which the bytecodes were compiled. The JVM is invoked by the java command. For example, to execute a Java application called Welcome, you’d type the command java Welcome in a command window to invoke the JVM, which would then initiate the steps necessary to execute the application. This begins Phase 3. IDEs typically provide a menu item, such as Run, that invokes the java command for you. Phase 3: Loading a Program into Memory In Phase 3, the JVM places the program in memory to execute it—this is known as loading (Fig. 1.8).The JVM’s class loader takes the .class files containing the program’s bytecodes and transfers them to primary memory. It also loads any of the .class files provided by Java that your program uses. The .class files can be loaded from a disk on your system or over a network (e.g., your local college or company network, or the Internet). Primary Memory Phase 3: Load Class loader reads Class Loader .class files ... Disk Fig. 1.8 | Typical Java development environment—loading phase. containing bytecodes from disk and puts those bytecodes in memory 20 Chapter 1 Introduction to Computers, the Internet and Java Phase 4: Bytecode Verification In Phase 4, as the classes are loaded, the bytecode verifier examines their bytecodes to ensure that they’re valid and do not violate Java’s security restrictions (Fig. 1.9). Java enforces strong security to make sure that Java programs arriving over the network do not damage your files or your system (as computer viruses and worms might). Primary Memory Phase 4: Verify Bytecode Verifier ... Bytecode verifier confirms that all bytecodes are valid and do not violate Java’s security restrictions Fig. 1.9 | Typical Java development environment—verification phase. Phase 5: Execution In Phase 5, the JVM executes the program’s bytecodes, thus performing the actions specified by the program (Fig. 1.10). In early Java versions, the JVM was simply an interpreter for Java bytecodes. Most Java programs would execute slowly, because the JVM would interpret and execute one bytecode at a time. Some modern computer architectures can execute several instructions in parallel. Today’s JVMs typically execute bytecodes using a combination of interpretation and so-called just-in-time (JIT) compilation. In this process, the JVM analyzes the bytecodes as they’re interpreted, searching for hot spots—parts of the bytecodes that execute frequently. For these parts, a just-in-time (JIT) compiler, such as Oracle’s Java HotSpot™ compiler, translates the bytecodes into the underlying computer’s machine language. When the JVM encounters these compiled parts again, the faster machine-language code executes. Thus Java programs actually go through two compilation phases—one in which source code is translated into bytecodes (for portability across JVMs on different computer platforms) and a second in which, during execution, the bytecodes are translated into machine language for the actual computer on which the program executes. Primary Memory Phase 5: Execute Java Virtual Machine (JVM) ... To execute the program, the JVM reads bytecodes and just-in-time (JIT) compiles (i.e., translates) them into a language that the computer can understand. As the program executes, it may store data values in primary memory. Fig. 1.10 | Typical Java development environment—execution phase. Problems That May Occur at Execution Time Programs might not work on the first try. Each of the preceding phases can fail because of various errors that we’ll discuss throughout this book. For example, an executing program 1.10 Test-Driving a Java Application 21 might try to divide by zero (an illegal operation for whole-number arithmetic in Java). This would cause the Java program to display an error message. If this occurred, you’d have to return to the edit phase, make the necessary corrections and proceed through the remaining phases again to determine that the corrections fixed the problem(s). [Note: Most programs in Java input or output data. When we say that a program displays a message, we normally mean that it displays that message on your computer’s screen. Messages and other data may be output to other devices, such as disks and hardcopy printers, or even to a network for transmission to other computers.] Common Programming Error 1.1 Errors such as division by zero occur as a program runs, so they’re called runtime errors or execution-time errors. Fatal runtime errors cause programs to terminate immediately without having successfully performed their jobs. Nonfatal runtime errors allow programs to run to completion, often producing incorrect results. 1.10 Test-Driving a Java Application In this section, you’ll run and interact with your first Java application. The Painter application, which you’ll build over the course of several exercises, allows you to drag the mouse to “paint.” The elements and functionality you see here are typical of what you’ll learn to program in this book. Using the Painter’s graphical user interface (GUI), you can control the drawing color, the shape to draw (line, rectangle or oval) and whether the shape is filled with the drawing color. You can also undo the last shape you added to the drawing or clear the entire drawing. [Note: We use fonts to distinguish between features. Our convention is to emphasize screen features like titles and menus (e.g., the File menu) in a semibold sans-serif Helvetica font and to emphasize nonscreen elements, such as file names, program code or input (e.g., ProgramName.java), in a sans-serif Lucida font.] The steps in this section show you how to execute the Painter app from a Command Prompt (Windows), Terminal (OS X) or shell (Linux) window on your system. Throughout the book, we’ll refer to these windows simply as command windows. Perform the following steps to use the Painter application to draw a smiley face: 1. Checking your setup. Read the Before You Begin section to confirm that you’ve set up Java properly on your computer, that you’ve copied the book’s examples to your hard drive and that you know how to open a command window on your system. 2. Changing to the completed application’s directory. Open a command window and use the cd command to change to the directory (also called a folder) for the Painter application. We assume that the book’s examples are located in C:\examples on Windows or in your user account’s Documents/examples folder on Linux/OS X. On Windows type cd C:\examples\ch01\painter, then press Enter. On Linux/ OS X, type cd ~/Documents/examples/ch01/painter, then press Enter. 3. Running the Painter application. Recall that the java command, followed by the name of the application’s .class file (in this case, Painter), executes the application. Type the command java Painter and press Enter to execute the app. Figure 1.11 shows the app running on Windows, Linux and OS X, respectively—we shortened the windows to save space. 22 Chapter 1 Introduction to Computers, the Internet and Java a) Painter app running on Windows Close button Select a color Select a shape Clear the entire drawing Specify whether a rectangle or oval is filled with color Undo the last shape that was added to the drawing b) Painter app running on Linux. Close button c) Painter app running on OS X. Close button Fig. 1.11 | Painter app executing in Windows 7, Linux and OS X. [Note: Java commands are case sensitive—that is, uppercase letters are different from lowercase letters. It’s important to type the name of this application as Painter with a capital P. Otherwise, the application will not execute. Specifying the .class extension when using the java command results in an error. Also, if you receive the error message, “Exception in thread "main" java.lang.NoClassDefFoundError: Painter," your system has a CLASSPATH problem. Please refer to the Before You Begin section for instructions to help you fix this problem.] 4. Drawing a filled yellow oval for the face. Select Yellow as the drawing color, Oval as the shape and check the Filled checkbox, then drag the mouse to draw a large oval (Fig. 1.12). 1.10 Test-Driving a Java Application 23 Fig. 1.12 | Drawing a filled yellow oval for the face. 5. Drawing blue eyes. Select Blue as the drawing color, then draw two small ovals as the eyes (Fig. 1.13). Fig. 1.13 | Drawing blue eyes. 6. Drawing black eyebrows and a nose. Select Black as the drawing color and Line as the shape, then draw eyebrows and a nose (Fig. 1.14). Lines do not have fill, so leaving the Filled checkbox checked has no effect when drawing lines. 24 Chapter 1 Introduction to Computers, the Internet and Java Fig. 1.14 | Drawing black eyebrows and a nose. 7. Drawing a magenta mouth. Select Magenta as the drawing color and Oval as the shape, then draw a mouth (Fig. 1.15). Fig. 1.15 | Drawing a magenta mouth. 8. Drawing a yellow oval on the mouth to make a smile. Select Yellow as the drawing color, then draw an oval to change the magenta oval into a smile (Fig. 1.16). 1.11 Internet and World Wide Web 25 Fig. 1.16 | Drawing a yellow oval on the mouth to make a smile. 9. Exiting the Painter application. To exit the Painter application, click the Close button (in the window’s upper-right corner on Windows and the upper-left corner on Linux and OS X). Closing the window causes the Painter application to terminate. 1.11 Internet and World Wide Web In the late 1960s, ARPA—the Advanced Research Projects Agency of the United States Department of Defense—rolled out plans for networking the main computer systems of approximately a dozen ARPA-funded universities and research institutions. The computers were to be connected with communications lines operating at speeds on the order of 50,000 bits per second, a stunning rate at a time when most people (of the few who even had networking access) were connecting over telephone lines to computers at a rate of 110 bits per second. Academic research was about to take a giant leap forward. ARPA proceeded to implement what quickly became known as the ARPANET, the precursor to today’s Internet. Today’s fastest Internet speeds are on the order of billions of bits per second with trillionbit-per-second speeds on the horizon! Things worked out differently from the original plan. Although the ARPANET enabled researchers to network their computers, its main benefit proved to be the capability for quick and easy communication via what came to be known as electronic mail (email). This is true even on today’s Internet, with e-mail, instant messaging, file transfer and social media such as Facebook and Twitter enabling billions of people worldwide to communicate quickly and easily. The protocol (set of rules) for communicating over the ARPANET became known as the Transmission Control Protocol (TCP). TCP ensured that messages, consisting of sequentially numbered pieces called packets, were properly routed from sender to receiver, arrived intact and were assembled in the correct order. 26 Chapter 1 Introduction to Computers, the Internet and Java 1.11.1 The Internet: A Network of Networks In parallel with the early evolution of the Internet, organizations worldwide were implementing their own networks for both intraorganization (that is, within an organization) and interorganization (that is, between organizations) communication. A huge variety of networking hardware and software appeared. One challenge was to enable these different networks to communicate with each other. ARPA accomplished this by developing the Internet Protocol (IP), which created a true “network of networks,” the current architecture of the Internet. The combined set of protocols is now called TCP/IP. Businesses rapidly realized that by using the Internet, they could improve their operations and offer new and better services to their clients. Companies started spending large amounts of money to develop and enhance their Internet presence. This generated fierce competition among communications carriers and hardware and software suppliers to meet the increased infrastructure demand. As a result, bandwidth—the information-carrying capacity of communications lines—on the Internet has increased tremendously, while hardware costs have plummeted. 1.11.2 The World Wide Web: Making the Internet User-Friendly The World Wide Web (simply called “the web”) is a collection of hardware and software associated with the Internet that allows computer users to locate and view multimedia-based documents (documents with various combinations of text, graphics, animations, audios and videos) on almost any subject. The introduction of the web was a relatively recent event. In 1989, Tim Berners-Lee of CERN (the European Organization for Nuclear Research) began to develop a technology for sharing information via “hyperlinked” text documents. BernersLee called his invention the HyperText Markup Language (HTML). He also wrote communication protocols such as HyperText Transfer Protocol (HTTP) to form the backbone of his new hypertext information system, which he referred to as the World Wide Web. In 1994, Berners-Lee founded an organization, called the World Wide Web Consortium (W3C, www.w3.org), devoted to developing web technologies. One of the W3C’s primary goals is to make the web universally accessible to everyone regardless of disabilities, language or culture. In this book, you’ll use Java to build web-based applications. 1.11.3 Web Services and Mashups In online Chapter 32, we include a substantial treatment of web services (Fig. 1.17). The applications-development methodology of mashups enables you to rapidly develop powerful software applications by combining (often free) complementary web services and other forms of information feeds. One of the first mashups combined the real-estate listings provided by www.craigslist.org with the mapping capabilities of Google Maps to offer maps that showed the locations of homes for sale or rent in a given area. Web services source How it’s used Google Maps Twitter Mapping services Microblogging Fig. 1.17 | Some popular web services (www.programmableweb.com/apis/ directory/1?sort=mashups). (Part 1 of 2.) www.allitebooks.com 1.11 Internet and World Wide Web Web services source How it’s used YouTube Facebook Instagram Foursquare LinkedIn Groupon Netflix eBay Wikipedia PayPal Last.fm Amazon eCommerce Salesforce.com Skype Microsoft Bing Flickr Zillow Yahoo Search WeatherBug Video search Social networking Photo sharing Mobile check-in Social networking for business Social commerce Movie rentals Internet auctions Collaborative encyclopedia Payments Internet radio Shopping for books and many other products Customer Relationship Management (CRM) Internet telephony Search Photo sharing Real-estate pricing Search Weather 27 Fig. 1.17 | Some popular web services (www.programmableweb.com/apis/ directory/1?sort=mashups). (Part 2 of 2.) 1.11.4 Ajax Ajax helps Internet-based applications perform like desktop applications—a difficult task, given that such applications suffer transmission delays as data is shuttled back and forth between your computer and server computers on the Internet. Using Ajax, applications like Google Maps have achieved excellent performance and approach the look-and-feel of desktop applications. Although we don’t discuss “raw” Ajax programming (which is quite complex) in this text, we do show in online Chapter 31 how to build Ajax-enabled applications using JavaServer Faces (JSF) Ajax-enabled components. 1.11.5 The Internet of Things The Internet is no longer just a network of computers—it’s an Internet of Things. A thing is any object with an IP address and the ability to send data automatically over a network—e.g., a car with a transponder for paying tolls, a heart monitor implanted in a human, a smart meter that reports energy usage, mobile apps that can track your movement and location, and smart thermostats that adjust room temperatures based on weather forecasts and activity in the home. You’ll use IP addresses to build networked applications in online Chapter 28. 28 Chapter 1 Introduction to Computers, the Internet and Java 1.12 Software Technologies Figure 1.18 lists a number of buzzwords that you’ll hear in the software development community. We’ve created Resource Centers on most of these topics, with more on the way. Technology Description Agile software development Agile software development is a set of methodologies that try to get software implemented faster and using fewer resources. Check out the Agile Alliance (www.agilealliance.org) and the Agile Manifesto (www.agilemanifesto.org). Refactoring involves reworking programs to make them clearer and easier to maintain while preserving their correctness and functionality. It’s widely employed with agile development methodologies. Many IDEs contain built-in refactoring tools to do major portions of the reworking automatically. Design patterns are proven architectures for constructing flexible and maintainable object-oriented software. The field of design patterns tries to enumerate those recurring patterns, encouraging software designers to reuse them to develop better-quality software using less time, money and effort. We discuss Java design patterns in the online Appendix N. LAMP is an acronym for the open-source technologies that many developers use to build web applications—it stands for Linux, Apache, MySQL and PHP (or Perl or Python—two other scripting languages). MySQL is an open-source database management system. PHP is the most popular opensource server-side “scripting” language for developing web applications. Apache is the most popular web server software. The equivalent for Windows development is WAMP—Windows, Apache, MySQL and PHP. Software has generally been viewed as a product; most software still is offered this way. If you want to run an application, you buy a software package from a software vendor—often a CD, DVD or web download. You then install that software on your computer and run it as needed. As new versions appear, you upgrade your software, often at considerable cost in time and money. This process can become cumbersome for organizations that must maintain tens of thousands of systems on a diverse array of computer equipment. With Software as a Service (SaaS), the software runs on servers elsewhere on the Internet. When that server is updated, all clients worldwide see the new capabilities—no local installation is needed. You access the service through a browser. Browsers are quite portable, so you can run the same applications on a wide variety of computers from anywhere in the world. Salesforce.com, Google, and Microsoft’s Office Live and Windows Live all offer SaaS. Platform as a Service (PaaS) provides a computing platform for developing and running applications as a service over the web, rather than installing the tools on your computer. Some PaaS providers are Google App Engine, Amazon EC2 and Windows Azure™. Refactoring Design patterns LAMP Software as a Service (SaaS) Platform as a Service (PaaS) Fig. 1.18 | Software technologies. (Part 1 of 2.) 1.12 Software Technologies 29 Technology Description Cloud computing SaaS and PaaS are examples of cloud computing. You can use software and data stored in the “cloud”—i.e., accessed on remote computers (or servers) via the Internet and available on demand—rather than having it stored on your desktop, notebook computer or mobile device. This allows you to increase or decrease computing resources to meet your needs at any given time, which is more cost effective than purchasing hardware to provide enough storage and processing power to meet occasional peak demands. Cloud computing also saves money by shifting the burden of managing these apps to the service provider. Software Development Kits (SDKs) include the tools and documentation developers use to program applications. For example, you’ll use the Java Development Kit (JDK) to build and run Java applications. Software Development Kit (SDK) Fig. 1.18 | Software technologies. (Part 2 of 2.) Software is complex. Large, real-world software applications can take many months or even years to design and implement. When large software products are under development, they typically are made available to the user communities as a series of releases, each more complete and polished than the last (Fig. 1.19). Version Description Alpha Alpha software is the earliest release of a software product that’s still under active development. Alpha versions are often buggy, incomplete and unstable and are released to a relatively small number of developers for testing new features, getting early feedback, etc. Beta versions are released to a larger number of developers later in the development process after most major bugs have been fixed and new features are nearly complete. Beta software is more stable, but still subject to change. Release candidates are generally feature complete, (mostly) bug free and ready for use by the community, which provides a diverse testing environment— the software is used on different systems, with varying constraints and for a variety of purposes. Any bugs that appear in the release candidate are corrected, and eventually the final product is released to the general public. Software companies often distribute incremental updates over the Internet. Software that’s developed using this approach (for example, Google search or Gmail) generally does not have version numbers. It’s hosted in the cloud (not installed on your computer) and is constantly evolving so that users always have the latest version. Beta Release candidates Final release Continuous beta Fig. 1.19 | Software product-release terminology. 30 Chapter 1 Introduction to Computers, the Internet and Java 1.13 Keeping Up-to-Date with Information Technologies Figure 1.20 lists key technical and business publications that will help you stay up-to-date with the latest news and trends and technology. You can also find a growing list of Internet- and web-related Resource Centers at www.deitel.com/ResourceCenters.html. Publication URL AllThingsD Bloomberg BusinessWeek CNET Communications of the ACM Computerworld Engadget eWeek Fast Company Fortune GigaOM Hacker News IEEE Computer Magazine InfoWorld Mashable PCWorld SD Times Slashdot Technology Review Techcrunch The Next Web The Verge Wired allthingsd.com www.businessweek.com news.cnet.com cacm.acm.org www.computerworld.com www.engadget.com www.eweek.com www.fastcompany.com/ money.cnn.com/magazines/fortune gigaom.com news.ycombinator.com www.computer.org/portal/web/computingnow/computer www.infoworld.com mashable.com www.pcworld.com www.sdtimes.com slashdot.org/ technologyreview.com techcrunch.com thenextweb.com www.theverge.com www.wired.com Fig. 1.20 | Technical and business publications. Self-Review Exercises 1.1 Fill in the blanks in each of the following statements: a) Computers process data under the control of sets of instructions called . b) The key logical units of the computer are the , , , , and . c) The three types of languages discussed in the chapter are , and . d) The programs that translate high-level language programs into machine language are called . Answers to Self-Review Exercises 31 e) f) is an operating system for mobile devices based on the Linux kernel and Java. software is generally feature complete, (supposedly) bug free and ready for use by the community. g) The Wii Remote, as well as many smartphones, use a(n) which allows the device to respond to motion. 1.2 Fill in the blanks in each of the following sentences about the Java environment: command from the JDK executes a Java application. a) The b) The command from the JDK compiles a Java program. file extension. c) A Java source code file must end with the d) When a Java program is compiled, the file produced by the compiler ends with the file extension. that are executed by the Java e) The file produced by the Java compiler contains Virtual Machine. 1.3 Fill in the blanks in each of the following statements (based on Section 1.5): a) Objects enable the design practice of —although they may know how to communicate with one another across well-defined interfaces, they normally are not allowed to know how other objects are implemented. , which contain fields and the set of b) Java programmers concentrate on creating methods that manipulate those fields and provide services to clients. c) The process of analyzing and designing a system from an object-oriented point of view is called . d) A new class of objects can be created conveniently by —the new class (called the subclass) starts with the characteristics of an existing class (called the superclass), possibly customizing them and adding unique characteristics of its own. e) is a graphical language that allows people who design software systems to use an industry-standard notation to represent them. of the object’s class. f) The size, shape, color and weight of an object are considered Answers to Self-Review Exercises 1.1 a) programs. b) input unit, output unit, memory unit, central processing unit, arithmetic and logic unit, secondary storage unit. c) machine languages, assembly languages, high-level languages. d) compilers. e) Android. f) Release candidate. g) accelerometer. 1.2 a) java. b) javac. c) .java. d) .class. e) bytecodes. 1.3 a) information hiding. b) classes. c) object-oriented analysis and design (OOAD). d) inheritance. e) The Unified Modeling Language (UML). f) attributes. Exercises 1.4 Fill in the blanks in each of the following statements: a) The logical unit that receives information from outside the computer for use by the . computer is the b) The process of instructing the computer to solve a problem is called . c) is a type of computer language that uses Englishlike abbreviations for machine-language instructions. is a logical unit that sends information which has already been processed by d) the computer to various devices so that it may be used outside the computer. e) and are logical units of the computer that retain information. f) is a logical unit of the computer that performs calculations. g) is a logical unit of the computer that makes logical decisions. 32 Chapter 1 Introduction to Computers, the Internet and Java h) languages are most convenient to the programmer for writing programs quickly and easily. . i) The only language a computer can directly understand is that computer’s j) is a logical unit of the computer that coordinates the activities of all the other logical units. 1.5 Fill in the blanks in each of the following statements: programming language is now used to develop large-scale enterprise apa) The plications, to enhance the functionality of web servers, to provide applications for consumer devices and for many other purposes. b) initially became widely known as the development language of the UNIX operating system. ensures that messages, consisting of sequentially numbered pieces called c) The bytes, were properly routed from sender to receiver, arrived intact and were assembled in the correct order. d) The programming language was developed by Bjarne Stroustrup in the early 1980s at Bell Laboratories. 1.6 Fill in the blanks in each of the following statements: , , , a) Java programs normally go through five phases— and . b) A(n) provides many tools that support the software development process, such as editors for writing and editing programs, debuggers for locating logic errors in programs, and many other features. c) The command java invokes the , which executes Java programs. is a software application that simulates a computer, but hides the underd) A(n) lying operating system and hardware from the programs that interact with it. e) The takes the .class files containing the program’s bytecodes and transfers them to primary memory. examines bytecodes to ensure that they’re valid. f) The 1.7 Explain the two compilation phases of Java programs. 1.8 One of the world’s most common objects is a wrist watch. Discuss how each of the following terms and concepts applies to the notion of a watch: object, attributes, behaviors, class, inheritance (consider, for example, an alarm clock), modeling, messages, encapsulation, interface and information hiding. Making a Difference Throughout the book we’ve included Making a Difference exercises in which you’ll be asked to work on problems that really matter to individuals, communities, countries and the world. For more information about worldwide organizations working to make a difference, and for related programming project ideas, visit our Making a Difference Resource Center at www.deitel.com/ makingadifference. 1.9 (Test-Drive: Carbon Footprint Calculator) Some scientists believe that carbon emissions, especially from the burning of fossil fuels, contribute significantly to global warming and that this can be combatted if individuals take steps to limit their use of carbon-based fuels. Organizations and individuals are increasingly concerned about their “carbon footprints.” Websites such as TerraPass http://www.terrapass.com/carbon-footprint-calculator/ and Carbon Footprint http://www.carbonfootprint.com/calculator.aspx Making a Difference 33 provide carbon-footprint calculators. Test-drive these calculators to determine your carbon footprint. Exercises in later chapters will ask you to program your own carbon-footprint calculator. To prepare for this, use the web to research the formulas for calculating carbon footprints. 1.10 (Test-Drive: Body Mass Index Calculator) Obesity causes significant increases in illnesses such as diabetes and heart disease. To determine whether a person is overweight or obese, you can use a measure called the body mass index (BMI). The United States Department of Health and Human Services provides a BMI calculator at http://www.nhlbi.nih.gov/guidelines/obesity/BMI/ bmicalc.htm. Use it to calculate your own BMI. A forthcoming exercise will ask you to program your own BMI calculator. To prepare for this, use the web to research the formulas for calculating BMI. 1.11 (Attributes of Hybrid Vehicles) In this chapter you learned some basics of classes. Now you’ll “flesh out” aspects of a class called “Hybrid Vehicle.” Hybrid vehicles are becoming increasingly popular, because they often get much better mileage than purely gasoline-powered vehicles. Browse the web and study the features of four or five of today’s popular hybrid cars, then list as many of their hybrid-related attributes as you can. Some common attributes include city-miles-per-gallon and highway-miles-per-gallon. Also list the attributes of the batteries (type, weight, etc.). 1.12 (Gender Neutrality) Many people want to eliminate sexism in all forms of communication. You’ve been asked to create a program that can process a paragraph of text and replace gender-specific words with gender-neutral ones. Assuming that you’ve been given a list of gender-specific words and their gender-neutral replacements (e.g., replace both “wife” and “husband” with “spouse,” “man” and “woman” with “person,” “daughter” and “son” with “child”), explain the procedure you’d use to read through a paragraph of text and manually perform these replacements. How might your procedure generate a strange term like “woperchild?” You’ll soon learn that a more formal term for “procedure” is “algorithm,” and that an algorithm specifies the steps to be performed and the order in which to perform them. We’ll show how to develop algorithms then convert them to Java programs which can be run on computers. 2 What’s in a name? That which we call a rose By any other name would smell as sweet. —William Shakespeare The chief merit of language is clearness. —Galen One person can make a difference and every person should try. —John F. Kennedy Objectives In this chapter you’ll: ■ Write simple Java applications. ■ Use input and output statements. ■ Learn about Java’s primitive types. ■ Understand basic memory concepts. ■ Use arithmetic operators. ■ Learn the precedence of arithmetic operators. ■ Write decision-making statements. ■ Use relational and equality operators. Introduction to Java Applications; Input/Output and Operators 2.1 Introduction 2.1 Introduction 2.2 Your First Program in Java: Printing a Line of Text 2.3 Modifying Your First Java Program 2.4 Displaying Text with printf 2.5 Another Application: Adding Integers 2.5.1 import Declarations 2.5.2 Declaring Class Addition 2.5.3 Declaring and Creating a Scanner to Obtain User Input from the Keyboard 2.5.4 Declaring Variables to Store Integers 2.5.5 Prompting the User for Input 35 2.5.6 Obtaining an int as Input from the User 2.5.7 Prompting for and Inputting a Second int 2.5.8 Using Variables in a Calculation 2.5.9 Displaying the Result of the Calculation 2.5.10 Java API Documentation 2.6 Memory Concepts 2.7 Arithmetic 2.8 Decision Making: Equality and Relational Operators 2.9 Wrap-Up Summary | Self-Review Exercises | Answers to Self-Review Exercises | Exercises | Making a Difference 2.1 Introduction This chapter introduces Java application programming. We begin with examples of programs that display (output) messages on the screen. We then present a program that obtains (inputs) two numbers from a user, calculates their sum and displays the result. You’ll learn how to instruct the computer to perform arithmetic calculations and save their results for later use. The last example demonstrates how to make decisions. The application compares two numbers, then displays messages that show the comparison results. You’ll use the JDK command-line tools to compile and run this chapter’s programs. If you prefer to use an integrated development environment (IDE), we’ve also posted Dive Into® videos at http://www.deitel.com/books/jhtp10/ for Eclipse, NetBeans and IntelliJ IDEA. 2.2 Your First Program in Java: Printing a Line of Text A Java application is a computer program that executes when you use the java command to launch the Java Virtual Machine (JVM). Later in this section we’ll discuss how to compile and run a Java application. First we consider a simple application that displays a line of text. Figure 2.1 shows the program followed by a box that displays its output. 1 2 3 4 5 6 7 8 9 10 11 // Fig. 2.1: Welcome1.java // Text-printing program. public class Welcome1 { // main method begins execution of Java application public static void main(String[] args) { System.out.println("Welcome to Java Programming!"); } // end method main } // end class Welcome1 Fig. 2.1 | Text-printing program. (Part 1 of 2.) 36 Chapter 2 Introduction to Java Applications; Input/Output and Operators Welcome to Java Programming! Fig. 2.1 | Text-printing program. (Part 2 of 2.) The program includes line numbers. We’ve added these for instructional purposes— they’re not part of a Java program. This example illustrates several important Java features. We’ll see that line 9 does the work—displaying the phrase Welcome to Java Programming! on the screen. Commenting Your Programs We insert comments to document programs and improve their readability. The Java compiler ignores comments, so they do not cause the computer to perform any action when the program is run. By convention, we begin every program with a comment indicating the figure number and filename. The comment in line 1 // Fig. 2.1: Welcome1.java begins with //, indicating that it’s an end-of-line comment—it terminates at the end of the line on which the // appears. An end-of-line comment need not begin a line; it also can begin in the middle of a line and continue until the end (as in lines 6, 10 and 11). Line 2 // Text-printing program. by our convention, is a comment that describes the purpose of the program. Java also has traditional comments, which can be spread over several lines as in /* This is a traditional comment. It can be split over multiple lines */ These begin and end with delimiters, /* and */. The compiler ignores all text between the delimiters. Java incorporated traditional comments and end-of-line comments from the C and C++ programming languages, respectively. We prefer using // comments. Java provides comments of a third type—Javadoc comments. These are delimited by /** and */. The compiler ignores all text between the delimiters. Javadoc comments enable you to embed program documentation directly in your programs. Such comments are the preferred Java documenting format in industry. The javadoc utility program (part of the JDK) reads Javadoc comments and uses them to prepare program documentation in HTML format. We demonstrate Javadoc comments and the javadoc utility in online Appendix G, Creating Documentation with javadoc. Common Programming Error 2.1 Forgetting one of the delimiters of a traditional or Javadoc comment is a syntax error. A syntax error occurs when the compiler encounters code that violates Java’s language rules (i.e., its syntax). These rules are similar to a natural language’s grammar rules specifying sentence structure. Syntax errors are also called compiler errors, compile-time errors or compilation errors, because the compiler detects them when compiling the program. When a syntax error is encountered, the compiler issues an error message. You must eliminate all compilation errors before your program will compile properly. 2.2 Your First Program in Java: Printing a Line of Text 37 Good Programming Practice 2.1 Some organizations require that every program begin with a comment that states the purpose of the program and the author, date and time when the program was last modified. Error-Prevention Tip 2.1 As you write new programs or modify existing ones, keep your comments up-to-date with the code. Programmers will often need to make changes to existing code to fix errors or to enhance capabilities. Updating your comments helps ensure that they accurately reflect what the code does. This will make your programs easier to understand and modify in the future. Programmers using or updating code with out-of-date comments might make incorrect assumptions about the code that could lead to errors or even security breaches. Using Blank Lines Line 3 is a blank line. Blank lines, space characters and tabs make programs easier to read. Together, they’re known as white space (or whitespace). The compiler ignores white space. Good Programming Practice 2.2 Use blank lines and spaces to enhance program readability. Declaring a Class Line 4 public class Welcome1 begins a class declaration for class Welcome1. Every Java program consists of at least one class that you (the programmer) define. The class keyword introduces a class declaration and is immediately followed by the class name (Welcome1). Keywords (sometimes called reserved words) are reserved for use by Java and are always spelled with all lowercase letters. The complete list of keywords is shown in Appendix C. In Chapters 2–7, every class we define begins with the public keyword. For now, we simply require public. You’ll learn more about public and non-public classes in Chapter 8. Filename for a public Class A public class must be placed in a file that has a filename of the form ClassName.java, so class Welcome1 is stored in the file Welcome1.java. Common Programming Error 2.2 A compilation error occurs if a public class’s filename is not exactly same name as the class (in terms of both spelling and capitalization) followed by the .java extension. Class Names and Identifiers By convention, class names begin with a capital letter and capitalize the first letter of each word they include (e.g., SampleClassName). A class name is an identifier—a series of characters consisting of letters, digits, underscores ( _ ) and dollar signs ($) that does not begin with a digit and does not contain spaces. Some valid identifiers are Welcome1, $value, _value, m_inputField1 and button7. The name 7button is not a valid identifier because it begins with a digit, and the name input field is not a valid identifier because it contains a space. Normally, an identifier that does not begin with a capital letter is not a class name. 38 Chapter 2 Introduction to Java Applications; Input/Output and Operators Java is case sensitive—uppercase and lowercase letters are distinct—so value and Value are different (but both valid) identifiers. Class Body A left brace (as in line 5), {, begins the body of every class declaration. A corresponding right brace (at line 11), }, must end each class declaration. Lines 6–10 are indented. Good Programming Practice 2.3 Indent the entire body of each class declaration one “level” between the left brace and the right brace that delimit the body of the class. This format emphasizes the class declaration’s structure and makes it easier to read. We use three spaces to form a level of indent—many programmers prefer two or four spaces. Whatever you choose, use it consistently. Error-Prevention Tip 2.2 When you type an opening left brace, {, immediately type the closing right brace, }, then reposition the cursor between the braces and indent to begin typing the body. This practice helps prevent errors due to missing braces. Many IDEs insert the closing right brace for you when you type the opening left brace. Common Programming Error 2.3 It’s a syntax error if braces do not occur in matching pairs. Good Programming Practice 2.4 IDEs typically indent code for you. The Tab key may also be used to indent code. You can configure each IDE to specify the number of spaces inserted when you press Tab. Declaring a Method Line 6 // main method begins execution of Java application is an end-of-line comment indicating the purpose of lines 7–10 of the program. Line 7 public static void main(String[] args) is the starting point of every Java application. The parentheses after the identifier main indicate that it’s a program building block called a method. Java class declarations normally contain one or more methods. For a Java application, one of the methods must be called main and must be defined as shown in line 7; otherwise, the Java Virtual Machine (JVM) will not execute the application. Methods perform tasks and can return information when they complete their tasks. We’ll explain the purpose of keyword static in Section 3.2.5. Keyword void indicates that this method will not return any information. Later, we’ll see how a method can return information. For now, simply mimic main’s first line in your Java applications. In line 7, the String[] args in parentheses is a required part of the method main’s declaration—we discuss this in Chapter 7. The left brace in line 8 begins the body of the method declaration. A corresponding right brace must end it (line 10). Line 9 in the method body is indented between the braces. 2.2 Your First Program in Java: Printing a Line of Text 39 Good Programming Practice 2.5 Indent the entire body of each method declaration one “level” between the braces that define the body of the method. This makes the structure of the method stand out and makes the method declaration easier to read. Performing Output with System.out.println Line 9 System.out.println("Welcome to Java Programming!"); instructs the computer to perform an action—namely, to display the characters contained between the double quotation marks (the quotation marks themselves are not displayed). Together, the quotation marks and the characters between them are a string—also known as a character string or a string literal. White-space characters in strings are not ignored by the compiler. Strings cannot span multiple lines of code. The System.out object—which is predefined for you—is known as the standard output object. It allows a Java application to display information in the command window from which it executes. In recent versions of Microsoft Windows, the command window is the Command Prompt. In UNIX/Linux/Mac OS X, the command window is called a terminal window or a shell. Many programmers call it simply the command line. Method System.out.println displays (or prints) a line of text in the command window. The string in the parentheses in line 9 is the argument to the method. When System.out.println completes its task, it positions the output cursor (the location where the next character will be displayed) at the beginning of the next line in the command window. This is similar to what happens when you press the Enter key while typing in a text editor—the cursor appears at the beginning of the next line in the document. The entire line 9, including System.out.println, the argument "Welcome to Java Programming!" in the parentheses and the semicolon (;), is called a statement. A method typically contains one or more statements that perform its task. Most statements end with a semicolon. When the statement in line 9 executes, it displays Welcome to Java Programming! in the command window. When learning how to program, sometimes it’s helpful to “break” a working program so you can familiarize yourself with the compiler’s syntax-error messages. These messages do not always state the exact problem in the code. When you encounter an error, it will give you an idea of what caused it. [Try removing a semicolon or brace from the program of Fig. 2.1, then recompile the program to see the error messages generated by the omission.] Error-Prevention Tip 2.3 When the compiler reports a syntax error, it may not be on the line that the error message indicates. First, check the line for which the error was reported. If you don’t find an error on that line, check several preceding lines. Using End-of-Line Comments on Right Braces for Readability As an aid to programming novices, we include an end-of-line comment after a closing brace that ends a method declaration and after a closing brace that ends a class declaration. For example, line 10 } // end method main 40 Chapter 2 Introduction to Java Applications; Input/Output and Operators indicates the closing brace of method main, and line 11 } // end class Welcome1 indicates the closing brace of class Welcome1. Each comment indicates the method or class that the right brace terminates. We’ll omit such ending comments after this chapter. Compiling Your First Java Application We’re now ready to compile and execute our program. We assume you’re using the Java Development Kit’s command-line tools, not an IDE. To help you compile and run your programs in an IDE, we provide online Dive Into® videos for the popular IDEs Eclipse, NetBeans and IntelliJ IDEA. These are located on the book’s website: http://www.deitel.com/books/jhtp10 To prepare to compile the program, open a command window and change to the directory where the program is stored. Many operating systems use the command cd to change directories. On Windows, for example, cd c:\examples\ch02\fig02_01 changes to the fig02_01 directory. On UNIX/Linux/Max OS X, the command cd ~/examples/ch02/fig02_01 changes to the fig02_01 directory. To compile the program, type javac Welcome1.java If the program contains no compilation errors, this command creates a new file called (known as the class file for Welcome1) containing the platform-independent Java bytecodes that represent our application. When we use the java command to execute the application on a given platform, the JVM will translate these bytecodes into instructions that are understood by the underlying operating system and hardware. Welcome1.class Common Programming Error 2.4 When using javac, if you receive a message such as “bad command or filename,” “javac: command not found” or “'javac' is not recognized as an internal or external command, operable program or batch file,” then your Java software installation was not completed properly. This indicates that the system’s PATH environment variable was not set properly. Carefully review the installation instructions in the Before You Begin section of this book. On some systems, after correcting the PATH, you may need to reboot your computer or open a new command window for these settings to take effect. Each syntax-error message contains the filename and line number where the error occurred. For example, Welcome1.java:6 indicates that an error occurred at line 6 in Welcome1.java. The rest of the message provides information about the syntax error. Common Programming Error 2.5 The compiler error message “class Welcome1 is public, should be declared in a file named Welcome1.java” indicates that the filename does not match the name of the public class in the file or that you typed the class name incorrectly when compiling the class. 2.3 Modifying Your First Java Program 41 Executing the Welcome1 Application The following instructions assume that the book’s examples are located in C:\examples on Windows or in your user account’s Documents/examples folder on Linux/OS X. To execute this program in a command window, change to the directory containing Welcome1.java—C:\examples\ch02\fig02_01 on Microsoft Windows or ~/Documents/ examples/ch02/fig02_01 on Linux/OS X. Next, type java Welcome1 and press Enter. This command launches the JVM, which loads the Welcome1.class file. The command omits the .class file-name extension; otherwise, the JVM will not execute the program. The JVM calls class Welcome1’s main method. Next, the statement at line 9 of main displays "Welcome to Java Programming!". Figure 2.2 shows the program executing in a Microsoft Windows Command Prompt window. [Note: Many environments show command windows with black backgrounds and white text. We adjusted these settings to make our screen captures more readable.] Error-Prevention Tip 2.4 When attempting to run a Java program, if you receive a message such as “Exception in thread "main" java.lang.NoClassDefFoundError: Welcome1,” your CLASSPATH environment variable has not been set properly. Please carefully review the installation instructions in the Before You Begin section of this book. On some systems, you may need to reboot your computer or open a new command window after configuring the CLASSPATH. You type this command to execute the application The program outputs to the screen Welcome to Java Programming! Fig. 2.2 | Executing Welcome1 from the Command Prompt. 2.3 Modifying Your First Java Program In this section, we modify the example in Fig. 2.1 to print text on one line by using multiple statements and to print text on several lines by using a single statement. Displaying a Single Line of Text with Multiple Statements Welcome to Java Programming! can be displayed several ways. Class Welcome2, shown in Fig. 2.3, uses two statements (lines 9–10) to produce the output shown in Fig. 2.1. [Note: From this point forward, we highlight with a yellow background the new and key features in each code listing, as we’ve done for lines 9–10.] The program is similar to Fig. 2.1, so we discuss only the changes here. Line 2 // Printing a line of text with multiple statements. 42 1 2 3 4 5 6 7 8 9 10 11 12 Chapter 2 Introduction to Java Applications; Input/Output and Operators // Fig. 2.3: Welcome2.java // Printing a line of text with multiple statements. public class Welcome2 { // main method begins execution of Java application public static void main(String[] args) { System.out.print("Welcome to "); System.out.println("Java Programming!"); } // end method main } // end class Welcome2 Welcome to Java Programming! Fig. 2.3 | Printing a line of text with multiple statements. is an end-of-line comment stating the purpose of the program. Line 4 begins the Welcome2 class declaration. Lines 9–10 of method main System.out.print("Welcome to "); System.out.println("Java Programming!"); display one line of text. The first statement uses System.out’s method print to display a string. Each print or println statement resumes displaying characters from where the last print or println statement stopped displaying characters. Unlike println, after displaying its argument, print does not position the output cursor at the beginning of the next line in the command window—the next character the program displays will appear immediately after the last character that print displays. So, line 10 positions the first character in its argument (the letter “J”) immediately after the last character that line 9 displays (the space character before the string’s closing double-quote character). Displaying Multiple Lines of Text with a Single Statement A single statement can display multiple lines by using newline characters, which indicate to System.out’s print and println methods when to position the output cursor at the beginning of the next line in the command window. Like blank lines, space characters and tab characters, newline characters are whitespace characters. The program in Fig. 2.4 outputs four lines of text, using newline characters to determine when to begin each new line. Most of the program is identical to those in Figs. 2.1 and 2.3. 1 2 3 4 5 6 7 8 // Fig. 2.4: Welcome3.java // Printing multiple lines of text with a single statement. public class Welcome3 { // main method begins execution of Java application public static void main(String[] args) { Fig. 2.4 | Printing multiple lines of text with a single statement. (Part 1 of 2.) 2.4 Displaying Text with printf 9 10 11 43 System.out.println("Welcome\nto\nJava\nProgramming!"); } // end method main } // end class Welcome3 Welcome to Java Programming! Fig. 2.4 | Printing multiple lines of text with a single statement. (Part 2 of 2.) Line 9 System.out.println("Welcome\nto\nJava\nProgramming!"); displays four lines of text in the command window. Normally, the characters in a string are displayed exactly as they appear in the double quotes. However, the paired characters \ and n (repeated three times in the statement) do not appear on the screen. The backslash (\) is an escape character, which has special meaning to System.out’s print and println methods. When a backslash appears in a string, Java combines it with the next character to form an escape sequence—\n represents the newline character. When a newline character appears in a string being output with System.out, the newline character causes the screen’s output cursor to move to the beginning of the next line in the command window. Figure 2.5 lists several common escape sequences and describes how they affect the display of characters in the command window. For the complete list of escape sequences, visit http://docs.oracle.com/javase/specs/jls/se7/html/ jls-3.html#jls-3.10.6 Escape sequence \n \t \r \\ \" Description Newline. Position the screen cursor at the beginning of the next line. Horizontal tab. Move the screen cursor to the next tab stop. Carriage return. Position the screen cursor at the beginning of the current line—do not advance to the next line. Any characters output after the carriage return overwrite the characters previously output on that line. Backslash. Used to print a backslash character. Double quote. Used to print a double-quote character. For example, System.out.println("\"in quotes\""); displays "in quotes". Fig. 2.5 | Some common escape sequences. 2.4 Displaying Text with printf The System.out.printf method (f means “formatted”) displays formatted data. Figure 2.6 uses this method to output on two lines the strings "Welcome to" and "Java Programming!". 44 1 2 3 4 5 6 7 8 9 10 11 12 Chapter 2 Introduction to Java Applications; Input/Output and Operators // Fig. 2.6: Welcome4.java // Displaying multiple lines with method System.out.printf. public class Welcome4 { // main method begins execution of Java application public static void main(String[] args) { System.out.printf("%s%n%s%n", "Welcome to", "Java Programming!"); } // end method main } // end class Welcome4 Welcome to Java Programming! Fig. 2.6 | Displaying multiple lines with method System.out.printf. Lines 9–10 System.out.printf("%s%n%s%n", "Welcome to", "Java Programming!"); call method System.out.printf to display the program’s output. The method call specifies three arguments. When a method requires multiple arguments, they’re placed in a comma-separated list. Calling a method is also referred to as invoking a method. Good Programming Practice 2.6 Place a space after each comma (,) in an argument list to make programs more readable. Lines 9–10 represent only one statement. Java allows large statements to be split over many lines. We indent line 10 to indicate that it’s a continuation of line 9. Common Programming Error 2.6 Splitting a statement in the middle of an identifier or a string is a syntax error. Method printf’s first argument is a format string that may consist of fixed text and format specifiers. Fixed text is output by printf just as it would be by print or println. Each format specifier is a placeholder for a value and specifies the type of data to output. Format specifiers also may include optional formatting information. Format specifiers begin with a percent sign (%) followed by a character that represents the data type. For example, the format specifier %s is a placeholder for a string. The format string in line 9 specifies that printf should output two strings, each followed by a newline character. At the first format specifier’s position, printf substitutes the value of the first argument after the format string. At each subsequent format specifier’s position, printf substitutes the value of the next argument. So this example substitutes "Welcome to" for the first %s and "Java Programming!" for the second %s. The output shows that two lines of text are displayed on two lines. Notice that instead of using the escape sequence \n, we used the %n format specifier, which is a line separator that’s portable across operating systems. You cannot use %n in the 2.5 Another Application: Adding Integers 45 argument to System.out.print or System.out.println; however, the line separator output by System.out.println after it displays its argument is portable across operating systems. Online Appendix I presents more details of formatting output with printf. 2.5 Another Application: Adding Integers Our next application reads (or inputs) two integers (whole numbers, such as –22, 7, 0 and 1024) typed by a user at the keyboard, computes their sum and displays it. This program must keep track of the numbers supplied by the user for the calculation later in the program. Programs remember numbers and other data in the computer’s memory and access that data through program elements called variables. The program of Fig. 2.7 demonstrates these concepts. In the sample output, we use bold text to identify the user’s input (i.e., 45 and 72). As in prior programs, Lines 1–2 state the figure number, filename and purpose of the program. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 // Fig. 2.7: Addition.java // Addition program that inputs two numbers then displays their sum. import java.util.Scanner; // program uses class Scanner public class Addition { // main method begins execution of Java application public static void main(String[] args) { // create a Scanner to obtain input from the command window Scanner input = new Scanner(System.in); int number1; // first number to add int number2; // second number to add int sum; // sum of number1 and number2 System.out.print("Enter first integer: "); // prompt number1 = input.nextInt(); // read first number from user System.out.print("Enter second integer: "); // prompt number2 = input.nextInt(); // read second number from user sum = number1 + number2; // add numbers, then store total in sum System.out.printf("Sum is %d%n", sum); // display sum } // end method main } // end class Addition Enter first integer: 45 Enter second integer: 72 Sum is 117 Fig. 2.7 | Addition program that inputs two numbers then displays their sum. 2.5.1 import Declarations A great strength of Java is its rich set of predefined classes that you can reuse rather than “reinventing the wheel.” These classes are grouped into packages—named groups of related 46 Chapter 2 Introduction to Java Applications; Input/Output and Operators classes—and are collectively referred to as the Java class library, or the Java Application Programming Interface (Java API). Line 3 import java.util.Scanner; // program uses class Scanner is an import declaration that helps the compiler locate a class that’s used in this program. It indicates that the program uses the predefined Scanner class (discussed shortly) from the package named java.util. The compiler then ensures that you use the class correctly. Common Programming Error 2.7 All import declarations must appear before the first class declaration in the file. Placing an import declaration inside or after a class declaration is a syntax error. Common Programming Error 2.8 Forgetting to include an import declaration for a class that must be imported results in a compilation error containing a message such as “cannot find symbol.” When this occurs, check that you provided the proper import declarations and that the names in them are correct, including proper capitalization. Software Engineering Observation 2.1 In each new Java version, the APIs typically contain new capabilities that fix bugs, improve performance or offer better means for accomplishing tasks. The corresponding older versions are no longer needed and should not be used. Such APIs are said to be deprecated and might be removed from later Java versions. You’ll often encounter deprecated APIs when browsing the online API documentation. The compiler will warn you when you compile code that uses deprecated APIs. If you compile your code with javac using the command-line argument -deprecation, the compiler will tell you which deprecated features you’re using. For each one, the online documentation (http://docs.oracle.com/javase/7/docs/api/) indicates and typically links to the new feature that replaces the deprecated one. 2.5.2 Declaring Class Addition Line 5 public class Addition begins the declaration of class Addition. The filename for this public class must be Addition.java. Remember that the body of each class declaration starts with an opening left brace (line 6) and ends with a closing right brace (line 27). The application begins execution with the main method (lines 8–26). The left brace (line 9) marks the beginning of method main’s body, and the corresponding right brace (line 26) marks its end. Method main is indented one level in the body of class Addition, and the code in the body of main is indented another level for readability. 2.5.3 Declaring and Creating a Scanner to Obtain User Input from the Keyboard A variable is a location in the computer’s memory where a value can be stored for use later in a program. All Java variables must be declared with a name and a type before they can be used. A variable’s name enables the program to access the value of the variable in memory. A 2.5 Another Application: Adding Integers 47 variable’s name can be any valid identifier—again, a series of characters consisting of letters, digits, underscores (_) and dollar signs ($) that does not begin with a digit and does not contain spaces. A variable’s type specifies what kind of information is stored at that location in memory. Like other statements, declaration statements end with a semicolon (;). Line 11 Scanner input = new Scanner(System.in); is a variable declaration statement that specifies the name (input) and type (Scanner) of a variable that’s used in this program. A Scanner enables a program to read data (e.g., numbers and strings) for use in a program. The data can come from many sources, such as the user at the keyboard or a file on disk. Before using a Scanner, you must create it and specify the source of the data. The = in line 11 indicates that Scanner variable input should be initialized (i.e., prepared for use in the program) in its declaration with the result of the expression to the right of the equals sign—new Scanner(System.in). This expression uses the new keyword to create a Scanner object that reads characters typed by the user at the keyboard. The standard input object, System.in, enables applications to read bytes of data typed by the user. The Scanner translates these bytes into types (like ints) that can be used in a program. 2.5.4 Declaring Variables to Store Integers The variable declaration statements in lines 13–15 int number1; // first number to add int number2; // second number to add int sum; // sum of number1 and number2 declare that variables number1, number2 and sum hold data of type int—they can hold integer values (whole numbers such as 72, –1127 and 0). These variables are not yet initialized. The range of values for an int is –2,147,483,648 to +2,147,483,647. [Note: The int values you use in a program may not contain commas.] Some other types of data are float and double, for holding real numbers, and char, for holding character data. Real numbers contain decimal points, such as in 3.4, 0.0 and –11.19. Variables of type char represent individual characters, such as an uppercase letter (e.g., A), a digit (e.g., 7), a special character (e.g., * or %) or an escape sequence (e.g., the tab character, \t). The types int, float, double and char are called primitive types. Primitive-type names are keywords and must appear in all lowercase letters. Appendix D summarizes the characteristics of the eight primitive types (boolean, byte, char, short, int, long, float and double). Several variables of the same type may be declared in a single declaration with the variable names separated by commas (i.e., a comma-separated list of variable names). For example, lines 13–15 can also be written as: int number1, // first number to add number2, // second number to add sum; // sum of number1 and number2 Good Programming Practice 2.7 Declare each variable in its own declaration. This format allows a descriptive comment to be inserted next to each variable being declared. 48 Chapter 2 Introduction to Java Applications; Input/Output and Operators Good Programming Practice 2.8 Choosing meaningful variable names helps a program to be self-documenting (i.e., one can understand the program simply by reading it rather than by reading associated documentation or creating and viewing an excessive number of comments). Good Programming Practice 2.9 By convention, variable-name identifiers begin with a lowercase letter, and every word in the name after the first word begins with a capital letter. For example, variable-name identifier firstNumber starts its second word, Number, with a capital N. This naming convention is known as camel case, because the uppercase letters stand out like a camel’s humps. 2.5.5 Prompting the User for Input Line 17 System.out.print("Enter first integer: "); // prompt uses System.out.print to display the message "Enter first integer: ". This message is called a prompt because it directs the user to take a specific action. We use method print here rather than println so that the user’s input appears on the same line as the prompt. Recall from Section 2.2 that identifiers starting with capital letters typically represent class names. Class System is part of package java.lang. Notice that class System is not imported with an import declaration at the beginning of the program. Software Engineering Observation 2.2 By default, package java.lang is imported in every Java program; thus, classes in java.lang are the only ones in the Java API that do not require an import declaration. 2.5.6 Obtaining an int as Input from the User Line 18 number1 = input.nextInt(); // read first number from user uses Scanner object input’s nextInt method to obtain an integer from the user at the keyboard. At this point the program waits for the user to type the number and press the Enter key to submit the number to the program. Our program assumes that the user enters a valid integer value. If not, a runtime logic error will occur and the program will terminate. Chapter 11, Exception Handling: A Deeper Look, discusses how to make your programs more robust by enabling them to handle such errors. This is also known as making your program fault tolerant. In line 18, we place the result of the call to method nextInt (an int value) in variable number1 by using the assignment operator, =. The statement is read as “number1 gets the value of input.nextInt().” Operator = is called a binary operator, because it has two operands—number1 and the result of the method call input.nextInt(). This statement is called an assignment statement, because it assigns a value to a variable. Everything to the right of the assignment operator, =, is always evaluated before the assignment is performed. Good Programming Practice 2.10 Place spaces on either side of a binary operator for readability. 2.5 Another Application: Adding Integers 49 2.5.7 Prompting for and Inputting a Second int Line 20 System.out.print("Enter second integer: "); // prompt prompts the user to enter the second integer. Line 21 number2 = input.nextInt(); // read second number from user reads the second integer and assigns it to variable number2. 2.5.8 Using Variables in a Calculation Line 23 sum = number1 + number2; // add numbers then store total in sum is an assignment statement that calculates the sum of the variables number1 and number2 then assigns the result to variable sum by using the assignment operator, =. The statement is read as “sum gets the value of number1 + number2.” When the program encounters the addition operation, it performs the calculation using the values stored in the variables number1 and number2. In the preceding statement, the addition operator is a binary operator—its two operands are the variables number1 and number2. Portions of statements that contain calculations are called expressions. In fact, an expression is any portion of a statement that has a value associated with it. For example, the value of the expression number1 + number2 is the sum of the numbers. Similarly, the value of the expression input.nextInt() is the integer typed by the user. 2.5.9 Displaying the Result of the Calculation After the calculation has been performed, line 25 System.out.printf("Sum is %d%n", sum); // display sum uses method System.out.printf to display the sum. The format specifier %d is a placeholder for an int value (in this case the value of sum)—the letter d stands for “decimal integer.” The remaining characters in the format string are all fixed text. So, method printf displays "Sum is ", followed by the value of sum (in the position of the %d format specifier) and a newline. Calculations can also be performed inside printf statements. We could have combined the statements at lines 23 and 25 into the statement System.out.printf("Sum is %d%n", (number1 + number2)); The parentheses around the expression number1 + number2 are optional—they’re included to emphasize that the value of the entire expression is output in the position of the %d format specifier. Such parentheses are said to be redundant. 2.5.10 Java API Documentation For each new Java API class we use, we indicate the package in which it’s located. This information helps you locate descriptions of each package and class in the Java API documentation. A web-based version of this documentation can be found at http://docs.oracle.com/javase/7/docs/api/index.html 50 Chapter 2 Introduction to Java Applications; Input/Output and Operators You can download it from the Additional Resources section at http://www.oracle.com/technetwork/java/javase/downloads/index.html Appendix F shows how to use this documentation. 2.6 Memory Concepts Variable names such as number1, number2 and sum actually correspond to locations in the computer’s memory. Every variable has a name, a type, a size (in bytes) and a value. In the addition program of Fig. 2.7, when the following statement (line 18) executes: number1 = input.nextInt(); // read first number from user the number typed by the user is placed into a memory location corresponding to the name number1. Suppose that the user enters 45. The computer places that integer value into location number1 (Fig. 2.8), replacing the previous value (if any) in that location. The previous value is lost, so this process is said to be destructive. number1 45 Fig. 2.8 | Memory location showing the name and value of variable number1. When the statement (line 21) number2 = input.nextInt(); // read second number from user executes, suppose that the user enters 72. The computer places that integer value into location number2. The memory now appears as shown in Fig. 2.9. number1 45 number2 72 Fig. 2.9 | Memory locations after storing values for number1 and number2. After the program of Fig. 2.7 obtains values for number1 and number2, it adds the values and places the total into variable sum. The statement (line 23) sum = number1 + number2; // add numbers, then store total in sum performs the addition, then replaces any previous value in sum. After sum has been calculated, memory appears as shown in Fig. 2.10. The values of number1 and number2 appear exactly as they did before they were used in the calculation of sum. These values were used, but not destroyed, as the computer performed the calculation. When a value is read from a memory location, the process is nondestructive. 2.7 Arithmetic number1 45 number2 72 sum 51 117 Fig. 2.10 | Memory locations after storing the sum of number1 and number2. 2.7 Arithmetic Most programs perform arithmetic calculations. The arithmetic operators are summarized in Fig. 2.11. Note the use of various special symbols not used in algebra. The asterisk (*) indicates multiplication, and the percent sign (%) is the remainder operator, which we’ll discuss shortly. The arithmetic operators in Fig. 2.11 are binary operators, because each operates on two operands. For example, the expression f + 7 contains the binary operator + and the two operands f and 7. Java operation Operator Algebraic expression Java expression Addition Subtraction Multiplication Division Remainder + f+7 p–c bm x / y or x-y or x ÷ y r mod s f + 7 – * / % p - c b * m x / y r % s Fig. 2.11 | Arithmetic operators. Integer division yields an integer quotient. For example, the expression 7 / 4 evaluates to 1, and the expression 17 / 5 evaluates to 3. Any fractional part in integer division is simply truncated (i.e., discarded)—no rounding occurs. Java provides the remainder operator, %, which yields the remainder after division. The expression x % y yields the remainder after x is divided by y. Thus, 7 % 4 yields 3, and 17 % 5 yields 2. This operator is most commonly used with integer operands but it can also be used with other arithmetic types. In this chapter’s exercises and in later chapters, we consider several interesting applications of the remainder operator, such as determining whether one number is a multiple of another. Arithmetic Expressions in Straight-Line Form Arithmetic expressions in Java must be written in straight-line form to facilitate entering programs into the computer. Thus, expressions such as “a divided by b” must be written as a / b, so that all constants, variables and operators appear in a straight line. The following algebraic notation is generally not acceptable to compilers: a -b 52 Chapter 2 Introduction to Java Applications; Input/Output and Operators Parentheses for Grouping Subexpressions Parentheses are used to group terms in Java expressions in the same manner as in algebraic expressions. For example, to multiply a times the quantity b + c, we write a * (b + c) If an expression contains nested parentheses, such as ((a + b) * c) the expression in the innermost set of parentheses (a + b in this case) is evaluated first. Rules of Operator Precedence Java applies the operators in arithmetic expressions in a precise sequence determined by the rules of operator precedence, which are generally the same as those followed in algebra: 1. Multiplication, division and remainder operations are applied first. If an expression contains several such operations, they’re applied from left to right. Multiplication, division and remainder operators have the same level of precedence. 2. Addition and subtraction operations are applied next. If an expression contains several such operations, the operators are applied from left to right. Addition and subtraction operators have the same level of precedence. These rules enable Java to apply operators in the correct order.1 When we say that operators are applied from left to right, we’re referring to their associativity. Some operators associate from right to left. Figure 2.12 summarizes these rules of operator precedence. A complete precedence chart is included in Appendix A. Operator(s) Operation(s) Order of evaluation (precedence) * / % Multiplication Division Remainder Addition Subtraction Assignment Evaluated first. If there are several operators of this type, they’re evaluated from left to right. + = Evaluated next. If there are several operators of this type, they’re evaluated from left to right. Evaluated last. Fig. 2.12 | Precedence of arithmetic operators. Sample Algebraic and Java Expressions Now let’s consider several expressions in light of the rules of operator precedence. Each example lists an algebraic expression and its Java equivalent. The following is an example of an arithmetic mean (average) of five terms: 1. We use simple examples to explain the order of evaluation of expressions. Subtle issues occur in the more complex expressions you’ll encounter later in the book. For more information on order of evaluation, see Chapter 15 of The Java™ Language Specification (http://docs.oracle.com/javase/ specs/jls/se7/html/index.html). 2.7 Arithmetic Algebra: a+b+c+d+e m = ------------------------------------5 Java: m = (a + b + c + d + e) / 5; 53 The parentheses are required because division has higher precedence than addition. The entire quantity (a + b + c + d + e) is to be divided by 5. If the parentheses are erroneously omitted, we obtain a + b + c + d + e / 5, which evaluates as e a + b + c + d + --5 Here’s an example of the equation of a straight line: Algebra: Java: y = mx + b y = m * x + b; No parentheses are required. The multiplication operator is applied first because multiplication has a higher precedence than addition. The assignment occurs last because it has a lower precedence than multiplication or addition. The following example contains remainder (%), multiplication, division, addition and subtraction operations: Algebra: Java: z = pr %q + w/x – y z = p 6 * r 1 % q 2 + w 4 / 3 x - y; 5 The circled numbers under the statement indicate the order in which Java applies the operators. The *, % and / operations are evaluated first in left-to-right order (i.e., they associate from left to right), because they have higher precedence than + and -. The + and operations are evaluated next. These operations are also applied from left to right. The assignment (=) operation is evaluated last. Evaluation of a Second-Degree Polynomial To develop a better understanding of the rules of operator precedence, consider the evaluation of an assignment expression that includes a second-degree polynomial ax2 + bx + c: y = 6 a * 1 x * 2 x + 4 b * x 3 + c; 5 The multiplication operations are evaluated first in left-to-right order (i.e., they associate from left to right), because they have higher precedence than addition. (Java has no arithmetic operator for exponentiation, so x2 is represented as x * x. Section 5.4 shows an alternative for performing exponentiation.) The addition operations are evaluated next from left to right. Suppose that a, b, c and x are initialized (given values) as follows: a = 2, b = 3, c = 7 and x = 5. Figure 2.13 illustrates the order in which the operators are applied. You can use redundant parentheses (unnecessary parentheses) to make an expression clearer. For example, the preceding statement might be parenthesized as follows: y = (a * x * x) + (b * x) + c; 54 Chapter 2 Introduction to Java Applications; Input/Output and Operators y = 2 * 5 * 5 + 3 * 5 + 7; Step 1. (Leftmost multiplication) 2 * 5 is 10 y = 10 * 5 + 3 * 5 + 7; Step 2. (Leftmost multiplication) 10 * 5 is 50 y = 50 + 3 * 5 + 7; Step 3. (Multiplication before addition) 3 * 5 is 15 y = 50 + 15 + 7; Step 4. (Leftmost addition) 50 + 15 is 65 y = 65 + 7; Step 5. (Last addition) 65 + 7 is 72 (Last operation—place 72 in y) y = 72 Step 6. Fig. 2.13 | Order in which a second-degree polynomial is evaluated. 2.8 Decision Making: Equality and Relational Operators A condition is an expression that can be true or false. This section introduces Java’s if selection statement, which allows a program to make a decision based on a condition’s value. For example, the condition “grade is greater than or equal to 60” determines whether a student passed a test. If the condition in an if statement is true, the body of the if statement executes. If the condition is false, the body does not execute. We’ll see an example shortly. Conditions in if statements can be formed by using the equality operators (== and !=) and relational operators (>, <, >= and <=) summarized in Fig. 2.14. Both equality operators have the same level of precedence, which is lower than that of the relational operators. The equality operators associate from left to right. The relational operators all have the same level of precedence and also associate from left to right. Algebraic operator Java equality or relational operator Sample Java condition Meaning of Java condition == x == y x is equal to y != x != y x is not equal to y Equality operators = ¹ Fig. 2.14 | Equality and relational operators. (Part 1 of 2.) 2.8 Decision Making: Equality and Relational Operators Algebraic operator Java equality or relational operator Sample Java condition Meaning of Java condition > x > y x is greater than y < x < y x is less than y >= x >= y x is greater than or equal to y <= x <= y x is less than or equal to y 55 Relational operators > < ≥ ≤ Fig. 2.14 | Equality and relational operators. (Part 2 of 2.) Figure 2.15 uses six if statements to compare two integers input by the user. If the condition in any of these if statements is true, the statement associated with that if statement executes; otherwise, the statement is skipped. We use a Scanner to input the integers from the user and store them in variables number1 and number2. The program compares the numbers and displays the results of the comparisons that are true. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 // Fig. 2.15: Comparison.java // Compare integers using if statements, relational operators // and equality operators. import java.util.Scanner; // program uses class Scanner public class Comparison { // main method begins execution of Java application public static void main(String[] args) { // create Scanner to obtain input from command line Scanner input = new Scanner(System.in); int number1; // first number to compare int number2; // second number to compare System.out.print("Enter first integer: "); // prompt number1 = input.nextInt(); // read first number from user System.out.print("Enter second integer: "); // prompt number2 = input.nextInt(); // read second number from user if (number1 == number2) System.out.printf("%d == %d%n", number1, number2); if (number1 != number2) System.out.printf("%d != %d%n", number1, number2); if (number1 < number2) System.out.printf("%d < %d%n", number1, number2); Fig. 2.15 | Compare integers using if statements, relational operators and equality operators. (Part 1 of 2.) 56 31 32 33 34 35 36 37 38 39 40 41 Chapter 2 Introduction to Java Applications; Input/Output and Operators if (number1 > number2) System.out.printf("%d > %d%n", number1, number2); if (number1 <= number2) System.out.printf("%d <= %d%n", number1, number2); if (number1 >= number2) System.out.printf("%d >= %d%n", number1, number2); } // end method main } // end class Comparison Enter first integer: 777 Enter second integer: 777 777 == 777 777 <= 777 777 >= 777 Enter first integer: 1000 Enter second integer: 2000 1000 != 2000 1000 < 2000 1000 <= 2000 Enter first integer: 2000 Enter second integer: 1000 2000 != 1000 2000 > 1000 2000 >= 1000 Fig. 2.15 | Compare integers using if statements, relational operators and equality operators. (Part 2 of 2.) The declaration of class Comparison begins at line 6 public class Comparison The class’s main method (lines 9–40) begins the execution of the program. Line 12 Scanner input = new Scanner(System.in); declares Scanner variable input and assigns it a Scanner that inputs data from the standard input (i.e., the keyboard). Lines 14–15 int number1; // first number to compare int number2; // second number to compare declare the int variables used to store the values input from the user. Lines 17–18 System.out.print("Enter first integer: "); // prompt number1 = input.nextInt(); // read first number from user 2.8 Decision Making: Equality and Relational Operators 57 prompt the user to enter the first integer and input the value, respectively. The value is stored in variable number1. Lines 20–21 System.out.print("Enter second integer: "); // prompt number2 = input.nextInt(); // read second number from user prompt the user to enter the second integer and input the value, respectively. The value is stored in variable number2. Lines 23–24 if (number1 == number2) System.out.printf("%d == %d%n", number1, number2); compare the values of number1 and number2 to determine whether they’re equal. An if statement always begins with keyword if, followed by a condition in parentheses. An if statement expects one statement in its body, but may contain multiple statements if they’re enclosed in a set of braces ({}). The indentation of the body statement shown here is not required, but it improves the program’s readability by emphasizing that the statement in line 24 is part of the if statement that begins at line 23. Line 24 executes only if the numbers stored in variables number1 and number2 are equal (i.e., the condition is true). The if statements in lines 26–27, 29–30, 32–33, 35–36 and 38–39 compare number1 and number2 using the operators !=, <, >, <= and >=, respectively. If the condition in one or more of the if statements is true, the corresponding body statement executes. Common Programming Error 2.9 Confusing the equality operator, ==, with the assignment operator, =, can cause a logic error or a compilation error. The equality operator should be read as “is equal to” and the assignment operator as “gets” or “gets the value of.” To avoid confusion, some people read the equality operator as “double equals” or “equals equals.” Good Programming Practice 2.11 Place only one statement per line in a program for readability. There’s no semicolon (;) at the end of the first line of each if statement. Such a semicolon would result in a logic error at execution time. For example, if (number1 == number2); // logic error System.out.printf("%d == %d%n", number1, number2); would actually be interpreted by Java as if (number1 == number2) ; // empty statement System.out.printf("%d == %d%n", number1, number2); where the semicolon on the line by itself—called the empty statement—is the statement to execute if the condition in the if statement is true. When the empty statement executes, no task is performed. The program then continues with the output statement, which always executes, regardless of whether the condition is true or false, because the output statement is not part of the if statement. 58 Chapter 2 Introduction to Java Applications; Input/Output and Operators White space Note the use of white space in Fig. 2.15. Recall that the compiler normally ignores white space. So, statements may be split over several lines and may be spaced according to your preferences without affecting a program’s meaning. It’s incorrect to split identifiers and strings. Ideally, statements should be kept small, but this is not always possible. Error-Prevention Tip 2.5 A lengthy statement can be spread over several lines. If a single statement must be split across lines, choose natural breaking points, such as after a comma in a comma-separated list, or after an operator in a lengthy expression. If a statement is split across two or more lines, indent all subsequent lines until the end of the statement. Operators Discussed So Far Figure 2.16 shows the operators discussed so far in decreasing order of precedence. All but the assignment operator, =, associate from left to right. The assignment operator, =, associates from right to left. An assignment expression’s value is whatever was assigned to the variable on the = operator’s left side—for example, the value of the expression x = 7 is 7. So an expression like x = y = 0 is evaluated as if it had been written as x = (y = 0), which first assigns the value 0 to variable y, then assigns the result of that assignment, 0, to x. Operators * / + - < <= == != = % > >= Associativity Type left to right left to right left to right left to right right to left multiplicative additive relational equality assignment Fig. 2.16 | Precedence and associativity of operators discussed. Good Programming Practice 2.12 When writing expressions containing many operators, refer to the operator precedence chart (Appendix A). Confirm that the operations in the expression are performed in the order you expect. If, in a complex expression, you’re uncertain about the order of evaluation, use parentheses to force the order, exactly as you’d do in algebraic expressions. 2.9 Wrap-Up In this chapter, you learned many important features of Java, including displaying data on the screen in a Command Prompt, inputting data from the keyboard, performing calculations and making decisions. The applications presented here introduced you to many basic programming concepts. As you’ll see in Chapter 3, Java applications typically contain just a few lines of code in method main—these statements normally create the objects that perform the work of the application. In Chapter 3, you’ll learn how to implement your own classes and use objects of those classes in applications. Summary 59 Summary Section 2.2 Your First Program in Java: Printing a Line of Text • A Java application (p. 35) executes when you use the java command to launch the JVM. • Comments (p. 36) document programs and improve their readability. The compiler ignores them. • A comment that begins with // is an end-of-line comment—it terminates at the end of the line on which it appears. • Traditional comments (p. 36) can be spread over several lines and are delimited by /* and */. • Javadoc comments (p. 36), delimited by /** and */, enable you to embed program documentation in your code. The javadoc utility program generates HTML pages based on these comments. • A syntax error (p. 36; also called a compiler error, compile-time error or compilation error) occurs when the compiler encounters code that violates Java’s language rules. It’s similar to a grammar error in a natural language. • Blank lines, space characters and tab characters are known as white space (p. 37). White space makes programs easier to read and is ignored by the compiler. • Keywords (p. 37) are reserved for use by Java and are always spelled with all lowercase letters. • Keyword class (p. 37) introduces a class declaration. • By convention, all class names in Java begin with a capital letter and capitalize the first letter of each word they include (e.g., SampleClassName). • A Java class name is an identifier—a series of characters consisting of letters, digits, underscores ( _ ) and dollar signs ($) that does not begin with a digit and does not contain spaces. • Java is case sensitive (p. 38)—that is, uppercase and lowercase letters are distinct. • The body of every class declaration (p. 38) is delimited by braces, { and }. • A public (p. 37) class declaration must be saved in a file with the same name as the class followed by the “.java” file-name extension. • Method main (p. 38) is the starting point of every Java application and must begin with public static void main(String[] args) otherwise, the JVM will not execute the application. • Methods perform tasks and return information when they complete them. Keyword void (p. 38) indicates that a method will perform a task but return no information. • Statements instruct the computer to perform actions. • A string (p. 39) in double quotes is sometimes called a character string or a string literal. • The standard output object (System.out; p. 39) displays characters in the command window. • Method System.out.println (p. 39) displays its argument (p. 39) in the command window followed by a newline character to position the output cursor to the beginning of the next line. • You compile a program with the command javac. If the program contains no syntax errors, a class file (p. 40) containing the Java bytecodes that represent the application is created. These bytecodes are interpreted by the JVM when you execute the program. • To run an application, type java followed by the name of the class that contains the main method. Section 2.3 Modifying Your First Java Program • (p. 42) displays its argument and positions the output cursor immediately after the last character displayed. • A backslash (\) in a string is an escape character (p. 43). Java combines it with the next character to form an escape sequence (p. 43). The escape sequence \n (p. 43) represents the newline character. System.out.print 60 Chapter 2 Introduction to Java Applications; Input/Output and Operators Section 2.4 Displaying Text with printf • System.out.printf method (p. 43; f means “formatted”) displays formatted data. • Method printf’s first argument is a format string (p. 44) containing fixed text and/or format specifiers. Each format specifier (p. 44) indicates the type of data to output and is a placeholder for a corresponding argument that appears after the format string. • Format specifiers begin with a percent sign (%) and are followed by a character that represents the data type. The format specifier %s (p. 44) is a placeholder for a string. • The %n format specifier (p. 44) is a portable line separator. You cannot use %n in the argument to System.out.print or System.out.println; however, the line separator output by System.out.println after it displays its argument is portable across operating systems. Section 2.5 Another Application: Adding Integers • An import declaration (p. 46) helps the compiler locate a class that’s used in a program. • Java’s rich set of predefined classes are grouped into packages (p. 45)—named groups of classes. These are referred to as the Java class library (p. 46), or the Java Application Programming Interface (Java API). • A variable (p. 46) is a location in the computer’s memory where a value can be stored for use later in a program. All variables must be declared with a name and a type before they can be used. • A variable’s name enables the program to access the variable’s value in memory. • A Scanner (package java.util; p. 47) enables a program to read data that the program will use. Before a Scanner can be used, the program must create it and specify the source of the data. • Variables should be initialized (p. 47) to prepare them for use in a program. • The expression new Scanner(System.in) creates a Scanner that reads from the standard input object (System.in; p. 47)—normally the keyboard. • Data type int (p. 47) is used to declare variables that will hold integer values. The range of values for an int is –2,147,483,648 to +2,147,483,647. • Types float and double (p. 47) specify real numbers with decimal points, such as 3.4 and –11.19. • Variables of type char (p. 47) represent individual characters, such as an uppercase letter (e.g., A), a digit (e.g., 7), a special character (e.g., * or %) or an escape sequence (e.g., tab, \t). • Types such as int, float, double and char are primitive types (p. 47). Primitive-type names are keywords; thus, they must appear in all lowercase letters. • A prompt (p. 48) directs the user to take a specific action. • Scanner method nextInt obtains an integer for use in a program. • The assignment operator, = (p. 48), enables the program to give a value to a variable. It’s called a binary operator (p. 48) because it has two operands. • Portions of statements that have values are called expressions (p. 49). • The format specifier %d (p. 49) is a placeholder for an int value. Section 2.6 Memory Concepts • Variable names (p. 50) correspond to locations in the computer’s memory. Every variable has a name, a type, a size and a value. • A value that’s placed in a memory location replaces the location’s previous value, which is lost. Section 2.7 Arithmetic • The arithmetic operators (p. 51) are + (addition), - (subtraction), * (multiplication), / (division) and % (remainder). Self-Review Exercises 61 • • • • • Integer division (p. 51) yields an integer quotient. The remainder operator, % (p. 51), yields the remainder after division. Arithmetic expressions must be written in straight-line form (p. 51). If an expression contains nested parentheses (p. 52), the innermost set is evaluated first. Java applies the operators in arithmetic expressions in a precise sequence determined by the rules of operator precedence (p. 52). • When we say that operators are applied from left to right, we’re referring to their associativity (p. 52). Some operators associate from right to left. • Redundant parentheses (p. 53) can make an expression clearer. Section 2.8 Decision Making: Equality and Relational Operators • The if statement (p. 54) makes a decision based on a condition’s value (true or false). • Conditions in if statements can be formed by using the equality (== and !=) and relational (>, <, >= and <=) operators (p. 54). • An if statement begins with keyword if followed by a condition in parentheses and expects one statement in its body. • The empty statement (p. 57) is a statement that does not perform a task. Self-Review Exercises 2.1 Fill in the blanks in each of the following statements: a) A(n) begins the body of every method, and a(n) ends the body of every method. statement to make decisions. b) You can use the c) begins an end-of-line comment. , and are called white space. d) e) are reserved for use by Java. f) Java applications begin execution at method . , and display information in a command wing) Methods dow. 2.2 State whether each of the following is true or false. If false, explain why. a) Comments cause the computer to print the text after the // on the screen when the program executes. b) All variables must be given a type when they’re declared. c) Java considers the variables number and NuMbEr to be identical. d) The remainder operator (%) can be used only with integer operands. e) The arithmetic operators *, /, %, + and - all have the same level of precedence. 2.3 Write statements to accomplish each of the following tasks: a) Declare variables c, thisIsAVariable, q76354 and number to be of type int. b) Prompt the user to enter an integer. c) Input an integer and assign the result to int variable value. Assume Scanner variable input can be used to read a value from the keyboard. d) Print "This is a Java program" on one line in the command window. Use method System.out.println. e) Print "This is a Java program" on two lines in the command window. The first line should end with Java. Use method System.out.printf and two %s format specifiers. f) If the variable number is not equal to 7, display "The variable number is not equal to 7". 62 Chapter 2 Introduction to Java Applications; Input/Output and Operators 2.4 Identify and correct the errors in each of the following statements: a) if (c < 7); System.out.println("c is less than 7"); b) if (c => 7) System.out.println("c is equal to or greater than 7"); 2.5 Write declarations, statements or comments that accomplish each of the following tasks: a) State that a program will calculate the product of three integers. b) Create a Scanner called input that reads values from the standard input. c) Declare the variables x, y, z and result to be of type int. d) Prompt the user to enter the first integer. e) Read the first integer from the user and store it in the variable x. f) Prompt the user to enter the second integer. g) Read the second integer from the user and store it in the variable y. h) Prompt the user to enter the third integer. i) Read the third integer from the user and store it in the variable z. j) Compute the product of the three integers contained in variables x, y and z, and assign the result to the variable result. k) Use System.out.printf to display the message "Product is" followed by the value of the variable result. 2.6 Using the statements you wrote in Exercise 2.5, write a complete program that calculates and prints the product of three integers. Answers to Self-Review Exercises 2.1 a) left brace ({), right brace (}). b) if. c) //. d) Space characters, newlines and tabs. e) Keywords. f) main. g) System.out.print, System.out.println and System.out.printf. 2.2 a) False. Comments do not cause any action to be performed when the program executes. They’re used to document programs and improve their readability. b) True. c) False. Java is case sensitive, so these variables are distinct. d) False. The remainder operator can also be used with noninteger operands in Java. e) False. The operators *, / and % are higher precedence than operators + and -. 2.3 a) int c, thisIsAVariable, q76354, number; or int c; int thisIsAVariable; int q76354; int number; b) c) d) e) f) System.out.print("Enter an integer: "); value = input.nextInt(); System.out.println("This is a Java program"); System.out.printf("%s%n%s%n", "This is a Java", "program"); if (number != 7) System.out.println("The variable number is not equal to 7"); 2.4 a) Error: Semicolon after the right parenthesis of the condition (c < 7) in the if. Correction: Remove the semicolon after the right parenthesis. [Note: As a result, the output statement will execute regardless of whether the condition in the if is true.] b) Error: The relational operator => is incorrect. Correction: Change => to >=. Answers to Self-Review Exercises 2.5 a) b) c) // Calculate the product of three integers Scanner input = new Scanner(System.in); int x, y, z, result; or int x; int y; int z; int result; d) e) f) g) h) i) j) k) 2.6 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 System.out.print("Enter first integer: "); x = input.nextInt(); System.out.print("Enter second integer: "); y = input.nextInt(); System.out.print("Enter third integer: "); z = input.nextInt(); result = x * y * z; System.out.printf("Product is %d%n", result); The solution to Self-Review Exercise 2.6 is as follows: // Ex. 2.6: Product.java // Calculate the product of three integers. import java.util.Scanner; // program uses Scanner public class Product { public static void main(String[] args) { // create Scanner to obtain input from command window Scanner input = new Scanner(System.in); int int int int x; // first number input by user y; // second number input by user z; // third number input by user result; // product of numbers System.out.print("Enter first integer: "); // prompt for input x = input.nextInt(); // read first integer System.out.print("Enter second integer: "); // prompt for input y = input.nextInt(); // read second integer System.out.print("Enter third integer: "); // prompt for input z = input.nextInt(); // read third integer result = x * y * z; // calculate product of numbers System.out.printf("Product is %d%n", result); } // end method main } // end class Product Enter first integer: 10 Enter second integer: 20 Enter third integer: 30 Product is 6000 63 64 Chapter 2 Introduction to Java Applications; Input/Output and Operators Exercises 2.7 Fill in the blanks in each of the following statements: a) are used to document a program and improve its readability. b) A decision can be made in a Java program with a(n) . c) Calculations are normally performed by statements. d) The arithmetic operators with the same precedence as multiplication are and . set of parene) When parentheses in an arithmetic expression are nested, the theses is evaluated first. f) A location in the computer’s memory that may contain different values at various times throughout the execution of a program is called a(n) . 2.8 Write Java statements that accomplish each of the following tasks: a) Display the message "Enter an integer: ", leaving the cursor on the same line. b) Assign the product of variables b and c to variable a. c) Use a comment to state that a program performs a sample payroll calculation. 2.9 State whether each of the following is true or false. If false, explain why. a) Java operators are evaluated from left to right. b) The following are all valid variable names: _under_bar_, m928134, t5, j7, her_sales$, his_$account_total, a, b$, c, z and z2. c) A valid Java arithmetic expression with no parentheses is evaluated from left to right. d) The following are all invalid variable names: 3g, 87, 67h2, h22 and 2h. 2.10 Assuming that x = 2 and y = 3, what does each of the following statements display? a) System.out.printf("x = %d%n", x); b) System.out.printf("Value of %d + %d is %d%n", x, x, (x + x)); c) System.out.printf("x ="); d) System.out.printf("%d = %d%n", (x + y), (y + x)); 2.11 Which of the following Java statements contain variables whose values are modified? a) p = i + j + k + 7; b) System.out.println("variables whose values are modified"); c) System.out.println("a = 5"); d) value = input.nextInt(); 2.12 Given that y = ax3 + 7, which of the following are correct Java statements for this equation? a) y = a * x * x * x + 7; b) y = a * x * x * (x + 7); c) y = (a * x) * x * (x + 7); d) y = (a * x) * x * x + 7; e) y = a * (x * x * x) + 7; f) y = a * x * (x * x + 7); 2.13 State the order of evaluation of the operators in each of the following Java statements, and show the value of x after each statement is performed: a) x = 7 + 3 * 6 / 2 - 1; b) x = 2 % 2 + 2 * 2 - 2 / 2; c) x = (3 * 9 * (3 + (9 * 3 / (3)))); 2.14 Write an application that displays the numbers 1 to 4 on the same line, with each pair of adjacent numbers separated by one space. Use the following techniques: a) Use one System.out.println statement. b) Use four System.out.print statements. c) Use one System.out.printf statement. Exercises 65 2.15 (Arithmetic) Write an application that asks the user to enter two integers, obtains them from the user and prints their sum, product, difference and quotient (division). Use the techniques shown in Fig. 2.7. 2.16 (Comparing Integers) Write an application that asks the user to enter two integers, obtains them from the user and displays the larger number followed by the words "is larger". If the numbers are equal, print the message "These numbers are equal". Use the techniques shown in Fig. 2.15. 2.17 (Arithmetic, Smallest and Largest) Write an application that inputs three integers from the user and displays the sum, average, product, smallest and largest of the numbers. Use the techniques shown in Fig. 2.15. [Note: The calculation of the average in this exercise should result in an integer representation of the average. So, if the sum of the values is 7, the average should be 2, not 2.3333….] 2.18 (Displaying Shapes with Asterisks) Write an application that displays a box, an oval, an arrow and a diamond using asterisks (*), as follows: ********* * * * * * * * * * * * * * * ********* *** * * * * * * * * * * * * * * *** * *** ***** * * * * * * * * * * * * * * * * * * * * * * 2.19 What does the following code print? 2.20 What does the following code print? System.out.printf("*%n**%n***%n****%n*****%n"); System.out.println("*"); System.out.println("***"); System.out.println("*****"); System.out.println("****"); System.out.println("**"); 2.21 What does the following code print? System.out.print("*"); System.out.print("***"); System.out.print("*****"); System.out.print("****"); System.out.println("**"); 2.22 What does the following code print? System.out.print("*"); System.out.println("***"); System.out.println("*****"); System.out.print("****"); System.out.println("**"); 2.23 What does the following code print? System.out.printf("%s%n%s%n%s%n", "*", "***", "*****"); 2.24 (Largest and Smallest Integers) Write an application that reads five integers and determines and prints the largest and smallest integers in the group. Use only the programming techniques you learned in this chapter. 66 Chapter 2 Introduction to Java Applications; Input/Output and Operators 2.25 (Odd or Even) Write an application that reads an integer and determines and prints whether it’s odd or even. [Hint: Use the remainder operator. An even number is a multiple of 2. Any multiple of 2 leaves a remainder of 0 when divided by 2.] 2.26 (Multiples) Write an application that reads two integers, determines whether the first is a multiple of the second and prints the result. [Hint: Use the remainder operator.] 2.27 (Checkerboard Pattern of Asterisks) Write an application that displays a checkerboard pattern, as follows: * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * 2.28 (Diameter, Circumference and Area of a Circle) Here’s a peek ahead. In this chapter, you learned about integers and the type int. Java can also represent floating-point numbers that contain decimal points, such as 3.14159. Write an application that inputs from the user the radius of a circle as an integer and prints the circle’s diameter, circumference and area using the floating-point value 3.14159 for π. Use the techniques shown in Fig. 2.7. [Note: You may also use the predefined constant Math.PI for the value of π. This constant is more precise than the value 3.14159. Class Math is defined in package java.lang. Classes in that package are imported automatically, so you do not need to import class Math to use it.] Use the following formulas (r is the radius): diameter = 2r circumference = 2πr area = πr2 Do not store the results of each calculation in a variable. Rather, specify each calculation as the value that will be output in a System.out.printf statement. The values produced by the circumference and area calculations are floating-point numbers. Such values can be output with the format specifier %f in a System.out.printf statement. You’ll learn more about floating-point numbers in Chapter 3. 2.29 (Integer Value of a Character) Here’s another peek ahead. In this chapter, you learned about integers and the type int. Java can also represent uppercase letters, lowercase letters and a considerable variety of special symbols. Every character has a corresponding integer representation. The set of characters a computer uses together with the corresponding integer representations for those characters is called that computer’s character set. You can indicate a character value in a program simply by enclosing that character in single quotes, as in 'A'. You can determine a character’s integer equivalent by preceding that character with (int), as in (int) 'A' An operator of this form is called a cast operator. (You’ll learn about cast operators in Chapter 4.) The following statement outputs a character and its integer equivalent: System.out.printf("The character %c has the value %d%n", 'A', ((int) 'A')); When the preceding statement executes, it displays the character A and the value 65 (from the Unicode® character set) as part of the string. The format specifier %c is a placeholder for a character (in this case, the character 'A'). Using statements similar to the one shown earlier in this exercise, write an application that displays the integer equivalents of some uppercase letters, lowercase letters, digits and special symbols. Display the integer equivalents of the following: A B C a b c 0 1 2 $ * + / and the blank character. Making a Difference 67 2.30 (Separating the Digits in an Integer) Write an application that inputs one number consisting of five digits from the user, separates the number into its individual digits and prints the digits separated from one another by three spaces each. For example, if the user types in the number 42339, the program should print 4 2 3 3 9 Assume that the user enters the correct number of digits. What happens when you enter a number with more than five digits? What happens when you enter a number with fewer than five digits? [Hint: It’s possible to do this exercise with the techniques you learned in this chapter. You’ll need to use both division and remainder operations to “pick off ” each digit.] 2.31 (Table of Squares and Cubes) Using only the programming techniques you learned in this chapter, write an application that calculates the squares and cubes of the numbers from 0 to 10 and prints the resulting values in table format, as shown below. number 0 1 2 3 4 5 6 7 8 9 10 square 0 1 4 9 16 25 36 49 64 81 100 cube 0 1 8 27 64 125 216 343 512 729 1000 2.32 (Negative, Positive and Zero Values) Write a program that inputs five numbers and determines and prints the number of negative numbers input, the number of positive numbers input and the number of zeros input. Making a Difference 2.33 (Body Mass Index Calculator) We introduced the body mass index (BMI) calculator in Exercise 1.10. The formulas for calculating BMI are weightInPounds × 703 BMI = -----------------------------------------------------------------------------------heightInInches × heightInInches or weightInKi log rams BMI = --------------------------------------------------------------------------------------heightInMeters × heightInMeters Create a BMI calculator that reads the user’s weight in pounds and height in inches (or, if you prefer, the user’s weight in kilograms and height in meters), then calculates and displays the user’s body mass index. Also, display the following information from the Department of Health and Human Services/National Institutes of Health so the user can evaluate his/her BMI: BMI VALUES Underweight: Normal: Overweight: Obese: less than 18.5 between 18.5 and 24.9 between 25 and 29.9 30 or greater 68 Chapter 2 Introduction to Java Applications; Input/Output and Operators [Note: In this chapter, you learned to use the int type to represent whole numbers. The BMI calculations when done with int values will both produce whole-number results. In Chapter 3 you’ll learn to use the double type to represent numbers with decimal points. When the BMI calculations are performed with doubles, they’ll both produce numbers with decimal points—these are called “floating-point” numbers.] 2.34 (World Population Growth Calculator) Use the web to determine the current world population and the annual world population growth rate. Write an application that inputs these values, then displays the estimated world population after one, two, three, four and five years. 2.35 (Car-Pool Savings Calculator) Research several car-pooling websites. Create an application that calculates your daily driving cost, so that you can estimate how much money could be saved by car pooling, which also has other advantages such as reducing carbon emissions and reducing traffic congestion. The application should input the following information and display the user’s cost per day of driving to work: a) Total miles driven per day. b) Cost per gallon of gasoline. c) Average miles per gallon. d) Parking fees per day. e) Tolls per day. Introduction to Classes, Objects, Methods and Strings 3 Your public servants serve you right. —Adlai E. Stevenson Nothing can have value without being an object of utility. —Karl Marx Objectives In this chapter you’ll learn: ■ How to declare a class and use it to create an object. ■ How to implement a class’s behaviors as methods. ■ How to implement a class’s attributes as instance variables. ■ How to call an object’s methods to make them perform their tasks. ■ What local variables of a method are and how they differ from instance variables. ■ What primitive types and reference types are. ■ How to use a constructor to initialize an object’s data. ■ How to represent and use numbers containing decimal points. 70 Chapter 3 Introduction to Classes, Objects, Methods and Strings 3.1 Introduction 3.2 Instance Variables, set Methods and get Methods 3.2.1 Account Class with an Instance Variable, a set Method and a get Method 3.2.2 AccountTest Class That Creates and Uses an Object of Class Account 3.2.3 Compiling and Executing an App with Multiple Classes 3.2.4 Account UML Class Diagram with an Instance Variable and set and get Methods 3.2.5 Additional Notes on Class AccountTest 3.2.6 Software Engineering with private Instance Variables and public set and get Methods 3.3 Primitive Types vs. Reference Types 3.4 Account Class: Initializing Objects with Constructors 3.4.1 Declaring an Account Constructor for Custom Object Initialization 3.4.2 Class AccountTest: Initializing Account Objects When They’re Created 3.5 Account Class with a Balance; Floating-Point Numbers 3.5.1 Account Class with a balance Instance Variable of Type double 3.5.2 AccountTest Class to Use Class Account 3.6 (Optional) GUI and Graphics Case Study: Using Dialog Boxes 3.7 Wrap-Up Summary | Self-Review Exercises | Answers to Self-Review Exercises | Exercises | Making a Difference 3.1 Introduction [Note: This chapter depends on the terminology and concepts of object-oriented programming introduced in Section 1.5, Introduction to Object Technology.] In Chapter 2, you worked with existing classes, objects and methods. You used the predefined standard output object System.out, invoking its methods print, println and printf to display information on the screen. You used the existing Scanner class to create an object that reads into memory integer data typed by the user at the keyboard. Throughout the book, you’ll use many more preexisting classes and objects—this is one of the great strengths of Java as an object-oriented programming language. In this chapter, you’ll learn how to create your own classes and methods. Each new class you create becomes a new type that can be used to declare variables and create objects. You can declare new classes as needed; this is one reason why Java is known as an extensible language. We present a case study on creating and using a simple, real-world bank account class—Account. Such a class should maintain as instance variables attributes such as its name and balance, and provide methods for tasks such as querying the balance (getBalance), making deposits that increase the balance (deposit) and making withdrawals that decrease the balance (withdraw). We’ll build the getBalance and deposit methods into the class in the chapter’s examples and you’ll add the withdraw method in the exercises. In Chapter 2 we used the data type int to represent integers. In this chapter, we introduce data type double to represent an account balance as a number that can contain a decimal point—such numbers are called floating-point numbers. [In Chapter 8, when we get a bit deeper into object technology, we’ll begin representing monetary amounts precisely with class BigDecimal (package java.math) as you should do when writing industrial-strength monetary applications.] 3.2 Instance Variables, set Methods and get Methods 71 Typically, the apps you develop in this book will consist of two or more classes. If you become part of a development team in industry, you might work on apps that contain hundreds, or even thousands, of classes. 3.2 Instance Variables, set Methods and get Methods In this section, you’ll create two classes—Account (Fig. 3.1) and AccountTest (Fig. 3.2). Class AccountTest is an application class in which the main method will create and use an Account object to demonstrate class Account’s capabilities. 3.2.1 Account Class with an Instance Variable, a set Method and a get Method Different accounts typically have different names. For this reason, class Account (Fig. 3.1) contains a name instance variable. A class’s instance variables maintain data for each object (that is, each instance) of the class. Later in the chapter we’ll add an instance variable named balance so we can keep track of how much money is in the account. Class Account contains two methods—method setName stores a name in an Account object and method getName obtains a name from an Account object. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 // Fig. 3.1: Account.java // Account class that contains a name instance variable // and methods to set and get its value. public class Account { private String name; // instance variable // method to set the name in the object public void setName(String name) { this.name = name; // store the name } // method to retrieve the name from the object public String getName() { return name; // return value of name to caller } } // end class Account Fig. 3.1 | Account class that contains a name instance variable and methods to set and get its value. Class Declaration The class declaration begins in line 5: public class Account The keyword public (which Chapter 8 explains in detail) is an access modifier. For now, we’ll simply declare every class public. Each public class declaration must be stored in a file having the same name as the class and ending with the .java filename extension; otherwise, 72 Chapter 3 Introduction to Classes, Objects, Methods and Strings a compilation error will occur. Thus, public classes Account and AccountTest (Fig. 3.2) must be declared in the separate files Account.java and AccountTest.java, respectively. Every class declaration contains the keyword class followed immediately by the class’s name—in this case, Account. Every class’s body is enclosed in a pair of left and right braces as in lines 6 and 20 of Fig. 3.1. Identifiers and Camel Case Naming Class names, method names and variable names are all identifiers and by convention all use the same camel case naming scheme we discussed in Chapter 2. Also by convention, class names begin with an initial uppercase letter, and method names and variable names begin with an initial lowercase letter. Instance Variable name Recall from Section 1.5 that an object has attributes, implemented as instance variables and carried with it throughout its lifetime. Instance variables exist before methods are called on an object, while the methods are executing and after the methods complete execution. Each object (instance) of the class has its own copy of the class’s instance variables. A class normally contains one or more methods that manipulate the instance variables belonging to particular objects of the class. Instance variables are declared inside a class declaration but outside the bodies of the class’s methods. Line 7 private String name; // instance variable declares instance variable name of type String outside the bodies of methods setName (lines 10–13) and getName (lines 16–19). String variables can hold character string values such as "Jane Green". If there are many Account objects, each has its own name. Because name is an instance variable, it can be manipulated by each of the class’s methods. Good Programming Practice 3.1 We prefer to list a class’s instance variables first in the class’s body, so that you see the names and types of the variables before they’re used in the class’s methods. You can list the class’s instance variables anywhere in the class outside its method declarations, but scattering the instance variables can lead to hard-to-read code. Access Modifiers public and private Most instance-variable declarations are preceded with the keyword private (as in line 7). Like public, private is an access modifier. Variables or methods declared with access modifier private are accessible only to methods of the class in which they’re declared. So, the variable name can be used only in each Account object’s methods (setName and getName in this case). You’ll soon see that this presents powerful software engineering opportunities. Method of Class Account Let’s walk through the code of setName’s method declaration (lines 10–13): setName public void setName(String name) This line is the method header { this.name = name; // store the name } 3.2 Instance Variables, set Methods and get Methods 73 We refer to the first line of each method declaration (line 10 in this case) as the method header. The method’s return type (which appears before the method name) specifies the type of data the method returns to its caller after performing its task. The return type void (line 10) indicates that setName will perform a task but will not return (i.e., give back) any information to its caller. In Chapter 2, you used methods that return information—for example, you used Scanner method nextInt to input an integer typed by the user at the keyboard. When nextInt reads a value from the user, it returns that value for use in the program. As you’ll soon see, Account method getName returns a value. Method setName receives parameter name of type String—which represents the name that will be passed to the method as an argument. You’ll see how parameters and arguments work together when we discuss the method call in line 21 of Fig. 3.2. Parameters are declared in a parameter list, which is located inside the parentheses that follow the method name in the method header. When there are multiple parameters, each is separated from the next by a comma. Each parameter must specify a type (in this case, String) followed by a variable name (in this case, name). Parameters Are Local Variables In Chapter 2, we declared all of an app’s variables in the main method. Variables declared in a particular method’s body (such as main) are local variables which can be used only in that method. Each method can access only its own local variables, not those of other methods. When a method terminates, the values of its local variables are lost. A method’s parameters also are local variables of the method. Method Body Every method body is delimited by a pair of braces (as in lines 11 and 13 of Fig. 3.1) containing one or more statements that perform the method’s task(s). In this case, the method body contains a single statement (line 12) that assigns the value of the name parameter (a String) to the class’s name instance variable, thus storing the account name in the object. If a method contains a local variable with the same name as an instance variable (as in lines 10 and 7, respectively), that method’s body will refer to the local variable rather than the instance variable. In this case, the local variable is said to shadow the instance variable in the method’s body. The method’s body can use the keyword this to refer to the shadowed instance variable explicitly, as shown on the left side of the assignment in line 12. setName Good Programming Practice 3.2 We could have avoided the need for keyword this here by choosing a different name for the parameter in line 10, but using the this keyword as shown in line 12 is a widely accepted practice to minimize the proliferation of identifier names. After line 12 executes, the method has completed its task, so it returns to its caller. As you’ll soon see, the statement in line 21 of main (Fig. 3.2) calls method setName. Method of Class Account Method getName (lines 16–19) getName public String getName() Keyword return passes the String name back to the method’s caller { return name; // return value of name to caller } 74 Chapter 3 Introduction to Classes, Objects, Methods and Strings returns a particular Account object’s name to the caller. The method has an empty parameter list, so it does not require additional information to perform its task. The method returns a String. When a method that specifies a return type other than void is called and completes its task, it must return a result to its caller. A statement that calls method getName on an Account object (such as the ones in lines 16 and 26 of Fig. 3.2) expects to receive the Account’s name—a String, as specified in the method declaration’s return type. The return statement in line 18 of Fig. 3.1 passes the String value of instance variable name back to the caller. For example, when the value is returned to the statement in lines 25–26 of Fig. 3.2, the statement uses that value to output the name. 3.2.2 AccountTest Class That Creates and Uses an Object of Class Account Next, we’d like to use class Account in an app and call each of its methods. A class that contains a main method begins the execution of a Java app. Class Account cannot execute by itself because it does not contain a main method—if you type java Account in the command window, you’ll get an error indicating “Main method not found in class Account.” To fix this problem, you must either declare a separate class that contains a main method or place a main method in class Account. Driver Class AccountTest To help you prepare for the larger programs you’ll encounter later in this book and in industry, we use a separate class AccountTest (Fig. 3.2) containing method main to test class Account. Once main begins executing, it may call other methods in this and other classes; those may, in turn, call other methods, and so on. Class AccountTest’s main method creates one Account object and calls its getName and setName methods. Such a class is sometimes called a driver class—just as a Person object drives a Car object by telling it what to do (go faster, go slower, turn left, turn right, etc.), class AccountTest drives an Account object, telling it what to do by calling its methods. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 // Fig. 3.2: AccountTest.java // Creating and manipulating an Account object. import java.util.Scanner; public class AccountTest { public static void main(String[] args) { // create a Scanner object to obtain input from the command window Scanner input = new Scanner(System.in); // create an Account object and assign it to myAccount Account myAccount = new Account(); // display initial value of name (null) System.out.printf("Initial name is: %s%n%n", myAccount.getName()); Fig. 3.2 | Creating and manipulating an Account object. (Part 1 of 2.) 3.2 Instance Variables, set Methods and get Methods 18 19 20 21 22 23 24 25 26 27 28 75 // prompt for and read name System.out.println("Please enter the name:"); String theName = input.nextLine(); // read a line of text myAccount.setName(theName); // put theName in myAccount System.out.println(); // outputs a blank line // display the name stored in object myAccount System.out.printf("Name in object myAccount is:%n%s%n", myAccount.getName()); } } // end class AccountTest Initial name is: null Please enter the name: Jane Green Name in object myAccount is: Jane Green Fig. 3.2 | Creating and manipulating an Account object. (Part 2 of 2.) Object for Receiving Input from the User Line 10 creates a Scanner object named input for inputting the name from the user. Line 19 prompts the user to enter a name. Line 20 uses the Scanner object’s nextLine method to read the name from the user and assign it to the local variable theName. You type the name and press Enter to submit it to the program. Pressing Enter inserts a newline character after the characters you typed. Method nextLine reads characters (including white-space characters, such as the blank in "Jane Green") until it encounters the newline, then returns a String containing the characters up to, but not including, the newline, which is discarded. Class Scanner provides various other input methods, as you’ll see throughout the book. A method similar to nextLine—named next—reads the next word. When you press Enter after typing some text, method next reads characters until it encounters a white-space character (such as a space, tab or newline), then returns a String containing the characters up to, but not including, the white-space character, which is discarded. All information after the first white-space character is not lost—it can be read by subsequent statements that call the Scanner’s methods later in the program. Scanner Instantiating an Object—Keyword new and Constructors Line 13 creates an Account object and assigns it to variable myAccount of type Account. Variable myAccount is initialized with the result of the class instance creation expression new Account(). Keyword new creates a new object of the specified class—in this case, Account. The parentheses to the right of Account are required. As you’ll learn in Section 3.4, those parentheses in combination with a class name represent a call to a constructor, which is similar to a method but is called implicitly by the new operator to initialize an object’s instance variables when the object is created. In Section 3.4, you’ll see how to place an argument in the parentheses to specify an initial value for an Account object’s name instance variable—you’ll enhance class Account to enable this. For now, we simply leave the parentheses empty. Line 10 contains a class instance creation expression for a Scanner object— 76 Chapter 3 Introduction to Classes, Objects, Methods and Strings the expression initializes the Scanner with System.in, which tells the Scanner where to read the input from (i.e., the keyboard). Calling Class Account’s getName Method Line 16 displays the initial name, which is obtained by calling the object’s getName method. Just as we can use object System.out to call its methods print, printf and println, we can use object myAccount to call its methods getName and setName. Line 16 calls getName using the myAccount object created in line 13, followed by a dot separator (.), then the method name getName and an empty set of parentheses because no arguments are being passed. When getName is called: 1. The app transfers program execution from the call (line 16 in main) to method getName’s declaration (lines 16–19 of Fig. 3.1). Because getName was called via the myAccount object, getName “knows” which object’s instance variable to manipulate. 2. Next, method getName performs its task—that is, it returns the name (line 18 of Fig. 3.1). When the return statement executes, program execution continues where getName was called (line 16 in Fig. 3.2). 3. System.out.printf displays the String returned by method getName, then the program continues executing at line 19 in main. Error-Prevention Tip 3.1 Never use as a format-control a string that was input from the user. When method System.out.printf evaluates the format-control string in its first argument, the method performs tasks based on the conversion specifier(s) in that string. If the format-control string were obtained from the user, a malicious user could supply conversion specifiers that would be executed by System.out.printf, possibly causing a security breach. null—the Default Initial Value for String Variables The first line of the output shows the name “null.” Unlike local variables, which are not automatically initialized, every instance variable has a default initial value—a value provided by Java when you do not specify the instance variable’s initial value. Thus, instance variables are not required to be explicitly initialized before they’re used in a program—unless they must be initialized to values other than their default values. The default value for an instance variable of type String (like name in this example) is null, which we discuss further in Section 3.3 when we consider reference types. Calling Class Account’s setName Method Line 21 calls myAccounts’s setName method. A method call can supply arguments whose values are assigned to the corresponding method parameters. In this case, the value of main’s local variable theName in parentheses is the argument that’s passed to setName so that the method can perform its task. When setName is called: 1. The app transfers program execution from line 21 in main to setName method’s declaration (lines 10–13 of Fig. 3.1), and the argument value in the call’s parentheses (theName) is assigned to the corresponding parameter (name) in the method header (line 10 of Fig. 3.1). Because setName was called via the myAccount object, setName “knows” which object’s instance variable to manipulate. 3.2 Instance Variables, set Methods and get Methods 77 2. Next, method setName performs its task—that is, it assigns the name parameter’s value to instance variable name (line 12 of Fig. 3.1). 3. When program execution reaches setName’s closing right brace, it returns to where setName was called (line 21 of Fig. 3.2), then continues at line 22 of Fig. 3.2. The number of arguments in a method call must match the number of parameters in the method declaration’s parameter list. Also, the argument types in the method call must be consistent with the types of the corresponding parameters in the method’s declaration. (As you’ll learn in Chapter 6, an argument’s type and its corresponding parameter’s type are not required to be identical.) In our example, the method call passes one argument of type String (theName)—and the method declaration specifies one parameter of type String (name, declared in line 10 of Fig. 3.1). So in this example, the type of the argument in the method call exactly matches the type of the parameter in the method header. Displaying the Name That Was Entered by the User Line 22 of Fig. 3.2 outputs a blank line. When the second call to method getName (line 26) executes, the name entered by the user in line 20 is displayed. When the statement at lines 25–26 completes execution, the end of method main is reached, so the program terminates. 3.2.3 Compiling and Executing an App with Multiple Classes You must compile the classes in Figs. 3.1 and 3.2 before you can execute the app. This is the first time you’ve created an app with multiple classes. Class AccountTest has a main method; class Account does not. To compile this app, first change to the directory that contains the app’s source-code files. Next, type the command javac Account.java AccountTest.java to compile both classes at once. If the directory containing the app includes only this app’s files, you can compile both classes with the command javac *.java The asterisk (*) in *.java indicates that all files in the current directory ending with the filename extension “.java” should be compiled. If both classes compile correctly—that is, no compilation errors are displayed—you can then run the app with the command java AccountTest 3.2.4 Account UML Class Diagram with an Instance Variable and set and get Methods We’ll often use UML class diagrams to summarize a class’s attributes and operations. In industry, UML diagrams help systems designers specify a system in a concise, graphical, programming-language-independent manner, before programmers implement the system in a specific programming language. Figure 3.3 presents a UML class diagram for class Account of Fig. 3.1. Top Compartment In the UML, each class is modeled in a class diagram as a rectangle with three compartments. In this diagram the top compartment contains the class name Account centered horizontally in boldface type. 78 Chapter 3 Introduction to Classes, Objects, Methods and Strings Account Top compartment – name : String Middle compartment + setName(name : String) + getName() : String Bottom compartment Fig. 3.3 | UML class diagram for class Account of Fig. 3.1. Middle Compartment The middle compartment contains the class’s attribute name, which corresponds to the instance variable of the same name in Java. Instance variable name is private in Java, so the UML class diagram lists a minus sign (–) access modifier before the attribute name. Following the attribute name are a colon and the attribute type, in this case String. Bottom Compartment The bottom compartment contains the class’s operations, setName and getName, which correspond to the methods of the same names in Java. The UML models operations by listing the operation name preceded by an access modifier, in this case + getName. This plus sign (+) indicates that getName is a public operation in the UML (because it’s a public method in Java). Operation getName does not have any parameters, so the parentheses following the operation name in the class diagram are empty, just as they are in the method’s declaration in line 16 of Fig. 3.1. Operation setName, also a public operation, has a String parameter called name. Return Types The UML indicates the return type of an operation by placing a colon and the return type after the parentheses following the operation name. Account method getName (Fig. 3.1) has a String return type. Method setName does not return a value (because it returns void in Java), so the UML class diagram does not specify a return type after the parentheses of this operation. Parameters The UML models a parameter a bit differently from Java by listing the parameter name, followed by a colon and the parameter type in the parentheses after the operation name. The UML has its own data types similar to those of Java, but for simplicity, we’ll use the Java data types. Account method setName (Fig. 3.1) has a String parameter named name, so Fig. 3.3 lists name : String between the parentheses following the method name. 3.2.5 Additional Notes on Class AccountTest Method main In Chapter 2, each class we declared had one method named main. Recall that main is a special method that’s always called automatically by the Java Virtual Machine (JVM) when you execute an app. You must call most other methods explicitly to tell them to perform their tasks. Lines 7–27 of Fig. 3.2 declare method main. A key part of enabling the JVM to locate and call method main to begin the app’s execution is the static keyword (line 7), which static 3.2 Instance Variables, set Methods and get Methods 79 indicates that main is a static method. A static method is special, because you can call it without first creating an object of the class in which the method is declared—in this case class AccountTest. We discuss static methods in detail in Chapter 6. Notes on import Declarations Notice the import declaration in Fig. 3.2 (line 3), which indicates to the compiler that the program uses class Scanner. As you learned in Chapter 2, classes System and String are in package java.lang, which is implicitly imported into every Java program, so all programs can use that package’s classes without explicitly importing them. Most other classes you’ll use in Java programs must be imported explicitly. There’s a special relationship between classes that are compiled in the same directory, like classes Account and AccountTest. By default, such classes are considered to be in the same package—known as the default package. Classes in the same package are implicitly imported into the source-code files of other classes in that package. Thus, an import declaration is not required when one class in a package uses another in the same package— such as when class AccountTest uses class Account. The import declaration in line 3 is not required if we refer to class Scanner throughout this file as java.util.Scanner, which includes the full package name and class name. This is known as the class’s fully qualified class name. For example, line 10 of Fig. 3.2 also could be written as java.util.Scanner input = new java.util.Scanner(System.in); Software Engineering Observation 3.1 The Java compiler does not require import declarations in a Java source-code file if the fully qualified class name is specified every time a class name is used. Most Java programmers prefer the more concise programming style enabled by import declarations. 3.2.6 Software Engineering with private Instance Variables and public set and get Methods As you’ll see, through the use of set and get methods, you can validate attempted modifications to private data and control how that data is presented to the caller—these are compelling software engineering benefits. We’ll discuss this in more detail in Section 3.5. If the instance variable were public, any client of the class—that is, any other class that calls the class’s methods—could see the data and do whatever it wanted with it, including setting it to an invalid value. You might think that even though a client of the class cannot directly access a private instance variable, the client can do whatever it wants with the variable through public set and get methods. You would think that you could peek at the private data any time with the public get method and that you could modify the private data at will through the public set method. But set methods can be programmed to validate their arguments and reject any attempts to set the data to bad values, such as a negative body temperature, a day in March out of the range 1 through 31, a product code not in the company’s product catalog, etc. And a get method can present the data in a different form. For example, a Grade class might store a grade as an int between 0 and 100, but a getGrade method might return a letter grade as a String, such as "A" for grades between 90 and 100, "B" for grades between 80 and 89, etc. Tightly controlling the access to and presentation of 80 Chapter 3 Introduction to Classes, Objects, Methods and Strings private data can greatly reduce errors, while increasing the robustness and security of your programs. Declaring instance variables with access modifier private is known as data hiding or information hiding. When a program creates (instantiates) an object of class Account, variable name is encapsulated (hidden) in the object and can be accessed only by methods of the object’s class. Software Engineering Observation 3.2 Precede each instance variable and method declaration with an access modifier. Generally, instance variables should be declared private and methods public. Later in the book, we’ll discuss why you might want to declare a method private. get N Conceptual View of an Account Object with Encapsulated Data You can think of an Account object as shown in Fig. 3.4. The private instance variable name is hidden inside the object (represented by the inner circle containing name) and protected by an outer layer of public methods (represented by the outer circle containing getName and setName). Any client code that needs to interact with the Account object can do so only by calling the public methods of the protective outer layer. e am Na me name se t Fig. 3.4 | Conceptual view of an Account object with its encapsulated private instance variable name and protective layer of public methods. 3.3 Primitive Types vs. Reference Types Java’s types are divided into primitive types and reference types. In Chapter 2, you worked with variables of type int—one of the primitive types. The other primitive types are boolean, byte, char, short, long, float and double, each of which we discuss in this book—these are summarized in Appendix D. All nonprimitive types are reference types, so classes, which specify the types of objects, are reference types. A primitive-type variable can hold exactly one value of its declared type at a time. For example, an int variable can store one integer at a time. When another value is assigned to that variable, the new value replaces the previous one—which is lost. Recall that local variables are not initialized by default. Primitive-type instance variables are initialized by default—instance variables of types byte, char, short, int, long, float and double are initialized to 0, and variables of type boolean are initialized to 3.4 Account Class: Initializing Objects with Constructors 81 false. You can specify your own initial value for a primitive-type variable by assigning the variable a value in its declaration, as in private int numberOfStudents = 10; Programs use variables of reference types (normally called references) to store the addresses of objects in the computer’s memory. Such a variable is said to refer to an object in the program. Objects that are referenced may each contain many instance variables. Line 10 of Fig. 3.2: Scanner input = new Scanner(System.in); creates an object of class Scanner, then assigns to the variable Scanner object. Line 13 of Fig. 3.2: input a reference to that Account myAccount = new Account(); creates an object of class Account, then assigns to the variable myAccount a reference to that object. Reference-type instance variables, if not explicitly initialized, are initialized by default to the value null—which represents a “reference to nothing.” That’s why the first call to getName in line 16 of Fig. 3.2 returns null—the value of name has not yet been set, so the default initial value null is returned. To call methods on an object, you need a reference to the object. In Fig. 3.2, the statements in method main use the variable myAccount to call methods getName (lines 16 and 26) and setName (line 21) to interact with the Account object. Primitive-type variables do not refer to objects, so such variables cannot be used to call methods. Account 3.4 Account Class: Initializing Objects with Constructors As mentioned in Section 3.2, when an object of class Account (Fig. 3.1) is created, its String instance variable name is initialized to null by default. But what if you want to provide a name when you create an Account object? Each class you declare can optionally provide a constructor with parameters that can be used to initialize an object of a class when the object is created. Java requires a constructor call for every object that’s created, so this is the ideal point to initialize an object’s instance variables. The next example enhances class Account (Fig. 3.5) with a constructor that can receive a name and use it to initialize instance variable name when an Account object is created (Fig. 3.6). 3.4.1 Declaring an Account Constructor for Custom Object Initialization When you declare a class, you can provide your own constructor to specify custom initialization for objects of your class. For example, you might want to specify a name for an Account object when the object is created, as in line 10 of Fig. 3.6: Account account1 = new Account("Jane Green"); In this case, the String argument "Jane Green" is passed to the Account object’s constructor and used to initialize the name instance variable. The preceding statement requires that the class provide a constructor that takes only a String parameter. Figure 3.5 contains a modified Account class with such a constructor. 82 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Chapter 3 Introduction to Classes, Objects, Methods and Strings // Fig. 3.5: Account.java // Account class with a constructor that initializes the name. public class Account { private String name; // instance variable // constructor initializes name with parameter name public Account(String name) // constructor name is class name { this.name = name; } // method to set the name public void setName(String name) { this.name = name; } // method to retrieve the name public String getName() { return name; } } // end class Account Fig. 3.5 | Account class with a constructor that initializes the name. Constructor Declaration Lines 9–12 of Fig. 3.5 declare Account’s constructor. A constructor must have the same name as the class. A constructor’s parameter list specifies that the constructor requires one or more pieces of data to perform its task. Line 9 indicates that the constructor has a String parameter called name. When you create a new Account object (as you’ll see in Fig. 3.6), you’ll pass a person’s name to the constructor, which will receive that name in the parameter name. The constructor will then assign name to instance variable name in line 11. Account Error-Prevention Tip 3.2 Even though it’s possible to do so, do not call methods from constructors. We’ll explain this in Chapter 10, Object-Oriented Programming: Polymorphism and Interfaces. Parameter name of Class Account’s Constructor and Method setName Recall from Section 3.2.1 that method parameters are local variables. In Fig. 3.5, the constructor and method setName both have a parameter called name. Although these parameters have the same identifier (name), the parameter in line 9 is a local variable of the constructor that’s not visible to method setName, and the one in line 15 is a local variable of setName that’s not visible to the constructor. 3.4.2 Class AccountTest: Initializing Account Objects When They’re Created The AccountTest program (Fig. 3.6) initializes two Account objects using the constructor. Line 10 creates and initializes the Account object account1. Keyword new requests 3.4 Account Class: Initializing Objects with Constructors 83 memory from the system to store the Account object, then implicitly calls the class’s constructor to initialize the object. The call is indicated by the parentheses after the class name, which contain the argument "Jane Green" that’s used to initialize the new object’s name. The class instance creation expression in line 10 returns a reference to the new object, which is assigned to the variable account1. Line 11 repeats this process, passing the argument "John Blue" to initialize the name for account2. Lines 14–15 use each object’s getName method to obtain the names and show that they were indeed initialized when the objects were created. The output shows different names, confirming that each Account maintains its own copy of instance variable name. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 // Fig. 3.6: AccountTest.java // Using the Account constructor to initialize the name instance // variable at the time each Account object is created. public class AccountTest { public static void main(String[] args) { // create two Account objects Account account1 = new Account("Jane Green"); Account account2 = new Account("John Blue"); // display initial value of name for each Account System.out.printf("account1 name is: %s%n", account1.getName()); System.out.printf("account2 name is: %s%n", account2.getName()); } } // end class AccountTest account1 name is: Jane Green account2 name is: John Blue Fig. 3.6 | Using the Account constructor to initialize the name instance variable at the time each Account object is created. Constructors Cannot Return Values An important difference between constructors and methods is that constructors cannot return values, so they cannot specify a return type (not even void). Normally, constructors are declared public—later in the book we’ll explain when to use private constructors. Default Constructor Recall that line 13 of Fig. 3.2 Account myAccount = new Account(); used new to create an Account object. The empty parentheses after “new Account” indicate a call to the class’s default constructor—in any class that does not explicitly declare a constructor, the compiler provides a default constructor (which always has no parameters). When a class has only the default constructor, the class’s instance variables are initialized to their default values. In Section 8.5, you’ll learn that classes can have multiple constructors. 84 Chapter 3 Introduction to Classes, Objects, Methods and Strings There’s No Default Constructor in a Class That Declares a Constructor If you declare a constructor for a class, the compiler will not create a default constructor for that class. In that case, you will not be able to create an Account object with the class instance creation expression new Account() as we did in Fig. 3.2—unless the custom constructor you declare takes no parameters. Software Engineering Observation 3.3 Unless default initialization of your class’s instance variables is acceptable, provide a custom constructor to ensure that your instance variables are properly initialized with meaningful values when each new object of your class is created. Adding the Constructor to Class Account’s UML Class Diagram The UML class diagram of Fig. 3.7 models class Account of Fig. 3.5, which has a constructor with a String name parameter. Like operations, the UML models constructors in the third compartment of a class diagram. To distinguish a constructor from the class’s operations, the UML requires that the word “constructor” be enclosed in guillemets (« and ») and placed before the constructor’s name. It’s customary to list constructors before other operations in the third compartment. Account – name : String «constructor» Account(name: String) + setName(name: String) + getName() : String Fig. 3.7 | UML class diagram for Account class of Fig. 3.5. 3.5 Account Class with a Balance; Floating-Point Numbers We now declare an Account class that maintains the balance of a bank account in addition to the name. Most account balances are not integers. So, class Account represents the account balance as a floating-point number—a number with a decimal point, such as 43.95, 0.0, –129.8873. [In Chapter 8, we’ll begin representing monetary amounts precisely with class BigDecimal as you should do when writing industrial-strength monetary applications.] Java provides two primitive types for storing floating-point numbers in memory— float and double. Variables of type float represent single-precision floating-point numbers and can hold up to seven significant digits. Variables of type double represent doubleprecision floating-point numbers. These require twice as much memory as float variables and can hold up to 15 significant digits—about double the precision of float variables. Most programmers represent floating-point numbers with type double. In fact, Java treats all floating-point numbers you type in a program’s source code (such as 7.33 and 0.0975) as double values by default. Such values in the source code are known as floatingpoint literals. See Appendix D, Primitive Types, for the precise ranges of values for floats and doubles. 3.5 Account Class with a Balance; Floating-Point Numbers 85 3.5.1 Account Class with a balance Instance Variable of Type double Our next app contains a version of class Account (Fig. 3.8) that maintains as instance variables the name and the balance of a bank account. A typical bank services many accounts, each with its own balance, so line 8 declares an instance variable balance of type double. Every instance (i.e., object) of class Account contains its own copies of both the name and the balance. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 // Fig. 3.8: Account.java // Account class with a double instance variable balance and a constructor // and deposit method that perform validation. public class Account { private String name; // instance variable private double balance; // instance variable // Account constructor that receives two parameters public Account(String name, double balance) { this.name = name; // assign name to instance variable name // validate that the balance is greater than 0.0; if it's not, // instance variable balance keeps its default initial value of 0.0 if (balance > 0.0) // if the balance is valid this.balance = balance; // assign it to instance variable balance } // method that deposits (adds) only a valid amount to the balance public void deposit(double depositAmount) { if (depositAmount > 0.0) // if the depositAmount is valid balance = balance + depositAmount; // add it to the balance } // method returns the account balance public double getBalance() { return balance; } // method that sets the name public void setName(String name) { this.name = name; } // method that returns the name public String getName() { Fig. 3.8 | deposit Account class with a double instance variable balance and a constructor and method that perform validation. (Part 1 of 2.) 86 43 44 45 Chapter 3 Introduction to Classes, Objects, Methods and Strings return name; // give value of name back to caller } // end method getName } // end class Account Fig. 3.8 | deposit Account class with a double instance variable balance and a constructor and method that perform validation. (Part 2 of 2.) Class Two-Parameter Constructor The class has a constructor and four methods. It’s common for someone opening an account to deposit money immediately, so the constructor (lines 11–19) now receives a second parameter—initialBalance of type double that represents the starting balance. Lines 17–18 ensure that initialBalance is greater than 0.0. If so, initialBalance’s value is assigned to instance variable balance. Otherwise, balance remains at 0.0—its default initial value. Account Class deposit Method Method deposit (lines 22–26) does not return any data when it completes its task, so its return type is void. The method receives one parameter named depositAmount—a double value that’s added to the balance only if the parameter value is valid (i.e., greater than zero). Line 25 first adds the current balance and depositAmount, forming a temporary sum which is then assigned to balance, replacing its prior value (recall that addition has a higher precedence than assignment). It’s important to understand that the calculation on the right side of the assignment operator in line 25 does not modify the balance—that’s why the assignment is necessary. Account Class getBalance Method Method getBalance (lines 29–32) allows clients of the class (i.e., other classes whose methods call the methods of this class) to obtain the value of a particular Account object’s balance. The method specifies return type double and an empty parameter list. Account Account’s Methods Can All Use balance Once again, the statements in lines 18, 25 and 31 use the variable balance even though it was not declared in any of the methods. We can use balance in these methods because it’s an instance variable of the class. 3.5.2 AccountTest Class to Use Class Account Class AccountTest (Fig. 3.9) creates two Account objects (lines 9–10) and initializes them with a valid balance of 50.00 and an invalid balance of -7.53, respectively—for the purpose of our examples, we assume that balances must be greater than or equal to zero. The calls to method System.out.printf in lines 13–16 output the account names and balances, which are obtained by calling each Account’s getName and getBalance methods. 1 2 3 // Fig. 3.9: AccountTest.java // Inputting and outputting floating-point numbers with Account objects. import java.util.Scanner; Fig. 3.9 | Inputting and outputting floating-point numbers with Account objects. (Part 1 of 3.) 3.5 Account Class with a Balance; Floating-Point Numbers 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 87 public class AccountTest { public static void main(String[] args) { Account account1 = new Account("Jane Green", 50.00); Account account2 = new Account("John Blue", -7.53); // display initial balance of each object System.out.printf("%s balance: $%.2f%n", account1.getName(), account1.getBalance()); System.out.printf("%s balance: $%.2f%n%n", account2.getName(), account2.getBalance()); // create a Scanner to obtain input from the command window Scanner input = new Scanner(System.in); System.out.print("Enter deposit amount for account1: "); // prompt double depositAmount = input.nextDouble(); // obtain user input System.out.printf("%nadding %.2f to account1 balance%n%n", depositAmount); account1.deposit(depositAmount); // add to account1’s balance // display balances System.out.printf("%s balance: $%.2f%n", account1.getName(), account1.getBalance()); System.out.printf("%s balance: $%.2f%n%n", account2.getName(), account2.getBalance()); System.out.print("Enter deposit amount for account2: "); // prompt depositAmount = input.nextDouble(); // obtain user input System.out.printf("%nadding %.2f to account2 balance%n%n", depositAmount); account2.deposit(depositAmount); // add to account2 balance // display balances System.out.printf("%s balance: $%.2f%n", account1.getName(), account1.getBalance()); System.out.printf("%s balance: $%.2f%n%n", account2.getName(), account2.getBalance()); } // end main } // end class AccountTest Jane Green balance: $50.00 John Blue balance: $0.00 Enter deposit amount for account1: 25.53 adding 25.53 to account1 balance Jane Green balance: $75.53 John Blue balance: $0.00 Fig. 3.9 | Inputting and outputting floating-point numbers with Account objects. (Part 2 of 3.) 88 Chapter 3 Introduction to Classes, Objects, Methods and Strings Enter deposit amount for account2: 123.45 adding 123.45 to account2 balance Jane Green balance: $75.53 John Blue balance: $123.45 Fig. 3.9 | Inputting and outputting floating-point numbers with Account objects. (Part 3 of 3.) Displaying the Account Objects’ Initial Balances When method getBalance is called for account1 from line 14, the value of account1’s balance is returned from line 31 of Fig. 3.8 and displayed by the System.out.printf statement (Fig. 3.9, lines 13–14). Similarly, when method getBalance is called for account2 from line 16, the value of the account2’s balance is returned from line 31 of Fig. 3.8 and displayed by the System.out.printf statement (Fig. 3.9, lines 15–16). The balance of account2 is initially 0.00, because the constructor rejected the attempt to start account2 with a negative balance, so the balance retains its default initial value. Formatting Floating-Point Numbers for Display Each of the balances is output by printf with the format specifier %.2f. The %f format specifier is used to output values of type float or double. The .2 between % and f represents the number of decimal places (2) that should be output to the right of the decimal point in the floating-point number—also known as the number’s precision. Any floatingpoint value output with %.2f will be rounded to the hundredths position—for example, 123.457 would be rounded to 123.46 and 27.33379 would be rounded to 27.33. Reading a Floating-Point Value from the User and Making a Deposit Line 21 (Fig. 3.9) prompts the user to enter a deposit amount for account1. Line 22 declares local variable depositAmount to store each deposit amount entered by the user. Unlike instance variables (such as name and balance in class Account), local variables (like depositAmount in main) are not initialized by default, so they normally must be initialized explicitly. As you’ll learn momentarily, variable depositAmount’s initial value will be determined by the user’s input. Common Programming Error 3.1 The Java compiler will issue a compilation error if you attempt to use the value of an uninitialized local variable. This helps you avoid dangerous execution-time logic errors. It’s always better to get the errors out of your programs at compilation time rather than execution time. Line 22 obtains the input from the user by calling Scanner object input’s nextDouble method, which returns a double value entered by the user. Lines 23–24 display the depositAmount. Line 25 calls object account1’s deposit method with the depositAmount as the method’s argument. When the method is called, the argument’s value is assigned to the parameter depositAmount of method deposit (line 22 of Fig. 3.8); then method deposit adds that value to the balance. Lines 28–31 (Fig. 3.9) output the names and balances of both Accounts again to show that only account1’s balance has changed. 3.5 Account Class with a Balance; Floating-Point Numbers 89 Line 33 prompts the user to enter a deposit amount for account2. Line 34 obtains the input from the user by calling Scanner object input’s nextDouble method. Lines 35–36 display the depositAmount. Line 37 calls object account2’s deposit method with depositAmount as the method’s argument; then method deposit adds that value to the balance. Finally, lines 40–43 output the names and balances of both Accounts again to show that only account2’s balance has changed. Duplicated Code in Method main The six statements at lines 13–14, 15–16, 28–29, 30–31, 40–41 and 42–43 are almost identical—they each output an Account’s name and balance. They differ only in the name of the Account object—account1 or account2. Duplicate code like this can create code maintenance problems when that code needs to be updated—if six copies of the same code all have the same error or update to be made, you must make that change six times, without making errors. Exercise 3.15 asks you to modify Fig. 3.9 to include a displayAccount method that takes as a parameter an Account object and outputs the object’s name and balance. You’ll then replace main’s duplicated statements with six calls to displayAccount, thus reducing the size of your program and improving its maintainability by having one copy of the code that displays an Account’s name and balance. Software Engineering Observation 3.4 Replacing duplicated code with calls to a method that contains one copy of that code can reduce the size of your program and improve its maintainability. UML Class Diagram for Class Account The UML class diagram in Fig. 3.10 concisely models class Account of Fig. 3.8. The diagram models in its second compartment the private attributes name of type String and balance of type double. Account – name : String – balance : double «constructor» Account(name : String, balance: double) + deposit(depositAmount : double) + getBalance() : double + setName(name : String) + getName() : String Fig. 3.10 | UML class diagram for Account class of Fig. 3.8. Class Account’s constructor is modeled in the third compartment with parameters name of type String and initialBalance of type double. The class’s four public methods also are modeled in the third compartment—operation deposit with a depositAmount parameter of type double, operation getBalance with a return type of double, operation setName with a name parameter of type String and operation getName with a return type of String. 90 Chapter 3 Introduction to Classes, Objects, Methods and Strings 3.6 (Optional) GUI and Graphics Case Study: Using Dialog Boxes This optional case study is designed for those who want to begin learning Java’s powerful capabilities for creating graphical user interfaces (GUIs) and graphics early in the book, before the deeper discussions of these topics later in the book. This case study features Java’s mature Swing technology, which as of this writing is still a bit more popular than the newer JavaFX technology presented in later chapters. This GUI and Graphics Case Study appears in 10 brief sections (see Fig. 3.11). Each section introduces new concepts and provides examples with screen captures that show sample interactions and results. In the first few sections, you’ll create your first graphical apps. In subsequent sections, you’ll use object-oriented programming concepts to create an app that draws a variety of shapes. When we formally introduce GUIs in Chapter 12, we use the mouse to choose exactly which shapes to draw and where to draw them. In Chapter 13, we add capabilities of the Java 2D graphics API to draw the shapes with different line thicknesses and fills. We hope you find this case study informative and entertaining. Location Title—Exercise(s) Section 3.6 Section 4.15 Using Dialog Boxes—Basic input and output with dialog boxes Creating Simple Drawings—Displaying and drawing lines on the screen Drawing Rectangles and Ovals—Using shapes to represent data Colors and Filled Shapes—Drawing a bull’s-eye and random graphics Drawing Arcs—Drawing spirals with arcs Using Objects with Graphics—Storing shapes as objects Displaying Text and Images Using Labels—Providing status information Drawing with Polymorphism—Identifying the similarities between shapes Expanding the Interface—Using GUI components and event handling Adding Java 2D—Using the Java 2D API to enhance drawings Section 5.11 Section 6.13 Section 7.17 Section 8.16 Section 9.7 Section 10.10 Exercise 12.17 Exercise 13.31 Fig. 3.11 | Summary of the GUI and Graphics Case Study in each chapter. Displaying Text in a Dialog Box The programs presented thus far display output in the command window. Many apps use windows or dialog boxes (also called dialogs) to display output. Web browsers such as Chrome, Firefox, Internet Explorer, Safari and Opera display web pages in their own windows. E-mail programs allow you to type and read messages in a window. Typically, dialog boxes are windows in which programs display important messages to users. Class JOptionPane provides prebuilt dialog boxes that enable programs to display windows containing messages—such windows are called message dialogs. Figure 3.12 displays the String "Welcome to Java" in a message dialog. 3.6 (Optional) GUI and Graphics Case Study: Using Dialog Boxes 1 2 3 4 5 6 7 8 9 10 11 12 91 // Fig. 3.12: Dialog1.java // Using JOptionPane to display multiple lines in a dialog box. import javax.swing.JOptionPane; public class Dialog1 { public static void main(String[] args) { // display a dialog with a message JOptionPane.showMessageDialog(null, "Welcome to Java"); } } // end class Dialog1 Fig. 3.12 | Using JOptionPane to display multiple lines in a dialog box. Class static Method showMessagDialog Line 3 indicates that the program uses class JOptionPane from package javax.swing. This package contains many classes that help you create graphical user interfaces (GUIs). GUI components facilitate data entry by a program’s user and presentation of outputs to the user. Line 10 calls JOptionPane method showMessageDialog to display a dialog box containing a message. The method requires two arguments. The first helps the Java app determine where to position the dialog box. A dialog is typically displayed from a GUI app with its own window. The first argument refers to that window (known as the parent window) and causes the dialog to appear centered over the app’s window. If the first argument is null, the dialog box is displayed at the center of your screen. The second argument is the String to display in the dialog box. JOptionPane Introducing static Methods JOptionPane method showMessageDialog is a so-called static method. Such methods often define frequently used tasks. For example, many programs display dialog boxes, and the code to do this is the same each time. Rather than requiring you to “reinvent the wheel” and create code to display a dialog, the designers of class JOptionPane declared a static method that performs this task for you. A static method is called by using its class name followed by a dot (.) and the method name, as in ClassName.methodName(arguments) Notice that you do not create an object of class JOptionPane to use its static method discuss static methods in more detail in Chapter 6. showMessageDialog.We Entering Text in a Dialog Figure 3.13 uses another predefined JOptionPane dialog called an input dialog that allows the user to enter data into a program. The program asks for the user’s name and responds with a message dialog containing a greeting and the name that the user entered. 92 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Chapter 3 Introduction to Classes, Objects, Methods and Strings // Fig. 3.13: NameDialog.java // Obtaining user input from a dialog. import javax.swing.JOptionPane; public class NameDialog { public static void main(String[] args) { // prompt user to enter name String name = JOptionPane.showInputDialog("What is your name?"); // create the message String message = String.format("Welcome, %s, to Java Programming!", name); // display the message to welcome the user by name JOptionPane.showMessageDialog(null, message); } // end main } // end class NameDialog Fig. 3.13 | Obtaining user input from a dialog. Class static Method showInputDialog Line 10 uses JOptionPane method showInputDialog to display an input dialog containing a prompt and a field (known as a text field) in which the user can enter text. Method showInputDialog’s argument is the prompt that indicates what the user should enter. The user types characters in the text field, then clicks the OK button or presses the Enter key to return the String to the program. Method showInputDialog returns a String containing the characters typed by the user. We store the String in variable name. If you press the dialog’s Cancel button or press the Esc key, the method returns null and the program displays the word “null” as the name. JOptionPane Class static Method format Lines 13–14 use static String method format to return a String containing a greeting with the user’s name. Method format works like method System.out.printf, except that format returns the formatted String rather than displaying it in a command window. Line 17 displays the greeting in a message dialog, just as we did in Fig. 3.12. String GUI and Graphics Case Study Exercise 3.1 Modify the addition program in Fig. 2.7 to use dialog-based input and output with the methods of class JOptionPane. Since method showInputDialog returns a String, you must convert the String the user enters to an int for use in calculations. The static method parseInt of class Integer (package java.lang) takes a String argument representing an integer and returns the value as an int. If the String does not contain a valid integer, the program will terminate with an error. 3.7 Wrap-Up 93 3.7 Wrap-Up In this chapter, you learned how to create your own classes and methods, create objects of those classes and call methods of those objects to perform useful actions. You declared instance variables of a class to maintain data for each object of the class, and you declared your own methods to operate on that data. You learned how to call a method to tell it to perform its task and how to pass information to a method as arguments whose values are assigned to the method’s parameters. You learned the difference between a local variable of a method and an instance variable of a class, and that only instance variables are initialized automatically. You also learned how to use a class’s constructor to specify the initial values for an object’s instance variables. You saw how to create UML class diagrams that model the methods, attributes and constructors of classes. Finally, you learned about floating-point numbers (numbers with decimal points)—how to store them with variables of primitive type double, how to input them with a Scanner object and how to format them with printf and format specifier %f for display purposes. [In Chapter 8, we’ll begin representing monetary amounts precisely with class BigDecimal.] You may have also begun the optional GUI and Graphics case study, learning how to write your first GUI applications. In the next chapter we begin our introduction to control statements, which specify the order in which a program’s actions are performed. You’ll use these in your methods to specify how they should order their tasks. Summary Section 3.2 Instance Variables, set Methods and get Methods • Each class you create becomes a new type that can be used to declare variables and create objects. • You can declare new classes as needed; this is one reason Java is known as an extensible language. Section 3.2.1 Account Class with an Instance Variable, a set Method and a get Method • Each class declaration that begins with the access modifier (p. 71) public must be stored in a file that has the same name as the class and ends with the .java filename extension. • Every class declaration contains keyword class followed immediately by the class’s name. • Class, method and variable names are identifiers. By convention all use camel case names. Class names begin with an uppercase letter, and method and variable names begin with a lowercase letter. • An object has attributes that are implemented as instance variables (p. 72) and carried with it throughout its lifetime. • Instance variables exist before methods are called on an object, while the methods are executing and after the methods complete execution. • A class normally contains one or more methods that manipulate the instance variables that belong to particular objects of the class. • Instance variables are declared inside a class declaration but outside the bodies of the class’s method declarations. • Each object (instance) of the class has its own copy of each of the class’s instance variables. • Most instance-variable declarations are preceded with the keyword private (p. 72), which is an access modifier. Variables or methods declared with access modifier private are accessible only to methods of the class in which they’re declared. 94 Chapter 3 Introduction to Classes, Objects, Methods and Strings • Parameters are declared in a comma-separated parameter list (p. 73), which is located inside the parentheses that follow the method name in the method declaration. Multiple parameters are separated by commas. Each parameter must specify a type followed by a variable name. • Variables declared in the body of a particular method are local variables and can be used only in that method. When a method terminates, the values of its local variables are lost. A method’s parameters are local variables of the method. • Every method’s body is delimited by left and right braces ({ and }). • Each method’s body contains one or more statements that perform the method’s task(s). • The method’s return type specifies the type of data returned to a method’s caller. Keyword void indicates that a method will perform a task but will not return any information. • Empty parentheses following a method name indicate that the method does not require any parameters to perform its task. • When a method that specifies a return type (p. 73) other than void is called and completes its task, the method must return a result to its calling method. • The return statement (p. 74) passes a value from a called method back to its caller. • Classes often provide public methods to allow the class’s clients to set or get private instance variables. The names of these methods need not begin with set or get, but this naming convention is recommended. Section 3.2.2 AccountTest Class That Creates and Uses an Object of Class Account • A class that creates an object of another class, then calls the object’s methods, is a driver class. • Scanner method nextLine (p. 75) reads characters until a newline character is encountered, then returns the characters as a String. • Scanner method next (p. 75) reads characters until any white-space character is encountered, then returns the characters as a String. • A class instance creation expression (p. 75) begins with keyword new and creates a new object. • A constructor is similar to a method but is called implicitly by the new operator to initialize an object’s instance variables at the time the object is created. • To call a method of an object, follow the object name with a dot separator (p. 76), the method name and a set of parentheses containing the method’s arguments. • Local variables are not automatically initialized. Every instance variable has a default initial value—a value provided by Java when you do not specify the instance variable’s initial value. • The default value for an instance variable of type String is null. • A method call supplies values—known as arguments—for each of the method’s parameters. Each argument’s value is assigned to the corresponding parameter in the method header. • The number of arguments in a method call must match the number of parameters in the method declaration’s parameter list. • The argument types in the method call must be consistent with the types of the corresponding parameters in the method’s declaration. Section 3.2.3 Compiling and Executing an App with Multiple Classes • The javac command can compile multiple classes at once. Simply list the source-code filenames after the command with each filename separated by a space from the next. If the directory containing the app includes only one app’s files, you can compile all of its classes with the command javac *.java. The asterisk (*) in *.java indicates that all files in the current directory ending with the filename extension “.java” should be compiled. Summary 95 Section 3.2.4 Account UML Class Diagram with an Instance Variable and set and get Methods • In the UML, each class is modeled in a class diagram (p. 77) as a rectangle with three compartments. The top one contains the class’s name centered horizontally in boldface. The middle one contains the class’s attributes, which correspond to instance variables in Java. The bottom one contains the class’s operations (p. 78), which correspond to methods and constructors in Java. • The UML represents instance variables as an attribute name, followed by a colon and the type. • Private attributes are preceded by a minus sign (–) in the UML. • The UML models operations by listing the operation name followed by a set of parentheses. A plus sign (+) in front of the operation name indicates that the operation is a public one in the UML (i.e., a public method in Java). • The UML models a parameter of an operation by listing the parameter name, followed by a colon and the parameter type between the parentheses after the operation name. • The UML indicates an operation’s return type by placing a colon and the return type after the parentheses following the operation name. • UML class diagrams do not specify return types for operations that do not return values. • Declaring instance variables private is known as data hiding or information hiding. Section 3.2.5 Additional Notes on Class AccountTest • You must call most methods other than main explicitly to tell them to perform their tasks. • A key part of enabling the JVM to locate and call method main to begin the app’s execution is the static keyword, which indicates that main is a static method that can be called without first creating an object of the class in which the method is declared. • Most classes you’ll use in Java programs must be imported explicitly. There’s a special relationship between classes that are compiled in the same directory. By default, such classes are considered to be in the same package—known as the default package. Classes in the same package are implicitly imported into the source-code files of other classes in that package. An import declaration is not required when one class in a package uses another in the same package. • An import declaration is not required if you always refer to a class with its fully qualified class name, which includes its package name and class name. Section 3.2.6 Software Engineering with private Instance Variables and public set and get Methods • Declaring instance variables private is known as data hiding or information hiding. Section 3.3 Primitive Types vs. Reference Types • Types in Java are divided into two categories—primitive types and reference types. The primitive types are boolean, byte, char, short, int, long, float and double. All other types are reference types, so classes, which specify the types of objects, are reference types. • A primitive-type variable can store exactly one value of its declared type at a time. • Primitive-type instance variables are initialized by default. Variables of types byte, char, short, int, long, float and double are initialized to 0. Variables of type boolean are initialized to false. • Reference-type variables (called references; p. 81) store the location of an object in the computer’s memory. Such variables refer to objects in the program. The object that’s referenced may contain many instance variables and methods. • Reference-type instance variables are initialized by default to the value null. 96 Chapter 3 Introduction to Classes, Objects, Methods and Strings • A reference to an object (p. 81) is required to invoke an object’s methods. A primitive-type variable does not refer to an object and therefore cannot be used to invoke a method. Section 3.4 Account Class: Initializing Objects with Constructors • Each class you declare can optionally provide a constructor with parameters that can be used to initialize an object of a class when the object is created. • Java requires a constructor call for every object that’s created. • Constructors can specify parameters but not return types. • If a class does not define constructors, the compiler provides a default constructor (p. 83) with no parameters, and the class’s instance variables are initialized to their default values. • If you declare a constructor for a class, the compiler will not create a default constructor for that class. • The UML models constructors in the third compartment of a class diagram. To distinguish a constructor from a class’s operations, the UML places the word “constructor” between guillemets (« and »; p. 84) before the constructor’s name. Section 3.5 Account Class with a Balance; Floating-Point Numbers and Type double • A floating-point number (p. 84) is a number with a decimal point. Java provides two primitive types for storing floating-point numbers in memory—float and double (p. 84). • Variables of type float represent single-precision floating-point numbers and have seven significant digits. Variables of type double represent double-precision floating-point numbers. These require twice as much memory as float variables and provide 15 significant digits—approximately double the precision of float variables. • Floating-point literals (p. 84) are of type double by default. • Scanner method nextDouble (p. 88) returns a double value. • The format specifier %f (p. 88) is used to output values of type float or double. The format specifier %.2f specifies that two digits of precision (p. 88) should be output to the right of the decimal point in the floating-point number. • The default value for an instance variable of type double is 0.0, and the default value for an instance variable of type int is 0. Self-Review Exercises 3.1 Fill in the blanks in each of the following: a) Each class declaration that begins with keyword must be stored in a file that has exactly the same name as the class and ends with the .java filename extension. in a class declaration is followed immediately by the class’s name. b) Keyword c) Keyword requests memory from the system to store an object, then calls the corresponding class’s constructor to initialize the object. and a(n) . d) Each parameter must specify both a(n) e) By default, classes that are compiled in the same directory are considered to be in the same package, known as the . f) Java provides two primitive types for storing floating-point numbers in memory: and . g) Variables of type double represent floating-point numbers. returns a double value. h) Scanner method i) Keyword public is an access . Answers to Self-Review Exercises 97 j) Return type indicates that a method will not return a value. reads characters until it encounters a newline character, k) Scanner method then returns those characters as a String. l) Class String is in package . is not required if you always refer to a class with its fully qualified class m) A(n) name. n) A(n) is a number with a decimal point, such as 7.33, 0.0975 or 1000.12345. _ -precision floating-point numbers. o) Variables of type float represent p) The format specifier is used to output values of type float or double. q) Types in Java are divided into two categories— types and types. 3.2 State whether each of the following is true or false. If false, explain why. a) By convention, method names begin with an uppercase first letter, and all subsequent words in the name begin with a capital first letter. b) An import declaration is not required when one class in a package uses another in the same package. c) Empty parentheses following a method name in a method declaration indicate that the method does not require any parameters to perform its task. d) A primitive-type variable can be used to invoke a method. e) Variables declared in the body of a particular method are known as instance variables and can be used in all methods of the class. f) Every method’s body is delimited by left and right braces ({ and }). g) Primitive-type local variables are initialized by default. h) Reference-type instance variables are initialized by default to the value null. i) Any class that contains public static void main(String[] args) can be used to execute an app. j) The number of arguments in the method call must match the number of parameters in the method declaration’s parameter list. k) Floating-point values that appear in source code are known as floating-point literals and are type float by default. 3.3 What is the difference between a local variable and an instance variable? 3.4 Explain the purpose of a method parameter. What is the difference between a parameter and an argument? Answers to Self-Review Exercises 3.1 a) public. b) class. c) new. d) type, name. e) default package. f) float, double. g) doubleprecision. h) nextDouble. i) modifier. j) void. k) nextLine. l) java.lang. m) import declaration. n) floating-point number. o) single. p) %f. q) primitive, reference. 3.2 a) False. By convention, method names begin with a lowercase first letter and all subsequent words in the name begin with a capital first letter. b) True. c) True. d) False. A primitive-type variable cannot be used to invoke a method—a reference to an object is required to invoke the object’s methods. e) False. Such variables are called local variables and can be used only in the method in which they’re declared. f) True. g) False. Primitive-type instance variables are initialized by default. Each local variable must explicitly be assigned a value. h) True. i) True. j) True. k) False. Such literals are of type double by default. 3.3 A local variable is declared in the body of a method and can be used only from the point at which it’s declared through the end of the method declaration. An instance variable is declared in a class, but not in the body of any of the class’s methods. Also, instance variables are accessible to all methods of the class. (We’ll see an exception to this in Chapter 8.) 98 Chapter 3 Introduction to Classes, Objects, Methods and Strings 3.4 A parameter represents additional information that a method requires to perform its task. Each parameter required by a method is specified in the method’s declaration. An argument is the actual value for a method parameter. When a method is called, the argument values are passed to the corresponding parameters of the method so that it can perform its task. Exercises 3.5 (Keyword new) What’s the purpose of keyword new? Explain what happens when you use it. 3.6 (Default Constructors) What is a default constructor? How are an object’s instance variables initialized if a class has only a default constructor? 3.7 (Instance Variables) Explain the purpose of an instance variable. 3.8 (Using Classes without Importing Them) Most classes need to be imported before they can be used in an app. Why is every app allowed to use classes System and String without first importing them? 3.9 (Using a Class without Importing It) Explain how a program could use class Scanner without importing it. 3.10 (set and get Methods) Explain why a class might provide a set method and a get method for an instance variable. (Modified Account Class) Modify class Account (Fig. 3.8) to provide a method called withthat withdraws money from an Account. Ensure that the withdrawal amount does not exceed the Account’s balance. If it does, the balance should be left unchanged and the method should print a message indicating "Withdrawal amount exceeded account balance." Modify class AccountTest (Fig. 3.9) to test method withdraw. 3.11 draw 3.12 (Invoice Class) Create a class called Invoice that a hardware store might use to represent an invoice for an item sold at the store. An Invoice should include four pieces of information as instance variables—a part number (type String), a part description (type String), a quantity of the item being purchased (type int) and a price per item (double). Your class should have a constructor that initializes the four instance variables. Provide a set and a get method for each instance variable. In addition, provide a method named getInvoiceAmount that calculates the invoice amount (i.e., multiplies the quantity by the price per item), then returns the amount as a double value. If the quantity is not positive, it should be set to 0. If the price per item is not positive, it should be set to 0.0. Write a test app named InvoiceTest that demonstrates class Invoice’s capabilities. 3.13 (Employee Class) Create a class called Employee that includes three instance variables—a first name (type String), a last name (type String) and a monthly salary (double). Provide a constructor that initializes the three instance variables. Provide a set and a get method for each instance variable. If the monthly salary is not positive, do not set its value. Write a test app named EmployeeTest that demonstrates class Employee’s capabilities. Create two Employee objects and display each object’s yearly salary. Then give each Employee a 10% raise and display each Employee’s yearly salary again. (Date Class) Create a class called Date that includes three instance variables—a month (type a day (type int) and a year (type int). Provide a constructor that initializes the three instance variables and assumes that the values provided are correct. Provide a set and a get method for each instance variable. Provide a method displayDate that displays the month, day and year separated by forward slashes (/). Write a test app named DateTest that demonstrates class Date’s capabilities. 3.14 int), (Removing Duplicated Code in Method main) In the AccountTest class of Fig. 3.9, method contains six statements (lines 13–14, 15–16, 28–29, 30–31, 40–41 and 42–43) that each display an Account object’s name and balance. Study these statements and you’ll notice that they differ 3.15 main Making a Difference 99 only in the Account object being manipulated—account1 or account2. In this exercise, you’ll define a new displayAccount method that contains one copy of that output statement. The method’s parameter will be an Account object and the method will output the object’s name and balance. You’ll then replace the six duplicated statements in main with calls to displayAccount, passing as an argument the specific Account object to output. Modify class AccountTest class of Fig. 3.9 to declare the following displayAccount method after the closing right brace of main and before the closing right brace of class AccountTest: public static void displayAccount(Account accountToDisplay) { // place the statement that displays // accountToDisplay's name and balance here } Replace the comment in the method’s body with a statement that displays accountToDisplay’s name and balance. Recall that main is a static method, so it can be called without first creating an object of the class in which main is declared. We also declared method displayAccount as a static method. When main needs to call another method in the same class without first creating an object of that class, the other method also must be declared static. Once you’ve completed displayAccount’s declaration, modify main to replace the statements that display each Account’s name and balance with calls to displayAccount—each receiving as its argument the account1 or account2 object, as appropriate. Then, test the updated AccountTest class to ensure that it produces the same output as shown in Fig. 3.9. Making a Difference 3.16 (Target-Heart-Rate Calculator) While exercising, you can use a heart-rate monitor to see that your heart rate stays within a safe range suggested by your trainers and doctors. According to the American Heart Association (AHA) (www.americanheart.org/presenter.jhtml?identifier=4736), the formula for calculating your maximum heart rate in beats per minute is 220 minus your age in years. Your target heart rate is a range that’s 50–85% of your maximum heart rate. [Note: These formulas are estimates provided by the AHA. Maximum and target heart rates may vary based on the health, fitness and gender of the individual. Always consult a physician or qualified health-care professional before beginning or modifying an exercise program.] Create a class called HeartRates. The class attributes should include the person’s first name, last name and date of birth (consisting of separate attributes for the month, day and year of birth). Your class should have a constructor that receives this data as parameters. For each attribute provide set and get methods. The class also should include a method that calculates and returns the person’s age (in years), a method that calculates and returns the person’s maximum heart rate and a method that calculates and returns the person’s target heart rate. Write a Java app that prompts for the person’s information, instantiates an object of class HeartRates and prints the information from that object—including the person’s first name, last name and date of birth—then calculates and prints the person’s age in (years), maximum heart rate and target-heart-rate range. 3.17 (Computerization of Health Records) A health-care issue that has been in the news lately is the computerization of health records. This possibility is being approached cautiously because of sensitive privacy and security concerns, among others. [We address such concerns in later exercises.] Computerizing health records could make it easier for patients to share their health profiles and histories among their various health-care professionals. This could improve the quality of health care, help avoid drug conflicts and erroneous drug prescriptions, reduce costs and, in emergencies, could save lives. In this exercise, you’ll design a “starter” HealthProfile class for a person. The class attributes should include the person’s first name, last name, gender, date of birth (consisting of separate 100 Chapter 3 Introduction to Classes, Objects, Methods and Strings attributes for the month, day and year of birth), height (in inches) and weight (in pounds). Your class should have a constructor that receives this data. For each attribute, provide set and get methods. The class also should include methods that calculate and return the user’s age in years, maximum heart rate and target-heart-rate range (see Exercise 3.16), and body mass index (BMI; see Exercise 2.33). Write a Java app that prompts for the person’s information, instantiates an object of class HealthProfile for that person and prints the information from that object—including the person’s first name, last name, gender, date of birth, height and weight—then calculates and prints the person’s age in years, BMI, maximum heart rate and target-heart-rate range. It should also display the BMI values chart from Exercise 2.33. Control Statements: Part 1; Assignment, ++ and -Operators 4 Let’s all move one place on. —Lewis Carroll How many apples fell on Newton’s head before he took the hint! —Robert Frost Objectives In this chapter you’ll: ■ Learn basic problem-solving techniques. ■ Develop algorithms through the process of top-down, stepwise refinement. ■ Use the if and if…else selection statements to choose between alternative actions. ■ Use the while repetition statement to execute statements in a program repeatedly. ■ Use counter-controlled repetition and sentinelcontrolled repetition. ■ Use the compound assignment operator, and the increment and decrement operators. ■ Learn about the portability of primitive data types. Chapter 4 Control Statements: Part 1; Assignment, ++ and -- Operators 102 4.1 4.2 4.3 4.4 4.5 4.6 Introduction Algorithms Pseudocode Control Structures if Single-Selection Statement if…else Double-Selection Statement 4.7 Student Class: Nested if…else Statements 4.8 while Repetition Statement 4.9 Formulating Algorithms: CounterControlled Repetition 4.10 Formulating Algorithms: SentinelControlled Repetition 4.11 Formulating Algorithms: Nested Control Statements 4.12 Compound Assignment Operators 4.13 Increment and Decrement Operators 4.14 Primitive Types 4.15 (Optional) GUI and Graphics Case Study: Creating Simple Drawings 4.16 Wrap-Up Summary | Self-Review Exercises | Answers to Self-Review Exercises | Exercises | Making a Difference 4.1 Introduction Before writing a program to solve a problem, you should have a thorough understanding of the problem and a carefully planned approach to solving it. When writing a program, you also should understand the available building blocks and employ proven programconstruction techniques. In this chapter and the next, we discuss these issues in presenting the theory and principles of structured programming. The concepts presented here are crucial in building classes and manipulating objects. We discuss Java’s if statement in additional detail, and introduce the if…else and while statements—all of these building blocks allow you to specify the logic required for methods to perform their tasks. We also introduce the compound assignment operator and the increment and decrement operators. Finally, we consider the portability of Java’s primitive types. 4.2 Algorithms Any computing problem can be solved by executing a series of actions in a specific order. A procedure for solving a problem in terms of 1. the actions to execute and 2. the order in which these actions execute is called an algorithm. The following example demonstrates that correctly specifying the order in which the actions execute is important. Consider the “rise-and-shine algorithm” followed by one executive for getting out of bed and going to work: (1) Get out of bed; (2) take off pajamas; (3) take a shower; (4) get dressed; (5) eat breakfast; (6) carpool to work. This routine gets the executive to work well prepared to make critical decisions. Suppose that the same steps are performed in a slightly different order: (1) Get out of bed; (2) take off pajamas; (3) get dressed; (4) take a shower; (5) eat breakfast; (6) carpool to work. In this case, our executive shows up for work soaking wet. Specifying the order in which statements (actions) execute in a program is called program control. This chapter investigates program control using Java’s control statements. 4.3 Pseudocode 103 4.3 Pseudocode Pseudocode is an informal language that helps you develop algorithms without having to worry about the strict details of Java language syntax. The pseudocode we present is particularly useful for developing algorithms that will be converted to structured portions of Java programs. The pseudocode we use in this book is similar to everyday English—it’s convenient and user friendly, but it’s not an actual computer programming language. You’ll see an algorithm written in pseudocode in Fig. 4.7. You may, of course, use your own native language(s) to develop your own pseudocode. Pseudocode does not execute on computers. Rather, it helps you “think out” a program before attempting to write it in a programming language, such as Java. This chapter provides several examples of using pseudocode to develop Java programs. The style of pseudocode we present consists purely of characters, so you can type pseudocode conveniently, using any text-editor program. A carefully prepared pseudocode program can easily be converted to a corresponding Java program. Pseudocode normally describes only statements representing the actions that occur after you convert a program from pseudocode to Java and the program is run on a computer. Such actions might include input, output or calculations. In our pseudocode, we typically do not include variable declarations, but some programmers choose to list variables and mention their purposes. 4.4 Control Structures Normally, statements in a program are executed one after the other in the order in which they’re written. This process is called sequential execution. Various Java statements, which we’ll soon discuss, enable you to specify that the next statement to execute is not necessarily the next one in sequence. This is called transfer of control. During the 1960s, it became clear that the indiscriminate use of transfers of control was the root of much difficulty experienced by software development groups. The blame was pointed at the goto statement (used in most programming languages of the time), which allows you to specify a transfer of control to one of a wide range of destinations in a program. [Note: Java does not have a goto statement; however, the word goto is reserved by Java and should not be used as an identifier in programs.] The research of Bohm and Jacopini1 had demonstrated that programs could be written without any goto statements. The challenge of the era for programmers was to shift their styles to “goto-less programming.” The term structured programming became almost synonymous with “goto elimination.” Not until the 1970s did most programmers start taking structured programming seriously. The results were impressive. Software development groups reported shorter development times, more frequent on-time delivery of systems and more frequent within-budget completion of software projects. The key to these successes was that structured programs were clearer, easier to debug and modify, and more likely to be bug free in the first place. Bohm and Jacopini’s work demonstrated that all programs could be written in terms of only three control structures—the sequence structure, the selection structure and the 1. C. Bohm, and G. Jacopini, “Flow Diagrams, Turing Machines, and Languages with Only Two Formation Rules,” Communications of the ACM, Vol. 9, No. 5, May 1966, pp. 336–371. 104 Chapter 4 Control Statements: Part 1; Assignment, ++ and -- Operators repetition structure. When we introduce Java’s control-structure implementations, we’ll refer to them in the terminology of the Java Language Specification as “control statements.” Sequence Structure in Java The sequence structure is built into Java. Unless directed otherwise, the computer executes Java statements one after the other in the order in which they’re written—that is, in sequence. The activity diagram in Fig. 4.1 illustrates a typical sequence structure in which two calculations are performed in order. Java lets you have as many actions as you want in a sequence structure. As we’ll soon see, anywhere a single action may be placed, we may place several actions in sequence. add grade to total add 1 to counter Corresponding Java statement: total = total + grade; Corresponding Java statement: counter = counter + 1; Fig. 4.1 | Sequence-structure activity diagram. A UML activity diagram models the workflow (also called the activity) of a portion of a software system. Such workflows may include a portion of an algorithm, like the sequence structure in Fig. 4.1. Activity diagrams are composed of symbols, such as actionstate symbols (rectangles with their left and right sides replaced with outward arcs), diamonds and small circles. These symbols are connected by transition arrows, which represent the flow of the activity—that is, the order in which the actions should occur. Like pseudocode, activity diagrams help you develop and represent algorithms. Activity diagrams clearly show how control structures operate. We use the UML in this chapter and Chapter 5 to show control flow in control statements. Online Chapters 33– 34 use the UML in a real-world automated-teller-machine case study. Consider the sequence-structure activity diagram in Fig. 4.1. It contains two action states, each containing an action expression—for example, “add grade to total” or “add 1 to counter”—that specifies a particular action to perform. Other actions might include calculations or input/output operations. The arrows in the activity diagram represent transitions, which indicate the order in which the actions represented by the action states occur. The program that implements the activities illustrated by the diagram in Fig. 4.1 first adds grade to total, then adds 1 to counter. The solid circle at the top of the activity diagram represents the initial state—the beginning of the workflow before the program performs the modeled actions. The solid circle surrounded by a hollow circle at the bottom of the diagram represents the final state—the end of the workflow after the program performs its actions. Figure 4.1 also includes rectangles with the upper-right corners folded over. These are UML notes (like comments in Java)—explanatory remarks that describe the purpose of sym- 4.5 if Single-Selection Statement 105 bols in the diagram. Figure 4.1 uses UML notes to show the Java code associated with each action state. A dotted line connects each note with the element it describes. Activity diagrams normally do not show the Java code that implements the activity. We do this here to illustrate how the diagram relates to Java code. For more information on the UML, see our optiona online object-oriented design case study (Chapters 33–34) or visit www.uml.org. Selection Statements in Java Java has three types of selection statements (discussed in this chapter and Chapter 5). The if statement either performs (selects) an action, if a condition is true, or skips it, if the condition is false. The if…else statement performs an action if a condition is true and performs a different action if the condition is false. The switch statement (Chapter 5) performs one of many different actions, depending on the value of an expression. The if statement is a single-selection statement because it selects or ignores a single action (or, as we’ll soon see, a single group of actions). The if…else statement is called a double-selection statement because it selects between two different actions (or groups of actions). The switch statement is called a multiple-selection statement because it selects among many different actions (or groups of actions). Repetition Statements in Java Java provides three repetition statements (also called iteration statements or looping statements) that enable programs to perform statements repeatedly as long as a condition (called the loop-continuation condition) remains true. The repetition statements are the while, do…while, for and enhanced for statements. (Chapter 5 presents the do…while and for statements and Chapter 7 presents the enhanced for statement.) The while and for statements perform the action (or group of actions) in their bodies zero or more times—if the loop-continuation condition is initially false, the action (or group of actions) will not execute. The do…while statement performs the action (or group of actions) in its body one or more times. The words if, else, switch, while, do and for are Java keywords. A complete list of Java keywords appears in Appendix C. Summary of Control Statements in Java Java has only three kinds of control structures, which from this point forward we refer to as control statements: the sequence statement, selection statements (three types) and repetition statements (three types). Every program is formed by combining as many of these statements as is appropriate for the algorithm the program implements. We can model each control statement as an activity diagram. Like Fig. 4.1, each diagram contains an initial state and a final state that represent a control statement’s entry point and exit point, respectively. Single-entry/single-exit control statements make it easy to build programs— we simply connect the exit point of one to the entry point of the next. We call this controlstatement stacking. We’ll learn that there’s only one other way in which control statements may be connected—control-statement nesting—in which one control statement appears inside another. Thus, algorithms in Java programs are constructed from only three kinds of control statements, combined in only two ways. This is the essence of simplicity. 4.5 if Single-Selection Statement Programs use selection statements to choose among alternative courses of action. For example, suppose that the passing grade on an exam is 60. The pseudocode statement 106 Chapter 4 Control Statements: Part 1; Assignment, ++ and -- Operators If student’s grade is greater than or equal to 60 Print “Passed” determines whether the condition “student’s grade is greater than or equal to 60” is true. If so, “Passed” is printed, and the next pseudocode statement in order is “performed.” (Remember, pseudocode is not a real programming language.) If the condition is false, the Print statement is ignored, and the next pseudocode statement in order is performed. The indentation of the second line of this selection statement is optional, but recommended, because it emphasizes the inherent structure of structured programs. The preceding pseudocode If statement may be written in Java as if (studentGrade >= 60) System.out.println("Passed"); The Java code corresponds closely to the pseudocode. This is one of the properties of pseudocode that makes it such a useful program development tool. UML Activity Diagram for an if Statement Figure 4.2 illustrates the single-selection if statement. This figure contains the most important symbol in an activity diagram—the diamond, or decision symbol, which indicates that a decision is to be made. The workflow continues along a path determined by the symbol’s associated guard conditions, which can be true or false. Each transition arrow emerging from a decision symbol has a guard condition (specified in square brackets next to the arrow). If a guard condition is true, the workflow enters the action state to which the transition arrow points. In Fig. 4.2, if the grade is greater than or equal to 60, the program prints “Passed,” then transitions to the activity’s final state. If the grade is less than 60, the program immediately transitions to the final state without displaying a message. [grade >= 60] print “Passed” [grade < 60] Fig. 4.2 | if single-selection statement UML activity diagram. The if statement is a single-entry/single-exit control statement. We’ll see that the activity diagrams for the remaining control statements also contain initial states, transition arrows, action states that indicate actions to perform, decision symbols (with associated guard conditions) that indicate decisions to be made, and final states. 4.6 if…else Double-Selection Statement The if single-selection statement performs an indicated action only when the condition is true; otherwise, the action is skipped. The if…else double-selection statement allows you to specify an action to perform when the condition is true and another action when the condition is false. For example, the pseudocode statement 4.6 if…else Double-Selection Statement 107 If student’s grade is greater than or equal to 60 Print “Passed” Else Print “Failed” prints “Passed” if the student’s grade is greater than or equal to 60, but prints “Failed” if it’s less than 60. In either case, after printing occurs, the next pseudocode statement in sequence is “performed.” The preceding If…Else pseudocode statement can be written in Java as if (grade >= 60) System.out.println("Passed"); else System.out.println("Failed"); The body of the else is also indented. Whatever indentation convention you choose should be applied consistently throughout your programs. Good Programming Practice 4.1 Indent both body statements of an if…else statement. Many IDEs do this for you. Good Programming Practice 4.2 If there are several levels of indentation, each level should be indented the same additional amount of space. UML Activity Diagram for an if…else Statement Figure 4.3 illustrates the flow of control in the if…else statement. Once again, the symbols in the UML activity diagram (besides the initial state, transition arrows and final state) represent action states and decisions. print “Failed” Fig. 4.3 | if…else [grade < 60] [grade >= 60] print “Passed” double-selection statement UML activity diagram. Nested if…else Statements A program can test multiple cases by placing if…else statements inside other if…else statements to create nested if…else statements. For example, the following pseudocode represents a nested if…else that prints A for exam grades greater than or equal to 90, B for grades 80 to 89, C for grades 70 to 79, D for grades 60 to 69 and F for all other grades: 108 Chapter 4 Control Statements: Part 1; Assignment, ++ and -- Operators If student’s grade is greater than or equal to 90 Print “A” else If student’s grade is greater than or equal to 80 Print “B” else If student’s grade is greater than or equal to 70 Print “C” else If student’s grade is greater than or equal to 60 Print “D” else Print “F” This pseudocode may be written in Java as if (studentGrade >= 90) System.out.println("A"); else if (studentGrade >= 80) System.out.println("B"); else if (studentGrade >= 70) System.out.println("C"); else if (studentGrade >= 60) System.out.println("D"); else System.out.println("F"); Error-Prevention Tip 4.1 In a nested if…else statement, ensure that you test for all possible cases. If variable studentGrade is greater than or equal to 90, the first four conditions in the nested if…else statement will be true, but only the statement in the if part of the first if…else statement will execute. After that statement executes, the else part of the “outermost” if…else statement is skipped. Many programmers prefer to write the preceding nested if…else statement as if (studentGrade >= 90) System.out.println("A"); else if (studentGrade >= 80) System.out.println("B"); else if (studentGrade >= 70) System.out.println("C"); else if (studentGrade >= 60) System.out.println("D"); else System.out.println("F"); 4.6 if…else Double-Selection Statement 109 The two forms are identical except for the spacing and indentation, which the compiler ignores. The latter form avoids deep indentation of the code to the right. Such indentation often leaves little room on a line of source code, forcing lines to be split. Dangling-else Problem The Java compiler always associates an else with the immediately preceding if unless told to do otherwise by the placement of braces ({ and }). This behavior can lead to what is referred to as the dangling-else problem. For example, if (x > 5) if (y > 5) System.out.println("x and y are > 5"); else System.out.println("x is <= 5"); appears to indicate that if x is greater than 5, the nested if statement determines whether is also greater than 5. If so, the string "x and y are > 5" is output. Otherwise, it appears that if x is not greater than 5, the else part of the if…else outputs the string "x is <= 5". Beware! This nested if…else statement does not execute as it appears. The compiler actually interprets the statement as y if (x > 5) if (y > 5) System.out.println("x and y are > 5"); else System.out.println("x is <= 5"); in which the body of the first if is a nested if…else. The outer if statement tests whether is greater than 5. If so, execution continues by testing whether y is also greater than 5. If the second condition is true, the proper string—"x and y are > 5"—is displayed. However, if the second condition is false, the string "x is <= 5" is displayed, even though we know that x is greater than 5. Equally bad, if the outer if statement’s condition is false, the inner if…else is skipped and nothing is displayed. To force the nested if…else statement to execute as it was originally intended, we must write it as follows: x if (x > 5) { if (y > 5) System.out.println("x and y are > 5"); } else System.out.println("x is <= 5"); The braces indicate that the second if is in the body of the first and that the else is associated with the first if. Exercises 4.27–4.28 investigate the dangling-else problem further. Blocks The if statement normally expects only one statement in its body. To include several statements in the body of an if (or the body of an else for an if…else statement), enclose the statements in braces. Statements contained in a pair of braces (such as the body of a 110 Chapter 4 Control Statements: Part 1; Assignment, ++ and -- Operators method) form a block. A block can be placed anywhere in a method that a single statement can be placed. The following example includes a block in the else part of an if…else statement: if (grade >= 60) System.out.println("Passed"); else { System.out.println("Failed"); System.out.println("You must take this course again."); } In this case, if grade is less than 60, the program executes both statements in the body of the else and prints Failed You must take this course again. Note the braces surrounding the two statements in the else clause. These braces are important. Without the braces, the statement System.out.println("You must take this course again."); would be outside the body of the else part of the if…else statement and would execute regardless of whether the grade was less than 60. Syntax errors (e.g., when one brace in a block is left out of the program) are caught by the compiler. A logic error (e.g., when both braces in a block are left out of the program) has its effect at execution time. A fatal logic error causes a program to fail and terminate prematurely. A nonfatal logic error allows a program to continue executing but causes it to produce incorrect results. Just as a block can be placed anywhere a single statement can be placed, it’s also possible to have an empty statement. Recall from Section 2.8 that the empty statement is represented by placing a semicolon (;) where a statement would normally be. Common Programming Error 4.1 Placing a semicolon after the condition in an if or if…else statement leads to a logic error in single-selection if statements and a syntax error in double-selection if…else statements (when the if-part contains an actual body statement). Conditional Operator (?:) Java provides the conditional operator (?:) that can be used in place of an if…else statement. This can make your code shorter and clearer. The conditional operator is Java’s only ternary operator (i.e., an operator that takes three operands). Together, the operands and the ?: symbol form a conditional expression. The first operand (to the left of the ?) is a boolean expression (i.e., a condition that evaluates to a boolean value—true or false), the second operand (between the ? and :) is the value of the conditional expression if the boolean expression is true and the third operand (to the right of the :) is the value of the conditional expression if the boolean expression evaluates to false. For example, the statement System.out.println(studentGrade >= 60 ? "Passed" : "Failed"); 4.7 Student Class: Nested if…else Statements 111 prints the value of println’s conditional-expression argument. The conditional expression in this statement evaluates to the string "Passed" if the boolean expression studentGrade >= 60 is true and to the string "Failed" if it’s false. Thus, this statement with the conditional operator performs essentially the same function as the if…else statement shown earlier in this section. The precedence of the conditional operator is low, so the entire conditional expression is normally placed in parentheses. We’ll see that conditional expressions can be used in some situations where if…else statements cannot. Error-Prevention Tip 4.2 Use expressions of the same type for the second and third operands of the ?: operator to avoid subtle errors. 4.7 Student Class: Nested if…else Statements The example of Figs. 4.4–4.5 demonstrates a nested if…else statement that determines a student’s letter grade based on the student’s average in a course. Class Student Class Student (Fig. 4.4) has features similar to those of class Account (discussed in Chapter 3). Class Student stores a student’s name and average and provides methods for manipulating these values. The class contains: • instance variable name of type String (line 5) to store a Student’s name • instance variable average of type double (line 6) to store a Student’s average in a course • a constructor (lines 9–18) that initializes the name and average—in Section 5.9, you’ll learn how to express lines 15–16 and 37–38 more concisely with logical operators that can test multiple conditions • methods setName and getName (lines 21–30) to set and get the Student’s name • methods setAverage and getAverage (lines 33–46) to set and get the Student’s average • method getLetterGrade (lines 49–65), which uses nested if…else statements to determine the Student’s letter grade based on the Student’s average The constructor and method setAverage each use nested if statements (lines 15–17 and 37–39) to validate the value used to set the average—these statements ensure that the value is greater than 0.0 and less than or equal to 100.0; otherwise, average’s value is left unchanged. Each if statement contains a simple condition. If the condition in line 15 is true, only then will the condition in line 16 be tested, and only if the conditions in both line 15 and line 16 are true will the statement in line 17 execute. Software Engineering Observation 4.1 Recall from Chapter 3 that you should not call methods from constructors (we’ll explain why in Chapter 10, Object-Oriented Programming: Polymorphism and Interfaces). For this reason, there is duplicated validation code in lines 15–17 and 37–39 of Fig. 4.4 and in subsequent examples. Chapter 4 Control Statements: Part 1; Assignment, ++ and -- Operators 112 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 // Fig. 4.4: Student.java // Student class that stores a student name and average. public class Student { private String name; private double average; // constructor initializes instance variables public Student(String name, double average) { this.name = name; // validate that average is > 0.0 and <= 100.0; otherwise, // keep instance variable average's default value (0.0) if (average > 0.0) if (average <= 100.0) this.average = average; // assign to instance variable } // sets the Student's name public void setName(String name) { this.name = name; } // retrieves the Student's name public String getName() { return name; } // sets the Student's average public void setAverage(double studentAverage) { // validate that average is > 0.0 and <= 100.0; otherwise, // keep instance variable average's current value if (average > 0.0) if (average <= 100.0) this.average = average; // assign to instance variable } // retrieves the Student's average public double getAverage() { return average; } // determines and returns the Student's letter grade public String getLetterGrade() { String letterGrade = ""; // initialized to empty String Fig. 4.4 | Student class that stores a student name and average. (Part 1 of 2.) 4.8 while Repetition Statement 53 54 55 56 57 58 59 60 61 62 63 64 65 66 113 if (average >= 90.0) letterGrade = "A"; else if (average >= 80.0) letterGrade = "B"; else if (average >= 70.0) letterGrade = "C"; else if (average >= 60.0) letterGrade = "D"; else letterGrade = "F"; return letterGrade; } } // end class Student Fig. 4.4 | Student class that stores a student name and average. (Part 2 of 2.) Class StudentTest To demonstrate the nested if…else statements in class Student’s getLetterGrade method, class StudentTest’s main method (Fig. 4.5) creates two Student objects (lines 7– 8). Next, lines 10–13 display each Student’s name and letter grade by calling the objects’ getName and getLetterGrade methods, respectively. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 // Fig. 4.5: StudentTest.java // Create and test Student objects. public class StudentTest { public static void main(String[] args) { Student account1 = new Student("Jane Green", 93.5); Student account2 = new Student("John Blue", 72.75); System.out.printf("%s's letter grade is: %s%n", account1.getName(), account1.getLetterGrade()); System.out.printf("%s's letter grade is: %s%n", account2.getName(), account2.getLetterGrade()); } } // end class StudentTest Jane Green's letter grade is: A John Blue's letter grade is: C Fig. 4.5 | Create and test Student objects. 4.8 while Repetition Statement A repetition statement allows you to specify that a program should repeat an action while some condition remains true. The pseudocode statement While there are more items on my shopping list Purchase next item and cross it off my list 114 Chapter 4 Control Statements: Part 1; Assignment, ++ and -- Operators describes the repetition during a shopping trip. The condition “there are more items on my shopping list” may be true or false. If it’s true, then the action “Purchase next item and cross it off my list” is performed. This action will be performed repeatedly while the condition remains true. The statement(s) contained in the While repetition statement constitute its body, which may be a single statement or a block. Eventually, the condition will become false (when the shopping list’s last item has been purchased and crossed off). At this point, the repetition terminates, and the first statement after the repetition statement executes. As an example of Java’s while repetition statement, consider a program segment that finds the first power of 3 larger than 100. Suppose that the int variable product is initialized to 3. After the following while statement executes, product contains the result: while (product <= 100) product = 3 * product; Each iteration of the while statement multiplies product by 3, so product takes on the values 9, 27, 81 and 243 successively. When product becomes 243, product <= 100 becomes false. This terminates the repetition, so the final value of product is 243. At this point, program execution continues with the next statement after the while statement. Common Programming Error 4.2 Not providing in the body of a while statement an action that eventually causes the condition in the while to become false normally results in a logic error called an infinite loop (the loop never terminates). UML Activity Diagram for a while Statement The UML activity diagram in Fig. 4.6 illustrates the flow of control in the preceding while statement. Once again, the symbols in the diagram (besides the initial state, transition arrows, a final state and three notes) represent an action state and a decision. This diagram introduces the UML’s merge symbol. The UML represents both the merge symbol and the decision symbol as diamonds. The merge symbol joins two flows of activity into one. In this diagram, the merge symbol joins the transitions from the initial state and from the action state, so they both flow into the decision that determines whether the loop should begin (or continue) executing. merge decision [product <= 100] triple product value [product > 100] Corresponding Java statement: product = 3 * product; Fig. 4.6 | while repetition statement UML activity diagram. 4.9 Formulating Algorithms: Counter-Controlled Repetition 115 The decision and merge symbols can be distinguished by the number of “incoming” and “outgoing” transition arrows. A decision symbol has one transition arrow pointing to the diamond and two or more pointing out from it to indicate possible transitions from that point. In addition, each transition arrow pointing out of a decision symbol has a guard condition next to it. A merge symbol has two or more transition arrows pointing to the diamond and only one pointing from the diamond, to indicate multiple activity flows merging to continue the activity. None of the transition arrows associated with a merge symbol has a guard condition. Figure 4.6 clearly shows the repetition of the while statement discussed earlier in this section. The transition arrow emerging from the action state points back to the merge, from which program flow transitions back to the decision that’s tested at the beginning of each iteration of the loop. The loop continues to execute until the guard condition product > 100 becomes true. Then the while statement exits (reaches its final state), and control passes to the next statement in sequence in the program. 4.9 Formulating Algorithms: Counter-Controlled Repetition To illustrate how algorithms are developed, we solve two variations of a problem that averages student grades. Consider the following problem statement: A class of ten students took a quiz. The grades (integers in the range 0–100) for this quiz are available to you. Determine the class average on the quiz. The class average is equal to the sum of the grades divided by the number of students. The algorithm for solving this problem on a computer must input each grade, keep track of the total of all grades input, perform the averaging calculation and print the result. Pseudocode Algorithm with Counter-Controlled Repetition Let’s use pseudocode to list the actions to execute and specify the order in which they should execute. We use counter-controlled repetition to input the grades one at a time. This technique uses a variable called a counter (or control variable) to control the number of times a set of statements will execute. Counter-controlled repetition is often called definite repetition, because the number of repetitions is known before the loop begins executing. In this example, repetition terminates when the counter exceeds 10. This section presents a fully developed pseudocode algorithm (Fig. 4.7) and a corresponding Java program (Fig. 4.8) that implements the algorithm. In Section 4.10, we demonstrate how to use pseudocode to develop such an algorithm from scratch. Note the references in the algorithm of Fig. 4.7 to a total and a counter. A total is a variable used to accumulate the sum of several values. A counter is a variable used to count—in this case, the grade counter indicates which of the 10 grades is about to be entered by the user. Variables used to store totals are normally initialized to zero before being used in a program. Software Engineering Observation 4.2 Experience has shown that the most difficult part of solving a problem on a computer is developing the algorithm for the solution. Once a correct algorithm has been specified, producing a working Java program from it is usually straightforward. 116 1 2 3 4 5 6 7 8 9 10 11 Chapter 4 Control Statements: Part 1; Assignment, ++ and -- Operators Set total to zero Set grade counter to one While grade counter is less than or equal to ten Prompt the user to enter the next grade Input the next grade Add the grade into the total Add one to the grade counter Set the class average to the total divided by ten Print the class average Fig. 4.7 | Pseudocode algorithm that uses counter-controlled repetition to solve the classaverage problem. Implementing Counter-Controlled Repetition In Fig. 4.8, class ClassAverage’s main method (lines 7–31) implements the class-averaging algorithm described by the pseudocode in Fig. 4.7—it allows the user to enter 10 grades, then calculates and displays the average. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 // Fig. 4.8: ClassAverage.java // Solving the class-average problem using counter-controlled repetition. import java.util.Scanner; // program uses class Scanner public class ClassAverage { public static void main(String[] args) { // create Scanner to obtain input from command window Scanner input = new Scanner(System.in); // initialization phase int total = 0; // initialize sum of grades entered by the user int gradeCounter = 1; // initialize # of grade to be entered next // processing phase uses counter-controlled repetition while (gradeCounter <= 10) // loop 10 times { System.out.print("Enter grade: "); // prompt int grade = input.nextInt(); // input next grade total = total + grade; // add grade to total gradeCounter = gradeCounter + 1; // increment counter by 1 } // termination phase int average = total / 10; // integer division yields integer result // display total and average of grades System.out.printf("%nTotal of all 10 grades is %d%n", total); Fig. 4.8 | Solving the class-average problem using counter-controlled repetition. (Part 1 of 2.) 4.9 Formulating Algorithms: Counter-Controlled Repetition 30 31 32 117 System.out.printf("Class average is %d%n", average); } } // end class ClassAverage Enter Enter Enter Enter Enter Enter Enter Enter Enter Enter grade: grade: grade: grade: grade: grade: grade: grade: grade: grade: 67 78 89 67 87 98 93 85 82 100 Total of all 10 grades is 846 Class average is 84 Fig. 4.8 | Solving the class-average problem using counter-controlled repetition. (Part 2 of 2.) Local Variables in Method main Line 10 declares and initializes Scanner variable input, which is used to read values entered by the user. Lines 13, 14, 20 and 26 declare local variables total, gradeCounter, grade and average, respectively, to be of type int. Variable grade stores the user input. These declarations appear in the body of method main. Recall that variables declared in a method body are local variables and can be used only from the line of their declaration to the closing right brace of the method declaration. A local variable’s declaration must appear before the variable is used in that method. A local variable cannot be accessed outside the method in which it’s declared. Variable grade, declared in the body of the while loop, can be used only in that block. Initialization Phase: Initializing Variables total and gradeCounter The assignments (in lines 13–14) initialize total to 0 and gradeCounter to 1. These initializations occur before the variables are used in calculations. Common Programming Error 4.3 Using the value of a local variable before it’s initialized results in a compilation error. All local variables must be initialized before their values are used in expressions. Error-Prevention Tip 4.3 Initialize each total and counter, either in its declaration or in an assignment statement. Totals are normally initialized to 0. Counters are normally initialized to 0 or 1, depending on how they’re used (we’ll show examples of when to use 0 and when to use 1). Processing Phase: Reading 10 Grades from the User Line 17 indicates that the while statement should continue looping (also called iterating) as long as gradeCounter’s value is less than or equal to 10. While this condition remains true, the while statement repeatedly executes the statements between the braces that delimit its body (lines 18–23). 118 Chapter 4 Control Statements: Part 1; Assignment, ++ and -- Operators Line 19 displays the prompt "Enter grade: ". Line 20 reads the grade entered by the user and assigns it to variable grade. Then line 21 adds the new grade entered by the user to the total and assigns the result to total, which replaces its previous value. Line 22 adds 1 to gradeCounter to indicate that the program has processed a grade and is ready to input the next grade from the user. Incrementing gradeCounter eventually causes it to exceed 10. Then the loop terminates, because its condition (line 17) becomes false. Termination Phase: Calculating and Displaying the Class Average When the loop terminates, line 26 performs the averaging calculation and assigns its result to the variable average. Line 29 uses System.out’s printf method to display the text "Total of all 10 grades is " followed by variable total’s value. Line 30 then uses printf to display the text "Class average is " followed by variable average’s value. When execution reaches line 31, the program terminates. Notice that this example contains only one class, with method main performing all the work. In this chapter and in Chapter 3, you’ve seen examples consisting of two classes—one containing instance variables and methods that perform tasks using those variables and one containing method main, which creates an object of the other class and calls its methods. Occasionally, when it does not make sense to try to create a reusable class to demonstrate a concept, we’ll place the program’s statements entirely within a single class’s main method. Notes on Integer Division and Truncation The averaging calculation performed by method main produces an integer result. The program’s output indicates that the sum of the grade values in the sample execution is 846, which, when divided by 10, should yield the floating-point number 84.6. However, the result of the calculation total / 10 (line 26 of Fig. 4.8) is the integer 84, because total and 10 are both integers. Dividing two integers results in integer division—any fractional part of the calculation is truncated (i.e., lost). In the next section we’ll see how to obtain a floating-point result from the averaging calculation. Common Programming Error 4.4 Assuming that integer division rounds (rather than truncates) can lead to incorrect results. For example, 7 ÷ 4, which yields 1.75 in conventional arithmetic, truncates to 1 in integer arithmetic, rather than rounding to 2. A Note About Arithmetic Overflow In Fig. 4.8, line 21 total = total + grade; // add grade to total added each grade entered by the user to the total. Even this simple statement has a potential problem—adding the integers could result in a value that’s too large to store in an int variable. This is known as arithmetic overflow and causes undefined behavior, which can lead to unintended results (http://en.wikipedia.org/wiki/Integer_overflow#Security_ ramifications). Figure 2.7’s Addition program had the same issue in line 23, which calculated the sum of two int values entered by the user: sum = number1 + number2; // add numbers, then store total in sum The maximum and minimum values that can be stored in an int variable are represented by the constants MIN_VALUE and MAX_VALUE, respectively, which are defined in class 4.10 Formulating Algorithms: Sentinel-Controlled Repetition 119 Integer. There are similar constants for the other integral types and for floating-point types. Each primitive type has a corresponding class type in package java.lang. You can see the values of these constants in each class’s online documentation. The online documentation for class Integer is located at: http://docs.oracle.com/javase/7/docs/api/java/lang/Integer.html It’s considered a good practice to ensure, before you perform arithmetic calculations like those in line 21 of Fig. 4.8 and line 23 of Fig. 2.7, that they will not overflow. The code for doing this is shown on the CERT website www.securecoding.cert.org—just search for guideline “NUM00-J.” The code uses the && (logical AND) and || (logical OR) operators, which are introduced in Chapter 5. In industrial-strength code, you should perform checks like these for all calculations. A Deeper Look at Receiving User Input Any time a program receives input from the user, various problems might occur. For example, in line 20 of Fig. 4.8 int grade = input.nextInt(); // input next grade we assume that the user will enter an integer grade in the range 0 to 100. However, the person entering a grade could enter an integer less than 0, an integer greater than 100, an integer outside the range of values that can be stored in an int variable, a number containing a decimal point or a value containing letters or special symbols that’s not even an integer. To ensure that inputs are valid, industrial-strength programs must test for all possible erroneous cases. A program that inputs grades should validate the grades by using range checking to ensure that hey are values from 0 to 100. You can then ask the user to reenter any value that’s out of range. If a program requires inputs from a specific set of values (e.g., nonsequential product codes), you can ensure that each input matches a value in the set. 4.10 Formulating Algorithms: Sentinel-Controlled Repetition Let’s generalize Section 4.9’s class-average problem. Consider the following problem: Develop a class-averaging program that processes grades for an arbitrary number of students each time it’s run. In the previous class-average example, the problem statement specified the number of students, so the number of grades (10) was known in advance. In this example, no indication is given of how many grades the user will enter during the program’s execution. The program must process an arbitrary number of grades. How can it determine when to stop the input of grades? How will it know when to calculate and print the class average? One way to solve this problem is to use a special value called a sentinel value (also called a signal value, a dummy value or a flag value) to indicate “end of data entry.” The user enters grades until all legitimate grades have been entered. The user then types the sentinel value to indicate that no more grades will be entered. Sentinel-controlled repetition is often called indefinite repetition because the number of repetitions is not known before the loop begins executing. 120 Chapter 4 Control Statements: Part 1; Assignment, ++ and -- Operators Clearly, a sentinel value must be chosen that cannot be confused with an acceptable input value. Grades on a quiz are nonnegative integers, so –1 is an acceptable sentinel value for this problem. Thus, a run of the class-average program might process a stream of inputs such as 95, 96, 75, 74, 89 and –1. The program would then compute and print the class average for the grades 95, 96, 75, 74 and 89; since –1 is the sentinel value, it should not enter into the averaging calculation. Developing the Pseudocode Algorithm with Top-Down, Stepwise Refinement: The Top and First Refinement We approach this class-average program with a technique called top-down, stepwise refinement, which is essential to the development of well-structured programs. We begin with a pseudocode representation of the top—a single statement that conveys the overall function of the program: Determine the class average for the quiz The top is, in effect, a complete representation of a program. Unfortunately, the top rarely conveys sufficient detail from which to write a Java program. So we now begin the refinement process. We divide the top into a series of smaller tasks and list these in the order in which they’ll be performed. This results in the following first refinement: Initialize variables Input, sum and count the quiz grades Calculate and print the class average This refinement uses only the sequence structure—the steps listed should execute in order, one after the other. Software Engineering Observation 4.3 Each refinement, as well as the top itself, is a complete specification of the algorithm— only the level of detail varies. Software Engineering Observation 4.4 Many programs can be divided logically into three phases: an initialization phase that initializes the variables; a processing phase that inputs data values and adjusts program variables accordingly; and a termination phase that calculates and outputs the final results. Proceeding to the Second Refinement The preceding Software Engineering Observation is often all you need for the first refinement in the top-down process. To proceed to the next level of refinement—that is, the second refinement—we commit to specific variables. In this example, we need a running total of the numbers, a count of how many numbers have been processed, a variable to receive the value of each grade as it’s input by the user and a variable to hold the calculated average. The pseudocode statement Initialize variables can be refined as follows: Initialize total to zero Initialize counter to zero 4.10 Formulating Algorithms: Sentinel-Controlled Repetition 121 Only the variables total and counter need to be initialized before they’re used. The variables average and grade (for the calculated average and the user input, respectively) need not be initialized, because their values will be replaced as they’re calculated or input. The pseudocode statement Input, sum and count the quiz grades requires repetition to successively input each grade. We do not know in advance how many grades will be entered, so we’ll use sentinel-controlled repetition. The user enters grades one at a time. After entering the last grade, the user enters the sentinel value. The program tests for the sentinel value after each grade is input and terminates the loop when the user enters the sentinel value. The second refinement of the preceding pseudocode statement is then Prompt the user to enter the first grade Input the first grade (possibly the sentinel) While the user has not yet entered the sentinel Add this grade into the running total Add one to the grade counter Prompt the user to enter the next grade Input the next grade (possibly the sentinel) In pseudocode, we do not use braces around the statements that form the body of the While structure. We simply indent the statements under the While to show that they belong to the While. Again, pseudocode is only an informal program development aid. The pseudocode statement Calculate and print the class average can be refined as follows: If the counter is not equal to zero Set the average to the total divided by the counter Print the average else Print “No grades were entered” We’re careful here to test for the possibility of division by zero—a logic error that, if undetected, would cause the program to fail or produce invalid output. The complete second refinement of the pseudocode for the class-average problem is shown in Fig. 4.9. Error-Prevention Tip 4.4 When performing division (/) or remainder (%) calculations in which the right operand could be zero, test for this and handle it (e.g., display an error message) rather than allowing the error to occur. 1 2 3 Initialize total to zero Initialize counter to zero Fig. 4.9 | Class-average pseudocode algorithm with sentinel-controlled repetition. (Part 1 of 2.) 122 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Chapter 4 Control Statements: Part 1; Assignment, ++ and -- Operators Prompt the user to enter the first grade Input the first grade (possibly the sentinel) While the user has not yet entered the sentinel Add this grade into the running total Add one to the grade counter Prompt the user to enter the next grade Input the next grade (possibly the sentinel) If the counter is not equal to zero Set the average to the total divided by the counter Print the average else Print “No grades were entered” Fig. 4.9 | Class-average pseudocode algorithm with sentinel-controlled repetition. (Part 2 of 2.) In Fig. 4.7 and Fig. 4.9, we included blank lines and indentation in the pseudocode to make it more readable. The blank lines separate the algorithms into their phases and set off control statements; the indentation emphasizes the bodies of the control statements. The pseudocode algorithm in Fig. 4.9 solves the more general class-average problem. This algorithm was developed after two refinements. Sometimes more are needed. Software Engineering Observation 4.5 Terminate the top-down, stepwise refinement process when you’ve specified the pseudocode algorithm in sufficient detail for you to convert the pseudocode to Java. Normally, implementing the Java program is then straightforward. Software Engineering Observation 4.6 Some programmers do not use program development tools like pseudocode. They feel that their ultimate goal is to solve the problem on a computer and that writing pseudocode merely delays the production of final outputs. Although this may work for simple and familiar problems, it can lead to serious errors and delays in large, complex projects. Implementing Sentinel-Controlled Repetition In Fig. 4.10, method main (lines 7–46) implements the pseudocode algorithm of Fig. 4.9. Although each grade is an integer, the averaging calculation is likely to produce a number with a decimal point—in other words, a real (floating-point) number. The type int cannot represent such a number, so this class uses type double to do so. You’ll also see that control statements may be stacked on top of one another (in sequence). The while statement (lines 22–30) is followed in sequence by an if…else statement (lines 34–45). Much of the code in this program is identical to that in Fig. 4.8, so we concentrate on the new concepts. 1 2 3 // Fig. 4.10: ClassAverage.java // Solving the class-average problem using sentinel-controlled repetition. import java.util.Scanner; // program uses class Scanner Fig. 4.10 | Solving the class-average problem using sentinel-controlled repetition. (Part 1 of 2.) 4.10 Formulating Algorithms: Sentinel-Controlled Repetition 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 123 public class ClassAverage { public static void main(String[] args) { // create Scanner to obtain input from command window Scanner input = new Scanner(System.in); // initialization phase int total = 0; // initialize sum of grades int gradeCounter = 0; // initialize # of grades entered so far // processing phase // prompt for input and read grade from user System.out.print("Enter grade or -1 to quit: "); int grade = input.nextInt(); // loop until sentinel value read from user while (grade != -1) { total = total + grade; // add grade to total gradeCounter = gradeCounter + 1; // increment counter // prompt for input and read next grade from user System.out.print("Enter grade or -1 to quit: "); grade = input.nextInt(); } // termination phase // if user entered at least one grade... if (gradeCounter != 0) { // use number with decimal point to calculate average of grades double average = (double) total / gradeCounter; // display total and average (with two digits of precision) System.out.printf("%nTotal of the %d grades entered is %d%n", gradeCounter, total); System.out.printf("Class average is %.2f%n", average); } else // no grades were entered, so output appropriate message System.out.println("No grades were entered"); } } // end class ClassAverage Enter Enter Enter Enter grade grade grade grade or or or or -1 -1 -1 -1 to to to to quit: quit: quit: quit: 97 88 72 -1 Total of the 3 grades entered is 257 Class average is 85.67 Fig. 4.10 | Solving the class-average problem using sentinel-controlled repetition. (Part 2 of 2.) 124 Chapter 4 Control Statements: Part 1; Assignment, ++ and -- Operators Program Logic for Sentinel-Controlled Repetition vs. Counter-Controlled Repetition Line 37 declares double variable average, which allows us to store the class average as a floating-point number. Line 14 initializes gradeCounter to 0, because no grades have been entered yet. Remember that this program uses sentinel-controlled repetition to input the grades. To keep an accurate record of the number of grades entered, the program increments gradeCounter only when the user enters a valid grade. Compare the program logic for sentinel-controlled repetition in this application with that for counter-controlled repetition in Fig. 4.8. In counter-controlled repetition, each iteration of the while statement (lines 17–23 of Fig. 4.8) reads a value from the user, for the specified number of iterations. In sentinel-controlled repetition, the program reads the first value (lines 18–19 of Fig. 4.10) before reaching the while. This value determines whether the program’s flow of control should enter the body of the while. If the condition of the while is false, the user entered the sentinel value, so the body of the while does not execute (i.e., no grades were entered). If, on the other hand, the condition is true, the body begins execution, and the loop adds the grade value to the total and increments the gradeCounter (lines 24–25). Then lines 28–29 in the loop body input the next value from the user. Next, program control reaches the closing right brace of the loop body at line 30, so execution continues with the test of the while’s condition (line 22). The condition uses the most recent grade input by the user to determine whether the loop body should execute again. The value of variable grade is always input from the user immediately before the program tests the while condition. This allows the program to determine whether the value just input is the sentinel value before the program processes that value (i.e., adds it to the total). If the sentinel value is input, the loop terminates, and the program does not add –1 to the total. Good Programming Practice 4.3 In a sentinel-controlled loop, prompts should remind the user of the sentinel. After the loop terminates, the if…else statement at lines 34–45 executes. The condition at line 34 determines whether any grades were input. If none were input, the else part (lines 44–45) of the if…else statement executes and displays the message "No grades were entered" and the method returns control to the calling method. Braces in a while statement Notice the while statement’s block in Fig. 4.10 (lines 23–30). Without the braces, the loop would consider its body to be only the first statement, which adds the grade to the total. The last three statements in the block would fall outside the loop body, causing the computer to interpret the code incorrectly as follows: while (grade != -1) total = total + grade; // add grade to total gradeCounter = gradeCounter + 1; // increment counter // prompt for input and read next grade from user System.out.print("Enter grade or -1 to quit: "); grade = input.nextInt(); The preceding code would cause an infinite loop in the program if the user did not input the sentinel -1 at line 19 (before the while statement). 4.10 Formulating Algorithms: Sentinel-Controlled Repetition 125 Common Programming Error 4.5 Omitting the braces that delimit a block can lead to logic errors, such as infinite loops. To prevent this problem, some programmers enclose the body of every control statement in braces, even if the body contains only a single statement. Explicitly and Implicitly Converting Between Primitive Types If at least one grade was entered, line 37 of Fig. 4.10 calculates the average of the grades. Recall from Fig. 4.8 that integer division yields an integer result. Even though variable average is declared as a double, if we had written the averaging calculation as double average = total / gradeCounter; it would lose the fractional part of the quotient before the result of the division is assigned to average. This occurs because total and gradeCounter are both integers, and integer division yields an integer result. Most averages are not whole numbers (e.g., 0, –22 and 1024). For this reason, we calculate the class average in this example as a floating-point number. To perform a floatingpoint calculation with integer values, we must temporarily treat these values as floatingpoint numbers for use in the calculation. Java provides the unary cast operator to accomplish this task. Line 37 of Fig. 4.10 uses the (double) cast operator—a unary operator— to create a temporary floating-point copy of its operand total (which appears to the right of the operator). Using a cast operator in this manner is called explicit conversion or type casting. The value stored in total is still an integer. The calculation now consists of a floating-point value (the temporary double copy of total) divided by the integer gradeCounter. Java can evaluate only arithmetic expressions in which the operands’ types are identical. To ensure this, Java performs an operation called promotion (or implicit conversion) on selected operands. For example, in an expression containing int and double values, the int values are promoted to double values for use in the expression. In this example, the value of gradeCounter is promoted to type double, then floating-point division is performed and the result of the calculation is assigned to average. As long as the (double) cast operator is applied to any variable in the calculation, the calculation will yield a double result. Later in this chapter, we discuss all the primitive types. You’ll learn more about the promotion rules in Section 6.7. Common Programming Error 4.6 A cast operator can be used to convert between primitive numeric types, such as int and double, and between related reference types (as we discuss in Chapter 10, Object-Oriented Programming: Polymorphism and Interfaces). Casting to the wrong type may cause compilation errors or runtime errors. A cast operator is formed by placing parentheses around any type’s name. The operator is a unary operator (i.e., an operator that takes only one operand). Java also supports unary versions of the plus (+) and minus (–) operators, so you can write expressions like 7 or +5. Cast operators associate from right to left and have the same precedence as other unary operators, such as unary + and unary -. This precedence is one level higher than that of the multiplicative operators *, / and %. (See the operator precedence chart in Appendix A.) We indicate the cast operator with the notation (type) in our precedence charts, to indicate that any type name can be used to form a cast operator. 126 Chapter 4 Control Statements: Part 1; Assignment, ++ and -- Operators Line 42 displays the class average. In this example, we display the class average rounded to the nearest hundredth. The format specifier %.2f in printf’s format control string indicates that variable average’s value should be displayed with two digits of precision to the right of the decimal point—indicated by.2 in the format specifier. The three grades entered during the sample execution (Fig. 4.10) total 257, which yields the average 85.666666…. Method printf uses the precision in the format specifier to round the value to the specified number of digits. In this program, the average is rounded to the hundredths position and is displayed as 85.67. Floating-Point Number Precision Floating-point numbers are not always 100% precise, but they have numerous applications. For example, when we speak of a “normal” body temperature of 98.6, we do not need to be precise to a large number of digits. When we read the temperature on a thermometer as 98.6, it may actually be 98.5999473210643. Calling this number simply 98.6 is fine for most applications involving body temperatures. Floating-point numbers often arise as a result of division, such as in this example’s class-average calculation. In conventional arithmetic, when we divide 10 by 3, the result is 3.3333333…, with the sequence of 3s repeating infinitely. The computer allocates only a fixed amount of space to hold such a value, so clearly the stored floating-point value can be only an approximation. Owing to the imprecise nature of floating-point numbers, type double is preferred over type float, because double variables can represent floating-point numbers more accurately. For this reason, we primarily use type double throughout the book. In some applications, the precision of float and double variables will be inadequate. For precise floating-point numbers (such as those required by monetary calculations), Java provides class BigDecimal (package java.math), which we’ll discuss in Chapter 8. Common Programming Error 4.7 Using floating-point numbers in a manner that assumes they’re represented precisely can lead to incorrect results. 4.11 Formulating Algorithms: Nested Control Statements For the next example, we once again formulate an algorithm by using pseudocode and topdown, stepwise refinement, and write a corresponding Java program. We’ve seen that control statements can be stacked on top of one another (in sequence). In this case study, we examine the only other structured way control statements can be connected—namely, by nesting one control statement within another. Consider the following problem statement: A college offers a course that prepares students for the state licensing exam for real-estate brokers. Last year, ten of the students who completed this course took the exam. The college wants to know how well its students did on the exam. You’ve been asked to write a program to summarize the results. You’ve been given a list of these 10 students. Next to each name is written a 1 if the student passed the exam or a 2 if the student failed. Your program should analyze the results of the exam as follows: 1. Input each test result (i.e., a 1 or a 2). Display the message “Enter result” on the screen each time the program requests another test result. 4.11 Formulating Algorithms: Nested Control Statements 127 2. Count the number of test results of each type. 3. Display a summary of the test results, indicating the number of students who passed and the number who failed. 4. If more than eight students passed the exam, print “Bonus to instructor!” After reading the problem statement carefully, we make the following observations: 1. The program must process test results for 10 students. A counter-controlled loop can be used, because the number of test results is known in advance. 2. Each test result has a numeric value—either a 1 or a 2. Each time it reads a test result, the program must determine whether it’s a 1 or a 2. We test for a 1 in our algorithm. If the number is not a 1, we assume that it’s a 2. (Exercise 4.24 considers the consequences of this assumption.) 3. Two counters are used to keep track of the exam results—one to count the number of students who passed the exam and one to count the number who failed. 4. After the program has processed all the results, it must decide whether more than eight students passed the exam. Let’s proceed with top-down, stepwise refinement. We begin with a pseudocode representation of the top: Analyze exam results and decide whether a bonus should be paid Once again, the top is a complete representation of the program, but several refinements are likely to be needed before the pseudocode can evolve naturally into a Java program. Our first refinement is Initialize variables Input the 10 exam results, and count passes and failures Print a summary of the exam results and decide whether a bonus should be paid Here, too, even though we have a complete representation of the entire program, further refinement is necessary. We now commit to specific variables. Counters are needed to record the passes and failures, a counter will be used to control the looping process and a variable is needed to store the user input. The variable in which the user input will be stored is not initialized at the start of the algorithm, because its value is read from the user during each iteration of the loop. The pseudocode statement Initialize variables can be refined as follows: Initialize passes to zero Initialize failures to zero Initialize student counter to one Notice that only the counters are initialized at the start of the algorithm. The pseudocode statement Input the 10 exam results, and count passes and failures 128 Chapter 4 Control Statements: Part 1; Assignment, ++ and -- Operators requires a loop that successively inputs the result of each exam. We know in advance that there are precisely 10 exam results, so counter-controlled looping is appropriate. Inside the loop (i.e., nested within the loop), a double-selection structure will determine whether each exam result is a pass or a failure and will increment the appropriate counter. The refinement of the preceding pseudocode statement is then While student counter is less than or equal to 10 Prompt the user to enter the next exam result Input the next exam result If the student passed Add one to passes Else Add one to failures Add one to student counter We use blank lines to isolate the If…Else control structure, which improves readability. The pseudocode statement Print a summary of the exam results and decide whether a bonus should be paid can be refined as follows: Print the number of passes Print the number of failures If more than eight students passed Print “Bonus to instructor!” Complete Second Refinement of Pseudocode and Conversion to Class Analysis The complete second refinement appears in Fig. 4.11. Notice that blank lines are also used to set off the While structure for program readability. This pseudocode is now sufficiently refined for conversion to Java. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Initialize passes to zero Initialize failures to zero Initialize student counter to one While student counter is less than or equal to 10 Prompt the user to enter the next exam result Input the next exam result If the student passed Add one to passes Else Add one to failures Add one to student counter Fig. 4.11 | Pseudocode for examination-results problem. (Part 1 of 2.) 4.11 Formulating Algorithms: Nested Control Statements 15 16 17 18 19 20 129 Print the number of passes Print the number of failures If more than eight students passed Print “Bonus to instructor!” Fig. 4.11 | Pseudocode for examination-results problem. (Part 2 of 2.) The Java class that implements the pseudocode algorithm and two sample executions are shown in Fig. 4.12. Lines 13, 14, 15 and 22 of main declare the variables that are used to process the examination results. Error-Prevention Tip 4.5 Initializing local variables when they’re declared helps you avoid any compilation errors that might arise from attempts to use uninitialized variables. While Java does not require that local-variable initializations be incorporated into declarations, it does require that local variables be initialized before their values are used in an expression. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 // Fig. 4.12: Analysis.java // Analysis of examination results using nested control statements. import java.util.Scanner; // class uses class Scanner public class Analysis { public static void main(String[] args) { // create Scanner to obtain input from command window Scanner input = new Scanner(System.in); // initializing variables in declarations int passes = 0; int failures = 0; int studentCounter = 1; // process 10 students using counter-controlled loop while (studentCounter <= 10) { // prompt user for input and obtain value from user System.out.print("Enter result (1 = pass, 2 = fail): "); int result = input.nextInt(); // if...else is nested in the while statement if (result == 1) passes = passes + 1; else failures = failures + 1; Fig. 4.12 | Analysis of examination results using nested control statements. (Part 1 of 2.) 130 30 31 32 33 34 35 36 37 38 39 40 41 Chapter 4 Control Statements: Part 1; Assignment, ++ and -- Operators // increment studentCounter so loop eventually terminates studentCounter = studentCounter + 1; } // termination phase; prepare and display results System.out.printf("Passed: %d%nFailed: %d%n", passes, failures); // determine whether more than 8 students passed if (passes > 8) System.out.println("Bonus to instructor!"); } } // end class Analysis Enter result (1 = pass, Enter result (1 = pass, Enter result (1 = pass, Enter result (1 = pass, Enter result (1 = pass, Enter result (1 = pass, Enter result (1 = pass, Enter result (1 = pass, Enter result (1 = pass, Enter result (1 = pass, Passed: 9 Failed: 1 Bonus to instructor! 2 2 2 2 2 2 2 2 2 2 = = = = = = = = = = fail): fail): fail): fail): fail): fail): fail): fail): fail): fail): 1 2 1 1 1 1 1 1 1 1 Enter result Enter result Enter result Enter result Enter result Enter result Enter result Enter result Enter result Enter result Passed: 6 Failed: 4 2 2 2 2 2 2 2 2 2 2 = = = = = = = = = = fail): fail): fail): fail): fail): fail): fail): fail): fail): fail): 1 2 1 2 1 2 2 1 1 1 (1 (1 (1 (1 (1 (1 (1 (1 (1 (1 = = = = = = = = = = pass, pass, pass, pass, pass, pass, pass, pass, pass, pass, Fig. 4.12 | Analysis of examination results using nested control statements. (Part 2 of 2.) The while statement (lines 18–32) loops 10 times. During each iteration, the loop inputs and processes one exam result. Notice that the if…else statement (lines 25–28) for processing each result is nested in the while statement. If the result is 1, the if…else statement increments passes; otherwise, it assumes the result is 2 and increments failures. Line 31 increments studentCounter before the loop condition is tested again at line 18. After 10 values have been input, the loop terminates and line 35 displays the number of passes and failures. The if statement at lines 38–39 determines whether more than eight students passed the exam and, if so, outputs the message "Bonus to instructor!". 4.12 Compound Assignment Operators 131 Figure 4.12 shows the input and output from two sample excutions of the program. During the first, the condition at line 38 of method main is true—more than eight students passed the exam, so the program outputs a message to bonus the instructor. 4.12 Compound Assignment Operators The compound assignment operators abbreviate assignment expressions. Statements like variable = variable operator expression; where operator is one of the binary operators +, -, *, / or % (or others we discuss later in the text) can be written in the form variable operator = expression; For example, you can abbreviate the statement c = c + 3; with the addition compound assignment operator, +=, as c += 3; The += operator adds the value of the expression on its right to the value of the variable on its left and stores the result in the variable on the left of the operator. Thus, the assignment expression c += 3 adds 3 to c. Figure 4.13 shows the arithmetic compound assignment operators, sample expressions using the operators and explanations of what the operators do. Assignment operator Sample expression Explanation Assigns Assume: int c = 3, d = 5, e = 4, f = 6, g = 12; += c += 7 c = c + 7 10 to c -= d -= 4 d = d - 4 1 to d *= e *= 5 e = e * 5 20 to e /= f /= 3 f = f / 3 2 to f %= g %= 9 g = g % 9 3 to g Fig. 4.13 | Arithmetic compound assignment operators. 4.13 Increment and Decrement Operators Java provides two unary operators (summarized in Fig. 4.14) for adding 1 to or subtracting 1 from the value of a numeric variable. These are the unary increment operator, ++, and the unary decrement operator, --. A program can increment by 1 the value of a variable called c using the increment operator, ++, rather than the expression c = c + 1 or c += 1. An increment or decrement operator that’s prefixed to (placed before) a variable is referred to as the prefix increment or prefix decrement operator, respectively. An increment or decrement operator that’s postfixed to (placed after) a variable is referred to as the postfix increment or postfix decrement operator, respectively. Chapter 4 Control Statements: Part 1; Assignment, ++ and -- Operators 132 Operator ++ ++ --- Operator name Sample expression prefix increment postfix increment prefix decrement postfix decrement ++a a++ --b b-- Explanation Increment a by 1, then use the new value of a in the expression in which a resides. Use the current value of a in the expression in which a resides, then increment a by 1. Decrement b by 1, then use the new value of b in the expression in which b resides. Use the current value of b in the expression in which b resides, then decrement b by 1. Fig. 4.14 | Increment and decrement operators. Using the prefix increment (or decrement) operator to add 1 to (or subtract 1 from) a variable is known as preincrementing (or predecrementing). This causes the variable to be incremented (decremented) by 1; then the new value of the variable is used in the expression in which it appears. Using the postfix increment (or decrement) operator to add 1 to (or subtract 1 from) a variable is known as postincrementing (or postdecrementing). This causes the current value of the variable to be used in the expression in which it appears; then the variable’s value is incremented (decremented) by 1. Good Programming Practice 4.4 Unlike binary operators, the unary increment and decrement operators should be placed next to their operands, with no intervening spaces. Difference Between Prefix Increment and Postfix Increment Operators Figure 4.15 demonstrates the difference between the prefix increment and postfix increment versions of the ++ increment operator. The decrement operator (--) works similarly. Line 9 initializes the variable c to 5, and line 10 outputs c’s initial value. Line 11 outputs the value of the expression c++. This expression postincrements the variable c, so c’s original value (5) is output, then c’s value is incremented (to 6). Thus, line 11 outputs c’s initial value (5) again. Line 12 outputs c’s new value (6) to prove that the variable’s value was indeed incremented in line 11. Line 17 resets c’s value to 5, and line 18 outputs c’s value. Line 19 outputs the value of the expression ++c. This expression preincrements c, so its value is incremented; then the new value (6) is output. Line 20 outputs c’s value again to show that the value of c is still 6 after line 19 executes. 1 2 3 4 5 // Fig. 4.15: Increment.java // Prefix increment and postfix increment operators. public class Increment { Fig. 4.15 | Prefix increment and postfix increment operators. (Part 1 of 2.) 4.13 Increment and Decrement Operators 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 133 public static void main(String[] args) { // demonstrate postfix increment operator int c = 5; System.out.printf("c before postincrement: %d%n", c); // prints 5 System.out.printf(" postincrementing c: %d%n", c++); // prints 5 System.out.printf(" c after postincrement: %d%n", c); // prints 6 System.out.println(); // skip a line // demonstrate prefix increment operator c = 5; System.out.printf(" c before preincrement: %d%n", c); // prints 5 System.out.printf(" preincrementing c: %d%n", ++c); // prints 6 System.out.printf(" c after preincrement: %d%n", c); // prints 6 } } // end class Increment c before postincrement: 5 postincrementing c: 5 c after postincrement: 6 c before preincrement: 5 preincrementing c: 6 c after preincrement: 6 Fig. 4.15 | Prefix increment and postfix increment operators. (Part 2 of 2.) Simplifying Statements with the Arithmetic Compound Assignment, Increment and Decrement Operators The arithmetic compound assignment operators and the increment and decrement operators can be used to simplify program statements. For example, the three assignment statements in Fig. 4.12 (lines 26, 28 and 31) passes = passes + 1; failures = failures + 1; studentCounter = studentCounter + 1; can be written more concisely with compound assignment operators as passes += 1; failures += 1; studentCounter += 1; with prefix increment operators as ++passes; ++failures; ++studentCounter; or with postfix increment operators as passes++; failures++; studentCounter++; Chapter 4 Control Statements: Part 1; Assignment, ++ and -- Operators 134 When incrementing or decrementing a variable in a statement by itself, the prefix increment and postfix increment forms have the same effect, and the prefix decrement and postfix decrement forms have the same effect. It’s only when a variable appears in the context of a larger expression that preincrementing and postincrementing the variable have different effects (and similarly for predecrementing and postdecrementing). Common Programming Error 4.8 Attempting to use the increment or decrement operator on an expression other than one to which a value can be assigned is a syntax error. For example, writing ++(x + 1) is a syntax error, because (x + 1) is not a variable. Operator Precedence and Associativity Figure 4.16 shows the precedence and associativity of the operators we’ve introduced. They’re shown from top to bottom in decreasing order of precedence. The second column describes the associativity of the operators at each level of precedence. The conditional operator (?:); the unary operators increment (++), decrement (--), plus (+) and minus (-); the cast operators and the assignment operators =, +=, -=, *=, /= and %= associate from right to left. All the other operators in the operator precedence chart in Fig. 4.16 associate from left to right. The third column lists the type of each group of operators. Good Programming Practice 4.5 Refer to the operator precedence and associativity chart (Appendix A) when writing expressions containing many operators. Confirm that the operators in the expression are performed in the order you expect. If you’re uncertain about the order of evaluation in a complex expression, break the expression into smaller statements or use parentheses to force the order of evaluation, exactly as you’d do in an algebraic expression. Be sure to observe that some operators such as assignment (=) associate right to left rather than left to right. Operators ++ -- ++ -- + * / % + - < <= == != - > >= -= *= (type) ?: = += /= %= Associativity Type right to left right to left left to right left to right left to right left to right right to left right to left unary postfix unary prefix multiplicative additive relational equality conditional assignment Fig. 4.16 | Precedence and associativity of the operators discussed so far. 4.14 Primitive Types The table in Appendix D lists the eight primitive types in Java. Like its predecessor languages C and C++, Java requires all variables to have a type. For this reason, Java is referred to as a strongly typed language. 4.15 (Optional) GUI and Graphics Case Study: Creating Simple Drawings 135 In C and C++, programmers frequently have to write separate versions of programs to support different computer platforms, because the primitive types are not guaranteed to be identical from computer to computer. For example, an int on one machine might be represented by 16 bits (2 bytes) of memory, on a second machine by 32 bits (4 bytes), and on another machine by 64 bits (8 bytes). In Java, int values are always 32 bits (4 bytes). Portability Tip 4.1 The primitive types in Java are portable across all computer platforms that support Java. Each type in Appendix D is listed with its size in bits (there are eight bits to a byte) and its range of values. Because the designers of Java want to ensure portability, they use internationally recognized standards for character formats (Unicode; for more information, visit www.unicode.org) and floating-point numbers (IEEE 754; for more information, visit grouper.ieee.org/groups/754/). Recall from Section 3.2 that variables of primitive types declared outside of a method as instance variables of a class are automatically assigned default values unless explicitly initialized. Instance variables of types char, byte, short, int, long, float and double are all given the value 0 by default. Instance variables of type boolean are given the value false by default. Reference-type instance variables are initialized by default to the value null. 4.15 (Optional) GUI and Graphics Case Study: Creating Simple Drawings An appealing feature of Java is its graphics support, which enables you to visually enhance your applications. We now introduce one of Java’s graphical capabilities—drawing lines. It also covers the basics of creating a window to display a drawing on the computer screen. Java’s Coordinate System To draw in Java, you must understand Java’s coordinate system (Fig. 4.17), a scheme for identifying points on the screen. By default, the upper-left corner of a GUI component has the coordinates (0, 0). A coordinate pair is composed of an x-coordinate (the horizontal coordinate) and a y-coordinate (the vertical coordinate). The x-coordinate is the horizontal location moving from left to right. The y-coordinate is the vertical location moving from top to bottom. The x-axis describes every horizontal coordinate, and the y-axis every vertical coordinate. Coordinates indicate where graphics should be displayed on a screen. Coordinate units are measured in pixels. The term pixel stands for “picture element.” A pixel is a display monitor’s smallest unit of resolution. +x (0, 0) +y x-axis (x, y) y-axis Fig. 4.17 | Java coordinate system. Units are measured in pixels. 136 Chapter 4 Control Statements: Part 1; Assignment, ++ and -- Operators First Drawing Application Our first drawing application simply draws two lines. Class DrawPanel (Fig. 4.18) performs the actual drawing, while class DrawPanelTest (Fig. 4.19) creates a window to display the drawing. In class DrawPanel, the import statements in lines 3–4 allow us to use class Graphics (from package java.awt), which provides various methods for drawing text and shapes onto the screen, and class JPanel (from package javax.swing), which provides an area on which we can draw. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 // Fig. 4.18: DrawPanel.java // Using drawLine to connect the corners of a panel. import java.awt.Graphics; import javax.swing.JPanel; public class DrawPanel extends JPanel { // draws an X from the corners of the panel public void paintComponent(Graphics g) { // call paintComponent to ensure the panel displays correctly super.paintComponent(g); int width = getWidth(); // total width int height = getHeight(); // total height // draw a line from the upper-left to the lower-right g.drawLine(0, 0, width, height); // draw a line from the lower-left to the upper-right g.drawLine(0, height, width, 0); } } // end class DrawPanel Fig. 4.18 | Using drawLine to connect the corners of a panel. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 // Fig. 4.19: DrawPanelTest.java // Creating JFrame to display DrawPanel. import javax.swing.JFrame; public class DrawPanelTest { public static void main(String[] args) { // create a panel that contains our drawing DrawPanel panel = new DrawPanel(); // create a new frame to hold the panel JFrame application = new JFrame(); // set the frame to exit when it is closed application.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); Fig. 4.19 | Creating JFrame to display DrawPanel. (Part 1 of 2.) 4.15 (Optional) GUI and Graphics Case Study: Creating Simple Drawings 17 18 19 20 21 22 137 application.add(panel); // add the panel to the frame application.setSize(250, 250); // set the size of the frame application.setVisible(true); // make the frame visible } } // end class DrawPanelTest Fig. 4.19 | Creating JFrame to display DrawPanel. (Part 2 of 2.) Line 6 uses the keyword extends to indicate that class DrawPanel is an enhanced type of JPanel. The keyword extends represents a so-called inheritance relationship in which our new class DrawPanel begins with the existing members (data and methods) from class JPanel. The class from which DrawPanel inherits, JPanel, appears to the right of keyword extends. In this inheritance relationship, JPanel is called the superclass and DrawPanel is called the subclass. This results in a DrawPanel class that has the attributes (data) and behaviors (methods) of class JPanel as well as the new features we’re adding in our DrawPanel class declaration—specifically, the ability to draw two lines along the diagonals of the panel. Inheritance is explained in detail in Chapter 9. For now, you should mimic our DrawPanel class when creating your own graphics programs. Method paintComponent Every JPanel, including our DrawPanel, has a paintComponent method (lines 9–22), which the system automatically calls every time it needs to display the DrawPanel. Method paintComponent must be declared as shown in line 9—otherwise, the system will not call it. This method is called when a JPanel is first displayed on the screen, when it’s covered then uncovered by a window on the screen, and when the window in which it appears is resized. Method paintComponent requires one argument, a Graphics object, that’s provided by the system when it calls paintComponent. This Graphics object is used to draw lines, rectangles, ovals and other graphics. The first statement in every paintComponent method you create should always be super.paintComponent(g); which ensures that the panel is properly rendered before we begin drawing on it. Next, lines 14–15 call methods that class DrawPanel inherits from JPanel. Because DrawPanel extends JPanel, DrawPanel can use any public methods of JPanel. Methods getWidth and getHeight return the JPanel’s width and height, respectively. Lines 14–15 store these values in the local variables width and height. Finally, lines 18 and 21 use the Graphics variable g to call method drawLine to draw the two lines. Method drawLine draws a line 138 Chapter 4 Control Statements: Part 1; Assignment, ++ and -- Operators between two points represented by its four arguments. The first two arguments are the xand y-coordinates for one endpoint, and the last two arguments are the coordinates for the other endpoint. If you resize the window, the lines will scale accordingly, because the arguments are based on the width and height of the panel. Resizing the window in this application causes the system to call paintComponent to redraw the DrawPanel’s contents. Class DrawPanelTest To display the DrawPanel on the screen, you must place it in a window. You create a window with an object of class JFrame. In DrawPanelTest.java (Fig. 4.19), line 3 imports class JFrame from package javax.swing. Line 10 in main creates a DrawPanel object, which contains our drawing, and line 13 creates a new JFrame that can hold and display our panel. Line 16 calls JFrame method setDefaultCloseOperation with the argument JFrame.EXIT_ON_CLOSE to indicate that the application should terminate when the user closes the window. Line 18 uses class JFrame’s add method to attach the DrawPanel to the JFrame. Line 19 sets the size of the JFrame. Method setSize takes two parameters that represent the width and height of the JFrame, respectively. Finally, line 20 displays the JFrame by calling its setVisible method with the argument true. When the JFrame is displayed, the DrawPanel’s paintComponent method (lines 9–22 of Fig. 4.18) is implicitly called, and the two lines are drawn (see the sample outputs in Fig. 4.19). Try resizing the window to see that the lines always draw based on the window’s current width and height. GUI and Graphics Case Study Exercises 4.1 Using loops and control statements to draw lines can lead to many interesting designs. a) Create the design in the left screen capture of Fig. 4.20. This design draws lines from the top-left corner, fanning them out until they cover the upper-left half of the panel. One approach is to divide the width and height into an equal number of steps (we found 15 steps worked well). The first endpoint of a line will always be in the top-left corner (0, 0). The second endpoint can be found by starting at the bottom-left corner and moving up one vertical step and right one horizontal step. Draw a line between the two endpoints. Continue moving up and to the right one step to find each successive endpoint. The figure should scale accordingly as you resize the window. Fig. 4.20 | Lines fanning from a corner. 4.16 Wrap-Up 139 b) Modify part (a) to have lines fan out from all four corners, as shown in the right screen capture of Fig. 4.20. Lines from opposite corners should intersect along the middle. 4.2 Figure 4.21 displays two additional designs created using while loops and drawLine. a) Create the design in the left screen capture of Fig. 4.21. Begin by dividing each edge into an equal number of increments (we chose 15 again). The first line starts in the topleft corner and ends one step right on the bottom edge. For each successive line, move down one increment on the left edge and right one increment on the bottom edge. Continue drawing lines until you reach the bottom-right corner. The figure should scale as you resize the window so that the endpoints always touch the edges. b) Modify your answer in part (a) to mirror the design in all four corners, as shown in the right screen capture of Fig. 4.21. Fig. 4.21 | Line art with loops and drawLine. 4.16 Wrap-Up This chapter presented basic problem solving for building classes and developing methods for these classes. We demonstrated how to construct an algorithm (i.e., an approach to solving a problem), then how to refine the algorithm through several phases of pseudocode development, resulting in Java code that can be executed as part of a method. The chapter showed how to use top-down, stepwise refinement to plan out the specific actions that a method must perform and the order in which the method must perform these actions. Only three types of control structures—sequence, selection and repetition—are needed to develop any problem-solving algorithm. Specifically, this chapter demonstrated the if single-selection statement, the if…else double-selection statement and the while repetition statement. These are some of the building blocks used to construct solutions to many problems. We used control-statement stacking to total and compute the average of a set of student grades with counter- and sentinel-controlled repetition, and we used control-statement nesting to analyze and make decisions based on a set of exam results. We introduced Java’s compound assignment operators and its increment and decrement operators. Finally, we discussed Java’s primitive types. In Chapter 5, we continue our discussion of control statements, introducing the for, do…while and switch statements. 140 Chapter 4 Control Statements: Part 1; Assignment, ++ and -- Operators Summary Section 4.1 Introduction • Before writing a program to solve a problem, you must have a thorough understanding of the problem and a carefully planned approach to solving it. You must also understand the building blocks that are available and employ proven program-construction techniques. Section 4.2 Algorithms • Any computing problem can be solved by executing a series of actions (p. 102) in a specific order. • A procedure for solving a problem in terms of the actions to execute and the order in which they execute is called an algorithm (p. 102). • Specifying the order in which statements execute in a program is called program control (p. 102). Section 4.3 Pseudocode • Pseudocode (p. 103) is an informal language that helps you develop algorithms without having to worry about the strict details of Java language syntax. • The pseudocode we use in this book is similar to everyday English—it’s convenient and user friendly, but it’s not an actual computer programming language. You may, of course, use your own native language(s) to develop your own pseudocode. • Pseudocode helps you “think out” a program before attempting to write it in a programming language, such as Java. • Carefully prepared pseudocode can easily be converted to a corresponding Java program. Section 4.4 Control Structures • Normally, statements in a program are executed one after the other in the order in which they’re written. This process is called sequential execution (p. 103). • Various Java statements enable you to specify that the next statement to execute is not necessarily the next one in sequence. This is called transfer of control (p. 103). • Bohm and Jacopini demonstrated that all programs could be written in terms of only three control structures (p. 103)—the sequence structure, the selection structure and the repetition structure. • The term “control structures” comes from the field of computer science. The Java Language Specification refers to “control structures” as “control statements” (p. 104). • The sequence structure is built into Java. Unless directed otherwise, the computer executes Java statements one after the other in the order in which they’re written—that is, in sequence. • Anywhere a single action may be placed, several actions may be placed in sequence. • Activity diagrams (p. 104) are part of the UML. An activity diagram models the workflow (p. 104; also called the activity) of a portion of a software system. • Activity diagrams are composed of symbols (p. 104)—such as action-state symbols, diamonds and small circles—that are connected by transition arrows, which represent the flow of the activity. • Action states (p. 104) contain action expressions that specify particular actions to perform. • The arrows in an activity diagram represent transitions, which indicate the order in which the actions represented by the action states occur. • The solid circle located at the top of an activity diagram represents the activity’s initial state (p. 104)—the beginning of the workflow before the program performs the modeled actions. • The solid circle surrounded by a hollow circle that appears at the bottom of the diagram represents the final state (p. 104)—the end of the workflow after the program performs its actions. Summary 141 • Rectangles with their upper-right corners folded over are UML notes (p. 104)—explanatory remarks that describe the purpose of symbols in the diagram. • Java has three types of selection statements (p. 105). • The if single-selection statement (p. 105) selects or ignores one or more actions. • The if…else double-selection statement selects between two actions or groups of actions. • The switch statement is called a multiple-selection statement (p. 105) because it selects among many different actions or groups of actions. • Java provides the while, do…while and for repetition (also called iteration or looping) statements that enable programs to perform statements repeatedly as long as a loop-continuation condition remains true. • The while and for statements perform the action(s) in their bodies zero or more times—if the loop-continuation condition (p. 105) is initially false, the action(s) will not execute. The do…while statement performs the action(s) in its body one or more times. • The words if, else, switch, while, do and for are Java keywords. Keywords cannot be used as identifiers, such as variable names. • Every program is formed by combining as many sequence, selection and repetition statements (p. 105) as is appropriate for the algorithm the program implements. • Single-entry/single-exit control statements (p. 105) are attached to one another by connecting the exit point of one to the entry point of the next. This is known as control-statement stacking. • A control statement may also be nested (p. 105) inside another control statement. Section 4.5 if Single-Selection Statement • Programs use selection statements to choose among alternative courses of action. • The single-selection if statement’s activity diagram contains the diamond symbol, which indicates that a decision is to be made. The workflow follows a path determined by the symbol’s associated guard conditions (p. 106). If a guard condition is true, the workflow enters the action state to which the corresponding transition arrow points. • The if statement is a single-entry/single-exit control statement. Section 4.6 if…else Double-Selection Statement • The if single-selection statement performs an indicated action only when the condition is true. • The if…else double-selection (p. 105) statement performs one action when the condition is true and another action when the condition is false. • A program can test multiple cases with nested if…else statements (p. 107). • The conditional operator (p. 110; ?:) is Java’s only ternary operator—it takes three operands. Together, the operands and the ?: symbol form a conditional expression (p. 110). • The Java compiler associates an else with the immediately preceding if unless told to do otherwise by the placement of braces. • The if statement expects one statement in its body. To include several statements in the body of an if (or the body of an else for an if…else statement), enclose the statements in braces. • A block (p. 110) of statements can be placed anywhere that a single statement can be placed. • A logic error (p. 110) has its effect at execution time. A fatal logic error (p. 110) causes a program to fail and terminate prematurely. A nonfatal logic error (p. 110) allows a program to continue executing, but causes it to produce incorrect results. 142 Chapter 4 Control Statements: Part 1; Assignment, ++ and -- Operators • Just as a block can be placed anywhere a single statement can be placed, you can also use an empty statement, represented by placing a semicolon (;) where a statement would normally be. Section 4.8 while Repetition Statement • The while repetition statement (p. 114) allows you to specify that a program should repeat an action while some condition remains true. • The UML’s merge (p. 114) symbol joins two flows of activity into one. • The decision and merge symbols can be distinguished by the number of incoming and outgoing transition arrows. A decision symbol has one transition arrow pointing to the diamond and two or more transition arrows pointing out from the diamond to indicate possible transitions from that point. Each transition arrow pointing out of a decision symbol has a guard condition. A merge symbol has two or more transition arrows pointing to the diamond and only one transition arrow pointing from the diamond, to indicate multiple activity flows merging to continue the activity. None of the transition arrows associated with a merge symbol has a guard condition. Section 4.9 Formulating Algorithms: Counter-Controlled Repetition • Counter-controlled repetition (p. 115) uses a variable called a counter (or control variable) to control the number of times a set of statements execute. • Counter-controlled repetition is often called definite repetition (p. 115), because the number of repetitions is known before the loop begins executing. • A total (p. 115) is a variable used to accumulate the sum of several values. Variables used to store totals are normally initialized to zero before being used in a program. • A local variable’s declaration must appear before the variable is used in that method. A local variable cannot be accessed outside the method in which it’s declared. • Dividing two integers results in integer division—the calculation’s fractional part is truncated. Section 4.10 Formulating Algorithms: Sentinel-Controlled Repetition • In sentinel-controlled repetition (p. 119), a special value called a sentinel value (also called a signal value, a dummy value or a flag value) is used to indicate “end of data entry.” • A sentinel value must be chosen that cannot be confused with an acceptable input value. • Top-down, stepwise refinement (p. 120) is essential to the development of well-structured programs. • Division by zero is a logic error. • To perform a floating-point calculation with integer values, cast one of the integers to type double. • Java knows how to evaluate only arithmetic expressions in which the operands’ types are identical. To ensure this, Java performs an operation called promotion on selected operands. • The unary cast operator is formed by placing parentheses around the name of a type. Section 4.12 Compound Assignment Operators • The compound assignment operators (p. 131) abbreviate assignment expressions. Statements of the form variable = variable operator expression; where operator is one of the binary operators +, -, *, / or %, can be written in the form variable operator= expression; • The += operator adds the value of the expression on the right of the operator to the value of the variable on the left of the operator and stores the result in the variable on the left of the operator. Self-Review Exercises 143 Section 4.13 Increment and Decrement Operators • The unary increment operator, ++, and the unary decrement operator, --, add 1 to or subtract 1 from the value of a numeric variable (p. 131). • An increment or decrement operator that’s prefixed (p. 131) to a variable is the prefix increment or prefix decrement operator, respectively. An increment or decrement operator that’s postfixed (p. 131) to a variable is the postfix increment or postfix decrement operator, respectively. • Using the prefix increment or decrement operator to add or subtract 1 is known as preincrementing or predecrementing, respectively. • Preincrementing or predecrementing a variable causes the variable to be incremented or decremented by 1; then the new value of the variable is used in the expression in which it appears. • Using the postfix increment or decrement operator to add or subtract 1 is known as postincrementing or postdecrementing, respectively. • Postincrementing or postdecrementing the variable causes its value to be used in the expression in which it appears; then the variable’s value is incremented or decremented by 1. • When incrementing or decrementing a variable in a statement by itself, the prefix and postfix increment have the same effect, and the prefix and postfix decrement have the same effect. Section 4.14 Primitive Types • Java requires all variables to have a type. Thus, Java is referred to as a strongly typed language (p. 134). • Java uses Unicode characters and IEEE 754 floating-point numbers. Self-Review Exercises 4.1 Fill in the blanks in each of the following statements: , a) All programs can be written in terms of three types of control structures: and . b) The statement is used to execute one action when a condition is true and another when that condition is false. repetition. c) Repeating a set of instructions a specific number of times is called d) When it’s not known in advance how many times a set of statements will be repeated, value can be used to terminate the repetition. a(n) e) The structure is built into Java; by default, statements execute in the order they appear. f) Instance variables of types char, byte, short, int, long, float and double are all given the value by default. g) Java is a(n) language; it requires all variables to have a type. to a variable, first the variable is incremented by h) If the increment operator is 1, then its new value is used in the expression. 4.2 State whether each of the following is true or false. If false, explain why. a) An algorithm is a procedure for solving a problem in terms of the actions to execute and the order in which they execute. b) A set of statements contained within a pair of parentheses is called a block. c) A selection statement specifies that an action is to be repeated while some condition remains true. d) A nested control statement appears in the body of another control statement. e) Java provides the arithmetic compound assignment operators +=, -=, *=, /= and %= for abbreviating assignment expressions. Chapter 4 Control Statements: Part 1; Assignment, ++ and -- Operators 144 f) The primitive types (boolean, char, byte, short, int, long, float and double) are portable across only Windows platforms. g) Specifying the order in which statements execute in a program is called program control. h) The unary cast operator (double) creates a temporary integer copy of its operand. i) Instance variables of type boolean are given the value true by default. j) Pseudocode helps you think out a program before attempting to write it in a programming language. 4.3 Write four different Java statements that each add 1 to integer variable x. 4.4 Write Java statements to accomplish each of the following tasks: a) Use one statement to assign the sum of x and y to z, then increment x by 1. b) Test whether variable count is greater than 10. If it is, print "Count is greater than 10". c) Use one statement to decrement the variable x by 1, then subtract it from variable total and store the result in variable total. d) Calculate the remainder after q is divided by divisor, and assign the result to q. Write this statement in two different ways. 4.5 Write a Java statement to accomplish each of the following tasks: a) Declare variables sum of type int and initialize it to 0. b) Declare variables x of type int and initialize it to 1. c) Add variable x to variable sum, and assign the result to variable sum. d) Print "The sum is: ", followed by the value of variable sum. 4.6 Combine the statements that you wrote in Exercise 4.5 into a Java application that calculates and prints the sum of the integers from 1 to 10. Use a while statement to loop through the calculation and increment statements. The loop should terminate when the value of x becomes 11. 4.7 Determine the value of the variables in the statement product *= x++; after the calculation is performed. Assume that all variables are type int and initially have the value 5. 4.8 Identify and correct the errors in each of the following sets of code: a) while (c <= 5) { product *= c; ++c; b) if (gender == 1) System.out.println("Woman"); else; System.out.println("Man"); 4.9 What is wrong with the following while statement? while (z >= 0) sum += z; Answers to Self-Review Exercises 4.1 a) sequence, selection, repetition. b) if…else. c) counter-controlled (or definite). d) sentinel, signal, flag or dummy. e) sequence. f) 0 (zero). g) strongly typed. h) prefixed. 4.2 a) True. b) False. A set of statements contained within a pair of braces ({ and }) is called a block. c) False. A repetition statement specifies that an action is to be repeated while some condition remains true. d) True. e) True. f) False. The primitive types (boolean, char, byte, short, int, long, float and double) are portable across all computer platforms that support Java. g) True. h) False. The unary cast operator (double) creates a temporary floating-point copy of its operand. i) False. Instance variables of type boolean are given the value false by default. j) True. Exercises 4.3 145 x = x + 1; x += 1; ++x; x++; 4.4 a) b) z = x++ + y; c) d) total -= --x; if (count > 10) System.out.println("Count is greater than 10"); q %= divisor; q = q % divisor; 4.5 4.6 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 a) b) c) d) int sum = 0; int x = 1; sum += x; or sum = sum + x; System.out.printf("The sum is: %d%n", sum); The program is as follows: // Exercise 4.6: Calculate.java // Calculate the sum of the integers from 1 to 10 public class Calculate { public static void main(String[] args) { int sum = 0; int x = 1; while (x <= 10) // while x is less than or equal to 10 { sum += x; // add x to sum ++x; // increment x } System.out.printf("The sum is: %d%n", sum); } } // end class Calculate The sum is: 55 = 25, x = 6 4.7 product 4.8 a) Error: The closing right brace of the while statement’s body is missing. Correction: Add a closing right brace after the statement ++c;. b) Error: The semicolon after else results in a logic error. The second output statement will always be executed. Correction: Remove the semicolon after else. 4.9 The value of the variable z is never changed in the while statement. Therefore, if the loopcontinuation condition (z >= 0) is true, an infinite loop is created. To prevent an infinite loop from occurring, z must be decremented so that it eventually becomes less than 0. Exercises 4.10 Compare and contrast the if single-selection statement and the while repetition statement. How are these two statements similar? How are they different? Chapter 4 Control Statements: Part 1; Assignment, ++ and -- Operators 146 4.11 Explain what happens when a Java program attempts to divide one integer by another. What happens to the fractional part of the calculation? How can you avoid that outcome? 4.12 Describe the two ways in which control statements can be combined. 4.13 What type of repetition would be appropriate for calculating the sum of the first 100 positive integers? What type would be appropriate for calculating the sum of an arbitrary number of positive integers? Briefly describe how each of these tasks could be performed. 4.14 What is the difference between preincrementing and postincrementing a variable? 4.15 Identify and correct the errors in each of the following pieces of code. [Note: There may be more than one error in each piece of code.] a) if (age >= 65); System.out.println("Age is greater than or equal to 65"); else System.out.println("Age is less than 65)"; b) int x = 1, total; while (x <= 10) { total += x; ++x; } c) while (x <= 100) total += x; ++x; d) while (y > 0) { System.out.println(y); ++y; 4.16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 What does the following program print? // Exercise 4.16: Mystery.java public class Mystery { public static void main(String[] args) { int x = 1; int total = 0; while (x <= 10) { int y = x * x; System.out.println(y); total += y; ++x; } System.out.printf("Total is %d%n", total); } } // end class Mystery For Exercise 4.17 through Exercise 4.20, perform each of the following steps: a) Read the problem statement. b) Formulate the algorithm using pseudocode and top-down, stepwise refinement. c) Write a Java program. Exercises 147 d) Test, debug and execute the Java program. e) Process three complete sets of data. 4.17 (Gas Mileage) Drivers are concerned with the mileage their automobiles get. One driver has kept track of several trips by recording the miles driven and gallons used for each tankful. Develop a Java application that will input the miles driven and gallons used (both as integers) for each trip. The program should calculate and display the miles per gallon obtained for each trip and print the combined miles per gallon obtained for all trips up to this point. All averaging calculations should produce floating-point results. Use class Scanner and sentinel-controlled repetition to obtain the data from the user. 4.18 (Credit Limit Calculator) Develop a Java application that determines whether any of several department-store customers has exceeded the credit limit on a charge account. For each customer, the following facts are available: a) account number b) balance at the beginning of the month c) total of all items charged by the customer this month d) total of all credits applied to the customer’s account this month e) allowed credit limit. The program should input all these facts as integers, calculate the new balance (= beginning balance + charges – credits), display the new balance and determine whether the new balance exceeds the customer’s credit limit. For those customers whose credit limit is exceeded, the program should display the message "Credit limit exceeded". 4.19 (Sales Commission Calculator) A large company pays its salespeople on a commission basis. The salespeople receive $200 per week plus 9% of their gross sales for that week. For example, a salesperson who sells $5,000 worth of merchandise in a week receives $200 plus 9% of $5000, or a total of $650. You’ve been supplied with a list of the items sold by each salesperson. The values of these items are as follows: Item 1 2 3 4 Value 239.99 129.75 99.95 350.89 Develop a Java application that inputs one salesperson’s items sold for last week and calculates and displays that salesperson’s earnings. There’s no limit to the number of items that can be sold. 4.20 (Salary Calculator) Develop a Java application that determines the gross pay for each of three employees. The company pays straight time for the first 40 hours worked by each employee and time and a half for all hours worked in excess of 40. You’re given a list of the employees, their number of hours worked last week and their hourly rates. Your program should input this information for each employee, then determine and display the employee’s gross pay. Use class Scanner to input the data. 4.21 (Find the Largest Number) The process of finding the largest value is used frequently in computer applications. For example, a program that determines the winner of a sales contest would input the number of units sold by each salesperson. The salesperson who sells the most units wins the contest. Write a pseudocode program, then a Java application that inputs a series of 10 integers and determines and prints the largest integer. Your program should use at least the following three variables: a) counter: A counter to count to 10 (i.e., to keep track of how many numbers have been input and to determine when all 10 numbers have been processed). b) number: The integer most recently input by the user. c) largest: The largest number found so far. 148 Chapter 4 Control Statements: Part 1; Assignment, ++ and -- Operators 4.22 (Tabular Output) Write a Java application that uses looping to print the following table of values: N 10*N 100*N 1000*N 1 2 3 4 5 10 20 30 40 50 100 200 300 400 500 1000 2000 3000 4000 5000 4.23 (Find the Two Largest Numbers) Using an approach similar to that for Exercise 4.21, find the two largest values of the 10 values entered. [Note: You may input each number only once.] 4.24 (Validating User Input) Modify the program in Fig. 4.12 to validate its inputs. For any input, if the value entered is other than 1 or 2, keep looping until the user enters a correct value. 4.25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 // Exercise 4.25: Mystery2.java public class Mystery2 { public static void main(String[] args) { int count = 1; while (count <= 10) { System.out.println(count % 2 == 1 ? "****" : "++++++++"); ++count; } } } // end class Mystery2 4.26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 What does the following program print? What does the following program print? // Exercise 4.26: Mystery3.java public class Mystery3 { public static void main(String[] args) { int row = 10; while (row >= 1) { int column = 1; while (column <= 10) { System.out.print(row % 2 == 1 ? "<" : ">"); ++column; } --row; System.out.println(); } } } // end class Mystery3 Exercises 149 4.27 (Dangling-else Problem) Determine the output for each of the given sets of code when x is 9 and y is 11 and when x is 11 and y is 9. The compiler ignores the indentation in a Java program. Also, the Java compiler always associates an else with the immediately preceding if unless told to do otherwise by the placement of braces ({}). On first glance, you may not be sure which if a particular else matches—this situation is referred to as the “dangling-else problem.” We’ve eliminated the indentation from the following code to make the problem more challenging. [Hint: Apply the indentation conventions you’ve learned.] a) if (x < 10) if (y > 10) System.out.println("*****"); else System.out.println("#####"); System.out.println("$$$$$"); b) if (x < 10) { if (y > 10) System.out.println("*****"); } else { System.out.println("#####"); System.out.println("$$$$$"); } 4.28 (Another Dangling-else Problem) Modify the given code to produce the output shown in each part of the problem. Use proper indentation techniques. Make no changes other than inserting braces and changing the indentation of the code. The compiler ignores indentation in a Java program. We’ve eliminated the indentation from the given code to make the problem more challenging. [Note: It’s possible that no modification is necessary for some of the parts.] if (y == 8) if (x == 5) System.out.println("@@@@@"); else System.out.println("#####"); System.out.println("$$$$$"); System.out.println("&&&&&"); a) Assuming that x = 5 and y = 8, the following output is produced: @@@@@ $$$$$ &&&&& b) Assuming that x = 5 and y = 8, the following output is produced: @@@@@ c) Assuming that x = 5 and y = 8, the following output is produced: @@@@@ d) Assuming that x = 5 and y = 7, the following output is produced. [Note: The last three output statements after the else are all part of a block.] ##### $$$$$ &&&&& 150 Chapter 4 Control Statements: Part 1; Assignment, ++ and -- Operators 4.29 (Square of Asterisks) Write an application that prompts the user to enter the size of the side of a square, then displays a hollow square of that size made of asterisks. Your program should work for squares of all side lengths between 1 and 20. 4.30 (Palindromes) A palindrome is a sequence of characters that reads the same backward as forward. For example, each of the following five-digit integers is a palindrome: 12321, 55555, 45554 and 11611. Write an application that reads in a five-digit integer and determines whether it’s a palindrome. If the number is not five digits long, display an error message and allow the user to enter a new value. 4.31 (Printing the Decimal Equivalent of a Binary Number) Write an application that inputs an integer containing only 0s and 1s (i.e., a binary integer) and prints its decimal equivalent. [Hint: Use the remainder and division operators to pick off the binary number’s digits one at a time, from right to left. In the decimal number system, the rightmost digit has a positional value of 1 and the next digit to the left a positional value of 10, then 100, then 1000, and so on. The decimal number 234 can be interpreted as 4 * 1 + 3 * 10 + 2 * 100. In the binary number system, the rightmost digit has a positional value of 1, the next digit to the left a positional value of 2, then 4, then 8, and so on. The decimal equivalent of binary 1101 is 1 * 1 + 0 * 2 + 1 * 4 + 1 * 8, or 1 + 0 + 4 + 8 or, 13.] 4.32 (Checkerboard Pattern of Asterisks) Write an application that uses only the output statements System.out.print("* "); System.out.print(" "); System.out.println(); to display the checkerboard pattern that follows. A System.out.println method call with no arguments causes the program to output a single newline character. [Hint: Repetition statements are required.] * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * 4.33 (Multiples of 2 with an Infinite Loop) Write an application that keeps displaying in the command window the multiples of the integer 2—namely, 2, 4, 8, 16, 32, 64, and so on. Your loop should not terminate (i.e., it should create an infinite loop). What happens when you run this program? 4.34 (What’s Wrong with This Code?) What is wrong with the following statement? Provide the correct statement to add one to the sum of x and y. System.out.println(++(x + y)); 4.35 (Sides of a Triangle) Write an application that reads three nonzero values entered by the user and determines and prints whether they could represent the sides of a triangle. 4.36 (Sides of a Right Triangle) Write an application that reads three nonzero integers and determines and prints whether they could represent the sides of a right triangle. 4.37 (Factorial) The factorial of a nonnegative integer n is written as n! (pronounced “n factorial”) and is defined as follows: Making a Difference 151 n! = n · (n – 1) · (n – 2) · … · 1 (for values of n greater than or equal to 1) and n! = 1 (for n = 0) For example, 5! = 5 · 4 · 3 · 2 · 1, which is 120. a) Write an application that reads a nonnegative integer and computes and prints its factorial. b) Write an application that estimates the value of the mathematical constant e by using the following formula. Allow the user to enter the number of terms to calculate. 1 1 1 e = 1 + ----- + ----- + ----- + … 1! 2! 3! c) Write an application that computes the value of ex by using the following formula. Allow the user to enter the number of terms to calculate. 2 3 x x x x e = 1 + ----- + ----- + ----- + … 1! 2! 3! Making a Difference 4.38 (Enforcing Privacy with Cryptography) The explosive growth of Internet communications and data storage on Internet-connected computers has greatly increased privacy concerns. The field of cryptography is concerned with coding data to make it difficult (and hopefully—with the most advanced schemes—impossible) for unauthorized users to read. In this exercise you’ll investigate a simple scheme for encrypting and decrypting data. A company that wants to send data over the Internet has asked you to write a program that will encrypt it so that it may be transmitted more securely. All the data is transmitted as four-digit integers. Your application should read a four-digit integer entered by the user and encrypt it as follows: Replace each digit with the result of adding 7 to the digit and getting the remainder after dividing the new value by 10. Then swap the first digit with the third, and swap the second digit with the fourth. Then print the encrypted integer. Write a separate application that inputs an encrypted four-digit integer and decrypts it (by reversing the encryption scheme) to form the original number. [Optional reading project: Research “public key cryptography” in general and the PGP (Pretty Good Privacy) specific public key scheme. You may also want to investigate the RSA scheme, which is widely used in industrial-strength applications.] 4.39 (World Population Growth) World population has grown considerably over the centuries. Continued growth could eventually challenge the limits of breathable air, drinkable water, arable cropland and other limited resources. There’s evidence that growth has been slowing in recent years and that world population could peak some time this century, then start to decline. For this exercise, research world population growth issues online. Be sure to investigate various viewpoints. Get estimates for the current world population and its growth rate (the percentage by which it’s likely to increase this year). Write a program that calculates world population growth each year for the next 75 years, using the simplifying assumption that the current growth rate will stay constant. Print the results in a table. The first column should display the year from year 1 to year 75. The second column should display the anticipated world population at the end of that year. The third column should display the numerical increase in the world population that would occur that year. Using your results, determine the year in which the population would be double what it is today, if this year’s growth rate were to persist. 5 The wheel is come full circle. —William Shakespeare All the evolution we know of proceeds from the vague to the definite. —Charles Sanders Peirce Objectives In this chapter you’ll: ■ Learn the essentials of counter-controlled repetition. ■ Use the for and do…while repetition statements to execute statements in a program repeatedly. ■ Understand multiple selection using the switch selection statement. ■ Use the break and continue program control statements to alter the flow of control. ■ Use the logical operators to form complex conditional expressions in control statements. Control Statements: Part 2; Logical Operators 5.1 Introduction 5.1 Introduction 5.2 Essentials of Counter-Controlled Repetition 5.3 for Repetition Statement 5.4 Examples Using the for Statement 5.5 do…while Repetition Statement 5.6 switch Multiple-Selection Statement 153 5.7 Class AutoPolicy Case Study: Strings in switch Statements 5.8 break and continue Statements 5.9 Logical Operators 5.10 Structured Programming Summary 5.11 (Optional) GUI and Graphics Case Study: Drawing Rectangles and Ovals 5.12 Wrap-Up Summary | Self-Review Exercises | Answers to Self-Review Exercises | Exercises | Making a Difference 5.1 Introduction This chapter continues our presentation of structured programming theory and principles by introducing all but one of Java’s remaining control statements. We demonstrate Java’s for, do…while and switch statements. Through a series of short examples using while and for, we explore the essentials of counter-controlled repetition. We use a switch statement to count the number of A, B, C, D and F grade equivalents in a set of numeric grades entered by the user. We introduce the break and continue program-control statements. We discuss Java’s logical operators, which enable you to use more complex conditional expressions in control statements. Finally, we summarize Java’s control statements and the proven problem-solving techniques presented in this chapter and Chapter 4. 5.2 Essentials of Counter-Controlled Repetition This section uses the while repetition statement introduced in Chapter 4 to formalize the elements required to perform counter-controlled repetition, which requires 1. a control variable (or loop counter) 2. the initial value of the control variable 3. the increment by which the control variable is modified each time through the loop (also known as each iteration of the loop) 4. the loop-continuation condition that determines if looping should continue. To see these elements of counter-controlled repetition, consider the application of Fig. 5.1, which uses a loop to display the numbers from 1 through 10. 1 2 3 4 5 6 7 // Fig. 5.1: WhileCounter.java // Counter-controlled repetition with the while repetition statement. public class WhileCounter { public static void main(String[] args) { Fig. 5.1 | Counter-controlled repetition with the while repetition statement. (Part 1 of 2.) 154 8 9 10 11 12 13 14 15 16 17 18 1 Chapter 5 Control Statements: Part 2; Logical Operators int counter = 1; // declare and initialize control variable while (counter <= 10) // loop-continuation condition { System.out.printf("%d ", counter); ++counter; // increment control variable } System.out.println(); } } // end class WhileCounter 2 3 4 5 6 7 8 9 10 Fig. 5.1 | Counter-controlled repetition with the while repetition statement. (Part 2 of 2.) In Fig. 5.1, the elements of counter-controlled repetition are defined in lines 8, 10 and 13. Line 8 declares the control variable (counter) as an int, reserves space for it in memory and sets its initial value to 1. Variable counter also could have been declared and initialized with the following local-variable declaration and assignment statements: int counter; // declare counter counter = 1; // initialize counter to 1 Line 12 displays control variable counter’s value during each iteration of the loop. Line 13 increments the control variable by 1 for each iteration of the loop. The loop-continuation condition in the while (line 10) tests whether the value of the control variable is less than or equal to 10 (the final value for which the condition is true). The program performs the body of this while even when the control variable is 10. The loop terminates when the control variable exceeds 10 (i.e., counter becomes 11). Common Programming Error 5.1 Because floating-point values may be approximate, controlling loops with floating-point variables may result in imprecise counter values and inaccurate termination tests. Error-Prevention Tip 5.1 Use integers to control counting loops. The program in Fig. 5.1 can be made more concise by initializing counter to 0 in line 8 and preincrementing counter in the while condition as follows: while (++counter <= 10) // loop-continuation condition System.out.printf("%d ", counter); This code saves a statement, because the while condition performs the increment before testing the condition. (Recall from Section 4.13 that the precedence of ++ is higher than that of <=.) Coding in such a condensed fashion takes practice, might make code more difficult to read, debug, modify and maintain, and typically should be avoided. Software Engineering Observation 5.1 “Keep it simple” is good advice for most of the code you’ll write. 5.3 for Repetition Statement 155 5.3 for Repetition Statement Section 5.2 presented the essentials of counter-controlled repetition. The while statement can be used to implement any counter-controlled loop. Java also provides the for repetition statement, which specifies the counter-controlled-repetition details in a single line of code. Figure 5.2 reimplements the application of Fig. 5.1 using for. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 1 // Fig. 5.2: ForCounter.java // Counter-controlled repetition with the for repetition statement. public class ForCounter { public static void main(String[] args) { // for statement header includes initialization, // loop-continuation condition and increment for (int counter = 1; counter <= 10; counter++) System.out.printf("%d ", counter); System.out.println(); } } // end class ForCounter 2 3 4 5 6 7 8 9 10 Fig. 5.2 | Counter-controlled repetition with the for repetition statement. When the for statement (lines 10–11) begins executing, the control variable counter is declared and initialized to 1. (Recall from Section 5.2 that the first two elements of counter-controlled repetition are the control variable and its initial value.) Next, the program checks the loop-continuation condition, counter <= 10, which is between the two required semicolons. Because the initial value of counter is 1, the condition initially is true. Therefore, the body statement (line 11) displays control variable counter’s value, namely 1. After executing the loop’s body, the program increments counter in the expression counter++, which appears to the right of the second semicolon. Then the loop-continuation test is performed again to determine whether the program should continue with the next iteration of the loop. At this point, the control variable’s value is 2, so the condition is still true (the final value is not exceeded)—thus, the program performs the body statement again (i.e., the next iteration of the loop). This process continues until the numbers 1 through 10 have been displayed and the counter’s value becomes 11, causing the loop-continuation test to fail and repetition to terminate (after 10 repetitions of the loop body). Then the program performs the first statement after the for—in this case, line 13. Figure 5.2 uses (in line 10) the loop-continuation condition counter <= 10. If you incorrectly specified counter < 10 as the condition, the loop would iterate only nine times. This is a common logic error called an off-by-one error. Common Programming Error 5.2 Using an incorrect relational operator or an incorrect final value of a loop counter in the loop-continuation condition of a repetition statement can cause an off-by-one error. 156 Chapter 5 Control Statements: Part 2; Logical Operators Error-Prevention Tip 5.2 Using the final value and operator <= in a loop’s condition helps avoid off-by-one errors. For a loop that outputs 1 to 10, the loop-continuation condition should be counter <= 10 rather than counter < 10 (which causes an off-by-one error) or counter < 11 (which is correct). Many programmers prefer so-called zero-based counting, in which to count 10 times, counter would be initialized to zero and the loop-continuation test would be counter < 10. Error-Prevention Tip 5.3 As Chapter 4 mentioned, integers can overflow, causing logic errors. A loop’s control variable also could overflow. Write your loop conditions carefully to prevent this. A Closer Look at the for Statement’s Header Figure 5.3 takes a closer look at the for statement in Fig. 5.2. The first line—including the keyword for and everything in parentheses after for (line 10 in Fig. 5.2)—is sometimes called the for statement header. The for header “does it all”—it specifies each item needed for counter-controlled repetition with a control variable. If there’s more than one statement in the body of the for, braces are required to define the body of the loop. for keyword Control variable Required semicolon Required semicolon for (int counter = 1; counter <= 10; counter++) Initial value of control variable Loop-continuation condition Incrementing of control variable o Fig. 5.3 | for statement header components. General Format of a for Statement The general format of the for statement is for (initialization; loopContinuationCondition; increment) statement where the initialization expression names the loop’s control variable and optionally provides its initial value, loopContinuationCondition determines whether the loop should continue executing and increment modifies the control variable’s value, so that the loop-continuation condition eventually becomes false. The two semicolons in the for header are required. If the loop-continuation condition is initially false, the program does not execute the for statement’s body. Instead, execution proceeds with the statement following the for. Representing a for Statement with an Equivalent while Statement The for statement often can be represented with an equivalent while statement as follows: initialization; while (loopContinuationCondition) { statement increment; } 5.3 for Repetition Statement 157 In Section 5.8, we show a case in which a for statement cannot be represented with an equivalent while statement. Typically, for statements are used for counter-controlled repetition and while statements for sentinel-controlled repetition. However, while and for can each be used for either repetition type. Scope of a for Statement’s Control Variable If the initialization expression in the for header declares the control variable (i.e., the control variable’s type is specified before the variable name, as in Fig. 5.2), the control variable can be used only in that for statement—it will not exist outside it. This restricted use is known as the variable’s scope. The scope of a variable defines where it can be used in a program. For example, a local variable can be used only in the method that declares it and only from the point of declaration through the end of the method. Scope is discussed in detail in Chapter 6, Methods: A Deeper Look. Common Programming Error 5.3 When a for statement’s control variable is declared in the initialization section of the for’s header, using the control variable after the for’s body is a compilation error. Expressions in a for Statement’s Header Are Optional All three expressions in a for header are optional. If the loopContinuationCondition is omitted, Java assumes that the loop-continuation condition is always true, thus creating an infinite loop. You might omit the initialization expression if the program initializes the control variable before the loop. You might omit the increment expression if the program calculates the increment with statements in the loop’s body or if no increment is needed. The increment expression in a for acts as if it were a standalone statement at the end of the for’s body. Therefore, the expressions counter = counter + 1 counter += 1 ++counter counter++ are equivalent increment expressions in a for statement. Many programmers prefer counbecause it’s concise and because a for loop evaluates its increment expression after its body executes, so the postfix increment form seems more natural. In this case, the variable being incremented does not appear in a larger expression, so preincrementing and postincrementing actually have the same effect. ter++ Common Programming Error 5.4 Placing a semicolon immediately to the right of the right parenthesis of a for header makes that for’s body an empty statement. This is normally a logic error. Error-Prevention Tip 5.4 Infinite loops occur when the loop-continuation condition in a repetition statement never becomes false. To prevent this situation in a counter-controlled loop, ensure that the control variable is modified during each iteration of the loop so that the loop-continuation condition will eventually become false. In a sentinel-controlled loop, ensure that the sentinel value is able to be input. 158 Chapter 5 Control Statements: Part 2; Logical Operators Placing Arithmetic Expressions in a for Statement’s Header The initialization, loop-continuation condition and increment portions of a for statement can contain arithmetic expressions. For example, assume that x = 2 and y = 10. If x and y are not modified in the body of the loop, the statement for (int j = x; j <= 4 * x * y; j += y / x) is equivalent to the statement for (int j = 2; j <= 80; j += 5) The increment of a for statement may also be negative, in which case it’s a decrement, and the loop counts downward. Using a for Statement’s Control Variable in the Statement’s Body Programs frequently display the control-variable value or use it in calculations in the loop body, but this use is not required. The control variable is commonly used to control repetition without being mentioned in the body of the for. Error-Prevention Tip 5.5 Although the value of the control variable can be changed in the body of a for loop, avoid doing so, because this practice can lead to subtle errors. UML Activity Diagram for the for Statement The for statement’s UML activity diagram is similar to that of the while statement (Fig. 4.6). Figure 5.4 shows the activity diagram of the for statement in Fig. 5.2. The diagram makes it clear that initialization occurs once before the loop-continuation test is evaluated the first time, and that incrementing occurs each time through the loop after the body statement executes. Initialize control variable int counter = [counter <= 10] 1 Display the counter value Increment the control variable [counter > 10] Determine whether looping should continue counter++ System.out.printf(“%d Fig. 5.4 | UML activity diagram for the for statement in Fig. 5.2. ”, counter); 5.4 Examples Using the for Statement 159 5.4 Examples Using the for Statement The following examples show techniques for varying the control variable in a for statement. In each case, we write only the appropriate for header. Note the change in the relational operator for the loops that decrement the control variable. a) Vary the control variable from 1 to 100 in increments of 1. for (int i = 1; i <= 100; i++) b) Vary the control variable from 100 to 1 in decrements of 1. for (int i = 100; i >= 1; i--) c) Vary the control variable from 7 to 77 in increments of 7. for (int i = 7; i <= 77; i += 7) d) Vary the control variable from 20 to 2 in decrements of 2. for (int i = 20; i >= 2; i -= 2) e) Vary the control variable over the values 2, 5, 8, 11, 14, 17, 20. for (int i = 2; i <= 20; i += 3) f) Vary the control variable over the values 99, 88, 77, 66, 55, 44, 33, 22, 11, 0. for (int i = 99; i >= 0; i -= 11) Common Programming Error 5.5 Using an incorrect relational operator in the loop-continuation condition of a loop that counts downward (e.g., using i <= 1 instead of i >= 1 in a loop counting down to 1) is usually a logic error. Common Programming Error 5.6 Do not use equality operators (!= or ==) in a loop-continuation condition if the loop’s control variable increments or decrements by more than 1. For example, consider the for statement header for (int counter = 1; counter != 10; counter += 2). The loop-continuation test counter != 10 never becomes false (resulting in an infinite loop) because counter increments by 2 after each iteration. Application: Summing the Even Integers from 2 to 20 We now consider two sample applications that demonstrate simple uses of for. The application in Fig. 5.5 uses a for statement to sum the even integers from 2 to 20 and store the result in an int variable called total. 1 2 3 4 5 // Fig. 5.5: Sum.java // Summing integers with the for statement. public class Sum { Fig. 5.5 | Summing integers with the for statement. (Part 1 of 2.) 160 6 7 8 9 10 11 12 13 14 15 16 Chapter 5 Control Statements: Part 2; Logical Operators public static void main(String[] args) { int total = 0; // total even integers from 2 through 20 for (int number = 2; number <= 20; number += 2) total += number; System.out.printf("Sum is %d%n", total); } } // end class Sum Sum is 110 Fig. 5.5 | Summing integers with the for statement. (Part 2 of 2.) The initialization and increment expressions can be comma-separated lists that enable you to use multiple initialization expressions or multiple increment expressions. For example, although this is discouraged, you could merge the body of the for statement in lines 11–12 of Fig. 5.5 into the increment portion of the for header by using a comma as follows: for (int number = 2; number <= 20; total += number, number += 2) ; // empty statement Good Programming Practice 5.1 For readability limit the size of control-statement headers to a single line if possible. Application: Compound-Interest Calculations Let’s use the for statement to compute compound interest. Consider the following problem: A person invests $1,000 in a savings account yielding 5% interest. Assuming that all the interest is left on deposit, calculate and print the amount of money in the account at the end of each year for 10 years. Use the following formula to determine the amounts: a = p (1 + r) n where p is the original amount invested (i.e., the principal) r is the annual interest rate (e.g., use 0.05 for 5%) n is the number of years a is the amount on deposit at the end of the nth year. The solution to this problem (Fig. 5.6) involves a loop that performs the indicated calculation for each of the 10 years the money remains on deposit. Lines 8–10 in method main declare double variables amount, principal and rate, and initialize principal to 1000.0 and rate to 0.05. Java treats floating-point constants like 1000.0 and 0.05 as type double. Similarly, Java treats whole-number constants like 7 and -22 as type int. 5.4 Examples Using the for Statement 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 161 // Fig. 5.6: Interest.java // Compound-interest calculations with for. public class Interest { public static void main(String[] args) { double amount; // amount on deposit at end of each year double principal = 1000.0; // initial amount before interest double rate = 0.05; // interest rate // display headers System.out.printf("%s%20s%n", "Year", "Amount on deposit"); // calculate amount on deposit for each of ten years for (int year = 1; year <= 10; ++year) { // calculate new amount for specified year amount = principal * Math.pow(1.0 + rate, year); // display the year and the amount System.out.printf("%4d%,20.2f%n", year, amount); } } } // end class Interest Year 1 2 3 4 5 6 7 8 9 10 Amount on deposit 1,050.00 1,102.50 1,157.63 1,215.51 1,276.28 1,340.10 1,407.10 1,477.46 1,551.33 1,628.89 Fig. 5.6 | Compound-interest calculations with for. Formatting Strings with Field Widths and Justification Line 13 outputs the headers for two columns of output. The first column displays the year and the second column the amount on deposit at the end of that year. We use the format specifier %20s to output the String "Amount on Deposit". The integer 20 between the % and the conversion character s indicates that the value should be displayed with a field width of 20—that is, printf displays the value with at least 20 character positions. If the value to be output is less than 20 character positions wide (17 characters in this example), the value is right justified in the field by default. If the year value to be output were more than four character positions wide, the field width would be extended to the right to accommodate the entire value—this would push the amount field to the right, upsetting the neat columns of our tabular output. To output values left justified, simply precede the field width with the minus sign (–) formatting flag (e.g., %-20s). 162 Chapter 5 Control Statements: Part 2; Logical Operators Performing the Interest Calculations with static Method pow of Class Math The for statement (lines 16–23) executes its body 10 times, varying control variable year from 1 to 10 in increments of 1. This loop terminates when year becomes 11. (Variable year represents n in the problem statement.) Classes provide methods that perform common tasks on objects. In fact, most methods must be called on a specific object. For example, to output text in Fig. 5.6, line 13 calls method printf on the System.out object. Some classes also provide methods that perform common tasks and do not require you to first create objects of those classes. These are called static methods. For example, Java does not include an exponentiation operator, so the designers of Java’s Math class defined static method pow for raising a value to a power. You can call a static method by specifying the class name followed by a dot (.) and the method name, as in ClassName.methodName(arguments) In Chapter 6, you’ll learn how to implement static methods in your own classes. We use static method pow of class Math to perform the compound-interest calculation in Fig. 5.6. Math.pow(x, y) calculates the value of x raised to the yth power. The method receives two double arguments and returns a double value. Line 19 performs the calculation a = p(1 + r) n, where a is amount, p is principal, r is rate and n is year. Class Math is defined in package java.lang, so you do not need to import class Math to use it. The body of the for statement contains the calculation 1.0 + rate, which appears as an argument to the Math.pow method. In fact, this calculation produces the same result each time through the loop, so repeating it in every iteration of the loop is wasteful. Performance Tip 5.1 In loops, avoid calculations for which the result never changes—such calculations should typically be placed before the loop. Many of today’s sophisticated optimizing compilers will place such calculations outside loops in the compiled code. Formatting Floating-Point Numbers After each calculation, line 22 outputs the year and the amount on deposit at the end of that year. The year is output in a field width of four characters (as specified by %4d). The amount is output as a floating-point number with the format specifier %,20.2f. The comma (,) formatting flag indicates that the floating-point value should be output with a grouping separator. The actual separator used is specific to the user’s locale (i.e., country). For example, in the United States, the number will be output using commas to separate every three digits and a decimal point to separate the fractional part of the number, as in 1,234.45. The number 20 in the format specification indicates that the value should be output right justified in a field width of 20 characters. The .2 specifies the formatted number’s precision—in this case, the number is rounded to the nearest hundredth and output with two digits to the right of the decimal point. A Warning about Displaying Rounded Values We declared variables amount, principal and rate to be of type double in this example. We’re dealing with fractional parts of dollars and thus need a type that allows decimal points in its values. Unfortunately, floating-point numbers can cause trouble. Here’s a simple explanation of what can go wrong when using double (or float) to represent dollar 5.5 do…while Repetition Statement 163 amounts (assuming that dollar amounts are displayed with two digits to the right of the decimal point): Two double dollar amounts stored in the machine could be 14.234 (which would normally be rounded to 14.23 for display purposes) and 18.673 (which would normally be rounded to 18.67 for display purposes). When these amounts are added, they produce the internal sum 32.907, which would normally be rounded to 32.91 for display purposes. Thus, your output could appear as 14.23 + 18.67 ------32.91 but a person adding the individual numbers as displayed would expect the sum to be 32.90. You’ve been warned! Error-Prevention Tip 5.6 Do not use variables of type double (or float) to perform precise monetary calculations. The imprecision of floating-point numbers can lead to errors. In the exercises, you’ll learn how to use integers to perform precise monetary calculations—Java also provides class java.math.BigDecimal for this purpose, which we demonstrate in Fig. 8.16. 5.5 do…while Repetition Statement The do…while repetition statement is similar to the while statement. In the while, the program tests the loop-continuation condition at the beginning of the loop, before executing the loop’s body; if the condition is false, the body never executes. The do…while statement tests the loop-continuation condition after executing the loop’s body; therefore, the body always executes at least once. When a do…while statement terminates, execution continues with the next statement in sequence. Figure 5.7 uses a do…while to output the numbers 1–10. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 // Fig. 5.7: DoWhileTest.java // do...while repetition statement. public class DoWhileTest { public static void main(String[] args) { int counter = 1; do { System.out.printf("%d ", counter); ++counter; } while (counter <= 10); // end do...while System.out.println(); } } // end class DoWhileTest Fig. 5.7 | do…while repetition statement. (Part 1 of 2.) 164 1 2 Chapter 5 Control Statements: Part 2; Logical Operators 3 4 Fig. 5.7 | 5 6 do…while 7 8 9 10 repetition statement. (Part 2 of 2.) Line 8 declares and initializes control variable counter. Upon entering the do…while statement, line 12 outputs counter’s value and line 13 increments counter. Then the program evaluates the loop-continuation test at the bottom of the loop (line 14). If the condition is true, the loop continues at the first body statement (line 12). If the condition is false, the loop terminates and the program continues at the next statement after the loop. UML Activity Diagram for the do…while Repetition Statement Figure 5.8 contains the UML activity diagram for the do…while statement. This diagram makes it clear that the loop-continuation condition is not evaluated until after the loop performs the action state at least once. Compare this activity diagram with that of the while statement (Fig. 4.6). System.out.printf( “%d ”, counter ); ++counter Determine whether looping should continue Fig. 5.8 | do…while Display the counter value Increment the control variable [counter <= 10] [counter > 10] repetition statement UML activity diagram. Braces in a do…while Repetition Statement It isn’t necessary to use braces in the do…while repetition statement if there’s only one statement in the body. However, many programmers include the braces, to avoid confusion between the while and do…while statements. For example, while (condition) is normally the first line of a while statement. A around a single-statement body appears as: do…while statement with no braces 5.6 switch Multiple-Selection Statement 165 do statement while (condition); which can be confusing. A reader may misinterpret the last line—while(condition);—as a while statement containing an empty statement (the semicolon by itself). Thus, the do…while statement with one body statement is usually written with braces as follows: do { statement } while (condition); Good Programming Practice 5.2 Always include braces in a do…while statement. This helps eliminate ambiguity between the while statement and a do…while statement containing only one statement. 5.6 switch Multiple-Selection Statement Chapter 4 discussed the if single-selection statement and the if…else double-selection statement. The switch multiple-selection statement performs different actions based on the possible values of a constant integral expression of type byte, short, int or char. As of Java SE 7, the expression may also be a String, which we discuss in Section 5.7. Using a switch Statement to Count A, B, C, D and F Grades Figure 5.9 calculates the class average of a set of numeric grades entered by the user, and uses a switch statement to determine whether each grade is the equivalent of an A, B, C, D or F and to increment the appropriate grade counter. The program also displays a summary of the number of students who received each grade. Like earlier versions of the class-average program, the main method of class LetterGrades (Fig. 5.9) declares local variables total (line 9) and gradeCounter (line 10) to keep track of the sum of the grades entered by the user and the number of grades entered, respectively. Lines 11–15 declare counter variables for each grade category. Note that the variables in lines 9–15 are explicitly initialized to 0. Method main has two key parts. Lines 26–56 read an arbitrary number of integer grades from the user using sentinel-controlled repetition, update instance variables total and gradeCounter, and increment an appropriate letter-grade counter for each grade entered. Lines 59–80 output a report containing the total of all grades entered, the average of the grades and the number of students who received each letter grade. Let’s examine these parts in more detail. 1 2 3 4 5 6 7 8 // Fig. 5.9: LetterGrades.java // LetterGrades class uses the switch statement to count letter grades. import java.util.Scanner; public class LetterGrades { public static void main(String[] args) { Fig. 5.9 | LetterGrades class uses the switch statement to count letter grades. (Part 1 of 3.) 166 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Fig. 5.9 | Chapter 5 Control Statements: Part 2; Logical Operators int int int int int int int total = 0; // sum of gradeCounter = 0; // aCount = 0; // count bCount = 0; // count cCount = 0; // count dCount = 0; // count fCount = 0; // count grades number of grades entered of A grades of B grades of C grades of D grades of F grades Scanner input = new Scanner(System.in); System.out.printf("%s%n%s%n %s%n %s%n", "Enter the integer grades in the range 0-100.", "Type the end-of-file indicator to terminate input:", "On UNIX/Linux/Mac OS X type d then press Enter", "On Windows type z then press Enter"); // loop until user enters the end-of-file indicator while (input.hasNext()) { int grade = input.nextInt(); // read grade total += grade; // add grade to total ++gradeCounter; // increment number of grades // increment appropriate letter-grade counter switch (grade / 10) { case 9: // grade was between 90 case 10: // and 100, inclusive ++aCount; break; // exits switch case 8: // grade was between 80 and 89 ++bCount; break; // exits switch case 7: // grade was between 70 and 79 ++cCount; break; // exits switch case 6: // grade was between 60 and 69 ++dCount; break; // exits switch default: // grade was less than 60 ++fCount; break; // optional; exits switch anyway } // end switch } // end while // display grade report System.out.printf("%nGrade Report:%n"); LetterGrades class uses the switch statement to count letter grades. (Part 2 of 3.) 5.6 switch Multiple-Selection Statement 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 167 // if user entered at least one grade... if (gradeCounter != 0) { // calculate average of all grades entered double average = (double) total / gradeCounter; // output summary of results System.out.printf("Total of the %d grades entered is %d%n", gradeCounter, total); System.out.printf("Class average is %.2f%n", average); System.out.printf("%n%s%n%s%d%n%s%d%n%s%d%n%s%d%n%s%d%n", "Number of students who received each grade:", "A: ", aCount, // display number of A grades "B: ", bCount, // display number of B grades "C: ", cCount, // display number of C grades "D: ", dCount, // display number of D grades "F: ", fCount); // display number of F grades } // end if else // no grades were entered, so output appropriate message System.out.println("No grades were entered"); } // end main } // end class LetterGrades Enter the integer grades in the range 0-100. Type the end-of-file indicator to terminate input: On UNIX/Linux/Mac OS X type d then press Enter On Windows type z then press Enter 99 92 45 57 63 71 76 85 90 100 ^Z Grade Report: Total of the 10 grades entered is 778 Class average is 77.80 Number of students who received each grade: A: 4 B: 1 C: 2 D: 1 F: 2 Fig. 5.9 | LetterGrades class uses the switch statement to count letter grades. (Part 3 of 3.) Reading Grades from the User Lines 19–23 prompt the user to enter integer grades and to type the end-of-file indicator to terminate the input. The end-of-file indicator is a system-dependent keystroke combi- 168 Chapter 5 Control Statements: Part 2; Logical Operators nation which the user enters to indicate that there’s no more data to input. In Chapter 15, Files, Streams and Object Serialization, you’ll see how the end-of-file indicator is used when a program reads its input from a file. On UNIX/Linux/Mac OS X systems, end-of-file is entered by typing the sequence d on a line by itself. This notation means to simultaneously press both the Ctrl key and the d key. On Windows systems, end-of-file can be entered by typing z [Note: On some systems, you must press Enter after typing the end-of-file key sequence. Also, Windows typically displays the characters ^Z on the screen when the end-of-file indicator is typed, as shown in the output of Fig. 5.9.] Portability Tip 5.1 The keystroke combinations for entering end-of-file are system dependent. The while statement (lines 26–56) obtains the user input. The condition at line 26 calls Scanner method hasNext to determine whether there’s more data to input. This method returns the boolean value true if there’s more data; otherwise, it returns false. The returned value is then used as the value of the condition in the while statement. Method hasNext returns false once the user types the end-of-file indicator. Line 28 inputs a grade value from the user. Line 29 adds grade to total. Line 30 increments gradeCounter. These variables are used to compute the average of the grades. Lines 33–55 use a switch statement to increment the appropriate letter-grade counter based on the numeric grade entered. Processing the Grades The switch statement (lines 33–55) determines which counter to increment. We assume that the user enters a valid grade in the range 0–100. A grade in the range 90–100 represents A, 80–89 represents B, 70–79 represents C, 60–69 represents D and 0–59 represents F. The switch statement consists of a block that contains a sequence of case labels and an optional default case. These are used in this example to determine which counter to increment based on the grade. When the flow of control reaches the switch, the program evaluates the expression in the parentheses (grade / 10) following keyword switch. This is the switch’s controlling expression. The program compares this expression’s value (which must evaluate to an integral value of type byte, char, short or int, or to a String) with each case label. The controlling expression in line 33 performs integer division, which truncates the fractional part of the result. Thus, when we divide a value from 0 to 100 by 10, the result is always a value from 0 to 10. We use several of these values in our case labels. For example, if the user enters the integer 85, the controlling expression evaluates to 8. The switch compares 8 with each case label. If a match occurs (case 8: at line 40), the program executes that case’s statements. For the integer 8, line 41 increments bCount, because a grade in the 80s is a B. The break statement (line 42) causes program control to proceed with the first statement after the switch—in this program, we reach the end of the while loop, so con- 5.6 switch Multiple-Selection Statement 169 trol returns to the loop-continuation condition in line 26 to determine whether the loop should continue executing. The cases in our switch explicitly test for the values 10, 9, 8, 7 and 6. Note the cases at lines 35–36 that test for the values 9 and 10 (both of which represent the grade A). Listing cases consecutively in this manner with no statements between them enables the cases to perform the same set of statements—when the controlling expression evaluates to 9 or 10, the statements in lines 37–38 will execute. The switch statement does not provide a mechanism for testing ranges of values, so every value you need to test must be listed in a separate case label. Each case can have multiple statements. The switch statement differs from other control statements in that it does not require braces around multiple statements in a case. without a break Statement Without break statements, each time a match occurs in the switch, the statements for that case and subsequent cases execute until a break statement or the end of the switch is encountered. This is often referred to as “falling through” to the statements in subsequent cases. (This feature is perfect for writing a concise program that displays the iterative song “The Twelve Days of Christmas” in Exercise 5.29.) case Common Programming Error 5.7 Forgetting a break statement when one is needed in a switch is a logic error. The default Case If no match occurs between the controlling expression’s value and a case label, the default case (lines 52–54) executes. We use the default case in this example to process all controlling-expression values that are less than 6—that is, all failing grades. If no match occurs and the switch does not contain a default case, program control simply continues with the first statement after the switch. Error-Prevention Tip 5.7 In a switch statement, ensure that you test for all possible values of the controlling expression. Displaying the Grade Report Lines 59–80 output a report based on the grades entered (as shown in the input/output window in Fig. 5.9). Line 62 determines whether the user entered at least one grade—this helps us avoid dividing by zero. If so, line 65 calculates the average of the grades. Lines 68– 77 then output the total of all the grades, the class average and the number of students who received each letter grade. If no grades were entered, line 80 outputs an appropriate message. The output in Fig. 5.9 shows a sample grade report based on 10 grades. Statement UML Activity Diagram Figure 5.10 shows the UML activity diagram for the general switch statement. Most switch statements use a break in each case to terminate the switch statement after processing the case. Figure 5.10 emphasizes this by including break statements in the activity diagram. The diagram makes it clear that the break statement at the end of a case causes control to exit the switch statement immediately. switch 170 Chapter 5 Control Statements: Part 2; Logical Operators [true] case a case a actions(s) break case b actions(s) break case z actions(s) break [false] [true] case b [false] ... [true] case z [false] default actions(s) Fig. 5.10 | switch multiple-selection statement UML activity diagram with break statements. The break statement is not required for the switch’s last case (or the optional case, when it appears last), because execution continues with the next statement after the switch. default Error-Prevention Tip 5.8 Provide a default case in switch statements. This focuses you on the need to process exceptional conditions. Good Programming Practice 5.3 Although each case and the default case in a switch can occur in any order, place the default case last. When the default case is listed last, the break for that case is not required. Notes on the Expression in Each case of a switch When using the switch statement, remember that each case must contain a constant integral expression—that is, any combination of integer constants that evaluates to a constant integer value (e.g., –7, 0 or 221)—or a String. An integer constant is simply an integer value. In addition, you can use character constants—specific characters in single quotes, such as 'A', '7' or '$'—which represent the integer values of characters and enum constants (introduced in Section 6.10). (Appendix B shows the integer values of the characters in the ASCII character set, which is a subset of the Unicode® character set used by Java.) The expression in each case can also be a constant variable—a variable containing a value which does not change for the entire program. Such a variable is declared with keyword final (discussed in Chapter 6). Java has a feature called enum types, which we also present in Chapter 6—enum type constants can also be used in case labels. 5.7 Class AutoPolicy Case Study: Strings in switch Statements 171 In Chapter 10, Object-Oriented Programming: Polymorphism and Interfaces, we present a more elegant way to implement switch logic—we use a technique called polymorphism to create programs that are often clearer, easier to maintain and easier to extend than programs using switch logic. 5.7 Class AutoPolicy Case Study: Strings in switch Statements Strings can be used as controlling expressions in switch statements, and String literals can be used in case labels. To demonstrate that Strings can be used as controlling expressions in switch statements and that String literals can be used in case labels, we’ll implement an app that meets the following requirements: You’ve been hired by an auto insurance company that serves these northeast states— Connecticut, Maine, Massachusetts, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island and Vermont. The company would like you to create a program that produces a report indicating for each of their auto insurance policies whether the policy is held in a state with “no-fault” auto insurance—Massachusetts, New Jersey, New York and Pennsylvania. The Java app that meets these requirements contains two classes—AutoPolicy (Fig. 5.11) and AutoPolicyTest (Fig. 5.12). Class AutoPolicy Class AutoPolicy (Fig. 5.11) represents an auto insurance policy. The class contains: • int instance variable accountNumber (line 5) to store the policy’s account number • String instance variable makeAndModel (line 6) to store the car’s make and model (such as a "Toyota Camry") • String instance variable state (line 7) to store a two-character state abbreviation representing the state in which the policy is held (e.g., "MA" for Massachusetts) • a constructor (lines 10–15) that initializes the class’s instance variables • methods setAccountNumber and getAccountNumber (lines 18–27) to set and get an AutoPolicy’s accountNumber instance variable • methods setMakeAndModel and getMakeAndModel (lines 30–39) to set and get an AutoPolicy’s makeAndModel instance variable • methods setState and getState (lines 42–51) to set and get an AutoPolicy’s state instance variable • method isNoFaultState (lines 54–70) to return a boolean value indicating whether the policy is held in a no-fault auto insurance state; note the method name—the naming convention for a get method that returns a boolean value is to begin the name with "is" rather than "get" (such a method is commonly called a predicate method). In method isNoFaultState, the switch statement’s controlling expression (line 59) is the String returned by AutoPolicy method getState. The switch statement compares the controlling expression’s value with the case labels (line 61) to determine whether the policy is held in Massachusetts, New Jersey, New York or Pennsylvania (the no-fault states). 172 Chapter 5 Control Statements: Part 2; Logical Operators If there’s a match, then line 62 sets local variable noFaultState to true and the switch statement terminates; otherwise, the default case sets noFaultState to false (line 65). Then method isNoFaultState returns local variable noFaultState’s value. For simplicity, we did not validate an AutoPolicy’s data in the constructor or set methods, and we assume that state abbreviations are always two uppercase letters. In addition, a real AutoPolicy class would likely contain many other instance variables and methods for data such as the account holder’s name, address, etc. In Exercise 5.30, you’ll be asked to enhance class AutoPolicy by validating its state abbreviation using techniques that you’ll learn in Section 5.9. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 // Fig. 5.11: AutoPolicy.java // Class that represents an auto insurance policy. public class AutoPolicy { private int accountNumber; // policy account number private String makeAndModel; // car that the policy applies to private String state; // two-letter state abbreviation // constructor public AutoPolicy(int accountNumber, String makeAndModel, String state) { this.accountNumber = accountNumber; this.makeAndModel = makeAndModel; this.state = state; } // sets the accountNumber public void setAccountNumber(int accountNumber) { this.accountNumber = accountNumber; } // returns the accountNumber public int getAccountNumber() { return accountNumber; } // sets the makeAndModel public void setMakeAndModel(String makeAndModel) { this.makeAndModel = makeAndModel; } // returns the makeAndModel public String getMakeAndModel() { return makeAndModel; } Fig. 5.11 | Class that represents an auto insurance policy. (Part 1 of 2.) 5.7 Class AutoPolicy Case Study: Strings in switch Statements 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 173 // sets the state public void setState(String state) { this.state = state; } // returns the state public String getState() { return state; } // predicate method returns whether the state has no-fault insurance public boolean isNoFaultState() { boolean noFaultState; // determine whether state has no-fault auto insurance switch (getState()) // get AutoPolicy object's state abbreviation { case "MA": case "NJ": case "NY": case "PA": noFaultState = true; break; default: noFaultState = false; break; } return noFaultState; } } // end class AutoPolicy Fig. 5.11 | Class that represents an auto insurance policy. (Part 2 of 2.) Class AutoPolicyTest Class AutoPolicyTest (Fig. 5.12) creates two AutoPolicy objects (lines 8–11 in main). Lines 14–15 pass each object to static method policyInNoFaultState (lines 20–28), which uses AutoPolicy methods to determine and display whether the object it receives represents a policy in a no-fault auto insurance state. 1 2 3 4 5 6 7 8 9 10 11 // Fig. 5.12: AutoPolicyTest.java // Demonstrating Strings in switch. public class AutoPolicyTest { public static void main(String[] args) { // create two AutoPolicy objects AutoPolicy policy1 = new AutoPolicy(11111111, "Toyota Camry", "NJ"); AutoPolicy policy2 = new AutoPolicy(22222222, "Ford Fusion", "ME"); Fig. 5.12 | Demonstrating Strings in switch. (Part 1 of 2.) 174 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 Chapter 5 Control Statements: Part 2; Logical Operators // display whether each policy is in a no-fault state policyInNoFaultState(policy1); policyInNoFaultState(policy2); } // method that displays whether an AutoPolicy // is in a state with no-fault auto insurance public static void policyInNoFaultState(AutoPolicy policy) { System.out.println("The auto policy:"); System.out.printf( "Account #: %d; Car: %s; State %s %s a no-fault state%n%n", policy.getAccountNumber(), policy.getMakeAndModel(), policy.getState(), (policy.isNoFaultState() ? "is": "is not")); } } // end class AutoPolicyTest The auto policy: Account #: 11111111; Car: Toyota Camry; State NJ is a no-fault state The auto policy: Account #: 22222222; Car: Ford Fusion; State ME is not a no-fault state Fig. 5.12 | Demonstrating Strings in switch. (Part 2 of 2.) 5.8 break and continue Statements In addition to selection and repetition statements, Java provides statements break (which we discussed in the context of the switch statement)and continue (presented in this section and online Appendix L) to alter the flow of control. The preceding section showed how break can be used to terminate a switch statement’s execution. This section discusses how to use break in repetition statements. Statement The break statement, when executed in a while, for, do…while or switch, causes immediate exit from that statement. Execution continues with the first statement after the control statement. Common uses of the break statement are to escape early from a loop or to skip the remainder of a switch (as in Fig. 5.9). Figure 5.13 demonstrates a break statement exiting a for. break 1 2 3 4 5 6 // Fig. 5.13: BreakTest.java // break statement exiting a for statement. public class BreakTest { public static void main(String[] args) { Fig. 5.13 | break statement exiting a for statement. (Part 1 of 2.) 5.8 break and continue Statements 7 8 9 10 11 12 13 14 15 16 17 18 19 175 int count; // control variable also used after loop terminates for (count = 1; count <= 10; count++) // loop 10 times { if (count == 5) break; // terminates loop if count is 5 System.out.printf("%d ", count); } System.out.printf("%nBroke out of loop at count = %d%n", count); } } // end class BreakTest 1 2 3 4 Broke out of loop at count = 5 Fig. 5.13 | break statement exiting a for statement. (Part 2 of 2.) When the if statement nested at lines 11–12 in the for statement (lines 9–15) detects that count is 5, the break statement at line 12 executes. This terminates the for statement, and the program proceeds to line 17 (immediately after the for statement), which displays a message indicating the value of the control variable when the loop terminated. The loop fully executes its body only four times instead of 10. Statement The continue statement, when executed in a while, for or do…while, skips the remaining statements in the loop body and proceeds with the next iteration of the loop. In while and do…while statements, the program evaluates the loop-continuation test immediately after the continue statement executes. In a for statement, the increment expression executes, then the program evaluates the loop-continuation test. continue 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 // Fig. 5.14: ContinueTest.java // continue statement terminating an iteration of a for statement. public class ContinueTest { public static void main(String[] args) { for (int count = 1; count <= 10; count++) // loop 10 times { if (count == 5) continue; // skip remaining code in loop body if count is 5 System.out.printf("%d ", count); } System.out.printf("%nUsed continue to skip printing 5%n"); } } // end class ContinueTest Fig. 5.14 | continue statement terminating an iteration of a for statement. (Part 1 of 2.) 176 Chapter 5 Control Statements: Part 2; Logical Operators 1 2 3 4 6 7 8 9 10 Used continue to skip printing 5 Fig. 5.14 | continue statement terminating an iteration of a for statement. (Part 2 of 2.) Figure 5.14 uses continue (line 10) to skip the statement at line 12 when the nested determines that count’s value is 5. When the continue statement executes, program control continues with the increment of the control variable in the for statement (line 7). In Section 5.3, we stated that while could be used in most cases in place of for. This is not true when the increment expression in the while follows a continue statement. In this case, the increment does not execute before the program evaluates the repetition-continuation condition, so the while does not execute in the same manner as the for. if Software Engineering Observation 5.2 Some programmers feel that break and continue violate structured programming. Since the same effects are achievable with structured programming techniques, these programmers do not use break or continue. Software Engineering Observation 5.3 There’s a tension between achieving quality software engineering and achieving the bestperforming software. Sometimes one of these goals is achieved at the expense of the other. For all but the most performance-intensive situations, apply the following rule of thumb: First, make your code simple and correct; then make it fast and small, but only if necessary. 5.9 Logical Operators The if, if…else, while, do…while and for statements each require a condition to determine how to continue a program’s flow of control. So far, we’ve studied only simple conditions, such as count <= 10, number != sentinelValue and total > 1000. Simple conditions are expressed in terms of the relational operators >, <, >= and <= and the equality operators == and !=, and each expression tests only one condition. To test multiple conditions in the process of making a decision, we performed these tests in separate statements or in nested if or if…else statements. Sometimes control statements require more complex conditions to determine a program’s flow of control. Java’s logical operators enable you to form more complex conditions by combining simple conditions. The logical operators are && (conditional AND), || (conditional OR), & (boolean logical AND), | (boolean logical inclusive OR), ^ (boolean logical exclusive OR) and ! (logical NOT). [Note: The &, | and ^ operators are also bitwise operators when they’re applied to integral operands. We discuss the bitwise operators in online Appendix K.] Conditional AND (&&) Operator Suppose we wish to ensure at some point in a program that two conditions are both true before we choose a certain path of execution. In this case, we can use the && (conditional AND) operator, as follows: if (gender == FEMALE && age >= 65) ++seniorFemales; 5.9 Logical Operators 177 This if statement contains two simple conditions. The condition gender == FEMALE compares variable gender to the constant FEMALE to determine whether a person is female. The condition age >= 65 might be evaluated to determine whether a person is a senior citizen. The if statement considers the combined condition gender == FEMALE && age >= 65 which is true if and only if both simple conditions are true. In this case, the if statement’s body increments seniorFemales by 1. If either or both of the simple conditions are false, the program skips the increment. Some programmers find that the preceding combined condition is more readable when redundant parentheses are added, as in: (gender == FEMALE) && (age >= 65) The table in Fig. 5.15 summarizes the && operator. The table shows all four possible combinations of false and true values for expression1 and expression2. Such tables are called truth tables. Java evaluates to false or true all expressions that include relational operators, equality operators or logical operators. expression1 expression2 expression1 && expression2 false false true true false true false true false false false true Fig. 5.15 | && (conditional AND) operator truth table. Conditional OR (||) Operator Now suppose we wish to ensure that either or both of two conditions are true before we choose a certain path of execution. In this case, we use the || (conditional OR) operator, as in the following program segment: if ((semesterAverage >= 90) || (finalExam >= 90)) System.out.println ("Student grade is A"); This statement also contains two simple conditions. The condition semesterAverage >= 90 evaluates to determine whether the student deserves an A in the course because of a sol- id performance throughout the semester. The condition finalExam >= 90 evaluates to determine whether the student deserves an A in the course because of an outstanding performance on the final exam. The if statement then considers the combined condition (semesterAverage >= 90) || (finalExam >= 90) and awards the student an A if either or both of the simple conditions are true. The only time the message "Student grade is A" is not printed is when both of the simple conditions are false. Figure 5.16 is a truth table for operator conditional OR (||). Operator && has a higher precedence than operator ||. Both operators associate from left to right. 178 Chapter 5 Control Statements: Part 2; Logical Operators expression1 expression2 expression1 || expression2 false false true true false true false true false true true true Fig. 5.16 | || (conditional OR) operator truth table. Short-Circuit Evaluation of Complex Conditions The parts of an expression containing && or || operators are evaluated only until it’s known whether the condition is true or false. Thus, evaluation of the expression (gender == FEMALE) && (age >= 65) stops immediately if gender is not equal to FEMALE (i.e., the entire expression is false) and continues if gender is equal to FEMALE (i.e., the entire expression could still be true if the condition age >= 65 is true). This feature of conditional AND and conditional OR expressions is called short-circuit evaluation. Common Programming Error 5.8 In expressions using operator &&, a condition—we’ll call this the dependent condition— may require another condition to be true for the evaluation of the dependent condition to be meaningful. In this case, the dependent condition should be placed after the && operator to prevent errors. Consider the expression (i != 0) && (10 / i == 2). The dependent condition (10 / i == 2) must appear after the && operator to prevent the possibility of division by zero. Boolean Logical AND (&) and Boolean Logical Inclusive OR (|) Operators The boolean logical AND (&) and boolean logical inclusive OR (|) operators are identical to the && and || operators, except that the & and | operators always evaluate both of their operands (i.e., they do not perform short-circuit evaluation). So, the expression (gender == 1) & (age >= 65) evaluates age >= 65 regardless of whether gender is equal to 1. This is useful if the right operand has a required side effect—a modification of a variable’s value. For example, the expression (birthday == true) | (++age >= 65) guarantees that the condition ++age >= 65 will be evaluated. Thus, the variable age is incremented, regardless of whether the overall expression is true or false. Error-Prevention Tip 5.9 For clarity, avoid expressions with side effects (such as assignments) in conditions. They can make code harder to understand and can lead to subtle logic errors. Error-Prevention Tip 5.10 Assignment (=) expressions generally should not be used in conditions. Every condition must result in a boolean value; otherwise, a compilation error occurs. In a condition, an assignment will compile only if a boolean expression is assigned to a boolean variable. 5.9 Logical Operators 179 Boolean Logical Exclusive OR (^) A simple condition containing the boolean logical exclusive OR (^) operator is true if and only if one of its operands is true and the other is false. If both are true or both are false, the entire condition is false. Figure 5.17 is a truth table for the boolean logical exclusive OR operator (^). This operator is guaranteed to evaluate both of its operands. expression1 expression2 expression1 ^ expression2 false false true true false true false true false true true false Fig. 5.17 | ^ (boolean logical exclusive OR) operator truth table. Logical Negation (!) Operator The ! (logical NOT, also called logical negation or logical complement) operator “reverses” the meaning of a condition. Unlike the logical operators &&, ||, &, | and ^, which are binary operators that combine two conditions, the logical negation operator is a unary operator that has only one condition as an operand. The operator is placed before a condition to choose a path of execution if the original condition (without the logical negation operator) is false, as in the program segment if (! (grade == sentinelValue)) System.out.printf("The next grade is %d%n", grade); which executes the printf call only if grade is not equal to sentinelValue. The parentheses around the condition grade == sentinelValue are needed because the logical negation operator has a higher precedence than the equality operator. In most cases, you can avoid using logical negation by expressing the condition differently with an appropriate relational or equality operator. For example, the previous statement may also be written as follows: if (grade != sentinelValue) System.out.printf("The next grade is %d%n", grade); This flexibility can help you express a condition in a more convenient manner. Figure 5.18 is a truth table for the logical negation operator. expression !expression false true true false Fig. 5.18 | ! (logical NOT) operator truth table. Logical Operators Example Figure 5.19 uses logical operators to produce the truth tables discussed in this section. The output shows the boolean expression that was evaluated and its result. We used the 180 Chapter 5 Control Statements: Part 2; Logical Operators format specifier to display the word “true” or the word “false” based on a boolean expression’s value. Lines 9–13 produce the truth table for &&. Lines 16–20 produce the truth table for ||. Lines 23–27 produce the truth table for &. Lines 30–35 produce the truth table for |. Lines 38–43 produce the truth table for ^. Lines 46–47 produce the truth table for !. %b 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 // Fig. 5.19: LogicalOperators.java // Logical operators. public class LogicalOperators { public static void main(String[] args) { // create truth table for && (conditional AND) operator System.out.printf("%s%n%s: %b%n%s: %b%n%s: %b%n%s: %b%n%n", "Conditional AND (&&)", "false && false", (false && false) "false && true", (false && true) , "true && false", (true && false) , "true && true", (true && true) ); // create truth table for || (conditional OR) operator System.out.printf("%s%n%s: %b%n%s: %b%n%s: %b%n%s: %b%n%n", "Conditional OR (||)", "false || false", (false || false) "false || true", (false || true) , "true || false", (true || false) , "true || true", (true || true) ); , , // create truth table for & (boolean logical AND) operator System.out.printf("%s%n%s: %b%n%s: %b%n%s: %b%n%s: %b%n%n", "Boolean logical AND (&)", "false & false", (false & false) "false & true", (false & true) , "true & false", (true & false) , "true & true", (true & true) ); , // create truth table for | (boolean logical inclusive OR) operator System.out.printf("%s%n%s: %b%n%s: %b%n%s: %b%n%s: %b%n%n", "Boolean logical inclusive OR (|)", "false | false", (false | false) , "false | true", (false | true) , "true | false", (true | false) , "true | true", (true | true) ); // create truth table for ^ (boolean logical exclusive OR) operator System.out.printf("%s%n%s: %b%n%s: %b%n%s: %b%n%s: %b%n%n", "Boolean logical exclusive OR (^)", "false ^ false", (false ^ false) , "false ^ true", (false ^ true) , "true ^ false", (true ^ false) , "true ^ true", (true ^ true) ); Fig. 5.19 | Logical operators. (Part 1 of 2.) 5.9 Logical Operators 45 46 47 48 49 181 // create truth table for ! (logical negation) operator System.out.printf("%s%n%s: %b%n%s: %b%n", "Logical NOT (!)", "!false", (!false) , "!true", (!true) ); } } // end class LogicalOperators Conditional AND (&&) false && false: false false && true: false true && false: false true && true: true Conditional OR (||) false || false: false false || true: true true || false: true true || true: true Boolean logical AND (&) false & false: false false & true: false true & false: false true & true: true Boolean logical inclusive OR (|) false | false: false false | true: true true | false: true true | true: true Boolean logical exclusive OR (^) false ^ false: false false ^ true: true true ^ false: true true ^ true: false Logical NOT (!) !false: true !true: false Fig. 5.19 | Logical operators. (Part 2 of 2.) Precedence and Associativity of the Operators Presented So Far Figure 5.20 shows the precedence and associativity of the Java operators introduced so far. The operators are shown from top to bottom in decreasing order of precedence. Operators Associativity Type ++ -- ++ -- + * / % right to left right to left left to right unary postfix unary prefix multiplicative - ! (type) Fig. 5.20 | Precedence/associativity of the operators discussed so far. (Part 1 of 2.) 182 Chapter 5 Control Statements: Part 2; Logical Operators Operators Associativity Type + - < <= == != left to right left to right left to right left to right left to right left to right left to right left to right right to left right to left additive relational equality boolean logical AND boolean logical exclusive OR boolean logical inclusive OR conditional AND conditional OR conditional assignment > >= -= *= & ^ | && || ?: = += /= %= Fig. 5.20 | Precedence/associativity of the operators discussed so far. (Part 2 of 2.) 5.10 Structured Programming Summary Just as architects design buildings by employing the collective wisdom of their profession, so should programmers design programs. Our field is much younger than architecture, and our collective wisdom is considerably sparser. We’ve learned that structured programming produces programs that are easier than unstructured programs to understand, test, debug, modify and even prove correct in a mathematical sense. Java Control Statements Are Single-Entry/Single-Exit Figure 5.21 uses UML activity diagrams to summarize Java’s control statements. The initial and final states indicate the single entry point and the single exit point of each control statement. Arbitrarily connecting individual symbols in an activity diagram can lead to unstructured programs. Therefore, the programming profession has chosen a limited set of control statements that can be combined in only two simple ways to build structured programs. For simplicity, Java includes only single-entry/single-exit control statements—there’s only one way to enter and only one way to exit each control statement. Connecting control statements in sequence to form structured programs is simple. The final state of one control statement is connected to the initial state of the next—that is, the control statements are placed one after another in a program in sequence. We call this control-statement stacking. The rules for forming structured programs also allow for control statements to be nested. Rules for Forming Structured Programs Figure 5.22 shows the rules for forming structured programs. The rules assume that action states may be used to indicate any action. The rules also assume that we begin with the simplest activity diagram (Fig. 5.23) consisting of only an initial state, an action state, a final state and transition arrows. 5.10 Structured Programming Summary Sequence 183 Selection if statement switch statement with breaks (single selection) (multiple selection) [t] [t] break [f] [f] ... [t] break [f] if…else statement (double selection) ... [f] [t] [t] break [f] default processing Repetition while statement do…while statement for statement initialization [t] [f] [t] [f] [t] increment [f] Fig. 5.21 | Java’s single-entry/single-exit sequence, selection and repetition statements. 184 Chapter 5 Control Statements: Part 2; Logical Operators Rules for forming structured programs 1. Begin with the simplest activity diagram (Fig. 5.23). 2. Any action state can be replaced by two action states in sequence. 3. Any action state can be replaced by any control statement (sequence of action states, if, if…else, switch, while, do…while or for). 4. Rules 2 and 3 can be applied as often as you like and in any order. Fig. 5.22 | Rules for forming structured programs. action state Fig. 5.23 | Simplest activity diagram. Applying the rules in Fig. 5.22 always results in a properly structured activity diagram with a neat, building-block appearance. For example, repeatedly applying rule 2 to the simplest activity diagram results in an activity diagram containing many action states in sequence (Fig. 5.24). Rule 2 generates a stack of control statements, so let’s call rule 2 the stacking rule. The vertical dashed lines in Fig. 5.24 are not part of the UML—we use them to separate the four activity diagrams that demonstrate rule 2 of Fig. 5.22 being applied. apply rule 2 apply rule 2 action state action state action state action state action state ... action state apply rule 2 action state action state o Fig. 5.24 | Repeatedly applying rule 2 of Fig. 5.22 to the simplest activity diagram. 5.10 Structured Programming Summary 185 Rule 3 is called the nesting rule. Repeatedly applying rule 3 to the simplest activity diagram results in one with neatly nested control statements. For example, in Fig. 5.25, the action state in the simplest activity diagram is replaced with a double-selection (if…else) statement. Then rule 3 is applied again to the action states in the double-selection statement, replacing each with a double-selection statement. The dashed action-state symbol around each double-selection statement represents the action state that was replaced. [Note: The dashed arrows and dashed action-state symbols shown in Fig. 5.25 are not part of the UML. They’re used here to illustrate that any action state can be replaced with a control statement.] apply rule 3 [f] [t] action state apply rule 3 action state [f] [f] action state [t] action state apply rule 3 action state [t] [f] action state [t] action state Fig. 5.25 | Repeatedly applying rule 3 of Fig. 5.22 to the simplest activity diagram. Rule 4 generates larger, more involved and more deeply nested statements. The diagrams that emerge from applying the rules in Fig. 5.22 constitute the set of all possible structured activity diagrams and hence the set of all possible structured programs. The beauty of the structured approach is that we use only seven simple single-entry/single-exit control statements and assemble them in only two simple ways. 186 Chapter 5 Control Statements: Part 2; Logical Operators If the rules in Fig. 5.22 are followed, an “unstructured’ activity diagram (like the one in Fig. 5.26) cannot be created. If you’re uncertain about whether a particular diagram is structured, apply the rules of Fig. 5.22 in reverse to reduce it to the simplest activity diagram. If you can reduce it, the original diagram is structured; otherwise, it’s not. action state action state action state action state Fig. 5.26 | “Unstructured” activity diagram. Three Forms of Control Structured programming promotes simplicity. Only three forms of control are needed to implement an algorithm: • sequence • selection • repetition The sequence structure is trivial. Simply list the statements to execute in the order in which they should execute. Selection is implemented in one of three ways: • if statement (single selection) • if…else statement (double selection) • switch statement (multiple selection) In fact, it’s straightforward to prove that the simple if statement is sufficient to provide any form of selection—everything that can be done with the if…else statement and the switch statement can be implemented by combining if statements (although perhaps not as clearly and efficiently). Repetition is implemented in one of three ways: • while statement • do…while statement • for statement [Note: There’s a fourth repetition statement—the enhanced for statement—that we discuss in Section 7.7.] It’s straightforward to prove that the while statement is sufficient to provide any form of repetition. Everything that can be done with do…while and for can be done with the while statement (although perhaps not as conveniently). Combining these results illustrates that any form of control ever needed in a Java program can be expressed in terms of 5.11 (Optional) GUI and Graphics Case Study: Drawing Rectangles and Ovals • sequence • if • while 187 statement (selection) statement (repetition) and that these can be combined in only two ways—stacking and nesting. Indeed, structured programming is the essence of simplicity. 5.11 (Optional) GUI and Graphics Case Study: Drawing Rectangles and Ovals This section demonstrates drawing rectangles and ovals, using the Graphics methods drawRect and drawOval, respectively. These methods are demonstrated in Fig. 5.27. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 // Fig. 5.27: Shapes.java // Drawing a cascade of shapes based on the user’s choice. import java.awt.Graphics; import javax.swing.JPanel; public class Shapes extends JPanel { private int choice; // user's choice of which shape to draw // constructor sets the user's choice public Shapes(int userChoice) { choice = userChoice; } // draws a cascade of shapes starting from the top-left corner public void paintComponent(Graphics g) { super.paintComponent(g); for (int i = 0; i < 10; i++) { // pick the shape based on the user's choice switch (choice) { case 1: // draw rectangles g.drawRect(10 + i * 10, 10 + i * 10, 50 + i * 10, 50 + i * 10); break; case 2: // draw ovals g.drawOval(10 + i * 10, 10 + i * 10, 50 + i * 10, 50 + i * 10); break; } } } } // end class Shapes Fig. 5.27 | Drawing a cascade of shapes based on the user’s choice. 188 Chapter 5 Control Statements: Part 2; Logical Operators Line 6 begins the class declaration for Shapes, which extends JPanel. Instance varichoice determines whether paintComponent should draw rectangles or ovals. The Shapes constructor initializes choice with the value passed in parameter userChoice. Method paintComponent performs the actual drawing. Remember, the first statement in every paintComponent method should be a call to super.paintComponent, as in line 19. Lines 21–35 loop 10 times to draw 10 shapes. The nested switch statement (lines 24– 34) chooses between drawing rectangles and drawing ovals. If choice is 1, then the program draws rectangles. Lines 27–28 call Graphics method drawRect. Method drawRect requires four arguments. The first two represent the x- and y-coordinates of the upper-left corner of the rectangle; the next two represent the rectangle’s width and height. In this example, we start at a position 10 pixels down and 10 pixels right of the top-left corner, and every iteration of the loop moves the upper-left corner another 10 pixels down and to the right. The width and the height of the rectangle start at 50 pixels and increase by 10 pixels in each iteration. If choice is 2, the program draws ovals. It creates an imaginary rectangle called a bounding rectangle and places inside it an oval that touches the midpoints of all four sides. Method drawOval (lines 31–32) requires the same four arguments as method drawRect. The arguments specify the position and size of the bounding rectangle for the oval. The values passed to drawOval in this example are exactly the same as those passed to drawRect in lines 27–28. Since the width and height of the bounding rectangle are identical in this example, lines 27–28 draw a circle. As an exercise, try modifying the program to draw both rectangles and ovals to see how drawOval and drawRect are related. able Class ShapesTest Figure 5.28 is responsible for handling input from the user and creating a window to display the appropriate drawing based on the user’s response. Line 3 imports JFrame to handle the display, and line 4 imports JOptionPane to handle the input. Lines 11–13 prompt the user with an input dialog and store the user’s response in variable input. Notice that when displaying multiple lines of prompt text in a JOptionPane, you must use \n to begin a new line of text, rather than %n. Line 15 uses Integer method parseInt to convert the String entered by the user to an int and stores the result in variable choice. Line 18 creates a Shapes object and passes the user’s choice to the constructor. Lines 20–25 perform the standard operations that create and set up a window in this case study—create a frame, set it to exit the application when closed, add the drawing to the frame, set the frame size and make it visible. 1 2 3 4 5 6 7 8 9 // Fig. 5.28: ShapesTest.java // Obtaining user input and creating a JFrame to display Shapes. import javax.swing.JFrame; //handle the display import javax.swing.JOptionPane; public class ShapesTest { public static void main(String[] args) { Fig. 5.28 | Obtaining user input and creating a JFrame to display Shapes. (Part 1 of 2.) 5.11 (Optional) GUI and Graphics Case Study: Drawing Rectangles and Ovals 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 189 // obtain user's choice String input = JOptionPane.showInputDialog( "Enter 1 to draw rectangles\n" + "Enter 2 to draw ovals"); int choice = Integer.parseInt(input); // convert input to int // create the panel with the user's input Shapes panel = new Shapes(choice); JFrame application = new JFrame(); // creates a new JFrame application.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); application.add(panel); application.setSize(300, 300); application.setVisible(true); } } // end class ShapesTest Fig. 5.28 | Obtaining user input and creating a JFrame to display Shapes. (Part 2 of 2.) GUI and Graphics Case Study Exercises 5.1 Draw 12 concentric circles in the center of a JPanel (Fig. 5.29). The innermost circle should have a radius of 10 pixels, and each successive circle should have a radius 10 pixels larger than the previous one. Begin by finding the center of the JPanel. To get the upper-left corner of a circle, move up one radius and to the left one radius from the center. The width and height of the bounding rectangle are both the same as the circle’s diameter (i.e., twice the radius). 190 Chapter 5 Control Statements: Part 2; Logical Operators Fig. 5.29 | Drawing concentric circles. 5.2 Modify Exercise 5.16 from the end-of-chapter exercises to read input using dialogs and to display the bar chart using rectangles of varying lengths. 5.12 Wrap-Up In this chapter, we completed our introduction to control statements, which enable you to control the flow of execution in methods. Chapter 4 discussed if, if…else and while. This chapter demonstrated for, do…while and switch. We showed that any algorithm can be developed using combinations of the sequence structure, the three types of selection statements—if, if…else and switch—and the three types of repetition statements—while, do…while and for. In this chapter and Chapter 4, we discussed how you can combine these building blocks to utilize proven program-construction and problem-solving techniques. You used the break statement to exit a switch statement and to immediately terminate a loop, and used a continue statement to terminate a loop’s current iteration and proceed with the loop’s next iteration. This chapter also introduced Java’s logical operators, which enable you to use more complex conditional expressions in control statements. In Chapter 6, we examine methods in greater depth. Summary Section 5.2 Essentials of Counter-Controlled Repetition • Counter-controlled repetition (p. 153) requires a control variable, the initial value of the control variable, the increment by which the control variable is modified each time through the loop (also known as each iteration of the loop) and the loop-continuation condition that determines whether looping should continue. • You can declare a variable and initialize it in the same statement. Section 5.3 for Repetition Statement • The while statement can be used to implement any counter-controlled loop. • The for statement (p. 155) specifies all the details of counter-controlled repetition in its header • When the for statement begins executing, its control variable is declared and initialized. If the loop-continuation condition is initially true, the body executes. After executing the loop’s body, the increment expression executes. Then the loop-continuation test is performed again to determine whether the program should continue with the next iteration of the loop. Summary 191 • The general format of the for statement is for (initialization; statement loopContinuationCondition; increment) where the initialization expression names the loop’s control variable and provides its initial value, loopContinuationCondition determines whether the loop should continue executing and increment modifies the control variable’s value, so that the loop-continuation condition eventually becomes false. The two semicolons in the for header are required. • Most for statements can be represented with equivalent while statements as follows: initialization; while (loopContinuationCondition) { statement increment; } • Typically, for statements are used for counter-controlled repetition and while statements for sentinel-controlled repetition. • If the initialization expression in the for header declares the control variable, the control variable can be used only in that for statement—it will not exist outside the for statement. • The expressions in a for header are optional. If the loopContinuationCondition is omitted, Java assumes that it’s always true, thus creating an infinite loop. You might omit the initialization expression if the control variable is initialized before the loop. You might omit the increment expression if the increment is calculated with statements in the loop’s body or if no increment is needed. • The increment expression in a for acts as if it’s a standalone statement at the end of the for’s body. • A for statement can count downward by using a negative increment—i.e., a decrement (p. 158). • If the loop-continuation condition is initially false, the for statement’s body does not execute. Section 5.4 Examples Using the for Statement • Java treats floating-point constants like 1000.0 and 0.05 as type double. Similarly, Java treats whole-number constants like 7 and -22 as type int. • The format specifier %4s outputs a String in a field width (p. 161) of 4—that is, printf displays the value with at least 4 character positions. If the value to be output is less than 4 character positions wide, the value is right justified (p. 161) in the field by default. If the value is greater than 4 character positions wide, the field width expands to accommodate the appropriate number of characters. To left justify (p. 161) the value, use a negative integer to specify the field width. • Math.pow(x, y) (p. 162) calculates the value of x raised to the yth power. The method receives two double arguments and returns a double value. • The comma (,) formatting flag (p. 162) in a format specifier indicates that a floating-point value should be output with a grouping separator (p. 162). The actual separator used is specific to the user’s locale (i.e., country). In the United States, the number will have commas separating every three digits and a decimal point separating the fractional part of the number, as in 1,234.45. • The . in a format specifier indicates that the integer to its right is the number’s precision. Section 5.5 do…while Repetition Statement • The do…while statement (p. 163) is similar to the while statement. In the while, the program tests the loop-continuation condition at the beginning of the loop, before executing its body; if the condition is false, the body never executes. The do…while statement tests the loop-continuation condition after executing the loop’s body; therefore, the body always executes at least once. 192 Chapter 5 Control Statements: Part 2; Logical Operators Section 5.6 switch Multiple-Selection Statement • The switch statement (p. 165) performs different actions based on the possible values of a constant integral expression (a constant value of type byte, short, int or char, but not long), or a String. • The end-of-file indicator is a system-dependent keystroke combination that terminates user input. On UNIX/Linux/Mac OS X systems, end-of-file is entered by typing the sequence d on a line by itself. This notation means to simultaneously press both the Ctrl key and the d key. On Windows systems, enter end-of-file by typing z. • Scanner method hasNext (p. 168) determines whether there’s more data to input. This method returns the boolean value true if there’s more data; otherwise, it returns false. As long as the end-of-file indicator has not been typed, method hasNext will return true. • The switch statement consists of a block that contains a sequence of case labels (p. 168) and an optional default case (p. 168). • In a switch, the program evaluates the controlling expression and compares its value with each case label. If a match occurs, the program executes the statements for that case. • Listing cases consecutively with no statements between them enables the cases to perform the same set of statements. • Every value you wish to test in a switch must be listed in a separate case label. • Each case can have multiple statements, and these need not be placed in braces. • A case’s statements typically end with a break statement (p. 168) that terminates the switch’s execution. • Without break statements, each time a match occurs in the switch, the statements for that case and subsequent cases execute until a break statement or the end of the switch is encountered. • If no match occurs between the controlling expression’s value and a case label, the optional default case executes. If no match occurs and the switch does not contain a default case, program control simply continues with the first statement after the switch. Section 5.7 Class AutoPolicy Case Study: Strings in switch Statements • Strings can be used in a switch statement’s controlling expression and case labels. Section 5.8 break and continue Statements • The break statement, when executed in a while, for, do…while or switch, causes immediate exit from that statement. • The continue statement (p. 174), when executed in a while, for or do…while, skips the loop’s remaining body statements and proceeds with its next iteration. In while and do…while statements, the program evaluates the loop-continuation test immediately. In a for statement, the increment expression executes, then the program evaluates the loop-continuation test. Section 5.9 Logical Operators • Simple conditions are expressed in terms of the relational operators >, <, >= and <= and the equality operators == and !=, and each expression tests only one condition. • Logical operators (p. 176) enable you to form more complex conditions by combining simple conditions. The logical operators are && (conditional AND), || (conditional OR), & (boolean logical AND), | (boolean logical inclusive OR), ^ (boolean logical exclusive OR) and ! (logical NOT). • To ensure that two conditions are both true, use the && (conditional AND) operator. If either or both of the simple conditions are false, the entire expression is false. • To ensure that either or both of two conditions are true, use the || (conditional OR) operator, which evaluates to true if either or both of its simple conditions are true. Self-Review Exercises 193 • A condition using && or || operators (p. 176) uses short-circuit evaluation (p. 178)—they’re evaluated only until it’s known whether the condition is true or false. • The & and | operators (p. 178) work identically to the && and || operators but always evaluate both operands. • A simple condition containing the boolean logical exclusive OR (^; p. 179) operator is true if and only if one of its operands is true and the other is false. If both operands are true or both are false, the entire condition is false. This operator is also guaranteed to evaluate both of its operands. • The unary ! (logical NOT; p. 179) operator “reverses” the value of a condition. Self-Review Exercises 5.1 Fill in the blanks in each of the following statements: statements are used for counter-controlled repetition and a) Typically, statements for sentinel-controlled repetition. b) The do…while statement tests the loop-continuation condition executing the loop’s body; therefore, the body always executes at least once. statement selects among multiple actions based on the possible values c) The of an integer variable or expression, or a String. d) The statement, when executed in a repetition statement, skips the remaining statements in the loop body and proceeds with the next iteration of the loop. operator can be used to ensure that two conditions are both true before e) The choosing a certain path of execution. f) If the loop-continuation condition in a for header is initially , the program does not execute the for statement’s body. g) Methods that perform common tasks and do not require objects are called methods. 5.2 State whether each of the following is true or false. If false, explain why. a) The default case is required in the switch selection statement. b) The break statement is required in the last case of a switch selection statement. c) The expression ((x > y) && (a < b)) is true if either x > y is true or a < b is true. d) An expression containing the || operator is true if either or both of its operands are true. e) The comma (,) formatting flag in a format specifier (e.g., %,20.2f) indicates that a value should be output with a thousands separator. f) To test for a range of values in a switch statement, use a hyphen (–) between the start and end values of the range in a case label. g) Listing cases consecutively with no statements between them enables the cases to perform the same set of statements. 5.3 Write a Java statement or a set of Java statements to accomplish each of the following tasks: a) Sum the odd integers between 1 and 99, using a for statement. Assume that the integer variables sum and count have been declared. b) Calculate the value of 2.5 raised to the power of 3, using the pow method. c) Print the integers from 1 to 20, using a while loop and the counter variable i. Assume that the variable i has been declared, but not initialized. Print only five integers per line. [Hint: Use the calculation i % 5. When the value of this expression is 0, print a newline character; otherwise, print a tab character. Assume that this code is an application. Use the System.out.println() method to output the newline character, and use the System.out.print('\t') method to output the tab character.] d) Repeat part (c), using a for statement. 194 5.4 Chapter 5 Control Statements: Part 2; Logical Operators Find the error in each of the following code segments, and explain how to correct it: a) i = 1; while (i <= 10); ++i; } b) for (k = 0.1; k != 1.0; k += 0.1) c) switch (n) System.out.println(k); { case 1: System.out.println("The number is 1"); case 2: System.out.println("The number is 2"); break; default: System.out.println("The number is not 1 or 2"); break; } d) The following code should print the values 1 to 10: n = 1; while (n < 10) System.out.println(n++); Answers to Self-Review Exercises 5.1 a) for, while. b) after. c) switch. d) continue. e) && (conditional AND). f) false. g) static. 5.2 a) False. The default case is optional. If no default action is needed, then there’s no need for a default case. b) False. The break statement is used to exit the switch statement. The break statement is not required for the last case in a switch statement. c) False. Both of the relational expressions must be true for the entire expression to be true when using the && operator. d) True. e) True. f) False. The switch statement does not provide a mechanism for testing ranges of values, so every value that must be tested should be listed in a separate case label. g) True. 5.3 a) sum = 0; for (count = 1; count <= 99; count += 2) sum += count; b) c) double result = Math.pow(2.5, 3); i = 1; while (i <= 20) { System.out.print(i); if (i % 5 == 0) System.out.println(); else System.out.print('\t'); ++i; } Exercises d) 195 for (i = 1; i <= 20; i++) { System.out.print(i); if (i % 5 == 0) System.out.println(); else System.out.print('\t'); } 5.4 a) Error: The semicolon after the while header causes an infinite loop, and there’s a missing left brace. Correction: Replace the semicolon by a {, or remove both the ; and the }. b) Error: Using a floating-point number to control a for statement may not work, because floating-point numbers are represented only approximately by most computers. Correction: Use an integer, and perform the proper calculation in order to get the values you desire: for (k = 1; k != 10; k++) System.out.println((double) k / 10); c) Error: The missing code is the break statement in the statements for the first case. Correction: Add a break statement at the end of the statements for the first case. This omission is not necessarily an error if you want the statement of case 2: to execute every time the case 1: statement executes. d) Error: An improper relational operator is used in the while’s continuation condition. Correction: Use <= rather than <, or change 10 to 11. Exercises 5.5 Describe the four basic elements of counter-controlled repetition. 5.6 Compare and contrast the while and for repetition statements. 5.7 Discuss a situation in which it would be more appropriate to use a than a while statement. Explain why. do…while 5.8 Compare and contrast the break and continue statements. 5.9 Find and correct the error(s) in each of the following segments of code: a) For (i = 100, i >= 1, i++) System.out.println(i); b) The following code should print whether integer value is odd or even: switch (value % 2) { case 0: System.out.println("Even integer"); case 1: System.out.println("Odd integer"); } c) The following code should output the odd integers from 19 to 1: for (i = 19; i >= 1; i += 2) System.out.println(i); statement 196 Chapter 5 Control Statements: Part 2; Logical Operators d) The following code should output the even integers from 2 to 100: counter = 2; do { System.out.println(counter); counter += 2; } While (counter < 100); 5.10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 What does the following program do? // Exercise 5.10: Printing.java public class Printing { public static void main(String[] args) { for (int i = 1; i <= 10; i++) { for (int j = 1; j <= 5; j++) System.out.print('@'); System.out.println(); } } } // end class Printing 5.11 (Find the Smallest Value) Write an application that finds the smallest of several integers. Assume that the first value read specifies the number of values to input from the user. 5.12 (Calculating the Product of Odd Integers) Write an application that calculates the product of the odd integers from 1 to 15. 5.13 (Factorials) Factorials are used frequently in probability problems. The factorial of a positive integer n (written n! and pronounced “n factorial”) is equal to the product of the positive integers from 1 to n. Write an application that calculates the factorials of 1 through 20. Use type long. Display the results in tabular format. What difficulty might prevent you from calculating the factorial of 100? 5.14 (Modified Compound-Interest Program) Modify the compound-interest application of Fig. 5.6 to repeat its steps for interest rates of 5%, 6%, 7%, 8%, 9% and 10%. Use a for loop to vary the interest rate. 5.15 (Triangle Printing Program) Write an application that displays the following patterns separately, one below the other. Use for loops to generate the patterns. All asterisks (*) should be printed by a single statement of the form System.out.print('*'); which causes the asterisks to print side by side. A statement of the form System.out.println(); can be used to move to the next line. A statement of the form System.out.print(' '); can be used to display a space for the last two patterns. There should be no other output statements in the program. [Hint: The last two patterns require that each line begin with an appropriate number of blank spaces.] (a) (b) (c) (d) * ** *** **** ***** ****** ******* ******** ********* ********** ********** ********* ******** ******* ****** ***** **** *** ** * ********** ********* ******** ******* ****** ***** **** *** ** * * ** *** **** ***** ****** ******* ******** ********* ********** Exercises 197 5.16 (Bar Chart Printing Program) One interesting application of computers is to display graphs and bar charts. Write an application that reads five numbers between 1 and 30. For each number that’s read, your program should display the same number of adjacent asterisks. For example, if your program reads the number 7, it should display *******. Display the bars of asterisks after you read all five numbers. 5.17 (Calculating Sales) An online retailer sells five products whose retail prices are as follows: Product 1, $2.98; product 2, $4.50; product 3, $9.98; product 4, $4.49 and product 5, $6.87. Write an application that reads a series of pairs of numbers as follows: a) product number b) quantity sold Your program should use a switch statement to determine the retail price for each product. It should calculate and display the total retail value of all products sold. Use a sentinel-controlled loop to determine when the program should stop looping and display the final results. 5.18 (Modified Compound-Interest Program) Modify the application in Fig. 5.6 to use only integers to calculate the compound interest. [Hint: Treat all monetary amounts as integral numbers of pennies. Then break the result into its dollars and cents portions by using the division and remainder operations, respectively. Insert a period between the dollars and the cents portions.] 5.19 Assume that i = 1, j = 2, k = 3 and m = 2. What does each of the following statements print? a) System.out.println(i == 1); b) System.out.println(j == 3); c) System.out.println((i >= 1) && (j < 4)); d) System.out.println((m <= 99) & (k < m)); e) System.out.println((j >= i) || (k == m)); f) System.out.println((k + m < j) | (3 - j >= k)); g) System.out.println(!(k > m)); (Calculating the Value of π) Calculate the value of π from the infinite series 4 4 4 4 4 π = 4 – --- + --- – --- + --- – ------ + … 3 5 7 9 11 Print a table that shows the value of π approximated by computing the first 200,000 terms of this series. How many terms do you have to use before you first get a value that begins with 3.14159? 5.20 5.21 (Pythagorean Triples) A right triangle can have sides whose lengths are all integers. The set of three integer values for the lengths of the sides of a right triangle is called a Pythagorean triple. The lengths of the three sides must satisfy the relationship that the sum of the squares of two of the sides is equal to the square of the hypotenuse. Write an application that displays a table of the Pythagorean triples for side1, side2 and the hypotenuse, all no larger than 500. Use a triple-nested for loop that tries all possibilities. This method is an example of “brute-force” computing. You’ll learn in more advanced computer science courses that for many interesting problems there’s no known algorithmic approach other than using sheer brute force. 5.22 (Modified Triangle Printing Program) Modify Exercise 5.15 to combine your code from the four separate triangles of asterisks such that all four patterns print side by side. [Hint: Make clever use of nested for loops.] 5.23 (De Morgan’s Laws) In this chapter, we discussed the logical operators &&, &, ||, |, ^ and !. De Morgan’s laws can sometimes make it more convenient for us to express a logical expression. These laws state that the expression !(condition1 && condition2) is logically equivalent to the expression (!condition1 || !condition2). Also, the expression !(condition1 || condition2) is logically equivalent to the expression (!condition1 && !condition2). Use De Morgan’s laws to write equivalent 198 Chapter 5 Control Statements: Part 2; Logical Operators expressions for each of the following, then write an application to show that both the original expression and the new expression in each case produce the same value: a) !(x < 5) && !(y >= 7) b) !(a == b) || !(g != 5) c) !((x <= 8) && (y > 4)) d) !((i > 4) || (j <= 6)) 5.24 (Diamond Printing Program) Write an application that prints the following diamond shape. You may use output statements that print a single asterisk (*), a single space or a single newline character. Maximize your use of repetition (with nested for statements), and minimize the number of output statements. * *** ***** ******* ********* ******* ***** *** * 5.25 (Modified Diamond Printing Program) Modify the application you wrote in Exercise 5.24 to read an odd number in the range 1 to 19 to specify the number of rows in the diamond. Your program should then display a diamond of the appropriate size. 5.26 A criticism of the break statement and the continue statement is that each is unstructured. Actually, these statements can always be replaced by structured statements, although doing so can be awkward. Describe in general how you’d remove any break statement from a loop in a program and replace it with some structured equivalent. [Hint: The break statement exits a loop from the body of the loop. The other way to exit is by failing the loop-continuation test. Consider using in the loop-continuation test a second test that indicates “early exit because of a ‘break’ condition.”] Use the technique you develop here to remove the break statement from the application in Fig. 5.13. 5.27 What does the following program segment do? for (i = 1; i <= 5; i++) { for (j = 1; j <= 3; j++) { for (k = 1; k <= 4; k++) System.out.print('*'); System.out.println(); } // end inner for System.out.println(); } // end outer for 5.28 Describe in general how you’d remove any continue statement from a loop in a program and replace it with some structured equivalent. Use the technique you develop here to remove the continue statement from the program in Fig. 5.14. 5.29 (“The Twelve Days of Christmas” Song) Write an application that uses repetition and switch statements to print the song “The Twelve Days of Christmas.” One switch statement should be used to print the day (“first,” “second,” and so on). A separate switch statement should be used Making a Difference 199 to print the remainder of each verse. Visit the website en.wikipedia.org/wiki/The_Twelve_Days_ of_Christmas_(song) for the lyrics of the song. 5.30 (Modified AutoPolicy Class) Modify class AutoPolicy in Fig. 5.11 to validate the two-letter state codes for the northeast states. The codes are: CT for Connecticut, MA for Massachusetts, ME for Maine, NH for New Hampshire, NJ for New Jersey, NY for New York, PA for Pennsylvania and VT for Vermont. In AutoPolicy method setState, use the logical OR (||) operator (Section 5.9) to create a compound condition in an if…else statement that compares the method’s argument with each two-letter code. If the code is incorrect, the else part of the if…else statement should display an error message. In later chapters, you’ll learn how to use exception handling to indicate that a method received an invalid value. Making a Difference 5.31 (Global Warming Facts Quiz) The controversial issue of global warming has been widely publicized by the film “An Inconvenient Truth,” featuring former Vice President Al Gore. Mr. Gore and a U.N. network of scientists, the Intergovernmental Panel on Climate Change, shared the 2007 Nobel Peace Prize in recognition of “their efforts to build up and disseminate greater knowledge about man-made climate change.” Research both sides of the global warming issue online (you might want to search for phrases like “global warming skeptics”). Create a five-question multiplechoice quiz on global warming, each question having four possible answers (numbered 1–4). Be objective and try to fairly represent both sides of the issue. Next, write an application that administers the quiz, calculates the number of correct answers (zero through five) and returns a message to the user. If the user correctly answers five questions, print “Excellent”; if four, print “Very good”; if three or fewer, print “Time to brush up on your knowledge of global warming,” and include a list of some of the websites where you found your facts. 5.32 (Tax Plan Alternatives; The “FairTax”) There are many proposals to make taxation fairer. Check out the FairTax initiative in the United States at www.fairtax.org. Research how the proposed FairTax works. One suggestion is to eliminate income taxes and most other taxes in favor of a 23% consumption tax on all products and services that you buy. Some FairTax opponents question the 23% figure and say that because of the way the tax is calculated, it would be more accurate to say the rate is 30%—check this carefully. Write a program that prompts the user to enter expenses in various expense categories they have (e.g., housing, food, clothing, transportation, education, health care, vacations), then prints the estimated FairTax that person would pay. 5.33 (Facebook User Base Growth) According to CNNMoney.com, Facebook hit one billion users in October 2012. Using the compound-growth technique you learned in Fig. 5.6 and assuming its user base grows at a rate of 4% per month, how many months will it take for Facebook to grow its user base to 1.5 billion users? How many months will it take for Facebook to grow its user base to two billion users? 6 Form ever follows function. —Louis Henri Sullivan E pluribus unum. (One composed of many.) —Virgil O! call back yesterday, bid time return. —William Shakespeare Answer me in one word. —William Shakespeare There is a point at which methods devour themselves. —Frantz Fanon Objectives In this chapter you’ll learn: ■ ■ ■ ■ ■ ■ ■ How static methods and fields are associated with classes rather than objects. How the method-call/return mechanism is supported by the method-call stack. About argument promotion and casting. How packages group related classes. How to use secure randomnumber generation to implement game-playing applications. How the visibility of declarations is limited to specific regions of programs. What method overloading is and how to create overloaded methods. Methods: A Deeper Look 6.1 Introduction 6.1 Introduction 6.2 Program Modules in Java 6.3 static Methods, static Fields and Class Math 6.4 Declaring Methods with Multiple Parameters 6.5 Notes on Declaring and Using Methods 6.6 Method-Call Stack and Stack Frames 6.7 Argument Promotion and Casting 201 6.8 Java API Packages 6.9 Case Study: Secure Random-Number Generation 6.10 Case Study: A Game of Chance; Introducing enum Types 6.11 Scope of Declarations 6.12 Method Overloading 6.13 (Optional) GUI and Graphics Case Study: Colors and Filled Shapes 6.14 Wrap-Up Summary | Self-Review Exercises | Answers to Self-Review Exercises | Exercises | Making a Difference 6.1 Introduction Experience has shown that the best way to develop and maintain a large program is to construct it from small, simple pieces, or modules. This technique is called divide and conquer. Methods, which we introduced in Chapter 3, help you modularize programs. In this chapter, we study methods in more depth. You’ll learn more about static methods, which can be called without the need for an object of the class to exist. You’ll also learn how Java is able to keep track of which method is currently executing, how local variables of methods are maintained in memory and how a method knows where to return after it completes execution. We’ll take a brief diversion into simulation techniques with random-number generation and develop a version of the dice game called craps that uses most of the programming techniques you’ve used to this point in the book. In addition, you’ll learn how to declare constants in your programs. Many of the classes you’ll use or create while developing applications will have more than one method of the same name. This technique, called overloading, is used to implement methods that perform similar tasks for arguments of different types or for different numbers of arguments. We continue our discussion of methods in Chapter 18, Recursion. Recursion provides an intriguing way of thinking about methods and algorithms. 6.2 Program Modules in Java You write Java programs by combining new methods and classes with predefined ones available in the Java Application Programming Interface (also referred to as the Java API or Java class library) and in various other class libraries. Related classes are typically grouped into packages so that they can be imported into programs and reused. You’ll learn how to group your own classes into packages in Section 21.4.10. The Java API provides a rich collection of predefined classes that contain methods for performing common mathematical calculations, string manipulations, character manipulations, input/output operations, database operations, networking operations, file processing, error checking and more. 202 Chapter 6 Methods: A Deeper Look Software Engineering Observation 6.1 Familiarize yourself with the rich collection of classes and methods provided by the Java API (http://docs.oracle.com/javase/7/docs/api/). Section 6.8 overviews several common packages. Online Appendix F explains how to navigate the API documentation. Don’t reinvent the wheel. When possible, reuse Java API classes and methods. This reduces program development time and avoids introducing programming errors. Divide and Conquer with Classes and Methods Classes and methods help you modularize a program by separating its tasks into self-contained units. The statements in the method bodies are written only once, are hidden from other methods and can be reused from several locations in a program. One motivation for modularizing a program into classes and methods is the divideand-conquer approach, which makes program development more manageable by constructing programs from small, simple pieces. Another is software reusability—using existing classes and methods as building blocks to create new programs. Often, you can create programs mostly from existing classes and methods rather than by building customized code. For example, in earlier programs, we did not define how to read data from the keyboard—Java provides these capabilities in the methods of class Scanner. A third motivation is to avoid repeating code. Dividing a program into meaningful classes and methods makes the program easier to debug and maintain. Software Engineering Observation 6.2 To promote software reusability, every method should be limited to performing a single, well-defined task, and the name of the method should express that task effectively. Error-Prevention Tip 6.1 A method that performs one task is easier to test and debug than one that performs many tasks. Software Engineering Observation 6.3 If you cannot choose a concise name that expresses a method’s task, your method might be attempting to perform too many tasks. Break such a method into several smaller ones. Hierarchical Relationship Between Method Calls As you know, a method is invoked by a method call, and when the called method completes its task, it returns control and possibly a result to the caller. An analogy to this program structure is the hierarchical form of management (Fig. 6.1). A boss (the caller) asks a worker (the called method) to perform a task and report back (return) the results after completing the task. The boss method does not know how the worker method performs its designated tasks. The worker may also call other worker methods, unbeknown to the boss. This “hiding” of implementation details promotes good software engineering. Figure 6.1 shows the boss method communicating with several worker methods in a hierarchical manner. The boss method divides its responsibilities among the various worker methods. Here, worker1 acts as a “boss method” to worker4 and worker5. Error-Prevention Tip 6.2 When you call a method that returns a value indicating whether the method performed its task successfully, be sure to check the return value of that method and, if that method was unsuccessful, deal with the issue appropriately. 6.3 static Methods, static Fields and Class Math 203 boss worker1 worker4 worker2 worker3 worker5 Fig. 6.1 | Hierarchical boss-method/worker-method relationship. 6.3 static Methods, static Fields and Class Math Although most methods execute in response to method calls on specific objects, this is not always the case. Sometimes a method performs a task that does not depend on an object. Such a method applies to the class in which it’s declared as a whole and is known as a static method or a class method. It’s common for classes to contain convenient static methods to perform common tasks. For example, recall that we used static method pow of class Math to raise a value to a power in Fig. 5.6. To declare a method as static, place the keyword static before the return type in the method’s declaration. For any class imported into your program, you can call the class’s static methods by specifying the name of the class in which the method is declared, followed by a dot (.) and the method name, as in ClassName.methodName(arguments) Class Methods We use various Math class methods here to present the concept of static methods. Class Math provides a collection of methods that enable you to perform common mathematical calculations. For example, you can calculate the square root of 900.0 with the static method call Math Math.sqrt(900.0) This expression evaluates to 30.0. Method sqrt takes an argument of type double and returns a result of type double. To output the value of the preceding method call in the command window, you might write the statement System.out.println(Math.sqrt(900.0)); In this statement, the value that sqrt returns becomes the argument to method println. There was no need to create a Math object before calling method sqrt. Also all Math class methods are static—therefore, each is called by preceding its name with the class name Math and the dot (.) separator. Software Engineering Observation 6.4 Class Math is part of the java.lang package, which is implicitly imported by the compiler, so it’s not necessary to import class Math to use its methods. 204 Chapter 6 Methods: A Deeper Look Method arguments may be constants, variables or expressions. If c = 13.0, d = 3.0 and then the statement f = 4.0, System.out.println(Math.sqrt(c + d * f)); calculates and prints the square root of 13.0 + 3.0 * 4.0 = 25.0—namely, 5.0. Figure 6.2 summarizes several Math class methods. In the figure, x and y are of type double. Method Description Example abs(x) absolute value of x abs(23.7) is 23.7 is 0.0 abs(-23.7) is 23.7 ceil(9.2) is 10.0 ceil(-9.8) is -9.0 cos(0.0) is 1.0 exp(1.0) is 2.71828 exp(2.0) is 7.38906 floor(9.2) is 9.0 floor(-9.8) is -10.0 log(Math.E) is 1.0 abs(0.0) ceil(x) cos(x) exp(x) floor(x) log(x) rounds x to the smallest integer not less than x trigonometric cosine of x (x in radians) exponential method e x rounds x to the largest integer not greater than x natural logarithm of x (base e) log(Math.E * Math.E) max(x, y) larger value of x and y is 2.0 is 12.7 is -2.3 min(2.3, 12.7) is 2.3 min(-2.3, -12.7) is -12.7 pow(2.0, 7.0) is 128.0 pow(9.0, 0.5) is 3.0 sin(0.0) is 0.0 sqrt(900.0) is 30.0 tan(0.0) is 0.0 max(2.3, 12.7) max(-2.3, -12.7) min(x, y) smaller value of x and y pow(x, y) x raised to the power y (i.e., x y ) sin(x) trigonometric sine of x (x in radians) square root of x trigonometric tangent of x (x in radians) sqrt(x) tan(x) Fig. 6.2 | Math class methods. Variables Recall from Section 3.2 that each object of a class maintains its own copy of every instance variable of the class. There are variables for which each object of a class does not need its own separate copy (as you’ll see momentarily). Such variables are declared static and are also known as class variables. When objects of a class containing static variables are created, all the objects of that class share one copy of those variables. Together a class’s static variables and instance variables are known as its fields. You’ll learn more about static fields in Section 8.11. static Class static Constants PI and E Class Math declares two constants, Math.PI and Math.E, that represent high-precision approximations to commonly used mathematical constants. Math.PI (3.141592653589793) is the ratio of a circle’s circumference to its diameter. Math.E (2.718281828459045) is the Math 6.4 Declaring Methods with Multiple Parameters 205 base value for natural logarithms (calculated with static Math method log). These constants are declared in class Math with the modifiers public, final and static. Making them public allows you to use them in your own classes. Any field declared with keyword final is constant—its value cannot change after the field is initialized. Making these fields static allows them to be accessed via the class name Math and a dot (.) separator, just as class Math’s methods are. Why Is Method main Declared static? When you execute the Java Virtual Machine (JVM) with the java command, the JVM attempts to invoke the main method of the class you specify—at this point no objects of the class have been created. Declaring main as static allows the JVM to invoke main without creating an instance of the class. When you execute your application, you specify its class name as an argument to the java command, as in java ClassName argument1 argument2 … The JVM loads the class specified by ClassName and uses that class name to invoke method In the preceding command, ClassName is a command-line argument to the JVM that tells it which class to execute. Following the ClassName, you can also specify a list of Strings (separated by spaces) as command-line arguments that the JVM will pass to your application. Such arguments might be used to specify options (e.g., a filename) to run the application. As you’ll learn in Chapter 7, Arrays and ArrayLists, your application can access those command-line arguments and use them to customize the application. main. 6.4 Declaring Methods with Multiple Parameters Methods often require more than one piece of information to perform their tasks. We now consider how to write your own methods with multiple parameters. Figure 6.3 uses a method called maximum to determine and return the largest of three double values. In main, lines 14–18 prompt the user to enter three double values, then read them from the user. Line 21 calls method maximum (declared in lines 28–41) to determine the largest of the three values it receives as arguments. When method maximum returns the result to line 21, the program assigns maximum’s return value to local variable result. Then line 24 outputs the maximum value. At the end of this section, we’ll discuss the use of operator + in line 24. 1 2 3 4 5 6 7 8 9 10 11 12 // Fig. 6.3: MaximumFinder.java // Programmer-declared method maximum with three double parameters. import java.util.Scanner; public class MaximumFinder { // obtain three floating-point values and locate the maximum value public static void main(String[] args) { // create Scanner for input from command window Scanner input = new Scanner(System.in); Fig. 6.3 | Programmer-declared method maximum with three double parameters. (Part 1 of 2.) 206 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 Chapter 6 Methods: A Deeper Look // prompt for and input three floating-point values System.out.print( "Enter three floating-point values separated by spaces: "); double number1 = input.nextDouble(); // read first double double number2 = input.nextDouble(); // read second double double number3 = input.nextDouble(); // read third double // determine the maximum value double result = maximum(number1, number2, number3); // display maximum value System.out.println("Maximum is: " + result); } // returns the maximum of its three double parameters public static double maximum(double x, double y, double z) { double maximumValue = x; // assume x is the largest to start // determine whether y is greater than maximumValue if (y > maximumValue) maximumValue = y; // determine whether z is greater than maximumValue if (z > maximumValue) maximumValue = z; return maximumValue; } } // end class MaximumFinder Enter three floating-point values separated by spaces: 9.35 2.74 5.1 Maximum is: 9.35 Enter three floating-point values separated by spaces: 5.8 12.45 8.32 Maximum is: 12.45 Enter three floating-point values separated by spaces: 6.46 4.12 10.54 Maximum is: 10.54 Fig. 6.3 | Programmer-declared method maximum with three double parameters. (Part 2 of 2.) The public and static Keywords Method maximum’s declaration begins with keyword public to indicate that the method is “available to the public”—it can be called from methods of other classes. The keyword static enables the main method (another static method) to call maximum as shown in line 21 without qualifying the method name with the class name MaximumFinder—static methods in the same class can call each other directly. Any other class that uses maximum must fully qualify the method name with the class name. 6.4 Declaring Methods with Multiple Parameters 207 Method maximum Consider maximum’s declaration (lines 28–41). Line 28 indicates that it returns a double value, that the method’s name is maximum and that the method requires three double parameters (x, y and z) to accomplish its task. Multiple parameters are specified as a comma-separated list. When maximum is called from line 21, the parameters x, y and z are initialized with copies of the values of arguments number1, number2 and number3, respectively. There must be one argument in the method call for each parameter in the method declaration. Also, each argument must be consistent with the type of the corresponding parameter. For example, a parameter of type double can receive values like 7.35, 22 or –0.03456, but not Strings like "hello" nor the boolean values true or false. Section 6.7 discusses the argument types that can be provided in a method call for each parameter of a primitive type. To determine the maximum value, we begin with the assumption that parameter x contains the largest value, so line 30 declares local variable maximumValue and initializes it with the value of parameter x. Of course, it’s possible that parameter y or z contains the actual largest value, so we must compare each of these values with maximumValue. The if statement at lines 33–34 determines whether y is greater than maximumValue. If so, line 34 assigns y to maximumValue. The if statement at lines 37–38 determines whether z is greater than maximumValue. If so, line 38 assigns z to maximumValue. At this point the largest of the three values resides in maximumValue, so line 40 returns that value to line 21. When program control returns to the point in the program where maximum was called, maximum’s parameters x, y and z no longer exist in memory. Software Engineering Observation 6.5 Methods can return at most one value, but the returned value could be a reference to an object that contains many values. Software Engineering Observation 6.6 Variables should be declared as fields only if they’re required for use in more than one method of the class or if the program should save their values between calls to the class’s methods. Common Programming Error 6.1 Declaring method parameters of the same type as float x, y instead of float x, float y is a syntax error—a type is required for each parameter in the parameter list. Implementing Method maximum by Reusing Method Math.max The entire body of our maximum method could also be implemented with two calls to Math.max, as follows: return Math.max(x, Math.max(y, z)); The first call to Math.max specifies arguments x and Math.max(y, z). Before any method can be called, its arguments must be evaluated to determine their values. If an argument is a method call, the method call must be performed to determine its return value. So, in the preceding statement, Math.max(y, z) is evaluated to determine the maximum of y and z. Then the result is passed as the second argument to the other call to Math.max, which returns the larger of its two arguments. This is a good example of software reuse—we find the largest of three values by reusing Math.max, which finds the larger of two values. Note how concise this code is compared to lines 30–38 of Fig. 6.3. 208 Chapter 6 Methods: A Deeper Look Assembling Strings with String Concatenation Java allows you to assemble String objects into larger strings by using operators + or +=. This is known as string concatenation. When both operands of operator + are String objects, operator + creates a new String object in which the characters of the right operand are placed at the end of those in the left operand—e.g., the expression "hello " + "there" creates the String "hello there". In line 24 of Fig. 6.3, the expression "Maximum is: " + result uses operator + with operands of types String and double. Every primitive value and object in Java can be represented as a String. When one of the + operator’s operands is a String, the other is converted to a String, then the two are concatenated. In line 24, the double value is converted to its String representation and placed at the end of the String "Maximum is: ". If there are any trailing zeros in a double value, these will be discarded when the number is converted to a String—for example 9.3500 would be represented as 9.35. Primitive values used in String concatenation are converted to Strings. A boolean concatenated with a String is converted to the String "true" or "false". All objects have a toString method that returns a String representation of the object. (We discuss toString in more detail in subsequent chapters.) When an object is concatenated with a String, the object’s toString method is implicitly called to obtain the String representation of the object. Method toString also can be called explicitly. You can break large String literals into several smaller Strings and place them on multiple lines of code for readability. In this case, the Strings can be reassembled using concatenation. We discuss the details of Strings in Chapter 14. Common Programming Error 6.2 It’s a syntax error to break a String literal across lines. If necessary, you can split a String into several smaller Strings and use concatenation to form the desired String. Common Programming Error 6.3 Confusing the + operator used for string concatenation with the + operator used for addition can lead to strange results. Java evaluates the operands of an operator from left to right. For example, if integer variable y has the value 5, the expression "y + 2 = " + y + 2 results in the string "y + 2 = 52", not "y + 2 = 7", because first the value of y (5) is concatenated to the string "y + 2 = ", then the value 2 is concatenated to the new larger string "y + 2 = 5". The expression "y + 2 = " + (y + 2) produces the desired result "y + 2 = 7". 6.5 Notes on Declaring and Using Methods There are three ways to call a method: 1. Using a method name by itself to call another method of the same class—such as maximum(number1, number2, number3) in line 21 of Fig. 6.3. 2. Using a variable that contains a reference to an object, followed by a dot (.) and the method name to call a non-static method of the referenced object—such as the method call in line 16 of Fig. 3.2, myAccount.getName(), which calls a method of class Account from the main method of AccountTest. Non-static methods are typically called instance methods. 6.6 Method-Call Stack and Stack Frames 209 3. Using the class name and a dot (.) to call a static method of a class—such as Math.sqrt(900.0) in Section 6.3. A static method can call other static methods of the same class directly (i.e., using the method name by itself) and can manipulate static variables in the same class directly. To access the class’s instance variables and instance methods, a static method must use a reference to an object of the class. Instance methods can access all fields (static variables and instance variables) and methods of the class. Recall that static methods relate to a class as a whole, whereas instance methods are associated with a specific instance (object) of the class and may manipulate the instance variables of that object. Many objects of a class, each with its own copies of the instance variables, may exist at the same time. Suppose a static method were to invoke an instance method directly. How would the static method know which object’s instance variables to manipulate? What would happen if no objects of the class existed at the time the instance method was invoked? Thus, Java does not allow a static method to directly access instance variables and instance methods of the same class. There are three ways to return control to the statement that calls a method. If the method does not return a result, control returns when the program flow reaches the method-ending right brace or when the statement return; is executed. If the method returns a result, the statement return expression; evaluates the expression, then returns the result to the caller. Common Programming Error 6.4 Declaring a method outside the body of a class declaration or inside the body of another method is a syntax error. Common Programming Error 6.5 Redeclaring a parameter as a local variable in the method’s body is a compilation error. Common Programming Error 6.6 Forgetting to return a value from a method that should return a value is a compilation error. If a return type other than void is specified, the method must contain a return statement that returns a value consistent with the method’s return type. Returning a value from a method whose return type has been declared void is a compilation error. 6.6 Method-Call Stack and Stack Frames To understand how Java performs method calls, we first need to consider a data structure (i.e., collection of related data items) known as a stack. You can think of a stack as analogous to a pile of dishes. When a dish is placed on the pile, it’s normally placed at the top (referred to as pushing the dish onto the stack). Similarly, when a dish is removed from the pile, it’s normally removed from the top (referred to as popping the dish off the stack). 210 Chapter 6 Methods: A Deeper Look Stacks are known as last-in, first-out (LIFO) data structures—the last item pushed onto the stack is the first item popped from the stack. When a program calls a method, the called method must know how to return to its caller, so the return address of the calling method is pushed onto the method-call stack. If a series of method calls occurs, the successive return addresses are pushed onto the stack in last-in, first-out order so that each method can return to its caller. The method-call stack also contains the memory for the local variables (including the method parameters) used in each invocation of a method during a program’s execution. This data, stored as a portion of the method-call stack, is known as the stack frame (or activation record) of the method call. When a method call is made, the stack frame for that method call is pushed onto the method-call stack. When the method returns to its caller, the stack frame for this method call is popped off the stack and those local variables are no longer known to the program. If a local variable holding a reference to an object is the only variable in the program with a reference to that object, then, when the stack frame containing that local variable is popped off the stack, the object can no longer be accessed by the program and will eventually be deleted from memory by the JVM during garbage collection, which we discuss in Section 8.10. Of course, a computer’s memory is finite, so only a certain amount can be used to store stack frames on the method-call stack. If more method calls occur than can have their stack frames stored, an error known as a stack overflow occurs—we’ll talk more about this in Chapter 11, Exception Handling: A Deeper Look. 6.7 Argument Promotion and Casting Another important feature of method calls is argument promotion—converting an argument’s value, if possible, to the type that the method expects to receive in its corresponding parameter. For example, a program can call Math method sqrt with an int argument even though a double argument is expected. The statement System.out.println(Math.sqrt(4)); correctly evaluates Math.sqrt(4) and prints the value 2.0. The method declaration’s parameter list causes Java to convert the int value 4 to the double value 4.0 before passing the value to method sqrt. Such conversions may lead to compilation errors if Java’s promotion rules are not satisfied. These rules specify which conversions are allowed—that is, which ones can be performed without losing data. In the sqrt example above, an int is converted to a double without changing its value. However, converting a double to an int truncates the fractional part of the double value—thus, part of the value is lost. Converting large integer types to small integer types (e.g., long to int, or int to short) may also result in changed values. The promotion rules apply to expressions containing values of two or more primitive types and to primitive-type values passed as arguments to methods. Each value is promoted to the “highest” type in the expression. Actually, the expression uses a temporary copy of each value—the types of the original values remain unchanged. Figure 6.4 lists the primitive types and the types to which each can be promoted. The valid promotions for a given type are always to a type higher in the table. For example, an int can be promoted to the higher types long, float and double. 6.8 Java API Packages 211 Converting values to types lower in the table of Fig. 6.4 will result in different values if the lower type cannot represent the value of the higher type (e.g., the int value 2000000 cannot be represented as a short, and any floating-point number with digits after its decimal point cannot be represented in an integer type such as long, int or short). Therefore, in cases where information may be lost due to conversion, the Java compiler requires you to use a cast operator (introduced in Section 4.10) to explicitly force the conversion to occur—otherwise a compilation error occurs. This enables you to “take control” from the compiler. You essentially say, “I know this conversion might cause loss of information, but for my purposes here, that’s fine.” Suppose method square calculates the square of an integer and thus requires an int argument. To call square with a double argument named doubleValue, we would be required to write the method call as square((int) doubleValue) This method call explicitly casts (converts) doubleValue’s value to a a temporary integer for use in method square. Thus, if doubleValue’s value is 4.5, the method receives the value 4 and returns 16, not 20.25. Common Programming Error 6.7 Casting a primitive-type value to another primitive type may change the value if the new type is not a valid promotion. For example, casting a floating-point value to an integer value may introduce truncation errors (loss of the fractional part) into the result. Type Valid promotions double None float double long or double long, float or double int, long, float or double int, long, float or double (but not char) short, int, long, float or double (but not char) None (boolean values are not considered to be numbers in Java) int char short byte boolean float Fig. 6.4 | Promotions allowed for primitive types. 6.8 Java API Packages As you’ve seen, Java contains many predefined classes that are grouped into categories of related classes called packages. Together, these are known as the Java Application Programming Interface (Java API), or the Java class library. A great strength of Java is the Java API’s thousands of classes. Some key Java API packages that we use in this book are described in Fig. 6.5, which represents only a small portion of the reusable components in the Java API. 212 Chapter 6 Methods: A Deeper Look Package Description java.awt.event The Java Abstract Window Toolkit Event Package contains classes and interfaces that enable event handling for GUI components in both the java.awt and javax.swing packages. (See Chapter 12, GUI Components: Part 1, and Chapter 22, GUI Components: Part 2.) java.awt.geom The Java 2D Shapes Package contains classes and interfaces for working with Java’s advanced two-dimensional graphics capabilities. (See Chapter 13, .) java.io The Java Input/Output Package contains classes and interfaces that enable programs to input and output data. (See Chapter 15, Files, Streams and Object Serialization.) java.lang The Java Language Package contains classes and interfaces (discussed throughout the book) that are required by many Java programs. This package is imported by the compiler into all programs. java.net The Java Networking Package contains classes and interfaces that enable programs to communicate via computer networks like the Internet. (See online Chapter 28, Networking.) java.security The Java Security Package contains classes and interfaces for enhancing application security. java.sql The JDBC Package contains classes and interfaces for working with databases. (See Chapter 24, Accessing Databases with JDBC.) java.util The Java Utilities Package contains utility classes and interfaces that enable storing and processing of large amounts of data. Many of these classes and interfaces have been updated to support Java SE 8’s new lambda capabilities. (See Chapter 16, Generic Collections.) java.util.concurrent The Java Concurrency Package contains utility classes and interfaces for implementing programs that can perform multiple tasks in parallel. (See Chapter 23, Concurrency.) javax.swing The Java Swing GUI Components Package contains classes and interfaces for Java’s Swing GUI components that provide support for portable GUIs. This package still uses some elements of the older java.awt package. (See Chapter 12, GUI Components: Part 1, and Chapter 22, GUI Components: Part 2.) javax.swing.event The Java Swing Event Package contains classes and interfaces that enable event handling (e.g., responding to button clicks) for GUI components in package javax.swing. (See Chapter 12, GUI Components: Part 1, and Chapter 22, GUI Components: Part 2.) javax.xml.ws The JAX-WS Package contains classes and interfaces for working with web services in Java. (See online Chapter 32, REST-Based Web Services.) javafx packages JavaFX is the preferred GUI technology for the future. We discuss these packages in Chapter 25, JavaFX GUI: Part 1 and in the online JavaFX GUI and multimedia chapters. Fig. 6.5 | Java API packages (a subset). (Part 1 of 2.) 6.9 Case Study: Secure Random-Number Generation Package 213 Description Some Java SE 8 Packages Used in This Book java.time The new Java SE 8 Date/Time API Package contains classes and interfaces for working with dates and times. These features are designed to replace the older date and time capabilities of package java.util. (See Chapter 23, Concurrency.) java.util.function and These packages contain classes and interfaces for working with Java java.util.stream SE 8’s functional programming capabilities. (See Chapter 17, Java SE 8 Lambdas and Streams.) Fig. 6.5 | Java API packages (a subset). (Part 2 of 2.) The set of packages available in Java is quite large. In addition to those summarized in Fig. 6.5, Java includes packages for complex graphics, advanced graphical user interfaces, printing, advanced networking, security, database processing, multimedia, accessibility (for people with disabilities), concurrent programming, cryptography, XML processing and many other capabilities. For an overview of the packages in Java, visit http://docs.oracle.com/javase/7/docs/api/overview-summary.html http://download.java.net/jdk8/docs/api/overview-summary.html You can locate additional information about a predefined Java class’s methods in the Java API documentation at http://docs.oracle.com/javase/7/docs/api/ When you visit this site, click the Index link to see an alphabetical listing of all the classes and methods in the Java API. Locate the class name and click its link to see the online description of the class. Click the METHOD link to see a table of the class’s methods. Each static method will be listed with the word “static” preceding its return type. 6.9 Case Study: Secure Random-Number Generation We now take a brief diversion into a popular type of programming application—simulation and game playing. In this and the next section, we develop a game-playing program with multiple methods. The program uses most of the control statements presented thus far in the book and introduces several new programming concepts. The element of chance can be introduced in a program via an object of class SecureRandom (package java.security). Such objects can produce random boolean, byte, float, double, int, long and Gaussian values. In the next several examples, we use objects of class SecureRandom to produce random values. Moving to Secure Random Numbers Recent editions of this book used Java’s Random class to obtain “random” values. This class produced deterministic values that could be predicted by malicious programmers. SecureRandom objects produce nondeterministic random numbers that cannot be predicted. Deterministic random numbers have been the source of many software security breaches. Most programming languages now have library features similar to Java’s Secure- 214 Chapter 6 Methods: A Deeper Look Random class for producing nondeterministic random numbers to help prevent such problems. From this point forward in the text, when we refer to “random numbers” we mean “secure random numbers.” Creating a SecureRandom Object A new secure random-number generator object can be created as follows: SecureRandom randomNumbers = new SecureRandom(); It can then be used to generate random values—we discuss only random int values here. For more information on the SecureRandom class, see docs.oracle.com/javase/7/docs/ api/java/security/SecureRandom.html. Obtaining a Random int Value Consider the following statement: int randomValue = randomNumbers.nextInt(); method nextInt generates a random int value. If it truly produces values at random, then every value in the range should have an equal chance (or probability) of being chosen each time nextInt is called. SecureRandom Changing the Range of Values Produced By nextInt The range of values produced by method nextInt generally differs from the range of values required in a particular Java application. For example, a program that simulates coin tossing might require only 0 for “heads” and 1 for “tails.” A program that simulates the rolling of a six-sided die might require random integers in the range 1–6. A program that randomly predicts the next type of spaceship (out of four possibilities) that will fly across the horizon in a video game might require random integers in the range 1–4. For cases like these, class SecureRandom provides another version of method nextInt that receives an int argument and returns a value from 0 up to, but not including, the argument’s value. For example, for coin tossing, the following statement returns 0 or 1. int randomValue = randomNumbers.nextInt(2); Rolling a Six-Sided Die To demonstrate random numbers, let’s develop a program that simulates 20 rolls of a sixsided die and displays the value of each roll. We begin by using nextInt to produce random values in the range 0–5, as follows: int face = randomNumbers.nextInt(6); The argument 6—called the scaling factor—represents the number of unique values that nextInt should produce (in this case six—0, 1, 2, 3, 4 and 5). This manipulation is called scaling the range of values produced by SecureRandom method nextInt. A six-sided die has the numbers 1–6 on its faces, not 0–5. So we shift the range of numbers produced by adding a shifting value—in this case 1—to our previous result, as in int face = 1 + randomNumbers.nextInt(6); The shifting value (1) specifies the first value in the desired range of random integers. The preceding statement assigns face a random integer in the range 1–6. 6.9 Case Study: Secure Random-Number Generation 215 Rolling a Six-Sided Die 20 Times Figure 6.6 shows two sample outputs which confirm that the results of the preceding calculation are integers in the range 1–6, and that each run of the program can produce a different sequence of random numbers. Line 3 imports class SecureRandom from the java.security package. Line 10 creates the SecureRandom object randomNumbers to produce random values. Line 16 executes 20 times in a loop to roll the die. The if statement (lines 21–22) in the loop starts a new line of output after every five numbers to create a neat, five-column format. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 // Fig. 6.6: RandomIntegers.java // Shifted and scaled random integers. import java.security.SecureRandom; // program uses class SecureRandom public class RandomIntegers { public static void main(String[] args) { // randomNumbers object will produce secure random numbers SecureRandom randomNumbers = new SecureRandom(); // loop 20 times for (int counter = 1; counter <= 20; counter++) { // pick random integer from 1 to 6 int face = 1 + randomNumbers.nextInt(6); System.out.printf("%d ", face); // display generated value // if counter is divisible by 5, start a new line of output if (counter % 5 == 0) System.out.println(); } } } // end class RandomIntegers 1 5 4 3 5 2 4 1 3 6 4 6 6 5 2 2 2 2 6 2 6 1 6 6 5 2 3 4 4 5 2 2 2 1 2 6 6 3 1 4 Fig. 6.6 | Shifted and scaled random integers. Rolling a Six-Sided Die 6,000,000 Times To show that the numbers produced by nextInt occur with approximately equal likelihood, let’s simulate 6,000,000 rolls of a die with the application in Fig. 6.7. Each integer from 1 to 6 should appear approximately 1,000,000 times. 216 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 Chapter 6 Methods: A Deeper Look // Fig. 6.7: RollDie.java // Roll a six-sided die 6,000,000 times. import java.security.SecureRandom; public class RollDie { public static void main(String[] args) { // randomNumbers object will produce secure random numbers SecureRandom randomNumbers = new SecureRandom(); int int int int int int frequency1 frequency2 frequency3 frequency4 frequency5 frequency6 = = = = = = 0; 0; 0; 0; 0; 0; // // // // // // count count count count count count of of of of of of 1s 2s 3s 4s 5s 6s rolled rolled rolled rolled rolled rolled // tally counts for 6,000,000 rolls of a die for (int roll = 1; roll <= 6000000; roll++) { int face = 1 + randomNumbers.nextInt(6); // number from 1 to 6 // use face value 1-6 to determine which counter to increment switch (face) { case 1: ++frequency1; // increment the 1s counter break; case 2: ++frequency2; // increment the 2s counter break; case 3: ++frequency3; // increment the 3s counter break; case 4: ++frequency4; // increment the 4s counter break; case 5: ++frequency5; // increment the 5s counter break; case 6: ++frequency6; // increment the 6s counter break; } } System.out.println("Face\tFrequency"); // output headers System.out.printf("1\t%d%n2\t%d%n3\t%d%n4\t%d%n5\t%d%n6\t%d%n", frequency1, frequency2, frequency3, frequency4, frequency5, frequency6); } } // end class RollDie Fig. 6.7 | Roll a six-sided die 6,000,000 times. (Part 1 of 2.) 6.9 Case Study: Secure Random-Number Generation Face 1 2 3 4 5 6 Frequency 999501 1000412 998262 1000820 1002245 998760 Face 1 2 3 4 5 6 Frequency 999647 999557 999571 1000376 1000701 1000148 217 Fig. 6.7 | Roll a six-sided die 6,000,000 times. (Part 2 of 2.) As the sample outputs show, scaling and shifting the values produced by nextInt enables the program to simulate rolling a six-sided die. The application uses nested control statements (the switch is nested inside the for) to determine the number of times each side of the die appears. The for statement (lines 20–46) iterates 6,000,000 times. During each iteration, line 22 produces a random value from 1 to 6. That value is then used as the controlling expression (line 25) of the switch statement (lines 25–45). Based on the face value, the switch statement increments one of the six counter variables during each iteration of the loop. This switch statement has no default case, because we have a case for every possible die value that the expression in line 22 could produce. Run the program, and observe the results. As you’ll see, every time you run this program, it produces different results. When we study arrays in Chapter 7, we’ll show an elegant way to replace the entire switch statement in this program with a single statement. Then, when we study Java SE 8’s new functional programming capabilities in Chapter 17, we’ll show how to replace the loop that rolls the dice, the switch statement and the statement that displays the results with a single statement! Generalized Scaling and Shifting of Random Numbers Previously, we simulated the rolling of a six-sided die with the statement int face = 1 + randomNumbers.nextInt(6); This statement always assigns to variable face an integer in the range 1 ≤ face ≤ 6. The width of this range (i.e., the number of consecutive integers in the range) is 6, and the starting number in the range is 1. In the preceding statement, the width of the range is determined by the number 6 that’s passed as an argument to SecureRandom method nextInt, and the starting number of the range is the number 1 that’s added to randomNumbers.nextInt(6). We can generalize this result as int number = shiftingValue + randomNumbers.nextInt(scalingFactor); where shiftingValue specifies the first number in the desired range of consecutive integers and scalingFactor specifies how many numbers are in the range. 218 Chapter 6 Methods: A Deeper Look It’s also possible to choose integers at random from sets of values other than ranges of consecutive integers. For example, to obtain a random value from the sequence 2, 5, 8, 11 and 14, you could use the statement int number = 2 + 3 * randomNumbers.nextInt(5); In this case, randomNumbers.nextInt(5) produces values in the range 0–4. Each value produced is multiplied by 3 to produce a number in the sequence 0, 3, 6, 9 and 12. We add 2 to that value to shift the range of values and obtain a value from the sequence 2, 5, 8, 11 and 14. We can generalize this result as int number = shiftingValue + differenceBetweenValues * randomNumbers.nextInt(scalingFactor); where shiftingValue specifies the first number in the desired range of values, differenceBetweenValues represents the constant difference between consecutive numbers in the sequence and scalingFactor specifies how many numbers are in the range. A Note About Performance Using SecureRandom instead of Random to achieve higher levels of security incurs a significant performance penalty. For “casual” applications, you might want to use class Random from package java.util—simply replace SecureRandom with Random. 6.10 Case Study: A Game of Chance; Introducing enum Types A popular game of chance is a dice game known as craps, which is played in casinos and back alleys throughout the world. The rules of the game are straightforward: You roll two dice. Each die has six faces, which contain one, two, three, four, five and six spots, respectively. After the dice have come to rest, the sum of the spots on the two upward faces is calculated. If the sum is 7 or 11 on the first throw, you win. If the sum is 2, 3 or 12 on the first throw (called “craps”), you lose (i.e., the “house” wins). If the sum is 4, 5, 6, 8, 9 or 10 on the first throw, that sum becomes your “point.” To win, you must continue rolling the dice until you “make your point” (i.e., roll that same point value). You lose by rolling a 7 before making your point. Figure 6.8 simulates the game of craps, using methods to implement the game’s logic. The main method (lines 21–65) calls the rollDice method (lines 68–81) as necessary to roll the dice and compute their sum. The sample outputs show winning and losing on the first roll, and winning and losing on a subsequent roll. 1 2 3 4 5 6 7 8 // Fig. 6.8: Craps.java // Craps class simulates the dice game craps. import java.security.SecureRandom; public class Craps { // create secure random number generator for use in method rollDice private static final SecureRandom randomNumbers = new SecureRandom(); Fig. 6.8 | Craps class simulates the dice game craps. (Part 1 of 3.) 6.10 Case Study: A Game of Chance; Introducing enum Types 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 219 // enum type with constants that represent the game status private enum Status { CONTINUE, WON, LOST }; // constants that represent common rolls of the dice private static final int SNAKE_EYES = 2; private static final int TREY = 3; private static final int SEVEN = 7; private static final int YO_LEVEN = 11; private static final int BOX_CARS = 12; // plays one game of craps public static void main(String[] args) { int myPoint = 0; // point if no win or loss on first roll Status gameStatus; // can contain CONTINUE, WON or LOST Fig. 6.8 | int sumOfDice = rollDice(); // first roll of the dice // determine game status and point based on first roll switch (sumOfDice) { case SEVEN: // win with 7 on first roll case YO_LEVEN: // win with 11 on first roll gameStatus = Status.WON; break; case SNAKE_EYES: // lose with 2 on first roll case TREY: // lose with 3 on first roll case BOX_CARS: // lose with 12 on first roll gameStatus = Status.LOST; break; default: // did not win or lose, so remember point gameStatus = Status.CONTINUE; // game is not over myPoint = sumOfDice; // remember the point System.out.printf("Point is %d%n", myPoint); break; } // while game is not complete while (gameStatus == Status.CONTINUE) // not WON or LOST { sumOfDice = rollDice(); // roll dice again // determine game status if (sumOfDice == myPoint) // win by making point gameStatus = Status.WON; else if (sumOfDice == SEVEN) // lose by rolling 7 before point gameStatus = Status.LOST; } Craps class simulates the dice game craps. (Part 2 of 3.) 220 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 Chapter 6 Methods: A Deeper Look // display won or lost message if (gameStatus == Status.WON) System.out.println("Player wins"); else System.out.println("Player loses"); } // roll dice, calculate sum and display results public static int rollDice() { // pick random die values int die1 = 1 + randomNumbers.nextInt(6); // first die roll int die2 = 1 + randomNumbers.nextInt(6); // second die roll int sum = die1 + die2; // sum of die values // display results of this roll System.out.printf("Player rolled %d + %d = %d%n", die1, die2, sum); return sum; } } // end class Craps Player rolled 5 + 6 = 11 Player wins Player rolled 5 + 4 = 9 Point is 9 Player rolled 4 + 2 = 6 Player rolled 3 + 6 = 9 Player wins Player rolled 1 + 2 = 3 Player loses Player rolled Point is 8 Player rolled Player rolled Player rolled Player loses Fig. 6.8 | Craps 2 + 6 = 8 5 + 1 = 6 2 + 1 = 3 1 + 6 = 7 class simulates the dice game craps. (Part 3 of 3.) Method rollDice In the rules of the game, the player must roll two dice on the first and all subsequent rolls. We declare method rollDice (lines 68–81) to roll the dice and compute and print their sum. Method rollDice is declared once, but it’s called from two places (lines 26 and 50) in main, which contains the logic for one complete game of craps. Method rollDice takes 6.10 Case Study: A Game of Chance; Introducing enum Types 221 no arguments, so it has an empty parameter list. Each time it’s called, rollDice returns the sum of the dice, so the return type int is indicated in the method header (line 68). Although lines 71 and 72 look the same (except for the die names), they do not necessarily produce the same result. Each of these statements produces a random value in the range 1– 6. Variable randomNumbers (used in lines 71–72) is not declared in the method. Instead it’s declared as a private static final variable of the class and initialized in line 8. This enables us to create one SecureRandom object that’s reused in each call to rollDice. If there were a program that contained multiple instances of class Craps, they’d all share this one SecureRandom object. Method main’s Local Variables The game is reasonably involved. The player may win or lose on the first roll, or may win or lose on any subsequent roll. Method main (lines 21–65) uses local variable myPoint (line 23) to store the “point” if the player doesn’t win or lose on the first roll, local variable gameStatus (line 24) to keep track of the overall game status and local variable sumOfDice (line 26) to hold the sum of the dice for the most recent roll. Variable myPoint is initialized to 0 to ensure that the application will compile. If you do not initialize myPoint, the compiler issues an error, because myPoint is not assigned a value in every case of the switch statement, and thus the program could try to use myPoint before it’s assigned a value. By contrast, gameStatus is assigned a value in every case of the switch statement (including the default case)—thus, it’s guaranteed to be initialized before it’s used, so we do not need to initialize it in line 24. Type Status Local variable gameStatus (line 24) is declared to be of a new type called Status (declared at line 11). Type Status is a private member of class Craps, because Status will be used only in that class. Status is a type called an enum type, which, in its simplest form, declares a set of constants represented by identifiers. An enum type is a special kind of class that’s introduced by the keyword enum and a type name (in this case, Status). As with classes, braces delimit an enum declaration’s body. Inside the braces is a comma-separated list of enum constants, each representing a unique value. The identifiers in an enum must be unique. You’ll learn more about enum types in Chapter 8. enum Good Programming Practice 6.1 Use only uppercase letters in the names of enum constants to make them stand out and remind you that they’re not variables. Variables of type Status can be assigned only the three constants declared in the enum (line 11) or a compilation error will occur. When the game is won, the program sets local variable gameStatus to Status.WON (lines 33 and 54). When the game is lost, the program sets local variable gameStatus to Status.LOST (lines 38 and 57). Otherwise, the program sets local variable gameStatus to Status.CONTINUE (line 41) to indicate that the game is not over and the dice must be rolled again. Good Programming Practice 6.2 Using enum constants (like Status.WON, Status.LOST and Status.CONTINUE) rather than literal values (such as 0, 1 and 2) makes programs easier to read and maintain. 222 Chapter 6 Methods: A Deeper Look Logic of the main Method Line 26 in main calls rollDice, which picks two random values from 1 to 6, displays the values of the first die, the second die and their sum, and returns the sum. Method main next enters the switch statement (lines 29–45), which uses the sumOfDice value from line 26 to determine whether the game has been won or lost, or should continue with another roll. The values that result in a win or loss on the first roll are declared as private static final int constants in lines 14–18. The identifier names use casino parlance for these sums. These constants, like enum constants, are declared by convention with all capital letters, to make them stand out in the program. Lines 31–34 determine whether the player won on the first roll with SEVEN (7) or YO_LEVEN (11). Lines 35–39 determine whether the player lost on the first roll with SNAKE_EYES (2), TREY (3), or BOX_CARS (12). After the first roll, if the game is not over, the default case (lines 40–44) sets gameStatus to Status.CONTINUE, saves sumOfDice in myPoint and displays the point. If we’re still trying to “make our point” (i.e., the game is continuing from a prior roll), lines 48–58 execute. Line 50 rolls the dice again. If sumOfDice matches myPoint (line 53), line 54 sets gameStatus to Status.WON, then the loop terminates because the game is complete. If sumOfDice is SEVEN (line 56), line 57 sets gameStatus to Status.LOST, and the loop terminates because the game is complete. When the game completes, lines 61–64 display a message indicating whether the player won or lost, and the program terminates. The program uses the various program-control mechanisms we’ve discussed. The Craps class uses two methods—main and rollDice (called twice from main)—and the switch, while, if…else and nested if control statements. Note also the use of multiple case labels in the switch statement to execute the same statements for sums of SEVEN and YO_LEVEN (lines 31–32) and for sums of SNAKE_EYES, TREY and BOX_CARS (lines 35–37). Why Some Constants Are Not Defined as enum Constants You might be wondering why we declared the sums of the dice as private static final int constants rather than as enum constants. The reason is that the program must compare the int variable sumOfDice (line 26) to these constants to determine the outcome of each roll. Suppose we declared enum Sum containing constants (e.g., Sum.SNAKE_EYES) representing the five sums used in the game, then used these constants in the switch statement (lines 29–45). Doing so would prevent us from using sumOfDice as the switch statement’s controlling expression, because Java does not allow an int to be compared to an enum constant. To achieve the same functionality as the current program, we would have to use a variable currentSum of type Sum as the switch’s controlling expression. Unfortunately, Java does not provide an easy way to convert an int value to a particular enum constant. This could be done with a separate switch statement. This would be cumbersome and would not improve the program’s readability (thus defeating the purpose of using an enum). 6.11 Scope of Declarations You’ve seen declarations of various Java entities, such as classes, methods, variables and parameters. Declarations introduce names that can be used to refer to such Java entities. The scope of a declaration is the portion of the program that can refer to the declared entity by its name. Such an entity is said to be “in scope” for that portion of the program. This section introduces several important scope issues. 6.11 Scope of Declarations 223 The basic scope rules are as follows: 1. The scope of a parameter declaration is the body of the method in which the declaration appears. 2. The scope of a local-variable declaration is from the point at which the declaration appears to the end of that block. 3. The scope of a local-variable declaration that appears in the initialization section of a for statement’s header is the body of the for statement and the other expressions in the header. 4. A method or field’s scope is the entire body of the class. This enables a class’s instance methods to use the fields and other methods of the class. Any block may contain variable declarations. If a local variable or parameter in a method has the same name as a field of the class, the field is hidden until the block terminates execution—this is called shadowing. To access a shadowed field in a block: • If the field is an instance variable, precede its name with the this keyword and a dot (.), as in this.x. • If the field is a static class variable, precede its name with the class’s name and a dot (.), as in ClassName.x. Figure 6.9 demonstrates scoping issues with fields and local variables. Line 7 declares and initializes the field x to 1. This field is shadowed in any block (or method) that declares a local variable named x. Method main (lines 11–23) declares a local variable x (line 13) and initializes it to 5. This local variable’s value is output to show that the field x (whose value is 1) is shadowed in main. The program declares two other methods—useLocalVariable (lines 26–35) and useField (lines 38–45)—that each take no arguments and return no results. Method main calls each method twice (lines 17–20). Method useLocalVariable declares local variable x (line 28). When useLocalVariable is first called (line 17), it creates local variable x and initializes it to 25 (line 28), outputs the value of x (lines 30–31), increments x (line 32) and outputs the value of x again (lines 33–34). When uselLocalVariable is called a second time (line 19), it recreates local variable x and reinitializes it to 25, so the output of each useLocalVariable call is identical. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 // Fig. 6.9: Scope.java // Scope class demonstrates field and local variable scopes. public class Scope { // field that is accessible to all methods of this class private static int x = 1; // method main creates and initializes local variable x // and calls methods useLocalVariable and useField public static void main(String[] args) { int x = 5; // method's local variable x shadows field x Fig. 6.9 | Scope class demonstrates field and local-variable scopes. (Part 1 of 2.) 224 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 Chapter 6 Methods: A Deeper Look System.out.printf("local x in main is %d%n", x); useLocalVariable(); // useLocalVariable has local x useField(); // useField uses class Scope's field x useLocalVariable(); // useLocalVariable reinitializes local x useField(); // class Scope's field x retains its value System.out.printf("%nlocal x in main is %d%n", x); } // create and initialize local variable x during each call public static void useLocalVariable() { int x = 25; // initialized each time useLocalVariable is called System.out.printf( "%nlocal x on entering method useLocalVariable is %d%n", x); ++x; // modifies this method's local variable x System.out.printf( "local x before exiting method useLocalVariable is %d%n", x); } // modify class Scope's field x during each call public static void useField() { System.out.printf( "%nfield x on entering method useField is %d%n", x); x *= 10; // modifies class Scope's field x System.out.printf( "field x before exiting method useField is %d%n", x); } } // end class Scope local x in main is 5 local x on entering method useLocalVariable is 25 local x before exiting method useLocalVariable is 26 field x on entering method useField is 1 field x before exiting method useField is 10 local x on entering method useLocalVariable is 25 local x before exiting method useLocalVariable is 26 field x on entering method useField is 10 field x before exiting method useField is 100 local x in main is 5 Fig. 6.9 | Scope class demonstrates field and local-variable scopes. (Part 2 of 2.) Method useField does not declare any local variables. Therefore, when it refers to x, field x (line 7) of the class is used. When method useField is first called (line 18), it outputs the value (1) of field x (lines 40–41), multiplies the field x by 10 (line 42) and outputs 6.12 Method Overloading 225 the value (10) of field x again (lines 43–44) before returning. The next time method useis called (line 20), the field has its modified value (10), so the method outputs 10, then 100. Finally, in method main, the program outputs the value of local variable x again (line 22) to show that none of the method calls modified main’s local variable x, because the methods all referred to variables named x in other scopes. Field Principle of Least Privilege In a general sense, “things” should have the capabilities they need to get their job done, but no more. An example is the scope of a variable. A variable should not be visible when it’s not needed. Good Programming Practice 6.3 Declare variables as close to where they’re first used as possible. 6.12 Method Overloading Methods of the same name can be declared in the same class, as long as they have different sets of parameters (determined by the number, types and order of the parameters)—this is called method overloading. When an overloaded method is called, the compiler selects the appropriate method by examining the number, types and order of the arguments in the call. Method overloading is commonly used to create several methods with the same name that perform the same or similar tasks, but on different types or different numbers of arguments. For example, Math methods abs, min and max (summarized in Section 6.3) are overloaded with four versions each: 1. One with two double parameters. 2. One with two float parameters. 3. One with two int parameters. 4. One with two long parameters. Our next example demonstrates declaring and invoking overloaded methods. We demonstrate overloaded constructors in Chapter 8. Declaring Overloaded Methods Class MethodOverload (Fig. 6.10) includes two overloaded versions of method square— one that calculates the square of an int (and returns an int) and one that calculates the square of a double (and returns a double). Although these methods have the same name and similar parameter lists and bodies, think of them simply as different methods. It may help to think of the method names as “square of int” and “square of double,” respectively. 1 2 3 4 5 // Fig. 6.10: MethodOverload.java // Overloaded method declarations. public class MethodOverload { Fig. 6.10 | Overloaded method declarations. (Part 1 of 2.) 226 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 Chapter 6 Methods: A Deeper Look // test overloaded square methods public static void main(String[] args) { System.out.printf("Square of integer 7 is %d%n", square(7)); System.out.printf("Square of double 7.5 is %f%n", square(7.5)); } // square method with int argument public static int square(int intValue) { System.out.printf("%nCalled square with int argument: %d%n", intValue); return intValue * intValue; } // square method with double argument public static double square(double doubleValue) { System.out.printf("%nCalled square with double argument: %f%n", doubleValue); return doubleValue * doubleValue; } } // end class MethodOverload Called square with int argument: 7 Square of integer 7 is 49 Called square with double argument: 7.500000 Square of double 7.5 is 56.250000 Fig. 6.10 | Overloaded method declarations. (Part 2 of 2.) Line 9 invokes method square with the argument 7. Literal integer values are treated as type int, so the method call in line 9 invokes the version of square at lines 14–19 that specifies an int parameter. Similarly, line 10 invokes method square with the argument 7.5. Literal floating-point values are treated as type double, so the method call in line 10 invokes the version of square at lines 22–27 that specifies a double parameter. Each method first outputs a line of text to prove that the proper method was called in each case. The values in lines 10 and 24 are displayed with the format specifier %f. We did not specify a precision in either case. By default, floating-point values are displayed with six digits of precision if the precision is not specified in the format specifier. Distinguishing Between Overloaded Methods The compiler distinguishes overloaded methods by their signatures—a combination of the method’s name and the number, types and order of its parameters, but not its return type. If the compiler looked only at method names during compilation, the code in Fig. 6.10 would be ambiguous—the compiler would not know how to distinguish between the two square methods (lines 14–19 and 22–27). Internally, the compiler uses longer method names that include the original method name, the types of each parameter and the exact order of the parameters to determine whether the methods in a class are unique in that class. 6.13 (Optional) GUI and Graphics Case Study: Colors and Filled Shapes 227 For example, in Fig. 6.10, the compiler might (internally) use the logical name “square of int” for the square method that specifies an int parameter and “square of double” for the square method that specifies a double parameter (the actual names the compiler uses are messier). If method1’s declaration begins as void method1(int a, float b) then the compiler might use the logical name “method1 of int and float.” If the parameters are specified as void method1(float a, int b) then the compiler might use the logical name “method1 of float and int.” The order of the parameter types is important—the compiler considers the preceding two method1 headers to be distinct. Return Types of Overloaded Methods In discussing the logical names of methods used by the compiler, we did not mention the return types of the methods. Method calls cannot be distinguished only by return type. If you had overloaded methods that differed only by their return types and you called one of the methods in a standalone statement as in: square(2); the compiler would not be able to determine the version of the method to call, because the return value is ignored. When two methods have the same signature and different return types, the compiler issues an error message indicating that the method is already defined in the class. Overloaded methods can have different return types if the methods have different parameter lists. Also, overloaded methods need not have the same number of parameters. Common Programming Error 6.8 Declaring overloaded methods with identical parameter lists is a compilation error regardless of whether the return types are different. 6.13 (Optional) GUI and Graphics Case Study: Colors and Filled Shapes Although you can create many interesting designs with just lines and basic shapes, class Graphics provides many more capabilities. The next two features we introduce are colors and filled shapes. Adding color enriches the drawings a user sees on the computer screen. Shapes can be filled with solid colors. Colors displayed on computer screens are defined by their red, green, and blue components (called RGB values) that have integer values from 0 to 255. The higher the value of a component color, the richer that color’s shade will be. Java uses class Color (package java.awt) to represent colors using their RGB values. For convenience, class Color contains various predefined static Color objects—BLACK, BLUE, CYAN, DARK_GRAY, GRAY, GREEN, LIGHT_GRAY, MAGENTA, ORANGE, PINK, RED, WHITE and YELLOW. Each can be accessed via the class name and a dot (.) as in Color.RED. You can create custom colors by passing the red-, green- and blue-component values to class Color’s constructor: public Color(int r, int g, int b) 228 Chapter 6 Methods: A Deeper Look Graphics methods fillRect and fillOval draw filled rectangles and ovals, respectively. These have the same parameters as drawRect and drawOval; the first two are the coordinates for the upper-left corner of the shape, while the next two determine the width and height. The example in Figs. 6.11–6.12 demonstrates colors and filled shapes by drawing and displaying a yellow smiley face on the screen. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 // Fig. 6.11: DrawSmiley.java // Drawing a smiley face using colors and filled shapes. import java.awt.Color; import java.awt.Graphics; import javax.swing.JPanel; public class DrawSmiley extends JPanel { public void paintComponent(Graphics g) { super.paintComponent(g); // draw the face g.setColor(Color.YELLOW); g.fillOval(10, 10, 200, 200); // draw the eyes g.setColor(Color.BLACK); g.fillOval(55, 65, 30, 30); g.fillOval(135, 65, 30, 30); // draw the mouth g.fillOval(50, 110, 120, 60); // "touch up" the mouth into a smile g.setColor(Color.YELLOW); g.fillRect(50, 110, 120, 30); g.fillOval(50, 120, 120, 40); } } // end class DrawSmiley Fig. 6.11 | Drawing a smiley face using colors and filled shapes. The import statements in lines 3–5 of Fig. 6.11 import classes Color, Graphics and JPanel. Class DrawSmiley (lines 7–30) uses class Color to specify drawing colors, and uses class Graphics to draw. Class JPanel again provides the area in which we draw. Line 14 in method paintComponent uses Graphics method setColor to set the current drawing color to Color.YELLOW. Method setColor requires one argument, the Color to set as the drawing color. In this case, we use the predefined object Color.YELLOW. Line 15 draws a circle with diameter 200 to represent the face—when the width and height arguments are identical, method fillOval draws a circle. Next, line 18 sets the color to Color.Black, and lines 19–20 draw the eyes. Line 23 draws the mouth as an oval, but this is not quite what we want. 6.13 (Optional) GUI and Graphics Case Study: Colors and Filled Shapes 229 To create a happy face, we’ll touch up the mouth. Line 26 sets the color to Color.YELLOW, so any shapes we draw will blend in with the face. Line 27 draws a rectangle that’s half the mouth’s height. This erases the top half of the mouth, leaving just the bottom half. To create a better smile, line 28 draws another oval to slightly cover the upper portion of the mouth. Class DrawSmileyTest (Fig. 6.12) creates and displays a JFrame containing the drawing. When the JFrame is displayed, the system calls method paintComponent to draw the smiley face. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 // Fig. 6.12: DrawSmileyTest.java // Test application that displays a smiley face. import javax.swing.JFrame; public class DrawSmileyTest { public static void main(String[] args) { DrawSmiley panel = new DrawSmiley(); JFrame application = new JFrame(); application.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); application.add(panel); application.setSize(230, 250); application.setVisible(true); } } // end class DrawSmileyTest Fig. 6.12 | Test application that displays a smiley face. GUI and Graphics Case Study Exercises 6.1 Using method fillOval, draw a bull’s-eye that alternates between two random colors, as in Fig. 6.13. Use the constructor Color(int r, int g, int b) with random arguments to generate random colors. 6.2 Create a program that draws 10 random filled shapes in random colors, positions and sizes (Fig. 6.14). Method paintComponent should contain a loop that iterates 10 times. In each iteration, the loop should determine whether to draw a filled rectangle or an oval, create a random color and choose coordinates and dimensions at random. The coordinates should be chosen based on the panel’s width and height. Lengths of sides should be limited to half the width or height of the window. 230 Chapter 6 Methods: A Deeper Look Fig. 6.13 | A bulls-eye with two alternating, random colors. Fig. 6.14 | Randomly generated shapes. 6.14 Wrap-Up In this chapter, you learned more about method declarations. You also learned the difference between instance methods and static methods and how to call static methods by preceding the method name with the name of the class in which it appears and the dot (.) separator. You learned how to use operators + and += to perform string concatenations. We discussed how the method-call stack and stack frames keep track of the methods that have been called and where each method must return to when it completes its task. We also discussed Java’s promotion rules for converting implicitly between primitive types and how to perform explicit conversions with cast operators. Next, you learned about some of the commonly used packages in the Java API. You saw how to declare named constants using both enum types and private static final variables. You used class SecureRandom to generate random numbers for simulations. You also learned about the scope of fields and local variables in a class. Finally, you learned that multiple methods in one class can be overloaded by providing methods with the same name and different signatures. Such methods can be used to perform the same or similar tasks using different types or different numbers of parameters. Summary 231 In Chapter 7, you’ll learn how to maintain lists and tables of data in arrays. You’ll see a more elegant implementation of the application that rolls a die 6,000,000 times. We’ll present two versions of a GradeBook case study that stores sets of student grades in a GradeBook object. You’ll also learn how to access an application’s command-line arguments that are passed to method main when an application begins execution. Summary Section 6.1 Introduction • Experience has shown that the best way to develop and maintain a large program is to construct it from small, simple pieces, or modules. This technique is called divide and conquer (p. 201). Section 6.2 Program Modules in Java • Methods are declared within classes. Classes are typically grouped into packages so they can be imported and reused. • Methods allow you to modularize a program by separating its tasks into self-contained units. The statements in a method are written only once and hidden from other methods. • Using existing methods as building blocks to create new programs is a form of software reusability (p. 202) that allows you to avoid repeating code within a program. Section 6.3 static Methods, static Fields and Class Math • A method call specifies the name of the method to call and provides the arguments that the called method requires to perform its task. When the method call completes, the method returns either a result, or simply control, to its caller. • A class may contain static methods to perform common tasks that do not require an object of the class. Any data a static method might require to perform its tasks can be sent to the method as arguments in a method call. A static method is called by specifying the name of the class in which the method is declared followed by a dot (.) and the method name, as in ClassName.methodName(arguments) • Class Math provides static methods for performing common mathematical calculations. • The constant Math.PI (p. 204; 3.141592653589793) is the ratio of a circle’s circumference to its diameter. The constant Math.E (p. 204; 2.718281828459045) is the base value for natural logarithms (calculated with static Math method log). • Math.PI and Math.E are declared with the modifiers public, final and static. Making them public allows you to use these fields in your own classes. A field declared with keyword final (p. 205) is constant—its value cannot be changed after it’s initialized. Both PI and E are declared final because their values never change. Making these fields static allows them to be accessed via the class name Math and a dot (.) separator, just like class Math’s methods. • All the objects of a class share one copy of the class’s static fields. Together the class variables (p. 204) and instance variables represent the fields of a class. • When you execute the Java Virtual Machine (JVM) with the java command, the JVM loads the class you specify and uses that class name to invoke method main. You can specify additional command-line arguments (p. 205) that the JVM will pass to your application. • You can place a main method in every class you declare—only the main method in the class you use to execute the application will be called by the java command. 232 Chapter 6 Methods: A Deeper Look Section 6.4 Declaring Methods with Multiple Parameters • When a method is called, the program makes a copy of the method’s argument values and assigns them to the method’s corresponding parameters. When program control returns to the point in the program where the method was called, the method’s parameters are removed from memory. • A method can return at most one value, but the returned value could be a reference to an object that contains many values. • Variables should be declared as fields of a class only if they’re required for use in more than one method of the class or if the program should save their values between calls to the class’s methods. • When a method has more than one parameter, the parameters are specified as a comma-separated list. There must be one argument in the method call for each parameter in the method declaration. Also, each argument must be consistent with the type of the corresponding parameter. If a method does not accept arguments, the parameter list is empty. • Strings can be concatenated (p. 208) using operator +, which places the characters of the right operand at the end of those in the left operand. • Every primitive value and object in Java can be represented as a String. When an object is concatenated with a String, the object is converted to a String, then the two Strings are concatenated. • If a boolean is concatenated with a String, the word "true" or the word "false" is used to represent the boolean value. • All objects in Java have a special method named toString that returns a String representation of the object’s contents. When an object is concatenated with a String, the JVM implicitly calls the object’s toString method to obtain the string representation of the object. • You can break large String literals into several smaller Strings and place them on multiple lines of code for readability, then reassemble the Strings using concatenation. Section 6.5 Notes on Declaring and Using Methods • There are three ways to call a method—using a method name by itself to call another method of the same class; using a variable that contains a reference to an object, followed by a dot (.) and the method name to call a method of the referenced object; and using the class name and a dot (.) to call a static method of a class. • There are three ways to return control to a statement that calls a method. If the method does not return a result, control returns when the program flow reaches the method-ending right brace or when the statement return; is executed. If the method returns a result, the statement return expression; evaluates the expression, then immediately returns the resulting value to the caller. Section 6.6 Method-Call Stack and Stack Frames • Stacks (p. 209) are known as last-in, first-out (LIFO) data structures—the last item pushed (inserted) onto the stack is the first item popped (removed) from the stack. • A called method must know how to return to its caller, so the return address of the calling method is pushed onto the method-call stack when the method is called. If a series of method calls occurs, the successive return addresses are pushed onto the stack in last-in, first-out order so that the last method to execute will be the first to return to its caller. • The method-call stack (p. 210) contains the memory for the local variables used in each invocation of a method during a program’s execution. This data is known as the method call’s stack Summary 233 frame or activation record. When a method call is made, the stack frame for that method call is pushed onto the method-call stack. When the method returns to its caller, its stack frame call is popped off the stack and the local variables are no longer known to the program. • If there are more method calls than can have their stack frames stored on the method-call stack, an error known as a stack overflow (p. 210) occurs. The application will compile correctly, but its execution causes a stack overflow. Section 6.7 Argument Promotion and Casting • Argument promotion (p. 210) converts an argument’s value to the type that the method expects to receive in its corresponding parameter. • Promotion rules (p. 210) apply to expressions containing values of two or more primitive types and to primitive-type values passed as arguments to methods. Each value is promoted to the “highest” type in the expression. In cases where information may be lost due to conversion, the Java compiler requires you to use a cast operator to explicitly force the conversion to occur. Section 6.9 Case Study: Secure Random-Number Generation • Objects of class SecureRandom (package java.security; p. 213) can produce nondeterministic random values. • SecureRandom method nextInt (p. 214) generates a random int value. • Class SecureRandom provides another version of method nextInt that receives an int argument and returns a value from 0 up to, but not including, the argument’s value. • Random numbers in a range (p. 214) can be generated with int number = shiftingValue + randomNumbers.nextInt(scalingFactor); where shiftingValue specifies the first number in the desired range of consecutive integers, and scalingFactor specifies how many numbers are in the range. • Random numbers can be chosen from nonconsecutive integer ranges, as in shiftingValue + differenceBetweenValues * randomNumbers.nextInt(scalingFactor); int number = where shiftingValue specifies the first number in the range of values, differenceBetweenValues represents the difference between consecutive numbers in the sequence and scalingFactor specifies how many numbers are in the range. Section 6.10 Case Study: A Game of Chance; Introducing enum Types • An enum type (p. 221) is introduced by the keyword enum and a type name. As with any class, braces ({ and }) delimit the body of an enum declaration. Inside the braces is a comma-separated list of enum constants, each representing a unique value. The identifiers in an enum must be unique. Variables of an enum type can be assigned only constants of that enum type. • Constants can also be declared as private static final variables. Such constants are declared by convention with all capital letters to make them stand out in the program. Section 6.11 Scope of Declarations • Scope (p. 222) is the portion of the program in which an entity, such as a variable or a method, can be referred to by its name. Such an entity is said to be “in scope” for that portion of the program. • The scope of a parameter declaration is the body of the method in which the declaration appears. • The scope of a local-variable declaration is from the point at which the declaration appears to the end of that block. 234 Chapter 6 Methods: A Deeper Look • The scope of a local-variable declaration that appears in the initialization section of a for statement’s header is the body of the for statement and the other expressions in the header. • The scope of a method or field of a class is the entire body of the class. This enables a class’s methods to use simple names to call the class’s other methods and to access the class’s fields. • Any block may contain variable declarations. If a local variable or parameter in a method has the same name as a field, the field is shadowed (p. 223) until the block terminates execution. Section 6.12 Method Overloading • Java allows overloaded methods (p. 225) in a class, as long as the methods have different sets of parameters (determined by the number, order and types of the parameters). • Overloaded methods are distinguished by their signatures (p. 226)—combinations of the methods’ names and the number, types and order of their parameters, but not their return types. Self-Review Exercises 6.1 Fill in the blanks in each of the following statements: a) A method is invoked with a(n) . b) A variable known only within the method in which it’s declared is called a(n) . statement in a called method can be used to pass the value of an expresc) The sion back to the calling method. d) The keyword indicates that a method does not return a value. of a stack. e) Data can be added or removed only from the f) Stacks are known as data structures; the last item pushed (inserted) onto the stack is the first item popped (removed) from the stack. , g) The three ways to return control from a called method to a caller are and . h) An object of class produces truly random numbers. i) The method-call stack contains the memory for local variables on each invocation of a method during a program’s execution. This data, stored as a portion of the method-call stack, is known as the or of the method call. j) If there are more method calls than can be stored on the method-call stack, an error occurs. known as a(n) k) The of a declaration is the portion of a program that can refer to the entity in the declaration by name. l) It’s possible to have several methods with the same name that each operate on different types or numbers of arguments. This feature is called method . 6.2 For the class Craps in Fig. 6.8, state the scope of each of the following entities: a) the variable randomNumbers. b) the variable die1. c) the method rollDice. d) the method main. e) the variable sumOfDice. 6.3 Write an application that tests whether the examples of the Math class method calls shown in Fig. 6.2 actually produce the indicated results. 6.4 Give the method header for each of the following methods: a) Method hypotenuse, which takes two double-precision, floating-point arguments side1 and side2 and returns a double-precision, floating-point result. b) Method smallest, which takes three integers x, y and z and returns an integer. Answers to Self-Review Exercises 235 c) Method instructions, which does not take any arguments and does not return a value. [Note: Such methods are commonly used to display instructions to a user.] d) Method intToFloat, which takes integer argument number and returns a float. 6.5 Find the error in each of the following program segments. Explain how to correct the error. a) void g() { System.out.println("Inside method g"); void h() { System.out.println("Inside method h"); } b) c) d) } int sum(int x, int y) { int result; result = x + y; } void f(float a); { float a; System.out.println(a); } void product() { int a = 6, b = 5, c = 4, result; result = a * b * c; System.out.printf("Result is %d%n", result); return result; } 6.6 Declare method sphereVolume to calculate and return the volume of the sphere. Use the following statement to calculate the volume: double volume = (4.0 / 3.0) * Math.PI * Math.pow(radius, 3) Write a Java application that prompts the user for the double radius of a sphere, calls sphereVolume to calculate the volume and displays the result. Answers to Self-Review Exercises 6.1 a) method call. b) local variable. c) return. d) void. e) top. f) last-in, first-out (LIFO). g) return; or return expression; or encountering the closing right brace of a method. h) SecureRandom. i) stack frame, activation record. j) stack overflow. k) scope. l) method overloading. 6.2 a) class body. b) block that defines method rollDice’s body. c) class body. d) class body. e) block that defines method main’s body. 6.3 1 2 3 4 5 6 The following solution demonstrates the Math class methods in Fig. 6.2: // Exercise 6.3: MathTest.java // Testing the Math class methods. public class MathTest { public static void main(String[] args) { 236 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 Chapter 6 Methods: A Deeper Look System.out.printf("Math.abs(23.7) = %f%n", Math.abs(23.7)); System.out.printf("Math.abs(0.0) = %f%n", Math.abs(0.0)); System.out.printf("Math.abs(-23.7) = %f%n", Math.abs(-23.7)); System.out.printf("Math.ceil(9.2) = %f%n", Math.ceil(9.2)); System.out.printf("Math.ceil(-9.8) = %f%n", Math.ceil(-9.8)); System.out.printf("Math.cos(0.0) = %f%n", Math.cos(0.0)); System.out.printf("Math.exp(1.0) = %f%n", Math.exp(1.0)); System.out.printf("Math.exp(2.0) = %f%n", Math.exp(2.0)); System.out.printf("Math.floor(9.2) = %f%n", Math.floor(9.2)); System.out.printf("Math.floor(-9.8) = %f%n", Math.floor(-9.8)); System.out.printf("Math.log(Math.E) = %f%n", Math.log(Math.E)); System.out.printf("Math.log(Math.E * Math.E) = %f%n", Math.log(Math.E * Math.E)); System.out.printf("Math.max(2.3, 12.7) = %f%n", Math.max(2.3, 12.7)); System.out.printf("Math.max(-2.3, -12.7) = %f%n", Math.max(-2.3, -12.7)); System.out.printf("Math.min(2.3, 12.7) = %f%n", Math.min(2.3, 12.7)); System.out.printf("Math.min(-2.3, -12.7) = %f%n", Math.min(-2.3, -12.7)); System.out.printf("Math.pow(2.0, 7.0) = %f%n", Math.pow(2.0, 7.0)); System.out.printf("Math.pow(9.0, 0.5) = %f%n", Math.pow(9.0, 0.5)); System.out.printf("Math.sin(0.0) = %f%n", Math.sin(0.0)); System.out.printf("Math.sqrt(900.0) = %f%n", Math.sqrt(900.0)); System.out.printf("Math.tan(0.0) = %f%n", Math.tan(0.0)); } // end main } // end class MathTest Math.abs(23.7) = 23.700000 Math.abs(0.0) = 0.000000 Math.abs(-23.7) = 23.700000 Math.ceil(9.2) = 10.000000 Math.ceil(-9.8) = -9.000000 Math.cos(0.0) = 1.000000 Math.exp(1.0) = 2.718282 Math.exp(2.0) = 7.389056 Math.floor(9.2) = 9.000000 Math.floor(-9.8) = -10.000000 Math.log(Math.E) = 1.000000 Math.log(Math.E * Math.E) = 2.000000 Math.max(2.3, 12.7) = 12.700000 Math.max(-2.3, -12.7) = -2.300000 Math.min(2.3, 12.7) = 2.300000 Math.min(-2.3, -12.7) = -12.700000 Math.pow(2.0, 7.0) = 128.000000 Math.pow(9.0, 0.5) = 3.000000 Math.sin(0.0) = 0.000000 Math.sqrt(900.0) = 30.000000 Math.tan(0.0) = 0.000000 6.4 6.5 a) b) c) d) double hypotenuse(double side1, double side2) int smallest(int x, int y, int z) void instructions() float intToFloat(int number) a) Error: Method h is declared within method g. Correction: Move the declaration of h outside the declaration of g. b) Error: The method is supposed to return an integer, but does not. Correction: Delete the variable result, and place the statement return x + y; in the method, or add the following statement at the end of the method body: return result; Exercises 237 c) Error: The semicolon after the right parenthesis of the parameter list is incorrect, and the parameter a should not be redeclared in the method. Correction: Delete the semicolon after the right parenthesis of the parameter list, and delete the declaration float a;. d) Error: The method returns a value when it’s not supposed to. Correction: Change the return type from void to int. 6.6 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 The following solution calculates the volume of a sphere, using the radius entered by the user: // Exercise 6.6: Sphere.java // Calculate the volume of a sphere. import java.util.Scanner; public class Sphere { // obtain radius from user and display volume of sphere public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.print("Enter radius of sphere: "); double radius = input.nextDouble(); System.out.printf("Volume is %f%n", sphereVolume(radius)); } // end method determineSphereVolume // calculate and return sphere volume public static double sphereVolume(double radius) { double volume = (4.0 / 3.0) * Math.PI * Math.pow(radius, 3); return volume; } // end method sphereVolume } // end class Sphere Enter radius of sphere: 4 Volume is 268.082573 Exercises 6.7 What is the value of x after each of the following statements is executed? a) x = Math.abs(7.5); b) x = Math.floor(7.5); c) x = Math.abs(0.0); d) x = Math.ceil(0.0); e) x = Math.abs(-6.4); f) x = Math.ceil(-6.4); g) x = Math.ceil(-Math.abs(-8 + Math.floor(-5.5))); 6.8 (Parking Charges) A parking garage charges a $2.00 minimum fee to park for up to three hours. The garage charges an additional $0.50 per hour for each hour or part thereof in excess of three hours. The maximum charge for any given 24-hour period is $10.00. Assume that no car parks for longer than 24 hours at a time. Write an application that calculates and displays the parking charges for each customer who parked in the garage yesterday. You should enter the hours parked for each customer. The program should display the charge for the current customer and should calculate and display the running total of yesterday’s receipts. It should use the method calculateCharges to determine the charge for each customer. 238 6.9 Chapter 6 Methods: A Deeper Look (Rounding Numbers) Math.floor can be used to round values to the nearest integer—e.g., y = Math.floor(x + 0.5); will round the number x to the nearest integer and assign the result to y. Write an application that reads double values and uses the preceding statement to round each of the numbers to the nearest integer. For each number processed, display both the original number and the rounded number. 6.10 (Rounding Numbers) To round numbers to specific decimal places, use a statement like y = Math.floor(x * 10 + 0.5) / 10; which rounds x to the tenths position (i.e., the first position to the right of the decimal point), or y = Math.floor(x * 100 + 0.5) / 100; which rounds x to the hundredths position (i.e., the second position to the right of the decimal point). Write an application that defines four methods for rounding a number x in various ways: a) roundToInteger(number) b) roundToTenths(number) c) roundToHundredths(number) d) roundToThousandths(number) For each value read, your program should display the original value, the number rounded to the nearest integer, the number rounded to the nearest tenth, the number rounded to the nearest hundredth and the number rounded to the nearest thousandth. 6.11 Answer each of the following questions: a) What does it mean to choose numbers “at random”? b) Why is the nextInt method of class SecureRandom useful for simulating games of chance? c) Why is it often necessary to scale or shift the values produced by a SecureRandom object? d) Why is computerized simulation of real-world situations a useful technique? 6.12 Write statements that assign random integers to the variable n in the following ranges: a) 1 ≤ n ≤ 2. b) 1 ≤ n ≤ 100. c) 0 ≤ n ≤ 9. d) 1000 ≤ n ≤ 1112. e) –1 ≤ n ≤ 1. f) –3 ≤ n ≤ 11. 6.13 Write statements that will display a random number from each of the following sets: a) 2, 4, 6, 8, 10. b) 3, 5, 7, 9, 11. c) 6, 10, 14, 18, 22. 6.14 (Exponentiation) Write a method integerPower(base, exponent) that returns the value of base exponent For example, integerPower(3, 4) calculates 34 (or 3 * 3 * 3 * 3). Assume that exponent is a positive, nonzero integer and that base is an integer. Use a for or while statement to control the calculation. Do not use any Math class methods. Incorporate this method into an application that reads integer values for base and exponent and performs the calculation with the integerPower method. 6.15 (Hypotenuse Calculations) Define a method hypotenuse that calculates the hypotenuse of a right triangle when the lengths of the other two sides are given. The method should take two arguments of type double and return the hypotenuse as a double. Incorporate this method into an Exercises 239 application that reads values for side1 and side2 and performs the calculation with the hypotenuse method. Use Math methods pow and sqrt to determine the length of the hypotenuse for each of the triangles in Fig. 6.15. [Note: Class Math also provides method hypot to perform this calculation.] Triangle Side 1 Side 2 1 2 3 3.0 4.0 5.0 12.0 8.0 15.0 Fig. 6.15 | Values for the sides of triangles in Exercise 6.15. 6.16 (Multiples) Write a method isMultiple that determines, for a pair of integers, whether the second integer is a multiple of the first. The method should take two integer arguments and return true if the second is a multiple of the first and false otherwise. [Hint: Use the remainder operator.] Incorporate this method into an application that inputs a series of pairs of integers (one pair at a time) and determines whether the second value in each pair is a multiple of the first. 6.17 (Even or Odd) Write a method isEven that uses the remainder operator (%) to determine whether an integer is even. The method should take an integer argument and return true if the integer is even and false otherwise. Incorporate this method into an application that inputs a sequence of integers (one at a time) and determines whether each is even or odd. 6.18 (Displaying a Square of Asterisks) Write a method squareOfAsterisks that displays a solid square (the same number of rows and columns) of asterisks whose side is specified in integer parameter side. For example, if side is 4, the method should display **** **** **** **** Incorporate this method into an application that reads an integer value for side from the user and outputs the asterisks with the squareOfAsterisks method. 6.19 (Displaying a Square of Any Character) Modify the method created in Exercise 6.18 to receive a second parameter of type char called fillCharacter. Form the square using the char provided as an argument. Thus, if side is 5 and fillCharacter is #, the method should display ##### ##### ##### ##### ##### Use the following statement (in which input is a Scanner object) to read a character from the user at the keyboard: char fill = input.next().charAt(0); 6.20 (Circle Area) Write an application that prompts the user for the radius of a circle and uses a method called circleArea to calculate the area of the circle. 6.21 (Separating Digits) Write methods that accomplish each of the following tasks: a) Calculate the integer part of the quotient when integer a is divided by integer b. b) Calculate the integer remainder when integer a is divided by integer b. 240 Chapter 6 Methods: A Deeper Look c) Use the methods developed in parts (a) and (b) to write a method displayDigits that receives an integer between 1 and 99999 and displays it as a sequence of digits, separating each pair of digits by two spaces. For example, the integer 4562 should appear as 4 5 6 2 Incorporate the methods into an application that inputs an integer and calls displayDigits by passing the method the integer entered. Display the results. 6.22 (Temperature Conversions) Implement the following integer methods: a) Method celsius returns the Celsius equivalent of a Fahrenheit temperature, using the calculation celsius = 5.0 / 9.0 * (fahrenheit - 32); b) Method fahrenheit returns the Fahrenheit equivalent of a Celsius temperature, using the calculation fahrenheit = 9.0 / 5.0 * celsius + 32; c) Use the methods from parts (a) and (b) to write an application that enables the user either to enter a Fahrenheit temperature and display the Celsius equivalent or to enter a Celsius temperature and display the Fahrenheit equivalent. 6.23 (Find the Minimum) Write a method minimum3 that returns the smallest of three floatingpoint numbers. Use the Math.min method to implement minimum3. Incorporate the method into an application that reads three values from the user, determines the smallest value and displays the result. 6.24 (Perfect Numbers) An integer number is said to be a perfect number if its factors, including 1 (but not the number itself), sum to the number. For example, 6 is a perfect number, because 6 = 1 + 2 + 3. Write a method isPerfect that determines whether parameter number is a perfect number. Use this method in an application that displays all the perfect numbers between 1 and 1000. Display the factors of each perfect number to confirm that the number is indeed perfect. Challenge the computing power of your computer by testing numbers much larger than 1000. Display the results. 6.25 (Prime Numbers) A positive integer is prime if it’s divisible by only 1 and itself. For example, 2, 3, 5 and 7 are prime, but 4, 6, 8 and 9 are not. The number 1, by definition, is not prime. a) Write a method that determines whether a number is prime. b) Use this method in an application that determines and displays all the prime numbers less than 10,000. How many numbers up to 10,000 do you have to test to ensure that you’ve found all the primes? c) Initially, you might think that n/2 is the upper limit for which you must test to see whether a number n is prime, but you need only go as high as the square root of n. Rewrite the program, and run it both ways. 6.26 (Reversing Digits) Write a method that takes an integer value and returns the number with its digits reversed. For example, given the number 7631, the method should return 1367. Incorporate the method into an application that reads a value from the user and displays the result. 6.27 (Greatest Common Divisor) The greatest common divisor (GCD) of two integers is the largest integer that evenly divides each of the two numbers. Write a method gcd that returns the greatest common divisor of two integers. [Hint: You might want to use Euclid’s algorithm. You can find information about it at en.wikipedia.org/wiki/Euclidean_algorithm.] Incorporate the method into an application that reads two values from the user and displays the result. 6.28 Write a method qualityPoints that inputs a student’s average and returns 4 if it’s 90–100, 3 if 80–89, 2 if 70–79, 1 if 60–69 and 0 if lower than 60. Incorporate the method into an application that reads a value from the user and displays the result. Making a Difference 241 6.29 (Coin Tossing) Write an application that simulates coin tossing. Let the program toss a coin each time the user chooses the “Toss Coin” menu option. Count the number of times each side of the coin appears. Display the results. The program should call a separate method flip that takes no arguments and returns a value from a Coin enum (HEADS and TAILS). [Note: If the program realistically simulates coin tossing, each side of the coin should appear approximately half the time.] 6.30 (Guess the Number) Write an application that plays “guess the number” as follows: Your program chooses the number to be guessed by selecting a random integer in the range 1 to 1000. The application displays the prompt Guess a number between 1 and 1000. The player inputs a first guess. If the player's guess is incorrect, your program should display Too high. Try again. or Too low. Try again. to help the player “zero in” on the correct answer. The program should prompt the user for the next guess. When the user enters the correct answer, display Congratulations. You guessed the number!, and allow the user to choose whether to play again. [Note: The guessing technique employed in this problem is similar to a binary search, which is discussed in Chapter 19, Searching, Sorting and Big O.] 6.31 (Guess the Number Modification) Modify the program of Exercise 6.30 to count the number of guesses the player makes. If the number is 10 or fewer, display Either you know the secret or you got lucky! If the player guesses the number in 10 tries, display Aha! You know the secret! If the player makes more than 10 guesses, display You should be able to do better! Why should it take no more than 10 guesses? Well, with each “good guess,” the player should be able to eliminate half of the numbers, then half of the remaining numbers, and so on. 6.32 (Distance Between Points) Write method distance to calculate the distance between two points (x1, y1) and (x2, y2). All numbers and return values should be of type double. Incorporate this method into an application that enables the user to enter the coordinates of the points. 6.33 (Craps Game Modification) Modify the craps program of Fig. 6.8 to allow wagering. Initialize variable bankBalance to 1000 dollars. Prompt the player to enter a wager. Check that wager is less than or equal to bankBalance, and if it’s not, have the user reenter wager until a valid wager is entered. Then, run one game of craps. If the player wins, increase bankBalance by wager and display the new bankBalance. If the player loses, decrease bankBalance by wager, display the new bankBalance, check whether bankBalance has become zero and, if so, display the message "Sorry. You busted!" As the game progresses, display various messages to create some “chatter,” such as "Oh, you're going for broke, huh?" or "Aw c'mon, take a chance!" or "You're up big. Now's the time to cash in your chips!". Implement the “chatter” as a separate method that randomly chooses the string to display. 6.34 (Table of Binary, Octal and Hexadecimal Numbers) Write an application that displays a table of the binary, octal and hexadecimal equivalents of the decimal numbers in the range 1 through 256. If you aren’t familiar with these number systems, read online Appendix J first. Making a Difference As computer costs decline, it becomes feasible for every student, regardless of economic circumstance, to have a computer and use it in school. This creates exciting possibilities for improving the educational experience of all students worldwide, as suggested by the next five exercises. [Note: Check out initiatives such as the One Laptop Per Child Project (www.laptop.org). Also, research “green” laptops—what are some key “going green” characteristics of these devices? Look into the Electronic Product Environmental Assessment Tool (www.epeat.net), which can help you assess the “greenness” of desktops, notebooks and monitors to help you decide which products to purchase.] 6.35 (Computer-Assisted Instruction) The use of computers in education is referred to as computer-assisted instruction (CAI). Write a program that will help an elementary school student learn 242 Chapter 6 Methods: A Deeper Look multiplication. Use a SecureRandom object to produce two positive one-digit integers. The program should then prompt the user with a question, such as How much is 6 times 7? The student then inputs the answer. Next, the program checks the student’s answer. If it’s correct, display the message "Very good!" and ask another multiplication question. If the answer is wrong, display the message "No. Please try again." and let the student try the same question repeatedly until the student finally gets it right. A separate method should be used to generate each new question. This method should be called once when the application begins execution and each time the user answers the question correctly. 6.36 (Computer-Assisted Instruction: Reducing Student Fatigue) One problem in CAI environments is student fatigue. This can be reduced by varying the computer’s responses to hold the student’s attention. Modify the program of Exercise 6.35 so that various comments are displayed for each answer as follows: Possible responses to a correct answer: Very good! Excellent! Nice work! Keep up the good work! Possible responses to an incorrect answer: No. Please try again. Wrong. Try once more. Don't give up! No. Keep trying. Use random-number generation to choose a number from 1 to 4 that will be used to select one of the four appropriate responses to each correct or incorrect answer. Use a switch statement to issue the responses. 6.37 (Computer-Assisted Instruction: Monitoring Student Performance) More sophisticated computer-assisted instruction systems monitor the student’s performance over a period of time. The decision to begin a new topic is often based on the student’s success with previous topics. Modify the program of Exercise 6.36 to count the number of correct and incorrect responses typed by the student. After the student types 10 answers, your program should calculate the percentage that are correct. If the percentage is lower than 75%, display "Please ask your teacher for extra help.", then reset the program so another student can try it. If the percentage is 75% or higher, display "Congratulations, you are ready to go to the next level!", then reset the program so another student can try it. 6.38 (Computer-Assisted Instruction: Difficulty Levels) Exercises 6.35–6.37 developed a computer-assisted instruction program to help teach an elementary school student multiplication. Modify the program to allow the user to enter a difficulty level. At a difficulty level of 1, the program should use only single-digit numbers in the problems; at a difficulty level of 2, numbers as large as two digits, and so on. 6.39 (Computer-Assisted Instruction: Varying the Types of Problems) Modify the program of Exercise 6.38 to allow the user to pick a type of arithmetic problem to study. An option of 1 means addition problems only, 2 means subtraction problems only, 3 means multiplication problems only, 4 means division problems only and 5 means a random mixture of all these types. 7 Arrays and ArrayLists Begin at the beginning, … and go on till you come to the end: then stop. —Lewis Carroll To go beyond is as wrong as to fall short. —Confucius Objectives In this chapter you’ll: ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Learn what arrays are. Use arrays to store data in and retrieve data from lists and tables of values. Declare arrays, initialize arrays and refer to individual elements of arrays. Iterate through arrays with the enhanced for statement. Pass arrays to methods. Declare and manipulate multidimensional arrays. Use variable-length argument lists. Read command-line arguments into a program. Build an object-oriented instructor gradebook class. Perform common array manipulations with the methods of class Arrays. Use class ArrayList to manipulate a dynamically resizable arraylike data structure. 244 Chapter 7 Arrays and ArrayLists 7.1 7.2 7.3 7.4 Introduction Arrays Declaring and Creating Arrays Examples Using Arrays 7.4.1 Creating and Initializing an Array 7.4.2 Using an Array Initializer 7.4.3 Calculating the Values to Store in an Array 7.4.4 Summing the Elements of an Array 7.4.5 Using Bar Charts to Display Array Data Graphically 7.4.6 Using the Elements of an Array as Counters 7.4.7 Using Arrays to Analyze Survey Results 7.5 Exception Handling: Processing the Incorrect Response 7.5.1 The try Statement 7.5.2 Executing the catch Block 7.5.3 toString Method of the Exception Parameter 7.7 7.8 7.9 7.10 7.11 7.12 7.13 7.14 7.15 7.16 Enhanced for Statement Passing Arrays to Methods Pass-By-Value vs. Pass-By-Reference Case Study: Class GradeBook Using an Array to Store Grades Multidimensional Arrays Case Study: Class GradeBook Using a Two-Dimensional Array Variable-Length Argument Lists Using Command-Line Arguments Class Arrays Introduction to Collections and Class ArrayList 7.17 (Optional) GUI and Graphics Case Study: Drawing Arcs 7.18 Wrap-Up 7.6 Case Study: Card Shuffling and Dealing Simulation Summary | Self-Review Exercises | Answers to Self-Review Exercises | Exercises | Special Section: Building Your Own Computer | Making a Difference 7.1 Introduction This chapter introduces data structures—collections of related data items. Array objects are data structures consisting of related data items of the same type. Arrays make it convenient to process related groups of values. Arrays remain the same length once they’re created. We study data structures in depth in Chapters 16–21. After discussing how arrays are declared, created and initialized, we present practical examples that demonstrate common array manipulations. We introduce Java’s exceptionhandling mechanism and use it to allow a program to continue executing when it attempts to access an array element that does not exist. We also present a case study that examines how arrays can help simulate the shuffling and dealing of playing cards in a card-game application. We introduce Java’s enhanced for statement, which allows a program to access the data in an array more easily than does the counter-controlled for statement presented in Section 5.3. We build two versions of an instructor GradeBook case study that use arrays to maintain sets of student grades in memory and analyze student grades. We show how to use variable-length argument lists to create methods that can be called with varying numbers of arguments, and we demonstrate how to process command-line arguments in method main. Next, we present some common array manipulations with static methods of class Arrays from the java.util package. Although commonly used, arrays have limited capabilities. For instance, you must specify an array’s size, and if at execution time you wish to modify it, you must do so by creating a new array. At the end of this chapter, we introduce one of Java’s prebuilt data 7.2 Arrays 245 structures from the Java API’s collection classes. These offer greater capabilities than traditional arrays. They’re reusable, reliable, powerful and efficient. We focus on the ArrayList collection. ArrayLists are similar to arrays but provide additional functionality, such as dynamic resizing as necessary to accommodate more or fewer elements. Java SE 8 After reading Chapter 17, Java SE 8 Lambdas and Streams, you’ll be able to reimplement many of Chapter 7’s examples in a more concise and elegant manner, and in a way that makes them easier to parallelize to improve performance on today’s multi-core systems. 7.2 Arrays An array is a group of variables (called elements or components) containing values that all have the same type. Arrays are objects, so they’re considered reference types. As you’ll soon see, what we typically think of as an array is actually a reference to an array object in memory. The elements of an array can be either primitive types or reference types (including arrays, as we’ll see in Section 7.11). To refer to a particular element in an array, we specify the name of the reference to the array and the position number of the element in the array. The position number of the element is called the element’s index or subscript. Logical Array Representation Figure 7.1 shows a logical representation of an integer array called c. This array contains 12 elements. A program refers to any one of these elements with an array-access expression that includes the name of the array followed by the index of the particular element in square brackets ([]). The first element in every array has index zero and is sometimes called the zeroth element. Thus, the elements of array c are c[0], c[1], c[2] and so on. The highest index in array c is 11, which is 1 less than 12—the number of elements in the array. Array names follow the same conventions as other variable names. Name of array (c) Index (or subscript) of the element in array c Fig. 7.1 | A 12-element array. c[ 0 ] -45 c[ 1 ] 6 c[ 2 ] 0 c[ 3 ] 72 c[ 4 ] 1543 c[ 5 ] -89 c[ 6 ] 0 c[ 7 ] 62 c[ 8 ] -3 c[ 9 ] 1 c[ 10 ] 6453 c[ 11 ] 78 246 Chapter 7 Arrays and ArrayLists An index must be a nonnegative integer. A program can use an expression as an index. For example, if we assume that variable a is 5 and variable b is 6, then the statement c[a + b] += 2; adds 2 to array element c[11]. An indexed array name is an array-access expression, which can be used on the left side of an assignment to place a new value into an array element. Common Programming Error 7.1 An index must be an int value or a value of a type that can be promoted to int—namely, byte, short or char, but not long; otherwise, a compilation error occurs. Let’s examine array c in Fig. 7.1 more closely. The name of the array is c. Every array object knows its own length and stores it in a length instance variable. The expression c.length returns array c’s length. Even though the length instance variable of an array is public, it cannot be changed because it’s a final variable. This array’s 12 elements are referred to as c[0], c[1], c[2], …, c[11]. The value of c[0] is -45, the value of c[1] is 6, the value of c[2] is 0, the value of c[7] is 62 and the value of c[11] is 78. To calculate the sum of the values contained in the first three elements of array c and store the result in variable sum, we would write sum = c[0] + c[1] + c[2]; To divide the value of c[6] by 2 and assign the result to the variable x, we would write x = c[6] / 2; 7.3 Declaring and Creating Arrays Array objects occupy space in memory. Like other objects, arrays are created with keyword new. To create an array object, you specify the type of the array elements and the number of elements as part of an array-creation expression that uses keyword new. Such an expression returns a reference that can be stored in an array variable. The following declaration and array-creation expression create an array object containing 12 int elements and store the array’s reference in the array variable named c: int[] c = new int[12]; This expression can be used to create the array in Fig. 7.1. When an array is created, each of its elements receives a default value—zero for the numeric primitive-type elements, false for boolean elements and null for references. As you’ll soon see, you can provide nondefault element values when you create an array. Creating the array in Fig. 7.1 can also be performed in two steps as follows: int[] c; // declare the array variable c = new int[12]; // create the array; assign to array variable In the declaration, the square brackets following the type indicate that c is a variable that will refer to an array (i.e., the variable will store an array reference). In the assignment statement, the array variable c receives the reference to a new array of 12 int elements. Common Programming Error 7.2 In an array declaration, specifying the number of elements in the square brackets of the declaration (e.g., int[12] c;) is a syntax error. 7.4 Examples Using Arrays 247 A program can create several arrays in a single declaration. The following declaration reserves 100 elements for b and 27 elements for x: String[] b = new String[100], x = new String[27]; When the type of the array and the square brackets are combined at the beginning of the declaration, all the identifiers in the declaration are array variables. In this case, variables b and x refer to String arrays. For readability, we prefer to declare only one variable per declaration. The preceding declaration is equivalent to: String[] b = new String[100]; // create array b String[] x = new String[27]; // create array x Good Programming Practice 7.1 For readability, declare only one variable per declaration. Keep each declaration on a separate line, and include a comment describing the variable being declared. When only one variable is declared in each declaration, the square brackets can be placed either after the type or after the array variable name, as in: String b[] = new String[100]; // create array b String x[] = new String[27]; // create array x but placing the square brackets after the type is preferred. Common Programming Error 7.3 Declaring multiple array variables in a single declaration can lead to subtle errors. Consider the declaration int[] a, b, c;. If a, b and c should be declared as array variables, then this declaration is correct—placing square brackets directly following the type indicates that all the identifiers in the declaration are array variables. However, if only a is intended to be an array variable, and b and c are intended to be individual int variables, then this declaration is incorrect—the declaration int a[], b, c; would achieve the desired result. A program can declare arrays of any type. Every element of a primitive-type array contains a value of the array’s declared element type. Similarly, in an array of a reference type, every element is a reference to an object of the array’s declared element type. For example, every element of an int array is an int value, and every element of a String array is a reference to a String object. 7.4 Examples Using Arrays This section presents several examples that demonstrate declaring arrays, creating arrays, initializing arrays and manipulating array elements. 7.4.1 Creating and Initializing an Array The application of Fig. 7.2 uses keyword new to create an array of 10 int elements, which are initially zero (the default initial value for int variables). Line 9 declares array—a reference capable of referring to an array of int elements—then initializes the variable with a reference to an array object containing 10 int elements. Line 11 outputs the column headings. The first column contains the index (0–9) of each array element, and the second column contains the default initial value (0) of each array element. 248 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Chapter 7 Arrays and ArrayLists // Fig. 7.2: InitArray.java // Initializing the elements of an array to default values of zero. public class InitArray { public static void main(String[] args) { // declare variable array and initialize it with an array object int[] array = new int[10]; // create the array object System.out.printf("%s%8s%n", "Index", "Value"); // column headings // output each array element's value for (int counter = 0; counter < array.length; counter++) System.out.printf("%5d%8d%n", counter, array[counter]); } } // end class InitArray Index 0 1 2 3 4 5 6 7 8 9 Value 0 0 0 0 0 0 0 0 0 0 Fig. 7.2 | Initializing the elements of an array to default values of zero. The for statement (lines 14–15) outputs the index (represented by counter) and value of each array element (represented by array[counter]). Control variable counter is initially 0—index values start at 0, so using zero-based counting allows the loop to access every element of the array. The for’s loop-continuation condition uses the expression array.length (line 14) to determine the length of the array. In this example, the length of the array is 10, so the loop continues executing as long as the value of control variable counter is less than 10. The highest index value of a 10-element array is 9, so using the less-than operator in the loop-continuation condition guarantees that the loop does not attempt to access an element beyond the end of the array (i.e., during the final iteration of the loop, counter is 9). We’ll soon see what Java does when it encounters such an out-of-range index at execution time. 7.4.2 Using an Array Initializer You can create an array and initialize its elements with an array initializer—a comma-separated list of expressions (called an initializer list) enclosed in braces. In this case, the array length is determined by the number of elements in the initializer list. For example, int[] n = { 10, 20, 30, 40, 50 }; creates a five-element array with index values 0–4. Element n[0] is initialized to 10, n[1] is initialized to 20, and so on. When the compiler encounters an array declaration that in- 7.4 Examples Using Arrays 249 cludes an initializer list, it counts the number of initializers in the list to determine the size of the array, then sets up the appropriate new operation “behind the scenes.” The application in Fig. 7.3 initializes an integer array with 10 values (line 9) and displays the array in tabular format. The code for displaying the array elements (lines 14–15) is identical to that in Fig. 7.2 (lines 15–16). 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 // Fig. 7.3: InitArray.java // Initializing the elements of an array with an array initializer. public class InitArray { public static void main(String[] args) { // initializer list specifies the initial value for each element int[] array = { 32, 27, 64, 18, 95, 14, 90, 70, 60, 37 }; System.out.printf("%s%8s%n", "Index", "Value"); // column headings // output each array element's value for (int counter = 0; counter < array.length; counter++) System.out.printf("%5d%8d%n", counter, array[counter]); } // end class InitArray Index 0 1 2 3 4 5 6 7 8 9 Value 32 27 64 18 95 14 90 70 60 37 Fig. 7.3 | Initializing the elements of an array with an array initializer. 7.4.3 Calculating the Values to Store in an Array The application in Fig. 7.4 creates a 10-element array and assigns to each element one of the even integers from 2 to 20 (2, 4, 6, …, 20). Then the application displays the array in tabular format. The for statement at lines 12–13 calculates an array element’s value by multiplying the current value of the control variable counter by 2, then adding 2. 1 2 3 4 5 // Fig. 7.4: InitArray.java // Calculating the values to be placed into the elements of an array. public class InitArray { Fig. 7.4 | Calculating the values to be placed into the elements of an array. (Part 1 of 2.) 250 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 Chapter 7 Arrays and ArrayLists public static void main(String[] args) { final int ARRAY_LENGTH = 10; // declare constant int[] array = new int[ARRAY_LENGTH]; // create array // calculate value for each array element for (int counter = 0; counter < array.length; counter++) array[counter] = 2 + 2 * counter; System.out.printf("%s%8s%n", "Index", "Value"); // column headings // output each array element's value for (int counter = 0; counter < array.length; counter++) System.out.printf("%5d%8d%n", counter, array[counter]); } } // end class InitArray Index 0 1 2 3 4 5 6 7 8 9 Value 2 4 6 8 10 12 14 16 18 20 Fig. 7.4 | Calculating the values to be placed into the elements of an array. (Part 2 of 2.) Line 8 uses the modifier final to declare the constant variable ARRAY_LENGTH with the value 10. Constant variables must be initialized before they’re used and cannot be modified thereafter. If you attempt to modify a final variable after it’s initialized in its declaration, the compiler issues an error message like cannot assign a value to final variable variableName Good Programming Practice 7.2 Constant variables also are called named constants. They often make programs more readable than programs that use literal values (e.g., 10)—a named constant such as ARRAY_LENGTH clearly indicates its purpose, whereas a literal value could have different meanings based on its context. Good Programming Practice 7.3 Multiword named constants should have each word separated from the next with an underscore ( _ ) as in ARRAY_LENGTH. Common Programming Error 7.4 Assigning a value to a final variable after it has been initialized is a compilation error. Similarly, attempting to access the value of a final variable before it’s initialized results in a compilation error like, “variable variableName might not have been initialized.” 7.4 Examples Using Arrays 251 7.4.4 Summing the Elements of an Array Often, the elements of an array represent a series of values to be used in a calculation. If, for example, they represent exam grades, a professor may wish to total the elements of the array and use that sum to calculate the class average for the exam. The GradeBook examples in Figs. 7.14 and 7.18 use this technique. Figure 7.5 sums the values contained in a 10-element integer array. The program declares, creates and initializes the array at line 8. The for statement performs the calculations. [Note: The values supplied as array initializers are often read into a program rather than specified in an initializer list. For example, an application could input the values from a user or from a file on disk (as discussed in Chapter 15, Files, Streams and Object Serialization). Reading the data into a program (rather than “hand coding” it into the program) makes the program more reusable, because it can be used with different sets of data.] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 // Fig. 7.5: SumArray.java // Computing the sum of the elements of an array. public class SumArray { public static void main(String[] args) { int[] array = { 87, 68, 94, 100, 83, 78, 85, 91, 76, 87 }; int total = 0; // add each element's value to total for (int counter = 0; counter < array.length; counter++) total += array[counter]; System.out.printf("Total of array elements: %d%n", total); } } // end class SumArray Total of array elements: 849 Fig. 7.5 | Computing the sum of the elements of an array. 7.4.5 Using Bar Charts to Display Array Data Graphically Many programs present data to users in a graphical manner. For example, numeric values are often displayed as bars in a bar chart. In such a chart, longer bars represent proportionally larger numeric values. One simple way to display numeric data graphically is with a bar chart that shows each numeric value as a bar of asterisks (*). Professors often like to examine the distribution of grades on an exam. A professor might graph the number of grades in each of several categories to visualize the grade distribution. Suppose the grades on an exam were 87, 68, 94, 100, 83, 78, 85, 91, 76 and 87. They include one grade of 100, two grades in the 90s, four grades in the 80s, two grades in the 70s, one grade in the 60s and no grades below 60. Our next application (Fig. 7.6) stores this grade distribution data in an array of 11 elements, each corresponding to a category of grades. For example, array[0] indicates the number of grades in the range 0–9, array[7] the number of grades in the range 70–79 and array[10] the number of 100 252 Chapter 7 Arrays and ArrayLists grades. The GradeBook classes later in the chapter (Figs. 7.14 and 7.18) contain code that calculates these grade frequencies based on a set of grades. For now, we manually create the array with the given grade frequencies. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 // Fig. 7.6: BarChart.java // Bar chart printing program. public class BarChart { public static void main(String[] args) { int[] array = { 0, 0, 0, 0, 0, 0, 1, 2, 4, 2, 1 }; System.out.println("Grade distribution:"); // for each array element, output a bar of the chart for (int counter = 0; counter < array.length; counter++) { // output bar label ("00-09: ", ..., "90-99: ", "100: ") if (counter == 10) System.out.printf("%5d: ", 100); else System.out.printf("%02d-%02d: ", counter * 10, counter * 10 + 9); // print bar of asterisks for (int stars = 0; stars < array[counter]; stars++) System.out.print("*"); System.out.println(); } } } // end class BarChart Grade distribution: 00-09: 10-19: 20-29: 30-39: 40-49: 50-59: 60-69: * 70-79: ** 80-89: **** 90-99: ** 100: * Fig. 7.6 | Bar chart printing program. The application reads the numbers from the array and graphs the information as a bar chart. It displays each grade range followed by a bar of asterisks indicating the number of grades in that range. To label each bar, lines 16–20 output a grade range (e.g., "70-79: ") based on the current value of counter. When counter is 10, line 17 outputs 100 with a 7.4 Examples Using Arrays 253 field width of 5, followed by a colon and a space, to align the label "100: " with the other bar labels. The nested for statement (lines 23–24) outputs the bars. Note the loop-continuation condition at line 23 (stars < array[counter]). Each time the program reaches the inner for, the loop counts from 0 up to array[counter], thus using a value in array to determine the number of asterisks to display. In this example, no students received a grade below 60, so array[0]–array[5] contain zeroes, and no asterisks are displayed next to the first six grade ranges. In line 19, the format specifier %02d indicates that an int value should be formatted as a field of two digits. The 0 flag in the format specifier displays a leading 0 for values with fewer digits than the field width (2). 7.4.6 Using the Elements of an Array as Counters Sometimes, programs use counter variables to summarize data, such as the results of a survey. In Fig. 6.7, we used separate counters in our die-rolling program to track the number of occurrences of each side of a six-sided die as the program rolled the die 6,000,000 times. An array version of this application is shown in Fig. 7.7. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 // Fig. 7.7: RollDie.java // Die-rolling program using arrays instead of switch. import java.security.SecureRandom; public class RollDie { public static void main(String[] args) { SecureRandom randomNumbers = new SecureRandom(); int[] frequency = new int[7]; // array of frequency counters // roll die 6,000,000 times; use die value as frequency index for (int roll = 1; roll <= 6000000; roll++) ++frequency[1 + randomNumbers.nextInt(6)]; System.out.printf("%s%10s%n", "Face", "Frequency"); // output each array element's value for (int face = 1; face < frequency.length; face++) System.out.printf("%4d%10d%n", face, frequency[face]); } } // end class RollDie Face Frequency 1 999690 2 999512 3 1000575 4 999815 5 999781 6 1000627 Fig. 7.7 | Die-rolling program using arrays instead of switch. Figure 7.7 uses the array frequency (line 10) to count the occurrences of each side of the die. The single statement in line 14 of this program replaces lines 22–45 of Fig. 6.7. Line 254 Chapter 7 Arrays and ArrayLists 14 uses the random value to determine which frequency element to increment during each iteration of the loop. The calculation in line 14 produces random numbers from 1 to 6, so the array frequency must be large enough to store six counters. However, we use a seven-element array in which we ignore frequency[0]—it’s more logical to have the face value 1 increment frequency[1] than frequency[0]. Thus, each face value is used as an index for array frequency. In line 14, the calculation inside the square brackets evaluates first to determine which element of the array to increment, then the ++ operator adds one to that element. We also replaced lines 49–51 from Fig. 6.7 by looping through array frequency to output the results (lines 19–20). When we study Java SE 8’s new functional programming capabilities in Chapter 17, we’ll show how to replace lines 13–14 and 19–20 with a single statement! 7.4.7 Using Arrays to Analyze Survey Results Our next example uses arrays to summarize data collected in a survey. Consider the following problem statement: Twenty students were asked to rate on a scale of 1 to 5 the quality of the food in the student cafeteria, with 1 being “awful” and 5 being “excellent.” Place the 20 responses in an integer array and determine the frequency of each rating. This is a typical array-processing application (Fig. 7.8). We wish to summarize the number of responses of each type (that is, 1–5). Array responses (lines 9–10) is a 20-element integer array containing the students’ survey responses. The last value in the array is intentionally an incorrect response (14). When a Java program executes, array element indices are checked for validity—all indices must be greater than or equal to 0 and less than the length of the array. Any attempt to access an element outside that range of indices results in a runtime error that’s known as an ArrayIndexOutOfBoundsException. At the end of this section, we’ll discuss the invalid response value, demonstrate array bounds checking and introduce Java’s exception-handling mechanism, which can be used to detect and handle an ArrayIndexOutOfBoundsException. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 // Fig. 7.8: StudentPoll.java // Poll analysis program. public class StudentPoll { public static void main(String[] args) { // student response array (more typically, input at runtime) int[] responses = { 1, 2, 5, 4, 3, 5, 2, 1, 3, 3, 1, 4, 3, 3, 3, 2, 3, 3, 2, 14 }; int[] frequency = new int[6]; // array of frequency counters // for each answer, select responses element and use that value // as frequency index to determine element to increment for (int answer = 0; answer < responses.length; answer++) { Fig. 7.8 | Poll analysis program. (Part 1 of 2.) 7.4 Examples Using Arrays 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 255 try { ++frequency[responses[answer]]; } catch (ArrayIndexOutOfBoundsException e) { System.out.println(e); // invokes toString method System.out.printf(" responses[%d] = %d%n%n", answer, responses[answer]); } } System.out.printf("%s%10s%n", "Rating", "Frequency"); // output each array element's value for (int rating = 1; rating < frequency.length; rating++) System.out.printf("%6d%10d%n", rating, frequency[rating]); } } // end class StudentPoll java.lang.ArrayIndexOutOfBoundsException: 14 responses[19] = 14 Rating Frequency 1 3 2 4 3 8 4 2 5 2 Fig. 7.8 | Poll analysis program. (Part 2 of 2.) The frequency Array We use the six-element array frequency (line 11) to count the number of occurrences of each response. Each element is used as a counter for one of the possible types of survey responses—frequency[1] counts the number of students who rated the food as 1, frequency[2] counts the number of students who rated the food as 2, and so on. Summarizing the Results The for statement (lines 15–27) reads the responses from the array responses one at a time and increments one of the counters frequency[1] to frequency[5]; we ignore frequency[0] because the survey responses are limited to the range 1–5. The key statement in the loop appears in line 19. This statement increments the appropriate frequency counter as determined by the value of responses[answer]. Let’s step through the first few iterations of the for statement: • When the counter answer is 0, responses[answer] is the value of responses[0] (that is, 1—see line 9). In this case, frequency[responses[answer]] is interpreted as frequency[1], and the counter frequency[1] is incremented by one. To evaluate the expression, we begin with the value in the innermost set of brackets (answer, currently 0). The value of answer is plugged into the expression, and the 256 Chapter 7 Arrays and ArrayLists next set of brackets (responses[answer]) is evaluated. That value is used as the index for the frequency array to determine which counter to increment (in this case, frequency[1]). • The next time through the loop answer is 1, responses[answer] is the value of (that is, 2—see line 9), so frequency[responses[answer]] is interpreted as frequency[2], causing frequency[2] to be incremented. responses[1] • When answer is 2, responses[answer] is the value of responses[2] (that is, 5— see line 9), so frequency[responses[answer]] is interpreted as frequency[5], causing frequency[5] to be incremented, and so on. Regardless of the number of responses processed, only a six-element array (in which we ignore element zero) is required to summarize the results, because all the correct response values are values from 1 to 5, and the index values for a six-element array are 0–5. In the program’s output, the Frequency column summarizes only 19 of the 20 values in the responses array—the last element of the array responses contains an (intentionally) incorrect response that was not counted. Section 7.5 discusses what happens when the program in Fig. 7.8 encounters the invalid response (14) in the last element of array responses. 7.5 Exception Handling: Processing the Incorrect Response An exception indicates a problem that occurs while a program executes. The name “exception” suggests that the problem occurs infrequently—if the “rule” is that a statement normally executes correctly, then the problem represents the “exception to the rule.” Exception handling helps you create fault-tolerant programs that can resolve (or handle) exceptions. In many cases, this allows a program to continue executing as if no problems were encountered. For example, the StudentPoll application still displays results (Fig. 7.8), even though one of the responses was out of range. More severe problems might prevent a program from continuing normal execution, instead requiring the program to notify the user of the problem, then terminate. When the JVM or a method detects a problem, such as an invalid array index or an invalid method argument, it throws an exception—that is, an exception occurs. Methods in your own classes can also throw exceptions, as you’ll learn in Chapter 8. 7.5.1 The try Statement To handle an exception, place any code that might throw an exception in a try statement (lines 17–26). The try block (lines 17–20) contains the code that might throw an exception, and the catch block (lines 21–26) contains the code that handles the exception if one occurs. You can have many catch blocks to handle different types of exceptions that might be thrown in the corresponding try block. When line 19 correctly increments an element of the frequency array, lines 21–26 are ignored. The braces that delimit the bodies of the try and catch blocks are required. 7.5.2 Executing the catch Block When the program encounters the invalid value 14 in the responses array, it attempts to add 1 to frequency[14], which is outside the bounds of the array—the frequency array 7.6 Case Study: Card Shuffling and Dealing Simulation 257 has only six elements (with indexes 0–5). Because array bounds checking is performed at execution time, the JVM generates an exception—specifically line 19 throws an ArrayIndexOutOfBoundsException to notify the program of this problem. At this point the try block terminates and the catch block begins executing—if you declared any local variables in the try block, they’re now out of scope (and no longer exist), so they’re not accessible in the catch block. The catch block declares an exception parameter (e) of type (IndexOutOfRangeException). The catch block can handle exceptions of the specified type. Inside the catch block, you can use the parameter’s identifier to interact with a caught exception object. Error-Prevention Tip 7.1 When writing code to access an array element, ensure that the array index remains greater than or equal to 0 and less than the length of the array. This would prevent ArrayIndexOutOfBoundsExceptions if your program is correct. Software Engineering Observation 7.1 Systems in industry that have undergone extensive testing are still likely to contain bugs. Our preference for industrial-strength systems is to catch and deal with runtime exceptions, such as ArrayIndexOutOfBoundsExceptions, to ensure that a system either stays up and running or degrades gracefully, and to inform the system’s developers of the problem. 7.5.3 toString Method of the Exception Parameter When lines 21–26 catch the exception, the program displays a message indicating the problem that occurred. Line 23 implicitly calls the exception object’s toString method to get the error message that’s implicitly stored in the exception object and display it. Once the message is displayed in this example, the exception is considered handled and the program continues with the next statement after the catch block’s closing brace. In this example, the end of the for statement is reached (line 27), so the program continues with the increment of the control variable in line 15. We discuss exception handling again in Chapter 8, and more deeply in Chapter 11. 7.6 Case Study: Card Shuffling and Dealing Simulation The examples in the chapter thus far have used arrays containing elements of primitive types. Recall from Section 7.2 that the elements of an array can be either primitive types or reference types. This section uses random-number generation and an array of referencetype elements, namely objects representing playing cards, to develop a class that simulates card shuffling and dealing. This class can then be used to implement applications that play specific card games. The exercises at the end of the chapter use the classes developed here to build a simple poker application. We first develop class Card (Fig. 7.9), which represents a playing card that has a face (e.g., "Ace", "Deuce", "Three", …, "Jack", "Queen", "King") and a suit (e.g., "Hearts", "Diamonds", "Clubs", "Spades"). Next, we develop the DeckOfCards class (Fig. 7.10), which creates a deck of 52 playing cards in which each element is a Card object. We then build a test application (Fig. 7.11) that demonstrates class DeckOfCards’s card shuffling and dealing capabilities. 258 Chapter 7 Arrays and ArrayLists Class Card Class Card (Fig. 7.9) contains two String instance variables—face and suit—that are used to store references to the face name and suit name for a specific Card. The constructor for the class (lines 10–14) receives two Strings that it uses to initialize face and suit. Method toString (lines 17–20) creates a String consisting of the face of the card, the String " of " and the suit of the card. Card’s toString method can be invoked explicitly to obtain a string representation of a Card object (e.g., "Ace of Spades"). The toString method of an object is called implicitly when the object is used where a String is expected (e.g., when printf outputs the object as a String using the %s format specifier or when the object is concatenated to a String using the + operator). For this behavior to occur, toString must be declared with the header shown in Fig. 7.9. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 // Fig. 7.9: Card.java // Card class represents a playing card. public class Card { private final String face; // face of card ("Ace", "Deuce", ...) private final String suit; // suit of card ("Hearts", "Diamonds", ...) // two-argument constructor initializes card's face and suit public Card(String cardFace, String cardSuit) { this.face = cardFace; // initialize face of card this.suit = cardSuit; // initialize suit of card } // return String representation of Card public String toString() { return face + " of " + suit; } } // end class Card Fig. 7.9 | Card class represents a playing card. Class DeckOfCards Class DeckOfCards (Fig. 7.10) declares as an instance variable a Card array named deck (line 7). An array of a reference type is declared like any other array. Class DeckOfCards also declares an integer instance variable currentCard (line 8) representing the sequence number (0–51) of the next Card to be dealt from the deck array, and a named constant NUMBER_OF_CARDS (line 9) indicating the number of Cards in the deck (52). 1 2 3 4 // Fig. 7.10: DeckOfCards.java // DeckOfCards class represents a deck of playing cards. import java.security.SecureRandom; Fig. 7.10 | DeckOfCards class represents a deck of playing cards. (Part 1 of 2.) 7.6 Case Study: Card Shuffling and Dealing Simulation 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 259 public class DeckOfCards { private Card[] deck; // array of Card objects private int currentCard; // index of next Card to be dealt (0-51) private static final int NUMBER_OF_CARDS = 52; // constant # of Cards // random number generator private static final SecureRandom randomNumbers = new SecureRandom(); // constructor fills deck of Cards public DeckOfCards() { String[] faces = { "Ace", "Deuce", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Jack", "Queen", "King" }; String[] suits = { "Hearts", "Diamonds", "Clubs", "Spades" }; deck = new Card[NUMBER_OF_CARDS]; // create array of Card objects currentCard = 0; // first Card dealt will be deck[0] // populate deck with Card objects for (int count = 0; count < deck.length; count++) deck[count] = new Card(faces[count % 13], suits[count / 13]); } // shuffle deck of Cards with one-pass algorithm public void shuffle() { // next call to method dealCard should start at deck[0] again currentCard = 0; // for each Card, pick another random Card (0-51) and swap them for (int first = 0; first < deck.length; first++) { // select a random number between 0 and 51 int second = randomNumbers.nextInt(NUMBER_OF_CARDS); // swap current Card with randomly selected Card Card temp = deck[first]; deck[first] = deck[second]; deck[second] = temp; } } // deal one Card public Card dealCard() { // determine whether Cards remain to be dealt if (currentCard < deck.length) return deck[currentCard++]; // return current Card in array else return null; // return null to indicate that all Cards were dealt } } // end class DeckOfCards Fig. 7.10 | DeckOfCards class represents a deck of playing cards. (Part 2 of 2.) 260 Chapter 7 Arrays and ArrayLists Constructor The class’s constructor instantiates the deck array (line 20) with NUMBER_OF_CARDS (52) elements. The elements of deck are null by default, so the constructor uses a for statement (lines 24–26) to fill the deck with Cards. The loop initializes control variable count to 0 and loops while count is less than deck.length, causing count to take on each integer value from 0 to 51 (the indices of the deck array). Each Card is instantiated and initialized with two Strings—one from the faces array (which contains the Strings "Ace" through "King") and one from the suits array (which contains the Strings "Hearts", "Diamonds", "Clubs" and "Spades"). The calculation count % 13 always results in a value from 0 to 12 (the 13 indices of the faces array in lines 16–17), and the calculation count / 13 always results in a value from 0 to 3 (the four indices of the suits array in line 18). When the deck array is initialized, it contains the Cards with faces "Ace" through "King" in order for each suit (all 13 "Hearts", then all the "Diamonds", then the "Clubs", then the "Spades"). We use arrays of Strings to represent the faces and suits in this example. In Exercise 7.34, we ask you to modify this example to use arrays of enum constants to represent the faces and suits. DeckOfCards Method shuffle Method shuffle (lines 30–46) shuffles the Cards in the deck. The method loops through all 52 Cards (array indices 0 to 51). For each Card, a number between 0 and 51 is picked randomly to select another Card. Next, the current Card object and the randomly selected Card object are swapped in the array. This exchange is performed by the three assignments in lines 42–44. The extra variable temp temporarily stores one of the two Card objects being swapped. The swap cannot be performed with only the two statements DeckOfCards deck[first] = deck[second]; deck[second] = deck[first]; If deck[first] is the "Ace" of "Spades" and deck[second] is the "Queen" of "Hearts", after the first assignment, both array elements contain the "Queen" of "Hearts" and the "Ace" of "Spades" is lost—so, the extra variable temp is needed. After the for loop terminates, the Card objects are randomly ordered. A total of only 52 swaps are made in a single pass of the entire array, and the array of Card objects is shuffled! [Note: It’s recommended that you use a so-called unbiased shuffling algorithm for real card games. Such an algorithm ensures that all possible shuffled card sequences are equally likely to occur. Exercise 7.35 asks you to research the popular unbiased Fisher-Yates shuffling algorithm and use it to reimplement the DeckOfCards method shuffle.] Method dealCard Method dealCard (lines 49–56) deals one Card in the array. Recall that currentCard indicates the index of the next Card to be dealt (i.e., the Card at the top of the deck). Thus, line 52 compares currentCard to the length of the deck array. If the deck is not empty (i.e., currentCard is less than 52), line 53 returns the “top” Card and postincrements currentCard to prepare for the next call to dealCard—otherwise, null is returned. Recall from Chapter 3 that null represents a “reference to nothing.” DeckOfCards Shuffling and Dealing Cards Figure 7.11 demonstrates class DeckOfCards. Line 9 creates a DeckOfCards object named myDeckOfCards. The DeckOfCards constructor creates the deck with the 52 Card objects 7.6 Case Study: Card Shuffling and Dealing Simulation 261 in order by suit and face. Line 10 invokes myDeckOfCards’s shuffle method to rearrange the Card objects. Lines 13–20 deal all 52 Cards and print them in four columns of 13 Cards each. Line 16 deals one Card object by invoking myDeckOfCards’s dealCard method, then displays the Card left justified in a field of 19 characters. When a Card is output as a String, the Card’s toString method (lines 17–20 of Fig. 7.9) is implicitly invoked. Lines 18–19 start a new line after every four Cards. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 // Fig. 7.11: DeckOfCardsTest.java // Card shuffling and dealing. public class DeckOfCardsTest { // execute application public static void main(String[] args) { DeckOfCards myDeckOfCards = new DeckOfCards(); myDeckOfCards.shuffle(); // place Cards in random order // print all 52 Cards in the order in which they are dealt for (int i = 1; i <= 52; i++) { // deal and display a Card System.out.printf("%-19s", myDeckOfCards.dealCard()); if (i % 4 == 0) // output a newline after every fourth card System.out.println(); } } } // end class DeckOfCardsTest Six of Spades Queen of Hearts Three of Diamonds Four of Spades Three of Clubs King of Clubs Queen of Clubs Three of Spades Ace of Spades Deuce of Spades Jack of Hearts Ace of Diamonds Five of Diamonds Eight of Spades Seven of Clubs Deuce of Clubs Ace of Clubs Deuce of Hearts Ten of Hearts Eight of Diamonds King of Diamonds Four of Diamonds Eight of Hearts Seven of Spades Queen of Diamonds Ten of Clubs Six of Clubs Nine of Spades Ace of Hearts Seven of Diamonds Five of Spades Three of Hearts Deuce of Diamonds Nine of Clubs Seven of Hearts Five of Hearts Four of Clubs Five of Clubs Jack of Spades Nine of Hearts King of Hearts Ten of Spades Four of Hearts Jack of Diamonds Six of Diamonds Ten of Diamonds Six of Hearts Eight of Clubs Queen of Spades Nine of Diamonds King of Spades Jack of Clubs Fig. 7.11 | Card shuffling and dealing. Preventing NullPointerExceptions In Fig. 7.10, we created a deck array of 52 Card references—each element of every reference-type array created with new is default initialized to null. Reference-type variables which are fields of a class are also initialized to null by default. A NullPointerException occurs when you try to call a method on a null reference. In industrial-strength code, ensuring that references are not null before you use them to call methods prevents NullPointerExceptions. 262 Chapter 7 Arrays and ArrayLists 7.7 Enhanced for Statement The enhanced for statement iterates through the elements of an array without using a counter, thus avoiding the possibility of “stepping outside” the array. We show how to use the enhanced for statement with the Java API’s prebuilt data structures (called collections) in Section 7.16. The syntax of an enhanced for statement is: for (parameter : arrayName) statement where parameter has a type and an identifier (e.g., int number), and arrayName is the array through which to iterate. The type of the parameter must be consistent with the type of the elements in the array. As the next example illustrates, the identifier represents successive element values in the array on successive iterations of the loop. Figure 7.12 uses the enhanced for statement (lines 12–13) to sum the integers in an array of student grades. The enhanced for’s parameter is of type int, because array contains int values—the loop selects one int value from the array during each iteration. The enhanced for statement iterates through successive values in the array one by one. The statement’s header can be read as “for each iteration, assign the next element of array to int variable number, then execute the following statement.” Thus, for each iteration, identifier number represents an int value in array. Lines 12–13 are equivalent to the following counter-controlled repetition used in lines 12–13 of Fig. 7.5 to total the integers in array, except that counter cannot be accessed in the enhanced for statement: for (int counter = 0; counter < array.length; counter++) total += array[counter]; 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 // Fig. 7.12: EnhancedForTest.java // Using the enhanced for statement to total integers in an array. public class EnhancedForTest { public static void main(String[] args) { int[] array = { 87, 68, 94, 100, 83, 78, 85, 91, 76, 87 }; int total = 0; // add each element's value to total for (int number : array) total += number; System.out.printf("Total of array elements: %d%n", total); } } // end class EnhancedForTest Total of array elements: 849 Fig. 7.12 | Using the enhanced for statement to total integers in an array. The enhanced for statement can be used only to obtain array elements—it cannot be used to modify elements. If your program needs to modify elements, use the traditional counter-controlled for statement. 7.8 Passing Arrays to Methods 263 The enhanced for statement can be used in place of the counter-controlled for statement whenever code looping through an array does not require access to the counter indicating the index of the current array element. For example, totaling the integers in an array requires access only to the element values—the index of each element is irrelevant. However, if a program must use a counter for some reason other than simply to loop through an array (e.g., to print an index number next to each array element value, as in the examples earlier in this chapter), use the counter-controlled for statement. Error-Prevention Tip 7.2 The enhanced for statement simplifies the code for iterating through an array making the code more readable and eliminating several error possibilities, such as improperly specifying the control variable’s initial value, the loop-continuation test and the increment expression. Java SE 8 The for statement and the enhanced for statement each iterate sequentially from a starting value to an ending value. In Chapter 17, Java SE 8 Lambdas and Streams, you’ll learn about class Stream and its forEach method. Working together, these provide an elegant, more concise and less error prone means for iterating through collections so that some of the iterations may occur in parallel with others to achieve better multi-core system performance. 7.8 Passing Arrays to Methods This section demonstrates how to pass arrays and individual array elements as arguments to methods. To pass an array argument to a method, specify the name of the array without any brackets. For example, if array hourlyTemperatures is declared as double[] hourlyTemperatures = new double[24]; then the method call modifyArray(hourlyTemperatures); passes the reference of array hourlyTemperatures to method modifyArray. Every array object “knows” its own length. Thus, when we pass an array object’s reference into a method, we need not pass the array length as an additional argument. For a method to receive an array reference through a method call, the method’s parameter list must specify an array parameter. For example, the method header for method modifyArray might be written as void modifyArray(double[] b) indicating that modifyArray receives the reference of a double array in parameter b. The method call passes array hourlyTemperature’s reference, so when the called method uses the array variable b, it refers to the same array object as hourlyTemperatures in the caller. When an argument to a method is an entire array or an individual array element of a reference type, the called method receives a copy of the reference. However, when an argument to a method is an individual array element of a primitive type, the called method receives a copy of the element’s value. Such primitive values are called scalars or scalar quantities. To pass an individual array element to a method, use the indexed name of the array element as an argument in the method call. 264 Chapter 7 Arrays and ArrayLists Figure 7.13 demonstrates the difference between passing an entire array and passing a primitive-type array element to a method. Notice that main invokes static methods modifyArray (line 19) and modifyElement (line 30) directly. Recall from Section 6.4 that a static method of a class can invoke other static methods of the same class directly. The enhanced for statement at lines 16–17 outputs the elements of array. Line 19 invokes method modifyArray, passing array as an argument. The method (lines 36–40) receives a copy of array’s reference and uses it to multiply each of array’s elements by 2. To prove that array’s elements were modified, lines 23–24 output array’s elements again. As the output shows, method modifyArray doubled the value of each element. We could not use the enhanced for statement in lines 38–39 because we’re modifying the array’s elements. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 // Fig. 7.13: PassArray.java // Passing arrays and individual array elements to methods. public class PassArray { // main creates array and calls modifyArray and modifyElement public static void main(String[] args) { int[] array = { 1, 2, 3, 4, 5 }; System.out.printf( "Effects of passing reference to entire array:%n" + "The values of the original array are:%n"); // output original array elements for (int value : array) System.out.printf(" %d", value); modifyArray(array); // pass array reference System.out.printf("%n%nThe values of the modified array are:%n"); // output modified array elements for (int value : array) System.out.printf(" %d", value); System.out.printf( "%n%nEffects of passing array element value:%n" + "array[3] before modifyElement: %d%n", array[3]); modifyElement(array[3]); // attempt to modify array[3] System.out.printf( "array[3] after modifyElement: %d%n", array[3]); } // multiply each element of an array by 2 public static void modifyArray(int[] array2) { for (int counter = 0; counter < array2.length; counter++) array2[counter] *= 2; } Fig. 7.13 | Passing arrays and individual array elements to methods. (Part 1 of 2.) 7.9 Pass-By-Value vs. Pass-By-Reference 41 42 43 44 45 46 47 48 49 265 // multiply argument by 2 public static void modifyElement(int element) { element *= 2; System.out.printf( "Value of element in modifyElement: %d%n", element); } } // end class PassArray Effects of passing reference to entire array: The values of the original array are: 1 2 3 4 5 The values of the modified array are: 2 4 6 8 10 Effects of passing array element value: array[3] before modifyElement: 8 Value of element in modifyElement: 16 array[3] after modifyElement: 8 Fig. 7.13 | Passing arrays and individual array elements to methods. (Part 2 of 2.) Figure 7.13 next demonstrates that when a copy of an individual primitive-type array element is passed to a method, modifying the copy in the called method does not affect the original value of that element in the calling method’s array. Lines 26–28 output the value of array[3] before invoking method modifyElement. Remember that the value of this element is now 8 after it was modified in the call to modifyArray. Line 30 calls method modifyElement and passes array[3] as an argument. Remember that array[3] is actually one int value (8) in array. Therefore, the program passes a copy of the value of array[3]. Method modifyElement (lines 43–48) multiplies the value received as an argument by 2, stores the result in its parameter element, then outputs the value of element (16). Since method parameters, like local variables, cease to exist when the method in which they’re declared completes execution, the method parameter element is destroyed when method modifyElement terminates. When the program returns control to main, lines 31–32 output the unmodified value of array[3] (i.e., 8). 7.9 Pass-By-Value vs. Pass-By-Reference The preceding example demonstrated how arrays and primitive-type array elements are passed as arguments to methods. We now take a closer look at how arguments in general are passed to methods. Two ways to pass arguments in method calls in many programming languages are pass-by-value and pass-by-reference (sometimes called call-by-value and call-by-reference). When an argument is passed by value, a copy of the argument’s value is passed to the called method. The called method works exclusively with the copy. Changes to the called method’s copy do not affect the original variable’s value in the caller. When an argument is passed by reference, the called method can access the argument’s value in the caller directly and modify that data, if necessary. Pass-by-reference improves performance by eliminating the need to copy possibly large amounts of data. 266 Chapter 7 Arrays and ArrayLists Unlike some other languages, Java does not allow you to choose pass-by-value or passby-reference—all arguments are passed by value. A method call can pass two types of values to a method—copies of primitive values (e.g., values of type int and double) and copies of references to objects. Objects themselves cannot be passed to methods. When a method modifies a primitive-type parameter, changes to the parameter have no effect on the original argument value in the calling method. For example, when line 30 in main of Fig. 7.13 passes array[3] to method modifyElement, the statement in line 45 that doubles the value of parameter element has no effect on the value of array[3] in main. This is also true for reference-type parameters. If you modify a reference-type parameter so that it refers to another object, only the parameter refers to the new object—the reference stored in the caller’s variable still refers to the original object. Although an object’s reference is passed by value, a method can still interact with the referenced object by calling its public methods using the copy of the object’s reference. Since the reference stored in the parameter is a copy of the reference that was passed as an argument, the parameter in the called method and the argument in the calling method refer to the same object in memory. For example, in Fig. 7.13, both parameter array2 in method modifyArray and variable array in main refer to the same array object in memory. Any changes made using the parameter array2 are carried out on the object that array references in the calling method. In Fig. 7.13, the changes made in modifyArray using array2 affect the contents of the array object referenced by array in main. Thus, with a reference to an object, the called method can manipulate the caller’s object directly. Performance Tip 7.1 Passing references to arrays, instead of the array objects themselves, makes sense for performance reasons. Because everything in Java is passed by value, if array objects were passed, a copy of each element would be passed. For large arrays, this would waste time and consume considerable storage for the copies of the elements. 7.10 Case Study: Class GradeBook Using an Array to Store Grades We now present the first part of our case study on developing a GradeBook class that instructors can use to maintain students’ grades on an exam and display a grade report that includes the grades, class average, lowest grade, highest grade and a grade distribution bar chart. The version of class GradeBook presented in this section stores the grades for one exam in a one-dimensional array. In Section 7.12, we present a version of class GradeBook that uses a two-dimensional array to store students’ grades for several exams. Storing Student Grades in an Array in Class GradeBook Class GradeBook (Fig. 7.14) uses an array of ints to store several students’ grades on a single exam. Array grades is declared as an instance variable (line 7), so each GradeBook object maintains its own set of grades. The constructor (lines 10–14) has two parameters— the name of the course and an array of grades. When an application (e.g., class GradeBookTest in Fig. 7.15) creates a GradeBook object, the application passes an existing int array to the constructor, which assigns the array’s reference to instance variable grades (line 13). The grades array’s size is determined by the length instance variable of the constructor’s 7.10 Case Study: Class GradeBook Using an Array to Store Grades 267 array parameter. Thus, a GradeBook object can process a variable number of grades. The grade values in the argument could have been input from a user, read from a file on disk (as discussed in Chapter 15) or come from a variety of other sources. In class GradeBookTest, we initialize an array with grade values (Fig. 7.15, line 10). Once the grades are stored in instance variable grades of class GradeBook, all the class’s methods can access the elements of grades. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 // Fig. 7.14: GradeBook.java // GradeBook class using an array to store test grades. public class GradeBook { private String courseName; // name of course this GradeBook represents private int[] grades; // array of student grades // constructor public GradeBook(String courseName, int[] grades) { this.courseName = courseName; this.grades = grades; } // method to set the course name public void setCourseName(String courseName) { this.courseName = courseName; } // method to retrieve the course name public String getCourseName() { return courseName; } // perform various operations on the data public void processGrades() { // output grades array outputGrades(); // call method getAverage to calculate the average grade System.out.printf("%nClass average is %.2f%n", getAverage()); // call methods getMinimum and getMaximum System.out.printf("Lowest grade is %d%nHighest grade is %d%n%n", getMinimum(), getMaximum()); // call outputBarChart to print grade distribution chart outputBarChart(); } Fig. 7.14 | GradeBook class using an array to store test grades. (Part 1 of 3.) 268 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 Chapter 7 Arrays and ArrayLists // find minimum grade public int getMinimum() { int lowGrade = grades[0]; // assume grades[0] is smallest // loop through grades array for (int grade : grades) { // if grade lower than lowGrade, assign it to lowGrade if (grade < lowGrade) lowGrade = grade; // new lowest grade } return lowGrade; } // find maximum grade public int getMaximum() { int highGrade = grades[0]; // assume grades[0] is largest // loop through grades array for (int grade : grades) { // if grade greater than highGrade, assign it to highGrade if (grade > highGrade) highGrade = grade; // new highest grade } return highGrade; } // determine average grade for test public double getAverage() { int total = 0; // sum grades for one student for (int grade : grades) total += grade; // return average of grades return (double) total / grades.length; } // output bar chart displaying grade distribution public void outputBarChart() { System.out.println("Grade distribution:"); Fig. 7.14 | // stores frequency of grades in each range of 10 grades int[] frequency = new int[11]; GradeBook class using an array to store test grades. (Part 2 of 3.) 7.10 Case Study: Class GradeBook Using an Array to Store Grades 269 97 // for each grade, increment the appropriate frequency 98 for (int grade : grades) 99 100 ++frequency[grade / 10]; 101 102 // for each grade frequency, print bar in chart 103 for (int count = 0; count < frequency.length; count++) 104 { 105 // output bar label ("00-09: ", ..., "90-99: ", "100: ") 106 if (count == 10) 107 System.out.printf("%5d: ", 100); 108 else 109 System.out.printf("%02d-%02d: ", 110 count * 10, count * 10 + 9); 111 112 // print bar of asterisks 113 for (int stars = 0; stars < frequency[count]; stars++) 114 System.out.print("*"); 115 116 System.out.println(); 117 } 118 } 119 120 // output the contents of the grades array 121 public void outputGrades() 122 { 123 System.out.printf("The grades are:%n%n"); 124 125 // output each student's grade for (int student = 0; student < grades.length; student++) 126 System.out.printf("Student %2d: %3d%n", 127 128 student + 1, grades[student]); 129 } 130 } // end class GradeBook Fig. 7.14 | GradeBook class using an array to store test grades. (Part 3 of 3.) Method processGrades (lines 29–43) contains a series of method calls that output a report summarizing the grades. Line 32 calls method outputGrades to print the contents of the array grades. Lines 126–128 in method outputGrades output the students’ grades. A counter-controlled for statement must be used in this case, because lines 127–128 use counter variable student’s value to output each grade next to a particular student number (see the output in Fig. 7.15). Although array indices start at 0, a professor might typically number students starting at 1. Thus, lines 127–128 output student + 1 as the student number to produce grade labels "Student 1: ", "Student 2: ", and so on. Method processGrades next calls method getAverage (line 35) to obtain the average of the grades in the array. Method getAverage (lines 78–88) uses an enhanced for statement to total the values in array grades before calculating the average. The parameter in the enhanced for’s header (e.g., int grade) indicates that for each iteration, the int variable grade takes on a value in the array grades. The averaging calculation in line 87 uses grades.length to determine the number of grades being averaged. 270 Chapter 7 Arrays and ArrayLists Lines 38–39 in method processGrades call methods getMinimum and getMaximum to determine the lowest and highest grades of any student on the exam, respectively. Each of these methods uses an enhanced for statement to loop through array grades. Lines 51– 56 in method getMinimum loop through the array. Lines 54–55 compare each grade to lowGrade; if a grade is less than lowGrade, lowGrade is set to that grade. When line 58 executes, lowGrade contains the lowest grade in the array. Method getMaximum (lines 62–75) works similarly to method getMinimum. Finally, line 42 in method processGrades calls outputBarChart to print a grade distribution chart using a technique similar to that in Fig. 7.6. In that example, we manually calculated the number of grades in each category (i.e., 0–9, 10–19, …, 90–99 and 100) by simply looking at a set of grades. Here, lines 99–100 use a technique similar to that in Figs. 7.7–7.8 to calculate the frequency of grades in each category. Line 96 declares and creates array frequency of 11 ints to store the frequency of grades in each category. For each grade in array grades, lines 99–100 increment the appropriate frequency array element. To determine which one to increment, line 100 divides the current grade by 10 using integer division—e.g., if grade is 85, line 100 increments frequency[8] to update the count of grades in the range 80–89. Lines 103–117 print the bar chart (as shown in Fig. 7.15) based on the values in array frequency. Like lines 23–24 of Fig. 7.6, lines 113–116 of Fig. 7.14 use a value in array frequency to determine the number of asterisks to display in each bar. Class GradeBookTest That Demonstrates Class GradeBook The application of Fig. 7.15 creates an object of class GradeBook (Fig. 7.14) using the int array gradesArray (declared and initialized in line 10). Lines 12–13 pass a course name and gradesArray to the GradeBook constructor. Lines 14–15 display a welcome message that includes the course name stored in the GradeBook object. Line 16 invokes the GradeBook object’s processGrades method. The output summarizes the 10 grades in myGradeBook. Software Engineering Observation 7.2 A test harness (or test application) is responsible for creating an object of the class being tested and providing it with data. This data could come from any of several sources. Test data can be placed directly into an array with an array initializer, it can come from the user at the keyboard, from a file (as you’ll see in Chapter 15), from a database (as you’ll see in Chapter 24) or from a network (as you’ll see in online Chapter 28). After passing this data to the class’s constructor to instantiate the object, the test harness should call upon the object to test its methods and manipulate its data. Gathering data in the test harness like this allows the class to be more reusable, able to manipulate data from several sources. 1 2 3 4 5 6 7 8 // Fig. 7.15: GradeBookTest.java // GradeBookTest creates a GradeBook object using an array of grades, // then invokes method processGrades to analyze them. public class GradeBookTest { // main method begins program execution public static void main(String[] args) { Fig. 7.15 | GradeBookTest creates a GradeBook object using an array of grades, then invokes method processGrades to analyze them. (Part 1 of 2.) 7.10 Case Study: Class GradeBook Using an Array to Store Grades 9 10 11 12 13 14 15 16 17 18 271 // array of student grades int[] gradesArray = { 87, 68, 94, 100, 83, 78, 85, 91, 76, 87 }; GradeBook myGradeBook = new GradeBook( "CS101 Introduction to Java Programming", gradesArray); System.out.printf("Welcome to the grade book for%n%s%n%n", myGradeBook.getCourseName()); myGradeBook.processGrades(); } } // end class GradeBookTest Welcome to the grade book for CS101 Introduction to Java Programming The grades are: Student 1: 87 Student 2: 68 Student 3: 94 Student 4: 100 Student 5: 83 Student 6: 78 Student 7: 85 Student 8: 91 Student 9: 76 Student 10: 87 Class average is 84.90 Lowest grade is 68 Highest grade is 100 Grade distribution: 00-09: 10-19: 20-29: 30-39: 40-49: 50-59: 60-69: * 70-79: ** 80-89: **** 90-99: ** 100: * Fig. 7.15 | GradeBookTest creates a GradeBook object using an array of grades, then invokes method processGrades to analyze them. (Part 2 of 2.) Java SE 8 In Chapter 17, Java SE 8 Lambdas and Streams, the example of Fig. 17.5 uses stream methods min, max, count and average to process the elements of an int array elegantly and concisely without having to write repetition statements. In Chapter 23, Concurrency, the example of Fig. 23.29 uses stream method summaryStatistics to perform all of these operations in one method call. 272 Chapter 7 Arrays and ArrayLists 7.11 Multidimensional Arrays Multidimensional arrays with two dimensions are often used to represent tables of values with data arranged in rows and columns. To identify a particular table element, you specify two indices. By convention, the first identifies the element’s row and the second its column. Arrays that require two indices to identify each element are called two-dimensional arrays. (Multidimensional arrays can have more than two dimensions.) Java does not support multidimensional arrays directly, but it allows you to specify one-dimensional arrays whose elements are also one-dimensional arrays, thus achieving the same effect. Figure 7.16 illustrates a two-dimensional array named a with three rows and four columns (i.e., a three-by-four array). In general, an array with m rows and n columns is called an m-by-n array. Column 0 Column 1 Column 2 Column 3 Row 0 a[ 0 ][ 0 ] a[ 0 ][ 1 ] a[ 0 ][ 2 ] a[ 0 ][ 3 ] Row 1 a[ 1 ][ 0 ] a[ 1 ][ 1 ] a[ 1 ][ 2 ] a[ 1 ][ 3 ] Row 2 a[ 2 ][ 0 ] a[ 2 ][ 1 ] a[ 2 ][ 2 ] a[ 2 ][ 3 ] Column index Row index Array name Fig. 7.16 | Two-dimensional array with three rows and four columns. Every element in array a is identified in Fig. 7.16 by an array-access expression of the form a[row][column]; a is the name of the array, and row and column are the indices that uniquely identify each element by row and column index. The names of the elements in row 0 all have a first index of 0, and the names of the elements in column 3 all have a second index of 3. Arrays of One-Dimensional Arrays Like one-dimensional arrays, multidimensional arrays can be initialized with array initializers in declarations. A two-dimensional array b with two rows and two columns could be declared and initialized with nested array initializers as follows: int[][] b = {{1, 2}, {3, 4}}; The initial values are grouped by row in braces. So 1 and 2 initialize b[0][0] and b[0][1], respectively, and 3 and 4 initialize b[1][0] and b[1][1], respectively. The compiler counts the number of nested array initializers (represented by sets of braces within the outer braces) to determine the number of rows in array b. The compiler counts the initializer values in the nested array initializer for a row to determine the number of columns in that row. As we’ll see momentarily, this means that rows can have different lengths. Multidimensional arrays are maintained as arrays of one-dimensional arrays. Therefore array b in the preceding declaration is actually composed of two separate one-dimensional arrays—one containing the values in the first nested initializer list {1, 2} and one containing the values in the second nested initializer list {3, 4}. Thus, array b itself is an array of two elements, each a one-dimensional array of int values. 7.11 Multidimensional Arrays 273 Two-Dimensional Arrays with Rows of Different Lengths The manner in which multidimensional arrays are represented makes them quite flexible. In fact, the lengths of the rows in array b are not required to be the same. For example, int[][] b = {{1, 2}, {3, 4, 5}}; creates integer array b with two elements (determined by the number of nested array initializers) that represent the rows of the two-dimensional array. Each element of b is a reference to a one-dimensional array of int variables. The int array for row 0 is a onedimensional array with two elements (1 and 2), and the int array for row 1 is a one-dimensional array with three elements (3, 4 and 5). Creating Two-Dimensional Arrays with Array-Creation Expressions A multidimensional array with the same number of columns in every row can be created with an array-creation expression. For example, the following line declares array b and assign it a reference to a three-by-four array: int[][] b = new int[3][4]; In this case, we use the literal values 3 and 4 to specify the number of rows and number of columns, respectively, but this is not required. Programs can also use variables to specify array dimensions, because new creates arrays at execution time—not at compile time. The elements of a multidimensional array are initialized when the array object is created. A multidimensional array in which each row has a different number of columns can be created as follows: int[][] b = new int[2][]; // create 2 rows b[0] = new int[5]; // create 5 columns for row 0 b[1] = new int[3]; // create 3 columns for row 1 The preceding statements create a two-dimensional array with two rows. Row 0 has five columns, and row 1 has three columns. Two-Dimensional Array Example: Displaying Element Values Figure 7.17 demonstrates initializing two-dimensional arrays with array initializers, and using nested for loops to traverse the arrays (i.e., manipulate every element of each array). Class InitArray’s main declares two arrays. The declaration of array1 (line 9) uses nested array initializers of the same length to initialize the first row to the values 1, 2 and 3, and the second row to the values 4, 5 and 6. The declaration of array2 (line 10) uses nested initializers of different lengths. In this case, the first row is initialized to two elements with the values 1 and 2, respectively. The second row is initialized to one element with the value 3. The third row is initialized to three elements with the values 4, 5 and 6, respectively. 1 2 3 4 5 // Fig. 7.17: InitArray.java // Initializing two-dimensional arrays. public class InitArray { Fig. 7.17 | Initializing two-dimensional arrays. (Part 1 of 2.) 274 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 Chapter 7 Arrays and ArrayLists // create and output two-dimensional arrays public static void main(String[] args) { int[][] array1 = {{1, 2, 3}, {4, 5, 6}}; int[][] array2 = {{1, 2}, {3}, {4, 5, 6}}; System.out.println("Values in array1 by row are"); outputArray(array1); // displays array1 by row System.out.printf("%nValues in array2 by row are%n"); outputArray(array2); // displays array2 by row } // output rows and columns of a two-dimensional array public static void outputArray(int[][] array) { // loop through array's rows for (int row = 0; row < array.length; row++) { // loop through columns of current row for (int column = 0; column < array[row].length; column++) System.out.printf("%d ", array[row][column]); System.out.println(); } } } // end class InitArray Values in array1 by row are 1 2 3 4 5 6 Values in array2 by row are 1 2 3 4 5 6 Fig. 7.17 | Initializing two-dimensional arrays. (Part 2 of 2.) Lines 13 and 16 call method outputArray (lines 20–31) to output the elements of and array2, respectively. Method outputArray’s parameter—int[][] array— indicates that the method receives a two-dimensional array. The nested for statement (lines 23–30) outputs the rows of a two-dimensional array. In the loop-continuation condition of the outer for statement, the expression array.length determines the number of rows in the array. In the inner for statement, the expression array[row].length determines the number of columns in the current row of the array. The inner for statement’s condition enables the loop to determine the exact number of columns in each row. We demonstrate nested enhanced for statements in Fig. 7.18. array1 Common Multidimensional-Array Manipulations Performed with for Statements Many common array manipulations use for statements. As an example, the following for statement sets all the elements in row 2 of array a in Fig. 7.16 to zero: 7.12 Case Study: Class GradeBook Using a Two-Dimensional Array 275 for (int column = 0; column < a[2].length; column++) a[2][column] = 0; We specified row 2; therefore, we know that the first index is always 2 (0 is the first row, and 1 is the second row). This for loop varies only the second index (i.e., the column index). If row 2 of array a contains four elements, then the preceding for statement is equivalent to the assignment statements a[2][0] a[2][1] a[2][2] a[2][3] = = = = 0; 0; 0; 0; The following nested for statement totals the values of all the elements in array a: int total = 0; for (int row = 0; row < a.length; row++) { for (int column = 0; column < a[row].length; column++) total += a[row][column]; } These nested for statements total the array elements one row at a time. The outer for statement begins by setting the row index to 0 so that the first row’s elements can be totaled by the inner for statement. The outer for then increments row to 1 so that the second row can be totaled. Then, the outer for increments row to 2 so that the third row can be totaled. The variable total can be displayed when the outer for statement terminates. In the next example, we show how to process a two-dimensional array in a similar manner using nested enhanced for statements. 7.12 Case Study: Class GradeBook Using a TwoDimensional Array In Section 7.10, we presented class GradeBook (Fig. 7.14), which used a one-dimensional array to store student grades on a single exam. In most semesters, students take several exams. Professors are likely to want to analyze grades across the entire semester, both for a single student and for the class as a whole. Storing Student Grades in a Two-Dimensional Array in Class GradeBook Figure 7.18 contains a GradeBook class that uses a two-dimensional array grades to store the grades of several students on multiple exams. Each row of the array represents a single student’s grades for the entire course, and each column represents the grades of all the students who took a particular exam. Class GradeBookTest (Fig. 7.19) passes the array as an argument to the GradeBook constructor. In this example, we use a ten-by-three array for ten students’ grades on three exams. Five methods perform array manipulations to process the grades. Each method is similar to its counterpart in the earlier one-dimensional array version of GradeBook (Fig. 7.14). Method getMinimum (lines 46–62) determines the lowest grade of any student for the semester. Method getMaximum (lines 65–83) determines the highest grade of any student for the semester. Method getAverage (lines 86–96) determines a particular student’s semester average. Method outputBarChart (lines 99–129) out- 276 Chapter 7 Arrays and ArrayLists puts a grade bar chart for the entire semester’s student grades. Method outputGrades (lines 132–156) outputs the array in a tabular format, along with each student’s semester average. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 // Fig. 7.18: GradeBook.java // GradeBook class using a two-dimensional array to store grades. public class GradeBook { private String courseName; // name of course this grade book represents private int[][] grades; // two-dimensional array of student grades // two-argument constructor initializes courseName and grades array public GradeBook(String courseName, int[][] grades) { this.courseName = courseName; this.grades = grades; } // method to set the course name public void setCourseName(String courseName) { this.courseName = courseName; } // method to retrieve the course name public String getCourseName() { return courseName; } // perform various operations on the data public void processGrades() { // output grades array outputGrades(); // call methods getMinimum and getMaximum System.out.printf("%n%s %d%n%s %d%n%n", "Lowest grade in the grade book is", getMinimum(), "Highest grade in the grade book is", getMaximum()); // output grade distribution chart of all grades on all tests outputBarChart(); } // find minimum grade public int getMinimum() { // assume first element of grades array is smallest int lowGrade = grades[0][0]; Fig. 7.18 | GradeBook class using a two-dimensional array to store grades. (Part 1 of 4.) 7.12 Case Study: Class GradeBook Using a Two-Dimensional Array 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 277 // loop through rows of grades array for (int[] studentGrades : grades) { // loop through columns of current row for (int grade : studentGrades) { // if grade less than lowGrade, assign it to lowGrade if (grade < lowGrade) lowGrade = grade; } } return lowGrade; } // find maximum grade public int getMaximum() { // assume first element of grades array is largest int highGrade = grades[0][0]; // loop through rows of grades array for (int[] studentGrades : grades) { // loop through columns of current row for (int grade : studentGrades) { // if grade greater than highGrade, assign it to highGrade if (grade > highGrade) highGrade = grade; } } return highGrade; } // determine average grade for particular set of grades public double getAverage(int[] setOfGrades) { int total = 0; // sum grades for one student for (int grade : setOfGrades) total += grade; // return average of grades return (double) total / setOfGrades.length; } // output bar chart displaying overall grade distribution public void outputBarChart() { Fig. 7.18 | GradeBook class using a two-dimensional array to store grades. (Part 2 of 4.) 278 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 Chapter 7 Arrays and ArrayLists System.out.println("Overall grade distribution:"); // stores frequency of grades in each range of 10 grades int[] frequency = new int[11]; // for each grade in GradeBook, increment the appropriate frequency for (int[] studentGrades : grades) { for (int grade : studentGrades) ++frequency[grade / 10]; } // for each grade frequency, print bar in chart for (int count = 0; count < frequency.length; count++) { // output bar label ("00-09: ", ..., "90-99: ", "100: ") if (count == 10) System.out.printf("%5d: ", 100); else System.out.printf("%02d-%02d: ", count * 10, count * 10 + 9); // print bar of asterisks for (int stars = 0; stars < frequency[count]; stars++) System.out.print("*"); System.out.println(); } } // output the contents of the grades array public void outputGrades() { System.out.printf("The grades are:%n%n"); System.out.print(" "); // align column heads Fig. 7.18 | // create a column heading for each of the tests for (int test = 0; test < grades[0].length; test++) System.out.printf("Test %d ", test + 1); System.out.println("Average"); // student average column heading // create rows/columns of text representing array grades for (int student = 0; student < grades.length; student++) { System.out.printf("Student %2d", student + 1); for (int test : grades[student]) // output student's grades System.out.printf("%8d", test); // call method getAverage to calculate student's average grade; // pass row of grades as the argument to getAverage double average = getAverage(grades[student]); GradeBook class using a two-dimensional array to store grades. (Part 3 of 4.) 7.12 Case Study: Class GradeBook Using a Two-Dimensional Array 279 154 System.out.printf("%9.2f%n", average); 155 } 156 } 157 } // end class GradeBook Fig. 7.18 | GradeBook class using a two-dimensional array to store grades. (Part 4 of 4.) Methods getMinimum and getMaximum Methods getMinimum, getMaximum, outputBarChart and outputGrades each loop through array grades by using nested for statements—for example, the nested enhanced for statement (lines 50–59) from the declaration of method getMinimum. The outer enhanced for statement iterates through the two-dimensional array grades, assigning successive rows to parameter studentGrades on successive iterations. The square brackets following the parameter name indicate that studentGrades refers to a one-dimensional int array—namely, a row in array grades containing one student’s grades. To find the lowest overall grade, the inner for statement compares the elements of the current onedimensional array studentGrades to variable lowGrade. For example, on the first iteration of the outer for, row 0 of grades is assigned to parameter studentGrades. The inner enhanced for statement then loops through studentGrades and compares each grade value with lowGrade. If a grade is less than lowGrade, lowGrade is set to that grade. On the second iteration of the outer enhanced for statement, row 1 of grades is assigned to studentGrades, and the elements of this row are compared with variable lowGrade. This repeats until all rows of grades have been traversed. When execution of the nested statement is complete, lowGrade contains the lowest grade in the two-dimensional array. Method getMaximum works similarly to method getMinimum. Method outputBarChart Method outputBarChart in Fig. 7.18 is nearly identical to the one in Fig. 7.14. However, to output the overall grade distribution for a whole semester, the method here uses nested enhanced for statements (lines 107–111) to create the one-dimensional array frequency based on all the grades in the two-dimensional array. The rest of the code in each of the two outputBarChart methods that displays the chart is identical. Method outputGrades Method outputGrades (lines 132–156) uses nested for statements to output values of the array grades and each student’s semester average. The output (Fig. 7.19) shows the result, which resembles the tabular format of a professor’s physical grade book. Lines 138–139 print the column headings for each test. We use a counter-controlled for statement here so that we can identify each test with a number. Similarly, the for statement in lines 144– 155 first outputs a row label using a counter variable to identify each student (line 146). Although array indices start at 0, lines 139 and 146 output test + 1 and student + 1, respectively, to produce test and student numbers starting at 1 (see the output in Fig. 7.19). The inner for statement (lines 148–149) uses the outer for statement’s counter variable student to loop through a specific row of array grades and output each student’s test grade. An enhanced for statement can be nested in a counter-controlled for statement, and vice versa. Finally, line 153 obtains each student’s semester average by passing the current row of grades (i.e., grades[student]) to method getAverage. 280 Chapter 7 Arrays and ArrayLists Method getAverage Method getAverage (lines 86–96) takes one argument—a one-dimensional array of test results for a particular student. When line 153 calls getAverage, the argument is grades[student], which specifies that a particular row of the two-dimensional array grades should be passed to getAverage. For example, based on the array created in Fig. 7.19, the argument grades[1] represents the three values (a one-dimensional array of grades) stored in row 1 of the two-dimensional array grades. Recall that a two-dimensional array is one whose elements are one-dimensional arrays. Method getAverage calculates the sum of the array elements, divides the total by the number of test results and returns the floating-point result as a double value (line 95). Class GradeBookTest That Demonstrates Class GradeBook Figure 7.19 creates an object of class GradeBook (Fig. 7.18) using the two-dimensional array of ints named gradesArray (declared and initialized in lines 10–19). Lines 21–22 pass a course name and gradesArray to the GradeBook constructor. Lines 23–24 display a welcome message containing the course name, then line 25 invokes myGradeBook’s processGrades method to display a report summarizing the students’ grades for the semester. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 // Fig. 7.19: GradeBookTest.java // GradeBookTest creates GradeBook object using a two-dimensional array // of grades, then invokes method processGrades to analyze them. public class GradeBookTest { // main method begins program execution public static void main(String[] args) { // two-dimensional array of student grades int[][] gradesArray = {{87, 96, 70}, {68, 87, 90}, {94, 100, 90}, {100, 81, 82}, {83, 65, 85}, {78, 87, 65}, {85, 75, 83}, {91, 94, 100}, {76, 72, 84}, {87, 93, 73}}; GradeBook myGradeBook = new GradeBook( "CS101 Introduction to Java Programming", gradesArray); System.out.printf("Welcome to the grade book for%n%s%n%n", myGradeBook.getCourseName()); myGradeBook.processGrades(); } } // end class GradeBookTest Fig. 7.19 | GradeBookTest creates GradeBook object using a two-dimensional array of grades, then invokes method processGrades to analyze them. (Part 1 of 2.) 7.13 Variable-Length Argument Lists 281 Welcome to the grade book for CS101 Introduction to Java Programming The grades are: Student 1 Student 2 Student 3 Student 4 Student 5 Student 6 Student 7 Student 8 Student 9 Student 10 Test 1 87 68 94 100 83 78 85 91 76 87 Test 2 96 87 100 81 65 87 75 94 72 93 Test 3 70 90 90 82 85 65 83 100 84 73 Average 84.33 81.67 94.67 87.67 77.67 76.67 81.00 95.00 77.33 84.33 Lowest grade in the grade book is 65 Highest grade in the grade book is 100 Overall grade distribution: 00-09: 10-19: 20-29: 30-39: 40-49: 50-59: 60-69: *** 70-79: ****** 80-89: *********** 90-99: ******* 100: *** Fig. 7.19 | GradeBookTest creates GradeBook object using a two-dimensional array of grades, then invokes method processGrades to analyze them. (Part 2 of 2.) 7.13 Variable-Length Argument Lists With variable-length argument lists, you can create methods that receive an unspecified number of arguments. A type followed by an ellipsis (...) in a method’s parameter list indicates that the method receives a variable number of arguments of that particular type. This use of the ellipsis can occur only once in a parameter list, and the ellipsis, together with its type and the parameter name, must be placed at the end of the parameter list. While you can use method overloading and array passing to accomplish much of what is accomplished with variable-length argument lists, using an ellipsis in a method’s parameter list is more concise. Figure 7.20 demonstrates method average (lines 7–16), which receives a variablelength sequence of doubles. Java treats the variable-length argument list as an array whose elements are all of the same type. So, the method body can manipulate the parameter numbers as an array of doubles. Lines 12–13 use the enhanced for loop to walk through the array and calculate the total of the doubles in the array. Line 15 accesses numbers.length to obtain the size of the numbers array for use in the averaging calculation. Lines 29, 31 and 33 in main call method average with two, three and four arguments, respectively. Method average has a variable-length argument list (line 7), so it can average as many 282 Chapter 7 Arrays and ArrayLists double arguments as the caller passes. The output shows that each call to method average returns the correct value. Common Programming Error 7.5 Placing an ellipsis indicating a variable-length argument list in the middle of a parameter list is a syntax error. An ellipsis may be placed only at the end of the parameter list. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 d1 d2 d3 d4 // Fig. 7.20: VarargsTest.java // Using variable-length argument lists. public class VarargsTest { // calculate average public static double average(double... numbers) { double total = 0.0; // calculate total using the enhanced for statement for (double d : numbers) total += d; return total / numbers.length; } public static void main(String[] args) { double d1 = 10.0; double d2 = 20.0; double d3 = 30.0; double d4 = 40.0; System.out.printf("d1 = %.1f%nd2 = %.1f%nd3 = %.1f%nd4 = %.1f%n%n", d1, d2, d3, d4); System.out.printf("Average of d1 and d2 is %.1f%n", average(d1, d2) ); System.out.printf("Average of d1, d2 and d3 is %.1f%n", average(d1, d2, d3) ); System.out.printf("Average of d1, d2, d3 and d4 is %.1f%n", average(d1, d2, d3, d4) ); } } // end class VarargsTest = = = = 10.0 20.0 30.0 40.0 Average of d1 and d2 is 15.0 Average of d1, d2 and d3 is 20.0 Average of d1, d2, d3 and d4 is 25.0 Fig. 7.20 | Using variable-length argument lists. 7.14 Using Command-Line Arguments 283 7.14 Using Command-Line Arguments It’s possible to pass arguments from the command line to an application via method main’s String[] parameter, which receives an array of Strings. By convention, this parameter is named args. When an application is executed using the java command, Java passes the command-line arguments that appear after the class name in the java command to the application’s main method as Strings in the array args. The number of command-line arguments is obtained by accessing the array’s length attribute. Common uses of commandline arguments include passing options and filenames to applications. Our next example uses command-line arguments to determine the size of an array, the value of its first element and the increment used to calculate the values of the array’s remaining elements. The command java InitArray 5 0 4 passes three arguments, 5, 0 and 4, to the application InitArray. Command-line arguments are separated by white space, not commas. When this command executes, InitArray’s main method receives the three-element array args (i.e., args.length is 3) in which args[0] contains the String "5", args[1] contains the String "0" and args[2] contains the String "4". The program determines how to use these arguments—in Fig. 7.21 we convert the three command-line arguments to int values and use them to initialize an array. When the program executes, if args.length is not 3, the program prints an error message and terminates (lines 9–12). Otherwise, lines 14–32 initialize and display the array based on the values of the command-line arguments. Line 16 gets args[0]—a String that specifies the array size—and converts it to an int value that the program uses to create the array in line 17. The static method parseInt of class Integer converts its String argument to an int. Lines 20–21 convert the args[1] and args[2] command-line arguments to int values and store them in initialValue and increment, respectively. Lines 24–25 calculate the value for each array element. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 // Fig. 7.21: InitArray.java // Initializing an array using command-line arguments. public class InitArray { public static void main(String[] args) { // check number of command-line arguments if (args.length != 3) System.out.printf( "Error: Please re-enter the entire command, including%n" + "an array size, initial value and increment.%n"); else { // get array size from first command-line argument int arrayLength = Integer.parseInt(args[0]); int[] array = new int[arrayLength]; Fig. 7.21 | Initializing an array using command-line arguments. (Part 1 of 2.) 284 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 Chapter 7 Arrays and ArrayLists // get initial value and increment from command-line arguments int initialValue = Integer.parseInt(args[1]); int increment = Integer.parseInt(args[2]); // calculate value for each array element for (int counter = 0; counter < array.length; counter++) array[counter] = initialValue + increment * counter; System.out.printf("%s%8s%n", "Index", "Value"); // display array index and value for (int counter = 0; counter < array.length; counter++) System.out.printf("%5d%8d%n", counter, array[counter]); } } } // end class InitArray java InitArray Error: Please re-enter the entire command, including an array size, initial value and increment. java InitArray 5 0 4 Index Value 0 0 1 4 2 8 3 12 4 16 java InitArray 8 1 2 Index Value 0 1 1 3 2 5 3 7 4 9 5 11 6 13 7 15 Fig. 7.21 | Initializing an array using command-line arguments. (Part 2 of 2.) The output of the first execution shows that the application received an insufficient number of command-line arguments. The second execution uses command-line arguments 5, 0 and 4 to specify the size of the array (5), the value of the first element (0) and the increment of each value in the array (4), respectively. The corresponding output shows that these values create an array containing the integers 0, 4, 8, 12 and 16. The output from the third execution shows that the command-line arguments 8, 1 and 2 produce an array whose 8 elements are the nonnegative odd integers from 1 to 15. 7.15 Class Arrays 285 7.15 Class Arrays Class Arrays helps you avoid reinventing the wheel by providing static methods for common array manipulations. These methods include sort for sorting an array (i.e., arranging elements into ascending order), binarySearch for searching a sorted array (i.e., determining whether an array contains a specific value and, if so, where the value is located), equals for comparing arrays and fill for placing values into an array. These methods are overloaded for primitive-type arrays and for arrays of objects. Our focus in this section is on using the built-in capabilities provided by the Java API. Chapter 19, Searching, Sorting and Big O, shows how to implement your own sorting and searching algorithms, a subject of great interest to computer-science researchers and students. Figure 7.22 uses Arrays methods sort, binarySearch, equals and fill, and shows how to copy arrays with class System’s static arraycopy method. In main, line 11 sorts the elements of array doubleArray. The static method sort of class Arrays orders the array’s elements in ascending order by default. We discuss how to sort in descending order later in the chapter. Overloaded versions of sort allow you to sort a specific range of elements within the array. Lines 12–15 output the sorted array. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 // Fig. 7.22: ArrayManipulations.java // Arrays class methods and System.arraycopy. import java.util.Arrays; public class ArrayManipulations { public static void main(String[] args) { // sort doubleArray into ascending order double[] doubleArray = { 8.4, 9.3, 0.2, 7.9, 3.4 }; Arrays.sort(doubleArray); System.out.printf("%ndoubleArray: "); Fig. 7.22 | for (double value : doubleArray) System.out.printf("%.1f ", value); // fill 10-element array with 7s int[] filledIntArray = new int[10]; Arrays.fill(filledIntArray, 7); displayArray(filledIntArray, "filledIntArray"); // copy array intArray into array intArrayCopy int[] intArray = { 1, 2, 3, 4, 5, 6 }; int[] intArrayCopy = new int[intArray.length]; System.arraycopy(intArray, 0, intArrayCopy, 0, intArray.length); displayArray(intArray, "intArray"); displayArray(intArrayCopy, "intArrayCopy"); // compare intArray and intArrayCopy for equality boolean b = Arrays.equals(intArray, intArrayCopy); System.out.printf("%n%nintArray %s intArrayCopy%n", (b ? "==" : "!=")); Arrays class methods and System.arraycopy. (Part 1 of 2.) 286 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 Chapter 7 Arrays and ArrayLists // compare intArray and filledIntArray for equality b = Arrays.equals(intArray, filledIntArray); System.out.printf("intArray %s filledIntArray%n", (b ? "==" : "!=")); // search intArray for the value 5 int location = Arrays.binarySearch(intArray, 5); if (location >= 0) System.out.printf( "Found 5 at element %d in intArray%n", location); else System.out.println("5 not found in intArray"); // search intArray for the value 8763 location = Arrays.binarySearch(intArray, 8763); if (location >= 0) System.out.printf( "Found 8763 at element %d in intArray%n", location); else System.out.println("8763 not found in intArray"); } // output values in each array public static void displayArray(int[] array, String description) { System.out.printf("%n%s: ", description); for (int value : array) System.out.printf("%d ", value); } } // end class ArrayManipulations doubleArray: 0.2 3.4 7.9 8.4 9.3 filledIntArray: 7 7 7 7 7 7 7 7 7 7 intArray: 1 2 3 4 5 6 intArrayCopy: 1 2 3 4 5 6 intArray == intArrayCopy intArray != filledIntArray Found 5 at element 4 in intArray 8763 not found in intArray Fig. 7.22 | Arrays class methods and System.arraycopy. (Part 2 of 2.) Line 19 calls static method fill of class Arrays to populate all 10 elements of with 7s. Overloaded versions of fill allow you to populate a specific range of elements with the same value. Line 20 calls our class’s displayArray method (declared at lines 59–65) to output the contents of filledIntArray. Line 25 copies the elements of intArray into intArrayCopy. The first argument (intArray) passed to System method arraycopy is the array from which elements are to filledIntArray 7.16 Introduction to Collections and Class ArrayList 287 be copied. The second argument (0) is the index that specifies the starting point in the range of elements to copy from the array. This value can be any valid array index. The third argument (intArrayCopy) specifies the destination array that will store the copy. The fourth argument (0) specifies the index in the destination array where the first copied element should be stored. The last argument specifies the number of elements to copy from the array in the first argument. In this case, we copy all the elements in the array. Lines 30 and 35 call static method equals of class Arrays to determine whether all the elements of two arrays are equivalent. If the arrays contain the same elements in the same order, the method returns true; otherwise, it returns false. Error-Prevention Tip 7.3 When comparing array contents, always use Arrays.equals(array1, array2), which compares the two arrays’ contents, rather than array1.equals(array2), which compares whether array1 and array2 refer to the same array object. Lines 40 and 49 call static method binarySearch of class Arrays to perform a binary search on intArray, using the second argument (5 and 8763, respectively) as the key. If value is found, binarySearch returns the index of the element; otherwise, binarySearch returns a negative value. The negative value returned is based on the search key’s insertion point—the index where the key would be inserted in the array if we were performing an insert operation. After binarySearch determines the insertion point, it changes its sign to negative and subtracts 1 to obtain the return value. For example, in Fig. 7.22, the insertion point for the value 8763 is the element with index 6 in the array. Method binarySearch changes the insertion point to –6, subtracts 1 from it and returns the value –7. Subtracting 1 from the insertion point guarantees that method binarySearch returns positive values (>= 0) if and only if the key is found. This return value is useful for inserting elements in a sorted array. Chapter 19 discusses binary searching in detail. Common Programming Error 7.6 Passing an unsorted array to binarySearch is a logic error—the value returned is undefined. Java SE 8—Class Arrays Method parallelSort The Arrays class now has several new “parallel” methods that take advantage of multi-core hardware. Arrays method parallelSort can sort large arrays more efficiently on multicore systems. In Section 23.12, we create a very large array and use features of the Java SE 8 Date/Time API to compare how long it takes to sort the array with methods sort and parallelSort. 7.16 Introduction to Collections and Class ArrayList The Java API provides several predefined data structures, called collections, used to store groups of related objects in memory. These classes provide efficient methods that organize, store and retrieve your data without requiring knowledge of how the data is being stored. This reduces application-development time. You’ve used arrays to store sequences of objects. Arrays do not automatically change their size at execution time to accommodate additional elements. The collection class 288 Chapter 7 Arrays and ArrayLists (package java.util) provides a convenient solution to this problem—it can dynamically change its size to accommodate more elements. The T (by convention) is a placeholder—when declaring a new ArrayList, replace it with the type of elements that you want the ArrayList to hold. For example, ArrayList ArrayList list; declares list as an ArrayList collection that can store only Strings. Classes with this kind of placeholder that can be used with any type are called generic classes. Only nonprimitive types can be used to declare variables and create objects of generic classes. However, Java provides a mechanism—known as boxing—that allows primitive values to be wrapped as objects for use with generic classes. So, for example, ArrayList integers; declares integers as an ArrayList that can store only Integers. When you place an int value into an ArrayList, the int value is boxed (wrapped) as an Integer object, and when you get an Integer object from an ArrayList, then assign the object to an int variable, the int value inside the object is unboxed (unwrapped). Additional generic collection classes and generics are discussed in Chapters 16 and 20, respectively. Figure 7.23 shows some common methods of class ArrayList. Method Description add Adds an element to the end of the ArrayList. Removes all the elements from the ArrayList. Returns true if the ArrayList contains the specified element; otherwise, returns false. Returns the element at the specified index. Returns the index of the first occurrence of the specified element in the ArrayList. Overloaded. Removes the first occurrence of the specified value or the element at the specified index. Returns the number of elements stored in the ArrayList. Trims the capacity of the ArrayList to the current number of elements. clear contains get indexOf remove size trimToSize Fig. 7.23 | Some methods and properties of class ArrayList. Demonstrating an ArrayList Figure 7.24 demonstrates some common ArrayList capabilities. Line 10 creates a new empty ArrayList of Strings with a default initial capacity of 10 elements. The capacity indicates how many items the ArrayList can hold without growing. ArrayList is implemented using a conventional array behind the scenes. When the ArrayList grows, it must create a larger internal array and copy each element to the new array. This is a time-consuming operation. It would be inefficient for the ArrayList to grow each time an element is added. Instead, it grows only when an element is added and the number of elements is equal to the capacity—i.e., there’s no space for the new element. 7.16 Introduction to Collections and Class ArrayList 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 289 // Fig. 7.24: ArrayListCollection.java // Generic ArrayList collection demonstration. import java.util.ArrayList; public class ArrayListCollection { public static void main(String[] args) { // create a new ArrayList of Strings with an initial capacity of 10 ArrayList items = new ArrayList(); items.add("red"); // append an item to the list items.add(0, "yellow"); // insert "yellow" at index 0 // header System.out.print( "Display list contents with counter-controlled loop:"); // display the colors in the list for (int i = 0; i < items.size(); i++) System.out.printf(" %s", items.get(i)); // display colors using enhanced for in the display method display(items, "%nDisplay list contents with enhanced for statement:"); items.add("green"); // add "green" to the end of the list items.add("yellow"); // add "yellow" to the end of the list display(items, "List with two new elements:"); items.remove("yellow"); // remove the first "yellow" display(items, "Remove first instance of yellow:"); items.remove(1); // remove item at index 1 display(items, "Remove second list element (green):"); // check if a value is in the List System.out.printf("\"red\" is %sin the list%n", items.contains("red") ? "": "not "); // display number of elements in the List System.out.printf("Size: %s%n", items.size()); } // display the ArrayList's elements on the console public static void display(ArrayList items, String header) { System.out.printf(header); // display header // display each element in items for (String item : items) System.out.printf(" %s", item); Fig. 7.24 | Generic ArrayList collection demonstration. (Part 1 of 2.) 290 54 55 56 Chapter 7 Arrays and ArrayLists System.out.println(); } } // end class ArrayListCollection Display list contents with counter-controlled loop: yellow red Display list contents with enhanced for statement: yellow red List with two new elements: yellow red green yellow Remove first instance of yellow: red green yellow Remove second list element (green): red yellow "red" is in the list Size: 2 Fig. 7.24 | Generic ArrayList collection demonstration. (Part 2 of 2.) The add method adds elements to the ArrayList (lines 12–13). The add method with one argument appends its argument to the end of the ArrayList. The add method with two arguments inserts a new element at the specified position. The first argument is an index. As with arrays, collection indices start at zero. The second argument is the value to insert at that index. The indices of all subsequent elements are incremented by one. Inserting an element is usually slower than adding an element to the end of the ArrayList Lines 20–21 display the items in the ArrayList. Method size returns the number of elements currently in the ArrayList. Method get (line 21) obtains the element at a specified index. Lines 24–25 display the elements again by invoking method display (defined at lines 46–55). Lines 27–28 add two more elements to the ArrayList, then line 29 displays the elements again to confirm that the two elements were added to the end of the collection. The remove method is used to remove an element with a specific value (line 31). It removes only the first such element. If no such element is in the ArrayList, remove does nothing. An overloaded version of the method removes the element at the specified index (line 34). When an element is removed, the indices of any elements after the removed element decrease by one. Line 39 uses the contains method to check if an item is in the ArrayList. The contains method returns true if the element is found in the ArrayList, and false otherwise. The method compares its argument to each element of the ArrayList in order, so using contains on a large ArrayList can be inefficient. Line 42 displays the ArrayList’s size. Java SE 7—Diamond (<>) Notation for Creating an Object of a Generic Class Consider line 10 of Fig. 7.24: ArrayList items = new ArrayList(); Notice that ArrayList appears in the variable declaration and in the class instance creation expression. Java SE 7 introduced the diamond (<>) notation to simplify statements like this. Using <> in a class instance creation expression for an object of a generic class tells the compiler to determine what belongs in the angle brackets. In Java SE 7 and higher, the preceding statement can be written as: ArrayList items = new ArrayList<>(); When the compiler encounters the diamond (<>) in the class instance creation expression, it uses the declaration of variable items to determine the ArrayList’s element type (String)—this is known as inferring the element type. 7.17 (Optional) GUI and Graphics Case Study: Drawing Arcs 291 7.17 (Optional) GUI and Graphics Case Study: Drawing Arcs Using Java’s graphics features, we can create complex drawings that would be more tedious to code line by line. In Figs. 7.25–7.26, we use arrays and repetition statements to draw a rainbow by using Graphics method fillArc. Drawing arcs in Java is similar to drawing ovals—an arc is simply a section of an oval. In Fig. 7.25, lines 10–11 declare and create two new color constants—VIOLET and INDIGO. As you may know, the colors of a rainbow are red, orange, yellow, green, blue, indigo and violet. Java has predefined constants only for the first five colors. Lines 15–17 initialize an array with the colors of the rainbow, starting with the innermost arcs first. The array begins with two Color.WHITE elements, which, as you’ll soon see, are for drawing the empty arcs at the center of the rainbow. The instance variables can be initialized when they’re declared, as shown in lines 10–17. The constructor (lines 20–23) contains a single statement that calls method setBackground (which is inherited from class JPanel) with the parameter Color.WHITE. Method setBackground takes a single Color argument and sets the background of the component to that color. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 // Fig. 7.25: DrawRainbow.java // Drawing a rainbow using arcs and an array of colors. import java.awt.Color; import java.awt.Graphics; import javax.swing.JPanel; public class DrawRainbow extends JPanel { // define indigo and violet private final static Color VIOLET = new Color(128, 0, 128); private final static Color INDIGO = new Color(75, 0, 130); // colors to use in the rainbow, starting from the innermost // The two white entries result in an empty arc in the center private Color[] colors = { Color.WHITE, Color.WHITE, VIOLET, INDIGO, Color.BLUE, Color.GREEN, Color.YELLOW, Color.ORANGE, Color.RED }; // constructor public DrawRainbow() { setBackground(Color.WHITE); // set the background to white } // draws a rainbow using concentric arcs public void paintComponent(Graphics g) { super.paintComponent(g); int radius = 20; // radius of an arc Fig. 7.25 | Drawing a rainbow using arcs and an array of colors. (Part 1 of 2.) 292 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 Chapter 7 Arrays and ArrayLists // draw the rainbow near the bottom-center int centerX = getWidth() / 2; int centerY = getHeight() - 10; // draws filled arcs starting with the outermost for (int counter = colors.length; counter > 0; counter--) { // set the color for the current arc g.setColor(colors[counter - 1]); // fill the arc from 0 to 180 degrees g.fillArc(centerX - counter * radius, centerY - counter * radius, counter * radius * 2, counter * radius * 2, 0, 180); } } } // end class DrawRainbow Fig. 7.25 | Drawing a rainbow using arcs and an array of colors. (Part 2 of 2.) Line 30 in paintComponent declares local variable radius, which determines the radius of each arc. Local variables centerX and centerY (lines 33–34) determine the location of the midpoint on the base of the rainbow. The loop at lines 37–46 uses control variable counter to count backward from the end of the array, drawing the largest arcs first and placing each successive smaller arc on top of the previous one. Line 40 sets the color to draw the current arc from the array. The reason we have Color.WHITE entries at the beginning of the array is to create the empty arc in the center. Otherwise, the center of the rainbow would just be a solid violet semicircle. You can change the individual colors and the number of entries in the array to create new designs. The fillArc method call at lines 43–45 draws a filled semicircle. Method fillArc requires six parameters. The first four represent the bounding rectangle in which the arc will be drawn. The first two of these specify the coordinates for the upper-left corner of the bounding rectangle, and the next two specify its width and height. The fifth parameter is the starting angle on the oval, and the sixth specifies the sweep, or the amount of arc to cover. The starting angle and sweep are measured in degrees, with zero degrees pointing right. A positive sweep draws the arc counterclockwise, while a negative sweep draws the arc clockwise. A method similar to fillArc is drawArc—it requires the same parameters as fillArc, but draws the edge of the arc rather than filling it. Class DrawRainbowTest (Fig. 7.26) creates and sets up a JFrame to display the rainbow. Once the program makes the JFrame visible, the system calls the paintComponent method in class DrawRainbow to draw the rainbow on the screen. 1 2 3 4 5 6 // Fig. 7.26: DrawRainbowTest.java // Test application to display a rainbow. import javax.swing.JFrame; public class DrawRainbowTest { Fig. 7.26 | Test application to display a rainbow. (Part 1 of 2.) 7.17 (Optional) GUI and Graphics Case Study: Drawing Arcs 7 8 9 10 11 12 13 14 15 16 17 293 public static void main(String[] args) { DrawRainbow panel = new DrawRainbow(); JFrame application = new JFrame(); application.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); application.add(panel); application.setSize(400, 250); application.setVisible(true); } } // end class DrawRainbowTest Fig. 7.26 | Test application to display a rainbow. (Part 2 of 2.) GUI and Graphics Case Study Exercise 7.1 (Drawing Spirals) In this exercise, you’ll draw spirals with methods drawLine and drawArc. a) Draw a square-shaped spiral (as in the left screen capture of Fig. 7.27), centered on the panel, using method drawLine. One technique is to use a loop that increases the line length after drawing every second line. The direction in which to draw the next line should follow a distinct pattern, such as down, left, up, right. b) Draw a circular spiral (as in the right screen capture of Fig. 7.27), using method drawArc to draw one semicircle at a time. Each successive semicircle should have a larger radius (as specified by the bounding rectangle’s width) and should continue drawing where the previous semicircle finished. Fig. 7.27 | Drawing a spiral using drawLine (left) and drawArc (right). 294 Chapter 7 Arrays and ArrayLists 7.18 Wrap-Up This chapter began our introduction to data structures, exploring the use of arrays to store data in and retrieve data from lists and tables of values. The chapter examples demonstrated how to declare an array, initialize an array and refer to individual elements of an array. The chapter introduced the enhanced for statement to iterate through arrays. We used exception handling to test for ArrayIndexOutOfBoundsExceptions that occur when a program attempts to access an array element outside the bounds of an array. We also illustrated how to pass arrays to methods and how to declare and manipulate multidimensional arrays. Finally, the chapter showed how to write methods that use variable-length argument lists and how to read arguments passed to a program from the command line. We introduced the ArrayList generic collection, which provides all the functionality and performance of arrays, along with other useful capabilities such as dynamic resizing. We used the add methods to add new items to the end of an ArrayList and to insert items in an ArrayList. The remove method was used to remove the first occurrence of a specified item, and an overloaded version of remove was used to remove an item at a specified index. We used the size method to obtain number of items in the ArrayList. We continue our coverage of data structures in Chapter 16, Generic Collections. Chapter 16 introduces the Java Collections Framework, which uses generics to allow you to specify the exact types of objects that a particular data structure will store. Chapter 16 also introduces Java’s other predefined data structures. Chapter 16 covers additional methods of class Arrays. You’ll be able to use some of the Arrays methods discussed in Chapter 16 after reading the current chapter, but some of the Arrays methods require knowledge of concepts presented later in the book. Chapter 20 discusses generics, which enable you to create general models of methods and classes that can be declared once, but used with many different data types. Chapter 21, Custom Generic Data Structures, shows how to build dynamic data structures, such as lists, queues, stacks and trees, that can grow and shrink as programs execute. We’ve now introduced the basic concepts of classes, objects, control statements, methods, arrays and collections. In Chapter 8, we take a deeper look at classes and objects. Summary Section 7.1 Introduction • Arrays (p. 244) are fixed-length data structures consisting of related data items of the same type. Section 7.2 Arrays • An array is a group of variables (called elements or components; p. 245) containing values that all have the same type. Arrays are objects, so they’re considered reference types. • A program refers to any one of an array’s elements with an array-access expression (p. 245) that includes the name of the array followed by the index of the particular element in square brackets ([]; p. 245). • The first element in every array has index zero (p. 245) and is sometimes called the zeroth element. • An index must be a nonnegative integer. A program can use an expression as an index. • An array object knows its own length and stores this information in a length instance variable (p. 246). Summary 295 Section 7.3 Declaring and Creating Arrays • To create an array object, specify the array’s element type and the number of elements as part of an array-creation expression (p. 246) that uses keyword new. • When an array is created, each element receives a default value—zero for numeric primitive-type elements, false for boolean elements and null for references. • In an array declaration, the type and the square brackets can be combined at the beginning of the declaration to indicate that all the identifiers in the declaration are array variables. • Every element of a primitive-type array contains a variable of the array’s declared type. Every element of a reference-type array is a reference to an object of the array’s declared type. Section 7.4 Examples Using Arrays • A program can create an array and initialize its elements with an array initializer (p. 248). • Constant variables (p. 250) are declared with keyword final, must be initialized before they’re used and cannot be modified thereafter. Section 7.5 Exception Handling: Processing the Incorrect Response • An exception indicates a problem that occurs while a program executes. The name “exception” suggests that the problem occurs infrequently—if the “rule” is that a statement normally executes correctly, then the problem represents the “exception to the rule.” • Exception handling (p. 256) enables you to create fault-tolerant programs. • When a Java program executes, the JVM checks array indices to ensure that they’re greater than or equal to 0 and less than the array’s length. If a program uses an invalid index, Java generates an exception (p. 256) to indicate that an error occurred in the program at execution time. • To handle an exception, place any code that might throw an exception (p. 256) in a try statement. • The try block (p. 256) contains the code that might throw an exception, and the catch block (p. 256) contains the code that handles the exception if one occurs. • You can have many catch blocks to handle different types of exceptions that might be thrown in the corresponding try block. • When a try block terminates, any variables declared in the try block go out of scope. • A catch block declares a type and an exception parameter. Inside the catch block, you can use the parameter’s identifier to interact with a caught exception object. • When a program is executed, array element indices are checked for validity—all indices must be greater than or equal to 0 and less than the length of the array. If an attempt is made to use an invalid index to access an element, an ArrayIndexOutOfRangeException (p. 257) exception occurs. • An exception object’s toString method returns the exception’s error message. Section 7.6 Case Study: Card Shuffling and Dealing Simulation • The toString method of an object is called implicitly when the object is used where a String is expected (e.g., when printf outputs the object as a String using the %s format specifier or when the object is concatenated to a String using the + operator). Section 7.7 Enhanced for Statement • The enhanced for statement (p. 262) allows you to iterate through the elements of an array or a collection without using a counter. The syntax of an enhanced for statement is: for (parameter : arrayName) statement 296 Chapter 7 Arrays and ArrayLists where parameter has a type and an identifier (e.g., int number), and arrayName is the array through which to iterate. • The enhanced for statement cannot be used to modify elements in an array. If a program needs to modify elements, use the traditional counter-controlled for statement. Section 7.8 Passing Arrays to Methods • When an argument is passed by value, a copy of the argument’s value is made and passed to the called method. The called method works exclusively with the copy. Section 7.9 Pass-By-Value vs. Pass-By-Reference • When an argument is passed by reference (p. 265), the called method can access the argument’s value in the caller directly and possibly modify it. • All arguments in Java are passed by value. A method call can pass two types of values to a method—copies of primitive values and copies of references to objects. Although an object’s reference is passed by value (p. 265), a method can still interact with the referenced object by calling its public methods using the copy of the object’s reference. • To pass an object reference to a method, simply specify in the method call the name of the variable that refers to the object. • When you pass an array or an individual array element of a reference type to a method, the called method receives a copy of the array or element’s reference. When you pass an individual element of a primitive type, the called method receives a copy of the element’s value. • To pass an individual array element to a method, use the indexed name of the array. Section 7.11 Multidimensional Arrays • Multidimensional arrays with two dimensions are often used to represent tables of values consisting of information arranged in rows and columns. • A two-dimensional array (p. 272) with m rows and n columns is called an m-by-n array. Such an array can be initialized with an array initializer of the form arrayType[][] arrayName = {{row1 initializer}, {row2 initializer}, …}; • Multidimensional arrays are maintained as arrays of separate one-dimensional arrays. As a result, the lengths of the rows in a two-dimensional array are not required to be the same. • A multidimensional array with the same number of columns in every row can be created with an array-creation expression of the form arrayType[][] arrayName = new arrayType[numRows][numColumns]; Section 7.13 Variable-Length Argument Lists • An argument type followed by an ellipsis (...; p. 281) in a method’s parameter list indicates that the method receives a variable number of arguments of that particular type. The ellipsis can occur only once in a method’s parameter list. It must be at the end of the parameter list. • A variable-length argument list (p. 281) is treated as an array within the method body. The number of arguments in the array can be obtained using the array’s length field. Section 7.14 Using Command-Line Arguments • Passing arguments to main (p. 283) from the command line is achieved by including a parameter of type String[] in the parameter list of main. By convention, main’s parameter is named args. • Java passes command-line arguments that appear after the class name in the java command to the application’s main method as Strings in the array args. Self-Review Exercises 297 Section 7.15 Class Arrays • Class Arrays (p. 285) provides static methods that peform common array manipulations, including sort to sort an array, binarySearch to search a sorted array, equals to compare arrays and fill to place items in an array. • Class System’s arraycopy method (p. 285) enables you to copy the elements of one array into another. Section 7.16 Introduction to Collections and Class ArrayList • The Java API’s collection classes provide efficient methods that organize, store and retrieve data without requiring knowledge of how the data is being stored. • An ArrayList (p. 288) is similar to an array but can be dynamically resized. • The add method (p. 290) with one argument appends an element to the end of an ArrayList. • The add method with two arguments inserts a new element at a specified position in an ArrayList. • The size method (p. 290) returns the number of elements currently in an ArrayList. • The remove method with a reference to an object as an argument removes the first element that matches the argument’s value. • The remove method with an integer argument removes the element at the specified index, and all elements above that index are shifted down by one. • The contains method returns true if the element is found in the ArrayList, and false otherwise. Self-Review Exercises 7.1 Fill in the blank(s) in each of the following statements: a) Lists and tables of values can be stored in and . b) An array is a group of (called elements or components) containing values that . all have the same c) The allows you to iterate through an array’s elements without using a counter. d) The number used to refer to a particular array element is called the element’s . array. e) An array that uses two indices is referred to as a(n) f) Use the enhanced for statement to walk through double array numbers. g) Command-line arguments are stored in . to receive the total number of arguments in a command h) Use the expression line. Assume that command-line arguments are stored in String[] args. i) Given the command java MyClass test, the first command-line argument is . in the parameter list of a method indicates that the method can receive j) A(n) a variable number of arguments. 7.2 Determine whether each of the following is true or false. If false, explain why. a) An array can store many different types of values. b) An array index should normally be of type float. c) An individual array element that’s passed to a method and modified in that method will contain the modified value when the called method completes execution. d) Command-line arguments are separated by commas. 7.3 Perform the following tasks for an array called fractions: a) Declare a constant ARRAY_SIZE that’s initialized to 10. b) Declare an array with ARRAY_SIZE elements of type double, and initialize the elements to 0. c) Refer to array element 4. d) Assign the value 1.667 to array element 9. e) Assign the value 3.333 to array element 6. 298 Chapter 7 Arrays and ArrayLists f) Sum all the elements of the array, using a for statement. Declare the integer variable x as a control variable for the loop. 7.4 Perform the following tasks for an array called table: a) Declare and create the array as an integer array that has three rows and three columns. Assume that the constant ARRAY_SIZE has been declared to be 3. b) How many elements does the array contain? c) Use a for statement to initialize each element of the array to the sum of its indices. Assume that the integer variables x and y are declared as control variables. 7.5 Find and correct the error in each of the following program segments: a) final int ARRAY_SIZE = 5; ARRAY_SIZE = 10; b) Assume int[] b = new int[10]; for (int i = 0; i <= b.length; i++) b[i] = 1; c) Assume int[][] a = {{1, 2}, {3, 4}}; a[1, 1] = 5; Answers to Self-Review Exercises 7.1 a) arrays, collections. b) variables, type. c) enhanced for statement. d) index (or subscript or position number). e) two-dimensional. f) for (double d : numbers). g) an array of Strings, called args by convention. h) args.length. i) test. j) ellipsis (...). 7.2 a) False. An array can store only values of the same type. b) False. An array index must be an integer or an integer expression. c) For individual primitive-type elements of an array: False. A called method receives and manipulates a copy of the value of such an element, so modifications do not affect the original value. If the reference of an array is passed to a method, however, modifications to the array elements made in the called method are indeed reflected in the original. For individual elements of a reference type: True. A called method receives a copy of the reference of such an element, and changes to the referenced object will be reflected in the original array element. d) False. Command-line arguments are separated by white space. 7.3 a) b) c) d) e) f) final int ARRAY_SIZE = 10; double[] fractions = new double[ARRAY_SIZE]; fractions[4] fractions[9] = 1.667; fractions[6] = 3.333; double total = 0.0; for (int x = 0; x < fractions.length; x++) total += fractions[x]; 7.4 a) int[][] table = b) Nine. c) for (int x = 0; new int[ARRAY_SIZE][ARRAY_SIZE]; x < table.length; x++) for (int y = 0; y < table[x].length; y++) table[x][y] = x + y; 7.5 a) Error: Assigning a value to a constant after it has been initialized. Correction: Assign the correct value to the constant in a final int ARRAY_SIZE declaration or declare another variable. Exercises 299 b) Error: Referencing an array element outside the bounds of the array (b[10]). Correction: Change the <= operator to <. c) Error: Array indexing is performed incorrectly. Correction: Change the statement to a[1][1] = 5;. Exercises 7.6 Fill in the blanks in each of the following statements: a) One-dimensional array p contains four elements. The names of those elements are , , and . b) Naming an array, stating its type and specifying the number of dimensions in the array the array. is called c) In a two-dimensional array, the first index identifies the of an element and the of an element. second index identifies the d) An m-by-n array contains rows, columns and elements. e) The name of the element in row 3 and column 5 of array d is . 7.7 Determine whether each of the following is true or false. If false, explain why. a) To refer to a particular location or element within an array, we specify the name of the array and the value of the particular element. b) An array declaration reserves space for the array. c) To indicate that 100 locations should be reserved for integer array p, you write the declaration p[100]; d) An application that initializes the elements of a 15-element array to zero must contain at least one for statement. e) An application that totals the elements of a two-dimensional array must contain nested for statements. 7.8 Write Java statements to accomplish each of the following tasks: a) Display the value of element 6 of array f. b) Initialize each of the five elements of one-dimensional integer array g to 8. c) Total the 100 elements of floating-point array c. d) Copy 11-element array a into the first portion of array b, which contains 34 elements. e) Determine and display the smallest and largest values contained in 99-element floatingpoint array w. 7.9 Consider a two-by-three integer array t. a) Write a statement that declares and creates t. b) How many rows does t have? c) How many columns does t have? d) How many elements does t have? e) Write access expressions for all the elements in row 1 of t. f) Write access expressions for all the elements in column 2 of t. g) Write a single statement that sets the element of t in row 0 and column 1 to zero. h) Write individual statements to initialize each element of t to zero. i) Write a nested for statement that initializes each element of t to zero. j) Write a nested for statement that inputs the values for the elements of t from the user. k) Write a series of statements that determines and displays the smallest value in t. l) Write a single printf statement that displays the elements of the first row of t. m) Write a statement that totals the elements of the third column of t. Do not use repetition. n) Write a series of statements that displays the contents of t in tabular format. List the column indices as headings across the top, and list the row indices at the left of each row. 300 Chapter 7 Arrays and ArrayLists 7.10 (Sales Commissions) Use a one-dimensional array to solve the following problem: A company pays its salespeople on a commission basis. The salespeople receive $200 per week plus 9% of their gross sales for that week. For example, a salesperson who grosses $5,000 in sales in a week receives $200 plus 9% of $5,000, or a total of $650. Write an application (using an array of counters) that determines how many of the salespeople earned salaries in each of the following ranges (assume that each salesperson’s salary is truncated to an integer amount): a) $200–299 b) $300–399 c) $400–499 d) $500–599 e) $600–699 f) $700–799 g) $800–899 h) $900–999 i) $1,000 and over Summarize the results in tabular format. 7.11 Write statements that perform the following one-dimensional-array operations: a) Set the 10 elements of integer array counts to zero. b) Add one to each of the 15 elements of integer array bonus. c) Display the five values of integer array bestScores in column format. 7.12 (Duplicate Elimination) Use a one-dimensional array to solve the following problem: Write an application that inputs five numbers, each between 10 and 100, inclusive. As each number is read, display it only if it’s not a duplicate of a number already read. Provide for the “worst case,” in which all five numbers are different. Use the smallest possible array to solve this problem. Display the complete set of unique values input after the user enters each new value. 7.13 Label the elements of three-by-five two-dimensional array which they’re set to zero by the following program segment: sales to indicate the order in for (int row = 0; row < sales.length; row++) { for (int col = 0; col < sales[row].length; col++) { sales[row][col] = 0; } } 7.14 (Variable-Length Argument List) Write an application that calculates the product of a series of integers that are passed to method product using a variable-length argument list. Test your method with several calls, each with a different number of arguments. 7.15 (Command-Line Arguments) Rewrite Fig. 7.2 so that the size of the array is specified by the first command-line argument. If no command-line argument is supplied, use 10 as the default size of the array. 7.16 (Using the Enhanced for Statement) Write an application that uses an enhanced for statement to sum the double values passed by the command-line arguments. [Hint: Use the static method parseDouble of class Double to convert a String to a double value.] 7.17 (Dice Rolling) Write an application to simulate the rolling of two dice. The application should use an object of class Random once to roll the first die and again to roll the second die. The sum of the two values should then be calculated. Each die can show an integer value from 1 to 6, so the sum of the values will vary from 2 to 12, with 7 being the most frequent sum, and 2 and 12 the least frequent. Figure 7.28 shows the 36 possible combinations of the two dice. Your application Exercises 301 should roll the dice 36,000,000 times. Use a one-dimensional array to tally the number of times each possible sum appears. Display the results in tabular format. 1 2 3 4 5 6 1 2 3 4 5 6 7 2 3 4 5 6 7 8 3 4 5 6 7 8 9 4 5 6 7 8 9 10 5 6 7 8 9 10 11 6 7 8 9 10 11 12 Fig. 7.28 | The 36 possible sums of two dice. 7.18 (Game of Craps) Write an application that runs 1,000,000 games of craps (Fig. 6.8) and answers the following questions: a) How many games are won on the first roll, second roll, …, twentieth roll and after the twentieth roll? b) How many games are lost on the first roll, second roll, …, twentieth roll and after the twentieth roll? c) What are the chances of winning at craps? [Note: You should discover that craps is one of the fairest casino games. What do you suppose this means?] d) What is the average length of a game of craps? e) Do the chances of winning improve with the length of the game? 7.19 (Airline Reservations System) A small airline has just purchased a computer for its new automated reservations system. You’ve been asked to develop the new system. You’re to write an application to assign seats on each flight of the airline’s only plane (capacity: 10 seats). Your application should display the following alternatives: Please type 1 for First Class and Please type 2 for Economy. If the user types 1, your application should assign a seat in the firstclass section (seats 1–5). If the user types 2, your application should assign a seat in the economy section (seats 6–10). Your application should then display a boarding pass indicating the person’s seat number and whether it’s in the first-class or economy section of the plane. Use a one-dimensional array of primitive type boolean to represent the seating chart of the plane. Initialize all the elements of the array to false to indicate that all the seats are empty. As each seat is assigned, set the corresponding element of the array to true to indicate that the seat is no longer available. Your application should never assign a seat that has already been assigned. When the economy section is full, your application should ask the person if it’s acceptable to be placed in the first-class section (and vice versa). If yes, make the appropriate seat assignment. If no, display the message "Next flight leaves in 3 hours." 7.20 (Total Sales) Use a two-dimensional array to solve the following problem: A company has four salespeople (1 to 4) who sell five different products (1 to 5). Once a day, each salesperson passes in a slip for each type of product sold. Each slip contains the following: a) The salesperson number b) The product number c) The total dollar value of that product sold that day Thus, each salesperson passes in between 0 and 5 sales slips per day. Assume that the information from all the slips for last month is available. Write an application that will read all this information for 302 Chapter 7 Arrays and ArrayLists last month’s sales and summarize the total sales by salesperson and by product. All totals should be stored in the two-dimensional array sales. After processing all the information for last month, display the results in tabular format, with each column representing a salesperson and each row representing a particular product. Cross-total each row to get the total sales of each product for last month. Cross-total each column to get the total sales by salesperson for last month. Your output should include these cross-totals to the right of the totaled rows and to the bottom of the totaled columns. 7.21 (Turtle Graphics) The Logo language made the concept of turtle graphics famous. Imagine a mechanical turtle that walks around the room under the control of a Java application. The turtle holds a pen in one of two positions, up or down. While the pen is down, the turtle traces out shapes as it moves, and while the pen is up, the turtle moves about freely without writing anything. In this problem, you’ll simulate the operation of the turtle and create a computerized sketchpad. Use a 20-by-20 array floor that’s initialized to zeros. Read commands from an array that contains them. Keep track of the current position of the turtle at all times and whether the pen is currently up or down. Assume that the turtle always starts at position (0, 0) of the floor with its pen up. The set of turtle commands your application must process are shown in Fig. 7.29. Command Meaning 1 Pen up Pen down 2 3 4 5,10 6 9 Turn right Turn left Move forward 10 spaces (replace 10 for a different number of spaces) Display the 20-by-20 array End of data (sentinel) Fig. 7.29 | Turtle graphics commands. Suppose that the turtle is somewhere near the center of the floor. The following “program” would draw and display a 12-by-12 square, leaving the pen in the up position: 2 5,12 3 5,12 3 5,12 3 5,12 1 6 9 As the turtle moves with the pen down, set the appropriate elements of array floor to 1s. When the 6 command (display the array) is given, wherever there’s a 1 in the array, display an asterisk or any character you choose. Wherever there’s a 0, display a blank. Write an application to implement the turtle graphics capabilities discussed here. Write several turtle graphics programs to draw interesting shapes. Add other commands to increase the power of your turtle graphics language. Exercises 303 7.22 (Knight’s Tour) An interesting puzzler for chess buffs is the Knight’s Tour problem, originally proposed by the mathematician Euler. Can the knight piece move around an empty chessboard and touch each of the 64 squares once and only once? We study this intriguing problem in depth here. The knight makes only L-shaped moves (two spaces in one direction and one space in a perpendicular direction). Thus, as shown in Fig. 7.30, from a square near the middle of an empty chessboard, the knight (labeled K) can make eight different moves (numbered 0 through 7). 0 1 2 3 4 5 6 7 0 1 2 2 1 3 0 K 3 4 4 7 5 5 6 6 7 Fig. 7.30 | The eight possible moves of the knight. a) Draw an eight-by-eight chessboard on a sheet of paper, and attempt a Knight’s Tour by hand. Put a 1 in the starting square, a 2 in the second square, a 3 in the third, and so on. Before starting the tour, estimate how far you think you’ll get, remembering that a full tour consists of 64 moves. How far did you get? Was this close to your estimate? b) Now let’s develop an application that will move the knight around a chessboard. The board is represented by an eight-by-eight two-dimensional array board. Each square is initialized to zero. We describe each of the eight possible moves in terms of its horizontal and vertical components. For example, a move of type 0, as shown in Fig. 7.30, consists of moving two squares horizontally to the right and one square vertically upward. A move of type 2 consists of moving one square horizontally to the left and two squares vertically upward. Horizontal moves to the left and vertical moves upward are indicated with negative numbers. The eight moves may be described by two one-dimensional arrays, horizontal and vertical, as follows: horizontal[0] horizontal[1] horizontal[2] horizontal[3] horizontal[4] horizontal[5] horizontal[6] horizontal[7] = = = = = = = = 2 1 -1 -2 -2 -1 1 2 vertical[0] vertical[1] vertical[2] vertical[3] vertical[4] vertical[5] vertical[6] vertical[7] = = = = = = = = -1 -2 -2 -1 1 2 2 1 Let the variables currentRow and currentColumn indicate the row and column, respectively, of the knight’s current position. To make a move of type moveNumber, where moveNumber is between 0 and 7, your application should use the statements currentRow += vertical[moveNumber]; currentColumn += horizontal[moveNumber]; Write an application to move the knight around the chessboard. Keep a counter that varies from 1 to 64. Record the latest count in each square the knight moves to. 304 Chapter 7 Arrays and ArrayLists Test each potential move to see if the knight has already visited that square. Test every potential move to ensure that the knight does not land off the chessboard. Run the application. How many moves did the knight make? c) After attempting to write and run a Knight’s Tour application, you’ve probably developed some valuable insights. We’ll use these insights to develop a heuristic (i.e., a common-sense rule) for moving the knight. Heuristics do not guarantee success, but a carefully developed heuristic greatly improves the chance of success. You may have observed that the outer squares are more troublesome than the squares nearer the center of the board. In fact, the most troublesome or inaccessible squares are the four corners. Intuition may suggest that you should attempt to move the knight to the most troublesome squares first and leave open those that are easiest to get to, so that when the board gets congested near the end of the tour, there will be a greater chance of success. We could develop an “accessibility heuristic” by classifying each of the squares according to how accessible it is and always moving the knight (using the knight’s Lshaped moves) to the most inaccessible square. We label a two-dimensional array accessibility with numbers indicating from how many squares each particular square is accessible. On a blank chessboard, each of the 16 squares nearest the center is rated as 8, each corner square is rated as 2, and the other squares have accessibility numbers of 3, 4 or 6 as follows: 2 3 4 4 4 4 3 2 3 4 6 6 6 6 4 3 4 6 8 8 8 8 6 4 4 6 8 8 8 8 6 4 4 6 8 8 8 8 6 4 4 6 8 8 8 8 6 4 3 4 6 6 6 6 4 3 2 3 4 4 4 4 3 2 Write a new version of the Knight’s Tour, using the accessibility heuristic. The knight should always move to the square with the lowest accessibility number. In case of a tie, the knight may move to any of the tied squares. Therefore, the tour may begin in any of the four corners. [Note: As the knight moves around the chessboard, your application should reduce the accessibility numbers as more squares become occupied. In this way, at any given time during the tour, each available square’s accessibility number will remain equal to precisely the number of squares from which that square may be reached.] Run this version of your application. Did you get a full tour? Modify the application to run 64 tours, one starting from each square of the chessboard. How many full tours did you get? d) Write a version of the Knight’s Tour application that, when encountering a tie between two or more squares, decides what square to choose by looking ahead to those squares reachable from the “tied” squares. Your application should move to the tied square for which the next move would arrive at a square with the lowest accessibility number. 7.23 (Knight’s Tour: Brute-Force Approaches) In part (c) of Exercise 7.22, we developed a solution to the Knight’s Tour problem. The approach used, called the “accessibility heuristic,” generates many solutions and executes efficiently. As computers continue to increase in power, we’ll be able to solve more problems with sheer computer power and relatively unsophisticated algorithms. Let’s call this approach “brute-force” problem solving. a) Use random-number generation to enable the knight to walk around the chessboard (in its legitimate L-shaped moves) at random. Your application should run one tour and display the final chessboard. How far did the knight get? Exercises 305 b) Most likely, the application in part (a) produced a relatively short tour. Now modify your application to attempt 1,000 tours. Use a one-dimensional array to keep track of the number of tours of each length. When your application finishes attempting the 1,000 tours, it should display this information in neat tabular format. What was the best result? c) Most likely, the application in part (b) gave you some “respectable” tours, but no full tours. Now let your application run until it produces a full tour. [Caution: This version of the application could run for hours on a powerful computer.] Once again, keep a table of the number of tours of each length, and display this table when the first full tour is found. How many tours did your application attempt before producing a full tour? How much time did it take? d) Compare the brute-force version of the Knight’s Tour with the accessibility-heuristic version. Which required a more careful study of the problem? Which algorithm was more difficult to develop? Which required more computer power? Could we be certain (in advance) of obtaining a full tour with the accessibility-heuristic approach? Could we be certain (in advance) of obtaining a full tour with the brute-force approach? Argue the pros and cons of brute-force problem solving in general. 7.24 (Eight Queens) Another puzzler for chess buffs is the Eight Queens problem, which asks the following: Is it possible to place eight queens on an empty chessboard so that no queen is “attacking” any other (i.e., no two queens are in the same row, in the same column or along the same diagonal)? Use the thinking developed in Exercise 7.22 to formulate a heuristic for solving the Eight Queens problem. Run your application. [Hint: It’s possible to assign a value to each square of the chessboard to indicate how many squares of an empty chessboard are “eliminated” if a queen is placed in that square. Each of the corners would be assigned the value 22, as demonstrated by Fig. 7.31. Once these “elimination numbers” are placed in all 64 squares, an appropriate heuristic might be as follows: Place the next queen in the square with the smallest elimination number. Why is this strategy intuitively appealing?] * * * * * * * * * * * * * * * * * * * * * * m Fig. 7.31 | The 22 squares eliminated by placing a queen in the upper left corner. 7.25 (Eight Queens: Brute-Force Approaches) In this exercise, you’ll develop several brute-force approaches to solving the Eight Queens problem introduced in Exercise 7.24. a) Use the random brute-force technique developed in Exercise 7.23 to solve the Eight Queens problem. b) Use an exhaustive technique (i.e., try all possible combinations of eight queens on the chessboard) to solve the Eight Queens problem. c) Why might the exhaustive brute-force approach not be appropriate for solving the Knight’s Tour problem? d) Compare and contrast the random brute-force and exhaustive brute-force approaches. 306 Chapter 7 Arrays and ArrayLists 7.26 (Knight’s Tour: Closed-Tour Test) In the Knight’s Tour (Exercise 7.22), a full tour occurs when the knight makes 64 moves, touching each square of the chessboard once and only once. A closed tour occurs when the 64th move is one move away from the square in which the knight started the tour. Modify the application you wrote in Exercise 7.22 to test for a closed tour if a full tour has occurred. 7.27 (Sieve of Eratosthenes) A prime number is any integer greater than 1 that’s evenly divisible only by itself and 1. The Sieve of Eratosthenes is a method of finding prime numbers. It operates as follows: a) Create a primitive-type boolean array with all elements initialized to true. Array elements with prime indices will remain true. All other array elements will eventually be set to false. b) Starting with array index 2, determine whether a given element is true. If so, loop through the remainder of the array and set to false every element whose index is a multiple of the index for the element with value true. Then continue the process with the next element with value true. For array index 2, all elements beyond element 2 in the array that have indices which are multiples of 2 (indices 4, 6, 8, 10, etc.) will be set to false; for array index 3, all elements beyond element 3 in the array that have indices which are multiples of 3 (indices 6, 9, 12, 15, etc.) will be set to false; and so on. When this process completes, the array elements that are still true indicate that the index is a prime number. These indices can be displayed. Write an application that uses an array of 1,000 elements to determine and display the prime numbers between 2 and 999. Ignore array elements 0 and 1. 7.28 (Simulation: The Tortoise and the Hare) In this problem, you’ll re-create the classic race of the tortoise and the hare. You’ll use random-number generation to develop a simulation of this memorable event. Our contenders begin the race at square 1 of 70 squares. Each square represents a possible position along the race course. The finish line is at square 70. The first contender to reach or pass square 70 is rewarded with a pail of fresh carrots and lettuce. The course weaves its way up the side of a slippery mountain, so occasionally the contenders lose ground. A clock ticks once per second. With each tick of the clock, your application should adjust the position of the animals according to the rules in Fig. 7.32. Use variables to keep track of the positions of the animals (i.e., position numbers are 1–70). Start each animal at position 1 (the “starting gate”). If an animal slips left before square 1, move it back to square 1. Animal Move type Percentage of the time Actual move Tortoise Fast plod Slip Slow plod Sleep Big hop Big slip Small hop Small slip 50% 20% 30% 20% 20% 10% 30% 20% 3 squares to the right 6 squares to the left 1 square to the right No move at all 9 squares to the right 12 squares to the left 1 square to the right 2 squares to the left Hare Fig. 7.32 | Rules for adjusting the positions of the tortoise and the hare. Exercises 307 Generate the percentages in Fig. 7.32 by producing a random integer i in the range 1 ≤ i ≤ 10. For the tortoise, perform a “fast plod” when 1 ≤ i ≤ 5, a “slip” when 6 ≤ i ≤ 7 or a “slow plod” when 8 ≤ i ≤ 10. Use a similar technique to move the hare. Begin the race by displaying BANG !!!!! AND THEY'RE OFF !!!!! Then, for each tick of the clock (i.e., each repetition of a loop), display a 70-position line showing the letter T in the position of the tortoise and the letter H in the position of the hare. Occasionally, the contenders will land on the same square. In this case, the tortoise bites the hare, and your application should display OUCH!!! beginning at that position. All output positions other than the T, the H or the OUCH!!! (in case of a tie) should be blank. After each line is displayed, test for whether either animal has reached or passed square 70. If so, display the winner and terminate the simulation. If the tortoise wins, display TORTOISE WINS!!! YAY!!! If the hare wins, display Hare wins. Yuch. If both animals win on the same tick of the clock, you may want to favor the tortoise (the “underdog”), or you may want to display It's a tie. If neither animal wins, perform the loop again to simulate the next tick of the clock. When you’re ready to run your application, assemble a group of fans to watch the race. You’ll be amazed at how involved your audience gets! Later in the book, we introduce a number of Java capabilities, such as graphics, images, animation, sound and multithreading. As you study those features, you might enjoy enhancing your tortoise-and-hare contest simulation. 7.29 (Fibonacci Series) The Fibonacci series 0, 1, 1, 2, 3, 5, 8, 13, 21, … begins with the terms 0 and 1 and has the property that each succeeding term is the sum of the two preceding terms. a) Write a method fibonacci(n) that calculates the nth Fibonacci number. Incorporate this method into an application that enables the user to enter the value of n. b) Determine the largest Fibonacci number that can be displayed on your system. c) Modify the application you wrote in part (a) to use double instead of int to calculate and return Fibonacci numbers, and use this modified application to repeat part (b). Exercises 7.30–7.34 are reasonably challenging. Once you’ve done them, you ought to be able to implement most popular card games easily. 7.30 (Card Shuffling and Dealing) Modify the application of Fig. 7.11 to deal a five-card poker hand. Then modify class DeckOfCards of Fig. 7.10 to include methods that determine whether a hand contains a) a pair b) two pairs c) three of a kind (e.g., three jacks) d) four of a kind (e.g., four aces) e) a flush (i.e., all five cards of the same suit) f) a straight (i.e., five cards of consecutive face values) g) a full house (i.e., two cards of one face value and three cards of another face value) [Hint: Add methods getFace and getSuit to class Card of Fig. 7.9.] 7.31 (Card Shuffling and Dealing) Use the methods developed in Exercise 7.30 to write an application that deals two five-card poker hands, evaluates each hand and determines which is better. 308 Chapter 7 Arrays and ArrayLists 7.32 (Project: Card Shuffling and Dealing) Modify the application developed in Exercise 7.31 so that it can simulate the dealer. The dealer’s five-card hand is dealt “face down,” so the player cannot see it. The application should then evaluate the dealer’s hand, and, based on the quality of the hand, the dealer should draw one, two or three more cards to replace the corresponding number of unneeded cards in the original hand. The application should then reevaluate the dealer’s hand. [Caution: This is a difficult problem!] 7.33 (Project: Card Shuffling and Dealing) Modify the application developed in Exercise 7.32 so that it can handle the dealer’s hand automatically, but the player is allowed to decide which cards of the player’s hand to replace. The application should then evaluate both hands and determine who wins. Now use this new application to play 20 games against the computer. Who wins more games, you or the computer? Have a friend play 20 games against the computer. Who wins more games? Based on the results of these games, refine your poker-playing application. (This, too, is a difficult problem.) Play 20 more games. Does your modified application play a better game? 7.34 (Project: Card Shuffling and Dealing) Modify the application of Figs. 7.9–7.11 to use Face and Suit enum types to represent the faces and suits of the cards. Declare each of these enum types as a public type in its own source-code file. Each Card should have a Face and a Suit instance variable. These should be initialized by the Card constructor. In class DeckOfCards, create an array of Faces that’s initialized with the names of the constants in the Face enum type and an array of Suits that’s initialized with the names of the constants in the Suit enum type. [Note: When you output an enum constant as a String, the name of the constant is displayed.] 7.35 (Fisher-Yates Shuffling Algorithm) Research the Fisher-Yates shuffling algorithm online, then use it to reimplement the shuffle method in Fig. 7.10. Special Section: Building Your Own Computer In the next several problems, we take a temporary diversion from the world of high-level language programming to “peel open” a computer and look at its internal structure. We introduce machinelanguage programming and write several machine-language programs. To make this an especially valuable experience, we then build a computer (through the technique of software-based simulation) on which you can execute your machine-language programs. 7.36 (Machine-Language Programming) Let’s create a computer called the Simpletron. As its name implies, it’s a simple machine, but powerful. The Simpletron runs programs written in the only language it directly understands: Simpletron Machine Language (SML). The Simpletron contains an accumulator—a special register in which information is put before the Simpletron uses that information in calculations or examines it in various ways. All the information in the Simpletron is handled in terms of words. A word is a signed four-digit decimal number, such as +3364, -1293, +0007 and -0001. The Simpletron is equipped with a 100-word memory, and these words are referenced by their location numbers 00, 01, …, 99. Before running an SML program, we must load, or place, the program into memory. The first instruction (or statement) of every SML program is always placed in location 00. The simulator will start executing at this location. Each instruction written in SML occupies one word of the Simpletron’s memory (so instructions are signed four-digit decimal numbers). We shall assume that the sign of an SML instruction is always plus, but the sign of a data word may be either plus or minus. Each location in the Simpletron’s memory may contain an instruction, a data value used by a program or an unused (and so undefined) area of memory. The first two digits of each SML instruction are the operation code specifying the operation to be performed. SML operation codes are summarized in Fig. 7.33. The last two digits of an SML instruction are the operand—the address of the memory location containing the word to which the operation applies. Let’s consider several simple SML programs. Special Section: Building Your Own Computer Operation code 309 Meaning Input/output operations: final int READ = 10; final int WRITE = 11; Read a word from the keyboard into a specific location in memory. Write a word from a specific location in memory to the screen. Load/store operations: final int LOAD = 20; final int STORE = 21; Load a word from a specific location in memory into the accumulator. Store a word from the accumulator into a specific location in memory. Arithmetic operations: final int ADD = 30; final int SUBTRACT = 31; final int DIVIDE = 32; final int MULTIPLY = 33; Add a word from a specific location in memory to the word in the accumulator (leave the result in the accumulator). Subtract a word from a specific location in memory from the word in the accumulator (leave the result in the accumulator). Divide a word from a specific location in memory into the word in the accumulator (leave result in the accumulator). Multiply a word from a specific location in memory by the word in the accumulator (leave the result in the accumulator). Transfer-of-control operations: final int BRANCH = 40; final int BRANCHNEG = 41; final int BRANCHZERO = 42; final int HALT = 43; Branch to a specific location in memory. Branch to a specific location in memory if the accumulator is negative. Branch to a specific location in memory if the accumulator is zero. Halt. The program has completed its task. Fig. 7.33 | Simpletron Machine Language (SML) operation codes. The first SML program (Fig. 7.34) reads two numbers from the keyboard and computes and displays their sum. The instruction +1007 reads the first number from the keyboard and places it into location 07 (which has been initialized to 0). Then instruction +1008 reads the next number into location 08. The load instruction, +2007, puts the first number into the accumulator, and the add instruction, +3008, adds the second number to the number in the accumulator. All SML arithmetic instructions leave their results in the accumulator. The store instruction, +2109, places the result back into memory location 09, from which the write instruction, +1109, takes the number and displays it (as a signed four-digit decimal number). The halt instruction, +4300, terminates execution. Location Number Instruction 00 01 02 03 04 +1007 (Read A) (Read B) (Load A) (Add B) (Store C) +1008 +2007 +3008 +2109 Fig. 7.34 | SML program that reads two integers and computes their sum. (Part 1 of 2.) 310 Chapter 7 Arrays and ArrayLists Location Number Instruction 05 06 07 08 09 +1109 (Write C) (Halt) (Variable A) (Variable B) (Result C) +4300 +0000 +0000 +0000 Fig. 7.34 | SML program that reads two integers and computes their sum. (Part 2 of 2.) The second SML program (Fig. 7.35) reads two numbers from the keyboard and determines and displays the larger value. Note the use of the instruction +4107 as a conditional transfer of control, much the same as Java’s if statement. Location Number Instruction 00 01 02 03 04 05 06 07 08 09 10 +1009 (Read A) (Read B) (Load A) (Subtract B) (Branch negative to 07) (Write A) (Halt) (Write B) (Halt) (Variable A) (Variable B) +1010 +2009 +3110 +4107 +1109 +4300 +1110 +4300 +0000 +0000 Fig. 7.35 | SML program that reads two integers and determines the larger. Now write SML programs to accomplish each of the following tasks: a) Use a sentinel-controlled loop to read 10 positive numbers. Compute and display their sum. b) Use a counter-controlled loop to read seven numbers, some positive and some negative, and compute and display their average. c) Read a series of numbers, and determine and display the largest number. The first number read indicates how many numbers should be processed. 7.37 (Computer Simulator) In this problem, you’re going to build your own computer. No, you’ll not be soldering components together. Rather, you’ll use the powerful technique of softwarebased simulation to create an object-oriented software model of the Simpletron of Exercise 7.36. Your Simpletron simulator will turn the computer you’re using into a Simpletron, and you’ll actually be able to run, test and debug the SML programs you wrote in Exercise 7.36. When you run your Simpletron simulator, it should begin by displaying: *** *** *** *** *** *** Welcome to Simpletron! *** Please enter your program one instruction (or data word) at a time. I will display the location number and a question mark (?). You then type the word for that location. Type -99999 to stop entering your program. *** *** *** *** *** Special Section: Building Your Own Computer 311 Your application should simulate the memory of the Simpletron with a one-dimensional array memory that has 100 elements. Now assume that the simulator is running, and let’s examine the dialog as we enter the program of Fig. 7.35 (Exercise 7.36): 00 01 02 03 04 05 06 07 08 09 10 11 ? ? ? ? ? ? ? ? ? ? ? ? +1009 +1010 +2009 +3110 +4107 +1109 +4300 +1110 +4300 +0000 +0000 -99999 Your program should display the memory location followed by a question mark. Each value to the right of a question mark is input by the user. When the sentinel value -99999 is input, the program should display the following: *** Program loading completed *** *** Program execution begins *** The SML program has now been placed (or loaded) in array memory. Now the Simpletron executes the SML program. Execution begins with the instruction in location 00 and, as in Java, continues sequentially, unless directed to some other part of the program by a transfer of control. Use the variable accumulator to represent the accumulator register. Use the variable instructionCounter to keep track of the location in memory that contains the instruction being performed. Use the variable operationCode to indicate the operation currently being performed (i.e., the left two digits of the instruction word). Use the variable operand to indicate the memory location on which the current instruction operates. Thus, operand is the rightmost two digits of the instruction currently being performed. Do not execute instructions directly from memory. Rather, transfer the next instruction to be performed from memory to a variable called instructionRegister. Then pick off the left two digits and place them in operationCode, and pick off the right two digits and place them in operand. When the Simpletron begins execution, the special registers are all initialized to zero. Now, let’s walk through execution of the first SML instruction, +1009 in memory location 00. This procedure is called an instruction-execution cycle. The instructionCounter tells us the location of the next instruction to be performed. We fetch the contents of that location from memory by using the Java statement instructionRegister = memory[instructionCounter]; The operation code and the operand are extracted from the instruction register by the statements operationCode = instructionRegister / 100; operand = instructionRegister % 100; Now the Simpletron must determine that the operation code is actually a read (versus a write, a load, and so on). A switch differentiates among the 12 operations of SML. In the switch statement, the behavior of various SML instructions is simulated as shown in Fig. 7.36. We discuss branch instructions shortly and leave the others to you. When the SML program completes execution, the name and contents of each register as well as the complete contents of memory should be displayed. Such a printout is often called a computer dump (no, a computer dump is not a place where old computers go). To help you program your dump method, a sample dump format is shown in Fig. 7.37. A dump after executing a Simpletron program would show the actual values of instructions and data values at the moment execution terminated. 312 Chapter 7 Arrays and ArrayLists Instruction Description read: Display the prompt "Enter an in location memory[operand]. load: add: halt: accumulator = memory[operand]; integer", then input the integer and store it accumulator += memory[operand]; This instruction displays the message *** Simpletron execution terminated *** Fig. 7.36 | Behavior of several SML instructions in the Simpletron. REGISTERS: accumulator instructionCounter instructionRegister operationCode operand +0000 00 +0000 00 00 MEMORY: 0 10 20 30 40 50 60 70 80 90 0 +0000 +0000 +0000 +0000 +0000 +0000 +0000 +0000 +0000 +0000 1 +0000 +0000 +0000 +0000 +0000 +0000 +0000 +0000 +0000 +0000 2 +0000 +0000 +0000 +0000 +0000 +0000 +0000 +0000 +0000 +0000 3 +0000 +0000 +0000 +0000 +0000 +0000 +0000 +0000 +0000 +0000 4 +0000 +0000 +0000 +0000 +0000 +0000 +0000 +0000 +0000 +0000 5 +0000 +0000 +0000 +0000 +0000 +0000 +0000 +0000 +0000 +0000 6 +0000 +0000 +0000 +0000 +0000 +0000 +0000 +0000 +0000 +0000 7 +0000 +0000 +0000 +0000 +0000 +0000 +0000 +0000 +0000 +0000 8 +0000 +0000 +0000 +0000 +0000 +0000 +0000 +0000 +0000 +0000 9 +0000 +0000 +0000 +0000 +0000 +0000 +0000 +0000 +0000 +0000 Fig. 7.37 | A sample dump. Let’s proceed with the execution of our program’s first instruction—namely, the +1009 in location 00. As we’ve indicated, the switch statement simulates this task by prompting the user to enter a value, reading the value and storing it in memory location memory[operand]. The value is then read into location 09. At this point, simulation of the first instruction is completed. All that remains is to prepare the Simpletron to execute the next instruction. Since the instruction just performed was not a transfer of control, we need merely increment the instruction-counter register as follows: instructionCounter++; This action completes the simulated execution of the first instruction. The entire process (i.e., the instruction-execution cycle) begins anew with the fetch of the next instruction to execute. Now let’s consider how the branching instructions—the transfers of control—are simulated. All we need to do is adjust the value in the instruction counter appropriately. Therefore, the unconditional branch instruction (40) is simulated within the switch as instructionCounter = operand; The conditional “branch if accumulator is zero” instruction is simulated as if (accumulator == 0) instructionCounter = operand; Special Section: Building Your Own Computer 313 At this point, you should implement your Simpletron simulator and run each of the SML programs you wrote in Exercise 7.36. If you desire, you may embellish SML with additional features and provide for these features in your simulator. Your simulator should check for various types of errors. During the program-loading phase, for example, each number the user types into the Simpletron’s memory must be in the range -9999 to +9999. Your simulator should test that each number entered is in this range and, if not, keep prompting the user to re-enter the number until the user enters a correct number. During the execution phase, your simulator should check for various serious errors, such as attempts to divide by zero, attempts to execute invalid operation codes, and accumulator overflows (i.e., arithmetic operations resulting in values larger than +9999 or smaller than -9999). Such serious errors are called fatal errors. When a fatal error is detected, your simulator should display an error message, such as *** Attempt to divide by zero *** *** Simpletron execution abnormally terminated *** and should display a full computer dump in the format we discussed previously. This treatment will help the user locate the error in the program. 7.38 (Simpletron Simulator Modifications) In Exercise 7.37, you wrote a software simulation of a computer that executes programs written in Simpletron Machine Language (SML). In this exercise, we propose several modifications and enhancements to the Simpletron Simulator. In the exercises of Chapter 21, we propose building a compiler that converts programs written in a high-level programming language (a variation of Basic) to Simpletron Machine Language. Some of the following modifications and enhancements may be required to execute the programs produced by the compiler: a) Extend the Simpletron Simulator’s memory to contain 1,000 memory locations to enable the Simpletron to handle larger programs. b) Allow the simulator to perform remainder calculations. This modification requires an additional SML instruction. c) Allow the simulator to perform exponentiation calculations. This modification requires an additional SML instruction. d) Modify the simulator to use hexadecimal rather than integer values to represent SML instructions. e) Modify the simulator to allow output of a newline. This modification requires an additional SML instruction. f) Modify the simulator to process floating-point values in addition to integer values. g) Modify the simulator to handle string input. [Hint: Each Simpletron word can be divided into two groups, each holding a two-digit integer. Each two-digit integer represents the ASCII (see Appendix B) decimal equivalent of a character. Add a machinelanguage instruction that will input a string and store the string, beginning at a specific Simpletron memory location. The first half of the word at that location will be a count of the number of characters in the string (i.e., the length of the string). Each succeeding half-word contains one ASCII character expressed as two decimal digits. The machinelanguage instruction converts each character into its ASCII equivalent and assigns it to a half-word.] h) Modify the simulator to handle output of strings stored in the format of part (g). [Hint: Add a machine-language instruction that will display a string, beginning at a certain Simpletron memory location. The first half of the word at that location is a count of the number of characters in the string (i.e., the length of the string). Each succeeding halfword contains one ASCII character expressed as two decimal digits. The machine-language instruction checks the length and displays the string by translating each two-digit number into its equivalent character.] 314 Chapter 7 Arrays and ArrayLists 7.39 (Enhanced GradeBook) Modify the GradeBook class of Fig. 7.18 so that the constructor accepts as parameters the number of students and the number of exams, then builds an appropriately sized two-dimensional array, rather than receiving a preinitialized two-dimensional array as it does now. Set each element of the new two-dimensional array to -1 to indicate that no grade has been entered for that element. Add a setGrade method that sets one grade for a particular student on a particular exam. Modify class GradeBookTest of Fig. 7.19 to input the number of students and number of exams for the GradeBook and to allow the instructor to enter one grade at a time. Making a Difference 7.40 (Polling) The Internet and the web are enabling more people to network, join a cause, voice opinions, and so on. Recent presidential candidates have used the Internet intensively to get out their messages and raise money for their campaigns. In this exercise, you’ll write a simple polling program that allows users to rate five social-consciousness issues from 1 (least important) to 10 (most important). Pick five causes that are important to you (e.g., political issues, global environmental issues). Use a one-dimensional array topics (of type String) to store the five causes. To summarize the survey responses, use a 5-row, 10-column two-dimensional array responses (of type int), each row corresponding to an element in the topics array. When the program runs, it should ask the user to rate each issue. Have your friends and family respond to the survey. Then have the program display a summary of the results, including: a) A tabular report with the five topics down the left side and the 10 ratings across the top, listing in each column the number of ratings received for each topic. b) To the right of each row, show the average of the ratings for that issue. c) Which issue received the highest point total? Display both the issue and the point total. d) Which issue received the lowest point total? Display both the issue and the point total. 8 Classes and Objects: A Deeper Look Is it a world to hide virtues in? —William Shakespeare But what, to serve our private ends, Forbids the cheating of our friends? —Charles Churchill This above all: to thine own self be true. —William Shakespeare. Objectives In this chapter you’ll: ■ Use the throw statement to indicate that a problem has occurred. ■ Use keyword this in a constructor to call another constructor in the same class. ■ Use static variables and methods. ■ Import static members of a class. ■ Use the enum type to create sets of constants with unique identifiers. ■ Declare enum constants with parameters. ■ Use BigDecimal for precise monetary calculations. 316 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9 8.10 Chapter 8 Classes and Objects: A Deeper Look Introduction Time Class Case Study Controlling Access to Members Referring to the Current Object’s Members with the this Reference Time Class Case Study: Overloaded Constructors Default and No-Argument Constructors Notes on Set and Get Methods Composition enum Types Garbage Collection 8.11 8.12 8.13 8.14 8.15 static Class Members static Import final Instance Variables Package Access Using BigDecimal for Precise Monetary Calculations 8.16 (Optional) GUI and Graphics Case Study: Using Objects with Graphics 8.17 Wrap-Up Summary | Self-Review Exercise | Answers to Self-Review Exercise | Exercises | Making a Difference 8.1 Introduction We now take a deeper look at building classes, controlling access to members of a class and creating constructors. We show how to throw an exception to indicate that a problem has occurred (Section 7.5 discussed catching exceptions). We use the this keyword to enable one constructor to conveniently call another constructor of the same class. We discuss composition—a capability that allows a class to have references to objects of other classes as members. We reexamine the use of set and get methods. Recall that Section 6.10 introduced the basic enum type to declare a set of constants. In this chapter, we discuss the relationship between enum types and classes, demonstrating that an enum type, like a class, can be declared in its own file with constructors, methods and fields. The chapter also discusses static class members and final instance variables in detail. We show a special relationship between classes in the same package. Finally, we demonstrate how to use class BigDecimal to perform precise monetary calculations. Two additional types of classes— nested classes and anonymous inner classes—are discussed in Chapter 12. 8.2 Time Class Case Study Our first example consists of two classes—Time1 (Fig. 8.1) and Time1Test (Fig. 8.2). Class Time1 represents the time of day. Class Time1Test’s main method creates one object of class Time1 and invokes its methods. The output of this program appears in Fig. 8.2. Class Declaration Class Time1’s private int instance variables hour, minute and second (lines 6–8) represent the time in universal-time format (24-hour clock format in which hours are in the range 0–23, and minutes and seconds are each in the range 0–59). Time1 contains public methods setTime (lines 12–25), toUniversalString (lines 28–31) and toString (lines 34–39). These methods are also called the public services or the public interface that the class provides to its clients. Time1 8.2 Time Class Case Study 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 317 // Fig. 8.1: Time1.java // Time1 class declaration maintains the time in 24-hour format. public class Time1 { private int hour; // 0 - 23 private int minute; // 0 - 59 private int second; // 0 - 59 // set a new time value using universal time; throw an // exception if the hour, minute or second is invalid public void setTime(int hour, int minute, int second) { // validate hour, minute and second if (hour < 0 || hour >= 24 || minute < 0 || minute >= 60 || second < 0 || second >= 60) { throw new IllegalArgumentException( "hour, minute and/or second was out of range"); } this.hour = hour; this.minute = minute; this.second = second; } // convert to String in universal-time format (HH:MM:SS) public String toUniversalString() { return String.format("%02d:%02d:%02d", hour, minute, second); } // convert to String in standard-time format (H:MM:SS AM or PM) public String toString() { return String.format("%d:%02d:%02d %s", ((hour == 0 || hour == 12) ? 12 : hour % 12), minute, second, (hour < 12 ? "AM" : "PM")); } } // end class Time1 Fig. 8.1 | Time1 class declaration maintains the time in 24-hour format. Default Constructor In this example, class Time1 does not declare a constructor, so the compiler supplies a default constructor. Each instance variable implicitly receives the default int value. Instance variables also can be initialized when they’re declared in the class body, using the same initialization syntax as with a local variable. Method setTime and Throwing Exceptions Method setTime (lines 12–25) is a public method that declares three int parameters and uses them to set the time. Lines 15–16 test each argument to determine whether the value is outside the proper range. The hour value must be greater than or equal to 0 and less than 318 Chapter 8 Classes and Objects: A Deeper Look 24, because universal-time format represents hours as integers from 0 to 23 (e.g., 1 PM is hour 13 and 11 PM is hour 23; midnight is hour 0 and noon is hour 12). Similarly, both minute and second values must be greater than or equal to 0 and less than 60. For values outside these ranges, setTime throws an exception of type IllegalArgumentException (lines 18–19), which notifies the client code that an invalid argument was passed to the method. As you learned in Section 7.5, you can use try...catch to catch exceptions and attempt to recover from them, which we’ll do in Fig. 8.2. The class instance creation expression in the throw statement (Fig. 8.1; line 18) creates a new object of type IllegalArgumentException. The parentheses following the class name indicate a call to the IllegalArgumentException constructor. In this case, we call the constructor that allows us to specify a custom error message. After the exception object is created, the throw statement immediately terminates method setTime and the exception is returned to the calling method that attempted to set the time. If the argument values are all valid, lines 22–24 assign them to the hour, minute and second instance variables. Software Engineering Observation 8.1 For a method like setTime in Fig. 8.1, validate all of the method’s arguments before using them to set instance variable values to ensure that the object’s data is modified only if all the arguments are valid. Method toUniversalString Method toUniversalString (lines 28–31) takes no arguments and returns a String in universal-time format, consisting of two digits each for the hour, minute and second—recall that you can use the 0 flag in a printf format specification (e.g., "%02d") to display leading zeros for a value that doesn’t use all the character positions in the specified field width. For example, if the time were 1:30:07 PM, the method would return 13:30:07. Line 30 uses static method format of class String to return a String containing the formatted hour, minute and second values, each with two digits and possibly a leading 0 (specified with the 0 flag). Method format is similar to method System.out.printf except that format returns a formatted String rather than displaying it in a command window. The formatted String is returned by method toUniversalString. Method toString Method toString (lines 34–39) takes no arguments and returns a String in standard-time format, consisting of the hour, minute and second values separated by colons and followed by AM or PM (e.g., 11:30:17 AM or 1:27:06 PM). Like method toUniversalString, method toString uses static String method format to format the minute and second as twodigit values, with leading zeros if necessary. Line 37 uses a conditional operator (?:) to determine the value for hour in the String—if the hour is 0 or 12 (AM or PM), it appears as 12; otherwise, it appears as a value from 1 to 11. The conditional operator in line 30 determines whether AM or PM will be returned as part of the String. Recall all objects in Java have a toString method that returns a String representation of the object. We chose to return a String containing the time in standard-time format. Method toString is called implicitly whenever a Time1 object appears in the code where a String is needed, such as the value to output with a %s format specifier in a call to System.out.printf. You may also call toString explicitly to obtain a String representation of a Time object. 8.2 Time Class Case Study 319 Using Class Time1 Class Time1Test (Fig. 8.2) uses class Time1. Line 9 declares and creates a Time1 object time. Operator new implicitly invokes class Time1’s default constructor, because Time1 does not declare any constructors. To confirm that the Time1 object was initialized properly, line 12 calls the private method displayTime (lines 35–39), which, in turn, calls the Time1 object’s toUniversalString and toString methods to output the time in universal-time format and standard-time format, respectively. Note that toString could have been called implicitly here rather than explicitly. Next, line 16 invokes method setTime of the time object to change the time. Then line 17 calls displayTime again to output the time in both formats to confirm that it was set correctly. Software Engineering Observation 8.2 Recall from Chapter 3 that methods declared with access modifier private can be called only by other methods of the class in which the private methods are declared. Such methods are commonly referred to as utility methods or helper methods because they’re typically used to support the operation of the class’s other methods. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 // Fig. 8.2: Time1Test.java // Time1 object used in an app. public class Time1Test { public static void main(String[] args) { // create and initialize a Time1 object Time1 time = new Time1(); // invokes Time1 constructor // output string representations of the time displayTime("After time object is created", time); System.out.println(); // change time and output updated time time.setTime(13, 27, 6); displayTime("After calling setTime", time); System.out.println(); // attempt to set time with invalid values try { time.setTime(99, 99, 99); // all values out of range } catch (IllegalArgumentException e) { System.out.printf("Exception: %s%n%n", e.getMessage()); } // display time after attempt to set invalid values displayTime("After calling setTime with invalid values", time); } Fig. 8.2 | Time1 object used in an app. (Part 1 of 2.) 320 33 34 35 36 37 38 39 40 Chapter 8 Classes and Objects: A Deeper Look // displays a Time1 object in 24-hour and 12-hour formats private static void displayTime(String header, Time1 t) { System.out.printf("%s%nUniversal time: %s%nStandard time: %s%n", header, t.toUniversalString(), t.toString()); } } // end class Time1Test After time object is created Universal time: 00:00:00 Standard time: 12:00:00 AM After calling setTime Universal time: 13:27:06 Standard time: 1:27:06 PM Exception: hour, minute and/or second was out of range After calling setTime with invalid values Universal time: 13:27:06 Standard time: 1:27:06 PM Fig. 8.2 | Time1 object used in an app. (Part 2 of 2.) Calling Time1 Method setTime with Invalid Values To illustrate that method setTime validates its arguments, line 23 calls method setTime with invalid arguments of 99 for the hour, minute and second. This statement is placed in a try block (lines 21–24) in case setTime throws an IllegalArgumentException, which it will do since the arguments are all invalid. When this occurs, the exception is caught at lines 25–28, and line 27 displays the exception’s error message by calling its getMessage method. Line 31 outputs the time again in both formats to confirm that setTime did not change the time when invalid arguments were supplied. Software Engineering of the Time1 Class Declaration Consider several issues of class design with respect to class Time1. The instance variables hour, minute and second are each declared private. The actual data representation used within the class is of no concern to the class’s clients. For example, it would be perfectly reasonable for Time1 to represent the time internally as the number of seconds since midnight or the number of minutes and seconds since midnight. Clients could use the same public methods and get the same results without being aware of this. (Exercise 8.5 asks you to represent the time in class Time2 of Fig. 8.5 as the number of seconds since midnight and show that indeed no change is visible to the clients of the class.) Software Engineering Observation 8.3 Classes simplify programming, because the client can use only a class’s public methods. Such methods are usually client oriented rather than implementation oriented. Clients are neither aware of, nor involved in, a class’s implementation. Clients generally care about what the class does but not how the class does it. 8.3 Controlling Access to Members 321 Software Engineering Observation 8.4 Interfaces change less frequently than implementations. When an implementation changes, implementation-dependent code must change accordingly. Hiding the implementation reduces the possibility that other program parts will become dependent on class implementation details. Java SE 8—Date/Time API This section’s example and several of this chapter’s later examples demonstrate various class-implementation concepts in classes that represent dates and times. In professional Java development, rather than building your own date and time classes, you’ll typically reuse the ones provided by the Java API. Though Java has always had classes for manipulating dates and times, Java SE 8 introduces a new Date/Time API—defined by the classes in the package java.time—applications built with Java SE 8 should use the Date/Time API’s capabilities, rather than those in earlier Java versions. The new API fixes various issues with the older classes and provides more robust, easier-to-use capabilities for manipulating dates, times, time zones, calendars and more. We use some Date/Time API features in Chapter 23. You can learn more about the Date/Time API’s classes at: download.java.net/jdk8/docs/api/java/time/package-summary.html 8.3 Controlling Access to Members The access modifiers public and private control access to a class’s variables and methods. In Chapter 9, we’ll introduce the additional access modifier protected. The primary purpose of public methods is to present to the class’s clients a view of the services the class provides (i.e., the class’s public interface). Clients need not be concerned with how the class accomplishes its tasks. For this reason, the class’s private variables and private methods (i.e., its implementation details) are not accessible to its clients. Figure 8.3 demonstrates that private class members are not accessible outside the class. Lines 9–11 attempt to access the private instance variables hour, minute and second of the Time1 object time. When this program is compiled, the compiler generates error messages that these private members are not accessible. This program assumes that the Time1 class from Fig. 8.1 is used. 1 2 3 4 5 6 7 8 9 10 11 12 13 // Fig. 8.3: MemberAccessTest.java // Private members of class Time1 are not accessible. public class MemberAccessTest { public static void main(String[] args) { Time1 time = new Time1(); // create and initialize Time1 object time.hour = 7; // error: hour has private access in Time1 time.minute = 15; // error: minute has private access in Time1 time.second = 30; // error: second has private access in Time1 } } // end class MemberAccessTest Fig. 8.3 | Private members of class Time1 are not accessible. (Part 1 of 2.) 322 Chapter 8 Classes and Objects: A Deeper Look MemberAccessTest.java:9: hour has private access in Time1 time.hour = 7; // error: hour has private access in Time1 ^ MemberAccessTest.java:10: minute has private access in Time1 time.minute = 15; // error: minute has private access in Time1 ^ MemberAccessTest.java:11: second has private access in Time1 time.second = 30; // error: second has private access in Time1 ^ 3 errors Fig. 8.3 | Private members of class Time1 are not accessible. (Part 2 of 2.) Common Programming Error 8.1 An attempt by a method that’s not a member of a class to access a private member of that class generates a compilation error. 8.4 Referring to the Current Object’s Members with the this Reference Every object can access a reference to itself with keyword this (sometimes called the this reference). When an instance method is called for a particular object, the method’s body implicitly uses keyword this to refer to the object’s instance variables and other methods. This enables the class’s code to know which object should be manipulated. As you’ll see in Fig. 8.4, you can also use keyword this explicitly in an instance method’s body. Section 8.5 shows another interesting use of keyword this. Section 8.11 explains why keyword this cannot be used in a static method. We now demonstrate implicit and explicit use of the this reference (Fig. 8.4). This example is the first in which we declare two classes in one file—class ThisTest is declared in lines 4–11, and class SimpleTime in lines 14–47. We do this to demonstrate that when you compile a .java file containing more than one class, the compiler produces a separate class file with the .class extension for every compiled class. In this case, two separate files are produced—SimpleTime.class and ThisTest.class. When one source-code (.java) file contains multiple class declarations, the compiler places both class files for those classes in the same directory. Note also in Fig. 8.4 that only class ThisTest is declared public. A source-code file can contain only one public class—otherwise, a compilation error occurs. Non-public classes can be used only by other classes in the same package. So, in this example, class SimpleTime can be used only by class ThisTest. 1 2 3 4 5 6 7 // Fig. 8.4: ThisTest.java // this used implicitly and explicitly to refer to members of an object. public class ThisTest { public static void main(String[] args) { Fig. 8.4 | this used implicitly and explicitly to refer to members of an object. (Part 1 of 2.) 8.4 Referring to the Current Object’s Members with the this Reference 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 323 SimpleTime time = new SimpleTime(15, 30, 19); System.out.println(time.buildString()); } } // end class ThisTest // class SimpleTime demonstrates the "this" reference class SimpleTime { private int hour; // 0-23 private int minute; // 0-59 private int second; // 0-59 // if the constructor uses parameter names identical to // instance variable names the "this" reference is // required to distinguish between the names public SimpleTime(int hour, int minute, int second) { this.hour = hour; // set "this" object's hour this.minute = minute; // set "this" object's minute this.second = second; // set "this" object's second } // use explicit and implicit "this" to call toUniversalString public String buildString() { return String.format("%24s: %s%n%24s: %s", "this.toUniversalString()", this.toUniversalString(), "toUniversalString()", toUniversalString()); } // convert to String in universal-time format (HH:MM:SS) public String toUniversalString() { // "this" is not required here to access instance variables, // because method does not have local variables with same // names as instance variables return String.format("%02d:%02d:%02d", this.hour, this.minute, this.second); } } // end class SimpleTime this.toUniversalString(): 15:30:19 toUniversalString(): 15:30:19 Fig. 8.4 | this used implicitly and explicitly to refer to members of an object. (Part 2 of 2.) Class SimpleTime (lines 14–47) declares three private instance variables—hour, and second (lines 16–18). The class’s constructor (lines 23–28) receives three int arguments to initialize a SimpleTime object. Once again, we used parameter names for the constructor (line 23) that are identical to the class’s instance-variable names (lines 16–18), so we use the this reference to refer to the instance variables in lines 25–27. minute 324 Chapter 8 Classes and Objects: A Deeper Look Error-Prevention Tip 8.1 Most IDEs will issue a warning if you say x = x; instead of this.x = x;. The statement x = x; is often called a no-op (no operation). Method buildString (lines 31–36) returns a String created by a statement that uses the this reference explicitly and implicitly. Line 34 uses it explicitly to call method toUniversalString. Line 35 uses it implicitly to call the same method. Both lines perform the same task. You typically will not use this explicitly to reference other methods within the current object. Also, line 45 in method toUniversalString explicitly uses the this reference to access each instance variable. This is not necessary here, because the method does not have any local variables that shadow the instance variables of the class. Performance Tip 8.1 Java conserves storage by maintaining only one copy of each method per class—this method is invoked by every object of the class. Each object, on the other hand, has its own copy of the class’s instance variables. Each method of the class implicitly uses this to determine the specific object of the class to manipulate. Class ThisTest’s main method (lines 6–10) demonstrates class SimpleTime. Line 8 creates an instance of class SimpleTime and invokes its constructor. Line 9 invokes the object’s buildString method, then displays the results. 8.5 Time Class Case Study: Overloaded Constructors As you know, you can declare your own constructor to specify how objects of a class should be initialized. Next, we demonstrate a class with several overloaded constructors that enable objects of that class to be initialized in different ways. To overload constructors, simply provide multiple constructor declarations with different signatures. Class Time2 with Overloaded Constructors The Time1 class’s default constructor in Fig. 8.1 initialized hour, minute and second to their default 0 values (i.e., midnight in universal time). The default constructor does not enable the class’s clients to initialize the time with nonzero values. Class Time2 (Fig. 8.5) contains five overloaded constructors that provide convenient ways to initialize objects. In this program, four of the constructors invoke a fifth, which in turn ensures that the value supplied for hour is in the range 0 to 23, and the values for minute and second are each in the range 0 to 59. The compiler invokes the appropriate constructor by matching the number, types and order of the types of the arguments specified in the constructor call with the number, types and order of the types of the parameters specified in each constructor declaration. Class Time2 also provides set and get methods for each instance variable. 1 2 3 4 5 // Fig. 8.5: Time2.java // Time2 class declaration with overloaded constructors. public class Time2 { Fig. 8.5 | Time2 class with overloaded constructors. (Part 1 of 4.) 8.5 Time Class Case Study: Overloaded Constructors 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 325 private int hour; // 0 - 23 private int minute; // 0 - 59 private int second; // 0 - 59 // Time2 no-argument constructor: // initializes each instance variable to zero public Time2() { this(0, 0, 0); // invoke constructor with three arguments } // Time2 constructor: hour supplied, minute and second defaulted to 0 public Time2(int hour) { this(hour, 0, 0); // invoke constructor with three arguments } // Time2 constructor: hour and minute supplied, second defaulted to 0 public Time2(int hour, int minute) { this(hour, minute, 0); // invoke constructor with three arguments } // Time2 constructor: hour, minute and second supplied public Time2(int hour, int minute, int second) { if (hour < 0 || hour >= 24) throw new IllegalArgumentException("hour must be 0-23"); if (minute < 0 || minute >= 60) throw new IllegalArgumentException("minute must be 0-59"); if (second < 0 || second >= 60) throw new IllegalArgumentException("second must be 0-59"); this.hour = hour; this.minute = minute; this.second = second; } // Time2 constructor: another Time2 object supplied public Time2(Time2 time) { // invoke constructor with three arguments this(time.getHour(), time.getMinute(), time.getSecond()); } // Set Methods // set a new time value using universal time; // validate the data public void setTime(int hour, int minute, int second) { Fig. 8.5 | Time2 class with overloaded constructors. (Part 2 of 4.) 326 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 Chapter 8 Classes and Objects: A Deeper Look if (hour < 0 || hour >= 24) throw new IllegalArgumentException("hour must be 0-23"); if (minute < 0 || minute >= 60) throw new IllegalArgumentException("minute must be 0-59"); if (second < 0 || second >= 60) throw new IllegalArgumentException("second must be 0-59"); this.hour = hour; this.minute = minute; this.second = second; } // validate public void { if (hour throw and set hour setHour(int hour) < 0 || hour >= 24) new IllegalArgumentException("hour must be 0-23"); this.hour = hour; } // validate and set minute public void setMinute(int minute) { if (minute < 0 && minute >= 60) throw new IllegalArgumentException("minute must be 0-59"); this.minute = minute; } // validate and set second public void setSecond(int second) { if (second >= 0 && second < 60) throw new IllegalArgumentException("second must be 0-59"); this.second = second; } // Get Methods // get hour value public int getHour() { return hour; } // get minute value public int getMinute() { return minute; } Fig. 8.5 | Time2 class with overloaded constructors. (Part 3 of 4.) 8.5 Time Class Case Study: Overloaded Constructors 327 111 112 // get second value 113 public int getSecond() 114 { 115 return second; 116 } 117 118 // convert to String in universal-time format (HH:MM:SS) 119 public String toUniversalString() 120 { 121 return String.format( 122 "%02d:%02d:%02d", getHour(), getMinute(), getSecond()); 123 } 124 125 // convert to String in standard-time format (H:MM:SS AM or PM) 126 public String toString() 127 { 128 return String.format("%d:%02d:%02d %s", 129 ((getHour() == 0 || getHour() == 12) ? 12 : getHour() % 12), 130 getMinute(), getSecond(), (getHour() < 12 ? "AM" : "PM")); 131 } 132 } // end class Time2 Fig. 8.5 | Time2 class with overloaded constructors. (Part 4 of 4.) Class Time2’s Constructors—Calling One Constructor from Another via this Lines 12–15 declare a so-called no-argument constructor that’s invoked without arguments. Once you declare any constructors in a class, the compiler will not provide a default constructor. This no-argument constructor ensures that class Time2’s clients can create Time2 objects with default values. Such a constructor simply initializes the object as specified in the constructor’s body. In the body, we introduce a use of this that’s allowed only as the first statement in a constructor’s body. Line 14 uses this in method-call syntax to invoke the Time2 constructor that takes three parameters (lines 30–44) with values of 0 for the hour, minute and second. Using this as shown here is a popular way to reuse initialization code provided by another of the class’s constructors rather than defining similar code in the no-argument constructor’s body. We use this syntax in four of the five Time2 constructors to make the class easier to maintain and modify. If we need to change how objects of class Time2 are initialized, only the constructor that the class’s other constructors call will need to be modified. Common Programming Error 8.2 It’s a compilation error when this is used in a constructor’s body to call another of the class’s constructors if that call is not the first statement in the constructor. It’s also a compilation error when a method attempts to invoke a constructor directly via this. Lines 18–21 declare a Time2 constructor with a single int parameter representing the which is passed with 0 for the minute and second to the constructor at lines 30–44. Lines 24–27 declare a Time2 constructor that receives two int parameters representing the hour and minute, which are passed with 0 for the second to the constructor at lines 30– 44. Like the no-argument constructor, each of these constructors invokes the constructor at lines 30–44 to minimize code duplication. Lines 30–44 declare the Time2 constructor hour, 328 Chapter 8 Classes and Objects: A Deeper Look that receives three int parameters representing the hour, minute and second. This constructor validates and initializes the instance variables. Lines 47–51 declare a Time2 constructor that receives a reference to another Time2 object. The values from the Time2 argument are passed to the three-argument constructor at lines 30–44 to initialize the hour, minute and second. Line 50 could have directly accessed the hour, minute and second values of the argument time with the expressions time.hour, time.minute and time.second—even though hour, minute and second are declared as private variables of class Time2. This is due to a special relationship between objects of the same class. We’ll see in a moment why it’s preferable to use the get methods. Software Engineering Observation 8.5 When one object of a class has a reference to another object of the same class, the first object can access all the second object’s data and methods (including those that are private). Class Time2’s setTime Method Method setTime (lines 56–70) throws an IllegalArgumentException (lines 59, 62 and 65) if any the method’s arguments is out of range. Otherwise, it sets Time2’s instance variables to the argument values (lines 67–69). Notes Regarding Class Time2’s set and get Methods and Constructors Time2’s get methods are called throughout the class. In particular, methods toUniversalString and toString call methods getHour, getMinute and getSecond in line 122 and lines 129–130, respectively. In each case, these methods could have accessed the class’s private data directly without calling the get methods. However, consider changing the representation of the time from three int values (requiring 12 bytes of memory) to a single int value representing the total number of seconds that have elapsed since midnight (requiring only four bytes of memory). If we made such a change, only the bodies of the methods that access the private data directly would need to change—in particular, the three-argument constructor, the setTime method and the individual set and get methods for the hour, minute and second. There would be no need to modify the bodies of methods toUniversalString or toString because they do not access the data directly. Designing the class in this manner reduces the likelihood of programming errors when altering the class’s implementation. Similarly, each Time2 constructor could include a copy of the appropriate statements from the three-argument constructor. Doing so may be slightly more efficient, because the extra constructor calls are eliminated. But, duplicating statements makes changing the class’s internal data representation more difficult. Having the Time2 constructors call the constructor with three arguments requires that any changes to the implementation of the threeargument constructor be made only once. Also, the compiler can optimize programs by removing calls to simple methods and replacing them with the expanded code of their declarations—a technique known as inlining the code, which improves program performance. Using Class Time2’s Overloaded Constructors Class Time2Test (Fig. 8.6) invokes the overloaded Time2 constructors (lines 8–12 and 24). Line 8 invokes the Time2 no-argument constructor. Lines 9–12 demonstrate passing arguments to the other Time2 constructors. Line 9 invokes the single-argument constructor that receives an int at lines 18–21 of Fig. 8.5. Line 10 invokes the two-argument constructor at lines 24–27 of Fig. 8.5. Line 11 invokes the three-argument constructor at lines 30–44 of 8.5 Time Class Case Study: Overloaded Constructors 329 Fig. 8.5. Line 12 invokes the single-argument constructor that takes a Time2 at lines 47–51 of Fig. 8.5. Next, the app displays the String representations of each Time2 object to confirm that it was initialized properly (lines 15–19). Line 24 attempts to initialize t6 by creating a new Time2 object and passing three invalid values to the constructor. When the constructor attempts to use the invalid hour value to initialize the object’s hour, an IllegalArgumentException occurs. We catch this exception at line 26 and display its error message, which results in the last line of the output. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 // Fig. 8.6: Time2Test.java // Overloaded constructors used to initialize Time2 objects. public class Time2Test { public static void main(String[] args) { Time2 t1 = new Time2(); // 00:00:00 Time2 t2 = new Time2(2); // 02:00:00 Time2 t3 = new Time2(21, 34); // 21:34:00 Time2 t4 = new Time2(12, 25, 42); // 12:25:42 Time2 t5 = new Time2(t4); // 12:25:42 System.out.println("Constructed with:"); displayTime("t1: all default arguments", t1); displayTime("t2: hour specified; default minute and second", t2); displayTime("t3: hour and minute specified; default second", t3); displayTime("t4: hour, minute and second specified", t4); displayTime("t5: Time2 object t4 specified", t5); // attempt to initialize t6 with invalid values try { Time2 t6 = new Time2(27, 74, 99); // invalid values } catch (IllegalArgumentException e) { System.out.printf("%nException while initializing t6: %s%n", e.getMessage()); } } // displays a Time2 object in 24-hour and 12-hour formats private static void displayTime(String header, Time2 t) { System.out.printf("%s%n %s%n %s%n", header, t.toUniversalString(), t.toString()); } } // end class Time2Test Constructed with: t1: all default arguments 00:00:00 12:00:00 AM Fig. 8.6 | Overloaded constructors used to initialize Time2 objects. (Part 1 of 2.) 330 Chapter 8 Classes and Objects: A Deeper Look t2: hour specified; default minute and second 02:00:00 2:00:00 AM t3: hour and minute specified; default second 21:34:00 9:34:00 PM t4: hour, minute and second specified 12:25:42 12:25:42 PM t5: Time2 object t4 specified 12:25:42 12:25:42 PM Exception while initializing t6: hour must be 0-23 Fig. 8.6 | Overloaded constructors used to initialize Time2 objects. (Part 2 of 2.) 8.6 Default and No-Argument Constructors Every class must have at least one constructor. If you do not provide any in a class’s declaration, the compiler creates a default constructor that takes no arguments when it’s invoked. The default constructor initializes the instance variables to the initial values specified in their declarations or to their default values (zero for primitive numeric types, false for boolean values and null for references). In Section 9.4.1, you’ll learn that the default constructor also performs another task. Recall that if your class declares constructors, the compiler will not create a default constructor. In this case, you must declare a no-argument constructor if default initialization is required. Like a default constructor, a no-argument constructor is invoked with empty parentheses. The Time2 no-argument constructor (lines 12–15 of Fig. 8.5) explicitly initializes a Time2 object by passing to the three-argument constructor 0 for each parameter. Since 0 is the default value for int instance variables, the no-argument constructor in this example could actually be declared with an empty body. In this case, each instance variable would receive its default value when the no-argument constructor is called. If we omit the no-argument constructor, clients of this class would not be able to create a Time2 object with the expression new Time2(). Error-Prevention Tip 8.2 Ensure that you do not include a return type in a constructor definition. Java allows other methods of the class besides its constructors to have the same name as the class and to specify return types. Such methods are not constructors and will not be called when an object of the class is instantiated. Common Programming Error 8.3 A compilation error occurs if a program attempts to initialize an object of a class by passing the wrong number or types of arguments to the class’s constructor. 8.7 Notes on Set and Get Methods As you know, a class’s private fields can be manipulated only by its methods. A typical manipulation might be the adjustment of a customer’s bank balance (e.g., a private in- 8.7 Notes on Set and Get Methods 331 stance variable of a class BankAccount) by a method computeInterest. Set methods are also commonly called mutator methods, because they typically change an object’s state— i.e., modify the values of instance variables. Get methods are also commonly called accessor methods or query methods. Set and Get Methods vs. public Data It would seem that providing set and get capabilities is essentially the same as making a class’s instance variables public. This is one of the subtleties that makes Java so desirable for software engineering. A public instance variable can be read or written by any method that has a reference to an object containing that variable. If an instance variable is declared private, a public get method certainly allows other methods to access it, but the get method can control how the client can access it. For example, a get method might control the format of the data it returns, shielding the client code from the actual data representation. A public set method can—and should—carefully scrutinize attempts to modify the variable’s value and throw an exception if necessary. For example, attempts to set the day of the month to 37 or a person’s weight to a negative value should be rejected. Thus, although set and get methods provide access to private data, the access is restricted by the implementation of the methods. This helps promote good software engineering. Software Engineering Observation 8.6 Classes should never have public nonconstant data, but declaring data public static final enables you to make constants available to clients of your class. For example, class Math offers public static final constants Math.E and Math.PI. Error-Prevention Tip 8.3 Do not provide public static final constants if the constants’ values are likely to change in future versions of your software. Validity Checking in Set Methods The benefits of data integrity do not follow automatically simply because instance variables are declared private—you must provide validity checking. A class’s set methods could determine that attempts were made to assign invalid data to objects of the class. Typically set methods have void return type and use exception handling to indicate attempts to assign invalid data. We discuss exception handling in detail in Chapter 11. Software Engineering Observation 8.7 When appropriate, provide public methods to change and retrieve the values of private instance variables. This architecture helps hide the implementation of a class from its clients, which improves program modifiability. Error-Prevention Tip 8.4 Using set and get methods helps you create classes that are easier to debug and maintain. If only one method performs a particular task, such as setting an instance variable in an object, it’s easier to debug and maintain the class. If the instance variable is not being set properly, the code that actually modifies instance variable is localized to one set method. Your debugging efforts can be focused on that one method. 332 Chapter 8 Classes and Objects: A Deeper Look Predicate Methods Another common use for accessor methods is to test whether a condition is true or false— such methods are often called predicate methods. An example would be class ArrayList’s isEmpty method, which returns true if the ArrayList is empty and false otherwise. A program might test isEmpty before attempting to read another item from an ArrayList. 8.8 Composition A class can have references to objects of other classes as members. This is called composition and is sometimes referred to as a has-a relationship. For example, an AlarmClock object needs to know the current time and the time when it’s supposed to sound its alarm, so it’s reasonable to include two references to Time objects in an AlarmClock object. A car has-a steering wheel, a break pedal and an accelerator pedal. Class Date This composition example contains classes Date (Fig. 8.7), Employee (Fig. 8.8) and EmployeeTest (Fig. 8.9). Class Date (Fig. 8.7) declares instance variables month, day and year (lines 6–8) to represent a date. The constructor receives three int parameters. Lines 17–19 validate the month—if it’s out-of-range, lines 18–19 throw an exception. Lines 22– 25 validate the day. If the day is incorrect based on the number of days in the particular month (except February 29th which requires special testing for leap years), lines 24–25 throw an exception. Lines 28–31 perform the leap year testing for February. If the month is February and the day is 29 and the year is not a leap year, lines 30–31 throw an exception. If no exceptions are thrown, then lines 33–35 initialize the Date’s instance variables and lines 38–38 output the this reference as a String. Since this is a reference to the current Date object, the object’s toString method (lines 42–45) is called implicitly to obtain the object’s String representation. In this example, we assume that the value for year is correct—an industrial-strength Date class should also validate the year. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 // Fig. 8.7: Date.java // Date class declaration. public class Date { private int month; // 1-12 private int day; // 1-31 based on month private int year; // any year private static final int[] daysPerMonth = { 0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 }; // constructor: confirm proper value for month and day given the year public Date(int month, int day, int year) { // check if month in range if (month <= 0 || month > 12) throw new IllegalArgumentException( "month (" + month + ") must be 1-12"); Fig. 8.7 | Date class declaration. (Part 1 of 2.) 8.8 Composition 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 333 // check if day in range for month if (day <= 0 || (day > daysPerMonth[month] && !(month == 2 && day == 29))) throw new IllegalArgumentException("day (" + day + ") out-of-range for the specified month and year"); // check for leap year if month is 2 and day is 29 if (month == 2 && day == 29 && !(year % 400 == 0 || (year % 4 == 0 && year % 100 != 0))) throw new IllegalArgumentException("day (" + day + ") out-of-range for the specified month and year"); this.month = month; this.day = day; this.year = year; System.out.printf( "Date object constructor for date %s%n", this); } // return a String of the form month/day/year public String toString() { return String.format("%d/%d/%d", month, day, year); } } // end class Date Fig. 8.7 | Date class declaration. (Part 2 of 2.) Class Employee Class Employee (Fig. 8.8) has instance variables firstName, lastName, birthDate and hireDate. Members firstName and lastName are references to String objects. Members birthDate and hireDate are references to Date objects. This demonstrates that a class can have as instance variables references to objects of other classes. The Employee constructor (lines 12–19) takes four parameters representing the first name, last name, birth date and hire date. The objects referenced by the parameters are assigned to the Employee object’s instance variables. When class Employee’s toString method is called, it returns a String containing the employee’s name and the String representations of the two Date objects. Each of these Strings is obtained with an implicit call to the Date class’s toString method. 1 2 3 4 5 6 7 8 9 // Fig. 8.8: Employee.java // Employee class with references to other objects. public class Employee { private String firstName; private String lastName; private Date birthDate; private Date hireDate; Fig. 8.8 | Employee class with references to other objects. (Part 1 of 2.) 334 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 Chapter 8 Classes and Objects: A Deeper Look // constructor to initialize name, birth date and hire date public Employee(String firstName, String lastName, Date birthDate, Date hireDate) { this.firstName = firstName; this.lastName = lastName; this.birthDate = birthDate; this.hireDate = hireDate; } // convert Employee to String format public String toString() { return String.format("%s, %s Hired: %s Birthday: %s", lastName, firstName, hireDate, birthDate); } } // end class Employee Fig. 8.8 | Employee class with references to other objects. (Part 2 of 2.) Class EmployeeTest Class EmployeeTest (Fig. 8.9) creates two Date objects to represent an Employee’s birthday and hire date, respectively. Line 10 creates an Employee and initializes its instance variables by passing to the constructor two Strings (representing the Employee’s first and last names) and two Date objects (representing the birthday and hire date). Line 12 implicitly invokes the Employee’s toString method to display the values of its instance variables and demonstrate that the object was initialized properly. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 // Fig. 8.9: EmployeeTest.java // Composition demonstration. public class EmployeeTest { public static void main(String[] args) { Date birth = new Date(7, 24, 1949); Date hire = new Date(3, 12, 1988); Employee employee = new Employee("Bob", "Blue", birth, hire); System.out.println(employee); } } // end class EmployeeTest Date object constructor for date 7/24/1949 Date object constructor for date 3/12/1988 Blue, Bob Hired: 3/12/1988 Birthday: 7/24/1949 Fig. 8.9 | Composition demonstration. 8.9 enum Types 335 8.9 enum Types In Fig. 6.8, we introduced the basic enum type, which defines a set of constants represented as unique identifiers. In that program the enum constants represented the game’s status. In this section we discuss the relationship between enum types and classes. Like classes, all enum types are reference types. An enum type is declared with an enum declaration, which is a comma-separated list of enum constants—the declaration may optionally include other components of traditional classes, such as constructors, fields and methods (as you’ll see momentarily). Each enum declaration declares an enum class with the following restrictions: 1. enum constants are implicitly final. 2. enum constants are implicitly static. 3. Any attempt to create an object of an compilation error. enum type with operator new results in a The enum constants can be used anywhere constants can be used, such as in the case labels of switch statements and to control enhanced for statements. Declaring Instance Variables, a Constructor and Methods in an enum Type Figure 8.10 demonstrates instance variables, a constructor and methods in an enum type. The enum declaration (lines 5–37) contains two parts—the enum constants and the other members of the enum type. The first part (lines 8–13) declares six constants. Each is optionally followed by arguments that are passed to the enum constructor (lines 20–24). Like the constructors you’ve seen in classes, an enum constructor can specify any number of parameters and can be overloaded. In this example, the enum constructor requires two String parameters. To properly initialize each enum constant, we follow it with parentheses containing two String arguments. The second part (lines 16–36) declares the other members of the enum type—two instance variables (lines 16–17), a constructor (lines 20–24) and two methods (lines 27–30 and 33–36). Lines 16–17 declare the instance variables title and copyrightYear. Each enum constant in enum type Book is actually an object of enum type Book that has its own copy of instance variables title and copyrightYear. The constructor (lines 20–24) takes two String parameters, one that specifies the book’s title and one that specifies its copyright year. Lines 22–23 assign these parameters to the instance variables. Lines 27–36 declare methods, which return the book title and copyright year, respectively. 1 2 3 4 5 6 7 8 9 10 // Fig. 8.10: Book.java // Declaring an enum type with a constructor and explicit instance fields // and accessors for these fields public enum Book { // declare constants of enum type JHTP("Java How to Program", "2015"), CHTP("C How to Program", "2013"), IW3HTP("Internet & World Wide Web How to Program", "2012"), Fig. 8.10 | Declaring an enum type with a constructor and explicit instance fields and accessors for these fields. (Part 1 of 2.) 336 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 Chapter 8 Classes and Objects: A Deeper Look CPPHTP("C++ How to Program", "2014"), VBHTP("Visual Basic How to Program", "2014"), CSHARPHTP("Visual C# How to Program", "2014"); // instance fields private final String title; // book title private final String copyrightYear; // copyright year // enum constructor Book(String title, String copyrightYear) { this.title = title; this.copyrightYear = copyrightYear; } // accessor for field title public String getTitle() { return title; } // accessor for field copyrightYear public String getCopyrightYear() { return copyrightYear; } } // end enum Book Fig. 8.10 | Declaring an enum type with a constructor and explicit instance fields and accessors for these fields. (Part 2 of 2.) Using enum type Book Figure 8.11 tests the enum type Book and illustrates how to iterate through a range of enum constants. For every enum, the compiler generates the static method values (called in line 12) that returns an array of the enum’s constants in the order they were declared. Lines 12–14 use the enhanced for statement to display all the constants declared in the enum Book. Line 14 invokes the enum Book’s getTitle and getCopyrightYear methods to get the title and copyright year associated with the constant. When an enum constant is converted to a String (e.g., book in line 13), the constant’s identifier is used as the String representation (e.g., JHTP for the first enum constant). 1 2 3 4 5 6 7 8 // Fig. 8.11: EnumTest.java // Testing enum type Book. import java.util.EnumSet; public class EnumTest { public static void main(String[] args) { Fig. 8.11 | Testing enum type Book. (Part 1 of 2.) 8.10 Garbage Collection 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 337 System.out.println("All books:"); // print all books in enum Book for (Book book : Book.values()) System.out.printf("%-10s%-45s%s%n", book, book.getTitle(), book.getCopyrightYear()); System.out.printf("%nDisplay a range of enum constants:%n"); // print first four books for (Book book : EnumSet.range(Book.JHTP, Book.CPPHTP)) System.out.printf("%-10s%-45s%s%n", book, book.getTitle(), book.getCopyrightYear()); } } // end class EnumTest All books: JHTP Java How to Program CHTP C How to Program IW3HTP Internet & World Wide Web How to Program CPPHTP C++ How to Program VBHTP Visual Basic How to Program CSHARPHTP Visual C# How to Program 2015 2013 2012 2014 2014 2014 Display a range of enum constants: JHTP Java How to Program CHTP C How to Program IW3HTP Internet & World Wide Web How to Program CPPHTP C++ How to Program 2015 2013 2012 2014 Fig. 8.11 | Testing enum type Book. (Part 2 of 2.) Lines 19–21 use the static method range of class EnumSet (declared in package to display a range of the enum Book’s constants. Method range takes two parameters—the first and the last enum constants in the range—and returns an EnumSet that contains all the constants between these two constants, inclusive. For example, the expression EnumSet.range(Book.JHTP, Book.CPPHTP) returns an EnumSet containing Book.JHTP, Book.CHTP, Book.IW3HTP and Book.CPPHTP. The enhanced for statement can be used with an EnumSet just as it can with an array, so lines 12–14 use it to display the title and copyright year of every book in the EnumSet. Class EnumSet provides several other static methods for creating sets of enum constants from the same enum type. java.util) Common Programming Error 8.4 In an enum declaration, it’s a syntax error to declare enum constants after the enum type’s constructors, fields and methods. 8.10 Garbage Collection Every object uses system resources, such as memory. We need a disciplined way to give resources back to the system when they’re no longer needed; otherwise, “resource leaks” might occur that would prevent resources from being reused by your program or possibly 338 Chapter 8 Classes and Objects: A Deeper Look by other programs. The JVM performs automatic garbage collection to reclaim the memory occupied by objects that are no longer used. When there are no more references to an object, the object is eligible to be collected. Collection typically occurs when the JVM executes its garbage collector, which may not happen for a while, or even at all before a program terminates. So, memory leaks that are common in other languages like C and C++ (because memory is not automatically reclaimed in those languages) are less likely in Java, but some can still happen in subtle ways. Resource leaks other than memory leaks can also occur. For example, an app may open a file on disk to modify its contents—if the app does not close the file, it must terminate before any other app can use the file. A Note about Class Object’s finalize Method Every class in Java has the methods of class Object (package java.lang), one of which is method finalize. (You’ll learn more about class Object in Chapter 9.) You should never use method finalize, because it can cause many problems and there’s uncertainty as to whether it will ever get called before a program terminates. The original intent of finalize was to allow the garbage collector to perform termination housekeeping on an object just before reclaiming the object’s memory. Now, it’s considered better practice for any class that uses system resources—such as files on disk— to provide a method that programmers can call to release resources when they’re no longer needed in a program. AutoClosable objects reduce the likelihood of resource leaks when you use them with the try-with-resources statement. As its name implies, an AutoClosable object is closed automatically, once a try-with-resources statement finishes using the object. We discuss this in more detail in Section 11.12. Software Engineering Observation 8.8 Many Java API classes (e.g., class Scanner and classes that read files from or write files to disk) provide close or dispose methods that programmers can call to release resources when they’re no longer needed in a program. 8.11 static Class Members Every object has its own copy of all the instance variables of the class. In certain cases, only one copy of a particular variable should be shared by all objects of a class. A static field— called a class variable—is used in such cases. A static variable represents classwide information—all objects of the class share the same piece of data. The declaration of a static variable begins with the keyword static. Motivating static Let’s motivate static data with an example. Suppose that we have a video game with Martians and other space creatures. Each Martian tends to be brave and willing to attack other space creatures when the Martian is aware that at least four other Martians are present. If fewer than five Martians are present, each of them becomes cowardly. Thus, each Martian needs to know the martianCount. We could endow class Martian with martianCount as an instance variable. If we do this, then every Martian will have a separate copy of the instance variable, and every time we create a new Martian, we’ll have to update the instance variable martianCount in every Martian object. This wastes space with the redundant copies, wastes time in updating the separate copies and is error prone. Instead, we declare mar- 8.11 static Class Members 339 tianCount to be static, making martianCount classwide data. Every Martian can see the as if it were an instance variable of class Martian, but only one copy of the is maintained. This saves space. We save time by having the Martian constructor increment the static martianCount—there’s only one copy, so we do not have to increment separate copies for each Martian object. martianCount static martianCount Software Engineering Observation 8.9 Use a static variable when all objects of a class must use the same copy of the variable. Class Scope Static variables have class scope—they can be used in all of the class’s methods. We can access a class’s public static members through a reference to any object of the class, or by qualifying the member name with the class name and a dot (.), as in Math.random(). A class’s private static class members can be accessed by client code only through methods of the class. Actually, static class members exist even when no objects of the class exist— they’re available as soon as the class is loaded into memory at execution time. To access a public static member when no objects of the class exist (and even when they do), prefix the class name and a dot (.) to the static member, as in Math.PI. To access a private static member when no objects of the class exist, provide a public static method and call it by qualifying its name with the class name and a dot. Software Engineering Observation 8.10 Static class variables and methods exist, and can be used, even if no objects of that class have been instantiated. Methods Cannot Directly Access Instance Variables and Instance Methods A static method cannot access a class’s instance variables and instance methods, because a static method can be called even when no objects of the class have been instantiated. For the same reason, the this reference cannot be used in a static method. The this reference must refer to a specific object of the class, and when a static method is called, there might not be any objects of its class in memory. static Common Programming Error 8.5 A compilation error occurs if a static method calls an instance method in the same class by using only the method name. Similarly, a compilation error occurs if a static method attempts to access an instance variable in the same class by using only the variable name. Common Programming Error 8.6 Referring to this in a static method is a compilation error. Tracking the Number of Employee Objects That Have Been Created Our next program declares two classes—Employee (Fig. 8.12) and EmployeeTest (Fig. 8.13). Class Employee declares private static variable count (Fig. 8.12, line 7) and public static method getCount (lines 36–39). The static variable count maintains a count of the number of objects of class Employee that have been created so far. This class 340 Chapter 8 Classes and Objects: A Deeper Look variable is initialized to zero in line 7. If a static variable is not initialized, the compiler assigns it a default value—in this case 0, the default value for type int. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 // Fig. 8.12: Employee.java // static variable used to maintain a count of the number of // Employee objects in memory. public class Employee { private static int count = 0; // number of Employees created private String firstName; private String lastName; // initialize Employee, add 1 to static count and // output String indicating that constructor was called public Employee(String firstName, String lastName) { this.firstName = firstName; this.lastName = lastName; ++count; // increment static count of employees System.out.printf("Employee constructor: %s %s; count = %d%n", firstName, lastName, count); } // get first name public String getFirstName() { return firstName; } // get last name public String getLastName() { return lastName; } // static method to get static count value public static int getCount() { return count; } } // end class Employee Fig. 8.12 | static variable used to maintain a count of the number of Employee objects in memory. When Employee objects exist, variable count can be used in any method of an Employee object—this example increments count in the constructor (line 18). The public static method getCount (lines 36–39) returns the number of Employee objects that have been created so far. When no objects of class Employee exist, client code can access variable count by calling method getCount via the class name, as in Employee.getCount(). When objects exist, method getCount can also be called via any reference to an Employee object. 8.11 static Class Members 341 Good Programming Practice 8.1 Invoke every static method by using the class name and a dot (.) to emphasize that the method being called is a static method. Class EmployeeTest EmployeeTest method main (Fig. 8.13) instantiates two Employee objects (lines 13–14). When each Employee object’s constructor is invoked, lines 15–16 of Fig. 8.12 assign the Employee’s first name and last name to instance variables firstName and lastName. These two statements do not make copies of the original String arguments. Actually, String objects in Java are immutable—they cannot be modified after they’re created. Therefore, it’s safe to have many references to one String object. This is not normally the case for objects of most other classes in Java. If String objects are immutable, you might wonder why we’re able to use operators + and += to concatenate String objects. String-concatenation actually results in a new String object containing the concatenated values. The original String objects are not modified. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 // Fig. 8.13: EmployeeTest.java // static member demonstration. public class EmployeeTest { public static void main(String[] args) { // show that count is 0 before creating Employees System.out.printf("Employees before instantiation: %d%n", Employee.getCount()); // create two Employees; count should be 2 Employee e1 = new Employee("Susan", "Baker"); Employee e2 = new Employee("Bob", "Blue"); // show that count is 2 after creating two Employees System.out.printf("%nEmployees after instantiation:%n"); System.out.printf("via e1.getCount(): %d%n", e1.getCount()); System.out.printf("via e2.getCount(): %d%n", e2.getCount()); System.out.printf("via Employee.getCount(): %d%n", Employee.getCount()); // get names of Employees System.out.printf("%nEmployee 1: %s %s%nEmployee 2: %s %s%n", e1.getFirstName(), e1.getLastName(), e2.getFirstName(), e2.getLastName()); } } // end class EmployeeTest Employees before instantiation: 0 Employee constructor: Susan Baker; count = 1 Employee constructor: Bob Blue; count = 2 Fig. 8.13 | static member demonstration. (Part 1 of 2.) 342 Chapter 8 Classes and Objects: A Deeper Look Employees after instantiation: via e1.getCount(): 2 via e2.getCount(): 2 via Employee.getCount(): 2 Employee 1: Susan Baker Employee 2: Bob Blue Fig. 8.13 | static member demonstration. (Part 2 of 2.) When main terminates, local variables e1 and e2 are discarded—remember that a local variable exists only until the block in which it’s declared completes execution. Because e1 and e2 were the only references to the Employee objects created in lines 13–14 (Fig. 8.13), these objects become “eligible for garbage collection” as main terminates. In a typical app, the garbage collector might eventually reclaim the memory for any objects that are eligible for collection. If any objects are not reclaimed before the program terminates, the operating system will reclaim the memory used by the program. The JVM does not guarantee when, or even whether, the garbage collector will execute. When it does, it’s possible that no objects or only a subset of the eligible objects will be collected. 8.12 static Import In Section 6.3, you learned about the static fields and methods of class Math. We access class Math’s static fields and methods by preceding each with the class name Math and a dot (.). A static import declaration enables you to import the static members of a class or interface so you can access them via their unqualified names in your class—that is, the class name and a dot (.) are not required when using an imported static member. Import Forms A static import declaration has two forms—one that imports a particular static member (which is known as single static import) and one that imports all static members of a class (known as static import on demand). The following syntax imports a particular static member: static import static packageName.ClassName.staticMemberName; where packageName is the package of the class (e.g., java.lang), ClassName is the name of the class (e.g., Math) and staticMemberName is the name of the static field or method (e.g., PI or abs). The following syntax imports all static members of a class: import static packageName.ClassName.*; The asterisk (*) indicates that all static members of the specified class should be available for use in the file. static import declarations import only static class members. Regular import statements should be used to specify the classes used in a program. Demonstrating static Import Figure 8.14 demonstrates a static import. Line 3 is a static import declaration, which imports all static fields and methods of class Math from package java.lang. Lines 9–12 access the Math class’s static fields E (line 11) and PI (line 12) and the static methods 8.13 final Instance Variables 343 sqrt (line 9) and ceil (line 10) without preceding the field names or method names with class name Math and a dot. Common Programming Error 8.7 A compilation error occurs if a program attempts to import two or more classes’ static methods that have the same signature or static fields that have the same name. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 // Fig. 8.14: StaticImportTest.java // Static import of Math class methods. import static java.lang.Math.*; public class StaticImportTest { public static void main(String[] args) { System.out.printf("sqrt(900.0) = %.1f%n", sqrt(900.0)); System.out.printf("ceil(-9.8) = %.1f%n", ceil(-9.8)); System.out.printf("E = %f%n", E); System.out.printf("PI = %f%n", PI); } } // end class StaticImportTest sqrt(900.0) = 30.0 ceil(-9.8) = -9.0 E = 2.718282 PI = 3.141593 Fig. 8.14 | static import of Math class methods. 8.13 final Instance Variables The principle of least privilege is fundamental to good software engineering. In the context of an app’s code, it states that code should be granted only the amount of privilege and access that it needs to accomplish its designated task, but no more. This makes your programs more robust by preventing code from accidentally (or maliciously) modifying variable values and calling methods that should not be accessible. Let’s see how this principle applies to instance variables. Some of them need to be modifiable and some do not. You can use the keyword final to specify that a variable is not modifiable (i.e., it’s a constant) and that any attempt to modify it is an error. For example, private final int INCREMENT; declares a final (constant) instance variable INCREMENT of type int. Such variables can be initialized when they’re declared. If they’re not, they must be initialized in every constructor of the class. Initializing constants in constructors enables each object of the class to have a different value for the constant. If a final variable is not initialized in its declaration or in every constructor, a compilation error occurs. 344 Chapter 8 Classes and Objects: A Deeper Look Software Engineering Observation 8.11 Declaring an instance variable as final helps enforce the principle of least privilege. If an instance variable should not be modified, declare it to be final to prevent modification. For example, in Fig. 8.8, the instance variables firstName, lastName, birthDate and hireDate are never modified after they’re initialized, so they should be declared final. We’ll enforce this practice in all programs going forward. You’ll see additional benefits of final in Chapter 23, Concurrency. Common Programming Error 8.8 Attempting to modify a final instance variable after it’s initialized is a compilation error. Error-Prevention Tip 8.5 Attempts to modify a final instance variable are caught at compilation time rather than causing execution-time errors. It’s always preferable to get bugs out at compilation time, if possible, rather than allow them to slip through to execution time (where experience has found that repair is often many times more expensive). Software Engineering Observation 8.12 A final field should also be declared static if it’s initialized in its declaration to a value that’s the same for all objects of the class. After this initialization, its value can never change. Therefore, we don’t need a separate copy of the field for every object of the class. Making the field static enables all objects of the class to share the final field. 8.14 Package Access If no access modifier (public, protected or private—we discuss protected in Chapter 9) is specified for a method or variable when it’s declared in a class, the method or variable is considered to have package access. In a program that consists of one class declaration, this has no specific effect. However, if a program uses multiple classes from the same package (i.e., a group of related classes), these classes can access each other’s packageaccess members directly through references to objects of the appropriate classes, or in the case of static members through the class name. Package access is rarely used. Figure 8.15 demonstrates package access. The app contains two classes in one sourcecode file—the PackageDataTest class containing main (lines 5–21) and the PackageData class (lines 24–41). Classes in the same source file are part of the same package. Consequently, class PackageDataTest is allowed to modify the package-access data of PackageData objects. When you compile this program, the compiler produces two separate .class files—PackageDataTest.class and PackageData.class. The compiler places the two .class files in the same directory. You can also place class PackageData (lines 24–41) in a separate source-code file. In the PackageData class declaration, lines 26–27 declare the instance variables number and string with no access modifiers—therefore, these are package-access instance variables. Class PackageDataTest’s main method creates an instance of the PackageData class (line 9) to demonstrate the ability to modify the PackageData instance variables directly (as shown in lines 15–16). The results of the modification can be seen in the output window. 8.15 Using BigDecimal for Precise Monetary Calculations 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 345 // Fig. 8.15: PackageDataTest.java // Package-access members of a class are accessible by other classes // in the same package. public class PackageDataTest { public static void main(String[] args) { PackageData packageData = new PackageData(); // output String representation of packageData System.out.printf("After instantiation:%n%s%n", packageData); // change package access data in packageData object packageData.number = 77; packageData.string = "Goodbye"; // output String representation of packageData System.out.printf("%nAfter changing values:%n%s%n", packageData); } } // end class PackageDataTest // class with package access instance variables class PackageData { int number; // package-access instance variable String string; // package-access instance variable // constructor public PackageData() { number = 0; string = "Hello"; } // return PackageData object String representation public String toString() { return String.format("number: %d; string: %s", number, string); } } // end class PackageData After instantiation: number: 0; string: Hello After changing values: number: 77; string: Goodbye Fig. 8.15 | Package-access members of a class are accessible by other classes in the same package. 8.15 Using BigDecimal for Precise Monetary Calculations In earlier chapters, we demonstrated monetary calculations using values of type double. In Chapter 5, we discussed the fact that some double values are represented approximately. 346 Chapter 8 Classes and Objects: A Deeper Look Any application that requires precise floating-point calculations—such as those in financial applications—should instead use class BigDecimal (from package java.math). Interest Calculations Using BigDecimal Figure 8.16 reimplements the interest calculation example of Fig. 5.6 using objects of class BigDecimal to perform the calculations. We also introduce class NumberFormat (package java.text) for formatting numeric values as locale-specific Strings—for example, in the U.S. locale, the value 1234.56, would be formatted as "1,234.56", whereas in many European locales it would be formatted as "1.234,56". 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 // Interest.java // Compound-interest calculations with BigDecimal. import java.math.BigDecimal; import java.text.NumberFormat; public class Interest { public static void main(String args[]) { // initial principal amount before interest BigDecimal principal = BigDecimal.valueOf(1000.0); BigDecimal rate = BigDecimal.valueOf(0.05); // interest rate // display headers System.out.printf("%s%20s%n", "Year", "Amount on deposit"); // calculate amount on deposit for each of ten years for (int year = 1; year <= 10; year++) { // calculate new amount for specified year BigDecimal amount = principal.multiply(rate.add(BigDecimal.ONE).pow(year)); // display the year and the amount System.out.printf("%4d%20s%n", year, NumberFormat.getCurrencyInstance().format(amount)); } } } // end class Interest Year 1 2 3 4 5 6 7 8 9 10 Amount on deposit $1,050.00 $1,102.50 $1,157.62 $1,215.51 $1,276.28 $1,340.10 $1,407.10 $1,477.46 $1,551.33 $1,628.89 Fig. 8.16 | Compound-interest calculations with BigDecimal. 8.15 Using BigDecimal for Precise Monetary Calculations 347 Creating BigDecimal Objects Lines 11–12 declare and initialize BigDecimal variables principal and rate using the BigDecimal static method valueOf that receives a double argument and returns a BigDecimal object that represents the exact value specified. Performing the Interest Calculations with BigDecimal Lines 21–22 perform the interest calculation using BigDecimal methods multiply, add and pow. The expression in line 22 evaluates as follows: 1. First, the expression rate.add(BigDecimal.ONE) adds 1 to the rate to produce a BigDecimal containing 1.05—this is equivalent to 1.0 + rate in line 19 of Fig. 5.6. The BigDecimal constant ONE represents the value 1. Class BigDecimal also provides the commonly used constants ZERO (0) and TEN (10). 2. Next, BigDecimal method pow is called on the preceding result to raise 1.05 to the power year—this is equivalent to passing 1.0 + rate and year to method Math.pow in line 19 of Fig. 5.6. 3. Finally, we call BigDecimal method multiply on the principal object, passing the preceding result as the argument. This returns a BigDecimal representing the amount on deposit at the end of the specified year. Since the expression rate.add(BigDecimal.ONE) produces the same value in each iteration of the loop, we could have simply initialized rate to 1.05 in line 12; however, we chose to mimic the precise calculations we used in line 19 of Fig. 5.6. Formatting Currency Values with NumberFormat During each iteration of the loop, line 26 NumberFormat.getCurrencyInstance().format(amount) evaluates as follows: 1. First, the expression uses NumberFormat’s static method getCurrencyInstance to get a NumberFormat that’s pre-configured to format numeric values as localespecific currency Strings—for example, in the U.S. locale, the numeric value 1628.89 is formatted as $1,628.89. Locale-specific formatting is an important part of internationalization—the process of customizing your applications for users’ various locales and spoken languages. 2. Next, the expression invokes method NumberFormat method format (on the object returned by getCurrencyInstance) to perform the formatting of the amount value. Method format then returns the locale-specific String representation, rounded to two-digits to the right of the decimal point. Rounding BigDecimal Values In addition to precise calculations, BigDecimal also gives you control over how values are rounded—by default all calculations are exact and no rounding occurs. If you do not specify how to round BigDecimal values and a given value cannot be represented exactly—such as the result of 1 divided by 3, which is 0.3333333…—an ArithmeticException occurs. Though we do not do so in this example, you can specify the rounding mode for BigDecimal by supplying a MathContext object (package java.math) to class BigDecimal’s 348 Chapter 8 Classes and Objects: A Deeper Look constructor when you create a BigDecimal. You may also provide a MathContext to various BigDecimal methods that perform calculations. Class MathContext contains several pre-configured MathContext objects that you can learn about at http://docs.oracle.com/javase/7/docs/api/java/math/MathContext.html By default, each pre-configured MathContext uses so called “bankers rounding” as explained for the RoundingMode constant HALF_EVEN at: http://docs.oracle.com/javase/7/docs/api/java/math/ RoundingMode.html#HALF_EVEN Scaling BigDecimal Values A BigDecimal’s scale is the number of digits to the right of its decimal point. If you need a BigDecimal rounded to a specific digit, you can call BigDecimal method setScale. For example, the following expression returns a BigDecimal with two digits to the right of the decimal point and using bankers rounding: amount.setScale(2, RoundingMode.HALF_EVEN) 8.16 (Optional) GUI and Graphics Case Study: Using Objects with Graphics Most of the graphics you’ve seen to this point did not vary with each program execution. Exercise 6.2 in Section 6.13 asked you to create a program that generated shapes and colors at random. In that exercise, the drawing changed every time the system called paintComponent. To create a more consistent drawing that remains the same each time it’s drawn, we must store information about the displayed shapes so that we can reproduce them each time the system calls paintComponent. To do this, we’ll create a set of shape classes that store information about each shape. We’ll make these classes “smart” by allowing objects of these classes to draw themselves by using a Graphics object. Class MyLine Figure 8.17 declares class MyLine, which has all these capabilities. Class MyLine imports classes Color and Graphics (lines 3–4). Lines 8–11 declare instance variables for the coordinates of the endpoints needed to draw a line, and line 12 declares the instance variable that stores the color of the line. The constructor at lines 15–22 takes five parameters, one for each instance variable that it initializes. Method draw at lines 25–29 requires a Graphics object and uses it to draw the line in the proper color and between the endpoints. 1 2 3 4 5 6 7 // Fig. 8.17: MyLine.java // MyLine class represents a line. import java.awt.Color; import java.awt.Graphics; public class MyLine { Fig. 8.17 | MyLine class represents a line. (Part 1 of 2.) 8.16 (Optional) GUI and Graphics Case Study: Using Objects with Graphics 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 private private private private private 349 int x1; // x-coordinate of first endpoint int y1; // y-coordinate of first endpoint int x2; // x-coordinate of second endpoint int y2; // y-coordinate of second endpoint Color color; // color of this line // constructor with input values public MyLine(int x1, int y1, int x2, int y2, Color color) { this.x1 = x1; this.y1 = y1; this.x2 = x2; this.y2 = y2; this.color = color; } // Draw the line in the specified color public void draw(Graphics g) { g.setColor(color); g.drawLine(x1, y1, x2, y2); } } // end class MyLine Fig. 8.17 | MyLine class represents a line. (Part 2 of 2.) Class DrawPanel In Fig. 8.18, we declare class DrawPanel, which will generate random objects of class MyLine. Line 12 declares the MyLine array lines to store the lines to draw. Inside the constructor (lines 15–37), line 17 sets the background color to Color.WHITE. Line 19 creates the array with a random length between 5 and 9. The loop at lines 22–36 creates a new MyLine for every element in the array. Lines 25–28 generate random coordinates for each line’s endpoints, and lines 31–32 generate a random color for the line. Line 35 creates a new MyLine object with the randomly generated values and stores it in the array. Method paintComponent iterates through the MyLine objects in array lines using an enhanced for statement (lines 45–46). Each iteration calls the draw method of the current MyLine object and passes it the Graphics object for drawing on the panel. 1 2 3 4 5 6 7 8 9 10 11 12 // Fig. 8.18: DrawPanel.java // Program that uses class MyLine // to draw random lines. import java.awt.Color; import java.awt.Graphics; import java.security.SecureRandom; import javax.swing.JPanel; public class DrawPanel extends JPanel { private SecureRandom randomNumbers = new SecureRandom(); private MyLine[] lines; // array of lines Fig. 8.18 | Program that uses class MyLine to draw random lines. (Part 1 of 2.) 350 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 Chapter 8 Classes and Objects: A Deeper Look // constructor, creates a panel with random shapes public DrawPanel() { setBackground(Color.WHITE); lines = new MyLine[5 + randomNumbers.nextInt(5)]; // create lines for (int count = 0; count < lines.length; count++) { // generate random coordinates int x1 = randomNumbers.nextInt(300); int y1 = randomNumbers.nextInt(300); int x2 = randomNumbers.nextInt(300); int y2 = randomNumbers.nextInt(300); // generate a random color Color color = new Color(randomNumbers.nextInt(256), randomNumbers.nextInt(256), randomNumbers.nextInt(256)); // add the line to the list of lines to be displayed lines[count] = new MyLine(x1, y1, x2, y2, color); } } // for each shape array, draw the individual shapes public void paintComponent(Graphics g) { super.paintComponent(g); // draw the lines for (MyLine line : lines) line.draw(g); } } // end class DrawPanel Fig. 8.18 | Program that uses class MyLine to draw random lines. (Part 2 of 2.) Class TestDraw Class TestDraw in Fig. 8.19 sets up a new window to display our drawing. Since we’re setting the coordinates for the lines only once in the constructor, the drawing does not change if paintComponent is called to refresh the drawing on the screen. 1 2 3 4 5 6 // Fig. 8.19: TestDraw.java // Creating a JFrame to display a DrawPanel. import javax.swing.JFrame; public class TestDraw { Fig. 8.19 | Creating a JFrame to display a DrawPanel. (Part 1 of 2.) 8.16 (Optional) GUI and Graphics Case Study: Using Objects with Graphics 7 8 9 10 11 12 13 14 15 16 17 351 public static void main(String[] args) { DrawPanel panel = new DrawPanel(); JFrame app = new JFrame(); app.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); app.add(panel); app.setSize(300, 300); app.setVisible(true); } } // end class TestDraw Fig. 8.19 | Creating a JFrame to display a DrawPanel. (Part 2 of 2.) GUI and Graphics Case Study Exercise 8.1 Extend the program in Figs. 8.17–8.19 to randomly draw rectangles and ovals. Create classes MyRectangle and MyOval. Both of these classes should include x1, y1, x2, y2 coordinates, a color and a boolean flag to determine whether the shape is filled. Declare a constructor in each class with arguments for initializing all the instance variables. To help draw rectangles and ovals, each class should provide methods getUpperLeftX, getUpperLeftY, getWidth and getHeight that calculate the upperleft x-coordinate, upper-left y-coordinate, width and height, respectively. The upper-left x-coordinate is the smaller of the two x-coordinate values, the upper-left y-coordinate is the smaller of the two ycoordinate values, the width is the absolute value of the difference between the two x-coordinate values, and the height is the absolute value of the difference between the two y-coordinate values. Class DrawPanel, which extends JPanel and handles the creation of the shapes, should declare three arrays, one for each shape type. The length of each array should be a random number between 1 and 5. The constructor of class DrawPanel will fill each array with shapes of random position, size, color and fill. In addition, modify all three shape classes to include the following: a) A constructor with no arguments that sets the shape’s coordinates to 0, the color of the shape to Color.BLACK, and the filled property to false (MyRectangle and MyOval only). b) Set methods for the instance variables in each class. The methods that set a coordinate value should verify that the argument is greater than or equal to zero before setting the coordinate—if it’s not, they should set the coordinate to zero. The constructor should call the set methods rather than initialize the local variables directly. c) Get methods for the instance variables in each class. Method draw should reference the coordinates by the get methods rather than access them directly. 352 Chapter 8 Classes and Objects: A Deeper Look 8.17 Wrap-Up In this chapter, we presented additional class concepts. The Time class case study showed a complete class declaration consisting of private data, overloaded public constructors for initialization flexibility, set and get methods for manipulating the class’s data, and methods that returned String representations of a Time object in two different formats. You also learned that every class can declare a toString method that returns a String representation of an object of the class and that method toString can be called implicitly whenever an object of a class appears in the code where a String is expected. We showed how to throw an exception to indicate that a problem has occurred. You learned that the this reference is used implicitly in a class’s instance methods to access the class’s instance variables and other instance methods. You also saw explicit uses of the this reference to access the class’s members (including shadowed fields) and how to use keyword this in a constructor to call another constructor of the class. We discussed the differences between default constructors provided by the compiler and no-argument constructors provided by the programmer. You learned that a class can have references to objects of other classes as members—a concept known as composition. You learned more about enum types and how they can be used to create a set of constants for use in a program. You learned about Java’s garbage-collection capability and how it (unpredictably) reclaims the memory of objects that are no longer used. The chapter explained the motivation for static fields in a class and demonstrated how to declare and use static fields and methods in your own classes. You also learned how to declare and initialize final variables. You learned that fields declared without an access modifier are given package access by default. You saw the relationship between classes in the same package that allows each class in a package to access the package-access members of other classes in the package. Finally, we demonstrated how to use class BigDecimal to perform precise monetary calculations. In the next chapter, you’ll learn about an important aspect of object-oriented programming in Java—inheritance. You’ll see that all classes in Java are related by inheritance, directly or indirectly, to the class called Object. You’ll also begin to understand how the relationships between classes enable you to build more powerful apps. Summary Section 8.2 Time Class Case Study • The public methods of a class are also known as the class’s public services or public interface (p. 316). They present to the class’s clients a view of the services the class provides. • A class’s private members are not accessible to its clients. • String class static method format (p. 318) is similar to method System.out.printf except that format returns a formatted String rather than displaying it in a command window. • All objects in Java have a toString method that returns a String representation of the object. Method toString is called implicitly when an object appears in code where a String is needed. Section 8.3 Controlling Access to Members • The access modifiers public and private control access to a class’s variables and methods. Summary 353 • The primary purpose of public methods is to present to the class’s clients a view of the services the class provides. Clients need not be concerned with how the class accomplishes its tasks. • A class’s private variables and private methods (i.e., its implementation details) are not accessible to its clients. Section 8.4 Referring to the Current Object’s Members with the this Reference • An instance method of an object implicitly uses keyword this (p. 322) to refer to the object’s instance variables and other methods. Keyword this can also be used explicitly. • The compiler produces a separate file with the .class extension for every compiled class. • If a local variable has the same name as a class’s field, the local variable shadows the field. You can use the this reference in a method to refer to the shadowed field explicitly. Section 8.5 Time Class Case Study: Overloaded Constructors • Overloaded constructors enable objects of a class to be initialized in different ways. The compiler differentiates overloaded constructors (p. 324) by their signatures. • To call one constructor of a class from another of the same class, you can use the this keyword followed by parentheses containing the constructor arguments. If used, such a constructor call must appear as the first statement in the constructor’s body. Section 8.6 Default and No-Argument Constructors • If no constructors are provided in a class, the compiler creates a default constructor. • If a class declares constructors, the compiler will not create a default constructor. In this case, you must declare a no-argument constructor (p. 327) if default initialization is required. Section 8.7 Notes on Set and Get Methods • Set methods are commonly called mutator methods (p. 331) because they typically change a value. Get methods are commonly called accessor methods (p. 331) or query methods. A predicate method (p. 332) tests whether a condition is true or false. Section 8.8 Composition • A class can have references to objects of other classes as members. This is called composition (p. 332) and is sometimes referred to as a has-a relationship. Section 8.9 enum Types • All enum types (p. 335) are reference types. An enum type is declared with an enum declaration, which is a comma-separated list of enum constants. The declaration may optionally include other components of traditional classes, such as constructors, fields and methods. • enum constants are implicitly final, because they declare constants that should not be modified. • enum constants are implicitly static. • Any attempt to create an object of an enum type with operator new results in a compilation error. • enum constants can be used anywhere constants can be used, such as in the case labels of switch statements and to control enhanced for statements. • Each enum constant in an enum declaration is optionally followed by arguments which are passed to the enum constructor. • For every enum, the compiler generates a static method called values (p. 336) that returns an array of the enum’s constants in the order in which they were declared. • EnumSet static method range (p. 337) receives the first and last enum constants in a range and returns an EnumSet that contains all the constants between these two constants, inclusive. 354 Chapter 8 Classes and Objects: A Deeper Look Section 8.10 Garbage Collection • The Java Virtual Machine (JVM) performs automatic garbage collection (p. 338) to reclaim the memory occupied by objects that are no longer in use. When there are no more references to an object, the object is eligible for garbage collection. The memory for such an object can be reclaimed when the JVM executes its garbage collector. Section 8.11 static Class Members • A static variable (p. 338) represents classwide information that’s shared among the class’s objects. • static variables have class scope. A class’s public static members can be accessed through a reference to any object of the class, or they can be accessed by qualifying the member name with the class name and a dot (.). Client code can access a class’s private static class members only through methods of the class. • static class members exist as soon as the class is loaded into memory. • A method declared static cannot access a class’s instance variables and instance methods, because a static method can be called even when no objects of the class have been instantiated. • The this reference cannot be used in a static method. Section 8.12 static Import • A static import declaration (p. 342) enables you to refer to imported static members without the class name and a dot (.). A single static import declaration imports one static member, and a static import on demand imports all static members of a class. Section 8.13 final Instance Variables • In the context of an app’s code, the principle of least privilege (p. 343) states that code should be granted only the amount of privilege and access that it needs to accomplish its designated task. • Keyword final specifies that a variable is not modifiable. Such variables must be initialized when they’re declared or by each of a class’s constructors. Section 8.14 Package Access • If no access modifier is specified for a method or variable when it’s declared in a class, the method or variable is considered to have package access (p. 344). Section 8.15 Using BigDecimal for Precise Monetary Calculations • Any application that requires precise floating-point calculations without rounding errors—such as those in financial applications—should instead use class BigDecimal (package java.math; p. 346). • BigDecimal static method valueOf (p. 347) with a double argument returns a BigDecimal that represents the exact value specified. • BigDecimal method add (p. 347) adds its argument BigDecimal to the BigDecimal on which the method is called and returns the result. • BigDecimal provides the constants ONE (1), ZERO (0) and TEN (10). • BigDecimal method pow (p. 347) raises its first argument to the power specified in its second argument. • BigDecimal method multiply (p. 347) multiplies its argument BigDecimal with the BigDecimal on which the method is called and returns the result. • Class NumberFormat (package java.text; p. 347) provides capabilities for formatting numeric values as locale-specific Strings. The class’s static method getCurrencyInstance returns a preconfigured NumberFormat for local-specific currency values. NumberFormat method format performs the formatting. Self-Review Exercise 355 • Locale-specific formatting is an important part of internationalization—the process of customizing your applications for users’ various locales and spoken languages. • BigDecimal gives you control over how values are rounded—by default all calculations are exact and no rounding occurs. If you do not specify how to round BigDecimal values and a given value cannot be represented exactly an ArithmeticException occurs. • You can specify the rounding mode for BigDecimal by supplying a MathContext object (package java.math) to class BigDecimal’s constructor when you create a BigDecimal. You may also provide a MathContext to various BigDecimal methods that perform calculations. By default, each pre-configured MathContext uses so called “bankers rounding.” • A BigDecimal’s scale is the number of digits to the right of its decimal point. If you need a BigDecimal rounded to a specific digit, you can call BigDecimal method setScale. Self-Review Exercise 8.1 Fill in the blanks in each of the following statements: a) A(n) imports all static members of a class. is similar to method System.out.printf, but reb) String class static method turns a formatted String rather than displaying a String in a command window. c) If a method contains a local variable with the same name as one of its class’s fields, the the field in that method’s scope. local variable d) The public methods of a class are also known as the class’s or . declaration specifies one class to import. e) A(n) f) If a class declares constructors, the compiler will not create a(n) . g) An object’s method is called implicitly when an object appears in code where a String is needed. h) Get methods are commonly called or . i) A(n) method tests whether a condition is true or false. that returns an j) For every enum, the compiler generates a static method called array of the enum’s constants in the order in which they were declared. k) Composition is sometimes referred to as a(n) relationship. declaration contains a comma-separated list of constants. l) A(n) m) A(n) variable represents classwide information that’s shared by all the objects of the class. declaration imports one static member. n) A(n) o) The states that code should be granted only the amount of privilege and access that it needs to accomplish its designated task. specifies that a variable is not modifiable after initialiation in a decp) Keyword laration or constructor. q) A(n) declaration imports only the classes that the program uses from a particular package. because they typically change a value. r) Set methods are commonly called s) Use class to perform precise monetary calculations. statement to indicate that a problem has occurred. t) Use the Answers to Self-Review Exercise 8.1 a) static import on demand. b) format. c) shadows. d) public services, public interface. e) single-type-import. f) default constructor. g) toString. h) accessor methods, query methods. i) predicate. j) values. k) has-a. l) enum. m) static. n) single static import. o) principle of least privilege. p) final. q) type-import-on-demand. r) mutator methods. s) BigDecimal. t) throw. 356 Chapter 8 Classes and Objects: A Deeper Look Exercises 8.2 (Based on Section 8.14) Explain the notion of package access in Java. Explain the negative aspects of package access. 8.3 What happens when a return type, even void, is specified for a constructor? 8.4 (Rectangle Class) Create a class Rectangle with attributes length and width, each of which defaults to 1. Provide methods that calculate the rectangle’s perimeter and area. It has set and get methods for both length and width. The set methods should verify that length and width are each floating-point numbers larger than 0.0 and less than 20.0. Write a program to test class Rectangle. 8.5 (Modifying the Internal Data Representation of a Class) It would be perfectly reasonable for the Time2 class of Fig. 8.5 to represent the time internally as the number of seconds since midnight rather than the three integer values hour, minute and second. Clients could use the same public methods and get the same results. Modify the Time2 class of Fig. 8.5 to implement the time as the number of seconds since midnight and show that no change is visible to the clients of the class. 8.6 (Savings Account Class) Create class SavingsAccount. Use a static variable annualInterestRate to store the annual interest rate for all account holders. Each object of the class contains a private instance variable savingsBalance indicating the amount the saver currently has on deposit. Provide method calculateMonthlyInterest to calculate the monthly interest by multiplying the savingsBalance by annualInterestRate divided by 12—this interest should be added to savingsBalance. Provide a static method modifyInterestRate that sets the annualInterestRate to a new value. Write a program to test class SavingsAccount. Instantiate two savingsAccount objects, saver1 and saver2, with balances of $2000.00 and $3000.00, respectively. Set annualInterestRate to 4%, then calculate the monthly interest for each of 12 months and print the new balances for both savers. Next, set the annualInterestRate to 5%, calculate the next month’s interest and print the new balances for both savers. 8.7 (Enhancing Class Time2) Modify class Time2 of Fig. 8.5 to include a tick method that increments the time stored in a Time2 object by one second. Provide method incrementMinute to increment the minute by one and method incrementHour to increment the hour by one. Write a program that tests the tick method, the incrementMinute method and the incrementHour method to ensure that they work correctly. Be sure to test the following cases: a) incrementing into the next minute, b) incrementing into the next hour and c) incrementing into the next day (i.e., 11:59:59 PM to 12:00:00 AM). 8.8 (Enhancing Class Date) Modify class Date of Fig. 8.7 to perform error checking on the initializer values for instance variables month, day and year (currently it validates only the month and day). Provide a method nextDay to increment the day by one. Write a program that tests method nextDay in a loop that prints the date during each iteration to illustrate that the method works correctly. Test the following cases: a) incrementing into the next month and b) incrementing into the next year. 8.9 Rewrite the code in Fig. 8.14 to use a separate import declaration for each static member of class Math that’s used in the example. 8.10 Write an enum type TrafficLight, whose constants (RED, GREEN, YELLOW) take one parameter—the duration of the light. Write a program to test the TrafficLight enum so that it displays the enum constants and their durations. 8.11 (Complex Numbers) Create a class called Complex for performing arithmetic with complex numbers. Complex numbers have the form realPart + imaginaryPart * i Exercises 357 where i is –1 Write a program to test your class. Use floating-point variables to represent the private data of the class. Provide a constructor that enables an object of this class to be initialized when it’s declared. Provide a no-argument constructor with default values in case no initializers are provided. Provide public methods that perform the following operations: a) Add two Complex numbers: The real parts are added together and the imaginary parts are added together. b) Subtract two Complex numbers: The real part of the right operand is subtracted from the real part of the left operand, and the imaginary part of the right operand is subtracted from the imaginary part of the left operand. c) Print Complex numbers in the form (realPart, imaginaryPart). 8.12 (Date and Time Class) Create class DateAndTime that combines the modified Time2 class of Exercise 8.7 and the modified Date class of Exercise 8.8. Modify method incrementHour to call method nextDay if the time is incremented into the next day. Modify methods toString and toUniversalString to output the date in addition to the time. Write a program to test the new class DateAndTime. Specifically, test incrementing the time to the next day. 8.13 (Set of Integers) Create class IntegerSet. Each IntegerSet object can hold integers in the range 0–100. The set is represented by an array of booleans. Array element a[i] is true if integer i is in the set. Array element a[j] is false if integer j is not in the set. The no-argument constructor initializes the array to the “empty set” (i.e., all false values). Provide the following methods: The static method union creates a set that’s the set-theoretic union of two existing sets (i.e., an element of the new set’s array is set to true if that element is true in either or both of the existing sets—otherwise, the new set’s element is set to false). The static method intersection creates a set which is the set-theoretic intersection of two existing sets (i.e., an element of the new set’s array is set to false if that element is false in either or both of the existing sets—otherwise, the new set’s element is set to true). Method insertElement inserts a new integer k into a set (by setting a[k] to true). Method deleteElement deletes integer m (by setting a[m] to false). Method toString returns a String containing a set as a list of numbers separated by spaces. Include only those elements that are present in the set. Use --- to represent an empty set. Method isEqualTo determines whether two sets are equal. Write a program to test class IntegerSet. Instantiate several IntegerSet objects. Test that all your methods work properly. 8.14 (Date Class) Create class Date with the following capabilities: a) Output the date in multiple formats, such as MM/DD/YYYY June 14, 1992 DDD YYYY b) Use overloaded constructors to create Date objects initialized with dates of the formats in part (a). In the first case the constructor should receive three integer values. In the second case it should receive a String and two integer values. In the third case it should receive two integer values, the first of which represents the day number in the year. [Hint: To convert the String representation of the month to a numeric value, compare Strings using the equals method. For example, if s1 and s2 are Strings, the method call s1.equals(s2) returns true if the Strings are identical and otherwise returns false.] 8.15 (Rational Numbers) Create a class called Rational for performing arithmetic with fractions. Write a program to test your class. Use integer variables to represent the private instance variables of the class—the numerator and the denominator. Provide a constructor that enables an object of 358 Chapter 8 Classes and Objects: A Deeper Look this class to be initialized when it’s declared. The constructor should store the fraction in reduced form. The fraction 2/4 is equivalent to 1/2 and would be stored in the object as 1 in the numerator and 2 in the denominator. Provide a no-argument constructor with default values in case no initializers are provided. Provide public methods that perform each of the following operations: a) Add two Rational numbers: The result of the addition should be stored in reduced form. Implement this as a static method. b) Subtract two Rational numbers: The result of the subtraction should be stored in reduced form. Implement this as a static method. c) Multiply two Rational numbers: The result of the multiplication should be stored in reduced form. Implement this as a static method. d) Divide two Rational numbers: The result of the division should be stored in reduced form. Implement this as a static method. e) Return a String representation of a Rational number in the form a/b, where a is the numerator and b is the denominator. f) Return a String representation of a Rational number in floating-point format. (Consider providing formatting capabilities that enable the user of the class to specify the number of digits of precision to the right of the decimal point.) 8.16 (Huge Integer Class) Create a class HugeInteger which uses a 40-element array of digits to store integers as large as 40 digits each. Provide methods parse, toString, add and subtract. Method parse should receive a String, extract each digit using method charAt and place the integer equivalent of each digit into the integer array. For comparing HugeInteger objects, provide the following methods: isEqualTo, isNotEqualTo, isGreaterThan, isLessThan, isGreaterThanOrEqualTo and isLessThanOrEqualTo. Each of these is a predicate method that returns true if the relationship holds between the two HugeInteger objects and returns false if the relationship does not hold. Provide a predicate method isZero. If you feel ambitious, also provide methods multiply, divide and remainder. [Note: Primitive boolean values can be output as the word “true” or the word “false” with format specifier %b.] 8.17 (Tic-Tac-Toe) Create a class TicTacToe that will enable you to write a program to play TicTac-Toe. The class contains a private 3-by-3 two-dimensional array. Use an enum type to represent the value in each cell of the array. The enum’s constants should be named X, O and EMPTY (for a position that does not contain an X or an O). The constructor should initialize the board elements to EMPTY. Allow two human players. Wherever the first player moves, place an X in the specified square, and place an O wherever the second player moves. Each move must be to an empty square. After each move, determine whether the game has been won and whether it’s a draw. If you feel ambitious, modify your program so that the computer makes the moves for one of the players. Also, allow the player to specify whether he or she wants to go first or second. If you feel exceptionally ambitious, develop a program that will play three-dimensional Tic-Tac-Toe on a 4-by-4-by-4 board [Note: This is an extremely challenging project!]. 8.18 (Account Class with BigDecimal balance) Rewrite the Account class of Section 3.5 to store the balance as a BigDecimal object and to perform all calculations using BigDecimals. Making a Difference 8.19 (Project: Emergency Response Class) The North American emergency response service, 9-1-1, connects callers to a local Public Service Answering Point (PSAP). Traditionally, the PSAP would ask the caller for identification information—including the caller’s address, phone number and the nature of the emergency, then dispatch the appropriate emergency responders (such as the police, an ambu- Making a Difference 359 lance or the fire department). Enhanced 9-1-1 (or E9-1-1) uses computers and databases to determine the caller’s physical address, directs the call to the nearest PSAP, and displays the caller’s phone number and address to the call taker. Wireless Enhanced 9-1-1 provides call takers with identification information for wireless calls. Rolled out in two phases, the first phase required carriers to provide the wireless phone number and the location of the cell site or base station transmitting the call. The second phase required carriers to provide the location of the caller (using technologies such as GPS). To learn more about 9-1-1, visit http://www.fcc.gov/pshs/services/911-services/Welcome.html and http://people.howstuffworks.com/9-1-1.htm. An important part of creating a class is determining the class’s attributes (instance variables). For this class design exercise, research 9-1-1 services on the Internet. Then, design a class called Emergency that might be used in an object-oriented 9-1-1 emergency response system. List the attributes that an object of this class might use to represent the emergency. For example, the class might include information on who reported the emergency (including their phone number), the location of the emergency, the time of the report, the nature of the emergency, the type of response and the status of the response. The class attributes should completely describe the nature of the problem and what’s happening to resolve that problem. 9 Say not you know another entirely, till you have divided an inheritance with him. —Johann Kasper Lavater This method is to define as the number of a class the class of all classes similar to the given class. —Bertrand Russell Objectives In this chapter you’ll: ■ ■ ■ ■ ■ ■ ■ Understand inheritance and how to use it to develop new classes based on existing classes. Learn the notions of superclasses and subclasses and the relationship between them. Use keyword extends to create a class that inherits attributes and behaviors from another class. Use access modifier protected in a superclass to give subclass methods access to these superclass members. Access superclass members with super from a subclass. Learn how constructors are used in inheritance hierarchies. Learn about the methods of class Object, the direct or indirect superclass of all classes. Object-Oriented Programming: Inheritance 9.1 Introduction 9.1 9.2 9.3 9.4 Introduction Superclasses and Subclasses protected Members Relationship Between Superclasses and Subclasses 9.4.1 Creating and Using a CommissionEmployee Class 9.4.2 Creating and Using a BasePlusCommissionEmployee Class 9.4.3 Creating a CommissionEmployee– 361 9.4.5 CommissionEmployee– BasePlusCommissionEmployee Inheritance Hierarchy Using private Instance Variables 9.5 Constructors in Subclasses 9.6 Class Object 9.7 (Optional) GUI and Graphics Case Study: Displaying Text and Images Using Labels 9.8 Wrap-Up BasePlusCommissionEmployee Inheritance Hierarchy 9.4.4 CommissionEmployee– BasePlusCommissionEmployee Inheritance Hierarchy Using protected Instance Variables Summary | Self-Review Exercises | Answers to Self-Review Exercises | Exercises 9.1 Introduction This chapter continues our discussion of object-oriented programming (OOP) by introducing inheritance, in which a new class is created by acquiring an existing class’s members and possibly embellishing them with new or modified capabilities. With inheritance, you can save time during program development by basing new classes on existing proven and debugged high-quality software. This also increases the likelihood that a system will be implemented and maintained effectively. When creating a class, rather than declaring completely new members, you can designate that the new class should inherit the members of an existing class. The existing class is called the superclass, and the new class is the subclass. (The C++ programming language refers to the superclass as the base class and the subclass as the derived class.) A subclass can become a superclass for future subclasses. A subclass can add its own fields and methods. Therefore, a subclass is more specific than its superclass and represents a more specialized group of objects. The subclass exhibits the behaviors of its superclass and can modify those behaviors so that they operate appropriately for the subclass. This is why inheritance is sometimes referred to as specialization. The direct superclass is the superclass from which the subclass explicitly inherits. An indirect superclass is any class above the direct superclass in the class hierarchy, which defines the inheritance relationships among classes—as you’ll see in Section 9.2, diagrams help you understand these relationships. In Java, the class hierarchy begins with class Object (in package java.lang), which every class in Java directly or indirectly extends (or “inherits from”). Section 9.6 lists the methods of class Object that are inherited by all other Java classes. Java supports only single inheritance, in which each class is derived from exactly one direct superclass. Unlike C++, Java does not support multiple inheritance (which occurs when a class is derived from more than one direct superclass). Chapter 10, Object-Oriented Programming: Polymorphism and Interfaces, explains how to use Java interfaces to realize many of the benefits of multiple inheritance while avoiding the associated problems. 362 Chapter 9 Object-Oriented Programming: Inheritance We distinguish between the is-a relationship and the has-a relationship. Is-a represents inheritance. In an is-a relationship, an object of a subclass can also be treated as an object of its superclass—e.g., a car is a vehicle. By contrast, has-a represents composition (see Chapter 8). In a has-a relationship, an object contains as members references to other objects— e.g., a car has a steering wheel (and a car object has a reference to a steering-wheel object). New classes can inherit from classes in class libraries. Organizations develop their own class libraries and can take advantage of others available worldwide. Some day, most new software likely will be constructed from standardized reusable components, just as automobiles and most computer hardware are constructed today. This will facilitate the rapid development of more powerful, abundant and economical software. 9.2 Superclasses and Subclasses Often, an object of one class is an object of another class as well. For example, a CarLoan is a Loan as are HomeImprovementLoans and MortgageLoans. Thus, in Java, class CarLoan can be said to inherit from class Loan. In this context, class Loan is a superclass and class CarLoan is a subclass. A CarLoan is a specific type of Loan, but it’s incorrect to claim that every Loan is a CarLoan—the Loan could be any type of loan. Figure 9.1 lists several simple examples of superclasses and subclasses—superclasses tend to be “more general” and subclasses “more specific.” Superclass Subclasses Student GraduateStudent, UndergraduateStudent Shape Circle, Triangle, Rectangle, Sphere, Cube Loan CarLoan, HomeImprovementLoan, MortgageLoan Employee Faculty, Staff BankAccount CheckingAccount, SavingsAccount Fig. 9.1 | Inheritance examples. Because every subclass object is an object of its superclass, and one superclass can have many subclasses, the set of objects represented by a superclass is often larger than the set of objects represented by any of its subclasses. For example, the superclass Vehicle represents all vehicles, including cars, trucks, boats, bicycles and so on. By contrast, subclass Car represents a smaller, more specific subset of vehicles. University Community Member Hierarchy Inheritance relationships form treelike hierarchical structures. A superclass exists in a hierarchical relationship with its subclasses. Let’s develop a sample class hierarchy (Fig. 9.2), also called an inheritance hierarchy. A university community has thousands of members, including employees, students and alumni. Employees are either faculty or staff members. Faculty members are either administrators (e.g., deans and department chairpersons) or teachers. The hierarchy could contain many other classes. For example, students can be graduate or undergraduate students. Undergraduate students can be freshmen, sophomores, juniors or seniors. 9.2 Superclasses and Subclasses 363 CommunityMember Employee Faculty Administrator Student Alumnus Staff Teacher Fig. 9.2 | Inheritance hierarchy UML class diagram for university CommunityMembers. Each arrow in the hierarchy represents an is-a relationship. As we follow the arrows upward in this class hierarchy, we can state, for example, that “an Employee is a CommunityMember” and “a Teacher is a Faculty member.” CommunityMember is the direct superclass of Employee, Student and Alumnus and is an indirect superclass of all the other classes in the diagram. Starting from the bottom, you can follow the arrows and apply the is-a relationship up to the topmost superclass. For example, an Administrator is a Faculty member, is an Employee, is a CommunityMember and, of course, is an Object. Shape Hierarchy Now consider the Shape inheritance hierarchy in Fig. 9.3. This hierarchy begins with superclass Shape, which is extended by subclasses TwoDimensionalShape and ThreeDimensionalShape—Shapes are either TwoDimensionalShapes or ThreeDimensionalShapes. The third level of this hierarchy contains specific types of TwoDimensionalShapes and ThreeDimensionalShapes. As in Fig. 9.2, we can follow the arrows from the bottom of the diagram to the topmost superclass in this class hierarchy to identify several is-a relationships. For example, a Triangle is a TwoDimensionalShape and is a Shape, while a Sphere is a ThreeDimensionalShape and is a Shape. This hierarchy could contain many other classes. For example, ellipses and trapezoids also are TwoDimensionalShapes. Shape TwoDimensionalShape Circle Square ThreeDimensionalShape Triangle Sphere Fig. 9.3 | Inheritance hierarchy UML class diagram for Shapes. Cube Tetrahedron 364 Chapter 9 Object-Oriented Programming: Inheritance Not every class relationship is an inheritance relationship. In Chapter 8, we discussed the has-a relationship, in which classes have members that are references to objects of other classes. Such relationships create classes by composition of existing classes. For example, given the classes Employee, BirthDate and TelephoneNumber, it’s improper to say that an Employee is a BirthDate or that an Employee is a TelephoneNumber. However, an Employee has a BirthDate, and an Employee has a TelephoneNumber. It’s possible to treat superclass objects and subclass objects similarly—their commonalities are expressed in the superclass’s members. Objects of all classes that extend a common superclass can be treated as objects of that superclass—such objects have an is-a relationship with the superclass. Later in this chapter and in Chapter 10, we consider many examples that take advantage of the is-a relationship. A subclass can customize methods that it inherits from its superclass. To do this, the subclass overrides (redefines) the superclass method with an appropriate implementation, as we’ll see in the chapter’s code examples. 9.3 protected Members Chapter 8 discussed access modifiers public and private. A class’s public members are accessible wherever the program has a reference to an object of that class or one of its subclasses. A class’s private members are accessible only within the class itself. In this section, we introduce the access modifier protected. Using protected access offers an intermediate level of access between public and private. A superclass’s protected members can be accessed by members of that superclass, by members of its subclasses and by members of other classes in the same package—protected members also have package access. All public and protected superclass members retain their original access modifier when they become members of the subclass—public members of the superclass become public members of the subclass, and protected members of the superclass become protected members of the subclass. A superclass’s private members are not accessible outside the class itself. Rather, they’re hidden from its subclasses and can be accessed only through the public or protected methods inherited from the superclass. Subclass methods can refer to public and protected members inherited from the superclass simply by using the member names. When a subclass method overrides an inherited superclass method, the superclass version of the method can be accessed from the subclass by preceding the superclass method name with keyword super and a dot (.) separator. We discuss accessing overridden members of the superclass in Section 9.4. Software Engineering Observation 9.1 Methods of a subclass cannot directly access private members of their superclass. A subclass can change the state of private superclass instance variables only through nonprivate methods provided in the superclass and inherited by the subclass. Software Engineering Observation 9.2 Declaring private instance variables helps you test, debug and correctly modify systems. If a subclass could access its superclass’s private instance variables, classes that inherit from that subclass could access the instance variables as well. This would propagate access to what should be private instance variables, and the benefits of information hiding would be lost. 9.4 Relationship Between Superclasses and Subclasses 365 9.4 Relationship Between Superclasses and Subclasses We now use an inheritance hierarchy containing types of employees in a company’s payroll application to discuss the relationship between a superclass and its subclass. In this company, commission employees (who will be represented as objects of a superclass) are paid a percentage of their sales, while base-salaried commission employees (who will be represented as objects of a subclass) receive a base salary plus a percentage of their sales. We divide our discussion of the relationship between these classes into five examples. The first declares class CommissionEmployee, which directly inherits from class Object and declares as private instance variables a first name, last name, social security number, commission rate and gross (i.e., total) sales amount. The second example declares class BasePlusCommissionEmployee, which also directly inherits from class Object and declares as private instance variables a first name, last name, social security number, commission rate, gross sales amount and base salary. We create this class by writing every line of code the class requires—we’ll soon see that it’s much more efficient to create it by inheriting from class CommissionEmployee. The third example declares a new BasePlusCommissionEmployee class that extends class CommissionEmployee (i.e., a BasePlusCommissionEmployee is a CommissionEmployee who also has a base salary). This software reuse lets us write much less code when developing the new subclass. In this example, class BasePlusCommissionEmployee attempts to access class CommissionEmployee’s private members—this results in compilation errors, because the subclass cannot access the superclass’s private instance variables. The fourth example shows that if CommissionEmployee’s instance variables are declared as protected, the BasePlusCommissionEmployee subclass can access that data directly. Both BasePlusCommissionEmployee classes contain identical functionality, but we show how the inherited version is easier to create and manage. After we discuss the convenience of using protected instance variables, we create the fifth example, which sets the CommissionEmployee instance variables back to private to enforce good software engineering. Then we show how the BasePlusCommissionEmployee subclass can use CommissionEmployee’s public methods to manipulate (in a controlled manner) the private instance variables inherited from CommissionEmployee. 9.4.1 Creating and Using a CommissionEmployee Class We begin by declaring class CommissionEmployee (Fig. 9.4). Line 4 begins the class declaration and indicates that class CommissionEmployee extends (i.e., inherits from) class Object (from package java.lang). This causes class CommissionEmployee to inherit the class Object’s methods—class Object does not have any fields. If you don’t explicitly specify which class a new class extends, the class extends Object implicitly. For this reason, you typically will not include “extends Object” in your code—we do so in this one example only for demonstration purposes. Overview of Class CommissionEmployee’s Methods and Instance Variables Class CommissionEmployee’s public services include a constructor (lines 13–34) and methods earnings (lines 87–90) and toString (lines 93–101). Lines 37–52 declare public get methods for the class’s final instance variables (declared in lines 6–8) firstName, lastName and socialSecurityNumber. These three instance variables are declared final because they do not need to be modified after they’re initialized—this is also why we do 366 Chapter 9 Object-Oriented Programming: Inheritance not provide corresponding set methods. Lines 55–84 declare public set and get methods for the class’s grossSales and commissionRate instance variables (declared in lines 9–10). The class declares its instance variables as private, so objects of other classes cannot directly access these variables. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 // Fig. 9.4: CommissionEmployee.java // CommissionEmployee class represents an employee paid a // percentage of gross sales. public class CommissionEmployee extends Object { private final String firstName; private final String lastName; private final String socialSecurityNumber; private double grossSales; // gross weekly sales private double commissionRate; // commission percentage Fig. 9.4 // five-argument constructor public CommissionEmployee(String firstName, String lastName, String socialSecurityNumber, double grossSales, double commissionRate) { // implicit call to Object's default constructor occurs here // if grossSales is invalid throw exception if (grossSales < 0.0) throw new IllegalArgumentException( "Gross sales must be >= 0.0"); // if commissionRate is invalid throw exception if (commissionRate <= 0.0 || commissionRate >= 1.0) throw new IllegalArgumentException( "Commission rate must be > 0.0 and < 1.0"); this.firstName = firstName; this.lastName = lastName; this.socialSecurityNumber = socialSecurityNumber; this.grossSales = grossSales; this.commissionRate = commissionRate; } // end constructor // return first name public String getFirstName() { return firstName; } // return last name public String getLastName() { return lastName; } | CommissionEmployee (Part 1 of 3.) class represents an employee paid a percentage of gross sales. 9.4 Relationship Between Superclasses and Subclasses 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 Fig. 9.4 367 // return social security number public String getSocialSecurityNumber() { return socialSecurityNumber; } // set gross sales amount public void setGrossSales(double grossSales) { if (grossSales < 0.0) throw new IllegalArgumentException( "Gross sales must be >= 0.0"); this.grossSales = grossSales; } // return gross sales amount public double getGrossSales() { return grossSales; } // set commission rate public void setCommissionRate(double commissionRate) { if (commissionRate <= 0.0 || commissionRate >= 1.0) throw new IllegalArgumentException( "Commission rate must be > 0.0 and < 1.0"); this.commissionRate = commissionRate; } // return commission rate public double getCommissionRate() { return commissionRate; } // calculate earnings public double earnings() { return commissionRate * grossSales; } // return String representation of CommissionEmployee object @Override // indicates that this method overrides a superclass method public String toString() { return String.format("%s: %s %s%n%s: %s%n%s: %.2f%n%s: %.2f", "commission employee", firstName, lastName, "social security number", socialSecurityNumber, | CommissionEmployee (Part 2 of 3.) class represents an employee paid a percentage of gross sales. 368 Chapter 9 Object-Oriented Programming: Inheritance 99 "gross sales", grossSales, "commission rate", commissionRate); 100 } 101 102 } // end class CommissionEmployee Fig. 9.4 | CommissionEmployee class represents an employee paid a percentage of gross sales. (Part 3 of 3.) Class CommissionEmployee’s Constructor Constructors are not inherited, so class CommissionEmployee does not inherit class Object’s constructor. However, a superclass’s constructors are still available to be called by subclasses. In fact, Java requires that the first task of any subclass constructor is to call its direct superclass’s constructor, either explicitly or implicitly (if no constructor call is specified), to ensure that the instance variables inherited from the superclass are initialized properly. The syntax for calling a superclass constructor explicitly is discussed in Section 9.4.3. In this example, class CommissionEmployee’s constructor calls class Object’s constructor implicitly. If the code does not include an explicit call to the superclass constructor, Java implicitly calls the superclass’s default or no-argument constructor. The comment in line 17 of Fig. 9.4 indicates where the implicit call to the superclass Object’s default constructor is made (you do not write the code for this call). Object’s default constructor does nothing. Even if a class does not have constructors, the default constructor that the compiler implicitly declares for the class will call the superclass’s default or no-argument constructor. After the implicit call to Object’s constructor, lines 20–22 and 25–27 validate the grossSales and commissionRate arguments. If these are valid (that is, the constructor does not throw an IllegalArgumentException), lines 29–33 assign the constructor’s arguments to the class’s instance variables. We did not validate the values of arguments firstName, lastName and socialSecurityNumber before assigning them to the corresponding instance variables. We could validate the first and last names—perhaps to ensure that they’re of a reasonable length. Similarly, a social security number could be validated using regular expressions (Section 14.7) to ensure that it contains nine digits, with or without dashes (e.g., 123-456789 or 123456789). Class CommissionEmployee’s earnings Method Method earnings (lines 87–90) calculates a CommissionEmployee’s earnings. Line 89 multiplies the commissionRate by the grossSales and returns the result. Class CommissionEmployee’s toString Method and the @Override Annotation Method toString (lines 93–101) is special—it’s one of the methods that every class inherits directly or indirectly from class Object (summarized in Section 9.6). Method toString returns a String representing an object. It’s called implicitly whenever an object must be converted to a String representation, such as when an object is output by printf or output by String method format via the %s format specifier. Class Object’s toString method returns a String that includes the name of the object’s class. It’s primarily a placeholder that can be overridden by a subclass to specify an appropriate String representation of the data in a subclass object. Method toString of class CommissionEmployee overrides (redefines) class Object’s toString method. When invoked, CommissionEmployee’s toString 9.4 Relationship Between Superclasses and Subclasses 369 method uses String method format to return a String containing information about the To override a superclass method, a subclass must declare a method with the same signature (method name, number of parameters, parameter types and order of parameter types) as the superclass method—Object’s toString method takes no parameters, so CommissionEmployee declares toString with no parameters. Line 93 uses the optional @Override annotation to indicate that the following method declaration (i.e., toString) should override an existing superclass method. This annotation helps the compiler catch a few common errors. For example, in this case, you intend to override superclass method toString, which is spelled with a lowercase “t” and an uppercase “S.” If you inadvertently use a lowercase “s,” the compiler will flag this as an error because the superclass does not contain a method named toString. If you didn’t use the @Override annotation, toString would be an entirely different method that would not be called if a CommissionEmployee were used where a String was needed. Another common overriding error is declaring the wrong number or types of parameters in the parameter list. This creates an unintentional overload of the superclass method, rather than overriding the existing method. If you then attempt to call the method (with the correct number and types of parameters) on a subclass object, the superclass’s version is invoked—potentially leading to subtle logic errors. When the compiler encounters a method declared with @Override, it compares the method’s signature with the superclass’s method signatures. If there isn’t an exact match, the compiler issues an error message, such as “method does not override or implement a method from a supertype.” You would then correct your method’s signature so that it matches one in the superclass. CommissionEmployee. Error-Prevention Tip 9.1 Though the @Override annotation is optional, declare overridden methods with it to ensure at compilation time that you defined their signatures correctly. It’s always better to find errors at compile time rather than at runtime. For this reason, the toString methods in Fig. 7.9 and in Chapter 8’s examples should have been declared with @Override. Common Programming Error 9.1 It’s a compilation error to override a method with a more restricted access modifier—a public superclass method cannot become a protected or private subclass method; a protected superclass method cannot become a private subclass method. Doing so would break the is-a relationship, which requires that all subclass objects be able to respond to method calls made to public methods declared in the superclass. If a public method, could be overridden as a protected or private method, the subclass objects would not be able to respond to the same method calls as superclass objects. Once a method is declared public in a superclass, the method remains public for all that class’s direct and indirect subclasses. Class CommissionEmployeeTest Figure 9.5 tests class CommissionEmployee. Lines 9–10 instantiate a CommissionEmployee object and invoke CommissionEmployee’s constructor (lines 13–34 of Fig. 9.4) to initialize it with "Sue" as the first name, "Jones" as the last name, "222-22-2222" as the social security number, 10000 as the gross sales amount ($10,000) and .06 as the commission rate (i.e., 6%). Lines 15–24 use CommissionEmployee’s get methods to retrieve the object’s instancevariable values for output. Lines 26–27 invoke the object’s setGrossSales and setCommissionRate methods to change the values of instance variables grossSales and commission- 370 Chapter 9 Object-Oriented Programming: Inheritance Rate. Lines 29–30 output the String representation of the updated CommissionEmployee. When an object is output using the %s format specifier, the object’s toString method is invoked implicitly to obtain the object’s String representation. [Note: In this chapter, we do not use the earnings method in each class, but it’s used extensively in Chapter 10.] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 // Fig. 9.5: CommissionEmployeeTest.java // CommissionEmployee class test program. public class CommissionEmployeeTest { public static void main(String[] args) { // instantiate CommissionEmployee object CommissionEmployee employee = new CommissionEmployee( "Sue", "Jones", "222-22-2222", 10000, .06); // get commission employee data System.out.println( "Employee information obtained by get methods:"); System.out.printf("%n%s %s%n", "First name is", employee.getFirstName()); System.out.printf("%s %s%n", "Last name is", employee.getLastName()); System.out.printf("%s %s%n", "Social security number is", employee.getSocialSecurityNumber()); System.out.printf("%s %.2f%n", "Gross sales is", employee.getGrossSales()); System.out.printf("%s %.2f%n", "Commission rate is", employee.getCommissionRate()); employee.setGrossSales(5000); employee.setCommissionRate(.1); System.out.printf("%n%s:%n%n%s%n", "Updated employee information obtained by toString", employee); } // end main } // end class CommissionEmployeeTest Employee information obtained by get methods: First name is Sue Last name is Jones Social security number is 222-22-2222 Gross sales is 10000.00 Commission rate is 0.06 Updated employee information obtained by toString: commission employee: Sue Jones social security number: 222-22-2222 gross sales: 5000.00 commission rate: 0.10 Fig. 9.5 | CommissionEmployee class test program. 9.4 Relationship Between Superclasses and Subclasses 371 9.4.2 Creating and Using a BasePlusCommissionEmployee Class We now discuss the second part of our introduction to inheritance by declaring and testing (a completely new and independent) class BasePlusCommissionEmployee (Fig. 9.6), which contains a first name, last name, social security number, gross sales amount, commission rate and base salary. Class BasePlusCommissionEmployee’s public services include a BasePlusCommissionEmployee constructor (lines 15–42) and methods earnings (lines 111–114) and toString (lines 117–126). Lines 45–108 declare public get and set methods for the class’s private instance variables (declared in lines 7–12) firstName, lastName, socialSecurityNumber, grossSales, commissionRate and baseSalary. These variables and methods encapsulate all the necessary features of a base-salaried commission employee. Note the similarity between this class and class CommissionEmployee (Fig. 9.4)—in this example, we’ll not yet exploit that similarity. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 // Fig. 9.6: BasePlusCommissionEmployee.java // BasePlusCommissionEmployee class represents an employee who receives // a base salary in addition to commission. public class BasePlusCommissionEmployee { private final String firstName; private final String lastName; private final String socialSecurityNumber; private double grossSales; // gross weekly sales private double commissionRate; // commission percentage private double baseSalary; // base salary per week // six-argument constructor public BasePlusCommissionEmployee(String firstName, String lastName, String socialSecurityNumber, double grossSales, double commissionRate, double baseSalary) { // implicit call to Object's default constructor occurs here // if grossSales is invalid throw exception if (grossSales < 0.0) throw new IllegalArgumentException( "Gross sales must be >= 0.0"); // if commissionRate is invalid throw exception if (commissionRate <= 0.0 || commissionRate >= 1.0) throw new IllegalArgumentException( "Commission rate must be > 0.0 and < 1.0"); // if baseSalary is invalid throw exception if (baseSalary < 0.0) throw new IllegalArgumentException( "Base salary must be >= 0.0"); Fig. 9.6 | BasePlusCommissionEmployee class represents an employee who receives a base salary in addition to a commission. (Part 1 of 3.) 372 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 Chapter 9 Object-Oriented Programming: Inheritance this.firstName = firstName; this.lastName = lastName; this.socialSecurityNumber = socialSecurityNumber; this.grossSales = grossSales; this.commissionRate = commissionRate; this.baseSalary = baseSalary; } // end constructor // return first name public String getFirstName() { return firstName; } // return last name public String getLastName() { return lastName; } // return social security number public String getSocialSecurityNumber() { return socialSecurityNumber; } // set gross sales amount public void setGrossSales(double grossSales) { if (grossSales < 0.0) throw new IllegalArgumentException( "Gross sales must be >= 0.0"); this.grossSales = grossSales; } // return gross sales amount public double getGrossSales() { return grossSales; } // set commission rate public void setCommissionRate(double commissionRate) { if (commissionRate <= 0.0 || commissionRate >= 1.0) throw new IllegalArgumentException( "Commission rate must be > 0.0 and < 1.0"); this.commissionRate = commissionRate; } Fig. 9.6 | BasePlusCommissionEmployee class represents an employee who receives a base salary in addition to a commission. (Part 2 of 3.) 9.4 Relationship Between Superclasses and Subclasses 373 88 // return commission rate 89 public double getCommissionRate() 90 { 91 return commissionRate; 92 } 93 94 // set base salary public void setBaseSalary(double baseSalary) 95 { 96 97 if (baseSalary < 0.0) throw new IllegalArgumentException( 98 "Base salary must be >= 0.0"); 99 100 this.baseSalary = baseSalary; 101 } 102 103 // return base salary 104 public double getBaseSalary() 105 { 106 return baseSalary; 107 } 108 109 110 // calculate earnings 111 public double earnings() 112 { 113 return baseSalary + (commissionRate * grossSales); 114 } 115 116 // return String representation of BasePlusCommissionEmployee 117 @Override 118 public String toString() 119 { 120 return String.format( 121 "%s: %s %s%n%s: %s%n%s: %.2f%n%s: %.2f%n%s: %.2f", 122 "base-salaried commission employee", firstName, lastName, 123 "social security number", socialSecurityNumber, 124 "gross sales", grossSales, "commission rate", commissionRate, 125 "base salary", baseSalary); 126 } 127 } // end class BasePlusCommissionEmployee Fig. 9.6 | BasePlusCommissionEmployee class represents an employee who receives a base salary in addition to a commission. (Part 3 of 3.) Class BasePlusCommissionEmployee does not specify “extends Object” in line 5, so the class implicitly extends Object. Also, like class CommissionEmployee’s constructor (lines 13–34 of Fig. 9.4), class BasePlusCommissionEmployee’s constructor invokes class Object’s default constructor implicitly, as noted in the comment in line 19. Class BasePlusCommissionEmployee’s earnings method (lines 111–114) returns the result of adding the BasePlusCommissionEmployee’s base salary to the product of the commission rate and the employee’s gross sales. Class BasePlusCommissionEmployee overrides Object method toString to return a String containing the BasePlusCommissionEmployee’s information. Once again, we use 374 Chapter 9 Object-Oriented Programming: Inheritance format specifier %.2f to format the gross sales, commission rate and base salary with two digits of precision to the right of the decimal point (line 121). Testing Class BasePlusCommissionEmployee Figure 9.7 tests class BasePlusCommissionEmployee. Lines 9–11 create a BasePlusCommissionEmployee object and pass "Bob", "Lewis", "333-33-3333", 5000, .04 and 300 to the constructor as the first name, last name, social security number, gross sales, commission rate and base salary, respectively. Lines 16–27 use BasePlusCommissionEmployee’s get methods to retrieve the values of the object’s instance variables for output. Line 29 invokes the object’s setBaseSalary method to change the base salary. Method setBaseSalary (Fig. 9.6, lines 95–102) ensures that instance variable baseSalary is not assigned a negative value. Line 33 of Fig. 9.7 invokes method toString explicitly to get the object’s String representation. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 // Fig. 9.7: BasePlusCommissionEmployeeTest.java // BasePlusCommissionEmployee test program. public class BasePlusCommissionEmployeeTest { public static void main(String[] args) { // instantiate BasePlusCommissionEmployee object BasePlusCommissionEmployee employee = new BasePlusCommissionEmployee( "Bob", "Lewis", "333-33-3333", 5000, .04, 300); // get base-salaried commission employee data System.out.println( "Employee information obtained by get methods:%n"); System.out.printf("%s %s%n", "First name is", employee.getFirstName()); System.out.printf("%s %s%n", "Last name is", employee.getLastName()); System.out.printf("%s %s%n", "Social security number is", employee.getSocialSecurityNumber()); System.out.printf("%s %.2f%n", "Gross sales is", employee.getGrossSales()); System.out.printf("%s %.2f%n", "Commission rate is", employee.getCommissionRate()); System.out.printf("%s %.2f%n", "Base salary is", employee.getBaseSalary()); employee.setBaseSalary(1000); System.out.printf("%n%s:%n%n%s%n", "Updated employee information obtained by toString", employee.toString()); } // end main } // end class BasePlusCommissionEmployeeTest Fig. 9.7 | BasePlusCommissionEmployee test program. (Part 1 of 2.) 9.4 Relationship Between Superclasses and Subclasses 375 Employee information obtained by get methods: First name is Bob Last name is Lewis Social security number is 333-33-3333 Gross sales is 5000.00 Commission rate is 0.04 Base salary is 300.00 Updated employee information obtained by toString: base-salaried commission employee: Bob Lewis social security number: 333-33-3333 gross sales: 5000.00 commission rate: 0.04 base salary: 1000.00 Fig. 9.7 | BasePlusCommissionEmployee test program. (Part 2 of 2.) Notes on Class BasePlusCommissionEmployee Much of class BasePlusCommissionEmployee’s code (Fig. 9.6) is similar, or identical, to that of class CommissionEmployee (Fig. 9.4). For example, private instance variables firstName and lastName and methods setFirstName, getFirstName, setLastName and getLastName are identical to those of class CommissionEmployee. The classes also both contain private instance variables socialSecurityNumber, commissionRate and grossSales, and corresponding get and set methods. In addition, the BasePlusCommissionEmployee constructor is almost identical to that of class CommissionEmployee, except that BasePlusCommissionEmployee’s constructor also sets the baseSalary. The other additions to class BasePlusCommissionEmployee are private instance variable baseSalary and methods setBaseSalary and getBaseSalary. Class BasePlusCommissionEmployee’s toString method is almost identical to that of class CommissionEmployee except that it also outputs instance variable baseSalary with two digits of precision to the right of the decimal point. We literally copied code from class CommissionEmployee and pasted it into class BasePlusCommissionEmployee, then modified class BasePlusCommissionEmployee to include a base salary and methods that manipulate the base salary. This “copy-and-paste” approach is often error prone and time consuming. Worse yet, it spreads copies of the same code throughout a system, creating code-maintenance problems—changes to the code would need to be made in multiple classes. Is there a way to “acquire” the instance variables and methods of one class in a way that makes them part of other classes without duplicating code? Next we answer this question, using a more elegant approach to building classes that emphasizes the benefits of inheritance. Software Engineering Observation 9.3 With inheritance, the instance variables and methods that are the same for all the classes in the hierarchy are declared in a superclass. Changes made to these common features in the superclass are inherited by the subclass. Without inheritance, changes would need to be made to all the source-code files that contain a copy of the code in question. 376 Chapter 9 Object-Oriented Programming: Inheritance 9.4.3 Creating a CommissionEmployee– BasePlusCommissionEmployee Inheritance Hierarchy Now we declare class BasePlusCommissionEmployee (Fig. 9.8) to extend class Commis(Fig. 9.4). A BasePlusCommissionEmployee object is a CommissionEmployee, because inheritance passes on class CommissionEmployee’s capabilities. Class BasePlusCommissionEmployee also has instance variable baseSalary (Fig. 9.8, line 6). Keyword extends (line 4) indicates inheritance. BasePlusCommissionEmployee inherits CommissionEmployee’s instance variables and methods. sionEmployee Software Engineering Observation 9.4 At the design stage in an object-oriented system, you’ll often find that certain classes are closely related. You should “factor out” common instance variables and methods and place them in a superclass. Then use inheritance to develop subclasses, specializing them with capabilities beyond those inherited from the superclass. Software Engineering Observation 9.5 Declaring a subclass does not affect its superclass’s source code. Inheritance preserves the integrity of the superclass. Only CommissionEmployee’s public and protected members are directly accessible in the subclass. The CommissionEmployee constructor is not inherited. So, the public BasePlusCommissionEmployee services include its constructor (lines 9–23), public methods inherited from CommissionEmployee, and methods setBaseSalary (lines 26– 33), getBaseSalary (lines 36–39), earnings (lines 42–47) and toString (lines 50–60). Methods earnings and toString override the corresponding methods in class CommissionEmployee because their superclass versions do not properly calculate a BasePlusCommissionEmployee’s earnings or return an appropriate String representation, respectively. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 // Fig. 9.8: BasePlusCommissionEmployee.java // private superclass members cannot be accessed in a subclass. public class BasePlusCommissionEmployee extends CommissionEmployee { private double baseSalary; // base salary per week Fig. 9.8 // six-argument constructor public BasePlusCommissionEmployee(String firstName, String lastName, String socialSecurityNumber, double grossSales, double commissionRate, double baseSalary) { // explicit call to superclass CommissionEmployee constructor super(firstName, lastName, socialSecurityNumber, grossSales, commissionRate); // if baseSalary is invalid throw exception if (baseSalary < 0.0) throw new IllegalArgumentException( "Base salary must be >= 0.0"); | private superclass members cannot be accessed in a subclass. (Part 1 of 3.) 9.4 Relationship Between Superclasses and Subclasses 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 377 this.baseSalary = baseSalary; } // set base salary public void setBaseSalary(double baseSalary) { if (baseSalary < 0.0) throw new IllegalArgumentException( "Base salary must be >= 0.0"); this.baseSalary = baseSalary; } // return base salary public double getBaseSalary() { return baseSalary; } // calculate earnings @Override public double earnings() { // not allowed: commissionRate and grossSales private in superclass return baseSalary + (commissionRate * grossSales); } // return String representation of BasePlusCommissionEmployee @Override public String toString() { // not allowed: attempts to access private superclass members return String.format( "%s: %s %s%n%s: %s%n%s: %.2f%n%s: %.2f%n%s: %.2f", "base-salaried commission employee", firstName, lastName, "social security number", socialSecurityNumber, "gross sales", grossSales, "commission rate", commissionRate, "base salary", baseSalary); } } // end class BasePlusCommissionEmployee BasePlusCommissionEmployee.java:46: error: commissionRate has private access in CommissionEmployee return baseSalary + (commissionRate * grossSales); ^ BasePlusCommissionEmployee.java:46: error: grossSales has private access in CommissionEmployee return baseSalary + (commissionRate * grossSales); ^ BasePlusCommissionEmployee.java:56: error: firstName has private access in CommissionEmployee "base-salaried commission employee", firstName, lastName, ^ Fig. 9.8 | private superclass members cannot be accessed in a subclass. (Part 2 of 3.) 378 Chapter 9 Object-Oriented Programming: Inheritance BasePlusCommissionEmployee.java:56: error: lastName has private access in CommissionEmployee "base-salaried commission employee", firstName, lastName, ^ BasePlusCommissionEmployee.java:57: error: socialSecurityNumber has private access in CommissionEmployee "social security number", socialSecurityNumber, ^ BasePlusCommissionEmployee.java:58: error: grossSales has private access in CommissionEmployee "gross sales", grossSales, "commission rate", commissionRate, ^ BasePlusCommissionEmployee.java:58: error: commissionRate has private access inCommissionEmployee "gross sales", grossSales, "commission rate", commissionRate, ^ Fig. 9.8 | private superclass members cannot be accessed in a subclass. (Part 3 of 3.) A Subclass’s Constructor Must Call Its Superclass’s Constructor Each subclass constructor must implicitly or explicitly call one of its superclass’s constructors to initialize the instance variables inherited from the superclass. Lines 14–15 in BasePlusCommissionEmployee’s six-argument constructor (lines 9–23) explicitly call class CommissionEmployee’s five-argument constructor (declared at lines 13–34 of Fig. 9.4) to initialize the superclass portion of a BasePlusCommissionEmployee object (i.e., variables firstName, lastName, socialSecurityNumber, grossSales and commissionRate). We do this by using the superclass constructor call syntax—keyword super, followed by a set of parentheses containing the superclass constructor arguments, which are used to initialize the superclass instance variables firstName, lastName, socialSecurityNumber, grossSales and commissionRate, respectively. If BasePlusCommissionEmployee’s constructor did not invoke the superclass’s constructor explicitly, the compiler would attempt to insert a call to the superclass’s default or no-argument constructor. Class CommissionEmployee does not have such a constructor, so the compiler would issue an error. The explicit superclass constructor call in lines 14–15 of Fig. 9.8 must be the first statement in the constructor’s body. When a superclass contains a no-argument constructor, you can use super() to call that constructor explicitly, but this is rarely done. Software Engineering Observation 9.6 You learned previously that you should not call a class’s instance methods from its constructors and that we’ll say why in Chapter 10. Calling a superclass constructor from a subclass constructor does not contradict this advice. Methods Earnings and toString The compiler generates errors for line 46 (Fig. 9.8) because CommissionEmployee’s instance variables commissionRate and grossSales are private—subclass BasePlusCommissionEmployee’s methods are not allowed to access superclass CommissionEmployee’s private instance variables. We used red text in Fig. 9.8 to indicate erroneous code. The compiler issues additional errors at lines 56–58 of BasePlusCommissionEmployee’s toString method for the same reason. The errors in BasePlusCommissionEmployee could have been prevented by using the get methods inherited from class CommissionEmployee. BasePlusCommissionEmployee 9.4 Relationship Between Superclasses and Subclasses 379 For example, line 46 could have called getCommissionRate and getGrossSales to access instance variables commissionRate and grossSales, respectively. Lines 56–58 also could have used appropriate get methods to retrieve the values of the superclass’s instance variables. CommissionEmployee’s private 9.4.4 CommissionEmployee–BasePlusCommissionEmployee Inheritance Hierarchy Using protected Instance Variables To enable class BasePlusCommissionEmployee to directly access superclass instance variables firstName, lastName, socialSecurityNumber, grossSales and commissionRate, we can declare those members as protected in the superclass. As we discussed in Section 9.3, a superclass’s protected members are accessible by all subclasses of that superclass. In the new CommissionEmployee class, we modified only lines 6–10 of Fig. 9.4 to declare the instance variables with the protected access modifier as follows: protected protected protected protected protected final String firstName; final String lastName; final String socialSecurityNumber; double grossSales; // gross weekly sales double commissionRate; // commission percentage The rest of the class declaration (which is not shown here) is identical to that of Fig. 9.4. We could have declared CommissionEmployee’s instance variables public to enable subclass BasePlusCommissionEmployee to access them. However, declaring public instance variables is poor software engineering because it allows unrestricted access to the these variables from any class, greatly increasing the chance of errors. With protected instance variables, the subclass gets access to the instance variables, but classes that are not subclasses and classes that are not in the same package cannot access these variables directly—recall that protected class members are also visible to other classes in the same package. Class BasePlusCommissionEmployee Class BasePlusCommissionEmployee (Fig. 9.9) extends the new version of class CommissionEmployee with protected instance variables. BasePlusCommissionEmployee objects inherit CommissionEmployee’s protected instance variables firstName, lastName, socialSecurityNumber, grossSales and commissionRate—all these variables are now protected members of BasePlusCommissionEmployee. As a result, the compiler does not generate errors when compiling line 45 of method earnings and lines 54–56 of method toString. If another class extends this version of class BasePlusCommissionEmployee, the new subclass also can access the protected members. 1 2 3 4 5 6 7 // Fig. 9.9: BasePlusCommissionEmployee.java // BasePlusCommissionEmployee inherits protected instance // variables from CommissionEmployee. public class BasePlusCommissionEmployee extends CommissionEmployee { private double baseSalary; // base salary per week Fig. 9.9 | BasePlusCommissionEmployee CommissionEmployee. (Part 1 of 2.) inherits protected instance variables from 380 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 Chapter 9 Object-Oriented Programming: Inheritance // six-argument constructor public BasePlusCommissionEmployee(String firstName, String lastName, String socialSecurityNumber, double grossSales, double commissionRate, double baseSalary) { super(firstName, lastName, socialSecurityNumber, grossSales, commissionRate); // if baseSalary is invalid throw exception if (baseSalary < 0.0) throw new IllegalArgumentException( "Base salary must be >= 0.0"); this.baseSalary = baseSalary; } // set base salary public void setBaseSalary(double baseSalary) { if (baseSalary < 0.0) throw new IllegalArgumentException( "Base salary must be >= 0.0"); this.baseSalary = baseSalary; } // return base salary public double getBaseSalary() { return baseSalary; } // calculate earnings @Override // indicates that this method overrides a superclass method public double earnings() { return baseSalary + (commissionRate * grossSales); } // return String representation of BasePlusCommissionEmployee @Override public String toString() { return String.format( "%s: %s %s%n%s: %s%n%s: %.2f%n%s: %.2f%n%s: %.2f", "base-salaried commission employee", firstName, lastName, "social security number", socialSecurityNumber, "gross sales", grossSales, "commission rate", commissionRate, "base salary", baseSalary); } } // end class BasePlusCommissionEmployee Fig. 9.9 | BasePlusCommissionEmployee CommissionEmployee. (Part 2 of 2.) inherits protected instance variables from 9.4 Relationship Between Superclasses and Subclasses 381 A Subclass Object Contains the Instance Variables of All of Its Superclasses When you create a BasePlusCommissionEmployee object, it contains all instance variables declared in the class hierarchy to that point—that is, those from classes Object (which does not have instance variables), CommissionEmployee and BasePlusCommissionEmployee. Class BasePlusCommissionEmployee does not inherit CommissionEmployee’s five-argument constructor, but explicitly invokes it (lines 14–15) to initialize the instance variables that BasePlusCommissionEmployee inherited from CommissionEmployee. Similarly, CommissionEmployee’s constructor implicitly calls class Object’s constructor. BasePlusCommissionEmployee’s constructor must explicitly call CommissionEmployee’s constructor because CommissionEmployee does not have a no-argument constructor that could be invoked implicitly. Testing Class BasePlusCommissionEmployee The BasePlusCommissionEmployeeTest class for this example is identical to that of Fig. 9.7 and produces the same output, so we do not show it here. Although the version of class BasePlusCommissionEmployee in Fig. 9.6 does not use inheritance and the version in Fig. 9.9 does, both classes provide the same functionality. The source code in Fig. 9.9 (59 lines) is considerably shorter than that in Fig. 9.6 (127 lines), because most of the class’s functionality is now inherited from CommissionEmployee—there’s now only one copy of the CommissionEmployee functionality. This makes the code easier to maintain, modify and debug, because the code related to a CommissionEmployee exists only in that class. Notes on Using protected Instance Variables In this example, we declared superclass instance variables as protected so that subclasses could access them. Inheriting protected instance variables enables direct access to the variables by subclasses. In most cases, however, it’s better to use private instance variables to encourage proper software engineering. Your code will be easier to maintain, modify and debug. Using protected instance variables creates several potential problems. First, the subclass object can set an inherited variable’s value directly without using a set method. Therefore, a subclass object can assign an invalid value to the variable, possibly leaving the object in an inconsistent state. For example, if we were to declare CommissionEmployee’s instance variable grossSales as protected, a subclass object (e.g., BasePlusCommissionEmployee) could then assign a negative value to grossSales. Another problem with using protected instance variables is that subclass methods are more likely to be written so that they depend on the superclass’s data implementation. In practice, subclasses should depend only on the superclass services (i.e., non-private methods) and not on the superclass data implementation. With protected instance variables in the superclass, we may need to modify all the subclasses of the superclass if the superclass implementation changes. For example, if for some reason we were to change the names of instance variables firstName and lastName to first and last, then we would have to do so for all occurrences in which a subclass directly references superclass instance variables firstName and lastName. Such a class is said to be fragile or brittle, because a small change in the superclass can “break” subclass implementation. You should be able to change the superclass implementation while still providing the same services to the subclasses. Of course, if the superclass services change, we must reimplement our subclasses. A third problem is that a class’s protected members 382 Chapter 9 Object-Oriented Programming: Inheritance are visible to all classes in the same package as the class containing the protected members—this is not always desirable. Software Engineering Observation 9.7 Use the protected access modifier when a superclass should provide a method only to its subclasses and other classes in the same package, but not to other clients. Software Engineering Observation 9.8 Declaring superclass instance variables private (as opposed to protected) enables the superclass implementation of these instance variables to change without affecting subclass implementations. Error-Prevention Tip 9.2 When possible, do not include protected instance variables in a superclass. Instead, include non-private methods that access private instance variables. This will help ensure that objects of the class maintain consistent states. 9.4.5 CommissionEmployee–BasePlusCommissionEmployee Inheritance Hierarchy Using private Instance Variables Let’s reexamine our hierarchy once more, this time using good software engineering practices. Class CommissionEmployee Class CommissionEmployee (Fig. 9.10) declares instance variables firstName, lastName, socialSecurityNumber, grossSales and commissionRate as private (lines 6–10) and provides public methods getFirstName, getLastName, getSocialSecurityNumber, setGrossSales, getGrossSales, setCommissionRate, getCommissionRate, earnings and toString for manipulating these values. Methods earnings (lines 87–90) and toString (lines 93–101) use the class’s get methods to obtain the values of its instance variables. If we decide to change the names of the instance variables, the earnings and toString declarations will not require modification—only the bodies of the get and set methods that directly manipulate the instance variables will need to change. These changes occur solely within the superclass—no changes to the subclass are needed. Localizing the effects of changes like this is a good software engineering practice. 1 2 3 4 5 6 7 8 9 10 // Fig. 9.10: CommissionEmployee.java // CommissionEmployee class uses methods to manipulate its // private instance variables. public class CommissionEmployee { private final String firstName; private final String lastName; private final String socialSecurityNumber; private double grossSales; // gross weekly sales private double commissionRate; // commission percentage Fig. 9.10 | CommissionEmployee variables. (Part 1 of 3.) class uses methods to manipulate its private instance 9.4 Relationship Between Superclasses and Subclasses 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 Fig. 9.10 // five-argument constructor public CommissionEmployee(String firstName, String lastName, String socialSecurityNumber, double grossSales, double commissionRate) { // implicit call to Object constructor occurs here // if grossSales is invalid throw exception if (grossSales < 0.0) throw new IllegalArgumentException( "Gross sales must be >= 0.0"); // if commissionRate is invalid throw exception if (commissionRate <= 0.0 || commissionRate >= 1.0) throw new IllegalArgumentException( "Commission rate must be > 0.0 and < 1.0"); this.firstName = firstName; this.lastName = lastName; this.socialSecurityNumber = socialSecurityNumber; this.grossSales = grossSales; this.commissionRate = commissionRate; } // end constructor // return first name public String getFirstName() { return firstName; } // return last name public String getLastName() { return lastName; } // return social security number public String getSocialSecurityNumber() { return socialSecurityNumber; } // set gross sales amount public void setGrossSales(double grossSales) { if (grossSales < 0.0) throw new IllegalArgumentException( "Gross sales must be >= 0.0"); this.grossSales = grossSales; } | CommissionEmployee variables. (Part 2 of 3.) class uses methods to manipulate its private instance 383 384 Chapter 9 Object-Oriented Programming: Inheritance 63 64 // return gross sales amount 65 public double getGrossSales() 66 { 67 return grossSales; 68 } 69 70 // set commission rate 71 public void setCommissionRate(double commissionRate) 72 { 73 if (commissionRate <= 0.0 || commissionRate >= 1.0) 74 throw new IllegalArgumentException( 75 "Commission rate must be > 0.0 and < 1.0"); 76 77 this.commissionRate = commissionRate; 78 } 79 80 // return commission rate 81 public double getCommissionRate() 82 { 83 return commissionRate; 84 } 85 86 // calculate earnings 87 public double earnings() 88 { 89 return getCommissionRate() * getGrossSales(); 90 } 91 92 // return String representation of CommissionEmployee object 93 @Override 94 public String toString() 95 { 96 return String.format("%s: %s %s%n%s: %s%n%s: %.2f%n%s: %.2f", 97 "commission employee", getFirstName(), getLastName(), 98 "social security number", getSocialSecurityNumber(), 99 "gross sales", getGrossSales(), 100 "commission rate", getCommissionRate()); 101 } 102 } // end class CommissionEmployee Fig. 9.10 | CommissionEmployee class uses methods to manipulate its private instance variables. (Part 3 of 3.) Class BasePlusCommissionEmployee Subclass BasePlusCommissionEmployee (Fig. 9.11) inherits CommissionEmployee’s nonprivate methods and can access (in a controlled way) the private superclass members via those methods. Class BasePlusCommissionEmployee has several changes that distinguish it from Fig. 9.9. Methods earnings (lines 43–47) and toString (lines 50–55) each invoke method getBaseSalary to obtain the base salary value, rather than accessing baseSalary directly. If we decide to rename instance variable baseSalary, only the bodies of method setBaseSalary and getBaseSalary will need to change. 9.4 Relationship Between Superclasses and Subclasses 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 // // // // 385 Fig. 9.11: BasePlusCommissionEmployee.java BasePlusCommissionEmployee class inherits from CommissionEmployee and accesses the superclass’s private data via inherited public methods. public class BasePlusCommissionEmployee extends CommissionEmployee { private double baseSalary; // base salary per week // six-argument constructor public BasePlusCommissionEmployee(String firstName, String lastName, String socialSecurityNumber, double grossSales, double commissionRate, double baseSalary) { super(firstName, lastName, socialSecurityNumber, grossSales, commissionRate); // if baseSalary is invalid throw exception if (baseSalary < 0.0) throw new IllegalArgumentException( "Base salary must be >= 0.0"); this.baseSalary = baseSalary; } // set base salary public void setBaseSalary(double baseSalary) { if (baseSalary < 0.0) throw new IllegalArgumentException( "Base salary must be >= 0.0"); this.baseSalary = baseSalary; } // return base salary public double getBaseSalary() { return baseSalary; } // calculate earnings @Override public double earnings() { return getBaseSalary() + super.earnings(); } // return String representation of BasePlusCommissionEmployee @Override public String toString() { Fig. 9.11 | BasePlusCommissionEmployee class inherits from CommissionEmployee and accesses the superclass’s private data via inherited public methods. (Part 1 of 2.) 386 53 54 55 56 Chapter 9 Object-Oriented Programming: Inheritance return String.format("%s %s%n%s: %.2f", "base-salaried", super.toString(), "base salary", getBaseSalary()); } } // end class BasePlusCommissionEmployee Fig. 9.11 class inherits from CommissionEmployee and accesses the superclass’s private data via inherited public methods. (Part 2 of 2.) | BasePlusCommissionEmployee Class BasePlusCommissionEmployee’s earnings Method Method earnings (lines 43–47) overrides class CommissionEmployee’s earnings method (Fig. 9.10, lines 87–90) to calculate a base-salaried commission employee’s earnings. The new version obtains the portion of the earnings based on commission alone by calling CommissionEmployee’s earnings method with super.earnings() (line 46), then adds the base salary to this value to calculate the total earnings. Note the syntax used to invoke an overridden superclass method from a subclass—place the keyword super and a dot (.) separator before the superclass method name. This method invocation is a good software engineering practice—if a method performs all or some of the actions needed by another method, call that method rather than duplicate its code. By having BasePlusCommissionEmployee’s earnings method invoke CommissionEmployee’s earnings method to calculate part of a BasePlusCommissionEmployee object’s earnings, we avoid duplicating the code and reduce code-maintenance problems. Common Programming Error 9.2 When a superclass method is overridden in a subclass, the subclass version often calls the superclass version to do a portion of the work. Failure to prefix the superclass method name with the keyword super and the dot (.) separator when calling the superclass’s method causes the subclass method to call itself, potentially creating an error called infinite recursion, which would eventually cause the method-call stack to overflow—a fatal runtime error. Recursion, used correctly, is a powerful capability discussed in Chapter 18. Class BasePlusCommissionEmployee’s toString Method Similarly, BasePlusCommissionEmployee’s toString method (Fig. 9.11, lines 50–55) overrides CommissionEmployee’s toString method (Fig. 9.10, lines 93–101) to return a String representation that’s appropriate for a base-salaried commission employee. The new version creates part of a BasePlusCommissionEmployee object’s String representation (i.e., the String "commission employee" and the values of class CommissionEmployee’s private instance variables) by calling CommissionEmployee’s toString method with the expression super.toString() (Fig. 9.11, line 54). BasePlusCommissionEmployee’s toString method then completes the remainder of a BasePlusCommissionEmployee object’s String representation (i.e., the value of class BasePlusCommissionEmployee’s base salary). Testing Class BasePlusCommissionEmployee Class BasePlusCommissionEmployeeTest performs the same manipulations on a BasePlusCommissionEmployee object as in Fig. 9.7 and produces the same output, so we do not show it here. Although each BasePlusCommissionEmployee class you’ve seen behaves identically, the version in Fig. 9.11 is the best engineered. By using inheritance and by calling methods that hide the data and ensure consistency, we’ve efficiently and effectively constructed a well-engineered class. 9.5 Constructors in Subclasses 387 9.5 Constructors in Subclasses As we explained, instantiating a subclass object begins a chain of constructor calls in which the subclass constructor, before performing its own tasks, explicitly uses super to call one of the constructors in its direct superclass or implicitly calls the superclass's default or no-argument constructor. Similarly, if the superclass is derived from another class—true of every class except Object—the superclass constructor invokes the constructor of the next class up the hierarchy, and so on. The last constructor called in the chain is always Object’s constructor. The original subclass constructor’s body finishes executing last. Each superclass’s constructor manipulates the superclass instance variables that the subclass object inherits. For example, consider again the CommissionEmployee–BasePlusCommissionEmployee hierarchy from Figs. 9.10–9.11. When an app creates a BasePlusCommissionEmployee object, its constructor is called. That constructor calls CommissionEmployee’s constructor, which in turn calls Object’s constructor. Class Object’s constructor has an empty body, so it immediately returns control to CommissionEmployee’s constructor, which then initializes the CommissionEmployee instance variables that are part of the BasePlusCommissionEmployee object. When CommissionEmployee’s constructor completes execution, it returns control to BasePlusCommissionEmployee’s constructor, which initializes the baseSalary. Software Engineering Observation 9.9 Java ensures that even if a constructor does not assign a value to an instance variable, the variable is still initialized to its default value (e.g., 0 for primitive numeric types, false for booleans, null for references). 9.6 Class Object As we discussed earlier in this chapter, all classes in Java inherit directly or indirectly from class Object (package java.lang), so its 11 methods (some are overloaded) are inherited by all other classes. Figure 9.12 summarizes Object’s methods. We discuss several Object methods throughout this book (as indicated in Fig. 9.12). Method Description equals This method compares two objects for equality and returns true if they’re equal and false otherwise. The method takes any Object as an argument. When objects of a particular class must be compared for equality, the class should override method equals to compare the contents of the two objects. For the requirements of implementing this method (which include also overriding method hashCode), refer to the method’s documentation at docs.oracle.com/javase/7/docs/api/java/ lang/Object.html#equals(java.lang.Object). The default equals implementation uses operator == to determine whether two references refer to the same object in memory. Section 14.3.3 demonstrates class String’s equals method and differentiates between comparing String objects with == and with equals. Hashcodes are int values used for high-speed storage and retrieval of information stored in a data structure that’s known as a hashtable (see Section 16.11). This method is also called as part of Object’s default toString method implementation. hashCode Fig. 9.12 | Object methods. (Part 1 of 2.) 388 Chapter 9 Object-Oriented Programming: Inheritance Method Description toString This method (introduced in Section 9.4.1) returns a String representation of an object. The default implementation of this method returns the package name and class name of the object’s class typically followed by a hexadecimal representation of the value returned by the object’s hashCode method. Methods notify, notifyAll and the three overloaded versions of wait are related to multithreading, which is discussed in Chapter 23. Every object in Java knows its own type at execution time. Method getClass (used in Sections 10.5 and 12.5) returns an object of class Class (package java.lang) that contains information about the object’s type, such as its class name (returned by Class method getName). This protected method is called by the garbage collector to perform termination housekeeping on an object just before the garbage collector reclaims the object’s memory. Recall from Section 8.10 that it’s unclear whether, or when, finalize will be called. For this reason, most programmers should avoid method finalize. This protected method, which takes no arguments and returns an Object reference, makes a copy of the object on which it’s called. The default implementation performs a so-called shallow copy—instance-variable values in one object are copied into another object of the same type. For reference types, only the references are copied. A typical overridden clone method’s implementation would perform a deep copy that creates a new object for each reference-type instance variable. Implementing clone correctly is difficult. For this reason, its use is discouraged. Some industry experts suggest that object serialization should be used instead. We discuss object serialization in Chapter 15. Recall from Chapter 7 that arrays are objects. As a result, like all other objects, arrays inherit the members of class Object. Every array has an overridden clone method that copies the array. However, if the array stores references to objects, the objects are not copied—a shallow copy is performed. wait, notify, notifyAll getClass finalize clone Fig. 9.12 | Object methods. (Part 2 of 2.) 9.7 (Optional) GUI and Graphics Case Study: Displaying Text and Images Using Labels Programs often use labels when they need to display information or instructions to the user in a graphical user interface. Labels are a convenient way of identifying GUI components on the screen and keeping the user informed about the current state of the program. In Java, an object of class JLabel (from package javax.swing) can display text, an image or both. The example in Fig. 9.13 demonstrates several JLabel features, including a plain text label, an image label and a label with both text and an image. 1 2 3 4 // Fig 9.13: LabelDemo.java // Demonstrates the use of labels. import java.awt.BorderLayout; import javax.swing.ImageIcon; Fig. 9.13 | JLabel with text and with images. (Part 1 of 2.) 9.7 Displaying Text and Images Using Labels 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 389 import javax.swing.JLabel; import javax.swing.JFrame; public class LabelDemo { public static void main(String[] args) { // Create a label with plain text JLabel northLabel = new JLabel("North"); // create an icon from an image so we can put it on a JLabel ImageIcon labelIcon = new ImageIcon("GUItip.gif"); // create a label with an Icon instead of text JLabel centerLabel = new JLabel(labelIcon); // create another label with an Icon JLabel southLabel = new JLabel(labelIcon); // set the label to display text (as well as an icon) southLabel.setText("South"); // create a frame to hold the labels JFrame application = new JFrame(); application.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); // add the labels to the frame; the second argument specifies // where on the frame to add the label application.add(northLabel, BorderLayout.NORTH); application.add(centerLabel, BorderLayout.CENTER); application.add(southLabel, BorderLayout.SOUTH); application.setSize(300, 300); application.setVisible(true); } // end main } // end class LabelDemo Fig. 9.13 | JLabel with text and with images. (Part 2 of 2.) Lines 3–6 import the classes we need to display JLabels. BorderLayout from package contains constants that specify where we can place GUI components in the java.awt 390 Chapter 9 Object-Oriented Programming: Inheritance JFrame. Class ImageIcon represents an image that can be displayed on a JLabel, and class represents the window that will contain all the labels. Line 13 creates a JLabel that displays its constructor argument—the string "North". Line 16 declares local variable labelIcon and assigns it a new ImageIcon. The constructor for ImageIcon receives a String that specifies the path to the image. Since we specify only a filename, Java assumes that it’s in the same directory as class LabelDemo. ImageIcon can load images in GIF, JPEG and PNG image formats. Line 19 declares and initializes local variable centerLabel with a JLabel that displays the labelIcon. Line 22 declares and initializes local variable southLabel with a JLabel similar to the one in line 19. However, line 25 calls method setText to change the text the label displays. Method setText can be called on any JLabel to change its text. This JLabel displays both the icon and the text. Line 28 creates the JFrame that displays the JLabels, and line 30 indicates that the program should terminate when the JFrame is closed. We attach the labels to the JFrame in lines 34–36 by calling an overloaded version of method add that takes two parameters. The first parameter is the component we want to attach, and the second is the region in which it should be placed. Each JFrame has an associated layout that helps the JFrame position the GUI components that are attached to it. The default layout for a JFrame is known as a BorderLayout and has five regions—NORTH (top), SOUTH (bottom), EAST (right side), WEST (left side) and CENTER. Each of these is declared as a constant in class BorderLayout. When calling method add with one argument, the JFrame places the component in the CENTER automatically. If a position already contains a component, then the new component takes its place. Lines 38 and 39 set the size of the JFrame and make it visible on screen. JFrame GUI and Graphics Case Study Exercise 9.1 Modify GUI and Graphics Case Study Exercise 8.1 to include a JLabel as a status bar that displays counts representing the number of each shape displayed. Class DrawPanel should declare a method that returns a String containing the status text. In main, first create the DrawPanel, then create the JLabel with the status text as an argument to the JLabel’s constructor. Attach the JLabel to the SOUTH region of the JFrame, as shown in Fig. 9.14. Fig. 9.14 | JLabel displaying shape statistics. 9.8 Wrap-Up 391 9.8 Wrap-Up This chapter introduced inheritance—the ability to create classes by acquiring an existing class’s members (without copying and pasting the code) and having the ability to embellish them with new capabilities. You learned the notions of superclasses and subclasses and used keyword extends to create a subclass that inherits members from a superclass. We showed how to use the @Override annotation to prevent unintended overloading by indicating that a method overrides a superclass method. We introduced the access modifier protected; subclass methods can directly access protected superclass members. You learned how to use super to access overridden superclass members. You also saw how constructors are used in inheritance hierarchies. Finally, you learned about the methods of class Object, the direct or indirect superclass of all Java classes. In Chapter 10, Object-Oriented Programming: Polymorphism and Interfaces, we build on our discussion of inheritance by introducing polymorphism—an object-oriented concept that enables us to write programs that conveniently handle, in a more general and convenient manner, objects of a wide variety of classes related by inheritance. After studying Chapter 10, you’ll be familiar with classes, objects, encapsulation, inheritance and polymorphism—the key technologies of object-oriented programming. Summary Section 9.1 Introduction • Inheritance (p. 361) reduces program-development time. • The direct superclass (p. 361) of a subclass is the one from which the subclass inherits. An indirect superclass (p. 361) of a subclass is two or more levels up the class hierarchy from that subclass. • In single inheritance (p. 361), a class is derived from one superclass. In multiple inheritance, a class is derived from more than one direct superclass. Java does not support multiple inheritance. • A subclass is more specific than its superclass and represents a smaller group of objects (p. 361). • Every object of a subclass is also an object of that class’s superclass. However, a superclass object is not an object of its class’s subclasses. • An is-a relationship (p. 362) represents inheritance. In an is-a relationship, an object of a subclass also can be treated as an object of its superclass. • A has-a relationship (p. 362) represents composition. In a has-a relationship, a class object contains references to objects of other classes. Section 9.2 Superclasses and Subclasses • Single-inheritance relationships form treelike hierarchical structures—a superclass exists in a hierarchical relationship with its subclasses. Section 9.3 protected Members • A superclass’s public members are accessible wherever the program has a reference to an object of that superclass or one of its subclasses. • A superclass’s private members can be accessed directly only within the superclass’s declaration. • A superclass’s protected members (p. 364) have an intermediate level of protection between public and private access. They can be accessed by members of the superclass, by members of its subclasses and by members of other classes in the same package. 392 Chapter 9 Object-Oriented Programming: Inheritance • A superclass’s private members are hidden in its subclasses and can be accessed only through the public or protected methods inherited from the superclass. • An overridden superclass method can be accessed from a subclass if the superclass method name is preceded by super (p. 364) and a dot (.) separator. Section 9.4 Relationship Between Superclasses and Subclasses • A subclass cannot access the private members of its superclass, but it can access the non-private members. • A subclass can invoke a constructor of its superclass by using the keyword super, followed by a set of parentheses containing the superclass constructor arguments. This must appear as the first statement in the subclass constructor’s body. • A superclass method can be overridden in a subclass to declare an appropriate implementation for the subclass. • The @Override annotation (p. 369) indicates that a method should override a superclass method. When the compiler encounters a method declared with @Override, it compares the method’s signature with the superclass’s method signatures. If there isn’t an exact match, the compiler issues an error message, such as “method does not override or implement a method from a supertype.” • Method toString takes no arguments and returns a String. The Object class’s toString method is normally overridden by a subclass. • When an object is output using the %s format specifier, the object’s toString method is called implicitly to obtain its String representation. Section 9.5 Constructors in Subclasses • The first task of a subclass constructor is to call its direct superclass’s constructor (p. 378) to ensure that the instance variables inherited from the superclass are initialized. Section 9.6 Class Object • See the table of class Object’s methods in Fig. 9.12. Self-Review Exercises 9.1 Fill in the blanks in each of the following statements: a) is a form of software reusability in which new classes acquire the members of existing classes and embellish those classes with new capabilities. members can be accessed in the superclass declaration and in b) A superclass’s subclass declarations. relationship, an object of a subclass can also be treated as an object of c) In a(n) its superclass. d) In a(n) relationship, a class object has references to objects of other classes as members. relationship with its subclasses. e) In single inheritance, a class exists in a(n) f) A superclass’s members are accessible anywhere that the program has a reference to an object of that superclass or to an object of one of its subclasses. is called implicitly or g) When an object of a subclass is instantiated, a superclass explicitly. keyword. h) Subclass constructors can call superclass constructors via the 9.2 State whether each of the following is true or false. If a statement is false, explain why. a) Superclass constructors are not inherited by subclasses. b) A has-a relationship is implemented via inheritance. Answers to Self-Review Exercises 393 c) A Car class has an is-a relationship with the SteeringWheel and Brakes classes. d) When a subclass redefines a superclass method by using the same signature, the subclass is said to overload that superclass method. Answers to Self-Review Exercises 9.1 a) Inheritance. b) public and protected. c) is-a or inheritance. d) has-a or composition. e) hierarchical. f) public. g) constructor. h) super. 9.2 a) True. b) False. A has-a relationship is implemented via composition. An is-a relationship is implemented via inheritance. c) False. This is an example of a has-a relationship. Class Car has an is-a relationship with class Vehicle. d) False. This is known as overriding, not overloading—an overloaded method has the same name, but a different signature. Exercises 9.3 (Using Composition Rather Than Inheritance) Many programs written with inheritance could be written with composition instead, and vice versa. Rewrite class BasePlusCommissionEmployee (Fig. 9.11) of the CommissionEmployee–BasePlusCommissionEmployee hierarchy to use composition rather than inheritance. 9.4 (Software Reuse) Discuss the ways in which inheritance promotes software reuse, saves time during program development and helps prevent errors. 9.5 (Student Inheritance Hierarchy) Draw an inheritance hierarchy for students at a university similar to the hierarchy shown in Fig. 9.2. Use Student as the superclass of the hierarchy, then extend Student with classes UndergraduateStudent and GraduateStudent. Continue to extend the hierarchy as deep (i.e., as many levels) as possible. For example, Freshman, Sophomore, Junior and Senior might extend UndergraduateStudent, and DoctoralStudent and MastersStudent might be subclasses of GraduateStudent. After drawing the hierarchy, discuss the relationships that exist between the classes. [Note: You do not need to write any code for this exercise.] 9.6 (Shape Inheritance Hierarchy) The world of shapes is much richer than the shapes included in the inheritance hierarchy of Fig. 9.3. Write down all the shapes you can think of—both two-dimensional and three-dimensional—and form them into a more complete Shape hierarchy with as many levels as possible. Your hierarchy should have class Shape at the top. Classes TwoDimensionalShape and ThreeDimensionalShape should extend Shape. Add additional subclasses, such as Quadrilateral and Sphere, at their correct locations in the hierarchy as necessary. 9.7 (protected vs. private) Some programmers prefer not to use protected access, because they believe it breaks the encapsulation of the superclass. Discuss the relative merits of using protected access vs. using private access in superclasses. (Quadrilateral Inheritance Hierarchy) Write an inheritance hierarchy for classes Quadriand Square. Use Quadrilateral as the superclass of the hierarchy. Create and use a Point class to represent the points in each shape. Make the hierarchy as deep (i.e., as many levels) as possible. Specify the instance variables and methods for each class. The private instance variables of Quadrilateral should be the x-y coordinate pairs for the four endpoints of the Quadrilateral. Write a program that instantiates objects of your classes and outputs each object’s area (except Quadrilateral). 9.8 lateral, Trapezoid, Parallelogram, Rectangle 9.9 (What Does Each Code Snippet Do?) a) Assume that the following method call is located in an overridden earnings method in a subclass: super.earnings() 394 Chapter 9 Object-Oriented Programming: Inheritance b) Assume that the following line of code appears before a method declaration: @Override c) Assume that the following line of code appears as the first statement in a constructor’s body: super(firstArgument, secondArgument); 9.10 (Write a Line of Code) Write a line of code that performs each of the following tasks: a) Specify that class PieceWorker inherits from class Employee. b) Call superclass Employee’s toString method from subclass PieceWorker’s toString method. c) Call superclass Employee’s constructor from subclass PieceWorker’s constructor—assume that the superclass constructor receives three Strings representing the first name, last name and social security number. 9.11 (Using super in a Constructor’s Body) Explain why you would use super in the first statement of a subclass constructor’s body. 9.12 (Using super in an Instance Method’s Body) Explain why you would use super in the body of a subclass’s instance method. (Calling get Methods in a Class’s Body) In Figs. 9.10–9.11 methods earnings and toeach call various get methods within the same class. Explain the benefits of calling these get methods within the classes. 9.13 String (Employee Hierarchy) In this chapter, you studied an inheritance hierarchy in which class inherited from class CommissionEmployee. However, not all types of employees are CommissionEmployees. In this exercise, you’ll create a more general Employee superclass that factors out the attributes and behaviors in class CommissionEmployee that are common to all Employees. The common attributes and behaviors for all Employees are firstName, lastName, socialSecurityNumber, getFirstName, getLastName, getSocialSecurityNumber and a portion of method toString. Create a new superclass Employee that contains these instance variables and methods and a constructor. Next, rewrite class CommissionEmployee from Section 9.4.5 as a subclass of Employee. Class CommissionEmployee should contain only the instance variables and methods that are not declared in superclass Employee. Class CommissionEmployee’s constructor should invoke class Employee’s constructor and CommissionEmployee’s toString method should invoke Employee’s toString method. Once you’ve completed these modifications, run the CommissionEmployeeTest and BasePlusCommissionEmployeeTest apps using these new classes to ensure that the apps still display the same results for a CommissionEmployee object and BasePlusCommissionEmployee object, respectively. 9.14 BasePlusCommissionEmployee (Creating a New Subclass of Employee) Other types of Employees might include Salariedwho get paid a fixed weekly salary, PieceWorkers who get paid by the number of pieces they produce or HourlyEmployees who get paid an hourly wage with time-and-a-half—1.5 times the hourly wage—for hours worked over 40 hours. Create class HourlyEmployee that inherits from class Employee (Exercise 9.14) and has instance variable hours (a double) that represents the hours worked, instance variable wage (a double) that represents the wages per hour, a constructor that takes as arguments a first name, a last name, a social security number, an hourly wage and the number of hours worked, set and get methods for manipulating the hours and wage, an earnings method to calculate an HourlyEmployee’s earnings based on the hours worked and a toString method that returns the HourlyEmployee’s String representation. Method setWage should ensure that wage is nonnegative, and setHours should ensure that the value of hours is between 0 and 168 (the total number of hours in a week). Use class HourlyEmployee in a test program that’s similar to the one in Fig. 9.5. 9.15 Employees Object-Oriented Programming: Polymorphism and Interfaces 10 General propositions do not decide concrete cases. —Oliver Wendell Holmes A philosopher of imposing stature doesn’t think in a vacuum. Even his most abstract ideas are, to some extent, conditioned by what is or is not known in the time when he lives. —Alfred North Whitehead Objectives In this chapter you’ll: ■ ■ ■ ■ ■ ■ ■ Learn the concept of polymorphism. Use overridden methods to effect polymorphism. Distinguish between abstract and concrete classes. Declare abstract methods to create abstract classes. Learn how polymorphism makes systems extensible and maintainable. Determine an object’s type at execution time. Declare and implement interfaces, and become familiar with the Java SE 8 interface enhancements. 396 Chapter 10 Object-Oriented Programming: Polymorphism and Interfaces 10.1 10.2 10.3 10.4 10.5 Introduction Polymorphism Examples Demonstrating Polymorphic Behavior Abstract Classes and Methods Case Study: Payroll System Using Polymorphism 10.5.1 Abstract Superclass Employee 10.5.2 Concrete Subclass SalariedEmployee 10.5.3 Concrete Subclass HourlyEmployee 10.5.4 Concrete Subclass CommissionEmployee 10.5.5 Indirect Concrete Subclass BasePlusCommissionEmployee 10.5.6 Polymorphic Processing, Operator instanceof and Downcasting 10.6 Allowed Assignments Between Superclass and Subclass Variables 10.7 final Methods and Classes 10.8 A Deeper Explanation of Issues with Calling Methods from Constructors 10.9 Creating and Using Interfaces 10.9.1 10.9.2 10.9.3 10.9.4 Developing a Payable Hierarchy Interface Payable Class Invoice Modifying Class Employee to Implement Interface Payable 10.9.5 Modifying Class SalariedEmployee for Use in the Payable Hierarchy 10.9.6 Using Interface Payable to Process Invoices and Employees Polymorphically 10.9.7 Some Common Interfaces of the Java API 10.10 Java SE 8 Interface Enhancements 10.10.1 default Interface Methods 10.10.2 static Interface Methods 10.10.3 Functional Interfaces 10.11 (Optional) GUI and Graphics Case Study: Drawing with Polymorphism 10.12 Wrap-Up Summary | Self-Review Exercises | Answers to Self-Review Exercises | Exercises | Making a Difference 10.1 Introduction We continue our study of object-oriented programming by explaining and demonstrating polymorphism with inheritance hierarchies. Polymorphism enables you to “program in the general” rather than “program in the specific.” In particular, polymorphism enables you to write programs that process objects that share the same superclass, either directly or indirectly, as if they were all objects of the superclass; this can simplify programming. Consider the following example of polymorphism. Suppose we create a program that simulates the movement of several types of animals for a biological study. Classes Fish, Frog and Bird represent the types of animals under investigation. Imagine that each class extends superclass Animal, which contains a method move and maintains an animal’s current location as x-y coordinates. Each subclass implements method move. Our program maintains an Animal array containing references to objects of the various Animal subclasses. To simulate the animals’ movements, the program sends each object the same message once per second—namely, move. Each specific type of Animal responds to a move message in its own way—a Fish might swim three feet, a Frog might jump five feet and a Bird might fly ten feet. Each object knows how to modify its x-y coordinates appropriately for its specific type of movement. Relying on each object to know how to “do the right thing” (i.e., do what’s appropriate for that type of object) in response to the same method call is the key concept of polymorphism. The same message (in this case, move) sent to a variety of objects has many forms of results—hence the term polymorphism. 10.1 Introduction 397 Implementing for Extensibility With polymorphism, we can design and implement systems that are easily extensible—new classes can be added with little or no modification to the general portions of the program, as long as the new classes are part of the inheritance hierarchy that the program processes generically. The new classes simply “plug right in.” The only parts of a program that must be altered are those that require direct knowledge of the new classes that we add to the hierarchy. For example, if we extend class Animal to create class Tortoise (which might respond to a move message by crawling one inch), we need to write only the Tortoise class and the part of the simulation that instantiates a Tortoise object. The portions of the simulation that tell each Animal to move generically can remain the same. Chapter Overview First, we discuss common examples of polymorphism. We then provide a simple example demonstrating polymorphic behavior. We use superclass references to manipulate both superclass objects and subclass objects polymorphically. We then present a case study that revisits the employee hierarchy of Section 9.4.5. We develop a simple payroll application that polymorphically calculates the weekly pay of several different types of employees using each employee’s earnings method. Though the earnings of each type of employee are calculated in a specific way, polymorphism allows us to process the employees “in the general.” In the case study, we enlarge the hierarchy to include two new classes—SalariedEmployee (for people paid a fixed weekly salary) and HourlyEmployee (for people paid an hourly salary and “time-and-a-half” for overtime). We declare a common set of functionality for all the classes in the updated hierarchy in an “abstract” class, Employee, from which “concrete” classes SalariedEmployee, HourlyEmployee and CommissionEmployee inherit directly and “concrete” class BasePlusCommissionEmployee inherits indirectly. As you’ll soon see, when we invoke each employee’s earnings method off a superclass Employee reference (regardless of the employee’s type), the correct earnings subclass calculation is performed, due to Java’s polymorphic capabilities. Programming in the Specific Occasionally, when performing polymorphic processing, we need to program “in the specific.” Our Employee case study demonstrates that a program can determine the type of an object at execution time and act on that object accordingly. In the case study, we’ve decided that BasePlusCommissionEmployees should receive 10% raises on their base salaries. So, we use these capabilities to determine whether a particular employee object is a BasePlusCommissionEmployee. If so, we increase that employee’s base salary by 10%. Interfaces The chapter continues with an introduction to Java interfaces, which are particularly useful for assigning common functionality to possibly unrelated classes. This allows objects of these classes to be processed polymorphically—objects of classes that implement the same interface can respond to all of the interface method calls. To demonstrate creating and using interfaces, we modify our payroll application to create a generalized accounts payable application that can calculate payments due for company employees and invoice amounts to be billed for purchased goods. 398 Chapter 10 Object-Oriented Programming: Polymorphism and Interfaces 10.2 Polymorphism Examples Let’s consider several additional examples of polymorphism. Quadrilaterals If class Rectangle is derived from class Quadrilateral, then a Rectangle object is a more specific version of a Quadrilateral. Any operation (e.g., calculating the perimeter or the area) that can be performed on a Quadrilateral can also be performed on a Rectangle. These operations can also be performed on other Quadrilaterals, such as Squares, Parallelograms and Trapezoids. The polymorphism occurs when a program invokes a method through a superclass Quadrilateral variable—at execution time, the correct subclass version of the method is called, based on the type of the reference stored in the superclass variable. You’ll see a simple code example that illustrates this process in Section 10.3. Space Objects in a Video Game Suppose we design a video game that manipulates objects of classes Martian, Venusian, Plutonian, SpaceShip and LaserBeam. Imagine that each class inherits from the superclass SpaceObject, which contains method draw. Each subclass implements this method. A screen manager maintains a collection (e.g., a SpaceObject array) of references to objects of the various classes. To refresh the screen, the screen manager periodically sends each object the same message—namely, draw. However, each object responds its own way, based on its class. For example, a Martian object might draw itself in red with green eyes and the appropriate number of antennae. A SpaceShip object might draw itself as a bright silver flying saucer. A LaserBeam object might draw itself as a bright red beam across the screen. Again, the same message (in this case, draw) sent to a variety of objects has “many forms” of results. A screen manager might use polymorphism to facilitate adding new classes to a system with minimal modifications to the system’s code. Suppose that we want to add Mercurian objects to our video game. To do so, we’d build a class Mercurian that extends SpaceObject and provides its own draw method implementation. When Mercurian objects appear in the SpaceObject collection, the screen-manager code invokes method draw, exactly as it does for every other object in the collection, regardless of its type. So the new Mercurian objects simply “plug right in” without any modification of the screen manager code by the programmer. Thus, without modifying the system (other than to build new classes and modify the code that creates new objects), you can use polymorphism to conveniently include additional types that were not even considered when the system was created. Software Engineering Observation 10.1 Polymorphism enables you to deal in generalities and let the execution-time environment handle the specifics. You can tell objects to behave in manners appropriate to those objects, without knowing their specific types, as long as they belong to the same inheritance hierarchy. Software Engineering Observation 10.2 Polymorphism promotes extensibility: Software that invokes polymorphic behavior is independent of the object types to which messages are sent. New object types that can respond to existing method calls can be incorporated into a system without modifying the base system. Only client code that instantiates new objects must be modified to accommodate new types. 10.3 Demonstrating Polymorphic Behavior 399 10.3 Demonstrating Polymorphic Behavior Section 9.4 created a class hierarchy, in which class BasePlusCommissionEmployee inherited from CommissionEmployee. The examples in that section manipulated CommissionEmployee and BasePlusCommissionEmployee objects by using references to them to invoke their methods—we aimed superclass variables at superclass objects and subclass variables at subclass objects. These assignments are natural and straightforward—superclass variables are intended to refer to superclass objects, and subclass variables are intended to refer to subclass objects. However, as you’ll soon see, other assignments are possible. In the next example, we aim a superclass reference at a subclass object. We then show how invoking a method on a subclass object via a superclass reference invokes the subclass functionality—the type of the referenced object, not the type of the variable, determines which method is called. This example demonstrates that an object of a subclass can be treated as an object of its superclass, enabling various interesting manipulations. A program can create an array of superclass variables that refer to objects of many subclass types. This is allowed because each subclass object is an object of its superclass. For instance, we can assign the reference of a BasePlusCommissionEmployee object to a superclass CommissionEmployee variable, because a BasePlusCommissionEmployee is a CommissionEmployee—so we can treat a BasePlusCommissionEmployee as a CommissionEmployee. As you’ll learn later in the chapter, you cannot treat a superclass object as a subclass object, because a superclass object is not an object of any of its subclasses. For example, we cannot assign the reference of a CommissionEmployee object to a subclass BasePlusCommissionEmployee variable, because a CommissionEmployee is not a BasePlusCommissionEmployee—a CommissionEmployee does not have a baseSalary instance variable and does not have methods setBaseSalary and getBaseSalary. The is-a relationship applies only up the hierarchy from a subclass to its direct (and indirect) superclasses, and not vice versa (i.e., not down the hierarchy from a superclass to its subclasses or indirect subclasses). The Java compiler does allow the assignment of a superclass reference to a subclass variable if we explicitly cast the superclass reference to the subclass type. Why would we ever want to perform such an assignment? A superclass reference can be used to invoke only the methods declared in the superclass—attempting to invoke subclass-only methods through a superclass reference results in compilation errors. If a program needs to perform a subclass-specific operation on a subclass object referenced by a superclass variable, the program must first cast the superclass reference to a subclass reference through a technique known as downcasting. This enables the program to invoke subclass methods that are not in the superclass. We demonstrate the mechanics of downcasting in Section 10.5. Software Engineering Observation 10.3 Although it’s allowed, you should generally avoid downcasting. The example in Fig. 10.1 demonstrates three ways to use superclass and subclass variables to store references to superclass and subclass objects. The first two are straightforward—as in Section 9.4, we assign a superclass reference to a superclass variable, and a subclass reference to a subclass variable. Then we demonstrate the relationship between subclasses and superclasses (i.e., the is-a relationship) by assigning a subclass reference to a superclass variable. This program uses classes CommissionEmployee and BasePlusCommissionEmployee from Fig. 9.10 and Fig. 9.11, respectively. 400 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 Chapter 10 Object-Oriented Programming: Polymorphism and Interfaces // Fig. 10.1: PolymorphismTest.java // Assigning superclass and subclass references to superclass and // subclass variables. public class PolymorphismTest { public static void main(String[] args) { // assign superclass reference to superclass variable CommissionEmployee commissionEmployee = new CommissionEmployee( "Sue", "Jones", "222-22-2222", 10000, .06); // assign subclass reference to subclass variable BasePlusCommissionEmployee basePlusCommissionEmployee = new BasePlusCommissionEmployee( "Bob", "Lewis", "333-33-3333", 5000, .04, 300); // invoke toString on superclass object using superclass variable System.out.printf("%s %s:%n%n%s%n%n", "Call CommissionEmployee's toString with superclass reference ", "to superclass object", commissionEmployee.toString()); // invoke toString on subclass object using subclass variable System.out.printf("%s %s:%n%n%s%n%n", "Call BasePlusCommissionEmployee's toString with subclass", "reference to subclass object", basePlusCommissionEmployee.toString()); // invoke toString on subclass object using superclass variable CommissionEmployee commissionEmployee2 = basePlusCommissionEmployee; System.out.printf("%s %s:%n%n%s%n", "Call BasePlusCommissionEmployee's toString with superclass", "reference to subclass object", commissionEmployee2.toString()); } // end main } // end class PolymorphismTest Call CommissionEmployee's toString with superclass reference to superclass object: commission employee: Sue Jones social security number: 222-22-2222 gross sales: 10000.00 commission rate: 0.06 Call BasePlusCommissionEmployee's toString with subclass reference to subclass object: base-salaried commission employee: Bob Lewis social security number: 333-33-3333 gross sales: 5000.00 commission rate: 0.04 base salary: 300.00 Fig. 10.1 | Assigning superclass and subclass references to superclass and subclass variables. (Part 1 of 2.) 10.4 Abstract Classes and Methods 401 Call BasePlusCommissionEmployee's toString with superclass reference to subclass object: base-salaried commission employee: Bob Lewis social security number: 333-33-3333 gross sales: 5000.00 commission rate: 0.04 base salary: 300.00 Fig. 10.1 | Assigning superclass and subclass references to superclass and subclass variables. (Part 2 of 2.) In Fig. 10.1, lines 10–11 create a CommissionEmployee object and assign its reference to a CommissionEmployee variable. Lines 14–16 create a BasePlusCommissionEmployee object and assign its reference to a BasePlusCommissionEmployee variable. These assignments are natural—for example, a CommissionEmployee variable’s primary purpose is to hold a reference to a CommissionEmployee object. Lines 19–21 use commissionEmployee to invoke toString explicitly. Because commissionEmployee refers to a CommissionEmployee object, superclass CommissionEmployee’s version of toString is called. Similarly, lines 24–27 use basePlusCommissionEmployee to invoke toString explicitly on the BasePlusCommissionEmployee object. This invokes subclass BasePlusCommissionEmployee’s version of toString. Lines 30–31 then assign the reference of subclass object basePlusCommissionEmployee to a superclass CommissionEmployee variable, which lines 32–34 use to invoke method toString. When a superclass variable contains a reference to a subclass object, and that reference is used to call a method, the subclass version of the method is called. Hence, commissionEmployee2.toString() in line 34 actually calls class BasePlusCommissionEmployee’s toString method. The Java compiler allows this “crossover” because an object of a subclass is an object of its superclass (but not vice versa). When the compiler encounters a method call made through a variable, the compiler determines if the method can be called by checking the variable’s class type. If that class contains the proper method declaration (or inherits one), the call is compiled. At execution time, the type of the object to which the variable refers determines the actual method to use. This process, called dynamic binding, is discussed in detail in Section 10.5. 10.4 Abstract Classes and Methods When we think of a class, we assume that programs will create objects of that type. Sometimes it’s useful to declare classes—called abstract classes—for which you never intend to create objects. Because they’re used only as superclasses in inheritance hierarchies, we refer to them as abstract superclasses. These classes cannot be used to instantiate objects, because, as we’ll soon see, abstract classes are incomplete. Subclasses must declare the “missing pieces” to become “concrete” classes, from which you can instantiate objects. Otherwise, these subclasses, too, will be abstract. We demonstrate abstract classes in Section 10.5. Purpose of Abstract Classes An abstract class’s purpose is to provide an appropriate superclass from which other classes can inherit and thus share a common design. In the Shape hierarchy of Fig. 9.3, for exam- 402 Chapter 10 Object-Oriented Programming: Polymorphism and Interfaces ple, subclasses inherit the notion of what it means to be a Shape—perhaps common attributes such as location, color and borderThickness, and behaviors such as draw, move, resize and changeColor. Classes that can be used to instantiate objects are called concrete classes. Such classes provide implementations of every method they declare (some of the implementations can be inherited). For example, we could derive concrete classes Circle, Square and Triangle from abstract superclass TwoDimensionalShape. Similarly, we could derive concrete classes Sphere, Cube and Tetrahedron from abstract superclass ThreeDimensionalShape. Abstract superclasses are too general to create real objects—they specify only what is common among subclasses. We need to be more specific before we can create objects. For example, if you send the draw message to abstract class TwoDimensionalShape, the class knows that two-dimensional shapes should be drawable, but it does not know what specific shape to draw, so it cannot implement a real draw method. Concrete classes provide the specifics that make it reasonable to instantiate objects. Not all hierarchies contain abstract classes. However, you’ll often write client code that uses only abstract superclass types to reduce the client code’s dependencies on a range of subclass types. For example, you can write a method with a parameter of an abstract superclass type. When called, such a method can receive an object of any concrete class that directly or indirectly extends the superclass specified as the parameter’s type. Abstract classes sometimes constitute several levels of a hierarchy. For example, the Shape hierarchy of Fig. 9.3 begins with abstract class Shape. On the next level of the hierarchy are abstract classes TwoDimensionalShape and ThreeDimensionalShape. The next level of the hierarchy declares concrete classes for TwoDimensionalShapes (Circle, Square and Triangle) and for ThreeDimensionalShapes (Sphere, Cube and Tetrahedron). Declaring an Abstract Class and Abstract Methods You make a class abstract by declaring it with keyword abstract. An abstract class normally contains one or more abstract methods. An abstract method is an instance method with keyword abstract in its declaration, as in public abstract void draw(); // abstract method Abstract methods do not provide implementations. A class that contains any abstract methods must be explicitly declared abstract even if that class contains some concrete (nonabstract) methods. Each concrete subclass of an abstract superclass also must provide concrete implementations of each of the superclass’s abstract methods. Constructors and static methods cannot be declared abstract. Constructors are not inherited, so an abstract constructor could never be implemented. Though non-private static methods are inherited, they cannot be overridden. Since abstract methods are meant to be overridden so that they can process objects based on their types, it would not make sense to declare a static method as abstract. Software Engineering Observation 10.4 An abstract class declares common attributes and behaviors (both abstract and concrete) of the various classes in a class hierarchy. An abstract class typically contains one or more abstract methods that subclasses must override if they are to be concrete. The instance variables and concrete methods of an abstract class are subject to the normal rules of inheritance. 10.4 Abstract Classes and Methods 403 Common Programming Error 10.1 Attempting to instantiate an object of an abstract class is a compilation error. Common Programming Error 10.2 Failure to implement a superclass’s abstract methods in a subclass is a compilation error unless the subclass is also declared abstract. Using Abstract Classes to Declare Variables Although we cannot instantiate objects of abstract superclasses, you’ll soon see that we can use abstract superclasses to declare variables that can hold references to objects of any concrete class derived from those abstract superclasses. We’ll use such variables to manipulate subclass objects polymorphically. You also can use abstract superclass names to invoke static methods declared in those abstract superclasses. Consider another application of polymorphism. A drawing program needs to display many shapes, including types of new shapes that you’ll add to the system after writing the drawing program. The drawing program might need to display shapes, such as Circles, Triangles, Rectangles or others, that derive from abstract class Shape. The drawing program uses Shape variables to manage the objects that are displayed. To draw any object in this inheritance hierarchy, the drawing program uses a superclass Shape variable containing a reference to the subclass object to invoke the object’s draw method. This method is declared abstract in superclass Shape, so each concrete subclass must implement method draw in a manner specific to that shape—each object in the Shape inheritance hierarchy knows how to draw itself. The drawing program does not have to worry about the type of each object or whether the program has ever encountered objects of that type. Layered Software Systems Polymorphism is particularly effective for implementing so-called layered software systems. In operating systems, for example, each type of physical device could operate quite differently from the others. Even so, commands to read or write data from and to devices may have a certain uniformity. For each device, the operating system uses a piece of software called a device driver to control all communication between the system and the device. The write message sent to a device-driver object needs to be interpreted specifically in the context of that driver and how it manipulates devices of a specific type. However, the write call itself really is no different from the write to any other device in the system—place some number of bytes from memory onto that device. An object-oriented operating system might use an abstract superclass to provide an “interface” appropriate for all device drivers. Then, through inheritance from that abstract superclass, subclasses are formed that all behave similarly. The device-driver methods are declared as abstract methods in the abstract superclass. The implementations of these abstract methods are provided in the concrete subclasses that correspond to the specific types of device drivers. New devices are always being developed, often long after the operating system has been released. When you buy a new device, it comes with a device driver provided by the device vendor. The device is immediately operational after you connect it to your computer and install the driver. This is another elegant example of how polymorphism makes systems extensible. 404 Chapter 10 Object-Oriented Programming: Polymorphism and Interfaces 10.5 Case Study: Payroll System Using Polymorphism This section reexamines the CommissionEmployee-BasePlusCommissionEmployee hierarchy that we explored throughout Section 9.4. Now we use an abstract method and polymorphism to perform payroll calculations based on an enhanced employee inheritance hierarchy that meets the following requirements: A company pays its employees on a weekly basis. The employees are of four types: Salaried employees are paid a fixed weekly salary regardless of the number of hours worked, hourly employees are paid by the hour and receive overtime pay (i.e., 1.5 times their hourly salary rate) for all hours worked in excess of 40 hours, commission employees are paid a percentage of their sales and base-salaried commission employees receive a base salary plus a percentage of their sales. For the current pay period, the company has decided to reward salaried-commission employees by adding 10% to their base salaries. The company wants you to write an application that performs its payroll calculations polymorphically. We use abstract class Employee to represent the general concept of an employee. The classes that extend Employee are SalariedEmployee, CommissionEmployee and HourlyEmployee. Class BasePlusCommissionEmployee—which extends CommissionEmployee— represents the last employee type. The UML class diagram in Fig. 10.2 shows the inheritance hierarchy for our polymorphic employee-payroll application. Abstract class name Employee is italicized—a convention of the UML. Employee SalariedEmployee CommissionEmployee HourlyEmployee BasePlusCommissionEmployee Fig. 10.2 | Employee hierarchy UML class diagram. Abstract superclass Employee declares the “interface” to the hierarchy—that is, the set of methods that a program can invoke on all Employee objects. We use the term “interface” here in a general sense to refer to the various ways programs can communicate with objects of any Employee subclass. Be careful not to confuse the general notion of an “interface” with the formal notion of a Java interface, the subject of Section 10.9. Each employee, regardless of the way his or her earnings are calculated, has a first name, a last name and a social security number, so private instance variables firstName, lastName and socialSecurityNumber appear in abstract superclass Employee. The diagram in Fig. 10.3 shows each of the five classes in the hierarchy down the left side and methods earnings and toString across the top. For each class, the diagram shows the desired results of each method. We do not list superclass Employee’s get methods because they’re not overridden in any of the subclasses—each of these methods is inherited and used “as is” by each subclass. 10.5 Case Study: Payroll System Using Polymorphism earnings Employee abstract SalariedEmployee weeklySalary HourlyEmployee CommissionEmployee if (hours <= 40) wage * hours else if (hours > 40) { 40 * wage + ( hours - 40 ) * wage * 1.5 } commissionRate * grossSales 405 toString firstName lastName social security number: SSN salaried employee: firstName lastName social security number: SSN weekly salary: weeklySalary hourly employee: firstName lastName social security number: SSN hourly wage: wage; hours worked: hours commission employee: firstName lastName social security number: SSN gross sales: grossSales; commission rate: commissionRate base salaried commission employee: BasePlusCommissionEmployee (commissionRate * grossSales) + baseSalary firstName lastName social security number: SSN gross sales: grossSales; commission rate: commissionRate; base salary: baseSalary Fig. 10.3 | Polymorphic interface for the Employee hierarchy classes. The following sections implement the Employee class hierarchy of Fig. 10.2. The first section implements abstract superclass Employee. The next four sections each implement one of the concrete classes. The last section implements a test program that builds objects of all these classes and processes those objects polymorphically. 10.5.1 Abstract Superclass Employee Class Employee (Fig. 10.4) provides methods earnings and toString, in addition to the get methods that return the values of Employee’s instance variables. An earnings method certainly applies generically to all employees. But each earnings calculation depends on the employee’s particular class. So we declare earnings as abstract in superclass Employee because a specific default implementation does not make sense for that method—there isn’t enough information to determine what amount earnings should return. Each subclass overrides earnings with an appropriate implementation. To calculate an employee’s earnings, the program assigns to a superclass Employee variable a reference to the employee’s object, then invokes the earnings method on that variable. We maintain an array of Employee variables, each holding a reference to an Employee object. You cannot use class Employee directly to create Employee objects, because Employee is an abstract class. Due to inheritance, however, all objects of all Employee subclasses may be thought of as Employee 406 Chapter 10 Object-Oriented Programming: Polymorphism and Interfaces objects. The program will iterate through the array and call method earnings for each Employee object. Java processes these method calls polymorphically. Declaring earnings as an abstract method in Employee enables the calls to earnings through Employee variables to compile and forces every direct concrete subclass of Employee to override earnings. Method toString in class Employee returns a String containing the first name, last name and social security number of the employee. As we’ll see, each subclass of Employee overrides method toString to create a String representation of an object of that class that contains the employee’s type (e.g., "salaried employee:") followed by the rest of the employee’s information. Let’s consider class Employee’s declaration (Fig. 10.4). The class includes a constructor that receives the first name, last name and social security number (lines 11–17); get methods that return the first name, last name and social security number (lines 20–23, 26–29 and 32–35, respectively); method toString (lines 38–43), which returns the String representation of an Employee; and abstract method earnings (line 46), which will be implemented by each of the concrete subclasses. The Employee constructor does not validate its parameters in this example; normally, such validation should be provided. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 // Fig. 10.4: Employee.java // Employee abstract superclass. public abstract class Employee { private final String firstName; private final String lastName; private final String socialSecurityNumber; // constructor public Employee(String firstName, String lastName, String socialSecurityNumber) { this.firstName = firstName; this.lastName = lastName; this.socialSecurityNumber = socialSecurityNumber; } // return first name public String getFirstName() { return firstName; } // return last name public String getLastName() { return lastName; } // return social security number public String getSocialSecurityNumber() { Fig. 10.4 | Employee abstract superclass. (Part 1 of 2.) 10.5 Case Study: Payroll System Using Polymorphism 34 35 36 37 38 39 40 41 42 43 44 45 46 47 407 return socialSecurityNumber; } // return String representation of Employee object @Override public String toString() { return String.format("%s %s%nsocial security number: %s", getFirstName(), getLastName(), getSocialSecurityNumber()); } // abstract method must be overridden by concrete subclasses public abstract double earnings(); // no implementation here } // end abstract class Employee Fig. 10.4 | Employee abstract superclass. (Part 2 of 2.) Why did we decide to declare earnings as an abstract method? It simply does not make sense to provide a specific implementation of this method in class Employee. We cannot calculate the earnings for a general Employee—we first must know the specific type of Employee to determine the appropriate earnings calculation. By declaring this method abstract, we indicate that each concrete subclass must provide an appropriate earnings implementation and that a program will be able to use superclass Employee variables to invoke method earnings polymorphically for any type of Employee. 10.5.2 Concrete Subclass SalariedEmployee Class SalariedEmployee (Fig. 10.5) extends class Employee (line 4) and overrides abstract method earnings (lines 38–42), which makes SalariedEmployee a concrete class. The class includes a constructor (lines 9–19) that receives a first name, a last name, a social security number and a weekly salary; a set method to assign a new nonnegative value to instance variable weeklySalary (lines 22–29); a get method to return weeklySalary’s value (lines 32– 35); a method earnings (lines 38–42) to calculate a SalariedEmployee’s earnings; and a method toString (lines 45–50), which returns a String including "salaried employee: " followed by employee-specific information produced by superclass Employee’s toString method and Salaried-Employee’s getWeeklySalary method. Class SalariedEmployee’s constructor passes the first name, last name and social security number to the Employee constructor (line 12) to initialize the private instance variables of the superclass. Once again, we’ve duplicated the weeklySalary validation code in the constructor and the setWeeklySalary method. Recall that more complex validation could be placed in a static class method that’s called from the constructor and the set method. Error-Prevention Tip 10.1 We’ve said that you should not call a class’s instance methods from its constructors—you can call static class methods and make the required call to one of the superclass’s constructors. If you follow this advice, you’ll avoid the problem of calling the class’s overridable methods either directly or indirectly, which can lead to runtime errors. Method earnings overrides Employee’s abstract method earnings to provide a concrete implementation that returns the SalariedEmployee’s weekly salary. If we do not 408 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 Chapter 10 Object-Oriented Programming: Polymorphism and Interfaces // Fig. 10.5: SalariedEmployee.java // SalariedEmployee concrete class extends abstract class Employee. public class SalariedEmployee extends Employee { private double weeklySalary; // constructor public SalariedEmployee(String firstName, String lastName, String socialSecurityNumber, double weeklySalary) { super(firstName, lastName, socialSecurityNumber); if (weeklySalary < 0.0) throw new IllegalArgumentException( "Weekly salary must be >= 0.0"); this.weeklySalary = weeklySalary; } // set salary public void setWeeklySalary(double weeklySalary) { if (weeklySalary < 0.0) throw new IllegalArgumentException( "Weekly salary must be >= 0.0"); this.weeklySalary = weeklySalary; } // return salary public double getWeeklySalary() { return weeklySalary; } // calculate earnings; override abstract method earnings in Employee @Override public double earnings() { return getWeeklySalary(); } // return String representation of SalariedEmployee object @Override public String toString() { return String.format("salaried employee: %s%n%s: $%,.2f", super.toString(), "weekly salary", getWeeklySalary()); } } // end class SalariedEmployee Fig. 10.5 | SalariedEmployee concrete class extends abstract class Employee. 10.5 Case Study: Payroll System Using Polymorphism 409 implement earnings, class SalariedEmployee must be declared abstract—otherwise, class SalariedEmployee will not compile. Of course, we want SalariedEmployee to be a concrete class in this example. Method toString (lines 45–50) overrides Employee method toString. If class SalariedEmployee did not override toString, SalariedEmployee would have inherited the Employee version of toString. In that case, SalariedEmployee’s toString method would simply return the employee’s full name and social security number, which does not adequately represent a SalariedEmployee. To produce a complete String representation of a SalariedEmployee, the subclass’s toString method returns "salaried employee: " followed by the superclass Employee-specific information (i.e., first name, last name and social security number) obtained by invoking the superclass’s toString method (line 49)—this is a nice example of code reuse. The String representation of a SalariedEmployee also contains the employee’s weekly salary obtained by invoking the class’s getWeeklySalary method. 10.5.3 Concrete Subclass HourlyEmployee Class HourlyEmployee (Fig. 10.6) also extends Employee (line 4). The class includes a constructor (lines 10–25) that receives a first name, a last name, a social security number, an hourly wage and the number of hours worked. Lines 28–35 and 44–51 declare set methods that assign new values to instance variables wage and hours, respectively. Method setWage (lines 28–35) ensures that wage is nonnegative, and method setHours (lines 44–51) ensures that the value of hours is between 0 and 168 (the total number of hours in a week) inclusive. Class HourlyEmployee also includes get methods (lines 38–41 and 54–57) to return the values of wage and hours, respectively; a method earnings (lines 60–67) to calculate an HourlyEmployee’s earnings; and a method toString (lines 70–76), which returns a String containing the employee’s type ("hourly employee: ") and the employee-specific information. The HourlyEmployee constructor, like the SalariedEmployee constructor, passes the first name, last name and social security number to the superclass Employee constructor (line 13) to initialize the private instance variables. In addition, method toString calls superclass method toString (line 74) to obtain the Employee-specific information (i.e., first name, last name and social security number)—this is another nice example of code reuse. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 // Fig. 10.6: HourlyEmployee.java // HourlyEmployee class extends Employee. public class HourlyEmployee extends Employee { private double wage; // wage per hour private double hours; // hours worked for week // constructor public HourlyEmployee(String firstName, String lastName, String socialSecurityNumber, double wage, double hours) { super(firstName, lastName, socialSecurityNumber); Fig. 10.6 | HourlyEmployee class extends Employee. (Part 1 of 3.) 410 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 Chapter 10 Object-Oriented Programming: Polymorphism and Interfaces if (wage < 0.0) // validate wage throw new IllegalArgumentException( "Hourly wage must be >= 0.0"); if ((hours < 0.0) || (hours > 168.0)) // validate hours throw new IllegalArgumentException( "Hours worked must be >= 0.0 and <= 168.0"); this.wage = wage; this.hours = hours; } // set wage public void setWage(double wage) { if (wage < 0.0) // validate wage throw new IllegalArgumentException( "Hourly wage must be >= 0.0"); this.wage = wage; } // return wage public double getWage() { return wage; } // set hours worked public void setHours(double hours) { if ((hours < 0.0) || (hours > 168.0)) // validate hours throw new IllegalArgumentException( "Hours worked must be >= 0.0 and <= 168.0"); this.hours = hours; } // return hours worked public double getHours() { return hours; } // calculate earnings; override abstract method earnings in Employee @Override public double earnings() { if (getHours() <= 40) // no overtime return getWage() * getHours(); else return 40 * getWage() + (getHours() - 40) * getWage() * 1.5; } Fig. 10.6 | HourlyEmployee class extends Employee. (Part 2 of 3.) 10.5 Case Study: Payroll System Using Polymorphism 68 69 70 71 72 73 74 75 76 77 411 // return String representation of HourlyEmployee object @Override public String toString() { return String.format("hourly employee: %s%n%s: $%,.2f; %s: %,.2f", super.toString(), "hourly wage", getWage(), "hours worked", getHours()); } } // end class HourlyEmployee Fig. 10.6 | HourlyEmployee class extends Employee. (Part 3 of 3.) 10.5.4 Concrete Subclass CommissionEmployee Class CommissionEmployee (Fig. 10.7) extends class Employee (line 4). The class includes a constructor (lines 10–25) that takes a first name, a last name, a social security number, a sales amount and a commission rate; set methods (lines 28–34 and 43–50) to assign valid new values to instance variables commissionRate and grossSales, respectively; get methods (lines 37–40 and 53–56) that retrieve the values of these instance variables; method earnings (lines 59–63) to calculate a CommissionEmployee’s earnings; and method toString (lines 66–73), which returns the employee’s type, namely, "commission employee: " and employee-specific information. The constructor also passes the first name, last name and social security number to superclass Employee’s constructor (line 14) to initialize Employee’s private instance variables. Method toString calls superclass method toString (line 70) to obtain the Employee-specific information (i.e., first name, last name and social security number). 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 // Fig. 10.7: CommissionEmployee.java // CommissionEmployee class extends Employee. public class CommissionEmployee extends Employee { private double grossSales; // gross weekly sales private double commissionRate; // commission percentage // constructor public CommissionEmployee(String firstName, String lastName, String socialSecurityNumber, double grossSales, double commissionRate) { super(firstName, lastName, socialSecurityNumber); Fig. 10.7 | if (commissionRate <= 0.0 || commissionRate >= 1.0) // validate throw new IllegalArgumentException( "Commission rate must be > 0.0 and < 1.0"); if (grossSales < 0.0) // validate throw new IllegalArgumentException("Gross sales must be >= 0.0"); CommissionEmployee class extends Employee. (Part 1 of 2.) 412 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 Chapter 10 Object-Oriented Programming: Polymorphism and Interfaces this.grossSales = grossSales; this.commissionRate = commissionRate; } // set gross sales amount public void setGrossSales(double grossSales) { if (grossSales < 0.0) // validate throw new IllegalArgumentException("Gross sales must be >= 0.0"); this.grossSales = grossSales; } // return gross sales amount public double getGrossSales() { return grossSales; } // set commission rate public void setCommissionRate(double commissionRate) { if (commissionRate <= 0.0 || commissionRate >= 1.0) // validate throw new IllegalArgumentException( "Commission rate must be > 0.0 and < 1.0"); this.commissionRate = commissionRate; } // return commission rate public double getCommissionRate() { return commissionRate; } // calculate earnings; override abstract method earnings in Employee @Override public double earnings() { return getCommissionRate() * getGrossSales(); } // return String representation of CommissionEmployee object @Override public String toString() { return String.format("%s: %s%n%s: $%,.2f; %s: %.2f", "commission employee", super.toString(), "gross sales", getGrossSales(), "commission rate", getCommissionRate()); } } // end class CommissionEmployee Fig. 10.7 | CommissionEmployee class extends Employee. (Part 2 of 2.) 10.5 Case Study: Payroll System Using Polymorphism 413 10.5.5 Indirect Concrete Subclass BasePlusCommissionEmployee Class BasePlusCommissionEmployee (Fig. 10.8) extends class CommissionEmployee (line 4) and therefore is an indirect subclass of class Employee. Class BasePlusCommissionEmployee has a constructor (lines 9–20) that receives a first name, a last name, a social security number, a sales amount, a commission rate and a base salary. It then passes all of these except the base salary to the CommissionEmployee constructor (lines 13–14) to initialize the superclass instance variables. BasePlusCommissionEmployee also contains a set method (lines 23–29) to assign a new value to instance variable baseSalary and a get method (lines 32–35) to return baseSalary’s value. Method earnings (lines 38–42) calculates a BasePlusCommissionEmployee’s earnings. Line 41 in method earnings calls superclass CommissionEmployee’s earnings method to calculate the commission-based portion of the employee’s earnings—this is another nice example of code reuse. BasePlusCommissionEmployee’s toString method (lines 45–51) creates a String representation of a BasePlusCommissionEmployee that contains "base-salaried", followed by the String obtained by invoking superclass CommissionEmployee’s toString method (line 49), then the base salary. The result is a String beginning with "base-salaried commission employee" followed by the rest of the BasePlusCommissionEmployee’s information. Recall that CommissionEmployee’s toString obtains the employee’s first name, last name and social security number by invoking the toString method of its superclass (i.e., Employee)—yet another example of code reuse. BasePlusCommissionEmployee’s toString initiates a chain of method calls that span all three levels of the Employee hierarchy. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 // Fig. 10.8: BasePlusCommissionEmployee.java // BasePlusCommissionEmployee class extends CommissionEmployee. public class BasePlusCommissionEmployee extends CommissionEmployee { private double baseSalary; // base salary per week // constructor public BasePlusCommissionEmployee(String firstName, String lastName, String socialSecurityNumber, double grossSales, double commissionRate, double baseSalary) { super(firstName, lastName, socialSecurityNumber, grossSales, commissionRate); if (baseSalary < 0.0) // validate baseSalary throw new IllegalArgumentException("Base salary must be >= 0.0"); this.baseSalary = baseSalary; } // set base salary public void setBaseSalary(double baseSalary) { if (baseSalary < 0.0) // validate baseSalary throw new IllegalArgumentException("Base salary must be >= 0.0"); Fig. 10.8 | BasePlusCommissionEmployee class extends CommissionEmployee. (Part 1 of 2.) 414 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 Chapter 10 Object-Oriented Programming: Polymorphism and Interfaces this.baseSalary = baseSalary; } // return base salary public double getBaseSalary() { return baseSalary; } // calculate earnings; override method earnings in CommissionEmployee @Override public double earnings() { return getBaseSalary() + super.earnings(); } // return String representation of BasePlusCommissionEmployee object @Override public String toString() { return String.format("%s %s; %s: $%,.2f", "base-salaried", super.toString(), "base salary", getBaseSalary()); } } // end class BasePlusCommissionEmployee Fig. 10.8 | BasePlusCommissionEmployee class extends CommissionEmployee. (Part 2 of 2.) 10.5.6 Polymorphic Processing, Operator instanceof and Downcasting To test our Employee hierarchy, the application in Fig. 10.9 creates an object of each of the four concrete classes SalariedEmployee, HourlyEmployee, CommissionEmployee and BasePlusCommissionEmployee. The program manipulates these objects nonpolymorphically, via variables of each object’s own type, then polymorphically, using an array of Employee variables. While processing the objects polymorphically, the program increases the base salary of each BasePlusCommissionEmployee by 10%—this requires determining the object’s type at execution time. Finally, the program polymorphically determines and outputs the type of each object in the Employee array. Lines 9–18 create objects of each of the four concrete Employee subclasses. Lines 22–30 output the String representation and earnings of each of these objects nonpolymorphically. Each object’s toString method is called implicitly by printf when the object is output as a String with the %s format specifier. 1 2 3 4 5 // Fig. 10.9: PayrollSystemTest.java // Employee hierarchy test program. public class PayrollSystemTest { Fig. 10.9 | Employee hierarchy test program. (Part 1 of 4.) 10.5 Case Study: Payroll System Using Polymorphism 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 415 public static void main(String[] args) { // create subclass objects SalariedEmployee salariedEmployee = new SalariedEmployee("John", "Smith", "111-11-1111", 800.00); HourlyEmployee hourlyEmployee = new HourlyEmployee("Karen", "Price", "222-22-2222", 16.75, 40); CommissionEmployee commissionEmployee = new CommissionEmployee( "Sue", "Jones", "333-33-3333", 10000, .06); BasePlusCommissionEmployee basePlusCommissionEmployee = new BasePlusCommissionEmployee( "Bob", "Lewis", "444-44-4444", 5000, .04, 300); Fig. 10.9 | System.out.println("Employees processed individually:"); System.out.printf("%n%s%n%s: $%,.2f%n%n", salariedEmployee, "earned", salariedEmployee.earnings()); System.out.printf("%s%n%s: $%,.2f%n%n", hourlyEmployee, "earned", hourlyEmployee.earnings()); System.out.printf("%s%n%s: $%,.2f%n%n", commissionEmployee, "earned", commissionEmployee.earnings()); System.out.printf("%s%n%s: $%,.2f%n%n", basePlusCommissionEmployee, "earned", basePlusCommissionEmployee.earnings()); // create four-element Employee array Employee[] employees = new Employee[4]; // initialize array with Employees employees[0] = salariedEmployee; employees[1] = hourlyEmployee; employees[2] = commissionEmployee; employees[3] = basePlusCommissionEmployee; System.out.printf("Employees processed polymorphically:%n%n"); // generically process each element in array employees for (Employee currentEmployee : employees) { System.out.println(currentEmployee); // invokes toString // determine whether element is a BasePlusCommissionEmployee if currentEmployee instanceof BasePlusCommissionEmployee() { // downcast Employee reference to // BasePlusCommissionEmployee reference BasePlusCommissionEmployee employee = (BasePlusCommissionEmployee) currentEmployee ; employee.setBaseSalary(1.10 * employee.getBaseSalary()); Employee hierarchy test program. (Part 2 of 4.) 416 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 Chapter 10 Object-Oriented Programming: Polymorphism and Interfaces System.out.printf( "new base salary with 10%% increase is: $%,.2f%n", employee.getBaseSalary()); } // end if System.out.printf( "earned $%,.2f%n%n", currentEmployee.earnings()); } // end for // get type name of each object in employees array for (int j = 0; j < employees.length; j++) System.out.printf("Employee %d is a %s%n", j, employees[j].getClass().getName()); } // end main } // end class PayrollSystemTest Employees processed individually: salaried employee: John Smith social security number: 111-11-1111 weekly salary: $800.00 earned: $800.00 hourly employee: Karen Price social security number: 222-22-2222 hourly wage: $16.75; hours worked: 40.00 earned: $670.00 commission employee: Sue Jones social security number: 333-33-3333 gross sales: $10,000.00; commission rate: 0.06 earned: $600.00 base-salaried commission employee: Bob Lewis social security number: 444-44-4444 gross sales: $5,000.00; commission rate: 0.04; base salary: $300.00 earned: $500.00 Employees processed polymorphically: salaried employee: John Smith social security number: 111-11-1111 weekly salary: $800.00 earned $800.00 hourly social hourly earned employee: Karen Price security number: 222-22-2222 wage: $16.75; hours worked: 40.00 $670.00 commission employee: Sue Jones social security number: 333-33-3333 gross sales: $10,000.00; commission rate: 0.06 earned $600.00 Fig. 10.9 | Employee hierarchy test program. (Part 3 of 4.) 10.5 Case Study: Payroll System Using Polymorphism 417 base-salaried commission employee: Bob Lewis social security number: 444-44-4444 gross sales: $5,000.00; commission rate: 0.04; base salary: $300.00 new base salary with 10% increase is: $330.00 earned $530.00 Employee Employee Employee Employee Fig. 10.9 | 0 1 2 3 is is is is a a a a SalariedEmployee HourlyEmployee CommissionEmployee BasePlusCommissionEmployee Employee hierarchy test program. (Part 4 of 4.) Creating the Array of Employees Line 33 declares employees and assigns it an array of four Employee variables. Line 36 assigns to employees[0] a reference to a SalariedEmployee object. Line 37 assigns to employees[1] a reference to an HourlyEmployee object. Line 38 assigns to employees[2] a reference to a CommissionEmployee object. Line 39 assigns to employee[3] a reference to a BasePlusCommissionEmployee object. These assignments are allowed, because a SalariedEmployee is an Employee, an HourlyEmployee is an Employee, a CommissionEmployee is an Employee and a BasePlusCommissionEmployee is an Employee. Therefore, we can assign the references of SalariedEmployee, HourlyEmployee, CommissionEmployee and BasePlusCommissionEmployee objects to superclass Employee variables, even though Employee is an abstract class. Polymorphically Processing Employees Lines 44–65 iterate through array employees and invoke methods toString and earnings with Employee variable currentEmployee, which is assigned the reference to a different Employee in the array on each iteration. The output illustrates that the specific methods for each class are indeed invoked. All calls to method toString and earnings are resolved at execution time, based on the type of the object to which currentEmployee refers. This process is known as dynamic binding or late binding. For example, line 46 implicitly invokes method toString of the object to which currentEmployee refers. As a result of dynamic binding, Java decides which class’s toString method to call at execution time rather than at compile time. Only the methods of class Employee can be called via an Employee variable (and Employee, of course, includes the methods of class Object). A superclass reference can be used to invoke only methods of the superclass—the subclass method implementations are invoked polymorphically. Performing Type-Specific Operations on BasePlusCommissionEmployees We perform special processing on BasePlusCommissionEmployee objects—as we encounter these objects at execution time, we increase their base salary by 10%. When processing objects polymorphically, we typically do not need to worry about the specifics, but to adjust the base salary, we do have to determine the specific type of Employee object at execution time. Line 49 uses the instanceof operator to determine whether a particular Employee object’s type is BasePlusCommissionEmployee. The condition in line 49 is true if the object referenced by currentEmployee is a BasePlusCommissionEmployee. This would also be true for any object of a BasePlusCommissionEmployee subclass because of the is-a rela- 418 Chapter 10 Object-Oriented Programming: Polymorphism and Interfaces tionship a subclass has with its superclass. Lines 53–54 downcast currentEmployee from type Employee to type BasePlusCommissionEmployee—this cast is allowed only if the object has an is-a relationship with BasePlusCommissionEmployee. The condition at line 49 ensures that this is the case. This cast is required if we’re to invoke subclass BasePlusCommissionEmployee methods getBaseSalary and setBaseSalary on the current Employee object—as you’ll see momentarily, attempting to invoke a subclass-only method directly on a superclass reference is a compilation error. Common Programming Error 10.3 Assigning a superclass variable to a subclass variable is a compilation error. Common Programming Error 10.4 When downcasting a reference, a ClassCastException occurs if the referenced object at execution time does not have an is-a relationship with the type specified in the cast operator. If the instanceof expression in line 49 is true, lines 53–60 perform the special processing required for the BasePlusCommissionEmployee object. Using BasePlusCommissionEmployee variable employee, line 56 invokes subclass-only methods getBaseSalary and setBaseSalary to retrieve and update the employee’s base salary with the 10% raise. Calling earnings Polymorphically Lines 63–64 invoke method earnings on currentEmployee, which polymorphically calls the appropriate subclass object’s earnings method. Obtaining the earnings of the SalariedEmployee, HourlyEmployee and CommissionEmployee polymorphically in lines 63– 64 produces the same results as obtaining these employees’ earnings individually in lines 22–27. The earnings amount obtained for the BasePlusCommissionEmployee in lines 63– 64 is higher than that obtained in lines 28–30, due to the 10% increase in its base salary. Getting Each Employee’s Class Name Lines 68–70 display each employee’s type as a String. Every object knows its own class and can access this information through the getClass method, which all classes inherit from class Object. Method getClass returns an object of type Class (from package java.lang), which contains information about the object’s type, including its class name. Line 70 invokes getClass on the current object to get its class. The result of the getClass call is used to invoke getName to get the object’s class name. Avoiding Compilation Errors with Downcasting In the previous example, we avoided several compilation errors by downcasting an Employee variable to a BasePlusCommissionEmployee variable in lines 53–54. If you remove the cast operator (BasePlusCommissionEmployee) from line 54 and attempt to assign Employee variable currentEmployee directly to BasePlusCommissionEmployee variable employee, you’ll receive an “incompatible types” compilation error. This error indicates that the attempt to assign the reference of superclass object currentEmployee to subclass variable employee is not allowed. The compiler prevents this assignment because a CommissionEmployee is not a BasePlusCommissionEmployee—the is-a relationship applies only between the subclass and its superclasses, not vice versa. 10.6 Allowed Assignments Between Superclass and Subclass Variables 419 Similarly, if lines 56 and 60 used superclass variable currentEmployee to invoke subclass-only methods getBaseSalary and setBaseSalary, we’d receive “cannot find symbol” compilation errors at these lines. Attempting to invoke subclass-only methods via a superclass variable is not allowed—even though lines 56 and 60 execute only if instanceof in line 49 returns true to indicate that currentEmployee holds a reference to a BasePlusCommissionEmployee object. Using a superclass Employee variable, we can invoke only methods found in class Employee—earnings, toString and Employee’s get and set methods. Software Engineering Observation 10.5 Although the actual method that’s called depends on the runtime type of the object to which a variable refers, a variable can be used to invoke only those methods that are members of that variable’s type, which the compiler verifies. 10.6 Allowed Assignments Between Superclass and Subclass Variables Now that you’ve seen a complete application that processes diverse subclass objects polymorphically, we summarize what you can and cannot do with superclass and subclass objects and variables. Although a subclass object also is a superclass object, the two classes are nevertheless different. As discussed previously, subclass objects can be treated as objects of their superclass. But because the subclass can have additional subclass-only members, assigning a superclass reference to a subclass variable is not allowed without an explicit cast—such an assignment would leave the subclass members undefined for the superclass object. We’ve discussed three proper ways to assign superclass and subclass references to variables of superclass and subclass types: 1. Assigning a superclass reference to a superclass variable is straightforward. 2. Assigning a subclass reference to a subclass variable is straightforward. 3. Assigning a subclass reference to a superclass variable is safe, because the subclass object is an object of its superclass. However, the superclass variable can be used to refer only to superclass members. If this code refers to subclass-only members through the superclass variable, the compiler reports errors. 10.7 final Methods and Classes We saw in Sections 6.3 and 6.10 that variables can be declared final to indicate that they cannot be modified after they’re initialized—such variables represent constant values. It’s also possible to declare methods, method parameters and classes with the final modifier. Final Methods Cannot Be Overridden A final method in a superclass cannot be overridden in a subclass—this guarantees that the final method implementation will be used by all direct and indirect subclasses in the hierarchy. Methods that are declared private are implicitly final, because it’s not possible to override them in a subclass. Methods that are declared static are also implicitly final. A final method’s declaration can never change, so all subclasses use the same method 420 Chapter 10 Object-Oriented Programming: Polymorphism and Interfaces implementation, and calls to final methods are resolved at compile time—this is known as static binding. Final Classes Cannot Be Superclasses A final class cannot extended to create a subclass. All methods in a final class are implicitly final. Class String is an example of a final class. If you were allowed to create a subclass of String, objects of that subclass could be used wherever Strings are expected. Since class String cannot be extended, programs that use Strings can rely on the functionality of String objects as specified in the Java API. Making the class final also prevents programmers from creating subclasses that might bypass security restrictions. We’ve now discussed declaring variables, methods and classes final, and we’ve emphasized that if something can be final it should be final. Compilers can perform various optimizations when they know something is final. When we study concurrency in Chapter 23, you’ll see that final variables make it much easier to parallelize your programs for use on today’s multi-core processors. For more insights on the use of final, visit http://docs.oracle.com/javase/tutorial/java/IandI/final.html Common Programming Error 10.5 Attempting to declare a subclass of a final class is a compilation error. Software Engineering Observation 10.6 In the Java API, the vast majority of classes are not declared final. This enables inheritance and polymorphism. However, in some cases, it’s important to declare classes final—typically for security reasons. Also, unless you carefully design a class for extension, you should declare the class as final to avoid (often subtle) errors. 10.8 A Deeper Explanation of Issues with Calling Methods from Constructors Do not call overridable methods from constructors. When creating a subclass object, this could lead to an overridden method being called before the subclass object is fully initialized. Recall that when you construct a subclass object, its constructor first calls one of the direct superclass’s constructors. If the superclass constructor calls an overridable method, the subclass’s version of that method will be called by the superclass constructor—before the subclass constructor’s body has a chance to execute. This could lead to subtle, difficult-todetect errors if the subclass method that was called depends on initialization that has not yet been performed in the subclass constructor’s body. It’s acceptable to call a static method from a constructor. For example, a constructor and a set method often perform the same validation for a particular instance variable. If the validation code is brief, it’s acceptible to duplicate it in the constructor and the set method. If lengthier validation is required, define a static validation method (typically a private helper method) then call it from the constructor and the set method. It’s also acceptable for a constructor to call a final instance method, provided that the method does not directly or indirectly call an overridable instance method. 10.9 Creating and Using Interfaces 421 10.9 Creating and Using Interfaces [Note: As written, this section and its code example apply through Java SE 7. Java SE 8’s interface enhancements are introduced in Section 10.10 and discussed in more detail in Chapter 17.] Our next example (Figs. 10.11–10.15) reexamines the payroll system of Section 10.5. Suppose that the company involved wishes to perform several accounting operations in a single accounts payable application—in addition to calculating the earnings that must be paid to each employee, the company must also calculate the payment due on each of several invoices (i.e., bills for goods purchased). Though applied to unrelated things (i.e., employees and invoices), both operations have to do with obtaining some kind of payment amount. For an employee, the payment refers to the employee’s earnings. For an invoice, the payment refers to the total cost of the goods listed on the invoice. Can we calculate such different things as the payments due for employees and invoices in a single application polymorphically? Does Java offer a capability requiring that unrelated classes implement a set of common methods (e.g., a method that calculates a payment amount)? Java interfaces offer exactly this capability. Standardizing Interactions Interfaces define and standardize the ways in which things such as people and systems can interact with one another. For example, the controls on a radio serve as an interface between radio users and a radio’s internal components. The controls allow users to perform only a limited set of operations (e.g., change the station, adjust the volume, choose between AM and FM), and different radios may implement the controls in different ways (e.g., using push buttons, dials, voice commands). The interface specifies what operations a radio must permit users to perform but does not specify how the operations are performed. Software Objects Communicate Via Interfaces Software objects also communicate via interfaces. A Java interface describes a set of methods that can be called on an object to tell it, for example, to perform some task or return some piece of information. The next example introduces an interface named Payable to describe the functionality of any object that must be “capable of being paid” and thus must offer a method to determine the proper payment amount due. An interface declaration begins with the keyword interface and contains only constants and abstract methods. Unlike classes, all interface members must be public, and interfaces may not specify any implementation details, such as concrete method declarations and instance variables. All methods declared in an interface are implicitly public abstract methods, and all fields are implicitly public, static and final. Good Programming Practice 10.1 According to the Java Language Specification, it’s proper style to declare an interface’s abstract methods without keywords public and abstract, because they’re redundant in interface-method declarations. Similarly, an interface’s constants should be declared without keywords public, static and final, because they, too, are redundant. Using an Interface To use an interface, a concrete class must specify that it implements the interface and must declare each method in the interface with the signature specified in the interface declaration. To specify that a class implements an interface, add the implements keyword and the 422 Chapter 10 Object-Oriented Programming: Polymorphism and Interfaces name of the interface to the end of your class declaration’s first line. A class that does not implement all the methods of the interface is an abstract class and must be declared abstract. Implementing an interface is like signing a contract with the compiler that states, “I will declare all the methods specified by the interface or I will declare my class abstract.” Common Programming Error 10.6 Failing to implement any method of an interface in a concrete class that implements the interface results in a compilation error indicating that the class must be declared abstract. Relating Disparate Types An interface is often used when disparate classes—i.e., classes that are not related by a class hierarchy—need to share common methods and constants. This allows objects of unrelated classes to be processed polymorphically—objects of classes that implement the same interface can respond to the same method calls. You can create an interface that describes the desired functionality, then implement this interface in any classes that require that functionality. For example, in the accounts payable application developed in this section, we implement interface Payable in any class that must be able to calculate a payment amount (e.g., Employee, Invoice). Interfaces vs. Abstract Classes An interface is often used in place of an abstract class when there’s no default implementation to inherit—that is, no fields and no default method implementations. Like public abstract classes, interfaces are typically public types. Like a public class, a public interface must be declared in a file with the same name as the interface and the .java filename extension. Software Engineering Observation 10.7 Many developers feel that interfaces are an even more important modeling technology than classes, especially with the new interface enhancements in Java SE 8 (see Section 10.10). Tagging Interfaces We’ll see in Chapter 15, Files, Streams and Object Serialization, the notion of tagging interfaces (also called marker interfaces)—empty interfaces that have no methods or constant values. They’re used to add is-a relationships to classes. For example, in Chapter 15 we’ll discuss a mechanism called object serialization, which can convert objects to byte representations and can convert those byte representations back to objects. To enable this mechanism to work with your objects, you simply have to mark them as Serializable by adding implements Serializable to the end of your class declaration’s first line. Then, all the objects of your class have the is-a relationship with Serializable. 10.9.1 Developing a Payable Hierarchy To build an application that can determine payments for employees and invoices alike, we first create interface Payable, which contains method getPaymentAmount that returns a double amount that must be paid for an object of any class that implements the interface. Method getPaymentAmount is a general-purpose version of method earnings of the Employee hierarchy—method earnings calculates a payment amount specifically for an Employee, while getPaymentAmount can be applied to a broad range of possibly unrelated objects. After declaring interface Payable, we introduce class Invoice, which implements 10.9 Creating and Using Interfaces 423 interface Payable. We then modify class Employee such that it also implements interface Finally, we update Employee subclass SalariedEmployee to “fit” into the Payable hierarchy by renaming SalariedEmployee method earnings as getPaymentAmount. Payable. Good Programming Practice 10.2 When declaring a method in an interface, choose a method name that describes the method’s purpose in a general manner, because the method may be implemented by many unrelated classes. Classes Invoice and Employee both represent things for which the company must be able to calculate a payment amount. Both classes implement the Payable interface, so a program can invoke method getPaymentAmount on Invoice objects and Employee objects alike. As we’ll soon see, this enables the polymorphic processing of Invoices and Employees required for the company’s accounts payable application. The UML class diagram in Fig. 10.10 shows the interface and class hierarchy used in our accounts payable application. The hierarchy begins with interface Payable. The UML distinguishes an interface from other classes by placing the word “interface” in guillemets (« and ») above the interface name. The UML expresses the relationship between a class and an interface through a relationship known as realization. A class is said to realize, or implement, the methods of an interface. A class diagram models a realization as a dashed arrow with a hollow arrowhead pointing from the implementing class to the interface. The diagram in Fig. 10.10 indicates that classes Invoice and Employee each realize interface Payable. As in the class diagram of Fig. 10.2, class Employee appears in italics, indicating that it’s an abstract class. Concrete class SalariedEmployee extends Employee, inheriting its superclass’s realization relationship with interface Payable. «interface» Payable Invoice Employee SalariedEmployee Fig. 10.10 | Payable hierarchy UML class diagram. 10.9.2 Interface Payable The declaration of interface Payable begins in Fig. 10.11 at line 4. Interface Payable contains public abstract method getPaymentAmount. Interface methods are always public and abstract, so they do not need to be declared as such. Interface Payable has only one method, but interfaces can have any number of methods. In addition, method getPaymentAmount has no parameters, but interface methods can have parameters. Interfaces may also contain final static constants. 424 1 2 3 4 5 6 7 Chapter 10 Object-Oriented Programming: Polymorphism and Interfaces // Fig. 10.11: Payable.java // Payable interface declaration. public interface Payable { double getPaymentAmount(); // calculate payment; no implementation } Fig. 10.11 | Payable interface declaration. 10.9.3 Class Invoice We now create class Invoice (Fig. 10.12) to represent a simple invoice that contains billing information for only one kind of part. The class declares private instance variables partNumber, partDescription, quantity and pricePerItem (in lines 6–9) that indicate the part number, a description of the part, the quantity of the part ordered and the price per item. Class Invoice also contains a constructor (lines 12–26), get and set methods (lines 29–69) that manipulate the class’s instance variables and a toString method (lines 72–78) that returns a String representation of an Invoice object. Methods setQuantity (lines 41–47) and setPricePerItem (lines 56–63) ensure that quantity and pricePerItem obtain only nonnegative values. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 // Fig. 10.12: Invoice.java // Invoice class that implements Payable. public class Invoice implements Payable { private final String partNumber; private final String partDescription; private int quantity; private double pricePerItem; // constructor public Invoice(String partNumber, String partDescription, int quantity, double pricePerItem) { if (quantity < 0) // validate quantity throw new IllegalArgumentException("Quantity must be >= 0"); if (pricePerItem < 0.0) // validate pricePerItem throw new IllegalArgumentException( "Price per item must be >= 0"); this.quantity = quantity; this.partNumber = partNumber; this.partDescription = partDescription; this.pricePerItem = pricePerItem; } // end constructor Fig. 10.12 | Invoice class that implements Payable. (Part 1 of 3.) 10.9 Creating and Using Interfaces 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 425 // get part number public String getPartNumber() { return partNumber; // should validate } // get description public String getPartDescription() { return partDescription; } // set quantity public void setQuantity(int quantity) { if (quantity < 0) // validate quantity throw new IllegalArgumentException("Quantity must be >= 0"); this.quantity = quantity; } // get quantity public int getQuantity() { return quantity; } // set price per item public void setPricePerItem(double pricePerItem) { if (pricePerItem < 0.0) // validate pricePerItem throw new IllegalArgumentException( "Price per item must be >= 0"); this.pricePerItem = pricePerItem; } // get price per item public double getPricePerItem() { return pricePerItem; } // return String representation of Invoice object @Override public String toString() { return String.format("%s: %n%s: %s (%s) %n%s: %d %n%s: $%,.2f", "invoice", "part number", getPartNumber(), getPartDescription(), "quantity", getQuantity(), "price per item", getPricePerItem()); } Fig. 10.12 | Invoice class that implements Payable. (Part 2 of 3.) 426 80 81 82 83 84 85 86 Chapter 10 Object-Oriented Programming: Polymorphism and Interfaces // method required to carry out contract with interface Payable @Override public double getPaymentAmount() { return getQuantity() * getPricePerItem(); // calculate total cost } } // end class Invoice Fig. 10.12 | Invoice class that implements Payable. (Part 3 of 3.) A Class Can Extend Only One Other Class But Can Implement Many Interfaces Line 4 indicates that class Invoice implements interface Payable. Like all classes, class Invoice also implicitly extends Object. Java does not allow subclasses to inherit from more than one superclass, but it allows a class to inherit from one superclass and implement as many interfaces as it needs. To implement more than one interface, use a comma-separated list of interface names after keyword implements in the class declaration, as in: public class ClassName extends SuperclassName implements FirstInterface, SecondInterface, … Software Engineering Observation 10.8 All objects of a class that implements multiple interfaces have the is-a relationship with each implemented interface type. Class Invoice implements the one abstract method in interface Payable—method is declared in lines 81–85. The method calculates the total payment required to pay the invoice. The method multiplies the values of quantity and pricePerItem (obtained through the appropriate get methods) and returns the result. This method satisfies the implementation requirement for this method in interface Payable—we’ve fulfilled the interface contract with the compiler. getPaymentAmount 10.9.4 Modifying Class Employee to Implement Interface Payable We now modify class Employee such that it implements interface Payable. Figure 10.13 contains the modified class, which is identical to that of Fig. 10.4 with two exceptions. First, line 4 of Fig. 10.13 indicates that class Employee now implements interface Payable. For this example, we renamed method earnings to getPaymentAmount throughout the Employee hierarchy. As with method earnings in the version of class Employee in Fig. 10.4, however, it does not make sense to implement method getPaymentAmount in class Employee because we cannot calculate the earnings payment owed to a general Employee—we must first know the specific type of Employee. In Fig. 10.4, we declared method earnings as abstract for this reason, so class Employee had to be declared abstract. This forced each Employee concrete subclass to override earnings with an implementation. 1 2 3 // Fig. 10.13: Employee.java // Employee abstract superclass that implements Payable. Fig. 10.13 | Employee abstract superclass that implements Payable. (Part 1 of 2.) 10.9 Creating and Using Interfaces 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 427 public abstract class Employee implements Payable { private final String firstName; private final String lastName; private final String socialSecurityNumber; // constructor public Employee(String firstName, String lastName, String socialSecurityNumber) { this.firstName = firstName; this.lastName = lastName; this.socialSecurityNumber = socialSecurityNumber; } // return first name public String getFirstName() { return firstName; } // return last name public String getLastName() { return lastName; } // return social security number public String getSocialSecurityNumber() { return socialSecurityNumber; } // return String representation of Employee object @Override public String toString() { return String.format("%s %s%nsocial security number: %s", getFirstName(), getLastName(), getSocialSecurityNumber()); } // Note: We do not implement Payable method getPaymentAmount here so // this class must be declared abstract to avoid a compilation error. } // end abstract class Employee Fig. 10.13 | Employee abstract superclass that implements Payable. (Part 2 of 2.) In Fig. 10.13, we handle this situation differently. Recall that when a class implements an interface, it makes a contract with the compiler stating either that the class will implement each method in the interface or the class will be declared abstract. Because class Employee does not provide a getPaymentAmount method, the class must be declared abstract. Any concrete subclass of the abstract class must implement the interface methods to fulfill the superclass’s contract with the compiler. If the subclass does not do so, it too must be declared 428 Chapter 10 Object-Oriented Programming: Polymorphism and Interfaces abstract. As indicated by the comments in lines 45–46, class Employee of Fig. 10.13 does not implement method getPaymentAmount, so the class is declared abstract. Each direct Employee subclass inherits the superclass’s contract to implement method getPaymentAmount and thus must implement this method to become a concrete class for which objects can be instantiated. A class that extends one of Employee’s concrete subclasses will inherit an implementation of getPaymentAmount and thus will also be a concrete class. 10.9.5 Modifying Class SalariedEmployee for Use in the Payable Hierarchy Figure 10.14 contains a modified SalariedEmployee class that extends Employee and fulfills superclass Employee’s contract to implement Payable method getPaymentAmount. This version of SalariedEmployee is identical to that of Fig. 10.5, but it replaces method earnings with method getPaymentAmount (lines 39–43). Recall that the Payable version of the method has a more general name to be applicable to possibly disparate classes. (If we included the remaining Employee subclasses from Section 10.5—HourlyEmployee, CommissionEmployee and BasePlusCommissionEmployee—in this example, their earnings methods should also be renamed getPaymentAmount. We leave these modifications to Exercise 10.15 and use only SalariedEmployee in our test program here. Exercise 10.16 asks you to implement interface Payable in the entire Employee class hierarchy of Figs. 10.4–10.9 without modifying the Employee subclasses.) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 // Fig. 10.14: SalariedEmployee.java // SalariedEmployee class that implements interface Payable. // method getPaymentAmount. public class SalariedEmployee extends Employee { private double weeklySalary; // constructor public SalariedEmployee(String firstName, String lastName, String socialSecurityNumber, double weeklySalary) { super(firstName, lastName, socialSecurityNumber); if (weeklySalary < 0.0) throw new IllegalArgumentException( "Weekly salary must be >= 0.0"); this.weeklySalary = weeklySalary; } // set salary public void setWeeklySalary(double weeklySalary) { if (weeklySalary < 0.0) throw new IllegalArgumentException( "Weekly salary must be >= 0.0"); Fig. 10.14 | SalariedEmployee getPaymentAmount. (Part 1 of 2.) class that implements interface Payable method 10.9 Creating and Using Interfaces 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 429 this.weeklySalary = weeklySalary; } // return salary public double getWeeklySalary() { return weeklySalary; } // calculate earnings; implement interface Payable method that was // abstract in superclass Employee @Override public double getPaymentAmount() { return getWeeklySalary(); } // return String representation of SalariedEmployee object @Override public String toString() { return String.format("salaried employee: %s%n%s: $%,.2f", super.toString(), "weekly salary", getWeeklySalary()); } } // end class SalariedEmployee Fig. 10.14 | SalariedEmployee getPaymentAmount. class that implements interface Payable method (Part 2 of 2.) When a class implements an interface, the same is-a relationship provided by inheritance applies. Class Employee implements Payable, so we can say that an Employee is a Payable. In fact, objects of any classes that extend Employee are also Payable objects. SalariedEmployee objects, for instance, are Payable objects. Objects of any subclasses of the class that implements the interface can also be thought of as objects of the interface type. Thus, just as we can assign the reference of a SalariedEmployee object to a superclass Employee variable, we can assign the reference of a SalariedEmployee object to an interface Payable variable. Invoice implements Payable, so an Invoice object also is a Payable object, and we can assign the reference of an Invoice object to a Payable variable. Software Engineering Observation 10.9 When a method parameter is declared with a superclass or interface type, the method processes the object passed as an argument polymorphically. Software Engineering Observation 10.10 Using a superclass reference, we can polymorphically invoke any method declared in the superclass and its superclasses (e.g., class Object). Using an interface reference, we can polymorphically invoke any method declared in the interface, its superinterfaces (one interface can extend another) and in class Object—a variable of an interface type must refer to an object to call methods, and all objects have the methods of class Object. 430 Chapter 10 Object-Oriented Programming: Polymorphism and Interfaces 10.9.6 Using Interface Payable to Process Invoices and Employees Polymorphically PayableInterfaceTest (Fig. 10.15) illustrates that interface Payable can be used to process a set of Invoices and Employees polymorphically in a single application. Line 9 declares payableObjects and assigns it an array of four Payable variables. Lines 12–13 assign the references of Invoice objects to the first two elements of payableObjects. Lines 14–17 then assign the references of SalariedEmployee objects to the remaining two elements of payableObjects. These assignments are allowed because an Invoice is a Payable, a SalariedEmployee is an Employee and an Employee is a Payable. Lines 23–29 use the enhanced for statement to polymorphically process each Payable object in payableObjects, printing the object as a String, along with the payment amount due. Line 27 invokes method toString via a Payable interface reference, even though toString is not declared in interface Payable—all references (including those of interface types) refer to objects that extend Object and therefore have a toString method. (Method toString also can be invoked implicitly here.) Line 28 invokes Payable method getPaymentAmount to obtain the payment amount for each object in payableObjects, regardless of the actual type of the object. The output reveals that each of the method calls in lines 27–28 invokes the appropriate class’s implementation of methods toString and getPaymentAmount. For instance, when currentPayable refers to an Invoice during the first iteration of the for loop, class Invoice’s toString and getPaymentAmount execute. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 // Fig. 10.15: PayableInterfaceTest.java // Payable interface test program processing Invoices and // Employees polymorphically. public class PayableInterfaceTest { public static void main(String[] args) { // create four-element Payable array Payable[] payableObjects = new Payable[4]; // populate array with objects that implement Payable payableObjects[0] = new Invoice("01234", "seat", 2, 375.00); payableObjects[1] = new Invoice("56789", "tire", 4, 79.95); payableObjects[2] = new SalariedEmployee("John", "Smith", "111-11-1111", 800.00); payableObjects[3] = new SalariedEmployee("Lisa", "Barnes", "888-88-8888", 1200.00); System.out.println( "Invoices and Employees processed polymorphically:"); // generically process each element in array payableObjects for (Payable currentPayable : payableObjects) { Fig. 10.15 | Payable interface test program processing Invoices and Employees polymorphically. (Part 1 of 2.) 10.9 Creating and Using Interfaces 25 26 27 28 29 30 31 431 // output currentPayable and its appropriate payment amount System.out.printf("%n%s %n%s: $%,.2f%n", currentPayable.toString(), // could invoke implicitly "payment due", currentPayable.getPaymentAmount()); } } // end main } // end class PayableInterfaceTest Invoices and Employees processed polymorphically: invoice: part number: 01234 (seat) quantity: 2 price per item: $375.00 payment due: $750.00 invoice: part number: 56789 (tire) quantity: 4 price per item: $79.95 payment due: $319.80 salaried employee: John Smith social security number: 111-11-1111 weekly salary: $800.00 payment due: $800.00 salaried employee: Lisa Barnes social security number: 888-88-8888 weekly salary: $1,200.00 payment due: $1,200.00 Fig. 10.15 | Payable interface test program processing Invoices and Employees polymorphically. (Part 2 of 2.) 10.9.7 Some Common Interfaces of the Java API You’ll use interfaces extensively when developing Java applications. The Java API contains numerous interfaces, and many of the Java API methods take interface arguments and return interface values. Figure 10.16 overviews a few of the more popular interfaces of the Java API that we use in later chapters. Interface Description Comparable Java contains several comparison operators (e.g., <, <=, >, >=, ==, !=) that allow you to compare primitive values. However, these operators cannot be used to compare objects. Interface Comparable is used to allow objects of a class that implements the interface to be compared to one another. Interface Comparable is commonly used for ordering objects in a collection such as an array. We use Comparable in Chapter 16, Generic Collections, and Chapter 20, Generic Classes and Methods. Fig. 10.16 | Common interfaces of the Java API. (Part 1 of 2.) 432 Chapter 10 Object-Oriented Programming: Polymorphism and Interfaces Interface Description Serializable An interface used to identify classes whose objects can be written to (i.e., serialized) or read from (i.e., deserialized) some type of storage (e.g., file on disk, database field) or transmitted across a network. We use Serializable in Chapter 15, Files, Streams and Object Serialization, and Chapter 28, Networking. Implemented by any class that represents a task to perform. Objects of such as class are often executed in parallel using a technique called multithreading (discussed in Chapter 23, Concurrency). The interface contains one method, run, which specifies the behavior of an object when executed. You work with graphical user interfaces (GUIs) every day. In your web browser, you might type the address of a website to visit, or you might click a button to return to a previous site. The browser responds to your interaction and performs the desired task. Your interaction is known as an event, and the code that the browser uses to respond to an event is known as an event handler. In Chapter 12, GUI Components: Part 1, and Chapter 22, GUI Components: Part 2, you’ll learn how to build GUIs and event handlers that respond to user interactions. Event handlers are declared in classes that implement an appropriate event-listener interface. Each event-listener interface specifies one or more methods that must be implemented to respond to user interactions. Implemented by classes that can be used with the try-with-resources statement (Chapter 11, Exception Handling: A Deeper Look) to help prevent resource leaks. Runnable GUI eventlistener interfaces AutoCloseable Fig. 10.16 | Common interfaces of the Java API. (Part 2 of 2.) 10.10 Java SE 8 Interface Enhancements This section introduces Java SE 8’s new interface features. We discuss these in more detail in later chapters. 10.10.1 default Interface Methods Prior to Java SE 8, interface methods could be only public abstract methods. This meant that an interface specified what operations an implementing class must perform but not how the class should perform them. In Java SE 8, interfaces also may contain public default methods with concrete default implementations that specify how operations are performed when an implementing class does not override the methods. If a class implements such an interface, the class also receives the interface’s default implementations (if any). To declare a default method, place the keyword default before the method’s return type and provide a concrete method implementation. Adding Methods to Existing Interfaces Prior to Java SE 8, adding methods to an interface would break any implementing classes that did not implement the new methods. Recall that if you didn’t implement each of an interface’s methods, you had to declare your class abstract. Any class that implements the original interface will not break when a default method is added—the class simply receives the new default method. When a class imple- 10.11 (Optional) GUI and Graphics Case Study: Drawing with Polymorphism 433 ments a Java SE 8 interface, the class “signs a contract” with the compiler that says, “I will declare all the abstract methods specified by the interface or I will declare my class abstract”—the implementing class is not required to override the interface’s default methods, but it can if necessary. Software Engineering Observation 10.11 Java SE 8 default methods enable you to evolve existing interfaces by adding new methods to those interfaces without breaking code that uses them. Interfaces vs. abstract Classes Prior to Java SE 8, an interface was typically used (rather than an abstract class) when there were no implementation details to inherit—no fields and no method implementations. With default methods, you can instead declare common method implementations in interfaces, which gives you more flexibility in designing your classes. 10.10.2 static Interface Methods Prior to Java SE 8, it was common to associate with an interface a class containing static helper methods for working with objects that implemented the interface. In Chapter 16, you’ll learn about class Collections which contains many static helper methods for working with objects that implement interfaces Collection, List, Set and more. For example, Collections method sort can sort objects of any class that implements interface List. With static interface methods, such helper methods can now be declared directly in interfaces rather than in separate classes. 10.10.3 Functional Interfaces As of Java SE 8, any interface containing only one abstract method is known as a functional interface. There are many such interfaces throughout the Java SE 7 APIs, and many new ones in Java SE 8. Some functional interfaces that you’ll use in this book include: (Chapter 12)—You’ll implement this interface to define a method that’s called when the user clicks a button. • ActionListener • Comparator (Chapter 16)—You’ll implement this interface to define a method that can compare two objects of a given type to determine whether the first object is less than, equal to or greater than the second. • Runnable (Chapter 23)—You’ll implement this interface to define a task that may be run in parallel with other parts of your program. Functional interfaces are used extensively with Java SE 8’s new lambda capabilities that we introduce in Chapter 17. In Chapter 12, you’ll often implement an interface by creating a so-called anonymous inner class that implements the interface’s method(s). In Java SE 8, lambdas provide a shorthand notation for creating anonymous methods that the compiler automatically translates into anonymous inner classes for you. 10.11 (Optional) GUI and Graphics Case Study: Drawing with Polymorphism You may have noticed in the drawing program created in GUI and Graphics Case Study Exercise 8.1 (and modified in GUI and Graphics Case Study Exercise 9.1) that shape 434 Chapter 10 Object-Oriented Programming: Polymorphism and Interfaces classes have many similarities. Using inheritance, we can “factor out” the common features from all three classes and place them in a single shape superclass. Then, using variables of the superclass type, we can manipulate shape objects polymorphically. Removing the redundant code will result in a smaller, more flexible program that’s easier to maintain. GUI and Graphics Case Study Exercises 10.1 Modify the MyLine, MyOval and MyRectangle classes of GUI and Graphics Case Study Exercise 8.1 and Exercise 9.1 to create the class hierarchy in Fig. 10.17. Classes of the MyShape hierarchy should be “smart” shape classes that know how to draw themselves (if provided with a Graphics object that tells them where to draw). Once the program creates an object from this hierarchy, it can manipulate it polymorphically for the rest of its lifetime as a MyShape. java.lang.Object MyShape MyLine Fig. 10.17 | MyShape MyOval MyRectangle hierarchy. In your solution, class MyShape in Fig. 10.17 must be abstract. Since MyShape represents any shape in general, you cannot implement a draw method without knowing specifically what shape it is. The data representing the coordinates and color of the shapes in the hierarchy should be declared as private members of class MyShape. In addition to the common data, class MyShape should declare the following methods: a) A no-argument constructor that sets all the coordinates of the shape to 0 and the color to Color.BLACK. b) A constructor that initializes the coordinates and color to the values of the arguments supplied. c) Set methods for the individual coordinates and color that allow the programmer to set any piece of data independently for a shape in the hierarchy. d) Get methods for the individual coordinates and color that allow the programmer to retrieve any piece of data independently for a shape in the hierarchy. e) The abstract method public abstract void draw(Graphics g); which the program’s paintComponent method will call to draw a shape on the screen. To ensure proper encapsulation, all data in class MyShape must be private. This requires declaring proper set and get methods to manipulate the data. Class MyLine should provide a noargument constructor and a constructor with arguments for the coordinates and color. Classes MyOval and MyRectangle should provide a no-argument constructor and a constructor with argu- 10.11 (Optional) GUI and Graphics Case Study: Drawing with Polymorphism 435 ments for the coordinates, color and determining whether the shape is filled. The no-argument constructor should, in addition to setting the default values, set the shape to be an unfilled shape. You can draw lines, rectangles and ovals if you know two points in space. Lines require x1, y1, x2 and y2 coordinates. The drawLine method of the Graphics class will connect the two points supplied with a line. If you have the same four coordinate values (x1, y1, x2 and y2) for ovals and rectangles, you can calculate the four arguments needed to draw them. Each requires an upper-left x-coordinate value (the smaller of the two x-coordinate values), an upper-left y-coordinate value (the smaller of the two y-coordinate values), a width (the absolute value of the difference between the two x-coordinate values) and a height (the absolute value of the difference between the two ycoordinate values). Rectangles and ovals should also have a filled flag that determines whether to draw the shape as a filled shape. There should be no MyLine, MyOval or MyRectangle variables in the program—only MyShape variables that contain references to MyLine, MyOval and MyRectangle objects. The program should generate random shapes and store them in an array of type MyShape. Method paintComponent should walk through the MyShape array and draw every shape, by polymorphically calling every shape’s draw method. Allow the user to specify (via an input dialog) the number of shapes to generate. The program will then generate and display the shapes along with a status bar that informs the user how many of each shape were created. 10.2 (Drawing Application Modification) In the preceding exercise, you created a MyShape hierarchy in which classes MyLine, MyOval and MyRectangle extend MyShape directly. If your hierarchy was properly designed, you should be able to see the similarities between the MyOval and MyRectangle classes. Redesign and reimplement the code for the MyOval and MyRectangle classes to “factor out” the common features into the abstract class MyBoundedShape to produce the hierarchy in Fig. 10.18. Class MyBoundedShape should declare two constructors that mimic those of class MyShape, only with an added parameter to specify whether the shape is filled. Class MyBoundedShape should also declare get and set methods for manipulating the filled flag and methods that calculate the upperleft x-coordinate, upper-left y-coordinate, width and height. Remember, the values needed to draw an oval or a rectangle can be calculated from two (x, y) coordinates. If designed properly, the new MyOval and MyRectangle classes should each have two constructors and a draw method. java.lang.Object MyShape MyLine MyBoundedShape MyOval Fig. 10.18 | MyShape hierarchy with MyBoundedShape. MyRectangle 436 Chapter 10 Object-Oriented Programming: Polymorphism and Interfaces 10.12 Wrap-Up This chapter introduced polymorphism—the ability to process objects that share the same superclass in a class hierarchy as if they were all objects of the superclass. We discussed how polymorphism makes systems extensible and maintainable, then demonstrated how to use overridden methods to effect polymorphic behavior. We introduced abstract classes, which allow you to provide an appropriate superclass from which other classes can inherit. You learned that an abstract class can declare abstract methods that each subclass must implement to become a concrete class and that a program can use variables of an abstract class to invoke the subclasses’ implementations of abstract methods polymorphically. You also learned how to determine an object’s type at execution time. We explained the notions of final methods and classes. Finally, the chapter discussed declaring and implementing an interface as a way for possibly disparate classes to implement common functionality, enabling objects of those classes to be processed polymorphically. You should now be familiar with classes, objects, encapsulation, inheritance, interfaces and polymorphism—the most essential aspects of object-oriented programming. In the next chapter, you’ll learn about exceptions, useful for handling errors during a program’s execution. Exception handling provides for more robust programs. Summary Section 10.1 Introduction • Polymorphism (p. 396) enables us to write programs that process objects that share the same superclass as if they were all objects of the superclass; this can simplify programming. • With polymorphism, we can design and implement systems that are easily extensible. The only parts of a program that must be altered to accommodate new classes are those that require direct knowledge of the new classes that you add to the hierarchy. Section 10.3 Demonstrating Polymorphic Behavior • When the compiler encounters a method call made through a variable, it determines if the method can be called by checking the variable’s class type. If that class contains the proper method declaration (or inherits one), the call is compiled. At execution time, the type of the object to which the variable refers determines the actual method to use. Section 10.4 Abstract Classes and Methods • Abstract classes (p. 401) cannot be used to instantiate objects, because they’re incomplete. • The primary purpose of an abstract class is to provide an appropriate superclass from which other classes can inherit and thus share a common design. • Classes that can be used to instantiate objects are called concrete classes (p. 402). They provide implementations of every method they declare (some of the implementations can be inherited). • Programmers often write client code that uses only abstract superclasses (p. 402) to reduce client code’s dependencies on specific subclass types. • Abstract classes sometimes constitute several levels of a hierarchy. • An abstract class normally contains one or more abstract methods (p. 402). • Abstract methods do not provide implementations. • A class that contains any abstract methods must be declared as an abstract class (p. 402). Each concrete subclass must provide implementations of each of the superclass’s abstract methods. Summary 437 • Constructors and static methods cannot be declared abstract. • Abstract superclass variables can hold references to objects of any concrete class derived from the superclass. Programs typically use such variables to manipulate subclass objects polymorphically. • Polymorphism is particularly effective for implementing layered software systems. Section 10.5 Case Study: Payroll System Using Polymorphism • A hierarchy designer can demand that each concrete subclass provide an appropriate method implementation by including an abstract method in a superclass. • Most method calls are resolved at execution time, based on the type of the object being manipulated. This process is known as dynamic binding (p. 417) or late binding. • A superclass variable can be used to invoke only methods declared in the superclass. • Operator instanceof (p. 417) determines if an object has the is-a relationship with a specific type. • Every object in Java knows its own class and can access it through Object method getClass (p. 418), which returns an object of type Class (package java.lang). • The is-a relationship applies only between the subclass and its superclasses, not vice versa. Section 10.7 final Methods and Classes • • • • A method that’s declared final (p. 419) in a superclass cannot be overridden in a subclass. Methods declared private are implicitly final, because you can’t override them in a subclass. Methods that are declared static are implicitly final. A final method’s declaration can never change, so all subclasses use the same implementation, and calls to final methods are resolved at compile time—this is known as static binding (p. 420). • The compiler can optimize programs by removing calls to final methods and inlining their expanded code at each method-call location. • A class that’s declared final cannot be extended (p. 420). • All methods in a final class are implicitly final. Section 10.9 Creating and Using Interfaces • • • • • • • • • An interface (p. 421) specifies what operations are allowed but not how they’re performed. A Java interface describes a set of methods that can be called on an object. An interface declaration begins with the keyword interface (p. 421). All interface members must be public, and interfaces may not specify any implementation details, such as concrete method declarations and instance variables. All methods declared in an interface are implicitly public abstract methods and all fields are implicitly public, static and final. To use an interface, a concrete class must specify that it implements (p. 421) the interface and must declare each interface method with the signature specified in the interface declaration. A class that does not implement all the interface’s methods must be declared abstract. Implementing an interface is like signing a contract with the compiler that states, “I will declare all the methods specified by the interface or I will declare my class abstract.” An interface is typically used when disparate (i.e., unrelated) classes need to share common methods and constants. This allows objects of unrelated classes to be processed polymorphically—objects of classes that implement the same interface can respond to the same method calls. You can create an interface that describes the desired functionality, then implement the interface in any classes that require that functionality. 438 Chapter 10 Object-Oriented Programming: Polymorphism and Interfaces • An interface is often used in place of an abstract class when there’s no default implementation to inherit—that is, no instance variables and no default method implementations. • Like public abstract classes, interfaces are typically public types, so they’re normally declared in files by themselves with the same name as the interface and the .java filename extension. • Java does not allow subclasses to inherit from more than one superclass, but it does allow a class to inherit from a superclass and implement more than one interface. • All objects of a class that implement multiple interfaces have the is-a relationship with each implemented interface type. • An interface can declare constants. The constants are implicitly public, static and final. Section 10.10 Java SE 8 Interface Enhancements • In Java SE 8, an interface may declare default methods—that is, public methods with concrete implementations that specify how an operation should be performed. • When a class implements an interface, the class receives the interface’s default concrete implementations if it does not override them. • To declare a default method in an interface, you simply place the keyword default before the method’s return type and provide a complete method body. • When you enhance an existing an interface with default methods—any classes that implemented the original interface will not break—it’ll simply receive the default method implementations. • With default methods, you can declare common method implementations in interfaces (rather than abstract classes), which gives you more flexibility in designing your classes. • As of Java SE 8, interfaces may now include public static methods. • As of Java SE 8, any interface containing only one method is known as a functional interface. There are many such interfaces throughout the Java APIs. • Functional interfaces are used extensively with Java SE 8’s new lambda capabilities. As you’ll see, lambdas provide a shorthand notation for creating anonymous methods. Self-Review Exercises 10.1 Fill in the blanks in each of the following statements: a) If a class contains at least one abstract method, it’s a(n) class. b) Classes from which objects can be instantiated are called classes. involves using a superclass variable to invoke methods on superclass and subc) class objects, enabling you to “program in the general.” d) Methods that are not interface methods and that do not provide implementations must be declared using keyword . e) Casting a reference stored in a superclass variable to a subclass type is called . 10.2 State whether each of the statements that follows is true or false. If false, explain why. a) All methods in an abstract class must be declared as abstract methods. b) Invoking a subclass-only method through a subclass variable is not allowed. c) If a superclass declares an abstract method, a subclass must implement that method. d) An object of a class that implements an interface may be thought of as an object of that interface type. 10.3 (Java SE 8 interfaces) Fill in the blanks in each of the following statements: a) In Java SE 8, an interface may declare —that is, public methods with concrete implementations that specify how an operation should be performed. b) As of Java SE 8, interfaces can now include helper methods. c) As of Java SE 8, any interface containing only one method is known as a(n) . Answers to Self-Review Exercises 439 Answers to Self-Review Exercises 10.1 a) abstract. b) concrete. c) Polymorphism. d) abstract. e) downcasting. 10.2 a) False. An abstract class can include methods with implementations and abstract methods. b) False. Trying to invoke a subclass-only method with a superclass variable is not allowed. c) False. Only a concrete subclass must implement the method. d) True. 10.3 a) default methods. b) static. c) functional interface. Exercises 10.4 How does polymorphism enable you to program “in the general” rather than “in the specific”? Discuss the key advantages of programming “in the general.” 10.5 What are abstract methods? Describe the circumstances in which an abstract method would be appropriate. 10.6 How does polymorphism promote extensibility? 10.7 Discuss three proper ways in which you can assign superclass and subclass references to variables of superclass and subclass types. 10.8 Compare and contrast abstract classes and interfaces. Why would you use an abstract class? Why would you use an interface? 10.9 (Java SE 8 Interfaces) Explain how default methods enable you to add new methods to an existing interface without breaking the classes that implemented the original interface. 10.10 (Java SE 8 Interfaces) What is a functional interface? 10.11 (Java SE 8 Interfaces) Why is it useful to be able to add static methods to interfaces? 10.12 (Payroll System Modification) Modify the payroll system of Figs. 10.4–10.9 to include private instance variable birthDate in class Employee. Use class Date of Fig. 8.7 to represent an employee’s birthday. Add get methods to class Date. Assume that payroll is processed once per month. Create an array of Employee variables to store references to the various employee objects. In a loop, calculate the payroll for each Employee (polymorphically), and add a $100.00 bonus to the person’s payroll amount if the current month is the one in which the Employee’s birthday occurs. Hierarchy) Implement the Shape hierarchy shown in Fig. 9.3. Each Twoshould contain method getArea to calculate the area of the two-dimensional shape. Each ThreeDimensionalShape should have methods getArea and getVolume to calculate the surface area and volume, respectively, of the three-dimensional shape. Create a program that uses an array of Shape references to objects of each concrete class in the hierarchy. The program should print a text description of the object to which each array element refers. Also, in the loop that processes all the shapes in the array, determine whether each shape is a TwoDimensionalShape or a ThreeDimensionalShape. If it’s a TwoDimensionalShape, display its area. If it’s a ThreeDimensionalShape, display its area and volume. 10.13 (Project: Shape DimensionalShape 10.14 (Payroll System Modification) Modify the payroll system of Figs. 10.4–10.9 to include an additional Employee subclass PieceWorker that represents an employee whose pay is based on the number of pieces of merchandise produced. Class PieceWorker should contain private instance variables wage (to store the employee’s wage per piece) and pieces (to store the number of pieces produced). Provide a concrete implementation of method earnings in class PieceWorker that calculates the employee’s earnings by multiplying the number of pieces produced by the wage per piece. Create an array of Employee variables to store references to objects of each concrete class in the new Employee hierarchy. For each Employee, display its String representation and earnings. 440 Chapter 10 Object-Oriented Programming: Polymorphism and Interfaces 10.15 (Accounts Payable System Modification) In this exercise, we modify the accounts payable application of Figs. 10.11–10.15 to include the complete functionality of the payroll application of Figs. 10.4–10.9. The application should still process two Invoice objects, but now should process one object of each of the four Employee subclasses. If the object currently being processed is a BasePlusCommissionEmployee, the application should increase the BasePlusCommissionEmployee’s base salary by 10%. Finally, the application should output the payment amount for each object. Complete the following steps to create the new application: a) Modify classes HourlyEmployee (Fig. 10.6) and CommissionEmployee (Fig. 10.7) to place them in the Payable hierarchy as subclasses of the version of Employee (Fig. 10.13) that implements Payable. [Hint: Change the name of method earnings to getPaymentAmount in each subclass so that the class satisfies its inherited contract with interface Payable.] b) Modify class BasePlusCommissionEmployee (Fig. 10.8) such that it extends the version of class CommissionEmployee created in part (a). c) Modify PayableInterfaceTest (Fig. 10.15) to polymorphically process two Invoices, one SalariedEmployee, one HourlyEmployee, one CommissionEmployee and one BasePlusCommissionEmployee. First output a String representation of each Payable object. Next, if an object is a BasePlusCommissionEmployee, increase its base salary by 10%. Finally, output the payment amount for each Payable object. 10.16 (Accounts Payable System Modification) It’s possible to include the functionality of the payroll application (Figs. 10.4–10.9) in the accounts payable application without modifying Employee subclasses SalariedEmployee, HourlyEmployee, CommissionEmployee or BasePlusCommissionEmployee. To do so, you can modify class Employee (Fig. 10.4) to implement interface Payable and declare method getPaymentAmount to invoke method earnings. Method getPaymentAmount would then be inherited by the subclasses in the Employee hierarchy. When getPaymentAmount is called for a particular subclass object, it polymorphically invokes the appropriate earnings method for that subclass. Reimplement Exercise 10.15 using the original Employee hierarchy from the payroll application of Figs. 10.4–10.9. Modify class Employee as described in this exercise, and do not modify any of class Employee’s subclasses. Making a Difference 10.17 (CarbonFootprint Interface: Polymorphism) Using interfaces, as you learned in this chapter, you can specify similar behaviors for possibly disparate classes. Governments and companies worldwide are becoming increasingly concerned with carbon footprints (annual releases of carbon dioxide into the atmosphere) from buildings burning various types of fuels for heat, vehicles burning fuels for power, and the like. Many scientists blame these greenhouse gases for the phenomenon called global warming. Create three small classes unrelated by inheritance—classes Building, Car and Bicycle. Give each class some unique appropriate attributes and behaviors that it does not have in common with other classes. Write an interface CarbonFootprint with a getCarbonFootprint method. Have each of your classes implement that interface, so that its getCarbonFootprint method calculates an appropriate carbon footprint for that class (check out a few websites that explain how to calculate carbon footprints). Write an application that creates objects of each of the three classes, places references to those objects in ArrayList, then iterates through the ArrayList, polymorphically invoking each object’s getCarbonFootprint method. For each object, print some identifying information and the object’s carbon footprint. 11 Exception Handling: A Deeper Look It is common sense to take a method and try it. If it fails, admit it frankly and try another. But above all, try something. —Franklin Delano Roosevelt O! throw away the worser part of it, And live the purer with the other half. —William Shakespeare If they’re running and they don’t look where they’re going I have to come out from somewhere and catch them. —Jerome David Salinger Objectives In this chapter you’ll: ■ ■ ■ ■ ■ ■ ■ ■ Learn what exceptions are and how they’re handled. Learn when to use exception handling. Use try blocks to delimit code in which exceptions might occur. Use throw to indicate a problem. Use catch blocks to specify exception handlers. Use the finally block to release resources. Understand the exception class hierarchy. Create user-defined exceptions. 442 Chapter 11 Exception Handling: A Deeper Look 11.1 Introduction 11.2 Example: Divide by Zero without Exception Handling 11.3 Example: Handling ArithmeticExceptions and InputMismatchExceptions 11.4 When to Use Exception Handling 11.5 Java Exception Hierarchy 11.6 finally Block 11.7 Stack Unwinding and Obtaining Information from an Exception Object 11.8 11.9 11.10 11.11 11.12 Chained Exceptions Declaring New Exception Types Preconditions and Postconditions Assertions try-with-Resources: Automatic Resource Deallocation 11.13 Wrap-Up Summary | Self-Review Exercises | Answers to Self-Review Exercises | Exercises 11.1 Introduction As you know from Chapter 7, an exception is an indication of a problem that occurs during a program’s execution. Exception handling enables you to create applications that can resolve (or handle) exceptions. In many cases, handling an exception allows a program to continute executing as if no problem had been encountered. The features presented in this chapter help you write robust and fault-tolerant programs that can deal with problems and continue executing or terminate gracefully. Java exception handling is based in part on the work of Andrew Koenig and Bjarne Stroustrup.1 First, we demonstrate basic exception-handling techniques by handling an exception that occurs when a method attempts to divide an integer by zero. Next, we introduce several classes at the top of Java’s exception-handling class hierarchy. As you’ll see, only classes that extend Throwable (package java.lang) directly or indirectly can be used with exception handling. We then show how to use chained exceptions—when you invoke a method that indicates an exception, you can throw another exception and chain the original one to the new one. This enables you to add application-specific information to the orginal exception. Next, we introduce preconditions and postconditions, which must be true when your methods are called and when they return, respectively. We then present assertions, which you can use at development time to help debug your code. We also discuss two exception-handling features that were introduced in Java SE 7—catching multiple exceptions with one catch handler and the new try-with-resources statement that automatically releases a resource after it’s used in the try block. This chapter focuses on the exception-handling concepts and presents several mechanical examples that demonstrate various features. As you’ll see in later chapters, many Java APIs methods throw exceptions that we handle in our code. Figure 11.1 shows some of the exception types you’ve already seen and others you’ll learn about. 1. A. Koenig and B. Stroustrup, “Exception Handling for C++ (revised),” Proceedings of the Usenix C++ Conference, pp. 149–176, San Francisco, April 1990. 11.2 Example: Divide by Zero without Exception Handling Chapter Sample of exceptions used Chapter 7 Chapters 8–10 Chapter 11 Chapter 15 ArrayIndexOutOfBoundsException 443 IllegalArgumentException ArithmeticException, InputMismatchException SecurityException, FileNotFoundException, IOException, ClassNotFoundException, IllegalStateException, FormatterClosedException, NoSuchElementException Chapter 16 ClassCastException, UnsupportedOperationException, NullPointerException, Chapter 20 Chapter 21 Chapter 23 Chapter 28 Chapter 24 Chapter 31 custom exception types custom exception types IllegalArgumentException, custom exception types InterruptedException, IllegalMonitorStateException, ExecutionException, CancellationException MalformedURLException, EOFException, SocketException, InterruptedException, UnknownHostException SQLException, IllegalStateException, PatternSyntaxException ClassCastException, SQLException Fig. 11.1 | Various exception types that you’ll see throughout this book 11.2 Example: Divide by Zero without Exception Handling First we demonstrate what happens when errors arise in an application that does not use exception handling. Figure 11.2 prompts the user for two integers and passes them to method quotient, which calculates the integer quotient and returns an int result. In this example, you’ll see that exceptions are thrown (i.e., the exception occurs) by a method when it detects a problem and is unable to handle it. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 // Fig. 11.2: DivideByZeroNoExceptionHandling.java // Integer division without exception handling. import java.util.Scanner; public class DivideByZeroNoExceptionHandling { // demonstrates throwing an exception when a divide-by-zero occurs public static int quotient(int numerator, int denominator) { return numerator / denominator; // possible division by zero } public static void main(String[] args) { Scanner scanner = new Scanner(System.in); Fig. 11.2 | Integer division without exception handling. (Part 1 of 2.) 444 16 17 18 19 20 21 22 23 24 25 26 Chapter 11 Exception Handling: A Deeper Look System.out.print("Please enter an integer numerator: "); int numerator = scanner.nextInt(); System.out.print("Please enter an integer denominator: "); int denominator = scanner.nextInt(); int result = quotient(numerator, denominator); System.out.printf( "%nResult: %d / %d = %d%n", numerator, denominator, result); } } // end class DivideByZeroNoExceptionHandling Please enter an integer numerator: 100 Please enter an integer denominator: 7 Result: 100 / 7 = 14 Please enter an integer numerator: 100 Please enter an integer denominator: 0 Exception in thread "main" java.lang.ArithmeticException: / by zero at DivideByZeroNoExceptionHandling.quotient( DivideByZeroNoExceptionHandling.java:10) at DivideByZeroNoExceptionHandling.main( DivideByZeroNoExceptionHandling.java:22) Please enter an integer numerator: 100 Please enter an integer denominator: hello Exception in thread "main" java.util.InputMismatchException at java.util.Scanner.throwFor(Unknown Source) at java.util.Scanner.next(Unknown Source) at java.util.Scanner.nextInt(Unknown Source) at java.util.Scanner.nextInt(Unknown Source) at DivideByZeroNoExceptionHandling.main( DivideByZeroNoExceptionHandling.java:20) Fig. 11.2 | Integer division without exception handling. (Part 2 of 2.) Stack Trace The first sample execution in Fig. 11.2 shows a successful division. In the second execution, the user enters the value 0 as the denominator. Several lines of information are displayed in response to this invalid input. This information is known as a stack trace, which includes the name of the exception (java.lang.ArithmeticException) in a descriptive message that indicates the problem that occurred and the method-call stack (i.e., the call chain) at the time it occurred. The stack trace includes the path of execution that led to the exception method by method. This helps you debug the program. Stack Trace for an ArithmeticException The first line specifies that an ArithmeticException has occurred. The text after the name of the exception (“/ by zero”) indicates that this exception occurred as a result of an attempt to divide by zero. Java does not allow division by zero in integer arithmetic. When this occurs, Java throws an ArithmeticException. ArithmeticExceptions can arise from 11.3 ArithmeticExceptions and InputMismatchExceptions 445 a number of different problems, so the extra data (“/ by zero”) provides more specific information. Java does allow division by zero with floating-point values. Such a calculation results in the value positive or negative infinity, which is represented in Java as a floatingpoint value (but displays as the string Infinity or -Infinity). If 0.0 is divided by 0.0, the result is NaN (not a number), which is also represented in Java as a floating-point value (but displays as NaN). If you need to compare a floating-point value to NaN, use the method isNaN of class Float (for float values) or of class Double (for double values). Classes Float and Double are in package java.lang. Starting from the last line of the stack trace, we see that the exception was detected in line 22 of method main. Each line of the stack trace contains the class name and method (e.g., DivideByZeroNoExceptionHandling.main) followed by the filename and line number (e.g., DivideByZeroNoExceptionHandling.java:22). Moving up the stack trace, we see that the exception occurs in line 10, in method quotient. The top row of the call chain indicates the throw point—the initial point at which the exception occurred. The throw point of this exception is in line 10 of method quotient. Stack Trace for an InputMismatchException In the third execution, the user enters the string "hello" as the denominator. Notice again that a stack trace is displayed. This informs us that an InputMismatchException has occurred (package java.util). Our prior examples that input numeric values assumed that the user would input a proper integer value. However, users sometimes make mistakes and input noninteger values. An InputMismatchException occurs when Scanner method nextInt receives a string that does not represent a valid integer. Starting from the end of the stack trace, we see that the exception was detected in line 20 of method main. Moving up the stack trace, we see that the exception occurred in method nextInt. Notice that in place of the filename and line number, we’re provided with the text Unknown Source. This means that the so-called debugging symbols that provide the filename and line number information for that method’s class were not available to the JVM—this is typically the case for the classes of the Java API. Many IDEs have access to the Java API source code and will display filenames and line numbers in stack traces. Program Termination In the sample executions of Fig. 11.2 when exceptions occur and stack traces are displayed, the program also exits. This does not always occur in Java. Sometimes a program may continue even though an exception has occurred and a stack trace has been printed. In such cases, the application may produce unexpected results. For example, a graphical user interface (GUI) application will often continue executing. In Fig. 11.2 both types of exceptions were detected in method main. In the next example, we’ll see how to handle these exceptions so that you can enable the program to run to normal completion. 11.3 Example: Handling ArithmeticExceptions and InputMismatchExceptions The application in Fig. 11.3, which is based on Fig. 11.2, uses exception handling to process any ArithmeticExceptions and InputMistmatchExceptions that arise. The application still prompts the user for two integers and passes them to method quotient, which calculates the quotient and returns an int result. This version of the application uses ex- 446 Chapter 11 Exception Handling: A Deeper Look ception handling so that if the user makes a mistake, the program catches and handles (i.e., deals with) the exception—in this case, allowing the user to re-enter the input. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 // Fig. 11.3: DivideByZeroWithExceptionHandling.java // Handling ArithmeticExceptions and InputMismatchExceptions. import java.util.InputMismatchException; import java.util.Scanner; public class DivideByZeroWithExceptionHandling { // demonstrates throwing an exception when a divide-by-zero occurs public static int quotient(int numerator, int denominator) throws ArithmeticException { return numerator / denominator; // possible division by zero } public static void main(String[] args) { Scanner scanner = new Scanner(System.in); boolean continueLoop = true; // determines if more input is needed do { try // read two numbers and calculate quotient { System.out.print("Please enter an integer numerator: "); int numerator = scanner.nextInt(); System.out.print("Please enter an integer denominator: "); int denominator = scanner.nextInt(); int result = quotient(numerator, denominator); System.out.printf("%nResult: %d / %d = %d%n", numerator, denominator, result); continueLoop = false; // input successful; end looping } catch (InputMismatchException inputMismatchException) { System.err.printf("%nException: %s%n", inputMismatchException); scanner.nextLine(); // discard input so user can try again System.out.printf( "You must enter integers. Please try again.%n%n"); } catch (ArithmeticException arithmeticException) { System.err.printf("%nException: %s%n", arithmeticException); System.out.printf( "Zero is an invalid denominator. Please try again.%n%n"); } } while (continueLoop); } } // end class DivideByZeroWithExceptionHandling Fig. 11.3 | Handling ArithmeticExceptions and InputMismatchExceptions. (Part 1 of 2.) 11.3 ArithmeticExceptions and InputMismatchExceptions 447 Please enter an integer numerator: 100 Please enter an integer denominator: 7 Result: 100 / 7 = 14 Please enter an integer numerator: 100 Please enter an integer denominator: 0 Exception: java.lang.ArithmeticException: / by zero Zero is an invalid denominator. Please try again. Please enter an integer numerator: 100 Please enter an integer denominator: 7 Result: 100 / 7 = 14 Please enter an integer numerator: 100 Please enter an integer denominator: hello Exception: java.util.InputMismatchException You must enter integers. Please try again. Please enter an integer numerator: 100 Please enter an integer denominator: 7 Result: 100 / 7 = 14 Fig. 11.3 | Handling ArithmeticExceptions and InputMismatchExceptions. (Part 2 of 2.) The first sample execution in Fig. 11.3 does not encounter any problems. In the second execution the user enters a zero denominator, and an ArithmeticException exception occurs. In the third execution the user enters the string "hello" as the denominator, and an InputMismatchException occurs. For each exception, the user is informed of the mistake and asked to try again, then is prompted for two new integers. In each sample execution, the program runs to completion successfully. Class InputMismatchException is imported in line 3. Class ArithmeticException does not need to be imported because it’s in package java.lang. Line 18 creates the boolean variable continueLoop, which is true if the user has not yet entered valid input. Lines 20–48 repeatedly ask users for input until a valid input is received. Enclosing Code in a try Block Lines 22–33 contain a try block, which encloses the code that might throw an exception and the code that should not execute if an exception occurs (i.e., if an exception occurs, the remaining code in the try block will be skipped). A try block consists of the keyword try followed by a block of code enclosed in curly braces. [Note: The term “try block” sometimes refers only to the block of code that follows the try keyword (not including the try keyword itself). For simplicity, we use the term “try block” to refer to the block of 448 Chapter 11 Exception Handling: A Deeper Look code that follows the try keyword, as well as the try keyword.] The statements that read the integers from the keyboard (lines 25 and 27) each use method nextInt to read an int value. Method nextInt throws an InputMismatchException if the value read in is not an integer. The division that can cause an ArithmeticException is not performed in the try block. Rather, the call to method quotient (line 29) invokes the code that attempts the division (line 12); the JVM throws an ArithmeticException object when the denominator is zero. Software Engineering Observation 11.1 Exceptions may surface through explicitly mentioned code in a try block, through deeply nested method calls initiated by code in a try block or from the Java Virtual Machine as it executes Java bytecodes. Catching Exceptions The try block in this example is followed by two catch blocks—one that handles an InputMismatchException (lines 34–41) and one that handles an ArithmeticException (lines 42–47). A catch block (also called a catch clause or exception handler) catches (i.e., receives) and handles an exception. A catch block begins with the keyword catch followed by a parameter in parentheses (called the exception parameter, discussed shortly) and a block of code enclosed in curly braces. [Note: The term “catch clause” is sometimes used to refer to the keyword catch followed by a block of code, whereas the term “catch block” refers to only the block of code following the catch keyword, but not including it. For simplicity, we use the term “catch block” to refer to the block of code following the catch keyword, as well as the keyword itself.] At least one catch block or a finally block (discussed in Section 11.6) must immediately follow the try block. Each catch block specifies in parentheses an exception parameter that identifies the exception type the handler can process. When an exception occurs in a try block, the catch block that executes is the first one whose type matches the type of the exception that occurred (i.e., the type in the catch block matches the thrown exception type exactly or is a direct or indirect superclass of it). The exception parameter’s name enables the catch block to interact with a caught exception object—e.g., to implicitly invoke the caught exception’s toString method (as in lines 37 and 44), which displays basic information about the exception. Notice that we use the System.err (standard error stream) object to output error messages. By default, System.err’s print methods, like those of System.out, display data to the command prompt. Line 38 of the first catch block calls Scanner method nextLine. Because an InputMismatchException occurred, the call to method nextInt never successfully read in the user’s data—so we read that input with a call to method nextLine. We do not do anything with the input at this point, because we know that it’s invalid. Each catch block displays an error message and asks the user to try again. After either catch block terminates, the user is prompted for input. We’ll soon take a deeper look at how this flow of control works in exception handling. Common Programming Error 11.1 It’s a syntax error to place code between a try block and its corresponding catch blocks. 11.3 ArithmeticExceptions and InputMismatchExceptions 449 Multi-catch It’s relatively common for a try block to be followed by several catch blocks to handle various types of exceptions. If the bodies of several catch blocks are identical, you can use the multi-catch feature (introduced in Java SE 7) to catch those exception types in a single catch handler and perform the same task. The syntax for a multi-catch is: catch (Type1 | Type2 | Type3 e) Each exception type is separated from the next with a vertical bar (|). The preceding line of code indicates that any of the types (or their subclasses) can be caught in the exception handler. Any number of Throwable types can be specified in a multi-catch. Uncaught Exceptions An uncaught exception is one for which there are no matching catch blocks. You saw uncaught exceptions in the second and third outputs of Fig. 11.2. Recall that when exceptions occurred in that example, the application terminated early (after displaying the exception’s stack trace). This does not always occur as a result of uncaught exceptions. Java uses a “multithreaded” model of program execution—each thread is a concurrent activity. One program can have many threads. If a program has only one thread, an uncaught exception will cause the program to terminate. If a program has multiple threads, an uncaught exception will terminate only the thread in which the exception occurred. In such programs, however, certain threads may rely on others, and if one thread terminates due to an uncaught exception, there may be adverse effects on the rest of the program. Chapter 23, Concurrency, discusses these issues in depth. Termination Model of Exception Handling If an exception occurs in a try block (such as an InputMismatchException being thrown as a result of the code at line 25 of Fig. 11.3), the try block terminates immediately and program control transfers to the first of the following catch blocks in which the exception parameter’s type matches the thrown exception’s type. In Fig. 11.3, the first catch block catches InputMismatchExceptions (which occur if invalid input is entered) and the second catch block catches ArithmeticExceptions (which occur if an attempt is made to divide by zero). After the exception is handled, program control does not return to the throw point, because the try block has expired (and its local variables have been lost). Rather, control resumes after the last catch block. This is known as the termination model of exception handling. Some languages use the resumption model of exception handling, in which, after an exception is handled, control resumes just after the throw point. Notice that we name our exception parameters (inputMismatchException and arithmeticException) based on their type. Java programmers often simply use the letter e as the name of their exception parameters. After executing a catch block, this program’s flow of control proceeds to the first statement after the last catch block (line 48 in this case). The condition in the do…while statement is true (variable continueLoop contains its initial value of true), so control returns to the beginning of the loop and the user is again prompted for input. This control statement will loop until valid input is entered. At that point, program control reaches line 32, which assigns false to variable continueLoop. The try block then terminates. If no exceptions are thrown in the try block, the catch blocks are skipped and control continues with the first statement after the catch blocks (we’ll learn about another possibility when 450 Chapter 11 Exception Handling: A Deeper Look we discuss the finally block in Section 11.6). Now the condition for the do…while loop is false, and method main ends. The try block and its corresponding catch and/or finally blocks form a try statement. Do not confuse the terms “try block” and “try statement”—the latter includes the try block as well as the following catch blocks and/or finally block. As with any other block of code, when a try block terminates, local variables declared in the block go out of scope and are no longer accessible; thus, the local variables of a try block are not accessible in the corresponding catch blocks. When a catch block terminates, local variables declared within the catch block (including the exception parameter of that catch block) also go out of scope and are destroyed. Any remaining catch blocks in the try statement are ignored, and execution resumes at the first line of code after the try…catch sequence—this will be a finally block, if one is present. Using the throws Clause In method quotient (Fig. 11.3, lines 9–13), line 10 is known as a throws clause. It specifies the exceptions the method might throw if problems occur. This clause, which must appear after the method’s parameter list and before the body, contains a comma-separated list of the exception types. Such exceptions may be thrown by statements in the method’s body or by methods called from there. We’ve added the throws clause to this application to indicate that this method might throw an ArithmeticException. Method quotient’s callers are thus informed that the method might throw an ArithmeticException. Some exception types, such as ArithmeticException, are not required to be listed in the throws clause. For those that are, the method can throw exceptions that have the is-a relationship with the classes listed in the throws clause. You’ll learn more about this in Section 11.5. Error-Prevention Tip 11.1 Read the online API documentation for a method before using it in a program. The documentation specifies the exceptions thrown by the method (if any) and indicates reasons why such exceptions may occur. Next, read the online API documentation for the specified exception classes. The documentation for an exception class typically contains potential reasons that such exceptions occur. Finally, provide for handling those exceptions in your program. When line 12 executes, if the denominator is zero, the JVM throws an ArithmeticException object. This object will be caught by the catch block at lines 42–47, which dis- plays basic information about the exception by implicitly invoking the exception’s method, then asks the user to try again. If the denominator is not zero, method quotient performs the division and returns the result to the point of invocation of method quotient in the try block (line 29). Lines 30–31 display the result of the calculation and line 32 sets continueLoop to false. In this case, the try block completes successfully, so the program skips the catch blocks and fails the condition at line 48, and method main completes execution normally. When quotient throws an ArithmeticException, quotient terminates and does not return a value, and quotient’s local variables go out of scope (and are destroyed). If quotient contained local variables that were references to objects and there were no other references to those objects, the objects would be marked for garbage collection. Also, when an exception occurs, the try block from which quotient was called terminates before lines 30–32 can execute. Here, too, if local variables were created in the try block prior to the exception’s being thrown, these variables would go out of scope. toString 11.4 When to Use Exception Handling 451 If an InputMismatchException is generated by lines 25 or 27, the try block terminates and execution continues with the catch block at lines 34–41. In this case, method quotient is not called. Then method main continues after the last catch block (line 48). 11.4 When to Use Exception Handling Exception handling is designed to process synchronous errors, which occur when a statement executes. Common examples we’ll see throughout the book are out-of-range array indices, arithmetic overflow (i.e., a value outside the representable range of values), division by zero, invalid method parameters and thread interruption (as we’ll see in Chapter 23). Exception handling is not designed to process problems associated with asynchronous events (e.g., disk I/O completions, network message arrivals, mouse clicks and keystrokes), which occur in parallel with, and independent of, the program’s flow of control. Software Engineering Observation 11.2 Incorporate your exception-handling and error-recovery strategy into your system from the inception of the design process—including these after a system has been implemented can be difficult. Software Engineering Observation 11.3 Exception handling provides a single, uniform technique for documenting, detecting and recovering from errors. This helps programmers working on large projects understand each other’s error-processing code. Software Engineering Observation 11.4 There’s a great variety of situations that generate exceptions—some exceptions are easier to recover from than others. 11.5 Java Exception Hierarchy All Java exception classes inherit directly or indirectly from class Exception, forming an inheritance hierarchy. You can extend this hierarchy with your own exception classes. Figure 11.4 shows a small portion of the inheritance hierarchy for class Throwable (a subclass of Object), which is the superclass of class Exception. Only Throwable objects can be used with the exception-handling mechanism. Class Throwable has two direct subclasses: Exception and Error. Class Exception and its subclasses—for instance, RuntimeException (package java.lang) and IOException (package java.io)—represent exceptional situations that can occur in a Java program and that can be caught by the application. Class Error and its subclasses represent abnormal situations that happen in the JVM. Most Errors happen infrequently and should not be caught by applications—it’s usually not possible for applications to recover from Errors. The Java exception hierarchy contains hundreds of classes. Information about Java’s exception classes can be found throughout the Java API. You can view Throwable’s documentation at docs.oracle.com/javase/7/docs/api/java/lang/Throwable.html. From there, you can look at this class’s subclasses to get more information about Java’s Exceptions and Errors. 452 Chapter 11 Exception Handling: A Deeper Look Throwable Exception RuntimeException Error AWTError IOException ClassCastException ThreadDeath NullPointerException VirtualMachineError ArithmeticException IndexOutOfBoundsException NoSuchElementException ArrayIndexOutOfBoundsException InputMismatchException Fig. 11.4 | Portion of class Throwable’s inheritance hierarchy. Checked vs. Unchecked Exceptions Java distinguishes between checked exceptions and unchecked exceptions. This distinction is important, because the Java compiler enforces special requirements for checked exceptions (discussed momentarily). An exception’s type determines whether it’s checked or unchecked. RuntimeExceptions Are Unchecked Exceptions All exception types that are direct or indirect subclasses of RuntimeException (package java.lang) are unchecked exceptions. These are typically caused by defects in your program’s code. Examples of unchecked exceptions include: • • ArrayIndexOutOfBoundsExceptions (discussed in Chapter 7)—You can avoid these by ensuring that your array indices are always greater than or equal to 0 and less than the array’s length. ArithmeticExceptions (shown in Fig. 11.3)—You can avoid the Arithmeticthat occurs when you divide by zero by checking the denominator to determine whether it’s 0 before performing the calculation. Exception Classes that inherit directly or indirectly from class Error (Fig. 11.4) are unchecked, because Errors are such serious problems that your program should not even attempt to deal with them. Checked Exceptions All classes that inherit from class Exception but not directly or indirectly from class RuntimeException are considered to be checked exceptions. Such exceptions are typically caused by conditions that are not under the control of the program—for example, in file processing, the program can’t open a file if it does not exist. 11.5 Java Exception Hierarchy 453 The Compiler and Checked Exceptions The compiler checks each method call and method declaration to determine whether the method throws a checked exception. If so, the compiler verifies that the checked exception is caught or is declared in a throws clause—this is known as the catch-or-declare requirement. We show how to catch or declare checked exceptions in the next several examples. Recall from Section 11.3 that the throws clause specifies the exceptions a method throws. Such exceptions are not caught in the method’s body. To satisfy the catch part of the catchor-declare requirement, the code that generates the exception must be wrapped in a try block and must provide a catch handler for the checked-exception type (or one of its superclasses). To satisfy the declare part of the catch-or-declare requirement, the method containing the code that generates the exception must provide a throws clause containing the checked-exception type after its parameter list and before its method body. If the catch-or-declare requirement is not satisfied, the compiler will issue an error message. This forces you to think about the problems that may occur when a method that throws checked exceptions is called. Error-Prevention Tip 11.2 You must deal with checked exceptions. This results in more robust code than would be created if you were able to simply ignore them. Common Programming Error 11.2 If a subclass method overrides a superclass method, it’s an error for the subclass method to list more exceptions in its throws clause than the superclass method does. However, a subclass’s throws clause can contain a subset of a superclass’s throws clause. Software Engineering Observation 11.5 If your method calls other methods that throw checked exceptions, those exceptions must be caught or declared. If an exception can be handled meaningfully in a method, the method should catch the exception rather than declare it. The Compiler and Unchecked Exceptions Unlike checked exceptions, the Java compiler does not examine the code to determine whether an unchecked exception is caught or declared. Unchecked exceptions typically can be prevented by proper coding. For example, the unchecked ArithmeticException thrown by method quotient (lines 9–13) in Fig. 11.3 can be avoided if the method ensures that the denominator is not zero before performing the division. Unchecked exceptions are not required to be listed in a method’s throws clause—even if they are, it’s not required that such exceptions be caught by an application. Software Engineering Observation 11.6 Although the compiler does not enforce the catch-or-declare requirement for unchecked exceptions, provide appropriate exception-handling code when it’s known that such exceptions might occur. For example, a program should process the NumberFormatException from Integer method parseInt, even though NumberFormatException is an indirect subclass of RuntimeException (and thus an unchecked exception). This makes your programs more robust. 454 Chapter 11 Exception Handling: A Deeper Look Catching Subclass Exceptions If a catch handler is written to catch superclass exception objects, it can also catch all objects of that class’s subclasses. This enables catch to handle related exceptions polymorphically. You can catch each subclass individually if those exceptions require different processing. Only the First Matching catch Executes If multiple catch blocks match a particular exception type, only the first matching catch block executes when an exception of that type occurs. It’s a compilation error to catch the exact same type in two different catch blocks associated with a particular try block. However, there can be several catch blocks that match an exception—i.e., several catch blocks whose types are the same as the exception type or a superclass of that type. For instance, we could follow a catch block for type ArithmeticException with a catch block for type Exception—both would match ArithmeticExceptions, but only the first matching catch block would execute. Common Programming Error 11.3 Placing a catch block for a superclass exception type before other catch blocks that catch subclass exception types would prevent those catch blocks from executing, so a compilation error occurs. Error-Prevention Tip 11.3 Catching subclass types individually is subject to error if you forget to test for one or more of the subclass types explicitly; catching the superclass guarantees that objects of all subclasses will be caught. Positioning a catch block for the superclass type after all other subclass catch blocks ensures that all subclass exceptions are eventually caught. Software Engineering Observation 11.7 In industry, throwing or catching type Exception is discouraged—we use it here simply to demonstrate exception-handling mechanics. In subsequent chapters, we generally throw and catch more specific exception types. 11.6 finally Block Programs that obtain certain resources must return them to the system to avoid so-called resource leaks. In programming languages such as C and C++, the most common resource leak is a memory leak. Java performs automatic garbage collection of memory no longer used by programs, thus avoiding most memory leaks. However, other types of resource leaks can occur. For example, files, database connections and network connections that are not closed properly after they’re no longer needed might not be available for use in other programs. Error-Prevention Tip 11.4 A subtle issue is that Java does not entirely eliminate memory leaks. Java will not garbagecollect an object until there are no remaining references to it. Thus, if you erroneously keep references to unwanted objects, memory leaks can occur. The finally block (which consists of the finally keyword, followed by code enclosed in curly braces), sometimes referred to as the finally clause, is optional. If it’s 11.6 finally Block present, it’s placed after the last catch block. If there are no block, if present, immediately follows the try block. catch blocks, the 455 finally When the finally Block Executes The finally block will execute whether or not an exception is thrown in the corresponding try block. The finally block also will execute if a try block exits by using a return, break or continue statement or simply by reaching its closing right brace. The one case in which the finally block will not execute is if the application exits early from a try block by calling method System.exit. This method, which we demonstrate in Chapter 15, immediately terminates an application. If an exception that occurs in a try block cannot be caught by one of that try block’s catch handlers, the program skips the rest of the try block and control proceeds to the finally block. Then the program passes the exception to the next outer try block—normally in the calling method—where an associated catch block might catch it. This process can occur through many levels of try blocks. Also, the exception could go uncaught (as we discussed in Section 11.3. If a catch block throws an exception, the finally block still executes. Then the exception is passed to the next outer try block—again, normally in the calling method. Releasing Resources in a finally Block Because a finally block always executes, it typically contains resource-release code. Suppose a resource is allocated in a try block. If no exception occurs, the catch blocks are skipped and control proceeds to the finally block, which frees the resource. Control then proceeds to the first statement after the finally block. If an exception occurs in the try block, the try block terminates. If the program catches the exception in one of the corresponding catch blocks, it processes the exception, then the finally block releases the resource and control proceeds to the first statement after the finally block. If the program doesn’t catch the exception, the finally block still releases the resource and an attempt is made to catch the exception in a calling method. Error-Prevention Tip 11.5 The finally block is an ideal place to release resources acquired in a try block (such as opened files), which helps eliminate resource leaks. Performance Tip 11.1 Always release a resource explicitly and at the earliest possible moment at which it’s no longer needed. This makes resources available for reuse as early as possible, thus improving resource utilization and program performance. Demonstrating the finally Block Figure 11.5 demonstrates that the finally block executes even if an exception is not thrown in the corresponding try block. The program contains static methods main (lines 6–18), throwException (lines 21–44) and doesNotThrowException (lines 47–64). Methods throwException and doesNotThrowException are declared static, so main can call them directly without instantiating a UsingExceptions object. 456 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 Chapter 11 Exception Handling: A Deeper Look // Fig. 11.5: UsingExceptions.java // try...catch...finally exception handling mechanism. public class UsingExceptions { public static void main(String[] args) { try { throwException(); } catch (Exception exception) // exception thrown by throwException { System.err.println("Exception handled in main"); } doesNotThrowException(); } // demonstrate try...catch...finally public static void throwException() throws Exception { try // throw an exception and immediately catch it { System.out.println("Method throwException"); throw new Exception(); // generate exception } catch (Exception exception) // catch exception thrown in try { System.err.println( "Exception handled in method throwException"); throw exception; // rethrow for further processing // code here would not be reached; would cause compilation errors } finally // executes regardless of what occurs in try...catch { System.err.println("Finally executed in throwException"); } // code here would not be reached; would cause compilation errors } // demonstrate finally when no exception occurs public static void doesNotThrowException() { try // try block does not throw an exception { System.out.println("Method doesNotThrowException"); } Fig. 11.5 | try…catch…finally exception-handling mechanism. (Part 1 of 2.) 11.6 finally Block 53 54 55 56 57 58 59 60 61 62 63 64 65 457 catch (Exception exception) // does not execute { System.err.println(exception); } finally // executes regardless of what occurs in try...catch { System.err.println( "Finally executed in doesNotThrowException"); } System.out.println("End of method doesNotThrowException"); } } // end class UsingExceptions Method throwException Exception handled in method throwException Finally executed in throwException Exception handled in main Method doesNotThrowException Finally executed in doesNotThrowException End of method doesNotThrowException Fig. 11.5 | try…catch…finally exception-handling mechanism. (Part 2 of 2.) System.out and System.err are streams—sequences of bytes. While System.out (known as the standard output stream) displays a program’s output, System.err (known as the standard error stream) displays a program’s errors. Output from these streams can be redirected (i.e., sent to somewhere other than the command prompt, such as to a file). Using two different streams enables you to easily separate error messages from other output. For instance, data output from System.err could be sent to a log file, while data output from System.out can be displayed on the screen. For simplicity, this chapter will not redirect output from System.err, but will display such messages to the command prompt. You’ll learn more about streams in Chapter 15. Throwing Exceptions Using the throw Statement Method main (Fig. 11.5) begins executing, enters its try block and immediately calls method throwException (line 10). Method throwException throws an Exception. The statement at line 26 is known as a throw statement—it’s executed to indicate that an exception has occurred. So far, you’ve caught only exceptions thrown by called methods. You can throw exceptions yourself by using the throw statement. Just as with exceptions thrown by the Java API’s methods, this indicates to client applications that an error has occurred. A throw statement specifies an object to be thrown. The operand of a throw can be of any class derived from class Throwable. Software Engineering Observation 11.8 When toString is invoked on any Throwable object, its resulting String includes the descriptive string that was supplied to the constructor, or simply the class name if no string was supplied. 458 Chapter 11 Exception Handling: A Deeper Look Software Engineering Observation 11.9 An exception can be thrown without containing information about the problem that occurred. In this case, simply knowing that an exception of a particular type occurred may provide sufficient information for the handler to process the problem correctly. Software Engineering Observation 11.10 Throw exceptions from constructors to indicate that the constructor parameters are not valid—this prevents an object from being created in an invalid state. Rethrowing Exceptions Line 32 of Fig. 11.5 rethrows the exception. Exceptions are rethrown when a catch block, upon receiving an exception, decides either that it cannot process that exception or that it can only partially process it. Rethrowing an exception defers the exception handling (or perhaps a portion of it) to another catch block associated with an outer try statement. An exception is rethrown by using the throw keyword, followed by a reference to the exception object that was just caught. Exceptions cannot be rethrown from a finally block, as the exception parameter (a local variable) from the catch block no longer exists. When a rethrow occurs, the next enclosing try block detects the rethrown exception, and that try block’s catch blocks attempt to handle it. In this case, the next enclosing try block is found at lines 8–11 in method main. Before the rethrown exception is handled, however, the finally block (lines 37–40) executes. Then method main detects the rethrown exception in the try block and handles it in the catch block (lines 12–15). Next, main calls method doesNotThrowException (line 17). No exception is thrown in doesNotThrowException’s try block (lines 49–52), so the program skips the catch block (lines 53–56), but the finally block (lines 57–61) nevertheless executes. Control proceeds to the statement after the finally block (line 63). Then control returns to main and the program terminates. Common Programming Error 11.4 If an exception has not been caught when control enters a finally block and the finally block throws an exception that’s not caught in the finally block, the first exception will be lost and the exception from the finally block will be returned to the calling method. Error-Prevention Tip 11.6 Avoid placing in a finally block code that can throw an exception. If such code is required, enclose the code in a try…catch within the finally block. Common Programming Error 11.5 Assuming that an exception thrown from a catch block will be processed by that catch block or any other catch block associated with the same try statement can lead to logic errors. Good Programming Practice 11.1 Exception handling removes error-processing code from the main line of a program’s code to improve program clarity. Do not place try…catch… finally around every statement that may throw an exception. This decreases readability. Rather, place one try block around a significant portion of your code, follow the try with catch blocks that handle each possible exception and follow the catch blocks with a single finally block (if one is required). 11.7 Stack Unwinding and Obtaining Information from an Exception Object 459 11.7 Stack Unwinding and Obtaining Information from an Exception Object When an exception is thrown but not caught in a particular scope, the method-call stack is “unwound,” and an attempt is made to catch the exception in the next outer try block. This process is called stack unwinding. Unwinding the method-call stack means that the method in which the exception was not caught terminates, all local variables in that method go out of scope and control returns to the statement that originally invoked that method. If a try block encloses that statement, an attempt is made to catch the exception. If a try block does not enclose that statement or if the exception is not caught, stack unwinding occurs again. Figure 11.6 demonstrates stack unwinding, and the exception handler in main shows how to access the data in an exception object. Stack Unwinding In main, the try block (lines 8–11) calls method1 (declared at lines 35–38), which in turn calls method2 (declared at lines 41–44), which in turn calls method3 (declared at lines 47– 50). Line 49 of method3 throws an Exception object—this is the throw point. Because the throw statement at line 49 is not enclosed in a try block, stack unwinding occurs— method3 terminates at line 49, then returns control to the statement in method2 that invoked method3 (i.e., line 43). Because no try block encloses line 43, stack unwinding occurs again—method2 terminates at line 43 and returns control to the statement in method1 that invoked method2 (i.e., line 37). Because no try block encloses line 37, stack unwinding occurs one more time—method1 terminates at line 37 and returns control to the statement in main that invoked method1 (i.e., line 10). The try block at lines 8–11 encloses this statement. The exception has not been handled, so the try block terminates and the first matching catch block (lines 12–31) catches and processes the exception. If there were no matching catch blocks, and the exception is not declared in each method that throws it, a compilation error would occur. Remember that this is not always the case—for unchecked exceptions, the application will compile, but it will run with unexpected results. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 // Fig. 11.6: UsingExceptions.java // Stack unwinding and obtaining data from an exception object. public class UsingExceptions { public static void main(String[] args) { try { method1(); } catch (Exception exception) // catch exception thrown in method1 { System.err.printf("%s%n%n", exception.getMessage()); exception.printStackTrace(); Fig. 11.6 | Stack unwinding and obtaining data from an exception object. (Part 1 of 2.) 460 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 Chapter 11 Exception Handling: A Deeper Look // obtain the stack-trace information StackTraceElement[] traceElements = exception.getStackTrace(); System.out.printf("%nStack trace from getStackTrace:%n"); System.out.println("Class\t\tFile\t\t\tLine\tMethod"); // loop through traceElements to get exception description for (StackTraceElement element : traceElements) { System.out.printf("%s\t", element.getClassName()); System.out.printf("%s\t", element.getFileName()); System.out.printf("%s\t", element.getLineNumber()); System.out.printf("%s%n", element.getMethodName()); } } } // end main // call method2; throw exceptions back to main public static void method1() throws Exception { method2(); } // call method3; throw exceptions back to method1 public static void method2() throws Exception { method3(); } // throw Exception back to method2 public static void method3() throws Exception { throw new Exception("Exception thrown in method3"); } } // end class UsingExceptions Exception thrown in method3 java.lang.Exception: Exception thrown in method3 at UsingExceptions.method3(UsingExceptions.java:49) at UsingExceptions.method2(UsingExceptions.java:43) at UsingExceptions.method1(UsingExceptions.java:37) at UsingExceptions.main(UsingExceptions.java:10) Stack trace from getStackTrace: Class File UsingExceptions UsingExceptions.java UsingExceptions UsingExceptions.java UsingExceptions UsingExceptions.java UsingExceptions UsingExceptions.java Line 49 43 37 10 Method method3 method2 method1 main Fig. 11.6 | Stack unwinding and obtaining data from an exception object. (Part 2 of 2.) 11.8 Chained Exceptions 461 Obtaining Data from an Exception Object All exceptions derive from class Throwable, which has a printStackTrace method that outputs to the standard error stream the stack trace (discussed in Section 11.2). Often this is helpful in testing and debugging. Class Throwable also provides a getStackTrace method that retrieves the stack-trace information that might be printed by printStackTrace. Class Throwable’s getMessage method returns the descriptive string stored in an exception. Error-Prevention Tip 11.7 An exception that’s not caught in an application causes Java’s default exception handler to run. This displays the name of the exception, a descriptive message that indicates the problem that occurred and a complete execution stack trace. In an application with a single thread of execution, the application terminates. In an application with multiple threads, the thread that caused the exception terminates. We discuss multithreading in Chapter 23. Error-Prevention Tip 11.8 method toString (inherited by all Throwable subclasses) returns a containing the name of the exception’s class and a descriptive message. Throwable String handler in Fig. 11.6 (lines 12–31) demonstrates getMessage, printand getStackTrace. If we wanted to output the stack-trace information to streams other than the standard error stream, we could use the information returned from getStackTrace and output it to another stream or use one of the overloaded versions of method printStackTrace. Sending data to other streams is discussed in Chapter 15. Line 14 invokes the exception’s getMessage method to get the exception description. Line 15 invokes the exception’s printStackTrace method to output the stack trace that indicates where the exception occurred. Line 18 invokes the exception’s getStackTrace method to obtain the stack-trace information as an array of StackTraceElement objects. Lines 24–30 get each StackTraceElement in the array and invoke its methods getClassName, getFileName, getLineNumber and getMethodName to get the class name, filename, line number and method name, respectively, for that StackTraceElement. Each StackTraceElement represents one method call on the method-call stack. The program’s output shows that output of printStackTrace follows the pattern: className.methodName(fileName:lineNumber), where className, methodName and fileName indicate the names of the class, method and file in which the exception occurred, respectively, and the lineNumber indicates where in the file the exception occurred. You saw this in the output for Fig. 11.2. Method getStackTrace enables custom processing of the exception information. Compare the output of printStackTrace with the output created from the StackTraceElements to see that both contain the same stack-trace information. The catch StackTrace Software Engineering Observation 11.11 Occasionally, you might want to ignore an exception by writing a catch handler with an empty body. Before doing so, ensure that the exception doesn’t indicate a condition that code higher up the stack might want to know about or recover from. 11.8 Chained Exceptions Sometimes a method responds to an exception by throwing a different exception type that’s specific to the current application. If a catch block throws a new exception, the orig- 462 Chapter 11 Exception Handling: A Deeper Look inal exception’s information and stack trace are lost. Earlier Java versions provided no mechanism to wrap the original exception information with the new exception’s information to provide a complete stack trace showing where the original problem occurred. This made debugging such problems particularly difficult. Chained exceptions enable an exception object to maintain the complete stack-trace information from the original exception. Figure 11.7 demonstrates chained exceptions. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 // Fig. 11.7: UsingChainedExceptions.java // Chained exceptions. public class UsingChainedExceptions { public static void main(String[] args) { try { method1(); } catch (Exception exception) // exceptions thrown from method1 { exception.printStackTrace(); } } // call method2; throw exceptions back to main public static void method1() throws Exception { try { method2(); } // end try catch (Exception exception) // exception thrown from method2 { throw new Exception("Exception thrown in method1", exception); } } // call method3; throw exceptions back to method1 public static void method2() throws Exception { try { method3(); } catch (Exception exception) // exception thrown from method3 { throw new Exception("Exception thrown in method2", exception); } } Fig. 11.7 | Chained exceptions. (Part 1 of 2.) 11.8 Chained Exceptions 44 45 46 47 48 49 463 // throw Exception back to method2 public static void method3() throws Exception { throw new Exception("Exception thrown in method3"); } } // end class UsingChainedExceptions java.lang.Exception: Exception thrown in method1 at UsingChainedExceptions.method1(UsingChainedExceptions.java:27) at UsingChainedExceptions.main(UsingChainedExceptions.java:10) Caused by: java.lang.Exception: Exception thrown in method2 at UsingChainedExceptions.method2(UsingChainedExceptions.java:40) at UsingChainedExceptions.method1(UsingChainedExceptions.java:23) ... 1 more Caused by: java.lang.Exception: Exception thrown in method3 at UsingChainedExceptions.method3(UsingChainedExceptions.java:47) at UsingChainedExceptions.method2(UsingChainedExceptions.java:36) ... 2 more Fig. 11.7 | Chained exceptions. (Part 2 of 2.) Program Flow of Control The program consists of four methods—main (lines 6–16), method1 (lines 19–29), method2 (lines 32–42) and method3 (lines 45–48). Line 10 in method main’s try block calls method1. Line 23 in method1’s try block calls method2. Line 36 in method2’s try block calls method3. In method3, line 47 throws a new Exception. Because this statement is not in a try block, method3 terminates, and the exception is returned to the calling method (method2) at line 36. This statement is in a try block; therefore, the try block terminates and the exception is caught at lines 38–41. Line 40 in the catch block throws a new exception. In this case, the Exception constructor with two arguments is called. The second argument represents the exception that was the original cause of the problem. In this program, that exception occurred at line 47. Because an exception is thrown from the catch block, method2 terminates and returns the new exception to the calling method (method1) at line 23. Once again, this statement is in a try block, so the try block terminates and the exception is caught at lines 25–28. Line 27 in the catch block throws a new exception and uses the exception that was caught as the second argument to the Exception constructor. Because an exception is thrown from the catch block, method1 terminates and returns the new exception to the calling method (main) at line 10. The try block in main terminates, and the exception is caught at lines 12–15. Line 14 prints a stack trace. Program Output Notice in the program output that the first three lines show the most recent exception that was thrown (i.e., the one from method1 at line 27). The next four lines indicate the exception that was thrown from method2 at line 40. Finally, the last four lines represent the exception that was thrown from method3 at line 47. Also notice that, as you read the output in reverse, it shows how many more chained exceptions remain. 464 Chapter 11 Exception Handling: A Deeper Look 11.9 Declaring New Exception Types Most Java programmers use existing classes from the Java API, third-party vendors and freely available class libraries (usually downloadable from the Internet) to build Java applications. The methods of those classes typically are declared to throw appropriate exceptions when problems occur. You write code that processes these existing exceptions to make your programs more robust. If you build classes that other programmers will use, it’s often appropriate to declare your own exception classes that are specific to the problems that can occur when another programmer uses your reusable classes. A New Exception Type Must Extend an Existing One A new exception class must extend an existing exception class to ensure that the class can be used with the exception-handling mechanism. An exception class is like any other class; however, a typical new exception class contains only four constructors: • one that takes no arguments and passes a default error message String to the superclass constructor • one that receives a customized error message as a String and passes it to the superclass constructor • one that receives a customized error message as a String and a Throwable (for chaining exceptions) and passes both to the superclass constructor • one that receives a Throwable (for chaining exceptions) and passes it to the superclass constructor. Good Programming Practice 11.2 Associating each type of serious execution-time malfunction with an appropriately named Exception class improves program clarity. Software Engineering Observation 11.12 When defining your own exception type, study the existing exception classes in the Java API and try to extend a related exception class. For example, if you’re creating a new class to represent when a method attempts a division by zero, you might extend class ArithmeticException because division by zero occurs during arithmetic. If the existing classes are not appropriate superclasses for your new exception class, decide whether your new class should be a checked or an unchecked exception class. If clients should be required to handle the exception, the new exception class should be a checked exception (i.e., extend Exception but not RuntimeException). The client application should be able to reasonably recover from such an exception. If the client code should be able to ignore the exception (i.e., the exception is an unchecked one), the new exception class should extend RuntimeException. Example of a Custom Exception Class In Chapter 21, Custom Generic Data Structures, we provide an example of a custom exception class. We declare a reusable class called List that’s capable of storing a list of references to objects. Some operations typically performed on a List are not allowed if the List is empty, such as removing an item from the front or back of the list. For this reason, some List methods throw exceptions of exception class EmptyListException. 11.10 Preconditions and Postconditions 465 Good Programming Practice 11.3 By convention, all exception-class names should end with the word Exception. 11.10 Preconditions and Postconditions Programmers spend significant amounts of time maintaining and debugging code. To facilitate these tasks and to improve the overall design, you can specify the expected states before and after a method’s execution. These states are called preconditions and postconditions, respectively. Preconditions A precondition must be true when a method is invoked. Preconditions describe constraints on method parameters and any other expectations the method has about the current state of a program just before it begins executing. If the preconditions are not met, then the method’s behavior is undefined—it may throw an exception, proceed with an illegal value or attempt to recover from the error. You should not expect consistent behavior if the preconditions are not satisfied. Postconditions A postcondition is true after the method successfully returns. Postconditions describe constraints on the return value and any other side effects the method may have. When defining a method, you should document all postconditions so that others know what to expect when they call your method, and you should make certain that your method honors all its postconditions if its preconditions are indeed met. Throwing Exceptions When Preconditions or Postconditions Are Not Met When their preconditions or postconditions are not met, methods typically throw exceptions. As an example, examine String method charAt, which has one int parameter—an index in the String. For a precondition, method charAt assumes that index is greater than or equal to zero and less than the length of the String. If the precondition is met, the postcondition states that the method will return the character at the position in the String specified by the parameter index. Otherwise, the method throws an IndexOutOfBoundsException. We trust that method charAt satisfies its postcondition, provided that we meet the precondition. We need not be concerned with the details of how the method actually retrieves the character at the index. Typically, a method’s preconditions and postconditions are described as part of its specification. When designing your own methods, you should state the preconditions and postconditions in a comment before the method declaration. 11.11 Assertions When implementing and debugging a class, it’s sometimes useful to state conditions that should be true at a particular point in a method. These conditions, called assertions, help ensure a program’s validity by catching potential bugs and identifying possible logic errors during development. Preconditions and postconditions are two types of assertions. Preconditions are assertions about a program’s state when a method is invoked, and postconditions are assertions about its state after a method finishes. 466 Chapter 11 Exception Handling: A Deeper Look While assertions can be stated as comments to guide you during program development, Java includes two versions of the assert statement for validating assertions programatically. The assert statement evaluates a boolean expression and, if false, throws an AssertionError (a subclass of Error). The first form of the assert statement is assert expression; which throws an AssertionError if expression is false. The second form is assert expression1 : expression2; which evaluates expression1 and throws an AssertionError with expression2 as the error message if expression1 is false. You can use assertions to implement preconditions and postconditions programmatically or to verify any other intermediate states that help you ensure that your code is working correctly. Figure 11.8 demonstrates the assert statement. Line 11 prompts the user to enter a number between 0 and 10, then line 12 reads the number. Line 15 determines whether the user entered a number within the valid range. If the number is out of range, the assert statement reports an error; otherwise, the program proceeds normally. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 // Fig. 11.8: AssertTest.java // Checking with assert that a value is within range import java.util.Scanner; public class AssertTest { public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.print("Enter a number between 0 and 10: "); int number = input.nextInt(); // assert that the value is >= 0 and <= 10 assert (number >= 0 && number <= 10) : "bad number: " + number; System.out.printf("You entered %d%n", number); } } // end class AssertTest Enter a number between 0 and 10: 5 You entered 5 Enter a number between 0 and 10: 50 Exception in thread "main" java.lang.AssertionError: bad number: 50 at AssertTest.main(AssertTest.java:15) Fig. 11.8 | Checking with assert that a value is within range. You use assertions primarily for debugging and identifying logic errors in an application. You must explicitly enable assertions when executing a program, because they reduce 11.12 try-with-Resources: Automatic Resource Deallocation performance and are unnecessary for the program’s user. To do so, use the mand’s -ea command-line option, as in java 467 com- java -ea AssertTest Software Engineering Observation 11.13 Users shouldn’t encounter AssertionErrors—these should be used only during program development. For this reason, you shouldn’t catch AssertionErrors. Instread, allow the program to terminate, so you can see the error message, then locate and fix the source of the problem. You should not use assert to indicate runtime problems in production code (as we did in Fig. 11.8 for demonstration purposes)—use the exception mechanism for this purpose. 11.12 try-with-Resources: Automatic Resource Deallocation Typically resource-release code should be placed in a finally block to ensure that a resource is released, regardless of whether there were exceptions when the resource was used in the corresponding try block. An alternative notation—the try-with-resources statement (introduced in Java SE 7)—simplifies writing code in which you obtain one or more resources, use them in a try block and release them in a corresponding finally block. For example, a file-processing application could process a file with a try-with-resources statement to ensure that the file is closed properly when it’s no longer needed—we demonstrate this in Chapter 15. Each resource must be an object of a class that implements the AutoCloseable interface, and thus provides a close method. The general form of a try-withresources statement is try (ClassName theObject = new ClassName()) { // use theObject here } catch (Exception e) { // catch exceptions that occur while using the resource } where ClassName is a class that implements the AutoCloseable interface. This code creates an object of type ClassName and uses it in the try block, then calls its close method to release any resources used by the object. The try-with-resources statement implicitly calls the theObject’s close method at the end of the try block. You can allocate multiple resources in the parentheses following try by separating them with a semicolon (;). You’ll see examples of the try-with-resources statement in Chapters 15 and 24. 11.13 Wrap-Up In this chapter, you learned how to use exception handling to deal with errors. You learned that exception handling enables you to remove error-handling code from the “main line” of the program’s execution. We showed how to use try blocks to enclose code that may throw an exception, and how to use catch blocks to deal with exceptions that may arise. You learned about the termination model of exception handling, which dictates that after an exception is handled, program control does not return to the throw point. We dis- 468 Chapter 11 Exception Handling: A Deeper Look cussed checked vs. unchecked exceptions, and how to specify with the throws clause the exceptions that a method might throw. You learned how to use the finally block to release resources whether or not an exception occurs. You also learned how to throw and rethrow exceptions. We showed how to obtain information about an exception using methods printStackTrace, getStackTrace and getMessage. Next, we presented chained exceptions, which allow you to wrap original exception information with new exception information. Then, we showed how to create your own exception classes. We introduced preconditions and postconditions to help programmers using your methods understand conditions that must be true when the method is called and when it returns, respectively. When preconditions and postconditions are not met, methods typically throw exceptions. We discussed the assert statement and how it can be used to help you debug your programs. In particular, assert can be used to ensure that preconditions and postconditions are met. We also introduced multi-catch for processing several types of exceptions in the same catch handler and the try-with-resources statement for automatically deallocating a resource after it’s used in the try block. In the next chapter, we take a deeper look at graphical user interfaces (GUIs). Summary Section 11.1 Introduction • An exception is an indication of a problem that occurs during a program’s execution. • Exception handling enables programmers to create applications that can resolve exceptions. Section 11.2 Example: Divide by Zero without Exception Handling • Exceptions are thrown (p. 443) when a method detects a problem and is unable to handle it. • An exception’s stack trace (p. 444) includes the name of the exception in a message that indicates the problem that occurred and the complete method-call stack at the time the exception occurred. • The point in the program at which an exception occurs is called the throw point (p. 445). Section 11.3 Example: Handling ArithmeticExceptions and InputMismatchExceptions • A try block (p. 447) encloses code that might throw an exception and code that should not execute if that exception occurs. • Exceptions may surface through explicitly mentioned code in a try block, through calls to other methods or even through deeply nested method calls initiated by code in the try block. • A catch block (p. 448) begins with keyword catch and an exception parameter followed by a block of code that handles the exception. This code executes when the try block detects the exception. • At least one catch block or a finally block (p. 448) must immediately follow the try block. • A catch block specifies in parentheses an exception parameter identifying the exception type to handle. The parameter’s name enables the catch block to interact with a caught exception object. • An uncaught exception (p. 449) is an exception that occurs for which there are no matching catch blocks. An uncaught exception will cause a program to terminate early if that program contains only one thread. Otherwise, only the thread in which the exception occurred will terminate. The rest of the program will run but possibly with adverse results. Summary 469 • Multi-catch (p. 449) enables you to catch multiple exception types in a single catch handler and perform the same task for each type of exception. The syntax for a multi-catch is: catch (Type1 | Type2 | Type3 e) • Each exception type is separated from the next with a vertical bar (|). • If an exception occurs in a try block, the try block terminates immediately and program control transfers to the first catch block with a parameter type that matches the thrown exception’s type. • After an exception is handled, program control does not return to the throw point, because the try block has expired. This is known as the termination model of exception handling (p. 449). • If there are multiple matching catch blocks when an exception occurs, only the first is executed. • A throws clause (p. 450) specifies a comma-separated list of exceptions that the method might throw, and appears after the method’s parameter list and before the method body. Section 11.4 When to Use Exception Handling • Exception handling processes synchronous errors (p. 451), which occur when a statement executes. • Exception handling is not designed to process problems associated with asynchronous events (p. 451), which occur in parallel with, and independent of, the program’s flow of control. Section 11.5 Java Exception Hierarchy • All Java exception classes inherit directly or indirectly from class Exception. • Programmers can extend the Java exception hierarchy with their own exception classes. • Class Throwable is the superclass of class Exception and is therefore also the superclass of all exceptions. Only Throwable objects can be used with the exception-handling mechanism. • Class Throwable (p. 451) has two subclasses: Exception and Error. • Class Exception and its subclasses represent problems that could occur in a Java program and be caught by the application. • Class Error and its subclasses represent problems that could happen in the Java runtime system. Errors happen infrequently and typically should not be caught by an application. • Java distinguishes between two categories of exceptions (p. 452): checked and unchecked. • The Java compiler does not check to determine if an unchecked exception is caught or declared. Unchecked exceptions typically can be prevented by proper coding. • Subclasses of RuntimeException represent unchecked exceptions. All exception types that inherit from class Exception but not from RuntimeException (p. 452) are checked. • If a catch block is written to catch exception objects of a superclass type, it can also catch all objects of that class’s subclasses. This allows for polymorphic processing of related exceptions. Section 11.6 finally Block • Programs that obtain certain types of resources must return them to the system to avoid so-called resource leaks (p. 454). Resource-release code typically is placed in a finally block (p. 454). • The finally block is optional. If it’s present, it’s placed after the last catch block. • The finally block will execute whether or not an exception is thrown in the corresponding try block or any of its corresponding catch blocks. • If an exception cannot be caught by one of that try block’s associated catch handlers, control proceeds to the finally block. Then the exception is passed to the next outer try block. • If a catch block throws an exception, the finally block still executes. Then the exception is passed to the next outer try block. 470 Chapter 11 Exception Handling: A Deeper Look • A throw statement (p. 457) can throw any Throwable object. • Exceptions are rethrown (p. 458) when a catch block, upon receiving an exception, decides either that it cannot process that exception or that it can only partially process it. Rethrowing an exception defers the exception handling (or perhaps a portion of it) to another catch block. • When a rethrow occurs, the next enclosing try block detects the rethrown exception, and that try block’s catch blocks attempt to handle it. Section 11.7 Stack Unwinding and Obtaining Information from an Exception Object • When an exception is thrown but not caught in a particular scope, the method-call stack is unwound, and an attempt is made to catch the exception in the next outer try statement. • Class Throwable offers a printStackTrace method that prints the method-call stack. Often, this is helpful in testing and debugging. • Class Throwable also provides a getStackTrace method that obtains the same stack-trace information that’s printed by printStackTrace (p. 461). • Class Throwable’s getMessage method (p. 461) returns the descriptive string stored in an exception. • Method getStackTrace (p. 461) obtains the stack-trace information as an array of StackTraceElement objects. Each StackTraceElement represents one method call on the method-call stack. • StackTraceElement methods (p. 461) getClassName, getFileName, getLineNumber and getMethodName get the class name, filename, line number and method name, respectively. Section 11.8 Chained Exceptions • Chained exceptions (p. 462) enable an exception object to maintain the complete stack-trace information, including information about previous exceptions that caused the current exception. Section 11.9 Declaring New Exception Types • A new exception class must extend an existing exception class to ensure that the class can be used with the exception-handling mechanism. Section 11.10 Preconditions and Postconditions • A method’s precondition (p. 465) must be true when the method is invoked. • A method’s postcondition (p. 465) is true after the method successfully returns. • When designing your own methods, you should state the preconditions and postconditions in a comment before the method declaration. Section 11.11 Assertions • Assertions (p. 465) help catch potential bugs and identify possible logic errors. • The assert statement (p. 466) allows for validating assertions programmatically. • To enable assertions at runtime, use the -ea switch when running the java command. Section 11.12 try-with-Resources: Automatic Resource Deallocation • The try-with-resources statement (p. 467) simplifies writing code in which you obtain a resource, use it in a try block and release the resource in a corresponding finally block. Instead, you allocate the resource in the parentheses following the try keyword and use the resource in the try block; then the statement implicitly calls the resource’s close method at the end of the try block. • Each resource must be an object of a class that implements the AutoCloseable interface (p. 467)—such a class has a close method. Self-Review Exercises 471 • You can allocate multiple resources in the parentheses following try by separating them with a semicolon (;). Self-Review Exercises 11.1 List five common examples of exceptions. 11.2 Why are exceptions particularly appropriate for dealing with errors produced by methods of classes in the Java API? 11.3 What is a “resource leak”? 11.4 If no exceptions are thrown in a block completes execution? try block, where does control proceed to when the 11.5 Give a key advantage of using catch(Exception exceptionName). 11.6 Should a conventional application catch Error objects? Explain. 11.7 What happens if no catch handler matches the type of a thrown object? 11.8 What happens if several catch blocks match the type of the thrown object? 11.9 Why would a programmer specify a superclass type as the type in a catch block? try 11.10 What is the key reason for using finally blocks? 11.11 What happens when a catch block throws an Exception? 11.12 What does the statement throw exceptionReference do in a catch block? 11.13 What happens to a local reference in a try block when that block throws an Exception? Answers to Self-Review Exercises 11.1 Memory exhaustion, array index out of bounds, arithmetic overflow, division by zero, invalid method parameters. 11.2 It’s unlikely that methods of classes in the Java API could perform error processing that would meet the unique needs of all users. 11.3 A “resource leak” occurs when an executing program does not properly release a resource when it’s no longer needed. 11.4 The catch blocks for that try statement are skipped, and the program resumes execution after the last catch block. If there’s a finally block, it’s executed first; then the program resumes execution after the finally block. 11.5 The form catch(Exception exceptionName) catches any type of exception thrown in a try block. An advantage is that no thrown Exception can slip by without being caught. You can handle the exception or rethrow it. 11.6 Errors are usually serious problems with the underlying Java system; most programs will not want to catch Errors because they will not be able to recover from them. 11.7 This causes the search for a match to continue in the next enclosing try statement. If there’s a finally block, it will be executed before the exception goes to the next enclosing try statement. If there are no enclosing try statements for which there are matching catch blocks and the exceptions are declared (or unchecked), a stack trace is printed and the current thread terminates early. If the exceptions are checked, but not caught or declared, compilation errors occur. 11.8 The first matching catch block after the try block is executed. 472 Chapter 11 Exception Handling: A Deeper Look 11.9 This enables a program to catch related types of exceptions and process them in a uniform manner. However, it’s often useful to process the subclass types individually for more precise exception handling. 11.10 The finally block is the preferred means for releasing resources to prevent resource leaks. 11.11 First, control passes to the finally block if there is one. Then the exception will be processed by a catch block (if one exists) associated with an enclosing try block (if one exists). 11.12 It rethrows the exception for processing by an exception handler of an enclosing try statement, after the finally block of the current try statement executes. 11.13 The reference goes out of scope. If the referenced object becomes unreachable, the object can be garbage collected. Exercises 11.14 (Exceptional Conditions) List the various exceptional conditions that have occurred in programs throughout this text so far. List as many additional exceptional conditions as you can. For each of these, describe briefly how a program typically would handle the exception by using the exception-handling techniques discussed in this chapter. Typical exceptions include division by zero and array index out of bounds. 11.15 (Exceptions and Constructor Failure) Until this chapter, we’ve found dealing with errors detected by constructors to be a bit awkward. Explain why exception handling is an effective means for dealing with constructor failure. 11.16 (Catching Exceptions with Superclasses) Use inheritance to create an exception superclass (called ExceptionA) and exception subclasses ExceptionB and ExceptionC, where ExceptionB inherits from ExceptionA and ExceptionC inherits from ExceptionB. Write a program to demonstrate that the catch block for type ExceptionA catches exceptions of types ExceptionB and ExceptionC. 11.17 (Catching Exceptions Using Class Exception) Write a program that demonstrates how various exceptions are caught with catch (Exception exception) This time, define classes ExceptionA (which inherits from class Exception) and ExceptionB (which inherits from class ExceptionA). In your program, create try blocks that throw exceptions of types ExceptionA, ExceptionB, NullPointerException and IOException. All exceptions should be caught with catch blocks specifying type Exception. 11.18 (Order of catch Blocks) Write a program demonstrating that the order of catch blocks is important. If you try to catch a superclass exception type before a subclass type, the compiler should generate errors. 11.19 (Constructor Failure) Write a program that shows a constructor passing information about constructor failure to an exception handler. Define class SomeClass, which throws an Exception in the constructor. Your program should try to create an object of type SomeClass and catch the exception that’s thrown from the constructor. 11.20 (Rethrowing Exceptions) Write a program that illustrates rethrowing an exception. Define methods someMethod and someMethod2. Method someMethod2 should initially throw an exception. Method someMethod should call someMethod2, catch the exception and rethrow it. Call someMethod from method main, and catch the rethrown exception. Print the stack trace of this exception. 11.21 (Catching Exceptions Using Outer Scopes) Write a program showing that a method with its own try block does not have to catch every possible error generated within the try. Some exceptions can slip through to, and be handled in, other scopes. 12 GUI Components: Part 1 Do you think I can listen all day to such stuff? —Lewis Carroll Even a minor event in the life of a child is an event of that child’s world and thus a world event. —Gaston Bachelard You pays your money and you takes your choice. —Punch Objectives In this chapter you’ll: ■ Learn how to use the Nimbus look-and-feel. ■ Build GUIs and handle events generated by user interactions with GUIs. ■ Understand the packages containing GUI components, event-handling classes and interfaces. ■ Create and manipulate buttons, labels, lists, text fields and panels. ■ Handle mouse events and keyboard events. ■ Use layout managers to arrange GUI components. 474 Chapter 12 GUI Components: Part 1 12.1 Introduction 12.2 Java’s Nimbus Look-and-Feel 12.3 Simple GUI-Based Input/Output with JOptionPane 12.4 Overview of Swing Components 12.5 Displaying Text and Images in a Window 12.6 Text Fields and an Introduction to Event Handling with Nested Classes 12.7 Common GUI Event Types and Listener Interfaces 12.8 How Event Handling Works 12.9 JButton 12.10 Buttons That Maintain State 12.10.1 JCheckBox 12.10.2 JRadioButton 12.11 JComboBox; Using an Anonymous Inner Class for Event Handling 12.12 JList 12.13 Multiple-Selection Lists 12.14 Mouse Event Handling 12.15 Adapter Classes 12.16 JPanel Subclass for Drawing with the Mouse 12.17 Key Event Handling 12.18 Introduction to Layout Managers 12.18.1 FlowLayout 12.18.2 BorderLayout 12.18.3 GridLayout 12.19 Using Panels to Manage More Complex Layouts 12.20 JTextArea 12.21 Wrap-Up Summary | Self-Review Exercises | Answers to Self-Review Exercises | Exercises | Making a Difference 12.1 Introduction A graphical user interface (GUI) presents a user-friendly mechanism for interacting with an application. A GUI (pronounced “GOO-ee”) gives an application a distinctive “lookand-feel.” GUIs are built from GUI components. These are sometimes called controls or widgets—short for window gadgets. A GUI component is an object with which the user interacts via the mouse, the keyboard or another form of input, such as voice recognition. In this chapter and Chapter 22, GUI Components: Part 2, you’ll learn about many of Java’s so-called Swing GUI components from the javax.swing package. We cover other GUI components as they’re needed throughout the book. In Chapter 25 and two online chapters, you’ll learn about JavaFX—Java’s latest APIs for GUI, graphics and multimedia. Look-and-Feel Observation 12.1 Providing different applications with consistent, intuitive user-interface components gives users a sense of familiarity with a new application, so that they can learn it more quickly and use it more productively. IDE Support for GUI Design Many IDEs provide GUI design tools with which you can specify a component’s size, location and other attributes in a visual manner by using the mouse, the keyboard and dragand-drop. The IDEs generate the GUI code for you. This greatly simplifies creating GUIs, but each IDE generates this code differently. For this reason, we wrote the GUI code by hand, as you’ll see in the source-code files for this chapter’s examples. We encourage you to build each GUI visually using your preferred IDE(s). 12.2 Java’s Nimbus Look-and-Feel 475 Sample GUI: The SwingSet3 Demo Application As an example of a GUI, consider Fig. 12.1, which shows the SwingSet3 demo application from the JDK demos and samples download at http://www.oracle.com/technetwork/ java/javase/downloads/index.html. This application is a nice way for you to browse through the various GUI components provided by Java’s Swing GUI APIs. Simply click a component name (e.g., JFrame, JTabbedPane, etc.) in the GUI Components area at the left of the window to see a demonstration of the GUI component in the right side of the window. The source code for each demo is shown in the text area at the bottom of the window. We’ve labeled a few of the GUI components in the application. At the top of the window is a title bar that contains the window’s title. Below that is a menu bar containing menus (File and View). In the top-right region of the window is a set of buttons—typically, users click buttons to perform tasks. In the GUI Components area of the window is a combo box; the user can click the down arrow at the right side of the box to select from a list of items. The menus, buttons and combo box are part of the application’s GUI. They enable you to interact with the application. menu title bar menu bar combo box text area button scroll bar Fig. 12.1 | SwingSet3 application demonstrates many of Java’s Swing GUI components. 12.2 Java’s Nimbus Look-and-Feel A GUI’s look consists of its visual aspects, such as its colors and fonts, and its feel consists of the components you use to interact with the GUI, such as buttons and menus. Together 476 Chapter 12 GUI Components: Part 1 these are known as the GUI’s look-and-feel. Swing has a cross-platform look-and-feel known as Nimbus. For GUI screen captures like Fig. 12.1, we’ve configured our systems to use Nimbus as the default look-and-feel. There are three ways that you can use Nimbus: 1. Set it as the default for all Java applications that run on your computer. 2. Set it as the look-and-feel at the time that you launch an application by passing a command-line argument to the java command. 3. Set it as the look-and-feel programatically in your application (see Section 22.6). To set Nimbus as the default for all Java applications, you must create a text file named swing.properties in the lib folder of both your JDK installation folder and your JRE installation folder. Place the following line of code in the file: swing.defaultlaf=com.sun.java.swing.plaf.nimbus.NimbusLookAndFeel In addition to the standalone JRE, there is a JRE nested in your JDK’s installation folder. If you’re using an IDE that depends on the JDK, you may also need to place the swing.properties file in the nested jre folder’s lib folder. If you prefer to select Nimbus on an application-by-application basis, place the following command-line argument after the java command and before the application’s name when you run the application: -Dswing.defaultlaf=com.sun.java.swing.plaf.nimbus.NimbusLookAndFeel 12.3 Simple GUI-Based Input/Output with JOptionPane The applications in Chapters 2–10 display text in the command window and obtain input from the command window. Most applications you use on a daily basis use windows or dialog boxes (also called dialogs) to interact with the user. For example, an e-mail program allows you to type and read messages in a window the program provides. Dialog boxes are windows in which programs display important messages to the user or obtain information from the user. Java’s JOptionPane class (package javax.swing) provides prebuilt dialog boxes for both input and output. These are displayed by invoking static JOptionPane methods. Figure 12.2 presents a simple addition application that uses two input dialogs to obtain integers from the user and a message dialog to display the sum of the integers the user enters. 1 2 3 4 5 6 7 8 9 10 11 // Fig. 12.2: Addition.java // Addition program that uses JOptionPane for input and output. import javax.swing.JOptionPane; public class Addition { public static void main(String[] args) { // obtain user input from JOptionPane input dialogs String firstNumber = JOptionPane.showInputDialog("Enter first integer"); Fig. 12.2 | Addition program that uses JOptionPane for input and output. (Part 1 of 2.) 12.3 Simple GUI-Based Input/Output with JOptionPane 12 13 14 15 16 17 18 19 20 21 22 23 24 25 477 String secondNumber = JOptionPane.showInputDialog("Enter second integer"); // convert String inputs to int values for use in a calculation int number1 = Integer.parseInt(firstNumber); int number2 = Integer.parseInt(secondNumber); int sum = number1 + number2; // display result in a JOptionPane message dialog JOptionPane.showMessageDialog(null, "The sum is " + sum, "Sum of Two Integers", JOptionPane.PLAIN_MESSAGE); } } // end class Addition (a) Input dialog displayed by lines 10–11 Prompt to the user Text field in which the user types a value When the user clicks OK, showInputDialog returns to the program the 100 typed by the user as a String; the program must convert the String to an int (b) Input dialog displayed by lines 12–13 (c) Message dialog displayed by lines 22–23—When the user clicks OK, the message dialog is dismissed (removed from the screen) Fig. 12.2 | Addition program that uses JOptionPane for input and output. (Part 2 of 2.) Input Dialogs Line 3 imports class JOptionPane. Lines 10–11 declare the local String variable firstNumber and assign it the result of the call to JOptionPane static method showInputDialog. This method displays an input dialog (see the screen capture in Fig. 12.2(a)), using the method’s String argument ("Enter first integer") as a prompt. Look-and-Feel Observation 12.2 The prompt in an input dialog typically uses sentence-style capitalization—a style that capitalizes only the first letter of the first word in the text unless the word is a proper noun (for example, Jones). The user types characters in the text field, then clicks OK or presses the Enter key to submit the String to the program. Clicking OK also dismisses (hides) the dialog. [Note: If you type in the text field and nothing appears, activate the text field by clicking it with the mouse.] Unlike Scanner, which can be used to input values of several types from the user at the keyboard, an input dialog can input only Strings. This is typical of most GUI 478 Chapter 12 GUI Components: Part 1 components. The user can type any characters in the input dialog’s text field. Our program assumes that the user enters a valid integer. If the user clicks Cancel, showInputDialog returns null. If the user either types a noninteger value or clicks the Cancel button in the input dialog, an exception will occur and the program will not operate correctly. Lines 12– 13 display another input dialog that prompts the user to enter the second integer. Each JOptionPane dialog that you display is a so called modal dialog—while the dialog is on the screen, the user cannot interact with the rest of the application. Look-and-Feel Observation 12.3 Do not overuse modal dialogs, as they can reduce the usability of your applications. Use a modal dialog only when it’s necessary to prevent users from interacting with the rest of an application until they dismiss the dialog. Converting Strings to int Values To perform the calculation, we convert the Strings that the user entered to int values. Recall that the Integer class’s static method parseInt converts its String argument to an int value and might throw a NumberFormatException. Lines 16–17 assign the converted values to local variables number1 and number2, and line 19 sums these values. Message Dialogs Lines 22–23 use JOptionPane static method showMessageDialog to display a message dialog (the last screen of Fig. 12.2) containing the sum. The first argument helps the Java application determine where to position the dialog box. A dialog is typically displayed from a GUI application with its own window. The first argument refers to that window (known as the parent window) and causes the dialog to appear centered over the parent (as we’ll do in Section 12.9). If the first argument is null, the dialog box is displayed at the center of your screen. The second argument is the message to display—in this case, the result of concatenating the String "The sum is " and the value of sum. The third argument—"Sum of Two Integers"—is the String that should appear in the title bar at the top of the dialog. The fourth argument—JOptionPane.PLAIN_MESSAGE—is the type of message dialog to display. A PLAIN_MESSAGE dialog does not display an icon to the left of the message. Class JOptionPane provides several overloaded versions of methods showInputDialog and showMessageDialog, as well as methods that display other dialog types. For complete information, visit http:// docs.oracle.com/javase/7/docs/api/javax/swing/JOptionPane.html. Look-and-Feel Observation 12.4 The title bar of a window typically uses book-title capitalization—a style that capitalizes the first letter of each significant word in the text and does not end with any punctuation (for example, Capitalization in a Book Title). Message Dialog Constants The constants that represent the message dialog types are shown in Fig. 12.3. All message dialog types except PLAIN_MESSAGE display an icon to the left of the message. These icons provide a visual indication of the message’s importance to the user. A QUESTION_MESSAGE icon is the default icon for an input dialog box (see Fig. 12.2). JOptionPane 12.4 Overview of Swing Components Message dialog type Icon Description ERROR_MESSAGE Indicates an error. INFORMATION_MESSAGE Indicates an informational message. WARNING_MESSAGE Warns of a potential problem. QUESTION_MESSAGE Poses a question. This dialog normally requires a response, such as clicking a Yes or a No button. PLAIN_MESSAGE Fig. 12.3 | no icon JOptionPane static 479 A dialog that contains a message, but no icon. constants for message dialogs. 12.4 Overview of Swing Components Though it’s possible to perform input and output using the JOptionPane dialogs, most GUI applications require more elaborate user interfaces. The remainder of this chapter discusses many GUI components that enable application developers to create robust GUIs. Figure 12.4 lists several basic Swing GUI components that we discuss. Component Description JLabel Displays uneditable text and/or icons. Typically receives input from the user. Triggers an event when clicked with the mouse. Specifies an option that can be selected or not selected. A drop-down list of items from which the user can make a selection. A list of items from which the user can make a selection by clicking on any one of them. Multiple elements can be selected. An area in which components can be placed and organized. JTextField JButton JCheckBox JComboBox JList JPanel Fig. 12.4 | Some basic Swing GUI components. Swing vs. AWT There are actually two sets of Java GUI components. In Java’s early days, GUIs were built with components from the Abstract Window Toolkit (AWT) in package java.awt. These look like the native GUI components of the platform on which a Java program executes. For example, a Button object displayed in a Java program running on Microsoft Windows looks like those in other Windows applications. On Apple Mac OS X, the Button looks like those in other Mac applications. Sometimes, even the manner in which a user can interact with an AWT component differs between platforms. The component’s appearance and the way in which the user interacts with it are known as its look-and-feel. 480 Chapter 12 GUI Components: Part 1 Look-and-Feel Observation 12.5 Swing GUI components allow you to specify a uniform look-and-feel for your application across all platforms or to use each platform’s custom look-and-feel. An application can even change the look-and-feel during execution to enable users to choose their own preferred look-and-feel. Lightweight vs. Heavyweight GUI Components Most Swing components are lightweight components—they’re written, manipulated and displayed completely in Java. AWT components are heavyweight components, because they rely on the local platform’s windowing system to determine their functionality and their look-and-feel. Several Swing components are heavyweight components. Superclasses of Swing’s Lightweight GUI Components The UML class diagram of Fig. 12.5 shows an inheritance hierarchy of classes from which lightweight Swing components inherit their common attributes and behaviors. Object Component Container JComponent Fig. 12.5 | Common superclasses of the lightweight Swing components. Class Component (package java.awt) is a superclass that declares the common features of GUI components in packages java.awt and javax.swing. Any object that is a Container (package java.awt) can be used to organize Components by attaching the Components to the Container. Containers can be placed in other Containers to organize a GUI. Class JComponent (package javax.swing) is a subclass of Container. JComponent is the superclass of all lightweight Swing components and declares their common attributes and behaviors. Because JComponent is a subclass of Container, all lightweight Swing components are also Containers. Some common features supported by JComponent include: 1. A pluggable look-and-feel for customizing the appearance of components (e.g., for use on particular platforms). You’ll see an example of this in Section 22.6. 2. Shortcut keys (called mnemonics) for direct access to GUI components through the keyboard. You’ll see an example of this in Section 22.4. 3. Brief descriptions of a GUI component’s purpose (called tool tips) that are displayed when the mouse cursor is positioned over the component for a short time. You’ll see an example of this in the next section. 4. Support for accessibility, such as braille screen readers for the visually impaired. 5. Support for user-interface localization—that is, customizing the user interface to display in different languages and use local cultural conventions. 12.5 Displaying Text and Images in a Window 481 12.5 Displaying Text and Images in a Window Our next example introduces a framework for building GUI applications. Several concepts in this framework will appear in many of our GUI applications. This is our first example in which the application appears in its own window. Most windows you’ll create that can contain Swing GUI components are instances of class JFrame or a subclass of JFrame. JFrame is an indirect subclass of class java.awt.Window that provides the basic attributes and behaviors of a window—a title bar at the top, and buttons to minimize, maximize and close the window. Since an application’s GUI is typically specific to the application, most of our examples will consist of two classes—a subclass of JFrame that helps us demonstrate new GUI concepts and an application class in which main creates and displays the application’s primary window. Labeling GUI Components A typical GUI consists of many components. GUI designers often provide text stating the purpose of each. Such text is known as a label and is created with a JLabel—a subclass of JComponent. A JLabel displays read-only text, an image, or both text and an image. Applications rarely change a label’s contents after creating it. Look-and-Feel Observation 12.6 Text in a JLabel normally uses sentence-style capitalization. The application of Figs. 12.6–12.7 demonstrates several JLabel features and presents the framework we use in most of our GUI examples. We did not highlight the code in this example, since most of it is new. [Note: There are many more features for each GUI component than we can cover in our examples. To learn the complete details of each GUI component, visit its page in the online documentation. For class JLabel, visit docs.oracle.com/javase/7/docs/api/javax/swing/JLabel.html.] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 // Fig. 12.6: LabelFrame.java // JLabels with text and icons. import java.awt.FlowLayout; // specifies how components are arranged import javax.swing.JFrame; // provides basic window features import javax.swing.JLabel; // displays text and images import javax.swing.SwingConstants; // common constants used with Swing import javax.swing.Icon; // interface used to manipulate images import javax.swing.ImageIcon; // loads images public class LabelFrame { private final JLabel private final JLabel private final JLabel extends JFrame label1; // JLabel with just text label2; // JLabel constructed with text and icon label3; // JLabel with added text and icon // LabelFrame constructor adds JLabels to JFrame public LabelFrame() { Fig. 12.6 | JLabels with text and icons. (Part 1 of 2.) 482 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 Chapter 12 GUI Components: Part 1 super("Testing JLabel"); setLayout(new FlowLayout()); // set frame layout // JLabel constructor with a string argument label1 = new JLabel("Label with text"); label1.setToolTipText("This is label1"); add(label1); // add label1 to JFrame // JLabel constructor with string, Icon and alignment arguments Icon bug = new ImageIcon(getClass().getResource( "bug1.png")); label2 = new JLabel("Label with text and icon", bug, SwingConstants.LEFT); label2.setToolTipText("This is label2"); add(label2); // add label2 to JFrame label3 = new JLabel(); // JLabel constructor no arguments label3.setText("Label with icon and text at bottom"); label3.setIcon(bug); // add icon to JLabel label3.setHorizontalTextPosition(SwingConstants.CENTER); label3.setVerticalTextPosition(SwingConstants.BOTTOM); label3.setToolTipText("This is label3"); add(label3); // add label3 to JFrame } } // end class LabelFrame Fig. 12.6 | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 JLabels with text and icons. (Part 2 of 2.) // Fig. 12.7: LabelTest.java // Testing LabelFrame. import javax.swing.JFrame; public class LabelTest { public static void main(String[] args) { LabelFrame labelFrame = new LabelFrame(); labelFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); labelFrame.setSize(260, 180); labelFrame.setVisible(true); } } // end class LabelTest Fig. 12.7 | Testing LabelFrame. 12.5 Displaying Text and Images in a Window 483 Class LabelFrame (Fig. 12.6) extends JFrame to inherit the features of a window. We’ll use an instance of class LabelFrame to display a window containing three JLabels. Lines 12–14 declare the three JLabel instance variables that are instantiated in the LabelFrame constructor (lines 17–41). Typically, the JFrame subclass’s constructor builds the GUI that’s displayed in the window when the application executes. Line 19 invokes superclass JFrame’s constructor with the argument "Testing JLabel". JFrame’s constructor uses this String as the text in the window’s title bar. Specifying the Layout When building a GUI, you must attach each GUI component to a container, such as a window created with a JFrame. Also, you typically must decide where to position each GUI component—known as specifying the layout. Java provides several layout managers that can help you position components, as you’ll learn later in this chapter and in Chapter 22. Many IDEs provide GUI design tools in which you can specify components’ exact sizes and locations in a visual manner by using the mouse; then the IDE will generate the GUI code for you. Such IDEs can greatly simplify GUI creation. To ensure that our GUIs can be used with any IDE, we did not use an IDE to create the GUI code. We use Java’s layout managers to size and position components. With the FlowLayout layout manager, components are placed in a container from left to right in the order in which they’re added. When no more components can fit on the current line, they continue to display left to right on the next line. If the container is resized, a FlowLayout reflows the components, possibly with fewer or more rows based on the new container width. Every container has a default layout, which we’re changing for LabelFrame to a FlowLayout (line 20). Method setLayout is inherited into class LabelFrame indirectly from class Container. The argument to the method must be an object of a class that implements the LayoutManager interface (e.g., FlowLayout). Line 20 creates a new FlowLayout object and passes its reference as the argument to setLayout. Creating and Attaching label1 Now that we’ve specified the window’s layout, we can begin creating and attaching GUI components to the window. Line 23 creates a JLabel object and passes "Label with text" to the constructor. The JLabel displays this text on the screen. Line 24 uses method setToolTipText (inherited by JLabel from JComponent) to specify the tool tip that’s displayed when the user positions the mouse cursor over the JLabel in the GUI. You can see a sample tool tip in the second screen capture of Fig. 12.7. When you execute this application, hover the mouse pointer over each JLabel to see its tool tip. Line 25 (Fig. 12.6) attaches label1 to the LabelFrame by passing label1 to the add method, which is inherited indirectly from class Container. Common Programming Error 12.1 If you do not explicitly add a GUI component to a container, the GUI component will not be displayed when the container appears on the screen. Look-and-Feel Observation 12.7 Use tool tips to add descriptive text to your GUI components. This text helps the user determine the GUI component’s purpose in the user interface. 484 Chapter 12 GUI Components: Part 1 The Icon Interface and Class ImageIcon Icons are a popular way to enhance the look-and-feel of an application and are also commonly used to indicate functionality. For example, the same icon is used to play most of today’s media on devices like DVD players and MP3 players. Several Swing components can display images. An icon is normally specified with an Icon (package javax.swing) argument to a constructor or to the component’s setIcon method. Class ImageIcon supports several image formats, including Graphics Interchange Format (GIF), Portable Network Graphics (PNG) and Joint Photographic Experts Group (JPEG). Line 28 declares an ImageIcon. The file bug1.png contains the image to load and store in the ImageIcon object. This image is included in the directory for this example. The ImageIcon object is assigned to Icon reference bug. Loading an Image Resource In line 28, the expression getClass().getResource("bug1.png") invokes method getClass (inherited indirectly from class Object) to retrieve a reference to the Class object that represents the LabelFrame class declaration. That reference is then used to invoke Class method getResource, which returns the location of the image as a URL. The ImageIcon constructor uses the URL to locate the image, then loads it into memory. As we discussed in Chapter 1, the JVM loads class declarations into memory, using a class loader. The class loader knows where each class it loads is located on disk. Method getResource uses the Class object’s class loader to determine the location of a resource, such as an image file. In this example, the image file is stored in the same location as the LabelFrame.class file. The techniques described here enable an application to load image files from locations that are relative to the class file’s location. Creating and Attaching label2 Lines 29–30 use another JLabel constructor to create a JLabel that displays the text "Label with text and icon" and the Icon bug created in line 28. The last constructor argument indicates that the label’s contents are left justified, or left aligned (i.e., the icon and text are at the left side of the label’s area on the screen). Interface SwingConstants (package javax.swing) declares a set of common integer constants (such as SwingConstants.LEFT, SwingConstants.CENTER and SwingConstants.RIGHT) that are used with many Swing components. By default, the text appears to the right of the image when a label contains both text and an image. The horizontal and vertical alignments of a JLabel can be set with methods setHorizontalAlignment and setVerticalAlignment, respectively. Line 31 specifies the tool-tip text for label2, and line 32 adds label2 to the JFrame. Creating and Attaching label3 Class JLabel provides methods to change a JLabel’s appearance after it’s been instantiated. Line 34 creates an empty JLabel with the no-argument constructor. Line 35 uses JLabel method setText to set the text displayed on the label. Method getText can be used to retrieve the JLabel’s current text. Line 36 uses JLabel method setIcon to specify the Icon to display. Method getIcon can be used to retrieve the current Icon displayed on a label. Lines 37–38 use JLabel methods setHorizontalTextPosition and setVerticalTextPosition to specify the text position in the label. In this case, the text will be centered horizontally and will appear at the bottom of the label. Thus, the Icon will appear above the text. The horizontal-position constants in SwingConstants are LEFT, CENTER and RIGHT (Fig. 12.8). 12.6 Text Fields and an Introduction to Event Handling with Nested Classes 485 The vertical-position constants in SwingConstants are TOP, CENTER and BOTTOM (Fig. 12.8). Line 39 (Fig. 12.6) sets the tool-tip text for label3. Line 40 adds label3 to the JFrame. Constant Description Horizontal-position constants LEFT Place text on the left CENTER Place text in the center RIGHT Place text on the right Constant Description Vertical-position constants TOP Place text at the top CENTER Place text in the center BOTTOM Place text at the bottom Fig. 12.8 | Positioning constants (static members of interface SwingConstants). Creating and Displaying a LabelFrame Window Class LabelTest (Fig. 12.7) creates an object of class LabelFrame (line 9), then specifies the default close operation for the window. By default, closing a window simply hides the window. However, when the user closes the LabelFrame window, we would like the application to terminate. Line 10 invokes LabelFrame’s setDefaultCloseOperation method (inherited from class JFrame) with constant JFrame.EXIT_ON_CLOSE as the argument to indicate that the program should terminate when the window is closed by the user. This line is important. Without it the application will not terminate when the user closes the window. Next, line 11 invokes LabelFrame’s setSize method to specify the width and height of the window in pixels. Finally, line 12 invokes LabelFrame’s setVisible method with the argument true to display the window on the screen. Try resizing the window to see how the FlowLayout changes the JLabel positions as the window width changes. 12.6 Text Fields and an Introduction to Event Handling with Nested Classes Normally, a user interacts with an application’s GUI to indicate the tasks that the application should perform. For example, when you write an e-mail in an e-mail application, clicking the Send button tells the application to send the e-mail to the specified e-mail addresses. GUIs are event driven. When the user interacts with a GUI component, the interaction—known as an event—drives the program to perform a task. Some common user interactions that cause an application to perform a task include clicking a button, typing in a text field, selecting an item from a menu, closing a window and moving the mouse. The code that performs a task in response to an event is called an event handler, and the process of responding to events is known as event handling. Let’s consider two other GUI components that can generate events—JTextFields and JPasswordFields (package javax.swing). Class JTextField extends class JTextComponent (package javax.swing.text), which provides many features common to Swing’s text-based components. Class JPasswordField extends JTextField and adds methods that are specific to processing passwords. Each of these components is a single-line area in which the user can enter text via the keyboard. Applications can also display text in a JTextField (see the output of Fig. 12.10). A JPasswordField shows that characters are being typed as the user enters them, but hides the actual characters with an echo character, assuming that they represent a password that should remain known only to the user. 486 Chapter 12 GUI Components: Part 1 When the user types in a JTextField or a JPasswordField, then presses Enter, an event occurs. Our next example demonstrates how a program can perform a task in response to that event. The techniques shown here are applicable to all GUI components that generate events. The application of Figs. 12.9–12.10 uses classes JTextField and JPasswordField to create and manipulate four text fields. When the user types in one of the text fields, then presses Enter, the application displays a message dialog box containing the text the user typed. You can type only in the text field that’s “in focus.” When you click a component, it receives the focus. This is important, because the text field with the focus is the one that generates an event when you press Enter. In this example, when you press Enter in the JPasswordField, the password is revealed. We begin by discussing the setup of the GUI, then discuss the event-handling code. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 // Fig. 12.9: TextFieldFrame.java // JTextFields and JPasswordFields. import java.awt.FlowLayout; import java.awt.event.ActionListener; import java.awt.event.ActionEvent; import javax.swing.JFrame; import javax.swing.JTextField; import javax.swing.JPasswordField; import javax.swing.JOptionPane; public class TextFieldFrame extends JFrame { private final JTextField textField1; // text field with private final JTextField textField2; // text field with private final JTextField textField3; // text field with private final JPasswordField passwordField; // password set size text text and size field with text // TextFieldFrame constructor adds JTextFields to JFrame public TextFieldFrame() { super("Testing JTextField and JPasswordField"); setLayout(new FlowLayout()); Fig. 12.9 | // construct text field with 10 columns textField1 = new JTextField(10); add(textField1); // add textField1 to JFrame // construct text field with default text textField2 = new JTextField("Enter text here"); add(textField2); // add textField2 to JFrame // construct text field with default text and 21 columns textField3 = new JTextField("Uneditable text field", 21); textField3.setEditable(false); // disable editing add(textField3); // add textField3 to JFrame JTextFields and JPasswordFields. (Part 1 of 2.) 12.6 Text Fields and an Introduction to Event Handling with Nested Classes 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 487 // construct password field with default text passwordField = new JPasswordField("Hidden text"); add(passwordField); // add passwordField to JFrame // register event handlers TextFieldHandler handler = new TextFieldHandler(); textField1.addActionListener(handler); textField2.addActionListener(handler); textField3.addActionListener(handler); passwordField.addActionListener(handler); } // private inner class for event handling private class TextFieldHandler implements ActionListener { // process text field events @Override public void actionPerformed(ActionEvent event) { String string = ""; // user pressed Enter in JTextField textField1 if (event.getSource() == textField1) string = String.format("textField1: %s", event.getActionCommand()); // user pressed Enter in JTextField textField2 else if (event.getSource() == textField2) string = String.format("textField2: %s", event.getActionCommand()); // user pressed Enter in JTextField textField3 else if (event.getSource() == textField3) string = String.format("textField3: %s", event.getActionCommand()); // user pressed Enter in JTextField passwordField else if (event.getSource() == passwordField) string = String.format("passwordField: %s", event.getActionCommand()); // display JTextField content JOptionPane.showMessageDialog(null, string); } } // end private inner class TextFieldHandler } // end class TextFieldFrame Fig. 12.9 | JTextFields and JPasswordFields. (Part 2 of 2.) Class TextFieldFrame extends JFrame and declares three JTextField variables and a variable (lines 13–16). Each of the corresponding text fields is instantiated and attached to the TextFieldFrame in the constructor (lines 19–47). JPasswordField 488 Chapter 12 GUI Components: Part 1 Creating the GUI Line 22 sets the TextFieldFrame’s layout to FlowLayout. Line 25 creates textField1 with 10 columns of text. A text column’s width in pixels is determined by the average width of a character in the text field’s current font. When text is displayed in a text field and the text is wider than the field itself, a portion of the text at the right side is not visible. If you’re typing in a text field and the cursor reaches the right edge, the text at the left edge is pushed off the left side of the field and is no longer visible. Users can use the left and right arrow keys to move through the complete text. Line 26 adds textField1 to the JFrame. Line 29 creates textField2 with the initial text "Enter text here" to display in the text field. The width of the field is determined by the width of the default text specified in the constructor. Line 30 adds textField2 to the JFrame. Line 33 creates textField3 and calls the JTextField constructor with two arguments—the default text "Uneditable text field" to display and the text field’s width in columns (21). Line 34 uses method setEditable (inherited by JTextField from class JTextComponent) to make the text field uneditable—i.e., the user cannot modify the text in the field. Line 35 adds textField3 to the JFrame. Line 38 creates passwordField with the text "Hidden text" to display in the text field. The width of the field is determined by the width of the default text. When you execute the application, notice that the text is displayed as a string of asterisks. Line 39 adds passwordField to the JFrame. Steps Required to Set Up Event Handling for a GUI Component This example should display a message dialog containing the text from a text field when the user presses Enter in that text field. Before an application can respond to an event for a particular GUI component, you must: 1. Create a class that represents the event handler and implements an appropriate interface—known as an event-listener interface. 2. Indicate that an object of the class from Step 1 should be notified when the event occurs—known as registering the event handler. Using a Nested Class to Implement an Event Handler All the classes discussed so far were so-called top-level classes—that is, they were not declared inside another class. Java allows you to declare classes inside other classes—these are called nested classes. Nested classes can be static or non-static. Non-static nested classes are called inner classes and are frequently used to implement event handlers. An inner-class object must be created by an object of the top-level class that contains the inner class. Each inner-class object implicitly has a reference to an object of its top-level class. The inner-class object is allowed to use this implicit reference to directly access all the variables and methods of the top-level class. A nested class that’s static does not require an object of its top-level class and does not implicitly have a reference to an object of the top-level class. As you’ll see in Chapter 13, Graphics and Java 2D, the Java 2D graphics API uses static nested classes extensively. Inner Class TextFieldHandler The event handling in this example is performed by an object of the private inner class TextFieldHandler (lines 50–81). This class is private because it will be used only to create event handlers for the text fields in top-level class TextFieldFrame. As with other class 12.6 Text Fields and an Introduction to Event Handling with Nested Classes 489 members, inner classes can be declared public, protected or private. Since event handlers tend to be specific to the application in which they’re defined, they’re often implemented as private inner classes or as anonymous inner classes (Section 12.11). GUI components can generate many events in response to user interactions. Each event is represented by a class and can be processed only by the appropriate type of event handler. Normally, a component’s supported events are described in the Java API documentation for that component’s class and its superclasses. When the user presses Enter in a JTextField or JPasswordField, an ActionEvent (package java.awt.event) occurs. Such an event is processed by an object that implements the interface ActionListener (package java.awt.event). The information discussed here is available in the Java API documentation for classes JTextField and ActionEvent. Since JPasswordField is a subclass of JTextField, JPasswordField supports the same events. To prepare to handle the events in this example, inner class TextFieldHandler implements interface ActionListener and declares the only method in that interface— actionPerformed (lines 53–80). This method specifies the tasks to perform when an ActionEvent occurs. So, inner class TextFieldHandler satisfies Step 1 listed earlier in this section. We’ll discuss the details of method actionPerformed shortly. Registering the Event Handler for Each Text Field In the TextFieldFrame constructor, line 42 creates a TextFieldHandler object and assigns it to variable handler. This object’s actionPerformed method will be called automatically when the user presses Enter in any of the GUI’s text fields. However, before this can occur, the program must register this object as the event handler for each text field. Lines 43–46 are the event-registration statements that specify handler as the event handler for the three JTextFields and the JPasswordField. The application calls JTextField method addActionListener to register the event handler for each component. This method receives as its argument an ActionListener object, which can be an object of any class that implements ActionListener. The object handler is an ActionListener, because class TextFieldHandler implements ActionListener. After lines 43–46 execute, the object handler listens for events. Now, when the user presses Enter in any of these four text fields, method actionPerformed (line 53–80) in class TextFieldHandler is called to handle the event. If an event handler is not registered for a particular text field, the event that occurs when the user presses Enter in that text field is consumed—i.e., it’s simply ignored by the application. Software Engineering Observation 12.1 The event listener for an event must implement the appropriate event-listener interface. Common Programming Error 12.2 If you forget to register an event-handler object for a particular GUI component’s event type, events of that type will be ignored. Details of Class TextFieldHandler’s actionPerformed Method In this example, we use one event-handling object’s actionPerformed method (lines 53–80) to handle the events generated by four text fields. Since we’d like to output the name of each text field’s instance variable for demonstration purposes, we must determine which text field 490 Chapter 12 GUI Components: Part 1 generated the event each time actionPerformed is called. The event source is the component with which the user interacted. When the user presses Enter while a text field or the password field has the focus, the system creates a unique ActionEvent object that contains information about the event that just occurred, such as the event source and the text in the text field. The system passes this ActionEvent object to the event listener’s actionPerformed method. Line 56 declares the String that will be displayed. The variable is initialized with the empty string—a String containing no characters. The compiler requires the variable to be initialized in case none of the branches of the nested if in lines 59–76 executes. ActionEvent method getSource (called in lines 59, 64, 69 and 74) returns a reference to the event source. The condition in line 59 asks, “Is the event source textField1?” This condition compares references with the == operator to determine if they refer to the same object. If they both refer to textField1, the user pressed Enter in textField1. Then, lines 60–61 create a String containing the message that line 79 displays in a message dialog. Line 61 uses ActionEvent method getActionCommand to obtain the text the user typed in the text field that generated the event. In this example, we display the text of the password in the JPasswordField when the user presses Enter in that field. Sometimes it’s necessary to programatically process the characters in a password. Class JPasswordField method getPassword returns the password’s characters as an array of type char. Class TextFieldTest Class TextFieldTest (Fig. 12.10) contains the main method that executes this application and displays an object of class TextFieldFrame. When you execute the application, even the uneditable JTextField (textField3) can generate an ActionEvent. To test this, click the text field to give it the focus, then press Enter. Also, the actual text of the password is displayed when you press Enter in the JPasswordField. Of course, you would normally not display the password! 1 2 3 4 5 6 7 8 9 10 11 12 13 14 // Fig. 12.10: TextFieldTest.java // Testing TextFieldFrame. import javax.swing.JFrame; public class TextFieldTest { public static void main(String[] args) { TextFieldFrame textFieldFrame = new TextFieldFrame(); textFieldFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); textFieldFrame.setSize(350, 100); textFieldFrame.setVisible(true); } } // end class TextFieldTest Fig. 12.10 | Testing TextFieldFrame. (Part 1 of 2.) 12.7 Common GUI Event Types and Listener Interfaces 491 Fig. 12.10 | Testing TextFieldFrame. (Part 2 of 2.) This application used a single object of class TextFieldHandler as the event listener for four text fields. Starting in Section 12.10, you’ll see that it’s possible to declare several event-listener objects of the same type and register each object for a separate GUI component’s event. This technique enables us to eliminate the if…else logic used in this example’s event handler by providing separate event handlers for each component’s events. Java SE 8: Implementing Event Listeners with Lambdas Recall that interfaces like ActionListener that have only one abstract method are functional interfaces in Java SE 8. In Section 17.9, we show a more concise way to implement such event-listener interfaces with Java SE 8 lambdas. 12.7 Common GUI Event Types and Listener Interfaces In Section 12.6, you learned that information about the event that occurs when the user presses Enter in a text field is stored in an ActionEvent object. Many different types of events can occur when the user interacts with a GUI. The event information is stored in an object of a class that extends AWTEvent (from package java.awt). Figure 12.11 illustrates a hierarchy containing many event classes from the package java.awt.event. Some of these are discussed in this chapter and Chapter 22. These event types are used with both 492 Chapter 12 GUI Components: Part 1 AWT and Swing components. Additional event types that are specific to Swing GUI components are declared in package javax.swing.event. ActionEvent Object AdjustmentEvent EventObject ItemEvent AWTEvent TextEvent ComponentEvent ContainerEvent FocusEvent PaintEvent WindowEvent InputEvent KeyEvent MouseEvent MouseWheelEvent Fig. 12.11 | Some event classes of package java.awt.event. Let’s summarize the three parts to the event-handling mechanism that you saw in Section 12.6—the event source, the event object and the event listener. The event source is the GUI component with which the user interacts. The event object encapsulates information about the event that occurred, such as a reference to the event source and any event-specific information that may be required by the event listener for it to handle the event. The event listener is an object that’s notified by the event source when an event occurs; in effect, it “listens” for an event, and one of its methods executes in response to the event. A method of the event listener receives an event object when the event listener is notified of the event. The event listener then uses the event object to respond to the event. This event-handling model is known as the delegation event model—an event’s processing is delegated to an object (the event listener) in the application. For each event-object type, there’s typically a corresponding event-listener interface. An event listener for a GUI event is an object of a class that implements one or more of the event-listener interfaces from packages java.awt.event and javax.swing.event. Many of the event-listener types are common to both Swing and AWT components. Such types are declared in package java.awt.event, and some of them are shown in Fig. 12.12. Additional event-listener types that are specific to Swing components are declared in package javax.swing.event. 12.8 How Event Handling Works 493 «interface» java.util.EventListener «interface» ActionListener «interface» AdjustmentListener «interface» ComponentListener «interface» ContainerListener «interface» FocusListener «interface» ItemListener «interface» KeyListener «interface» MouseListener «interface» MouseMotionListener «interface» TextListener «interface» WindowListener Fig. 12.12 | Some common event-listener interfaces of package java.awt.event. Each event-listener interface specifies one or more event-handling methods that must be declared in the class that implements the interface. Recall from Section 10.9 that any class which implements an interface must declare all the abstract methods of that interface; otherwise, the class is an abstract class and cannot be used to create objects. When an event occurs, the GUI component with which the user interacted notifies its registered listeners by calling each listener’s appropriate event-handling method. For example, when the user presses the Enter key in a JTextField, the registered listener’s actionPerformed method is called. In the next section, we complete our discussion of how the event handling works in the preceding example. 12.8 How Event Handling Works Let’s illustrate how the event-handling mechanism works, using textField1 from the example of Fig. 12.9. We have two remaining open questions from Section 12.7: 1. How did the event handler get registered? 2. How does the GUI component know to call actionPerformed rather than some other event-handling method? The first question is answered by the event registration performed in lines 43–46 of Fig. 12.9. Figure 12.13 diagrams JTextField variable textField1, TextFieldHandler variable handler and the objects to which they refer. Registering Events Every JComponent has an instance variable called listenerList that refers to an object of class EventListenerList (package javax.swing.event). Each object of a JComponent 494 Chapter 12 GUI Components: Part 1 subclass maintains references to its registered listeners in the listenerList. For simplicity, we’ve diagramed listenerList as an array below the JTextField object in Fig. 12.13. textField1 handler JTextField object TextFieldHandler object public void actionPerformed( ActionEvent event ) { // event handled here } listenerList ... This reference is created by the statement textField1.addActionListener( handler ); Fig. 12.13 | Event registration for JTextField textField1. When the following statement (line 43 of Fig. 12.9) executes textField1.addActionListener(handler); a new entry containing a reference to the TextFieldHandler object is placed in textField1’s listenerList. Although not shown in the diagram, this new entry also includes the listener’s type (ActionListener). Using this mechanism, each lightweight Swing component maintains its own list of listeners that were registered to handle the component’s events. Event-Handler Invocation The event-listener type is important in answering the second question: How does the GUI component know to call actionPerformed rather than another method? Every GUI component supports several event types, including mouse events, key events and others. When an event occurs, the event is dispatched only to the event listeners of the appropriate type. Dispatching is simply the process by which the GUI component calls an event-handling method on each of its listeners that are registered for the event type that occurred. Each event type has one or more corresponding event-listener interfaces. For example, ActionEvents are handled by ActionListeners, MouseEvents by MouseListeners and MouseMotionListeners, and KeyEvents by KeyListeners. When an event occurs, the GUI component receives (from the JVM) a unique event ID specifying the event type. The GUI component uses the event ID to decide the listener type to which the event should be dispatched and to decide which method to call on each listener object. For an ActionEvent, the event is dispatched to every registered ActionListener’s actionPerformed method (the only method in interface ActionListener). For a MouseEvent, the event is dispatched to every registered MouseListener or MouseMotionListener, depending on the mouse event that 12.9 JButton 495 occurs. The MouseEvent’s event ID determines which of the several mouse event-handling methods are called. All these decisions are handled for you by the GUI components. All you need to do is register an event handler for the particular event type that your application requires, and the GUI component will ensure that the event handler’s appropriate method gets called when the event occurs. We discuss other event types and event-listener interfaces as they’re needed with each new component we introduce. Performance Tip 12.1 GUIs should always remain responsive to the user. Performing a long-running task in an event handler prevents the user from interacting with the GUI until that task completes. Section 23.11 demonstrates techniques prevent such problems. 12.9 JButton A button is a component the user clicks to trigger a specific action. A Java application can use several types of buttons, including command buttons, checkboxes, toggle buttons and radio buttons. Figure 12.14 shows the inheritance hierarchy of the Swing buttons we cover in this chapter. As you can see, all the button types are subclasses of AbstractButton (package javax.swing), which declares the common features of Swing buttons. In this section, we concentrate on buttons that are typically used to initiate a command. JComponent AbstractButton JButton JToggleButton JCheckBox JRadioButton Fig. 12.14 | Swing button hierarchy. A command button (see Fig. 12.16’s output) generates an ActionEvent when the user clicks it. Command buttons are created with class JButton. The text on the face of a JButton is called a button label. Look-and-Feel Observation 12.8 The text on buttons typically uses book-title capitalization. Look-and-Feel Observation 12.9 A GUI can have many JButtons, but each button label should be unique in the portion of the GUI that’s currently displayed. Having more than one JButton with the same label makes the JButtons ambiguous to the user. 496 Chapter 12 GUI Components: Part 1 The application of Figs. 12.15 and 12.16 creates two JButtons and demonstrates that can display Icons. Event handling for the buttons is performed by a single instance of inner class ButtonHandler (Fig. 12.15, lines 39–48). JButtons 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 // Fig. 12.15: ButtonFrame.java // Command buttons and action events. import java.awt.FlowLayout; import java.awt.event.ActionListener; import java.awt.event.ActionEvent; import javax.swing.JFrame; import javax.swing.JButton; import javax.swing.Icon; import javax.swing.ImageIcon; import javax.swing.JOptionPane; public class ButtonFrame extends JFrame { private final JButton plainJButton; // button with just text private final JButton fancyJButton; // button with icons // ButtonFrame adds JButtons to JFrame public ButtonFrame() { super("Testing Buttons"); setLayout(new FlowLayout()); plainJButton = new JButton("Plain Button"); // button with text add(plainJButton); // add plainJButton to JFrame Icon bug1 = new ImageIcon(getClass().getResource("bug1.gif")); Icon bug2 = new ImageIcon(getClass().getResource("bug2.gif")); fancyJButton = new JButton("Fancy Button", bug1); // set image fancyJButton.setRolloverIcon(bug2); // set rollover image add(fancyJButton); // add fancyJButton to JFrame // create new ButtonHandler for button event handling ButtonHandler handler = new ButtonHandler(); fancyJButton.addActionListener(handler); plainJButton.addActionListener(handler); } // inner class for button event handling private class ButtonHandler implements ActionListener { // handle button event @Override public void actionPerformed(ActionEvent event) { JOptionPane.showMessageDialog(ButtonFrame.this, String.format( "You pressed: %s", event.getActionCommand())); } } } // end class ButtonFrame Fig. 12.15 | Command buttons and action events. 12.9 JButton 1 2 3 4 5 6 7 8 9 10 11 12 13 14 497 // Fig. 12.16: ButtonTest.java // Testing ButtonFrame. import javax.swing.JFrame; public class ButtonTest { public static void main(String[] args) { ButtonFrame buttonFrame = new ButtonFrame(); buttonFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); buttonFrame.setSize(275, 110); buttonFrame.setVisible(true); } } // end class ButtonTest Fig. 12.16 | Testing ButtonFrame. Lines 14–15 declare JButton variables plainJButton and fancyJButton. The corresponding objects are instantiated in the constructor. Line 23 creates plainJButton with the button label "Plain Button". Line 24 adds the JButton to the JFrame. A JButton can display an Icon. To provide the user with an extra level of visual interaction with the GUI, a JButton can also have a rollover Icon—an Icon that’s displayed when the user positions the mouse over the JButton. The icon on the JButton changes as the mouse moves in and out of the JButton’s area on the screen. Lines 26–27 create two 498 Chapter 12 GUI Components: Part 1 objects that represent the default Icon and rollover Icon for the JButton created at line 28. Both statements assume that the image files are stored in the same directory as the application. Images are commonly placed in the same directory as the application or a subdirectory like images). These image files have been provided for you with the example. Line 28 creates fancyButton with the text "Fancy Button" and the icon bug1. By default, the text is displayed to the right of the icon. Line 29 uses setRolloverIcon (inherited from class AbstractButton) to specify the image displayed on the JButton when the user positions the mouse over it. Line 30 adds the JButton to the JFrame. ImageIcon Look-and-Feel Observation 12.10 Because class AbstractButton supports displaying text and images on a button, all subclasses of AbstractButton also support displaying text and images. Look-and-Feel Observation 12.11 Rollover icons provide visual feedback indicating that an action will occur when when a JButton is clicked. JButtons, like JTextFields, generate ActionEvents that can be processed by any object. Lines 33–35 create an object of private inner class ButtonHandler and use addActionListener to register it as the event handler for each JButton. Class ButtonHandler (lines 39–48) declares actionPerformed to display a message dialog box containing the label for the button the user pressed. For a JButton event, ActionEvent method getActionCommand returns the label on the JButton. ActionListener Accessing the this Reference in an Object of a Top-Level Class from an Inner Class When you execute this application and click one of its buttons, notice that the message dialog that appears is centered over the application’s window. This occurs because the call to JOptionPane method showMessageDialog (lines 45–46) uses ButtonFrame.this rather than null as the first argument. When this argument is not null, it represents the so-called parent GUI component of the message dialog (in this case the application window is the parent component) and enables the dialog to be centered over that component when the dialog is displayed. ButtonFrame.this represents the this reference of the object of toplevel class ButtonFrame. Software Engineering Observation 12.2 When used in an inner class, keyword this refers to the current inner-class object being manipulated. An inner-class method can use its outer-class object’s this by preceding this with the outer-class name and a dot (.) separator, as in ButtonFrame.this. 12.10 Buttons That Maintain State The Swing GUI components contain three types of state buttons—JToggleButton, JCheckBox and JRadioButton—that have on/off or true/false values. Classes JCheckBox and JRadioButton are subclasses of JToggleButton (Fig. 12.14). A JRadioButton is different from a JCheckBox in that normally several JRadioButtons are grouped together and 12.10 Buttons That Maintain State 499 are mutually exclusive—only one in the group can be selected at any time, just like the buttons on a car radio. We first discuss class JCheckBox. 12.10.1 JCheckBox The application of Figs. 12.17–12.18 uses two JCheckBoxes to select the desired font style of the text displayed in a JTextField. When selected, one applies a bold style and the other an italic style. If both are selected, the style is bold and italic. When the application initially executes, neither JCheckBox is checked (i.e., they’re both false), so the font is plain. Class CheckBoxTest (Fig. 12.18) contains the main method that executes this application. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 // Fig. 12.17: CheckBoxFrame.java // JCheckBoxes and item events. import java.awt.FlowLayout; import java.awt.Font; import java.awt.event.ItemListener; import java.awt.event.ItemEvent; import javax.swing.JFrame; import javax.swing.JTextField; import javax.swing.JCheckBox; public class CheckBoxFrame extends JFrame { private final JTextField textField; // displays text in changing fonts private final JCheckBox boldJCheckBox; // to select/deselect bold private final JCheckBox italicJCheckBox; // to select/deselect italic // CheckBoxFrame constructor adds JCheckBoxes to JFrame public CheckBoxFrame() { super("JCheckBox Test"); setLayout(new FlowLayout()); // set up JTextField and set its font textField = new JTextField("Watch the font style change", 20); textField.setFont(new Font("Serif", Font.PLAIN, 14)); add(textField); // add textField to JFrame boldJCheckBox = new JCheckBox("Bold"); italicJCheckBox = new JCheckBox("Italic"); add(boldJCheckBox); // add bold checkbox to JFrame add(italicJCheckBox); // add italic checkbox to JFrame // register listeners for JCheckBoxes CheckBoxHandler handler = new CheckBoxHandler(); boldJCheckBox.addItemListener(handler); italicJCheckBox.addItemListener(handler); } // private inner class for ItemListener event handling private class CheckBoxHandler implements ItemListener { Fig. 12.17 | JCheckBoxes and item events. (Part 1 of 2.) 500 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 Chapter 12 GUI Components: Part 1 // respond to checkbox events @Override public void itemStateChanged(ItemEvent event) { Font font = null; // stores the new Font // determine which CheckBoxes are checked and create Font if (boldJCheckBox.isSelected() && italicJCheckBox.isSelected()) font = new Font("Serif", Font.BOLD + Font.ITALIC, 14); else if (boldJCheckBox.isSelected()) font = new Font("Serif", Font.BOLD, 14); else if (italicJCheckBox.isSelected()) font = new Font("Serif", Font.ITALIC, 14); else font = new Font("Serif", Font.PLAIN, 14); textField.setFont(font); } } } // end class CheckBoxFrame Fig. 12.17 | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 JCheckBoxes and item events. (Part 2 of 2.) // Fig. 12.18: CheckBoxTest.java // Testing CheckBoxFrame. import javax.swing.JFrame; public class CheckBoxTest { public static void main(String[] args) { CheckBoxFrame checkBoxFrame = new CheckBoxFrame(); checkBoxFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); checkBoxFrame.setSize(275, 100); checkBoxFrame.setVisible(true); } } // end class CheckBoxTest Fig. 12.18 | Testing CheckBoxFrame. After the JTextField is created and initialized (Fig. 12.17, line 24), line 25 uses method setFont (inherited by JTextField indirectly from class Component) to set the font of the JTextField to a new object of class Font (package java.awt). The new Font is ini- 12.10 Buttons That Maintain State 501 tialized with "Serif" (a generic font name that represents a font such as Times and is supported on all Java platforms), Font.PLAIN style and 14-point size. Next, lines 28–29 create two JCheckBox objects. The String passed to the JCheckBox constructor is the checkbox label that appears to the right of the JCheckBox by default. When the user clicks a JCheckBox, an ItemEvent occurs. This event can be handled by an ItemListener object, which must implement method itemStateChanged. In this example, the event handling is performed by an instance of private inner class CheckBoxHandler (lines 40–60). Lines 34–36 create an instance of class CheckBoxHandler and register it with method addItemListener as the listener for both the JCheckBox objects. CheckBoxHandler method itemStateChanged (lines 43–59) is called when the user clicks the either boldJCheckBox or italicJCheckBox. In this example, we do not determine which JCheckBox was clicked—we use both of their states to determine the font to display. Line 49 uses JCheckBox method isSelected to determine if both JCheckBoxes are selected. If so, line 50 creates a bold italic font by adding the Font constants Font.BOLD and Font.ITALIC for the font-style argument of the Font constructor. Line 51 determines whether the boldJCheckBox is selected, and if so line 52 creates a bold font. Line 53 determines whether the italicJCheckBox is selected, and if so line 54 creates an italic font. If none of the preceding conditions are true, line 56 creates a plain font using the Font constant Font.PLAIN. Finally, line 58 sets textField’s new font, which changes the font in the JTextField on the screen. Relationship Between an Inner Class and Its Top-Level Class Class CheckBoxHandler used variables boldJCheckBox (lines 49 and 51), italicJCheckBox (lines 49 and 53) and textField (line 58) even though they are not declared in the inner class. Recall that an inner class has a special relationship with its top-level class—it’s allowed to access all the variables and methods of the top-level class. CheckBoxHandler method itemStateChanged (line 43–59) uses this relationship to determine which JCheckBoxes are checked and to set the font on the JTextField. Notice that none of the code in inner class CheckBoxHandler requires an explicit reference to the top-level class object. 12.10.2 JRadioButton Radio buttons (declared with class JRadioButton) are similar to checkboxes in that they have two states—selected and not selected (also called deselected). However, radio buttons normally appear as a group in which only one button can be selected at a time (see the output of Fig. 12.20). Radio buttons are used to represent mutually exclusive options (i.e., multiple options in the group cannot be selected at the same time). The logical relationship between radio buttons is maintained by a ButtonGroup object (package javax.swing), which itself is not a GUI component. A ButtonGroup object organizes a group of buttons and is not itself displayed in a user interface. Rather, the individual JRadioButton objects from the group are displayed in the GUI. The application of Figs. 12.19–12.20 is similar to that of Figs. 12.17–12.18. The user can alter the font style of a JTextField’s text. The application uses radio buttons that permit only a single font style in the group to be selected at a time. Class RadioButtonTest (Fig. 12.20) contains the main method that executes this application. 502 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 Chapter 12 GUI Components: Part 1 // Fig. 12.19: RadioButtonFrame.java // Creating radio buttons using ButtonGroup and JRadioButton. import java.awt.FlowLayout; import java.awt.Font; import java.awt.event.ItemListener; import java.awt.event.ItemEvent; import javax.swing.JFrame; import javax.swing.JTextField; import javax.swing.JRadioButton; import javax.swing.ButtonGroup; public class RadioButtonFrame extends JFrame { private final JTextField textField; // used to display font changes private final Font plainFont; // font for plain text private final Font boldFont; // font for bold text private final Font italicFont; // font for italic text private final Font boldItalicFont; // font for bold and italic text private final JRadioButton plainJRadioButton; // selects plain text private final JRadioButton boldJRadioButton; // selects bold text private final JRadioButton italicJRadioButton; // selects italic text private final JRadioButton boldItalicJRadioButton; // bold and italic private final ButtonGroup radioGroup; // holds radio buttons // RadioButtonFrame constructor adds JRadioButtons to JFrame public RadioButtonFrame() { super("RadioButton Test"); setLayout(new FlowLayout()); textField = new JTextField("Watch the font style change", 25); add(textField); // add textField to JFrame // create radio buttons plainJRadioButton = new JRadioButton("Plain", true); boldJRadioButton = new JRadioButton("Bold", false); italicJRadioButton = new JRadioButton("Italic", false); boldItalicJRadioButton = new JRadioButton("Bold/Italic", false); add(plainJRadioButton); // add plain button to JFrame add(boldJRadioButton); // add bold button to JFrame add(italicJRadioButton); // add italic button to JFrame add(boldItalicJRadioButton); // add bold and italic button // create logical relationship between JRadioButtons radioGroup = new ButtonGroup(); // create ButtonGroup radioGroup.add(plainJRadioButton); // add plain to group radioGroup.add(boldJRadioButton); // add bold to group radioGroup.add(italicJRadioButton); // add italic to group radioGroup.add(boldItalicJRadioButton); // add bold and italic // create font objects plainFont = new Font("Serif", Font.PLAIN, 14); boldFont = new Font("Serif", Font.BOLD, 14); Fig. 12.19 | Creating radio buttons using ButtonGroup and JRadioButton. (Part 1 of 2.) 12.10 Buttons That Maintain State 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 503 italicFont = new Font("Serif", Font.ITALIC, 14); boldItalicFont = new Font("Serif", Font.BOLD + Font.ITALIC, 14); textField.setFont(plainFont); // register events for JRadioButtons plainJRadioButton.addItemListener( new RadioButtonHandler(plainFont)); boldJRadioButton.addItemListener( new RadioButtonHandler(boldFont)); italicJRadioButton.addItemListener( new RadioButtonHandler(italicFont)); boldItalicJRadioButton.addItemListener( new RadioButtonHandler(boldItalicFont)); } // private inner class to handle radio button events private class RadioButtonHandler implements ItemListener { private Font font; // font associated with this listener public RadioButtonHandler(Font f) { font = f; } // handle radio button events @Override public void itemStateChanged(ItemEvent event) { textField.setFont(font); } } } // end class RadioButtonFrame Fig. 12.19 | Creating radio buttons using ButtonGroup and JRadioButton. (Part 2 of 2.) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 // Fig. 12.20: RadioButtonTest.java // Testing RadioButtonFrame. import javax.swing.JFrame; public class RadioButtonTest { public static void main(String[] args) { RadioButtonFrame radioButtonFrame = new RadioButtonFrame(); radioButtonFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); radioButtonFrame.setSize(300, 100); radioButtonFrame.setVisible(true); } } // end class RadioButtonTest Fig. 12.20 | Testing RadioButtonFrame. (Part 1 of 2.) 504 Chapter 12 GUI Components: Part 1 Fig. 12.20 | Testing RadioButtonFrame. (Part 2 of 2.) Lines 35–42 (Fig. 12.19) in the constructor create four JRadioButton objects and add them to the JFrame. Each JRadioButton is created with a constructor call like that in line 35. This constructor specifies the label that appears to the right of the JRadioButton by default and the initial state of the JRadioButton. A true second argument indicates that the JRadioButton should appear selected when it’s displayed. Line 45 instantiates ButtonGroup object radioGroup. This object is the “glue” that forms the logical relationship between the four JRadioButton objects and allows only one of the four to be selected at a time. It’s possible that no JRadioButtons in a ButtonGroup are selected, but this can occur only if no preselected JRadioButtons are added to the ButtonGroup and the user has not selected a JRadioButton yet. Lines 46–49 use ButtonGroup method add to associate each of the JRadioButtons with radioGroup. If more than one selected JRadioButton object is added to the group, the selected one that was added first will be selected when the GUI is displayed. JRadioButtons, like JCheckBoxes, generate ItemEvents when they’re clicked. Lines 59–66 create four instances of inner class RadioButtonHandler (declared at lines 70–85). In this example, each event-listener object is registered to handle the ItemEvent generated when the user clicks a particular JRadioButton. Notice that each RadioButtonHandler object is initialized with a particular Font object (created in lines 52–55). Class RadioButtonHandler (line 70–85) implements interface ItemListener so it can handle ItemEvents generated by the JRadioButtons. The constructor stores the Font object it receives as an argument in the event-listener object’s instance variable font (declared at line 72). When the user clicks a JRadioButton, radioGroup turns off the previously selected JRadioButton, and method itemStateChanged (lines 80–84) sets the font in the JTextField to the Font stored in the JRadioButton’s corresponding event-listener object. Notice that line 83 of inner class RadioButtonHandler uses the top-level class’s textField instance variable to set the font. 12.11 JComboBox; Using an Anonymous Inner Class for Event Handling A combo box (sometimes called a drop-down list) enables the user to select one item from a list (Fig. 12.22). Combo boxes are implemented with class JComboBox, which extends 12.11 JComboBox; Using an Anonymous Inner Class for Event Handling 505 class JComponent. JComboBox is a generic class, like the class ArrayList (Chapter 7). When you create a JComboBox, you specify the type of the objects that it manages—the JComboBox then displays a String representation of each object. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 // Fig. 12.21: ComboBoxFrame.java // JComboBox that displays a list of image names. import java.awt.FlowLayout; import java.awt.event.ItemListener; import java.awt.event.ItemEvent; import javax.swing.JFrame; import javax.swing.JLabel; import javax.swing.JComboBox; import javax.swing.Icon; import javax.swing.ImageIcon; public class ComboBoxFrame extends JFrame { private final JComboBox imagesJComboBox; // holds icon names private final JLabel label; // displays selected icon private static final String[] names = {"bug1.gif", "bug2.gif", "travelbug.gif", "buganim.gif"}; private final Icon[] icons = { new ImageIcon(getClass().getResource(names[0])), new ImageIcon(getClass().getResource(names[1])), new ImageIcon(getClass().getResource(names[2])), new ImageIcon(getClass().getResource(names[3]))}; // ComboBoxFrame constructor adds JComboBox to JFrame public ComboBoxFrame() { super("Testing JComboBox"); setLayout(new FlowLayout()); // set frame layout imagesJComboBox = new JComboBox(names); // set up JComboBox imagesJComboBox.setMaximumRowCount(3); // display three rows imagesJComboBox.addItemListener( new ItemListener() // anonymous inner class { // handle JComboBox event @Override public void itemStateChanged(ItemEvent event) { // determine whether item selected if (event.getStateChange() == ItemEvent.SELECTED) label.setIcon(icons[ imagesJComboBox.getSelectedIndex()]); } } // end anonymous inner class ); // end call to addItemListener Fig. 12.21 | JComboBox that displays a list of image names. (Part 1 of 2.) 506 49 50 51 52 53 Chapter 12 GUI Components: Part 1 add(imagesJComboBox); // add combo box to JFrame label = new JLabel(icons[0]); // display first icon add(label); // add label to JFrame } } // end class ComboBoxFrame Fig. 12.21 | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 JComboBox that displays a list of image names. (Part 2 of 2.) // Fig. 12.22: ComboBoxTest.java // Testing ComboBoxFrame. import javax.swing.JFrame; public class ComboBoxTest { public static void main(String[] args) { ComboBoxFrame comboBoxFrame = new ComboBoxFrame(); comboBoxFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); comboBoxFrame.setSize(350, 150); comboBoxFrame.setVisible(true); } } // end class ComboBoxTest Scroll box Scrollbar to scroll through the items in the list Scroll arrows Fig. 12.22 | Testing ComboBoxFrame. JComboBoxes generate ItemEvents just as JCheckBoxes and JRadioButtons do. This example also demonstrates a special form of inner class that’s used frequently in event handling. The application (Figs. 12.21–12.22) uses a JComboBox to provide a list of four image filenames from which the user can select one image to display. When the user selects a name, the application displays the corresponding image as an Icon on a JLabel. Class ComboBoxTest (Fig. 12.22) contains the main method that executes this application. The screen captures for this application show the JComboBox list after the selection was made to illustrate which image filename was selected. 12.11 JComboBox; Using an Anonymous Inner Class for Event Handling 507 Lines 19–23 (Fig. 12.21) declare and initialize array icons with four new ImageIcon objects. String array names (lines 17–18) contains the names of the four image files that are stored in the same directory as the application. At line 31, the constructor initializes a JComboBox object with the Strings in array names as the elements in the list. Each item in the list has an index. The first item is added at index 0, the next at index 1 and so forth. The first item added to a JComboBox appears as the currently selected item when the JComboBox is displayed. Other items are selected by clicking the JComboBox, then selecting an item from the list that appears. Line 32 uses JComboBox method setMaximumRowCount to set the maximum number of elements that are displayed when the user clicks the JComboBox. If there are additional items, the JComboBox provides a scrollbar (see the first screen) that allows the user to scroll through all the elements in the list. The user can click the scroll arrows at the top and bottom of the scrollbar to move up and down through the list one element at a time, or else drag the scroll box in the middle of the scrollbar up and down. To drag the scroll box, position the mouse cursor on it, hold the mouse button down and move the mouse. In this example, the drop-down list is too short to drag the scroll box, so you can click the up and down arrows or use your mouse’s wheel to scroll through the four items in the list. Line 49 attaches the JComboBox to the ComboBoxFrame’s FlowLayout (set in line 29). Line 50 creates the JLabel that displays ImageIcons and initializes it with the first ImageIcon in array icons. Line 51 attaches the JLabel to the ComboBoxFrame’s FlowLayout. Look-and-Feel Observation 12.12 Set the maximum row count for a JComboBox to a number of rows that prevents the list from expanding outside the bounds of the window in which it’s used. Using an Anonymous Inner Class for Event Handling Lines 34–46 are one statement that declares the event listener’s class, creates an object of that class and registers it as imagesJComboBox’s ItemEvent listener. This event-listener object is an instance of an anonymous inner class—a class that’s declared without a name and typically appears inside a method declaration. As with other inner classes, an anonymous inner class can access its top-level class’s members. However, an anonymous inner class has limited access to the local variables of the method in which it’s declared. Since an anonymous inner class has no name, one object of the class must be created at the point where the class is declared (starting at line 35). Software Engineering Observation 12.3 An anonymous inner class declared in a method can access the instance variables and methods of the top-level class object that declared it, as well as the method’s final local variables, but cannot access the method’s non-final local variables. As of Java SE 8, anonymous inner classes may also access a methods “effectively final” local variables—see Chapter 17 for more information. Lines 34–47 are a call to imagesJComboBox’s addItemListener method. The argument to this method must be an object that is an ItemListener (i.e., any object of a class that implements ItemListener). Lines 35–46 are a class-instance creation expression that declares an anonymous inner class and creates one object of that class. A reference to that object is then passed as the argument to addItemListener. The syntax ItemListener() 508 after Chapter 12 GUI Components: Part 1 new begins the declaration of an anonymous inner class that implements interface This is similar to beginning a class declaration with ItemListener. public class MyHandler implements ItemListener The opening left brace at line 36 and the closing right brace at line 46 delimit the body of the anonymous inner class. Lines 38–45 declare the ItemListener’s itemStateChanged method. When the user makes a selection from imagesJComboBox, this method sets label’s Icon. The Icon is selected from array icons by determining the index of the selected item in the JComboBox with method getSelectedIndex in line 44. For each item selected from a JComboBox, another item is first deselected—so two ItemEvents occur when an item is selected. We wish to display only the icon for the item the user just selected. For this reason, line 42 determines whether ItemEvent method getStateChange returns ItemEvent.SELECTED. If so, lines 43–44 set label’s icon. Software Engineering Observation 12.4 Like any other class, when an anonymous inner class implements an interface, the class must implement every abstract method in the interface. The syntax shown in lines 35–46 for creating an event handler with an anonymous inner class is similar to the code that would be generated by a Java integrated development environment (IDE). Typically, an IDE enables you to design a GUI visually, then it generates code that implements the GUI. You simply insert statements in the event-handling methods that declare how to handle each event. Java SE 8: Implementing Anonymous Inner Classes with Lambdas In Section 17.9, we show how to use Java SE 8 lambdas to create event handlers. As you’ll learn, the compiler translates a lambda into an object of an anonymous inner class. 12.12 JList A list displays a series of items from which the user may select one or more items (see the output of Fig. 12.24). Lists are created with class JList, which directly extends class JComponent. Class JList—which like JComboBox is a generic class—supports single-selection lists (which allow only one item to be selected at a time) and multiple-selection lists (which allow any number of items to be selected). In this section, we discuss single-selection lists. The application of Figs. 12.23–12.24 creates a JList containing 13 color names. When a color name is clicked in the JList, a ListSelectionEvent occurs and the application changes the background color of the application window to the selected color. Class ListTest (Fig. 12.24) contains the main method that executes this application. 1 2 3 4 5 6 // Fig. 12.23: ListFrame.java // JList that displays a list of colors. import java.awt.FlowLayout; import java.awt.Color; import javax.swing.JFrame; import javax.swing.JList; Fig. 12.23 | JList that displays a list of colors. (Part 1 of 2.) 12.12 JList 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 import import import import javax.swing.JScrollPane; javax.swing.event.ListSelectionListener; javax.swing.event.ListSelectionEvent; javax.swing.ListSelectionModel; public class ListFrame extends JFrame { private final JList colorJList; // list to display colors private static final String[] colorNames = {"Black", "Blue", "Cyan", "Dark Gray", "Gray", "Green", "Light Gray", "Magenta", "Orange", "Pink", "Red", "White", "Yellow"}; private static final Color[] colors = {Color.BLACK, Color.BLUE, Color.CYAN, Color.DARK_GRAY, Color.GRAY, Color.GREEN, Color.LIGHT_GRAY, Color.MAGENTA, Color.ORANGE, Color.PINK, Color.RED, Color.WHITE, Color.YELLOW}; // ListFrame constructor add JScrollPane containing JList to JFrame public ListFrame() { super("List Test"); setLayout(new FlowLayout()); colorJList = new JList(colorNames); // list of colorNames colorJList.setVisibleRowCount(5); // display five rows at once // do not allow multiple selections colorJList.setSelectionMode(ListSelectionModel.SINGLE_SELECTION); // add a JScrollPane containing JList to frame add(new JScrollPane(colorJList)); colorJList.addListSelectionListener( new ListSelectionListener() // anonymous inner class { // handle list selection events @Override public void valueChanged(ListSelectionEvent event) { getContentPane().setBackground( colors[colorJList.getSelectedIndex()]); } } ); } } // end class ListFrame Fig. 12.23 | 1 2 3 4 509 JList that displays a list of colors. (Part 2 of 2.) // Fig. 12.24: ListTest.java // Selecting colors from a JList. import javax.swing.JFrame; Fig. 12.24 | Selecting colors from a JList. (Part 1 of 2.) 510 5 6 7 8 9 10 11 12 13 14 Chapter 12 GUI Components: Part 1 public class ListTest { public static void main(String[] args) { ListFrame listFrame = new ListFrame(); // create ListFrame listFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); listFrame.setSize(350, 150); listFrame.setVisible(true); } } // end class ListTest Fig. 12.24 | Selecting colors from a JList. (Part 2 of 2.) Line 29 (Fig. 12.23) creates JList object colorJList. The argument to the JList constructor is the array of Objects (in this case Strings) to display in the list. Line 30 uses JList method setVisibleRowCount to determine the number of items visible in the list. Line 33 uses JList method setSelectionMode to specify the list’s selection mode. Class ListSelectionModel (of package javax.swing) declares three constants that specify a JList’s selection mode—SINGLE_SELECTION (which allows only one item to be selected at a time), SINGLE_INTERVAL_SELECTION (for a multiple-selection list that allows selection of several contiguous items) and MULTIPLE_INTERVAL_SELECTION (for a multiple-selection list that does not restrict the items that can be selected). Unlike a JComboBox, a JList does not provide a scrollbar if there are more items in the list than the number of visible rows. In this case, a JScrollPane object is used to provide the scrolling capability. Line 36 adds a new instance of class JScrollPane to the JFrame. The JScrollPane constructor receives as its argument the JComponent that needs scrolling functionality (in this case, colorJList). Notice in the screen captures that a scrollbar created by the JScrollPane appears at the right side of the JList. By default, the scrollbar appears only when the number of items in the JList exceeds the number of visible items. Lines 38–49 use JList method addListSelectionListener to register an object that implements ListSelectionListener (package javax.swing.event) as the listener for the JList’s selection events. Once again, we use an instance of an anonymous inner class (lines 39–48) as the listener. In this example, when the user makes a selection from colorJList, method valueChanged (line 42–47) should change the background color of the ListFrame to the selected color. This is accomplished in lines 45–46. Note the use of JFrame method getContentPane in line 45. Each JFrame actually consists of three layers—the background, the content pane and the glass pane. The content pane appears in front of the background and is where the GUI components in the JFrame are displayed. The glass pane is used to display tool tips and other items that should appear in front of the GUI components on the screen. The content pane completely hides the background of the JFrame; thus, to change the background color behind the GUI components, you must change the content pane’s background color. Method getContentPane returns a reference to the 12.13 Multiple-Selection Lists 511 JFrame’s content pane (an object of class Container). In line 45, we then use that reference to call method setBackground, which sets the content pane’s background color to an element in the colors array. The color is selected from the array by using the selected item’s index. JList method getSelectedIndex returns the selected item’s index. As with arrays and JComboBoxes, JList indexing is zero based. 12.13 Multiple-Selection Lists A multiple-selection list enables the user to select many items from a JList (see the output of Fig. 12.26). A SINGLE_INTERVAL_SELECTION list allows selecting a contiguous range of items. To do so, click the first item, then press and hold the Shift key while clicking the last item in the range. A MULTIPLE_INTERVAL_SELECTION list (the default) allows continuous range selection as described for a SINGLE_INTERVAL_SELECTION list. Such a list also allows miscellaneous items to be selected by pressing and holding the Ctrl key while clicking each item to select. To deselect an item, press and hold the Ctrl key while clicking the item a second time. The application of Figs. 12.25–12.26 uses multiple-selection lists to copy items from one JList to another. One list is a MULTIPLE_INTERVAL_SELECTION list and the other is a SINGLE_INTERVAL_SELECTION list. When you execute the application, try using the selection techniques described previously to select items in both lists. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 // Fig. 12.25: MultipleSelectionFrame.java // JList that allows multiple selections. import java.awt.FlowLayout; import java.awt.event.ActionListener; import java.awt.event.ActionEvent; import javax.swing.JFrame; import javax.swing.JList; import javax.swing.JButton; import javax.swing.JScrollPane; import javax.swing.ListSelectionModel; public class MultipleSelectionFrame extends JFrame { private final JList colorJList; // list to hold color names private final JList copyJList; // list to hold copied names private JButton copyJButton; // button to copy selected names private static final String[] colorNames = {"Black", "Blue", "Cyan", "Dark Gray", "Gray", "Green", "Light Gray", "Magenta", "Orange", "Pink", "Red", "White", "Yellow"}; // MultipleSelectionFrame constructor public MultipleSelectionFrame() { super("Multiple Selection Lists"); setLayout(new FlowLayout()); colorJList = new JList(colorNames); // list of color names colorJList.setVisibleRowCount(5); // show five rows Fig. 12.25 | JList that allows multiple selections. (Part 1 of 2.) 512 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 Chapter 12 GUI Components: Part 1 colorJList.setSelectionMode( ListSelectionModel.MULTIPLE_INTERVAL_SELECTION); add(new JScrollPane(colorJList)); // add list with scrollpane copyJButton = new JButton("Copy >>>"); copyJButton.addActionListener( new ActionListener() // anonymous inner class { // handle button event @Override public void actionPerformed(ActionEvent event) { // place selected values in copyJList copyJList.setListData( colorJList.getSelectedValuesList().toArray( new String[0])); } } ); add(copyJButton); // add copy button to JFrame copyJList = new JList(); // list to hold copied color names copyJList.setVisibleRowCount(5); // show 5 rows copyJList.setFixedCellWidth(100); // set width copyJList.setFixedCellHeight(15); // set height copyJList.setSelectionMode( ListSelectionModel.SINGLE_INTERVAL_SELECTION); add(new JScrollPane(copyJList)); // add list with scrollpane } } // end class MultipleSelectionFrame Fig. 12.25 | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 JList that allows multiple selections. (Part 2 of 2.) // Fig. 12.26: MultipleSelectionTest.java // Testing MultipleSelectionFrame. import javax.swing.JFrame; public class MultipleSelectionTest { public static void main(String[] args) { MultipleSelectionFrame multipleSelectionFrame = new MultipleSelectionFrame(); multipleSelectionFrame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE); multipleSelectionFrame.setSize(350, 150); multipleSelectionFrame.setVisible(true); } } // end class MultipleSelectionTest Fig. 12.26 | Testing MultipleSelectionFrame. (Part 1 of 2.) 12.14 Mouse Event Handling 513 Fig. 12.26 | Testing MultipleSelectionFrame. (Part 2 of 2.) Line 27 of Fig. 12.25 creates JList colorJList and initializes it with the Strings in the array colorNames. Line 28 sets the number of visible rows in colorJList to 5. Lines 29–30 specify that colorJList is a MULTIPLE_INTERVAL_SELECTION list. Line 31 adds a new JScrollPane containing colorJList to the JFrame. Lines 51–57 perform similar tasks for copyJList, which is declared as a SINGLE_INTERVAL_SELECTION list. If a JList does not contain items, it will not diplay in a FlowLayout. For this reason, lines 53–54 use JList methods setFixedCellWidth and setFixedCellHeight to set copyJList’s width to 100 pixels and the height of each item in the JList to 15 pixels, respectively. Normally, an event generated by another GUI component (known as an external event) specifies when the multiple selections in a JList should be processed. In this example, the user clicks the JButton called copyJButton to trigger the event that copies the selected items in colorJList to copyJList. Lines 34–47 declare, create and register an ActionListener for the copyJButton. When the user clicks copyJButton, method actionPerformed (lines 38–45) uses JList method setListData to set the items displayed in copyJList. Lines 43–44 call colorJList’s method getSelectedValuesList, which returns a List (because the JList was created as a JList) representing the selected items in colorJList. We call the List’s toArray method to convert this into an array of Strings that can be passed as the argument to copyJList’s setListData method. List method toArray receives as its argument an array representing the type of array that the method will return. You’ll learn more about List and toArray in Chapter 16. You might be wondering why copyJList can be used in line 42 even though the application does not create the object to which it refers until line 49. Remember that method actionPerformed (lines 38–45) does not execute until the user presses the copyJButton, which cannot occur until after the constructor completes execution and the application displays the GUI. At that point in the application’s execution, copyJList is already initialized with a new JList object. 12.14 Mouse Event Handling This section presents the MouseListener and MouseMotionListener event-listener interfaces for handling mouse events. Mouse events can be processed for any GUI component that derives from java.awt.Component. The methods of interfaces MouseListener and MouseMotionListener are summarized in Figure 12.27. Package javax.swing.event contains interface MouseInputListener, which extends interfaces MouseListener and MouseMotionListener to create a single interface containing all the MouseListener and MouseMotionListener methods. The MouseListener and MouseMotionListener meth- 514 Chapter 12 GUI Components: Part 1 ods are called when the mouse interacts with a Component if appropriate event-listener objects are registered for that Component. Each of the mouse event-handling methods receives as an argument a MouseEvent object that contains information about the mouse event that occurred, including the xand y-coordinates of its location. These coordinates are measured from the upper-left corner of the GUI component on which the event occurred. The x-coordinates start at 0 and increase from left to right. The y-coordinates start at 0 and increase from top to bottom. The methods and constants of class InputEvent (MouseEvent’s superclass) enable you to determine which mouse button the user clicked. MouseListener and MouseMotionListener interface methods Methods of interface MouseListener public void mousePressed(MouseEvent event) Called when a mouse button is pressed while the mouse cursor is on a component. public void mouseClicked(MouseEvent event) Called when a mouse button is pressed and released while the mouse cursor remains stationary on a component. Always preceded by a call to mousePressed and mouseReleased. public void mouseReleased(MouseEvent event) Called when a mouse button is released after being pressed. Always preceded by a call to mousePressed and one or more calls to mouseDragged. public void mouseEntered(MouseEvent event) Called when the mouse cursor enters the bounds of a component. public void mouseExited(MouseEvent event) Called when the mouse cursor leaves the bounds of a component. Methods of interface MouseMotionListener public void mouseDragged(MouseEvent event) Called when the mouse button is pressed while the mouse cursor is on a component and the mouse is moved while the mouse button remains pressed. Always preceded by a call to mousePressed. All drag events are sent to the component on which the user began to drag the mouse. public void mouseMoved(MouseEvent event) Called when the mouse is moved (with no mouse buttons pressed) when the mouse cursor is on a component. All move events are sent to the component over which the mouse is currently positioned. Fig. 12.27 | MouseListener and MouseMotionListener interface methods. Software Engineering Observation 12.5 Calls to mouseDragged are sent to the MouseMotionListener for the Component on which the drag started. Similarly, the mouseReleased call at the end of a drag operation is sent to the MouseListener for the Component on which the drag operation started. 12.14 Mouse Event Handling 515 Java also provides interface MouseWheelListener to enable applications to respond to the rotation of a mouse wheel. This interface declares method mouseWheelMoved, which receives a MouseWheelEvent as its argument. Class MouseWheelEvent (a subclass of MouseEvent) contains methods that enable the event handler to obtain information about the amount of wheel rotation. Tracking Mouse Events on a JPanel The MouseTracker application (Figs. 12.28–12.29) demonstrates the MouseListener and MouseMotionListener interface methods. The event-handler class (lines 36–97 of Fig. 12.28) implements both interfaces. You must declare all seven methods from these two interfaces when your class implements them both. Each mouse event in this example displays a String in the JLabel called statusBar that is attached to the bottom of the window. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 // Fig. 12.28: MouseTrackerFrame.java // Mouse event handling. import java.awt.Color; import java.awt.BorderLayout; import java.awt.event.MouseListener; import java.awt.event.MouseMotionListener; import java.awt.event.MouseEvent; import javax.swing.JFrame; import javax.swing.JLabel; import javax.swing.JPanel; public class MouseTrackerFrame extends JFrame { private final JPanel mousePanel; // panel in which mouse events occur private final JLabel statusBar; // displays event information // MouseTrackerFrame constructor sets up GUI and // registers mouse event handlers public MouseTrackerFrame() { super("Demonstrating Mouse Events"); mousePanel = new JPanel(); mousePanel.setBackground(Color.WHITE); add(mousePanel, BorderLayout.CENTER); // add panel to JFrame statusBar = new JLabel("Mouse outside JPanel"); add(statusBar, BorderLayout.SOUTH); // add label to JFrame // create and register listener for mouse and mouse motion events MouseHandler handler = new MouseHandler(); mousePanel.addMouseListener(handler); mousePanel.addMouseMotionListener(handler); } Fig. 12.28 | Mouse event handling. (Part 1 of 3.) 516 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 Chapter 12 GUI Components: Part 1 private class MouseHandler implements MouseListener, MouseMotionListener { // MouseListener event handlers // handle event when mouse released immediately after press @Override public void mouseClicked(MouseEvent event)’ { statusBar.setText(String.format("Clicked at [%d, %d]", event.getX(), event.getY())); } // handle event when mouse pressed @Override public void mousePressed(MouseEvent event) { statusBar.setText(String.format("Pressed at [%d, %d]", event.getX(), event.getY())); } // handle event when mouse released @Override public void mouseReleased(MouseEvent event) { statusBar.setText(String.format("Released at [%d, %d]", event.getX(), event.getY())); } // handle event when mouse enters area @Override public void mouseEntered(MouseEvent event) { statusBar.setText(String.format("Mouse entered at [%d, %d]", event.getX(), event.getY())); mousePanel.setBackground(Color.GREEN); } // handle event when mouse exits area @Override public void mouseExited(MouseEvent event) { statusBar.setText("Mouse outside JPanel"); mousePanel.setBackground(Color.WHITE); } // MouseMotionListener event handlers // handle event when user drags mouse with button pressed @Override public void mouseDragged(MouseEvent event) { statusBar.setText(String.format("Dragged at [%d, %d]", event.getX(), event.getY())); } Fig. 12.28 | Mouse event handling. (Part 2 of 3.) 12.14 Mouse Event Handling 89 90 91 92 93 94 95 96 97 98 517 // handle event when user moves mouse @Override public void mouseMoved(MouseEvent event) { statusBar.setText(String.format("Moved at [%d, %d]", event.getX(), event.getY())); } } // end inner class MouseHandler } // end class MouseTrackerFrame Fig. 12.28 | Mouse event handling. (Part 3 of 3.) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 // Fig. 12.29: MouseTrackerFrame.java // Testing MouseTrackerFrame. import javax.swing.JFrame; public class MouseTracker { public static void main(String[] args) { MouseTrackerFrame mouseTrackerFrame = new MouseTrackerFrame(); mouseTrackerFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); mouseTrackerFrame.setSize(300, 100); mouseTrackerFrame.setVisible(true); } } // end class MouseTracker Fig. 12.29 | Testing MouseTrackerFrame. Line 23 creates JPanel mousePanel. This JPanel’s mouse events aretracked by the app. Line 24 sets mousePanel’s background color to white. When the user moves the mouse into the mousePanel, the application will change mousePanel’s background color to green. When the user moves the mouse out of the mousePanel, the application will change the background color back to white. Line 25 attaches mousePanel to the JFrame. As you’ve learned, you typically must specify the layout of the GUI components in a JFrame. In that section, we intro- 518 Chapter 12 GUI Components: Part 1 duced the layout manager FlowLayout. Here we use the default layout of a JFrame’s content pane—BorderLayout, which arranges component NORTH, SOUTH, EAST, WEST and CENTER regions. NORTH corresponds to the container’s top. This example uses the CENTER and SOUTH regions. Line 25 uses a two-argument version of method add to place mousePanel in the CENTER region. The BorderLayout automatically sizes the component in the CENTER to use all the space in the JFrame that is not occupied by components in the other regions. Section 12.18.2 discusses BorderLayout in more detail. Lines 27–28 in the constructor declare JLabel statusBar and attach it to the JFrame’s SOUTH region. This JLabel occupies the width of the JFrame. The region’s height is determined by the JLabel. Line 31 creates an instance of inner class MouseHandler (lines 36–97) called handler that responds to mouse events. Lines 32–33 register handler as the listener for mousePanel’s mouse events. Methods addMouseListener and addMouseMotionListener are inherited indirectly from class Component and can be used to register MouseListeners and MouseMotionListeners, respectively. A MouseHandler object is a MouseListener and is a MouseMotionListener because the class implements both interfaces. We chose to implement both interfaces here to demonstrate a class that implements more than one interface, but we could have implemented interface MouseInputListener instead. When the mouse enters and exits mousePanel’s area, methods mouseEntered (lines 65–71) and mouseExited (lines 74–79) are called, respectively. Method mouseEntered displays a message in the statusBar indicating that the mouse entered the JPanel and changes the background color to green. Method mouseExited displays a message in the statusBar indicating that the mouse is outside the JPanel (see the first sample output window) and changes the background color to white. The other five events display a string in the statusBar that includes the event and the coordinates at which it occurred. MouseEvent methods getX and getY return the x- and ycoordinates, respectively, of the mouse at the time the event occurred. 12.15 Adapter Classes Many event-listener interfaces, such as MouseListener and MouseMotionListener, contain multiple methods. It’s not always desirable to declare every method in an event-listener interface. For instance, an application may need only the mouseClicked handler from MouseListener or the mouseDragged handler from MouseMotionListener. Interface WindowListener specifies seven window event-handling methods. For many of the listener interfaces that have multiple methods, packages java.awt.event and javax.swing.event provide event-listener adapter classes. An adapter class implements an interface and provides a default implementation (with an empty method body) of each method in the interface. Figure 12.30 shows several java.awt.event adapter classes and the interfaces they implement. You can extend an adapter class to inherit the default implementation of every method and subsequently override only the method(s) you need for event handling. Software Engineering Observation 12.6 When a class implements an interface, the class has an is-a relationship with that interface. All direct and indirect subclasses of that class inherit this interface. Thus, an object of a class that extends an event-adapter class is an object of the corresponding eventlistener type (e.g., an object of a subclass of MouseAdapter is a MouseListener). 12.15 Adapter Classes Event-adapter class in java.awt.event Implements interface ComponentAdapter ComponentListener ContainerAdapter ContainerListener FocusAdapter FocusListener KeyAdapter KeyListener MouseAdapter MouseListener MouseMotionAdapter MouseMotionListener WindowAdapter WindowListener 519 Fig. 12.30 | Event-adapter classes and the interfaces they implement. Extending MouseAdapter The application of Figs. 12.31–12.32 demonstrates how to determine the number of mouse clicks (i.e., the click count) and how to distinguish between the different mouse buttons. The event listener in this application is an object of inner class MouseClickHandler (Fig. 12.31, lines 25–46) that extends MouseAdapter, so we can declare just the mouseClicked method we need in this example. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 // Fig. 12.31: MouseDetailsFrame.java // Demonstrating mouse clicks and distinguishing between mouse buttons. import java.awt.BorderLayout; import java.awt.event.MouseAdapter; import java.awt.event.MouseEvent; import javax.swing.JFrame; import javax.swing.JLabel; public class MouseDetailsFrame extends JFrame { private String details; // String displayed in the statusBar private final JLabel statusBar; // JLabel at bottom of window // constructor sets title bar String and register mouse listener public MouseDetailsFrame() { super("Mouse clicks and buttons"); statusBar = new JLabel("Click the mouse"); add(statusBar, BorderLayout.SOUTH); addMouseListener(new MouseClickHandler()); // add handler } // inner class to handle mouse events private class MouseClickHandler extends MouseAdapter { // handle mouse-click event and determine which button was pressed @Override public void mouseClicked(MouseEvent event) { Fig. 12.31 | Demonstrating mouse clicks and distinguishing between mouse buttons. (Part 1 of 2.) 520 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 Chapter 12 GUI Components: Part 1 int xPos = event.getX(); // get x-position of mouse int yPos = event.getY(); // get y-position of mouse details = String.format("Clicked %d time(s)", event.getClickCount()); if (event.isMetaDown()) // right mouse button details += " with right mouse button"; else if (event.isAltDown()) // middle mouse button details += " with center mouse button"; else // left mouse button details += " with left mouse button"; statusBar.setText(details); // display message in statusBar } } } // end class MouseDetailsFrame Fig. 12.31 | Demonstrating mouse clicks and distinguishing between mouse buttons. (Part 2 of 2.) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 // Fig. 12.32: MouseDetails.java // Testing MouseDetailsFrame. import javax.swing.JFrame; public class MouseDetails { public static void main(String[] args) { MouseDetailsFrame mouseDetailsFrame = new MouseDetailsFrame(); mouseDetailsFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); mouseDetailsFrame.setSize(400, 150); mouseDetailsFrame.setVisible(true); } } // end class MouseDetails Fig. 12.32 | Testing MouseDetailsFrame. 12.15 Adapter Classes 521 Common Programming Error 12.3 If you extend an adapter class and misspell the name of the method you’re overriding, and you do not declare the method with @Override, your method simply becomes another method in the class. This is a logic error that is difficult to detect, since the program will call the empty version of the method inherited from the adapter class. A user of a Java application may be on a system with a one-, two- or three-button mouse. Class MouseEvent inherits several methods from class InputEvent that can distinguish among mouse buttons on a multibutton mouse or can mimic a multibutton mouse with a combined keystroke and mouse-button click. Figure 12.33 shows the InputEvent methods used to distinguish among mouse-button clicks. Java assumes that every mouse contains a left mouse button. Thus, it’s simple to test for a left-mouse-button click. However, users with a one- or two-button mouse must use a combination of keystrokes and mouse-button clicks at the same time to simulate the missing buttons on the mouse. In the case of a one- or two-button mouse, a Java application assumes that the center mouse button is clicked if the user holds down the Alt key and clicks the left mouse button on a two-button mouse or the only mouse button on a one-button mouse. In the case of a onebutton mouse, a Java application assumes that the right mouse button is clicked if the user holds down the Meta key (sometimes called the Command key or the “Apple” key on a Mac) and clicks the mouse button. InputEvent method Returns true when the user clicks the right mouse button on a mouse with two or three buttons. To simulate a right-mousebutton click on a one-button mouse, the user can hold down the Meta key on the keyboard and click the mouse button. Returns true when the user clicks the middle mouse button on a mouse with three buttons. To simulate a middle-mouse-button click on a one- or two-button mouse, the user can press the Alt key and click the only or left mouse button, respectively. isMetaDown() isAltDown() Fig. 12.33 | Description InputEvent methods that help determine whether the right or center mouse button was clicked. Line 21 of Fig. 12.31 registers a MouseListener for the MouseDetailsFrame. The event listener is an object of class MouseClickHandler, which extends MouseAdapter. This enables us to declare only method mouseClicked (lines 28–45). This method first captures the coordinates where the event occurred and stores them in local variables xPos and yPos (lines 31–32). Lines 34–35 create a String called details containing the number of consecutive mouse clicks, which is returned by MouseEvent method getClickCount at line 35. Lines 37–42 use methods isMetaDown and isAltDown to determine which mouse button the user clicked and append an appropriate String to details in each case. The resulting String is displayed in the statusBar. Class MouseDetails (Fig. 12.32) contains the main method that executes the application. Try clicking with each of your mouse’s buttons repeatedly to see the click count increment. 522 Chapter 12 GUI Components: Part 1 12.16 JPanel Subclass for Drawing with the Mouse Section 12.14 showed how to track mouse events in a JPanel. In this section, we use a JPanel as a dedicated drawing area in which the user can draw by dragging the mouse. In addition, this section demonstrates an event listener that extends an adapter class. Method paintComponent Lightweight Swing components that extend class JComponent (such as JPanel) contain method paintComponent, which is called when a lightweight Swing component is displayed. By overriding this method, you can specify how to draw shapes using Java’s graphics capabilities. When customizing a JPanel for use as a dedicated drawing area, the subclass should override method paintComponent and call the superclass version of paintComponent as the first statement in the body of the overridden method to ensure that the component displays correctly. The reason is that subclasses of JComponent support transparency. To display a component correctly, the program must determine whether the component is transparent. The code that determines this is in superclass JComponent’s paintComponent implementation. When a component is transparent, paintComponent will not clear its background when the program displays the component. When a component is opaque, paintComponent clears the component’s background before the component is displayed. The transparency of a Swing lightweight component can be set with method setOpaque (a false argument indicates that the component is transparent). Error-Prevention Tip 12.1 In a JComponent subclass’s paintComponent method, the first statement should always call the superclass’s paintComponent method to ensure that an object of the subclass displays correctly. Common Programming Error 12.4 If an overridden paintComponent method does not call the superclass’s version, the subclass component may not display properly. If an overridden paintComponent method calls the superclass’s version after other drawing is performed, the drawing will be erased. Defining the Custom Drawing Area The Painter application of Figs. 12.34–12.35 demonstrates a customized subclass of JPanel that’s used to create a dedicated drawing area. The application uses the mouseDragged event handler to create a simple drawing application. The user can draw pictures by dragging the mouse on the JPanel. This example does not use method mouseMoved, so our event-listener class (the anonymous inner class at lines 20–29 of Fig. 12.34) extends MouseMotionAdapter. Since this class already declares both mouseMoved and mouseDragged, we can simply override mouseDragged to provide the event handling this application requires. 1 2 3 4 5 // Fig. 12.34: PaintPanel.java // Adapter class used to implement event handlers. import java.awt.Point; import java.awt.Graphics; import java.awt.event.MouseEvent; Fig. 12.34 | Adapter class used to implement event handlers. (Part 1 of 2.) 12.16 JPanel Subclass for Drawing with the Mouse 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 523 import java.awt.event.MouseMotionAdapter; import java.util.ArrayList; import javax.swing.JPanel; public class PaintPanel extends JPanel { // list of Point references private final ArrayList points = new ArrayList<>(); // set up GUI and register mouse event handler public PaintPanel() { // handle frame mouse motion event addMouseMotionListener( new MouseMotionAdapter() // anonymous inner class { // store drag coordinates and repaint @Override public void mouseDragged(MouseEvent event) { points.add(event.getPoint()); repaint(); // repaint JFrame } } ); } // draw ovals in a 4-by-4 bounding box at specified locations on window @Override public void paintComponent(Graphics g) { super.paintComponent(g); // clears drawing area // draw all points for (Point point : points) g.fillOval(point.x, point.y, 4, 4); } } // end class PaintPanel Fig. 12.34 | Adapter class used to implement event handlers. (Part 2 of 2.) Class PaintPanel (Fig. 12.34) extends JPanel to create the dedicated drawing area. Class Point (package java.awt) represents an x-y coordinate. We use objects of this class to store the coordinates of each mouse drag event. Class Graphics is used to draw. In this example, we use an ArrayList of Points (line 13) to store the location at which each mouse drag event occurs. As you’ll see, method paintComponent uses these Points to draw. Lines 19–30 register a MouseMotionListener to listen for the PaintPanel’s mouse motion events. Lines 20–29 create an object of an anonymous inner class that extends the adapter class MouseMotionAdapter. Recall that MouseMotionAdapter implements MouseMotionListener, so the anonymous inner class object is a MouseMotionListener. The anonymous inner class inherits default mouseMoved and mouseDragged implementations, so it already implements all the interface’s methods. However, the default implementa- 524 Chapter 12 GUI Components: Part 1 tions do nothing when they’re called. So, we override method mouseDragged at lines 23– 28 to capture the coordinates of a mouse drag event and store them as a Point object. Line 26 invokes the MouseEvent’s getPoint method to obtain the Point where the event occurred and stores it in the ArrayList. Line 27 calls method repaint (inherited indirectly from class Component) to indicate that the PaintPanel should be refreshed on the screen as soon as possible with a call to the PaintPanel’s paintComponent method. Method paintComponent (lines 34–42), which receives a Graphics parameter, is called automatically any time the PaintPanel needs to be displayed on the screen—such as when the GUI is first displayed—or refreshed on the screen—such as when method repaint is called or when the GUI component has been hidden by another window on the screen and subsequently becomes visible again. Look-and-Feel Observation 12.13 Calling repaint for a Swing GUI component indicates that the component should be refreshed on the screen as soon as possible. The component’s background is cleared only if the component is opaque. JComponent method setOpaque can be passed a boolean argument indicating whether the component is opaque (true) or transparent (false). Line 37 invokes the superclass version of paintComponent to clear the PaintPanel’s background (JPanels are opaque by default). Lines 40–41 draw an oval at the location specified by each Point in the ArrayList. Graphics method fillOval draws a solid oval. The method’s four parameters represent a rectangular area (called the bounding box) in which the oval is displayed. The first two parameters are the upper-left x-coordinate and the upper-left y-coordinate of the rectangular area. The last two coordinates represent the rectangular area’s width and height. Method fillOval draws the oval so it touches the middle of each side of the rectangular area. In line 41, the first two arguments are specified by using class Point’s two public instance variables—x and y. You’ll learn more Graphics features in Chapter 13. Look-and-Feel Observation 12.14 Drawing on any GUI component is performed with coordinates that are measured from the upper-left corner (0, 0) of that GUI component, not the upper-left corner of the screen. Using the Custom JPanel in an Application Class Painter (Fig. 12.35) contains the main method that executes this application. Line 14 creates a PaintPanel object on which the user can drag the mouse to draw. Line 15 attaches the PaintPanel to the JFrame. 1 2 3 4 5 6 7 8 // Fig. 12.35: Painter.java // Testing PaintPanel. import java.awt.BorderLayout; import javax.swing.JFrame; import javax.swing.JLabel; public class Painter { Fig. 12.35 | Testing PaintPanel. (Part 1 of 2.) 12.17 Key Event Handling 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 525 public static void main(String[] args) { // create JFrame JFrame application = new JFrame("A simple paint program"); PaintPanel paintPanel = new PaintPanel(); application.add(paintPanel, BorderLayout.CENTER); // create a label and place it in SOUTH of BorderLayout application.add(new JLabel("Drag the mouse to draw"), BorderLayout.SOUTH); application.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); application.setSize(400, 200); application.setVisible(true); } } // end class Painter Fig. 12.35 | Testing PaintPanel. (Part 2 of 2.) 12.17 Key Event Handling This section presents the KeyListener interface for handling key events. Key events are generated when keys on the keyboard are pressed and released. A class that implements KeyListener must provide declarations for methods keyPressed, keyReleased and keyTyped, each of which receives a KeyEvent as its argument. Class KeyEvent is a subclass of InputEvent. Method keyPressed is called in response to pressing any key. Method keyTyped is called in response to pressing any key that is not an action key. (The action keys are any arrow key, Home, End, Page Up, Page Down, any function key, etc.) Method keyReleased is called when the key is released after any keyPressed or keyTyped event. The application of Figs. 12.36–12.37 demonstrates the KeyListener methods. Class KeyDemoFrame implements the KeyListener interface, so all three methods are declared in the application. The constructor (Fig. 12.36, lines 17–28) registers the application to handle its own key events by using method addKeyListener at line 27. Method addKeyListener is declared in class Component, so every subclass of Component can notify KeyListener objects of key events for that Component. 1 2 3 // Fig. 12.36: KeyDemoFrame.java // Key event handling. import java.awt.Color; Fig. 12.36 | Key event handling. (Part 1 of 3.) 526 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 Chapter 12 GUI Components: Part 1 import import import import java.awt.event.KeyListener; java.awt.event.KeyEvent; javax.swing.JFrame; javax.swing.JTextArea; public class KeyDemoFrame extends JFrame implements KeyListener { private final String line1 = ""; // first line of text area private final String line2 = ""; // second line of text area private final String line3 = ""; // third line of text area private final JTextArea textArea; // text area to display output // KeyDemoFrame constructor public KeyDemoFrame() { super("Demonstrating Keystroke Events"); textArea = new JTextArea(10, 15); // set up JTextArea textArea.setText("Press any key on the keyboard..."); textArea.setEnabled(false); textArea.setDisabledTextColor(Color.BLACK); add(textArea); // add text area to JFrame addKeyListener(this); // allow frame to process key events } // handle press of any key @Override public void keyPressed(KeyEvent event) { line1 = String.format("Key pressed: %s", KeyEvent.getKeyText(event.getKeyCode())); // show pressed key setLines2and3(event); // set output lines two and three } // handle release of any key @Override public void keyReleased(KeyEvent event) { line1 = String.format("Key released: %s", KeyEvent.getKeyText(event.getKeyCode())); // show released key setLines2and3(event); // set output lines two and three } // handle press of an action key @Override public void keyTyped(KeyEvent event) { line1 = String.format("Key typed: %s", event.getKeyChar()); setLines2and3(event); // set output lines two and three } Fig. 12.36 | Key event handling. (Part 2 of 3.) 12.17 Key Event Handling 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 // set second and third lines of output private void setLines2and3(KeyEvent event) { line2 = String.format("This key is %san action key", (event.isActionKey() ? "" : "not ")); String temp = KeyEvent.getKeyModifiersText(event.getModifiers()); line3 = String.format("Modifier keys pressed: %s", (temp.equals("") ? "none" : temp)); // output modifiers textArea.setText(String.format("%s\n%s\n%s\n", line1, line2, line3)); // output three lines of text } } // end class KeyDemoFrame Fig. 12.36 | Key event handling. (Part 3 of 3.) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 527 // Fig. 12.37: KeyDemo.java // Testing KeyDemoFrame. import javax.swing.JFrame; public class KeyDemo { public static void main(String[] args) { KeyDemoFrame keyDemoFrame = new KeyDemoFrame(); keyDemoFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); keyDemoFrame.setSize(350, 100); keyDemoFrame.setVisible(true); } } // end class KeyDemo Fig. 12.37 | Testing KeyDemoFrame. (Part 1 of 2.) 528 Chapter 12 GUI Components: Part 1 Fig. 12.37 | Testing KeyDemoFrame. (Part 2 of 2.) At line 25, the constructor adds the JTextArea textArea (where the application’s output is displayed) to the JFrame. A JTextArea is a multiline area in which you can display text. (We discuss JTextAreas in more detail in Section 12.20.) Notice in the screen captures that textArea occupies the entire window. This is due to the JFrame’s default BorderLayout (discussed in Section 12.18.2 and demonstrated in Fig. 12.41). When a single Component is added to a BorderLayout, the Component occupies the entire Container. Line 23 disables the JTextArea so the user cannot type in it. This causes the text in the JTextArea to become gray. Line 24 uses method setDisabledTextColor to change the text color in the JTextArea to black for readability. Methods keyPressed (lines 31–37) and keyReleased (lines 40–46) use KeyEvent method getKeyCode to get the virtual key code of the pressed key. Class KeyEvent contains virtual key-code constants that represent every key on the keyboard. These constants can be compared with getKeyCode’s return value to test for individual keys on the keyboard. The value returned by getKeyCode is passed to static KeyEvent method getKeyText, which returns a string containing the name of the key that was pressed. For a complete list of virtual key constants, see the online documentation for class KeyEvent (package java.awt.event). Method keyTyped (lines 49–54) uses KeyEvent method getKeyChar (which returns a char) to get the Unicode value of the character typed. All three event-handling methods finish by calling method setLines2and3 (lines 57– 69) and passing it the KeyEvent object. This method uses KeyEvent method isActionKey (line 60) to determine whether the key in the event was an action key. Also, InputEvent method getModifiers is called (line 62) to determine whether any modifier keys (such as Shift, Alt and Ctrl) were pressed when the key event occurred. The result of this method is passed to static KeyEvent method getKeyModifiersText, which produces a String containing the names of the pressed modifier keys. [Note: If you need to test for a specific key on the keyboard, class KeyEvent provides a key constant for each one. These constants can be used from the key event handlers to determine whether a particular key was pressed. Also, to determine whether the Alt, Ctrl, Meta and Shift keys are pressed individually, InputEvent methods isAltDown, isControlDown, isMetaDown and isShiftDown each return a boolean indicating whether the particular key was pressed during the key event.] 12.18 Introduction to Layout Managers Layout managers arrange GUI components in a container for presentation purposes. You can use the layout managers for basic layout capabilities instead of determining every GUI component’s exact position and size. This functionality enables you to concentrate on the basic look-and-feel and lets the layout managers process most of the layout details. All layout managers implement the interface LayoutManager (in package java.awt). Class Con- 12.18 Introduction to Layout Managers 529 tainer’s setLayout method takes an object that implements the LayoutManager interface as an argument. There are basically three ways for you to arrange components in a GUI: 1. Absolute positioning: This provides the greatest level of control over a GUI’s appearance. By setting a Container’s layout to null, you can specify the absolute position of each GUI component with respect to the upper-left corner of the Container by using Component methods setSize and setLocation or setBounds. If you do this, you also must specify each GUI component’s size. Programming a GUI with absolute positioning can be tedious, unless you have an integrated development environment (IDE) that can generate the code for you. 2. Layout managers: Using layout managers to position elements can be simpler and faster than creating a GUI with absolute positioning, and makes your GUIs more resizable, but you lose some control over the size and the precise positioning of each component. 3. Visual programming in an IDE: IDEs provide tools that make it easy to create GUIs. Each IDE typically provides a GUI design tool that allows you to drag and drop GUI components from a tool box onto a design area. You can then position, size and align GUI components as you like. The IDE generates the Java code that creates the GUI. In addition, you can typically add event-handling code for a particular component by double-clicking the component. Some design tools also allow you to use the layout managers described in this chapter and in Chapter 22. Look-and-Feel Observation 12.15 Most Java IDEs provide GUI design tools for visually designing a GUI; the tools then write Java code that creates the GUI. Such tools often provide greater control over the size, position and alignment of GUI components than do the built-in layout managers. Look-and-Feel Observation 12.16 It’s possible to set a Container’s layout to null, which indicates that no layout manager should be used. In a Container without a layout manager, you must position and size the components and take care that, on resize events, all components are repositioned as necessary. A component’s resize events can be processed by a ComponentListener. Figure 12.38 summarizes the layout managers presented in this chapter. A couple of additional layout managers are discussed in Chapter 22. Layout manager Description FlowLayout Default for javax.swing.JPanel. Places components sequentially, left to right, in the order they were added. It’s also possible to specify the order of the components by using the Container method add, which takes a Component and an integer index position as arguments. Default for JFrames (and other windows). Arranges the components into five areas: NORTH, SOUTH, EAST, WEST and CENTER. Arranges the components into rows and columns. BorderLayout GridLayout Fig. 12.38 | Layout managers. 530 Chapter 12 GUI Components: Part 1 12.18.1 FlowLayout FlowLayout is the simplest layout manager. GUI components are placed in a container from left to right in the order in which they’re added to the container. When the edge of the container is reached, components continue to display on the next line. Class FlowLayout allows GUI components to be left aligned, centered (the default) and right aligned. The application of Figs. 12.39–12.40 creates three JButton objects and adds them to the application, using a FlowLayout. The components are center aligned by default. When the user clicks Left, the FlowLayout’s alignment is changed to left aligned. When the user clicks Right, the FlowLayout’s alignment is changed to right aligned. When the user clicks Center, the FlowLayout’s alignment is changed to center aligned. The sample output windows show each alignment. The last sample output shows the centered alignment after the window has been resized to a smaller width so that the button Right flows onto a new line. As seen previously, a container’s layout is set with method setLayout of class Container. Line 25 (Fig. 12.39) sets the layout manager to the FlowLayout declared at line 23. Normally, the layout is set before any GUI components are added to a container. Look-and-Feel Observation 12.17 Each individual container can have only one layout manager, but multiple containers in the same application can each use different layout managers. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 // Fig. 12.39: FlowLayoutFrame.java // FlowLayout allows components to flow over multiple lines. import java.awt.FlowLayout; import java.awt.Container; import java.awt.event.ActionListener; import java.awt.event.ActionEvent; import javax.swing.JFrame; import javax.swing.JButton; public class FlowLayoutFrame extends JFrame { private final JButton leftJButton; // button to set alignment left private final JButton centerJButton; // button to set alignment center private final JButton rightJButton; // button to set alignment right private final FlowLayout layout; // layout object private final Container container; // container to set layout // set up GUI and register button listeners public FlowLayoutFrame() { super("FlowLayout Demo"); layout = new FlowLayout(); container = getContentPane(); // get container to layout setLayout(layout); // set up leftJButton and register listener leftJButton = new JButton("Left"); add(leftJButton); // add Left button to frame Fig. 12.39 | FlowLayout allows components to flow over multiple lines. (Part 1 of 2.) 12.18 Introduction to Layout Managers 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 leftJButton.addActionListener( new ActionListener() // anonymous inner class { // process leftJButton event @Override public void actionPerformed(ActionEvent event) { layout.setAlignment(FlowLayout.LEFT); // realign attached components layout.layoutContainer(container); } } ); // set up centerJButton and register listener centerJButton = new JButton("Center"); add(centerJButton); // add Center button to frame centerJButton.addActionListener( new ActionListener() // anonymous inner class { // process centerJButton event @Override public void actionPerformed(ActionEvent event) { layout.setAlignment(FlowLayout.CENTER); // realign attached components layout.layoutContainer(container); } } ); // set up rightJButton and register listener rightJButton = new JButton("Right"); add(rightJButton); // add Right button to frame rightJButton.addActionListener( new ActionListener() // anonymous inner class { // process rightJButton event @Override public void actionPerformed(ActionEvent event) { layout.setAlignment(FlowLayout.RIGHT); // realign attached components layout.layoutContainer(container); } } ); } // end FlowLayoutFrame constructor } // end class FlowLayoutFrame Fig. 12.39 | FlowLayout allows components to flow over multiple lines. (Part 2 of 2.) 531 532 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Chapter 12 GUI Components: Part 1 // Fig. 12.40: FlowLayoutDemo.java // Testing FlowLayoutFrame. import javax.swing.JFrame; public class FlowLayoutDemo { public static void main(String[] args) { FlowLayoutFrame flowLayoutFrame = new FlowLayoutFrame(); flowLayoutFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); flowLayoutFrame.setSize(300, 75); flowLayoutFrame.setVisible(true); } } // end class FlowLayoutDemo Fig. 12.40 | Testing FlowLayoutFrame. Each button’s event handler is specified with a separate anonymous inner-class object (lines 30–43, 48–61 and 66–79, respectively), and method actionPerformed in each case executes two statements. For example, line 37 in the event handler for leftJButton uses FlowLayout method setAlignment to change the alignment for the FlowLayout to a leftaligned (FlowLayout.LEFT) FlowLayout. Line 40 uses LayoutManager interface method layoutContainer (which is inherited by all layout managers) to specify that the JFrame should be rearranged based on the adjusted layout. According to which button was clicked, the actionPerformed method for each button sets the FlowLayout’s alignment to FlowLayout.LEFT (line 37), FlowLayout.CENTER (line 55) or FlowLayout.RIGHT (line 73). 12.18.2 BorderLayout The BorderLayout layout manager (the default layout manager for a JFrame) arranges components into five regions: NORTH, SOUTH, EAST, WEST and CENTER. NORTH corresponds to the top of the container. Class BorderLayout extends Object and implements interface LayoutManager2 (a subinterface of LayoutManager that adds several methods for enhanced layout processing). A BorderLayout limits a Container to containing at most five components—one in each region. The component placed in each region can be a container to which other components are attached. The components placed in the NORTH and SOUTH regions extend hor- 12.18 Introduction to Layout Managers 533 izontally to the sides of the container and are as tall as the components placed in those regions. The EAST and WEST regions expand vertically between the NORTH and SOUTH regions and are as wide as the components placed in those regions. The component placed in the CENTER region expands to fill all remaining space in the layout (which is the reason the JTextArea in Fig. 12.37 occupies the entire window). If all five regions are occupied, the entire container’s space is covered by GUI components. If the NORTH or SOUTH region is not occupied, the GUI components in the EAST, CENTER and WEST regions expand vertically to fill the remaining space. If the EAST or WEST region is not occupied, the GUI component in the CENTER region expands horizontally to fill the remaining space. If the CENTER region is not occupied, the area is left empty—the other GUI components do not expand to fill the remaining space. The application of Figs. 12.41–12.42 demonstrates the BorderLayout layout manager by using five JButtons. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 // Fig. 12.41: BorderLayoutFrame.java // BorderLayout containing five buttons. import java.awt.BorderLayout; import java.awt.event.ActionListener; import java.awt.event.ActionEvent; import javax.swing.JFrame; import javax.swing.JButton; public class BorderLayoutFrame extends JFrame implements ActionListener { private final JButton[] buttons; // array of buttons to hide portions private static final String[] names = {"Hide North", "Hide South", "Hide East", "Hide West", "Hide Center"}; private final BorderLayout layout; // set up GUI and event handling public BorderLayoutFrame() { super("BorderLayout Demo"); layout = new BorderLayout(5, 5); // 5 pixel gaps setLayout(layout); buttons = new JButton[names.length]; // create JButtons and register listeners for them for (int count = 0; count < names.length; count++) { buttons[count] = new JButton(names[count]); buttons[count].addActionListener(this); } add(buttons[0], add(buttons[1], add(buttons[2], add(buttons[3], add(buttons[4], BorderLayout.NORTH); BorderLayout.SOUTH); BorderLayout.EAST); BorderLayout.WEST); BorderLayout.CENTER); } Fig. 12.41 | BorderLayout containing five buttons. (Part 1 of 2.) 534 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 Chapter 12 GUI Components: Part 1 // handle button events @Override public void actionPerformed(ActionEvent event) { // check event source and lay out content pane correspondingly for (JButton button : buttons) { if (event.getSource() == button) button.setVisible(false); // hide the button that was clicked else button.setVisible(true); // show other buttons } layout.layoutContainer(getContentPane()); // lay out content pane } } // end class BorderLayoutFrame Fig. 12.41 | BorderLayout containing five buttons. (Part 2 of 2.) Line 21 of Fig. 12.41 creates a BorderLayout. The constructor arguments specify the number of pixels between components that are arranged horizontally (horizontal gap space) and between components that are arranged vertically (vertical gap space), respectively. The default is one pixel of gap space horizontally and vertically. Line 22 uses method setLayout to set the content pane’s layout to layout. We add Components to a BorderLayout with another version of Container method add that takes two arguments—the Component to add and the region in which the Component should appear. For example, line 32 specifies that buttons[0] should appear in the NORTH region. The components can be added in any order, but only one component should be added to each region. Look-and-Feel Observation 12.18 If no region is specified when adding a Component to a BorderLayout, the layout manager assumes that the Component should be added to region BorderLayout.CENTER. Common Programming Error 12.5 When more than one component is added to a region in a BorderLayout, only the last component added to that region will be displayed. There’s no error that indicates this problem. Class BorderLayoutFrame implements ActionListener directly in this example, so the will handle the events of the JButtons. For this reason, line 29 passes the this reference to the addActionListener method of each JButton. When the user clicks a particular JButton in the layout, method actionPerformed (lines 40–53) executes. The enhanced for statement at lines 44–50 uses an if…else to hide the particular JButton that generated the event. Method setVisible (inherited into JButton from class Component) is called with a false argument (line 47) to hide the JButton. If the current JButton in the array is not the one that generated the event, method setVisible is called with a true argument (line 49) to ensure that the JButton is displayed on the screen. Line 52 uses Layout- BorderLayoutFrame 12.18 Introduction to Layout Managers 535 method layoutContainer to recalculate the layout of the content pane. Notice in the screen captures of Fig. 12.42 that certain regions in the BorderLayout change shape as JButtons are hidden and displayed in other regions. Try resizing the application window to see how the various regions resize based on the window’s width and height. For more complex layouts, group components in JPanels, each with a separate layout manager. Place the JPanels on the JFrame using either the default BorderLayout or some other layout. Manager 1 2 3 4 5 6 7 8 9 10 11 12 13 14 // Fig. 12.42: BorderLayoutDemo.java // Testing BorderLayoutFrame. import javax.swing.JFrame; public class BorderLayoutDemo { public static void main(String[] args) { BorderLayoutFrame borderLayoutFrame = new BorderLayoutFrame(); borderLayoutFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); borderLayoutFrame.setSize(300, 200); borderLayoutFrame.setVisible(true); } } // end class BorderLayoutDemo horizontal gap vertical gap Fig. 12.42 | Testing BorderLayoutFrame. 536 Chapter 12 GUI Components: Part 1 12.18.3 GridLayout The GridLayout layout manager divides the container into a grid so that components can be placed in rows and columns. Class GridLayout inherits directly from class Object and implements interface LayoutManager. Every Component in a GridLayout has the same width and height. Components are added to a GridLayout starting at the top-left cell of the grid and proceeding left to right until the row is full. Then the process continues left to right on the next row of the grid, and so on. The application of Figs. 12.43–12.44 demonstrates the GridLayout layout manager by using six JButtons. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 // Fig. 12.43: GridLayoutFrame.java // GridLayout containing six buttons. import java.awt.GridLayout; import java.awt.Container; import java.awt.event.ActionListener; import java.awt.event.ActionEvent; import javax.swing.JFrame; import javax.swing.JButton; public class GridLayoutFrame extends JFrame implements ActionListener { private final JButton[] buttons; // array of buttons private static final String[] names = { "one", "two", "three", "four", "five", "six" }; private boolean toggle = true; // toggle between two layouts private final Container container; // frame container private final GridLayout gridLayout1; // first gridlayout private final GridLayout gridLayout2; // second gridlayout // no-argument constructor public GridLayoutFrame() { super("GridLayout Demo"); gridLayout1 = new GridLayout(2, 3, 5, 5); // 2 by 3; gaps of 5 gridLayout2 = new GridLayout(3, 2); // 3 by 2; no gaps container = getContentPane(); setLayout(gridLayout1); buttons = new JButton[names.length]; for (int count = 0; count < names.length; count++) { buttons[count] = new JButton(names[count]); buttons[count].addActionListener(this); // register listener add(buttons[count]); // add button to JFrame } } // handle button events by toggling between layouts @Override public void actionPerformed(ActionEvent event) { Fig. 12.43 | GridLayout containing six buttons. (Part 1 of 2.) 12.18 Introduction to Layout Managers 42 43 44 45 46 47 48 49 50 if (toggle) // set layout based on toggle container.setLayout(gridLayout2); else container.setLayout(gridLayout1); toggle = !toggle; container.validate(); // re-lay out container } } // end class GridLayoutFrame Fig. 12.43 | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 537 GridLayout containing six buttons. (Part 2 of 2.) // Fig. 12.44: GridLayoutDemo.java // Testing GridLayoutFrame. import javax.swing.JFrame; public class GridLayoutDemo { public static void main(String[] args) { GridLayoutFrame gridLayoutFrame = new GridLayoutFrame(); gridLayoutFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); gridLayoutFrame.setSize(300, 200); gridLayoutFrame.setVisible(true); } } // end class GridLayoutDemo Fig. 12.44 | Testing GridLayoutFrame. Lines 24–25 (Fig. 12.43) create two GridLayout objects. The GridLayout constructor used at line 24 specifies a GridLayout with 2 rows, 3 columns, 5 pixels of horizontal-gap space between Components in the grid and 5 pixels of vertical-gap space between Components in the grid. The GridLayout constructor used at line 25 specifies a GridLayout with 3 rows and 2 columns that uses the default gap space (1 pixel). The JButton objects in this example initially are arranged using gridLayout1 (set for the content pane at line 27 with method setLayout). The first component is added to the first column of the first row. The next component is added to the second column of the first row, and so on. When a JButton is pressed, method actionPerformed (lines 39–49) is called. Every call to actionPerformed toggles the layout between gridLayout2 and gridLayout1, using boolean variable toggle to determine the next layout to set. Line 48 shows another way to reformat a container for which the layout has changed. Container method validate recomputes the container’s layout based on the current layout manager for the Container and the current set of displayed GUI components. 538 Chapter 12 GUI Components: Part 1 12.19 Using Panels to Manage More Complex Layouts Complex GUIs (like Fig. 12.1) often require that each component be placed in an exact location. They often consist of multiple panels, with each panel’s components arranged in a specific layout. Class JPanel extends JComponent and JComponent extends class Container, so every JPanel is a Container. Thus, every JPanel may have components, including other panels, attached to it with Container method add. The application of Figs. 12.45–12.46 demonstrates how a JPanel can be used to create a more complex layout in which several JButtons are placed in the SOUTH region of a BorderLayout. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 // Fig. 12.45: PanelFrame.java // Using a JPanel to help lay out components. import java.awt.GridLayout; import java.awt.BorderLayout; import javax.swing.JFrame; import javax.swing.JPanel; import javax.swing.JButton; public class PanelFrame extends JFrame { private final JPanel buttonJPanel; // panel to hold buttons private final JButton[] buttons; // no-argument constructor public PanelFrame() { super("Panel Demo"); buttons = new JButton[5]; buttonJPanel = new JPanel(); buttonJPanel.setLayout(new GridLayout(1, buttons.length)); // create and add buttons for (int count = 0; count < buttons.length; count++) { buttons[count] = new JButton("Button " + (count + 1)); buttonJPanel.add(buttons[count]); // add button to panel } add(buttonJPanel, BorderLayout.SOUTH); // add panel to JFrame } } // end class PanelFrame Fig. 12.45 | JPanel with five JButtons in a GridLayout attached to the SOUTH region of a BorderLayout. 1 2 3 4 // Fig. 12.46: PanelDemo.java // Testing PanelFrame. import javax.swing.JFrame; Fig. 12.46 | Testing PanelFrame. (Part 1 of 2.) 12.20 JTextArea 5 6 7 8 9 10 11 12 13 14 539 public class PanelDemo extends JFrame { public static void main(String[] args) { PanelFrame panelFrame = new PanelFrame(); panelFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); panelFrame.setSize(450, 200); panelFrame.setVisible(true); } } // end class PanelDemo Fig. 12.46 | Testing PanelFrame. (Part 2 of 2.) After JPanel buttonJPanel is declared (line 11 of Fig. 12.45) and created (line 19), line 20 sets buttonJPanel’s layout to a GridLayout of one row and five columns (there are five JButtons in array buttons). Lines 23–27 add the JButtons in the array to the JPanel. Line 26 adds the buttons directly to the JPanel—class JPanel does not have a content pane, unlike a JFrame. Line 29 uses the JFrame’s default BorderLayout to add buttonJPanel to the SOUTH region. The SOUTH region is as tall as the buttons on buttonJPanel. A JPanel is sized to the components it contains. As more components are added, the JPanel grows (according to the restrictions of its layout manager) to accommodate the components. Resize the window to see how the layout manager affects the size of the JButtons. 12.20 JTextArea A JTextArea provides an area for manipulating multiple lines of text. Like class JTextField, JTextArea is a subclass of JTextComponent, which declares common methods for JTextFields, JTextAreas and several other text-based GUI components. The application in Figs. 12.47–12.48 demonstrates JTextAreas. One JTextArea displays text that the user can select. The other is uneditable by the user and is used to display the text the user selected in the first JTextArea. Unlike JTextFields, JTextAreas do not have action events—when you press Enter while typing in a JTextArea, the cursor simply moves to the next line. As with multiple-selection JLists (Section 12.13), an external event from another GUI component indicates when to process the text in a JTextArea. For example, when typing an e-mail message, you normally click a Send button to send the text of the message to the recipient. Similarly, when editing a document in a word processor, you normally save the file by selecting a Save or Save As… menu item. In this program, the button Copy >>> generates the external event that copies the selected text in the left JTextArea and displays it in the right JTextArea. 540 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 Chapter 12 GUI Components: Part 1 // Fig. 12.47: TextAreaFrame.java // Copying selected text from one JText area to another. import java.awt.event.ActionListener; import java.awt.event.ActionEvent; import javax.swing.Box; import javax.swing.JFrame; import javax.swing.JTextArea; import javax.swing.JButton; import javax.swing.JScrollPane; public class TextAreaFrame extends JFrame { private final JTextArea textArea1; // displays demo string private final JTextArea textArea2; // highlighted text is copied here private final JButton copyJButton; // initiates copying of text // no-argument constructor public TextAreaFrame() { super("TextArea Demo"); Box box = Box.createHorizontalBox(); // create box String demo = "This is a demo string to\n" + "illustrate copying text\nfrom one textarea to \n" + "another textarea using an\nexternal event\n"; textArea1 = new JTextArea(demo, 10, 15); box.add(new JScrollPane(textArea1)); // add scrollpane copyJButton = new JButton("Copy >>>"); // create copy button box.add(copyJButton); // add copy button to box copyJButton.addActionListener( new ActionListener() // anonymous inner class { // set text in textArea2 to selected text from textArea1 @Override public void actionPerformed(ActionEvent event) { textArea2.setText(textArea1.getSelectedText()); } } ); textArea2 = new JTextArea(10, 15); textArea2.setEditable(false); box.add(new JScrollPane(textArea2)); // add scrollpane add(box); // add box to frame } } // end class TextAreaFrame Fig. 12.47 | Copying selected text from one JTextArea to another. In the constructor (lines 18–48), line 21 creates a Box container (package to organize the GUI components. Box is a subclass of Container that uses javax.swing) 12.20 JTextArea 1 2 3 4 5 6 7 8 9 10 11 12 13 14 541 // Fig. 12.48: TextAreaDemo.java // Testing TextAreaFrame. import javax.swing.JFrame; public class TextAreaDemo { public static void main(String[] args) { TextAreaFrame textAreaFrame = new TextAreaFrame(); textAreaFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); textAreaFrame.setSize(425, 200); textAreaFrame.setVisible(true); } } // end class TextAreaDemo Fig. 12.48 | Testing TextAreaFrame. a BoxLayout layout manager (discussed in detail in Section 22.9) to arrange the GUI components either horizontally or vertically. Box’s static method createHorizontalBox creates a Box that arranges components from left to right in the order that they’re attached. Lines 26 and 43 create JTextAreas textArea1 and textArea2. Line 26 uses JTextArea’s three-argument constructor, which takes a String representing the initial text and two ints specifying that the JTextArea has 10 rows and 15 columns. Line 43 uses JTextArea’s two-argument constructor, specifying that the JTextArea has 10 rows and 15 columns. Line 26 specifies that demo should be displayed as the default JTextArea content. A JTextArea does not provide scrollbars if it cannot display its complete contents. So, line 27 creates a JScrollPane object, initializes it with textArea1 and attaches it to container box. By default, horizontal and vertical scrollbars appear as necessary in a JScrollPane. Lines 29–41 create JButton object copyJButton with the label "Copy >>>", add copyJButton to container box and register the event handler for copyJButton’s ActionEvent. This button provides the external event that determines when the program should copy the selected text in textArea1 to textArea2. When the user clicks copyJButton, line 38 in actionPerformed indicates that method getSelectedText (inherited into JTextArea from JTextComponent) should return the selected text from textArea1. The user selects text by dragging the mouse over the desired text to highlight it. Method setText changes the text in textArea2 to the string returned by getSelectedText. Lines 43–45 create textArea2, set its editable property to false and add it to container box. Line 47 adds box to the JFrame. Recall from Section 12.18.2 that the default layout of a JFrame is a BorderLayout and that the add method by default attaches its argument to the CENTER of the BorderLayout. 542 Chapter 12 GUI Components: Part 1 When text reaches the right edge of a JTextArea the text can wrap to the next line. This is referred to as line wrapping. By default, JTextArea does not wrap lines. Look-and-Feel Observation 12.19 To provide line wrapping functionality for a JTextArea, invoke JTextArea method setLineWrap with a true argument. Scrollbar Policies This example uses a JScrollPane to provide scrolling for a JTextArea. By default, JScrollPane displays scrollbars only if they’re required. You can set the horizontal and vertical scrollbar policies of a JScrollPane when it’s constructed. If a program has a reference to a JScrollPane, the program can use JScrollPane methods setHorizontalScrollBarPolicy and setVerticalScrollBarPolicy to change the scrollbar policies at any time. Class JScrollPane declares the constants JScrollPane JScrollPane.VERTICAL_SCROLLBAR_ALWAYS JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS to indicate that a scrollbar should always appear, constants JScrollPane.VERTICAL_SCROLLBAR_AS_NEEDED JScrollPane.HORIZONTAL_SCROLLBAR_AS_NEEDED to indicate that a scrollbar should appear only if necessary (the defaults) and constants JScrollPane.VERTICAL_SCROLLBAR_NEVER JScrollPane.HORIZONTAL_SCROLLBAR_NEVER to indicate that a scrollbar should never appear. If the horizontal scrollbar policy is set to JScrollPane.HORIZONTAL_SCROLLBAR_NEVER, a JTextArea attached to the JScrollPane will automatically wrap lines. 12.21 Wrap-Up In this chapter, you learned many GUI components and how to handle their events. You also learned about nested classes, inner classes and anonymous inner classes. You saw the special relationship between an inner-class object and an object of its top-level class. You learned how to use JOptionPane dialogs to obtain text input from the user and how to display messages to the user. You also learned how to create applications that execute in their own windows. We discussed class JFrame and components that enable a user to interact with an application. We also showed you how to display text and images to the user. You learned how to customize JPanels to create custom drawing areas, which you’ll use extensively in the next chapter. You saw how to organize components on a window using layout managers and how to creating more complex GUIs by using JPanels to organize components. Finally, you learned about the JTextArea component in which a user can enter text and an application can display text. In Chapter 22, you’ll learn about more advanced GUI components, such as sliders, menus and more complex layout managers. In the next chapter, you’ll learn how to add graphics to your GUI application. Graphics allow you to draw shapes and text with colors and styles. Summary 543 Summary Section 12.1 Introduction • A graphical user interface (GUI; p. 474) presents a user-friendly mechanism for interacting with an application. A GUI gives an application a distinctive look-and-feel (p. 474). • Providing different applications with consistent, intuitive user-interface components gives users a sense of familarity with a new application, so that they can learn it more quickly. • GUIs are built from GUI components (p. 474)—sometimes called controls or widgets. Section 12.2 Java’s Nimbus Look-and-Feel • As of Java SE 6 update 10, Java comes bundled with a new, elegant, cross-platform look-and-feel known as Nimbus (p. 476). • To set Nimbus as the default for all Java applications, create a swing.properties text file in the lib folder of your JDK and JRE installation folders. Place the following line of code in the file: swing.defaultlaf=com.sun.java.swing.plaf.nimbus.NimbusLookAndFeel • To select Nimbus on an application-by-application basis, place the following command-line argument after the java command and before the application’s name when you run the application: -Dswing.defaultlaf=com.sun.java.swing.plaf.nimbus.NimbusLookAndFeel Section 12.3 Simple GUI-Based Input/Output with JOptionPane • Most applications use windows or dialog boxes (p. 476) to interact with the user. • Class JOptionPane (p. 476) of package javax.swing (p. 474) provides prebuilt dialog boxes for both input and output. JOptionPane static method showInputDialog (p. 477) displays an input dialog (p. 476). • A prompt typically uses sentence-style capitalization—capitalizing only the first letter of the first word in the text unless the word is a proper noun. • An input dialog can input only input Strings. This is typical of most GUI components. • JOptionPane static method showMessageDialog (p. 478) displays a message dialog (p. 476). Section 12.4 Overview of Swing Components • Most Swing GUI components (p. 474) are located in package javax.swing. • Together, the appearance and the way in which the user interacts with the application are known as that application’s look-and-feel. Swing GUI components allow you to specify a uniform lookand-feel for your application across all platforms or to use each platform’s custom look-and-feel. • Lightweight Swing components are not tied to actual GUI components supported by the underlying platform on which an application executes. • Several Swing components are heavyweight components (p. 480) that require direct interaction with the local windowing system (p. 480), which may restrict their appearance and functionality. • Class Component (p. 480) of package java.awt declares many of the attributes and behaviors common to the GUI components in packages java.awt (p. 479) and javax.swing. • Class Container (p. 480) of package java.awt is a subclass of Component. Components are attached to Containers so the Components can be organized and displayed on the screen. • Class JComponent (p. 480) of package javax.swing is a subclass of Container. JComponent is the superclass of all lightweight Swing components and declares their common attributes and behaviors. • Some common JComponent features include a pluggable look-and-feel (p. 480), shortcut keys called mnemonics (p. 480), tool tips (p. 480), support for assistive technologies and support for user-interface localization (p. 480). 544 Chapter 12 GUI Components: Part 1 Section 12.5 Displaying Text and Images in a Window • Class JFrame provides the basic attributes and behaviors of a window. • A JLabel (p. 481) displays read-only text, an image, or both text and an image. Text in a JLabel normally uses sentence-style capitalization. • Each GUI component must be attached to a container, such as a window created with a JFrame (p. 483). • Many IDEs provide GUI design tools (p. 529) in which you can specify the exact size and location of a component by using the mouse; then the IDE will generate the GUI code for you. • JComponent method setToolTipText (p. 483) specifies the tool tip that’s displayed when the user positions the mouse cursor over a lightweight component (p. 480). • Container method add attaches a GUI component to a Container. • Class ImageIcon (p. 484) supports several image formats, including GIF, PNG and JPEG. • Method getClass of class Object (p. 484) retrieves a reference to the Class object that represents the the class declaration for the object on which the method is called. • Class method getResource (p. 484) returns the location of its argument as a URL. The method getResource uses the Class object’s class loader to determine the location of the resource. • The horizontal and vertical alignments of a JLabel can be set with methods setHorizontalAlignment (p. 484) and setVerticalAlignment (p. 484), respectively. • JLabel methods setText (p. 484) and getText (p. 484) set and get the text displayed on a label. • JLabel methods setIcon (p. 484) and getIcon (p. 484) set and get the Icon (p. 484) on a label. • JLabel methods setHorizontalTextPosition (p. 484) and setVerticalTextPosition (p. 484) specify the text position in the label. • JFrame method setDefaultCloseOperation (p. 485) with constant JFrame.EXIT_ON_CLOSE as the argument indicates that the program should terminate when the window is closed by the user. • Component method setSize (p. 485) specifies the width and height of a component. • Component method setVisible (p. 485) with the argument true displays a JFrame on the screen. Section 12.6 Text Fields and an Introduction to Event Handling with Nested Classes • GUIs are event driven—when the user interacts with a GUI component, events (p. 485) drive the program to perform tasks. • An event handler (p. 485) performs a task in response to an event. • Class JTextField (p. 485) extends JTextComponent (p. 485) of package javax.swing.text, which provides common text-based component features. Class JPasswordField (p. 485) extends JTextField and adds several methods that are specific to processing passwords. • A JPasswordField (p. 485) shows that characters are being typed as the user enters them, but hides the actual characters with echo characters (p. 485). • A component receives the focus (p. 486) when the user clicks the component. • JTextComponent method setEditable (p. 488) can be used to make a text field uneditable. • To respond to an event for a particular GUI component, you must create a class that represents the event handler and implements an appropriate event-listener interface (p. 488), then register an object of the event-handling class as the event handler (p. 488). • Non-static nested classes (p. 488) are called inner classes and are frequently used for event handling. • An object of a non-static inner class (p. 488) must be created by an object of the top-level class (p. 488) that contains the inner class. Summary 545 • An inner-class object can directly access the instance variables and methods of its top-level class. • A nested class that’s static does not require an object of its top-level class and does not implicitly have a reference to an object of the top-level class. • Pressing Enter in a JTextField or JPasswordField generates an ActionEvent (p. 489) that can be handled by an ActionListener (p. 489) of package java.awt.event. • JTextField method addActionListener (p. 489) registers an event handler for a text field’s ActionEvent. • The GUI component with which the user interacts is the event source (p. 490). • An ActionEvent object contains information about the event that just occurred, such as the event source and the text in the text field. • ActionEvent method getSource returns a reference to the event source. ActionEvent method getActionCommand (p. 490) returns the text the user typed in a text field or the label on a JButton. • JPasswordField method getPassword (p. 490) returns the password the user typed. Section 12.7 Common GUI Event Types and Listener Interfaces • Each event-object type typically has a corresponding event-listener interface that specifies one or more event-handling methods, which must be declared in the class that implements the interface. Section 12.8 How Event Handling Works • When an event occurs, the GUI component with which the user interacted notifies its registered listeners by calling each listener’s appropriate event-handling method. • Every GUI component supports several event types. When an event occurs, the event is dispatched (p. 494) only to the event listeners of the appropriate type. Section 12.9 JButton • A button is a component the user clicks to trigger an action. All the button types are subclasses of AbstractButton (p. 495; package javax.swing). Button labels (p. 495) typically use book-title capitalization (p. 495). • Command buttons (p. 495) are created with class JButton. • A JButton can display an Icon. A JButton can also have a rollover Icon (p. 497)—an Icon that’s displayed when the user positions the mouse over the button. • Method setRolloverIcon (p. 498) of class AbstractButton specifies the image displayed on a button when the user positions the mouse over it. Section 12.10 Buttons That Maintain State • There are three Swing state button types—JToggleButton (p. 498), JCheckBox (p. 498) and JRadioButton (p. 498). • Classes JCheckBox and JRadioButton are subclasses of JToggleButton. • Component method setFont (p. 500) sets the component’s font to a new Font object (p. 500) of package java.awt. • Clicking a JCheckBox causes an ItemEvent (p. 501) that can be handled by an ItemListener (p. 501) which defines method itemStateChanged (p. 501). Method addItemListener registers the listener for the ItemEvent of a JCheckBox or JRadioButton object. • JCheckBox method isSelected determines whether a JCheckBox is selected. • JRadioButtons have two states—selected and not selected. Radio buttons (p. 495) normally appear as a group (p. 501) in which only one button can be selected at a time. 546 Chapter 12 GUI Components: Part 1 • JRadioButtons are used to represent mutually exclusive options (p. 501). • The logical relationship between JRadioButtons is maintained by a ButtonGroup object (p. 501). • ButtonGroup method add (p. 504) associates each JRadioButton with a ButtonGroup. If more than one selected JRadioButton object is added to a group, the selected one that was added first will be selected when the GUI is displayed. • JRadioButtons generate ItemEvents when they’re clicked. Section 12.11 JComboBox; Using an Anonymous Inner Class for Event Handling • A (p. 504) provides a list of items from which the user can make a single selection. generate ItemEvents. Each item in a JComboBox has an index (p. 507). The first item added to a JComboBox appears as the currently selected item when the JComboBox is displayed. JComboBox method setMaximumRowCount (p. 507) sets the maximum number of elements that are displayed when the user clicks the JComboBox. An anonymous inner class (p. 507) is a class without a name and typically appears inside a method declaration. One object of the anonymous inner class must be created when the class is declared. JComboBox method getSelectedIndex (p. 508) returns the index of the selected item. JComboBox JComboBoxes • • • • Section 12.12 JList • A JList displays a series of items from which the user may select one or more items. Class JList supports single-selection lists (p. 508) and multiple-selection lists. • When the user clicks an item in a JList, a ListSelectionEvent (p. 508) occurs. JList method addListSelectionListener (p. 510) registers a ListSelectionListener (p. 510) for a JList’s selection events. A ListSelectionListener of package javax.swing.event(p. 492) must implement method valueChanged. • JList method setVisibleRowCount (p. 510) specifies the number of visible items in the list. • JList method setSelectionMode (p. 510) specifies a list’s selection mode. • A JList can be attached to a JScrollPane (p. 510) to provide a scrollbar for the JList. • JFrame method getContentPane (p. 510) returns a reference to the JFrame’s content pane where GUI components are displayed. • JList method getSelectedIndex (p. 511) returns the selected item’s index. Section 12.13 Multiple-Selection Lists • A multiple-selection list (p. 511) enables the user to select many items from a JList. • JList method setFixedCellWidth (p. 513) sets a JList’s width. Method setFixedCellHeight (p. 513) sets the height of each item in a JList. • Normally, an external event (p. 513) generated by another GUI component (such as a JButton) specifies when the multiple selections in a JList should be processed. • JList method setListData (p. 513) sets the items displayed in a JList. JList method getSelectedValues (p. 513) returns an array of Objects representing the selected items in a JList. Section 12.14 Mouse Event Handling • The MouseListener (p. 513) and MouseMotionListener (p. 513) event-listener interfaces are used to handle mouse events (p. 513). Mouse events can be trapped for any GUI component that extends Component. Summary • Interface MouseInputListener (p. 513) of package javax.swing.event 547 extends interfaces MouseListener and MouseMotionListener to create a single interface containing all their methods. • Each mouse event-handling method receives a MouseEvent object (p. 494) that contains information about the event, including the x- and y-coordinates where the event occurred. Coordinates are measured from the upper-left corner of the GUI component on which the event occurred. • The methods and constants of class InputEvent (p. 514; MouseEvent’s superclass) enable an application to determine which mouse button the user clicked. • Interface MouseWheelListener (p. 515) enables applications to respond to mouse-wheel events. Section 12.15 Adapter Classes • An adapter class (p. 518) implements an interface and provides default implementations of its methods. When you extend an adapter class, you can override just the method(s) you need. • MouseEvent method getClickCount (p. 521) returns the number of consecutive mouse-button clicks. Methods isMetaDown (p. 528) and isAltDown (p. 521) determine which button was clicked. Section 12.16 JPanel Subclass for Drawing with the Mouse • • • • • • • • • • JComponents method paintComponent (p. 522) is called when a lightweight Swing component is displayed. Override this method to specify how to draw shapes using Java’s graphics capabilities. When overriding paintComponent, call the superclass version as the first statement in the body. Subclasses of JComponent support transparency. When a component is opaque (p. 522), paintComponent clears its background before the component is displayed. The transparency of a Swing lightweight component can be set with method setOpaque (p. 522; a false argument indicates that the component is transparent). Class Point (p. 523) package java.awt represents an x-y coordinate. Class Graphics (p. 523) is used to draw. MouseEvent method getPoint (p. 524) obtains the Point where a mouse event occurred. Method repaint (p. 524), inherited indirectly from class Component, indicates that a component should be refreshed on the screen as soon as possible. Method paintComponent receives a Graphics parameter and is called automatically whenever a lightweight component needs to be displayed on the screen. Graphics method fillOval (p. 524) draws a solid oval. The first two arguments are the upper-left x-y coordinate of the bounding box, and the last two are the bounding box’s width and height. Section 12.17 Key Event Handling • Interface KeyListener (p. 494) is used to handle key events (p. 494) that are generated when keys on the keyboard are pressed and released. Method addKeyListener of class Component (p. 525) registers a KeyListener. • KeyEvent (p. 494) method getKeyCode (p. 528) gets the virtual key code (p. 528) of the pressed key. Class KeyEvent contains virtual key-code constants that represent every key on the keyboard. • KeyEvent method getKeyText (p. 528) returns a string containing the name of the pressed key. • KeyEvent method getKeyChar (p. 528) gets the Unicode value of the character typed. • KeyEvent method isActionKey (p. 528) determines whether the key in an event was an action key (p. 525). • InputEvent method getModifiers (p. 528) determines whether any modifier keys (such as Shift, Alt and Ctrl) were pressed when the key event occurred. 548 • Chapter 12 GUI Components: Part 1 KeyEvent method getKeyModifiersText (p. 528) returns a string containing the pressed modifi- er keys. Section 12.18 Introduction to Layout Managers • Layout managers (p. 528) arrange GUI components in a container for presentation purposes. • All layout managers implement the interface LayoutManager (p. 528) of package java.awt. • Container method setLayout specifies the layout of a container. • FlowLayout places components left to right in the order in which they’re added to the container. When the container’s edge is reached, components continue to display on the next line. FlowLayout allows GUI components to be left aligned, centered (the default) and right aligned. • FlowLayout method setAlignment (p. 532) changes the alignment for a FlowLayout. • BorderLayout (p. 532) the default for a JFrame) arranges components into five regions: NORTH, SOUTH, EAST, WEST and CENTER. NORTH corresponds to the top of the container. • A BorderLayout limits a Container to containing at most five components—one in each region. • GridLayout (p. 536) divides a container into a grid of rows and columns. • Container method validate (p. 537) recomputes a container’s layout based on the current layout manager for the Container and the current set of displayed GUI components. Section 12.19 Using Panels to Manage More Complex Layouts • Complex GUIs often consist of multiple panels with different layouts. Every JPanel may have components, including other panels, attached to it with Container method add. Section 12.20 JTextArea • A JTextArea (p. 539)—a subclass of JTextComponent—may contain multiple lines of text. • Class Box (p. 540) is a subclass of Container that uses a BoxLayout layout manager (p. 541) to arrange the GUI components either horizontally or vertically. • Box static method createHorizontalBox (p. 541) creates a Box that arranges components from left to right in the order that they’re attached. • Method getSelectedText (p. 541) returns the selected text from a JTextArea. • You can set the horizontal and vertical scrollbar policies (p. 542) of a JScrollPane when it’s constructed. JScrollPane methods setHorizontalScrollBarPolicy (p. 542) and setVerticalScrollBarPolicy (p. 542) can be used to change the scrollbar policies at any time. Self-Review Exercises 12.1 Fill in the blanks in each of the following statements: is called when the mouse is moved with no buttons pressed and an a) Method event listener is registered to handle the event. b) Text that cannot be modified by the user is called text. arranges GUI components in a Container. c) A(n) d) The add method for attaching GUI components is a method of class . e) GUI is an acronym for . is used to specify the layout manager for a container. f) Method g) A mouseDragged method call is preceded by a(n) method call and followed by a(n) method call. contains methods that display message dialogs and input dialogs. h) Class i) An input dialog capable of receiving input from the user is displayed with method of class . Answers to Self-Review Exercises 549 j) A dialog capable of displaying a message to the user is displayed with method . of class k) Both JTextFields and JTextAreas directly extend class . 12.2 Determine whether each statement is true or false. If false, explain why. a) BorderLayout is the default layout manager for a JFrame’s content pane. b) When the mouse cursor is moved into the bounds of a GUI component, method mouseOver is called. c) A JPanel cannot be added to another JPanel. d) In a BorderLayout, two buttons added to the NORTH region will be placed side by side. e) A maximum of five components can be added to a BorderLayout. f) Inner classes are not allowed to access the members of the enclosing class. g) A JTextArea’s text is always read-only. h) Class JTextArea is a direct subclass of class Component. 12.3 Find the error(s) in each of the following statements, and explain how to correct it (them): a) buttonName = JButton("Caption"); b) JLabel aLabel, JLabel; c) txtField = new JTextField(50, "Default Text"); d) setLayout(new BorderLayout()); button1 = new JButton("North Star"); button2 = new JButton("South Pole"); add(button1); add(button2); Answers to Self-Review Exercises 12.1 a) mouseMoved. b) uneditable (read-only). c) layout manager. d) Container. e) graphical user interface. f) setLayout. g) mousePressed, mouseReleased. h) JOptionPane. i) showInputDialog, JOptionPane. j) showMessageDialog, JOptionPane. k) JTextComponent. 12.2 a) True. b) False. Method mouseEntered is called. c) False. A JPanel can be added to another JPanel, because JPanel is an indirect subclass of Component. So, a JPanel is a Component. Any Component can be added to a Container. d) False. Only the last button added will be displayed. Remember that only one component should be added to each region in a BorderLayout. e) True. [Note: Panels containing multiple components can be added to each region.] f) False. Inner classes have access to all members of the enclosing class declaration. g) False. JTextAreas are editable by default. h) False. JTextArea derives from class JTextComponent. 12.3 a) new is needed to create an object. b) JLabel is a class name and cannot be used as a variable name. c) The arguments passed to the constructor are reversed. The String must be passed first. d) BorderLayout has been set, and components are being added without specifying the region, so both are added to the center region. Proper add statements might be add(button1, BorderLayout.NORTH); add(button2, BorderLayout.SOUTH); Exercises 12.4 Fill in the blanks in each of the following statements: a) The JTextField class directly extends class . 550 Chapter 12 GUI Components: Part 1 b) Container method attaches a GUI component to a container. is called when a mouse button is released (without moving the c) Method mouse). d) The class is used to create a group of JRadioButtons. 12.5 Determine whether each statement is true or false. If false, explain why. a) Only one layout manager can be used per Container. b) GUI components can be added to a Container in any order in a BorderLayout. c) JRadioButtons provide a series of mutually exclusive options (i.e., only one can be true at a time). d) Graphics method setFont is used to set the font for text fields. e) A JList displays a scrollbar if there are more items in the list than can be displayed. f) A Mouse object has a method called mouseDragged. 12.6 Determine whether each statement is true or false. If false, explain why. a) A JPanel is a JComponent. b) A JPanel is a Component. c) A JLabel is a Container. d) A JList is a JPanel. e) An AbstractButton is a JButton. f) A JTextField is an Object. g) ButtonGroup is a subclass of JComponent. 12.7 Find any errors in each of the following lines of code, and explain how to correct them. a) import javax.swing.JFrame b) panelObject.GridLayout(8, 8); c) container.setLayout(new FlowLayout(FlowLayout.DEFAULT)); d) container.add(eastButton, EAST); face 12.8 Create the following GUI. You do not have to provide any functionality. 12.9 Create the following GUI. You do not have to provide any functionality. 12.10 Create the following GUI. You do not have to provide any functionality. Exercises 551 12.11 Create the following GUI. You do not have to provide any functionality. 12.12 (Temperature Conversion) Write a temperature-conversion application that converts from Fahrenheit to Celsius. The Fahrenheit temperature should be entered from the keyboard (via a JTextField). A JLabel should be used to display the converted temperature. Use the following formula for the conversion: 5 Celsius = --- × (Fahrenheit – 32) 9 12.13 (Temperature-Conversion Modification) Enhance the temperature-conversion application of Exercise 12.12 by adding the Kelvin temperature scale. The application should also allow the user to make conversions between any two scales. Use the following formula for the conversion between Kelvin and Celsius (in addition to the formula in Exercise 12.12): Kelvin = Celsius + 273.15 12.14 (Guess-the-Number Game) Write an application that plays “guess the number” as follows: Your application chooses the number to be guessed by selecting an integer at random in the range 1–1000. The application then displays the following in a label: I have a number between 1 and 1000. Can you guess my number? Please enter your first guess. A JTextField should be used to input the guess. As each guess is input, the background color should change to either red or blue. Red indicates that the user is getting “warmer,” and blue, “colder.” A JLabel should display either "Too High" or "Too Low" to help the user zero in. When the user gets the correct answer, "Correct!" should be displayed, and the JTextField used for input should be changed to be uneditable. A JButton should be provided to allow the user to play the game again. When the JButton is clicked, a new random number should be generated and the input JTextField changed to be editable. 12.15 (Displaying Events) It’s often useful to display the events that occur during the execution of an application. This can help you understand when the events occur and how they’re generated. Write an application that enables the user to generate and process every event discussed in this chapter. The application should provide methods from the ActionListener, ItemListener, ListSelectionListener, MouseListener, MouseMotionListener and KeyListener interfaces to display messages when the events occur. Use method toString to convert the event objects received in each event handler into Strings that can be displayed. Method toString creates a String containing all the information in the event object. 12.16 (GUI-Based Craps Game) Modify the application of Section 6.10 to provide a GUI that enables the user to click a JButton to roll the dice. The application should also display four JLabels and four JTextFields, with one JLabel for each JTextField. The JTextFields should be used to display the values of each die and the sum of the dice after each roll. The point should be displayed in the fourth JTextField when the user does not win or lose on the first roll and should continue to be displayed until the game is lost. 552 Chapter 12 GUI Components: Part 1 (Optional) GUI and Graphics Case Study Exercise: Expanding the Interface 12.17 (Interactive Drawing Application) In this exercise, you’ll implement a GUI application that uses the MyShape hierarchy from GUI and Graphics Case Study Exercise 10.2 to create an interactive drawing application. You’ll create two classes for the GUI and provide a test class that launches the application. The classes of the MyShape hierarchy require no additional changes. The first class to create is a subclass of JPanel called DrawPanel, which represents the area on which the user draws the shapes. Class DrawPanel should have the following instance variables: a) An array shapes of type MyShape that will store all the shapes the user draws. b) An integer shapeCount that counts the number of shapes in the array. c) An integer shapeType that determines the type of shape to draw. d) A MyShape currentShape that represents the current shape the user is drawing. e) A Color currentColor that represents the current drawing color. f) A boolean filledShape that determines whether to draw a filled shape. g) A JLabel statusLabel that represents the status bar. The status bar will display the coordinates of the current mouse position. Class DrawPanel should also declare the following methods: a) Overridden method paintComponent that draws the shapes in the array. Use instance variable shapeCount to determine how many shapes to draw. Method paintComponent should also call currentShape’s draw method, provided that currentShape is not null. b) Set methods for the shapeType, currentColor and filledShape. c) Method clearLastShape should clear the last shape drawn by decrementing instance variable shapeCount. Ensure that shapeCount is never less than zero. d) Method clearDrawing should remove all the shapes in the current drawing by setting shapeCount to zero. Methods clearLastShape and clearDrawing should call repaint (inherited from JPanel) to refresh the drawing on the DrawPanel by indicating that the system should call method paintComponent. Class DrawPanel should also provide event handling to enable the user to draw with the mouse. Create a single inner class that both extends MouseAdapter and implements MouseMotionListener to handle all mouse events in one class. In the inner class, override method mousePressed so that it assigns currentShape a new shape of the type specified by shapeType and initializes both points to the mouse position. Next, override method mouseReleased to finish drawing the current shape and place it in the array. Set the second point of currentShape to the current mouse position and add currentShape to the array. Instance variable shapeCount determines the insertion index. Set currentShape to null and call method repaint to update the drawing with the new shape. Override method mouseMoved to set the text of the statusLabel so that it displays the mouse coordinates—this will update the label with the coordinates every time the user moves (but does not drag) the mouse within the DrawPanel. Next, override method mouseDragged so that it sets the second point of the currentShape to the current mouse position and calls method repaint. This will allow the user to see the shape while dragging the mouse. Also, update the JLabel in mouseDragged with the current position of the mouse. Create a constructor for DrawPanel that has a single JLabel parameter. In the constructor, initialize statusLabel with the value passed to the parameter. Also initialize array shapes with 100 entries, shapeCount to 0, shapeType to the value that represents a line, currentShape to null and currentColor to Color.BLACK. The constructor should then set the background color of the DrawPanel to Color.WHITE and register the MouseListener and MouseMotionListener so the JPanel properly handles mouse events. Next, create a JFrame subclass called DrawFrame that provides a GUI that enables the user to control various aspects of drawing. For the layout of the DrawFrame, we recommend a BorderLay- Making a Difference 553 out, with the components in the NORTH region, the main drawing panel in the CENTER region, and a status bar in the SOUTH region, as in Fig. 12.49. In the top panel, create the components listed below. Each component’s event handler should call the appropriate method in class DrawPanel. a) A button to undo the last shape drawn. b) A button to clear all shapes from the drawing. c) A combo box for selecting the color from the 13 predefined colors. d) A combo box for selecting the shape to draw. e) A checkbox that specifies whether a shape should be filled or unfilled. Declare and create the interface components in DrawFrame’s constructor. You’ll need to create the status bar JLabel before you create the DrawPanel, so you can pass the JLabel as an argument to DrawPanel’s constructor. Finally, create a test class that initializes and displays the DrawFrame to execute the application. Fig. 12.49 | Interface for drawing shapes. 12.18 (GUI-Based Version of the ATM Case Study) Reimplement the Optional ATM Case Study of Chapters 33–34 as a GUI-based application. Use GUI components to approximate the ATM user interface shown in Fig. 33.1. For the cash dispenser and the deposit slot use JButtons labeled Remove Cash and Insert Envelope. This will enable the application to receive events indicating when the user takes the cash and inserts a deposit envelope, respectively. Making a Difference 12.19 (Ecofont) Ecofont (www.ecofont.eu/ecofont_en.html)—developed by SPRANQ (a Netherlands-based company)—is a free, open-source computer font designed to reduce by as much as 20% the amount of ink used for printing, thus reducing also the number of ink cartridges used and the environmental impact of the manufacturing and shipping processes (using less energy, less fuel for shipping, and so on). The font, based on sans-serif Verdana, has small circular “holes” in the letters that are not visible in smaller sizes—such as the 9- or 10-point type frequently used. Download Ecofont, then install the font file Spranq_eco_sans_regular.ttf using the instructions from the Ecofont website. Next, develop a GUI-based program that allows you to type in a text string to be displayed in the Ecofont. Create Increase Font Size and Decrease Font Size buttons that allow you 554 Chapter 12 GUI Components: Part 1 to scale up or down by one point at a time. Start with a default font size of 9 points. As you scale up, you’ll be able to see the holes in the letters more clearly. As you scale down, the holes will be less apparent. What is the smallest font size at which you begin to notice the holes? 12.20 (Typing Tutor: Tuning a Crucial Skill in the Computer Age) Typing quickly and correctly is an essential skill for working effectively with computers and the Internet. In this exercise, you’ll build a GUI application that can help users learn to “touch type” (i.e., type correctly without looking at the keyboard). The application should display a virtual keyboard (Fig. 12.50) and should allow the user to watch what he or she is typing on the screen without looking at the actual keyboard. Use JButtons to represent the keys. As the user presses each key, the application highlights the corresponding JButton on the GUI and adds the character to a JTextArea that shows what the user has typed so far. [Hint: To highlight a JButton, use its setBackground method to change its background color. When the key is released, reset its original background color. You can obtain the JButton’s original background color with the getBackground method before you change its color.] Fig. 12.50 | Typing tutor. You can test your program by typing a pangram—a phrase that contains every letter of the alphabet at least once—such as “The quick brown fox jumped over a lazy dog.” You can find other pangrams on the web. To make the program more interesting you could monitor the user’s accuracy. You could have the user type specific phrases that you’ve prestored in your program and that you display on the screen above the virtual keyboard. You could keep track of how many keystrokes the user types correctly and how many are typed incorrectly. You could also keep track of which keys the user is having difficulty with and display a report showing those keys. 13 Graphics and Java 2D Treat nature in terms of the cylinder, the sphere, the cone, all in perspective. —Paul Cézanne Colors, like features, follow the changes of the emotions. —Pablo Picasso Nothing ever becomes real till it is experienced—even a proverb is no proverb to you till your life has illustrated it. —John Keats Objectives In this chapter you’ll: ■ Understand graphics contexts and graphics objects. ■ Manipulate colors and fonts. ■ Use methods of class Graphics to draw various shapes. ■ Use methods of class Graphics2D from the Java 2D API to draw various shapes. ■ Specify Paint and Stroke characteristics of shapes displayed with Graphics2D. 556 Chapter 13 Graphics and Java 2D 13.1 Introduction 13.2 Graphics Contexts and Graphics Objects 13.3 Color Control 13.4 Manipulating Fonts 13.5 Drawing Lines, Rectangles and Ovals 13.6 13.7 13.8 13.9 Drawing Arcs Drawing Polygons and Polylines Java 2D API Wrap-Up Summary | Self-Review Exercises | Answers to Self-Review Exercises | Exercises | Making a Difference 13.1 Introduction In this chapter, we overview several of Java’s capabilities for drawing two-dimensional shapes, controlling colors and controlling fonts. Part of Java’s initial appeal was its support for graphics that enabled programmers to visually enhance their applications. Java contains more sophisticated drawing capabilities as part of the Java 2D API (presented in this chapter) and its successor technology JavaFX (presented in Chapter 25 and two online chapters). This chapter begins by introducing many of Java’s original drawing capabilities. Next we present several of the more powerful Java 2D capabilities, such as controlling the style of lines used to draw shapes and the way shapes are filled with colors and patterns. The classes that were part of Java’s original graphics capabilities are now considered to be part of the Java 2D API. Figure 13.1 shows a portion of the class hierarchy that includes various graphics classes and Java 2D API classes and interfaces covered in this chapter. Class Color contains methods and constants for manipulating colors. Class JComponent contains method paintComponent, which is used to draw graphics on a component. Class Font contains methods and constants for manipulating fonts. Class FontMetrics contains methods for obtaining font information. Class Graphics contains methods for drawing strings, lines, rectangles and other shapes. Class Graphics2D, which extends class Graphics, is used for drawing with the Java 2D API. Class Polygon contains methods for creating polygons. The bottom half of the figure lists several classes and interfaces from the Java 2D API. Class BasicStroke helps specify the drawing characteristics of lines. Classes GradientPaint and TexturePaint help specify the characteristics for filling shapes with colors or patterns. Classes GeneralPath, Line2D, Arc2D, Ellipse2D, Rectangle2D and RoundRectangle2D represent several Java 2D shapes. To begin drawing in Java, we must first understand Java’s coordinate system (Fig. 13.2), which is a scheme for identifying every point on the screen. By default, the upper-left corner of a GUI component (e.g., a window) has the coordinates (0, 0). A coordinate pair is composed of an x-coordinate (the horizontal coordinate) and a y-coordinate (the vertical coordinate). The x-coordinate is the horizontal distance moving right from the left edge of the screen. The y-coordinate is the vertical distance moving down from the top of the screen. The x-axis describes every horizontal coordinate, and the y-axis every vertical coordinate. The coordinates are used to indicate where graphics should be displayed on a screen. Coordinate units are measured in pixels (which stands for “picture elements”). A pixel is a display monitor’s smallest unit of resolution. 13.1 Introduction 557 java.lang.Object java.awt.Color java.awt.Component java.awt.Container javax.swing.JComponent java.awt.Font java.awt.FontMetrics java.awt.Graphics java.awt.Graphics2D java.awt.Polygon java.awt.BasicStroke java.awt.GradientPaint java.awt.TexturePaint «interface» java.awt.Paint «interface» java.awt.Shape «interface» java.awt.Stroke java.awt.geom.GeneralPath java.awt.geom.Line2D java.awt.geom.RectangularShape java.awt.geom.Arc2D java.awt.geom.Ellipse2D java.awt.geom.Rectangle2D java.awt.geom.RoundRectangle2D Fig. 13.1 | Classes and interfaces used in this chapter from Java’s original graphics capabilities and from the Java 2D API. Portability Tip 13.1 Different display monitors have different resolutions (i.e., the density of the pixels varies). This can cause graphics to appear in different sizes on different monitors or on the same monitor with different settings. 558 Chapter 13 Graphics and Java 2D +x (0, 0) x-axis (x, y) +y y-axis Fig. 13.2 | Java coordinate system. Units are measured in pixels. 13.2 Graphics Contexts and Graphics Objects A graphics context enables drawing on the screen. A Graphics object manages a graphics context and draws pixels on the screen that represent text and other graphical objects (e.g., lines, ellipses, rectangles and other polygons). Graphics objects contain methods for drawing, font manipulation, color manipulation and the like. Class Graphics is an abstract class (i.e., you cannot instantiate Graphics objects). This contributes to Java’s portability. Because drawing is performed differently on every platform that supports Java, there cannot be only one implementation of the drawing capabilities across all systems. When Java is implemented on a particular platform, a subclass of Graphics is created that implements the drawing capabilities. This implementation is hidden by class Graphics, which supplies the interface that enables us to use graphics in a platform-independent manner. Recall from Chapter 12 that class Component is the superclass for many of the classes in package java.awt. Class JComponent (package javax.swing), which inherits indirectly from class Component, contains a paintComponent method that can be used to draw graphics. Method paintComponent takes a Graphics object as an argument. This object is passed to the paintComponent method by the system when a lightweight Swing component needs to be repainted. The header for the paintComponent method is public void paintComponent(Graphics g) Parameter g receives a reference to an instance of the system-specific subclass of Graphics. The preceding method header should look familiar to you—it’s the same one we used in some of the applications in Chapter 12. Actually, class JComponent is a superclass of JPanel. Many capabilities of class JPanel are inherited from class JComponent. You seldom call method paintComponent directly, because drawing graphics is an event-driven process. As we mentioned in Chapter 11, Java uses a multithreaded model of program execution. Each thread is a parallel activity. Each program can have many threads. When you create a GUI-based application, one of those threads is known as the eventdispatch thread (EDT)—it’s used to process all GUI events. All manipulation of the onscreen GUI components must be performed in that thread. When a GUI application executes, the application container calls method paintComponent (in the event-dispatch thread) for each lightweight component as the GUI is displayed. For paintComponent to be called again, an event must occur (such as covering and uncovering the component with another window). 13.3 Color Control 559 If you need paintComponent to execute (i.e., if you want to update the graphics drawn on a Swing component), you can call method repaint, which returns void, takes no arguments and is inherited by all JComponents indirectly from class Component (package java.awt). 13.3 Color Control Class Color declares methods and constants for manipulating colors in a Java program. The predeclared color constants are summarized in Fig. 13.3, and several color methods and constructors are summarized in Fig. 13.4. Two of the methods in Fig. 13.4 are Graphics methods that are specific to colors. Color constant RGB value public static final Color RED 255, 0, 0 0, 255, 0 0, 0, 255 255, 200, 0 255, 175, 175 0, 255, 255 255, 0, 255 255, 255, 0 0, 0, 0 255, 255, 255 128, 128, 128 192, 192, 192 64, 64, 64 public static final Color GREEN public static final Color BLUE public static final Color ORANGE public static final Color PINK public static final Color CYAN public static final Color MAGENTA public static final Color YELLOW public static final Color BLACK public static final Color WHITE public static final Color GRAY public static final Color LIGHT_GRAY public static final Color DARK_GRAY Fig. 13.3 | Method Color Color constants and their RGB values. Description constructors and methods public Color(int r, int g, int b) Creates a color based on red, green and blue components expressed as integers from 0 to 255. public Color(float r, float g, float b) Creates a color based on red, green and blue components expressed as floatingpoint values from 0.0 to 1.0. public int getRed() Returns a value between 0 and 255 representing the red content. Fig. 13.4 | Color methods and color-related Graphics methods. (Part 1 of 2.) 560 Chapter 13 Graphics and Java 2D Method Description public int getGreen() Returns a value between 0 and 255 representing the green content. public int getBlue() Returns a value between 0 and 255 representing the blue content. Graphics methods for manipulating Colors public Color getColor() Returns Color object representing current color for the graphics context. public void setColor(Color c) Sets the current color for drawing with the graphics context. Fig. 13.4 | Color methods and color-related Graphics methods. (Part 2 of 2.) Every color is created from a red, a green and a blue value. Together these are called RGB values. All three RGB components can be integers in the range from 0 to 255, or all three can be floating-point values in the range 0.0 to 1.0. The first RGB component specifies the amount of red, the second the amount of green and the third the amount of blue. The larger the value, the greater the amount of that particular color. Java enables you to choose from 256 × 256 × 256 (approximately 16.7 million) colors. Not all computers are capable of displaying all these colors. The screen will display the closest color it can. Two of class Color’s constructors are shown in Fig. 13.4—one that takes three int arguments and one that takes three float arguments, with each argument specifying the amount of red, green and blue. The int values must be in the range 0–255 and the float values in the range 0.0–1.0. The new Color object will have the specified amounts of red, green and blue. Color methods getRed, getGreen and getBlue return integer values from 0 to 255 representing the amounts of red, green and blue, respectively. Graphics method getColor returns a Color object representing the Graphics object’s current drawing color. Graphics method setColor sets the current drawing color. Drawing in Different Colors Figures 13.5–13.6 demonstrate several methods from Fig. 13.4 by drawing filled rectangles and Strings in several different colors. When the application begins execution, class ColorJPanel’s paintComponent method (lines 10–37 of Fig. 13.5) is called to paint the window. Line 17 uses Graphics method setColor to set the drawing color. Method setColor receives a Color object. The expression new Color(255, 0, 0) creates a new Color object that represents red (red value 255, and 0 for the green and blue values). Line 18 uses Graphics method fillRect to draw a filled rectangle in the current color. Method fillRect draws a rectangle based on its four arguments. The first two integer values represent the upper-left x-coordinate and upper-left y-coordinate, where the Graphics object begins drawing the rectangle. The third and fourth arguments are nonnegative integers that represent the width and the height of the rectangle in pixels, respectively. A rectangle drawn using method fillRect is filled by the current color of the Graphics object. 13.3 Color Control 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 // Fig. 13.5: ColorJPanel.java // Changing drawing colors. import java.awt.Graphics; import java.awt.Color; import javax.swing.JPanel; public class ColorJPanel extends JPanel { // draw rectangles and Strings in different colors @Override public void paintComponent(Graphics g) { super.paintComponent(g); this.setBackground(Color.WHITE); // set new drawing color using integers g.setColor(new Color(255, 0, 0)); g.fillRect(15, 25, 100, 20); g.drawString("Current RGB: " + g.getColor(), 130, 40); // set new drawing color using floats g.setColor(new Color(0.50f, 0.75f, 0.0f)); g.fillRect(15, 50, 100, 20); g.drawString("Current RGB: " + g.getColor(), 130, 65); // set new drawing color using static Color objects g.setColor(Color.BLUE); g.fillRect(15, 75, 100, 20); g.drawString("Current RGB: " + g.getColor(), 130, 90); // display individual RGB values Color color = Color.MAGENTA; g.setColor(color); g.fillRect(15, 100, 100, 20); g.drawString("RGB values: " + color.getRed() + ", " + color.getGreen() + ", " + color.getBlue(), 130, 115); } } // end class ColorJPanel Fig. 13.5 | Changing drawing colors. 1 2 3 4 5 6 7 8 9 // Fig. 13.6: ShowColors.java // Demonstrating Colors. import javax.swing.JFrame; public class ShowColors { // execute application public static void main(String[] args) { Fig. 13.6 | Demonstrating Colors. (Part 1 of 2.) 561 562 10 11 12 13 14 15 16 17 18 19 Chapter 13 Graphics and Java 2D // create frame for ColorJPanel JFrame frame = new JFrame("Using colors"); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); ColorJPanel colorJPanel = new ColorJPanel(); frame.add(colorJPanel); frame.setSize(400, 180); frame.setVisible(true); } } // end class ShowColors Fig. 13.6 | Demonstrating Colors. (Part 2 of 2.) Line 19 uses Graphics method drawString to draw a String in the current color. The expression g.getColor() retrieves the current color from the Graphics object. We then concatenate the Color with string "Current RGB: ", resulting in an implicit call to class Color’s toString method. The String representation of a Color contains the class name and package (java.awt.Color) and the red, green and blue values. Look-and-Feel Observation 13.1 People perceive colors differently. Choose your colors carefully to ensure that your application is readable, both for people who can perceive color and for those who are color blind. Try to avoid using many different colors in close proximity. Lines 22–24 and 27–29 perform the same tasks again. Line 22 uses the Color constructor with three float arguments to create a dark green color (0.50f for red, 0.75f for green and 0.0f for blue). Note the syntax of the values. The letter f appended to a floating-point literal indicates that the literal should be treated as type float. Recall that by default, floating-point literals are treated as type double. Line 27 sets the current drawing color to one of the predeclared Color constants (Color.BLUE). The Color constants are static, so they’re created when class Color is loaded into memory at execution time. The statement in lines 35–36 makes calls to Color methods getRed, getGreen and getBlue on the predeclared Color.MAGENTA constant. Method main of class ShowColors (lines 8–18 of Fig. 13.6) creates the JFrame that will contain a ColorJPanel object where the colors will be displayed. Software Engineering Observation 13.1 To change the color, you must create a new Color object (or use one of the predeclared Color constants). Like String objects, Color objects are immutable (not modifiable). 13.3 Color Control 563 The JColorChooser component (package javax.swing) enables application users to select colors. Figures 13.7–13.8 demonstrate a JColorChooser dialog. When you click the Change Color button, a JColorChooser dialog appears. When you select a color and press the dialog’s OK button, the background color of the application window changes. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 // Fig. 13.7: ShowColors2JFrame.java // Choosing colors with JColorChooser. import java.awt.BorderLayout; import java.awt.Color; import java.awt.event.ActionEvent; import java.awt.event.ActionListener; import javax.swing.JButton; import javax.swing.JFrame; import javax.swing.JColorChooser; import javax.swing.JPanel; public class ShowColors2JFrame extends JFrame { private final JButton changeColorJButton; private Color color = Color.LIGHT_GRAY; private final JPanel colorJPanel; // set up GUI public ShowColors2JFrame() { super("Using JColorChooser"); // create JPanel for display color colorJPanel = new JPanel(); colorJPanel.setBackground(color); // set up changeColorJButton and register its event handler changeColorJButton = new JButton("Change Color"); changeColorJButton.addActionListener( new ActionListener() // anonymous inner class { // display JColorChooser when user clicks button @Override public void actionPerformed(ActionEvent event) { color = JColorChooser.showDialog( ShowColors2JFrame.this, "Choose a color", color); // set default color, if no color is returned if (color == null) color = Color.LIGHT_GRAY; // change content pane's background color colorJPanel.setBackground(color); } // end method actionPerformed } // end anonymous inner class ); // end call to addActionListener Fig. 13.7 | Choosing colors with JColorChooser. (Part 1 of 2.) 564 48 49 50 51 52 53 54 55 Chapter 13 Graphics and Java 2D add(colorJPanel, BorderLayout.CENTER); add(changeColorJButton, BorderLayout.SOUTH); setSize(400, 130); setVisible(true); } // end ShowColor2JFrame constructor } // end class ShowColors2JFrame Fig. 13.7 | Choosing colors with JColorChooser. (Part 2 of 2.) 1 2 3 4 5 6 7 8 9 10 11 12 13 // Fig. 13.8: ShowColors2.java // Choosing colors with JColorChooser. import javax.swing.JFrame; public class ShowColors2 { // execute application public static void main(String[] args) { ShowColors2JFrame application = new ShowColors2JFrame(); application.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); } } // end class ShowColors2 (a) Initial application window (b) JColorChooser window Select a color from one of the color swatches (c) Application window after changing JPanel’s background color Fig. 13.8 | Choosing colors with JColorChooser. Class JColorChooser provides static method showDialog, which creates a JColor13.7 Chooser object, attaches it to a dialog box and displays the dialog. Lines 36–37 of Fig. 13.3 Color Control 565 invoke this method to display the color-chooser dialog. Method showDialog returns the selected Color object, or null if the user presses Cancel or closes the dialog without pressing OK. The method takes three arguments—a reference to its parent Component, a String to display in the title bar of the dialog and the initial selected Color for the dialog. The parent component is a reference to the window from which the dialog is displayed (in this case the JFrame, with the reference name frame). The dialog will be centered on the parent. If the parent is null, the dialog is centered on the screen. While the color-chooser dialog is on the screen, the user cannot interact with the parent component until the dialog is dismissed. This type of dialog is called a modal dialog. After the user selects a color, lines 40–41 determine whether color is null, and, if so, set color to Color.LIGHT_GRAY. Line 44 invokes method setBackground to change the background color of the JPanel. Method setBackground is one of the many Component methods that can be used on most GUI components. The user can continue to use the Change Color button to change the background color of the application. Figure 13.8 contains method main, which executes the program. Figure 13.8(b) shows the default JColorChooser dialog that allows the user to select a color from a variety of color swatches. There are three tabs across the top of the dialog— Swatches, HSB and RGB. These represent three different ways to select a color. The HSB tab allows you to select a color based on hue, saturation and brightness—values that are used to define the amount of light in a color. Visit http://en.wikipedia.org/wiki/HSL_and_HSV for more information on HSB. The RGB tab allows you to select a color by using sliders to select the red, green and blue components. The HSB and RGB tabs are shown in Fig. 13.9. Fig. 13.9 | HSB and RGB tabs of the JColorChooser dialog. (Part 1 of 2.) 566 Chapter 13 Graphics and Java 2D Sliders to select the red, green and blue color components Fig. 13.9 | HSB and RGB tabs of the JColorChooser dialog. (Part 2 of 2.) 13.4 Manipulating Fonts This section introduces methods and constants for manipulating fonts. Most font methods and font constants are part of class Font. Some constructors, methods and constants of class Font and class Graphics are summarized in Fig. 13.10. Method or constant Font Description constants, constructors and methods public static final int PLAIN public static final int BOLD public static final int ITALIC public Font(String name, int style, int size) public int getStyle() public int getSize() public String getName() public String getFamily() public boolean isPlain() public boolean isBold() public boolean isItalic() Fig. 13.10 | Font-related A constant representing a plain font style. A constant representing a bold font style. A constant representing an italic font style. Creates a Font object with the specified font name, style and size. Returns an int indicating the current font style. Returns an int indicating the current font size. Returns the current font name as a string. Returns the font’s family name as a string. Returns true if the font is plain, else false. Returns true if the font is bold, else false. Returns true if the font is italic, else false. methods and constants. (Part 1 of 2.) 13.4 Manipulating Fonts Method or constant Graphics 567 Description methods for manipulating Fonts Returns a Font object reference representing the current font. Sets the current font to the font, style and size specified by the Font object reference f. public Font getFont() public void setFont(Font f) Fig. 13.10 | Font-related methods and constants. (Part 2 of 2.) Class Font’s constructor takes three arguments—the font name, font style and font size. The font name is any font currently supported by the system on which the program is running, such as standard Java fonts Monospaced, SansSerif and Serif. The font style is Font.PLAIN, Font.ITALIC or Font.BOLD (each is a static field of class Font). Font styles can be used in combination (e.g., Font.ITALIC + Font.BOLD). The font size is measured in points. A point is 1/72 of an inch. Graphics method setFont sets the current drawing font—the font in which text will be displayed—to its Font argument. Portability Tip 13.2 The number of fonts varies across systems. Java provides five font names—Serif, Monospaced, SansSerif, Dialog and DialogInput—that can be used on all Java platforms. The Java runtime environment (JRE) on each platform maps these logical font names to actual fonts installed on the platform. The actual fonts used may vary by platform. The application of Figs. 13.11–13.12 displays text in four different fonts, with each font in a different size. Figure 13.11 uses the Font constructor to initialize Font objects (in lines 17, 21, 25 and 30) that are each passed to Graphics method setFont to change the drawing font. Each call to the Font constructor passes a font name (Serif, Monospaced or SansSerif) as a string, a font style (Font.PLAIN, Font.ITALIC or Font.BOLD) and a font size. Once Graphics method setFont is invoked, all text displayed following the call will appear in the new font until the font is changed. Each font’s information is displayed in lines 18, 22, 26 and 31–32 using method drawString. The coordinates passed to drawString correspond to the lower-left corner of the baseline of the font. Line 29 changes the drawing color to red, so the next string displayed appears in red. Lines 31–32 display information about the final Font object. Method getFont of class Graphics returns a Font object representing the current font. Method getName returns the current font name as a string. Method getSize returns the font size in points. Software Engineering Observation 13.2 To change the font, you must create a new Font object. Font objects are immutable—class Font has no set methods to change the characteristics of the current font. Figure 13.12 contains the main method, which creates a JFrame to display a Font15), which displays the graphics JPanel. We add a FontJPanel object to this JFrame (line created in Fig. 13.11. 568 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 Chapter 13 Graphics and Java 2D // Fig. 13.11: FontJPanel.java // Display strings in different fonts and colors. import java.awt.Font; import java.awt.Color; import java.awt.Graphics; import javax.swing.JPanel; public class FontJPanel extends JPanel { // display Strings in different fonts and colors @Override public void paintComponent(Graphics g) { super.paintComponent(g); // set font to Serif (Times), bold, 12pt and draw a string g.setFont(new Font("Serif", Font.BOLD, 12)); g.drawString("Serif 12 point bold.", 20, 30); // set font to Monospaced (Courier), italic, 24pt and draw a string g.setFont(new Font("Monospaced", Font.ITALIC, 24)); g.drawString("Monospaced 24 point italic.", 20, 50); // set font to SansSerif (Helvetica), plain, 14pt and draw a string g.setFont(new Font("SansSerif", Font.PLAIN, 14)); g.drawString("SansSerif 14 point plain.", 20, 70); // set font to Serif (Times), bold/italic, 18pt and draw a string g.setColor(Color.RED); g.setFont(new Font("Serif", Font.BOLD + Font.ITALIC, 18)); g.drawString(g.getFont().getName() + " " + g.getFont().getSize() + " point bold italic.", 20, 90); } } // end class FontJPanel Fig. 13.11 | Display strings in different fonts and colors. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 // Fig. 13.12: Fonts.java // Using fonts. import javax.swing.JFrame; public class Fonts { // execute application public static void main(String[] args) { // create frame for FontJPanel JFrame frame = new JFrame("Using fonts"); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); FontJPanel fontJPanel = new FontJPanel(); frame.add(fontJPanel); Fig. 13.12 | Using fonts. (Part 1 of 2.) 13.4 Manipulating Fonts 16 17 18 19 569 frame.setSize(420, 150); frame.setVisible(true); } } // end class Fonts Fig. 13.12 | Using fonts. (Part 2 of 2.) Font Metrics Sometimes it’s necessary to get information about the current drawing font, such as its name, style and size. Several Font methods used to get font information are summarized in Fig. 13.10. Method getStyle returns an integer value representing the current style. The integer value returned is either Font.PLAIN, Font.ITALIC, Font.BOLD or the combination of Font.ITALIC and Font.BOLD. Method getFamily returns the name of the font family to which the current font belongs. The name of the font family is platform specific. Font methods are also available to test the style of the current font, and these too are summarized in Fig. 13.10. Methods isPlain, isBold and isItalic return true if the current font style is plain, bold or italic, respectively. Figure 13.13 illustrates some of the common font metrics, which provide precise information about a font, such as height, descent (the amount a character dips below the baseline), ascent (the amount a character rises above the baseline) and leading (the difference between the descent of one line of text and the ascent of the line of text below it— that is, the interline spacing). leading height ascent descent baseline Fig. 13.13 | Font metrics. Class FontMetrics declares several methods for obtaining font metrics. These methods and Graphics method getFontMetrics are summarized in Fig. 13.14. The application of Figs. 13.15–13.16 uses the methods of Fig. 13.14 to obtain font metric information for two fonts. 570 Chapter 13 Graphics and Java 2D Method Description FontMetrics methods Returns the ascent of a font in points. Returns the descent of a font in points. Returns the leading of a font in points. Returns the height of a font in points. public int getAscent() public int getDescent() public int getLeading() public int getHeight() Graphics methods for getting a Font’s FontMetrics public FontMetrics getFontMetrics() Returns the FontMetrics object for the current drawing Font. public FontMetrics getFontMetrics(Font f) Returns the FontMetrics object for the specified Font argument. Fig. 13.14 | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 FontMetrics and Graphics methods for obtaining font metrics. // Fig. 13.15: MetricsJPanel.java // FontMetrics and Graphics methods useful for obtaining font metrics. import java.awt.Font; import java.awt.FontMetrics; import java.awt.Graphics; import javax.swing.JPanel; public class MetricsJPanel extends JPanel { // display font metrics @Override public void paintComponent(Graphics g) { super.paintComponent(g); g.setFont(new Font("SansSerif", Font.BOLD, 12)); FontMetrics metrics = g.getFontMetrics(); g.drawString("Current font: " + g.getFont(), 10, 30); g.drawString("Ascent: " + metrics.getAscent(), 10, 45); g.drawString("Descent: " + metrics.getDescent(), 10, 60); g.drawString("Height: " + metrics.getHeight(), 10, 75); g.drawString("Leading: " + metrics.getLeading(), 10, 90); Font font = new Font("Serif", Font.ITALIC, 14); metrics = g.getFontMetrics(font); g.setFont(font); g.drawString("Current font: " + font, 10, 120); g.drawString("Ascent: " + metrics.getAscent(), 10, 135); g.drawString("Descent: " + metrics.getDescent(), 10, 150); g.drawString("Height: " + metrics.getHeight(), 10, 165); g.drawString("Leading: " + metrics.getLeading(), 10, 180); } } // end class MetricsJPanel Fig. 13.15 | FontMetrics and Graphics methods useful for obtaining font metrics. 13.5 Drawing Lines, Rectangles and Ovals 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 571 // Fig. 13.16: Metrics.java // Displaying font metrics. import javax.swing.JFrame; public class Metrics { // execute application public static void main(String[] args) { // create frame for MetricsJPanel JFrame frame = new JFrame("Demonstrating FontMetrics"); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); MetricsJPanel metricsJPanel = new MetricsJPanel(); frame.add(metricsJPanel); frame.setSize(510, 240); frame.setVisible(true); } } // end class Metrics Fig. 13.16 | Displaying font metrics. Line 16 of Fig. 13.15 creates and sets the current drawing font to a SansSerif, bold, 12-point font. Line 17 uses Graphics method getFontMetrics to obtain the FontMetrics object for the current font. Line 18 outputs the String representation of the Font returned by g.getFont(). Lines 19–22 use FontMetric methods to obtain the ascent, descent, height and leading for the font. Line 24 creates a new Serif, italic, 14-point font. Line 25 uses a second version of Graphics method getFontMetrics, which accepts a Font argument and returns a corresponding FontMetrics object. Lines 28–31 obtain the ascent, descent, height and leading for the font. The font metrics are slightly different for the two fonts. 13.5 Drawing Lines, Rectangles and Ovals This section presents Graphics methods for drawing lines, rectangles and ovals. The methods and their parameters are summarized in Fig. 13.17. For each drawing method that requires a width and height parameter, the width and height must be nonnegative values. Otherwise, the shape will not display. 572 Chapter 13 Graphics and Java 2D Method Description public void drawLine(int x1, int y1, int x2, int y2) Draws a line between the point (x1, y1) and the point (x2, y2). public void drawRect(int x, int y, int width, int height) Draws a rectangle of the specified width and height. The rectangle’s top-left corner is located at (x, y). Only the outline of the rectangle is drawn using the Graphics object’s color—the body of the rectangle is not filled with this color. public void fillRect(int x, int y, int width, int height) Draws a filled rectangle in the current color with the specified width and height. The rectangle’s top-left corner is located at (x, y). public void clearRect(int x, int y, int width, int height) Draws a filled rectangle with the specified width and height in the current background color. The rectangle’s top-left corner is located at (x, y). This method is useful if you want to remove a portion of an image. public void drawRoundRect(int x, int y, int width, int height, int arcWidth, int arcHeight) Draws a rectangle with rounded corners in the current color with the specified width and height. The arcWidth and arcHeight determine the rounding of the corners (see Fig. 13.20). Only the outline of the shape is drawn. public void fillRoundRect(int x, int y, int width, int height, int arcWidth, int arcHeight) Draws a filled rectangle in the current color with rounded corners with the specified width and height. The arcWidth and arcHeight determine the rounding of the corners (see Fig. 13.20). public void draw3DRect(int x, int y, int width, int height, boolean b) Draws a three-dimensional rectangle in the current color with the specified width and height. The rectangle’s top-left corner is located at (x, y). The rectangle appears raised when b is true and lowered when b is false. Only the outline of the shape is drawn. public void fill3DRect(int x, int y, int width, int height, boolean b) Draws a filled three-dimensional rectangle in the current color with the specified width and height. The rectangle’s top-left corner is located at (x, y). The rectangle appears raised when b is true and lowered when b is false. public void drawOval(int x, int y, int width, int height) Draws an oval in the current color with the specified width and height. The bounding rectangle’s top-left corner is located at (x, y). The oval touches all four sides of the bounding rectangle at the center of each side (see Fig. 13.21). Only the outline of the shape is drawn. public void fillOval(int x, int y, int width, int height) Draws a filled oval in the current color with the specified width and height. The bounding rectangle’s top-left corner is located at (x, y). The oval touches the center of all four sides of the bounding rectangle (see Fig. 13.21). Fig. 13.17 | Graphics methods that draw lines, rectangles and ovals. 13.5 Drawing Lines, Rectangles and Ovals 573 The application of Figs. 13.18–13.19 demonstrates drawing a variety of lines, rectangles, three-dimensional rectangles, rounded rectangles and ovals. In Fig. 13.18, line 17 draws a red line, line 20 draws an empty blue rectangle and line 21 draws a filled blue rectangle. Methods fillRoundRect (line 24) and drawRoundRect (line 25) draw rectangles with rounded corners. Their first two arguments specify the coordinates of the upper-left corner of the bounding rectangle—the area in which the rounded rectangle will be drawn. The upper-left corner coordinates are not the edge of the rounded rectangle, but the coordinates where the edge would be if the rectangle had square corners. The third and fourth arguments specify the width and height of the rectangle. The last two arguments determine the horizontal and vertical diameters of the arc (i.e., the arc width and arc height) used to represent the corners. Figure 13.20 labels the arc width, arc height, width and height of a rounded rectangle. Using the same value for the arc width and arc height produces a quarter-circle at each 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 // Fig. 13.18: LinesRectsOvalsJPanel.java // Drawing lines, rectangles and ovals. import java.awt.Color; import java.awt.Graphics; import javax.swing.JPanel; public class LinesRectsOvalsJPanel extends JPanel { // display various lines, rectangles and ovals @Override public void paintComponent(Graphics g) { super.paintComponent(g); this.setBackground(Color.WHITE); g.setColor(Color.RED); g.drawLine(5, 30, 380, 30); g.setColor(Color.BLUE); g.drawRect(5, 40, 90, 55); g.fillRect(100, 40, 90, 55); g.setColor(Color.CYAN); g.fillRoundRect(195, 40, 90, 55, 50, 50); g.drawRoundRect(290, 40, 90, 55, 20, 20); g.setColor(Color.GREEN); g.draw3DRect(5, 100, 90, 55, true); g.fill3DRect(100, 100, 90, 55, false); g.setColor(Color.MAGENTA); g.drawOval(195, 100, 90, 55); g.fillOval(290, 100, 90, 55); } } // end class LinesRectsOvalsJPanel Fig. 13.18 | Drawing lines, rectangles and ovals. 574 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Chapter 13 Graphics and Java 2D // Fig. 13.19: LinesRectsOvals.java // Testing LinesRectsOvalsJPanel. import java.awt.Color; import javax.swing.JFrame; public class LinesRectsOvals { // execute application public static void main(String[] args) { // create frame for LinesRectsOvalsJPanel JFrame frame = new JFrame("Drawing lines, rectangles and ovals"); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); LinesRectsOvalsJPanel linesRectsOvalsJPanel = new LinesRectsOvalsJPanel(); linesRectsOvalsJPanel.setBackground(Color.WHITE); frame.add(linesRectsOvalsJPanel); frame.setSize(400, 210); frame.setVisible(true); } } // end class LinesRectsOvals drawLine fillRoundRect drawRect drawRoundRect fillRect drawOval draw3DRect fillOval fill3DRect Fig. 13.19 | Testing LinesRectsOvalsJPanel. (x, y) arc height arc width height width Fig. 13.20 | Arc width and arc height for rounded rectangles. 13.6 Drawing Arcs 575 corner. When the arc width, arc height, width and height have the same values, the result is a circle. If the values for width and height are the same and the values of arcWidth and arcHeight are 0, the result is a square. Methods draw3DRect (Fig. 13.18, line 28) and fill3DRect (line 29) take the same arguments. The first two specify the top-left corner of the rectangle. The next two arguments specify the width and height of the rectangle, respectively. The last argument determines whether the rectangle is raised (true) or lowered (false). The three-dimensional effect of draw3DRect appears as two edges of the rectangle in the original color and two edges in a slightly darker color. The three-dimensional effect of fill3DRect appears as two edges of the rectangle in the original drawing color and the fill and other two edges in a slightly darker color. Raised rectangles have the original drawing color edges at the top and left of the rectangle. Lowered rectangles have the original drawing color edges at the bottom and right of the rectangle. The three-dimensional effect is difficult to see in some colors. Methods drawOval and fillOval (lines 32–33) take the same four arguments. The first two specify the top-left coordinate of the bounding rectangle that contains the oval. The last two specify the width and height of the bounding rectangle, respectively. Figure 13.21 shows an oval bounded by a rectangle. The oval touches the center of all four sides of the bounding rectangle. (The bounding rectangle is not displayed on the screen.) (x,y) height width Fig. 13.21 | Oval bounded by a rectangle. 13.6 Drawing Arcs An arc is drawn as a portion of an oval. Arc angles are measured in degrees. Arcs sweep (i.e., move along a curve) from a starting angle through the number of degrees specified by their arc angle. The starting angle indicates in degrees where the arc begins. The arc angle specifies the total number of degrees through which the arc sweeps. Figure 13.22 illustrates two arcs. The left set of axes shows an arc sweeping from zero degrees to approximately 110 degrees. Arcs that sweep in a counterclockwise direction are measured in positive degrees. The set of axes on the right shows an arc sweeping from zero degrees to approximately –110 degrees. Arcs that sweep in a clockwise direction are measured in negative degrees. Note the dashed boxes around the arcs in Fig. 13.22. When drawing an arc, we specify a bounding rectangle for an oval. The arc will sweep along part of the oval. Graphics methods drawArc and fillArc for drawing arcs are summarized in Fig. 13.23. 576 Chapter 13 Graphics and Java 2D Positive angles 90º 180º Negative angles 90º 0º 180º 270º 0º 270º Fig. 13.22 | Positive and negative arc angles. Method Description public void drawArc(int x, int y, int width, int height, int startAngle, int arcAngle) Draws an arc relative to the bounding rectangle’s top-left x- and y-coordinates with the specified width and height. The arc segment is drawn starting at startAngle and sweeps arcAngle degrees. public void fillArc(int x, int y, int width, int height, int startAngle, int arcAngle) Draws a filled arc (i.e., a sector) relative to the bounding rectangle’s top-left x- and y-coordinates with the specified width and height. The arc segment is drawn starting at startAngle and sweeps arcAngle degrees. Fig. 13.23 | Graphics methods for drawing arcs. Figures 13.24–13.25 demonstrate the arc methods of Fig. 13.23. The application draws six arcs (three unfilled and three filled). To illustrate the bounding rectangle that helps determine where the arc appears, the first three arcs are displayed inside a red rectangle that has the same x, y, width and height arguments as the arcs. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 // Fig. 13.24: ArcsJPanel.java // Arcs displayed with drawArc and fillArc. import java.awt.Color; import java.awt.Graphics; import javax.swing.JPanel; public class ArcsJPanel extends JPanel { // draw rectangles and arcs @Override public void paintComponent(Graphics g) { super.paintComponent(g); Fig. 13.24 | Arcs displayed with drawArc and fillArc. (Part 1 of 2.) 13.6 Drawing Arcs 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 // start at 0 and sweep 360 degrees g.setColor(Color.RED); g.drawRect(15, 35, 80, 80); g.setColor(Color.BLACK); g.drawArc(15, 35, 80, 80, 0, 360); // start at 0 and sweep 110 degrees g.setColor(Color.RED); g.drawRect(100, 35, 80, 80); g.setColor(Color.BLACK); g.drawArc(100, 35, 80, 80, 0, 110); // start at 0 and sweep -270 degrees g.setColor(Color.RED); g.drawRect(185, 35, 80, 80); g.setColor(Color.BLACK); g.drawArc(185, 35, 80, 80, 0, -270); // start at 0 and sweep 360 degrees g.fillArc(15, 120, 80, 40, 0, 360); // start at 270 and sweep -90 degrees g.fillArc(100, 120, 80, 40, 270, -90); // start at 0 and sweep -270 degrees g.fillArc(185, 120, 80, 40, 0, -270); } } // end class ArcsJPanel Fig. 13.24 | Arcs displayed with drawArc and fillArc. (Part 2 of 2.) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 // Fig. 13.25: DrawArcs.java // Drawing arcs. import javax.swing.JFrame; public class DrawArcs { // execute application public static void main(String[] args) { // create frame for ArcsJPanel JFrame frame = new JFrame("Drawing Arcs"); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); ArcsJPanel arcsJPanel = new ArcsJPanel(); frame.add(arcsJPanel); frame.setSize(300, 210); frame.setVisible(true); } } // end class DrawArcs Fig. 13.25 | Drawing arcs. (Part 1 of 2.) 577 578 Chapter 13 Graphics and Java 2D Fig. 13.25 | Drawing arcs. (Part 2 of 2.) 13.7 Drawing Polygons and Polylines Polygons are closed multisided shapes composed of straight-line segments. Polylines are sequences of connected points. Figure 13.26 discusses methods for drawing polygons and polylines. Some methods require a Polygon object (package java.awt). Class Polygon’s constructors are also described in Fig. 13.26. The application of Figs. 13.27–13.28 draws polygons and polylines. Method Description Graphics methods for drawing polygons public void drawPolygon(int[] xPoints, int[] yPoints, int points) Draws a polygon. The x-coordinate of each point is specified in the xPoints array and the y-coordinate of each point in the yPoints array. The last argument specifies the number of points. This method draws a closed polygon. If the last point is different from the first, the polygon is closed by a line that connects the last point to the first. public void drawPolyline(int[] xPoints, int[] yPoints, int points) Draws a sequence of connected lines. The x-coordinate of each point is specified in the xPoints array and the y-coordinate of each point in the yPoints array. The last argument specifies the number of points. If the last point is different from the first, the polyline is not closed. public void drawPolygon(Polygon p) Draws the specified polygon. public void fillPolygon(int[] xPoints, int[] yPoints, int points) Draws a filled polygon. The x-coordinate of each point is specified in the xPoints array and the y-coordinate of each point in the yPoints array. The last argument specifies the number of points. This method draws a closed polygon. If the last point is different from the first, the polygon is closed by a line that connects the last point to the first. public void fillPolygon(Polygon p) Draws the specified filled polygon. The polygon is closed. Fig. 13.26 | Graphics methods for polygons and class Polygon methods. (Part 1 of 2.) 13.7 Drawing Polygons and Polylines Method Polygon 579 Description constructors and methods public Polygon() Constructs a new polygon object. The polygon does not contain any points. public Polygon(int[] xValues, int[] yValues, int numberOfPoints) Constructs a new polygon object. The polygon has numberOfPoints sides, with each point consisting of an x-coordinate from xValues and a y-coordinate from yValues. public void addPoint(int x, int y) Adds pairs of x- and y-coordinates to the Polygon. Fig. 13.26 | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 Graphics methods for polygons and class Polygon methods. (Part 2 of 2.) // Fig. 13.27: PolygonsJPanel.java // Drawing polygons. import java.awt.Graphics; import java.awt.Polygon; import javax.swing.JPanel; public class PolygonsJPanel extends JPanel { // draw polygons and polylines @Override public void paintComponent(Graphics g) { super.paintComponent(g); // draw polygon with Polygon object int[] xValues = {20, 40, 50, 30, 20, 15}; int[] yValues = {50, 50, 60, 80, 80, 60}; Polygon polygon1 = new Polygon(xValues, yValues, 6); g.drawPolygon(polygon1); // draw polylines with two arrays int[] xValues2 = {70, 90, 100, 80, 70, 65, 60}; int[] yValues2 = {100, 100, 110, 110, 130, 110, 90}; g.drawPolyline(xValues2, yValues2, 7); // fill polygon with two arrays int[] xValues3 = {120, 140, 150, 190}; int[] yValues3 = {40, 70, 80, 60}; g.fillPolygon(xValues3, yValues3, 4); // draw filled polygon Polygon polygon2 = new polygon2.addPoint(165, polygon2.addPoint(175, polygon2.addPoint(270, with Polygon object Polygon(); 135); 150); 200); Fig. 13.27 | Polygons displayed with drawPolygon and fillPolygon. (Part 1 of 2.) 580 36 37 38 39 40 Chapter 13 Graphics and Java 2D polygon2.addPoint(200, 220); polygon2.addPoint(130, 180); g.fillPolygon(polygon2); } } // end class PolygonsJPanel Fig. 13.27 | Polygons displayed with drawPolygon and fillPolygon. (Part 2 of 2.) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 // Fig. 13.28: DrawPolygons.java // Drawing polygons. import javax.swing.JFrame; public class DrawPolygons { // execute application public static void main(String[] args) { // create frame for PolygonsJPanel JFrame frame = new JFrame("Drawing Polygons"); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); PolygonsJPanel polygonsJPanel = new PolygonsJPanel(); frame.add(polygonsJPanel); frame.setSize(280, 270); frame.setVisible(true); } } // end class DrawPolygons Result of line 28 Result of line 18 Result of line 23 Result of line 37 Fig. 13.28 | Drawing polygons. Lines 16–17 of Fig. 13.27 create two int arrays and use them to specify the points for The Polygon constructor call in line 18 receives array xValues, which contains the x-coordinate of each point; array yValues, which contains the y-coordinate of each point; and 6 (the number of points in the polygon). Line 19 displays polygon1 by passing it as an argument to Graphics method drawPolygon. Lines 22–23 create two int arrays and use them to specify the points for a series of connected lines. Array xValues2 contains the x-coordinate of each point and array yValues2 the y-coordinate of each point. Line 24 uses Graphics method drawPolyline to Polygon polygon1. 13.8 Java 2D API 581 display the series of connected lines specified with the arguments xValues2, yValues2 and (the number of points). Lines 27–28 create two int arrays and use them to specify the points of a polygon. Array xValues3 contains the x-coordinate of each point and array yValues3 the y-coordinate of each point. Line 29 displays a polygon by passing to Graphics method fillPolygon the two arrays (xValues3 and yValues3) and the number of points to draw (4). 7 Common Programming Error 13.1 An ArrayIndexOutOfBoundsException is thrown if the number of points specified in the third argument to method drawPolygon or method fillPolygon is greater than the number of elements in the arrays of coordinates that specify the polygon to display. Line 32 creates Polygon polygon2 with no points. Lines 33–37 use Polygon method to add pairs of x- and y-coordinates to the Polygon. Line 38 displays Polygon polygon2 by passing it to Graphics method fillPolygon. addPoint 13.8 Java 2D API The Java 2D API provides advanced two-dimensional graphics capabilities for programmers who require detailed and complex graphical manipulations. The API includes features for processing line art, text and images in packages java.awt, java.awt.image, java.awt.color, java.awt.font, java.awt.geom, java.awt.print and java.awt.image.renderable. The capabilities of the API are far too broad to cover in this textbook. For an overview, visit http://docs.oracle.com/javase/7/docs/technotes/guides/2d/. In this section, we overview several Java 2D capabilities. Drawing with the Java 2D API is accomplished with a Graphics2D reference (package java.awt). Graphics2D is an abstract subclass of class Graphics, so it has all the graphics capabilities demonstrated earlier in this chapter. In fact, the actual object used to draw in every paintComponent method is an instance of a subclass of Graphics2D that is passed to method paintComponent and accessed via the superclass Graphics. To access Graphics2D capabilities, we must cast the Graphics reference (g) passed to paintComponent into a Graphics2D reference with a statement such as Graphics2D g2d = (Graphics2D) g; The next two examples use this technique. Lines, Rectangles, Round Rectangles, Arcs and Ellipses This example demonstrates several Java 2D shapes from package java.awt.geom, including Line2D.Double, Rectangle2D.Double, RoundRectangle2D.Double, Arc2D.Double and Ellipse2D.Double. Note the syntax of each class name. Each class represents a shape with dimensions specified as double values. There’s a separate version of each represented with float values (e.g., Ellipse2D.Float). In each case, Double is a public static nested class of the class specified to the left of the dot (e.g., Ellipse2D). To use the static nested class, we simply qualify its name with the outer-class name. In Figs. 13.29–13.30, we draw Java 2D shapes and modify their drawing characteristics, such as changing line thickness, filling shapes with patterns and drawing dashed lines. These are just a few of the many capabilities provided by Java 2D. 582 Chapter 13 Graphics and Java 2D Line 25 of Fig. 13.29 casts the Graphics reference received by paintComponent to a reference and assigns it to g2d to allow access to the Java 2D features. Graphics2D 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 // Fig. 13.29: ShapesJPanel.java // Demonstrating some Java 2D shapes. import java.awt.Color; import java.awt.Graphics; import java.awt.BasicStroke; import java.awt.GradientPaint; import java.awt.TexturePaint; import java.awt.Rectangle; import java.awt.Graphics2D; import java.awt.geom.Ellipse2D; import java.awt.geom.Rectangle2D; import java.awt.geom.RoundRectangle2D; import java.awt.geom.Arc2D; import java.awt.geom.Line2D; import java.awt.image.BufferedImage; import javax.swing.JPanel; public class ShapesJPanel extends JPanel { // draw shapes with Java 2D API @Override public void paintComponent(Graphics g) { super.paintComponent(g); Graphics2D g2d = (Graphics2D) g; // cast g to Graphics2D // draw 2D ellipse filled with a blue-yellow gradient g2d.setPaint(new GradientPaint(5, 30, Color.BLUE, 35, 100, Color.YELLOW, true)); g2d.fill(new Ellipse2D.Double(5, 30, 65, 100)); // draw 2D rectangle in red g2d.setPaint(Color.RED); g2d.setStroke(new BasicStroke(10.0f)); g2d.draw(new Rectangle2D.Double(80, 30, 65, 100)); // draw 2D rounded rectangle with a buffered background BufferedImage buffImage = new BufferedImage(10, 10, BufferedImage.TYPE_INT_RGB); // obtain Graphics2D from buffImage and draw on it Graphics2D gg = buffImage.createGraphics(); gg.setColor(Color.YELLOW); gg.fillRect(0, 0, 10, 10); gg.setColor(Color.BLACK); gg.drawRect(1, 1, 6, 6); gg.setColor(Color.BLUE); gg.fillRect(1, 1, 3, 3); gg.setColor(Color.RED); Fig. 13.29 | Demonstrating some Java 2D shapes. (Part 1 of 2.) 13.8 Java 2D API 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 gg.fillRect(4, 4, 3, 3); // draw a filled rectangle // paint buffImage onto the JFrame g2d.setPaint(new TexturePaint(buffImage, new Rectangle(10, 10))); g2d.fill( new RoundRectangle2D.Double(155, 30, 75, 100, 50, 50)); // draw 2D pie-shaped arc in white g2d.setPaint(Color.WHITE); g2d.setStroke(new BasicStroke(6.0f)); g2d.draw( new Arc2D.Double(240, 30, 75, 100, 0, 270, Arc2D.PIE)); // draw 2D lines in green and yellow g2d.setPaint(Color.GREEN); g2d.draw(new Line2D.Double(395, 30, 320, 150)); // draw 2D line using stroke float[] dashes = {10}; // specify dash pattern g2d.setPaint(Color.YELLOW); g2d.setStroke(new BasicStroke(4, BasicStroke.CAP_ROUND, BasicStroke.JOIN_ROUND, 10, dashes, 0)); g2d.draw(new Line2D.Double(320, 30, 395, 150)); } } // end class ShapesJPanel Fig. 13.29 | Demonstrating some Java 2D shapes. (Part 2 of 2.) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 // Fig. 13.30: Shapes.java // Testing ShapesJPanel. import javax.swing.JFrame; public class Shapes { // execute application public static void main(String[] args) { // create frame for ShapesJPanel JFrame frame = new JFrame("Drawing 2D shapes"); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); // create ShapesJPanel ShapesJPanel shapesJPanel = new ShapesJPanel(); frame.add(shapesJPanel); frame.setSize(425, 200); frame.setVisible(true); } } // end class Shapes Fig. 13.30 | Testing ShapesJPanel. (Part 1 of 2.) 583 584 Chapter 13 Graphics and Java 2D Fig. 13.30 | Testing ShapesJPanel. (Part 2 of 2.) Ovals, Gradient Fills and Paint Objects The first shape we draw is an oval filled with gradually changing colors. Lines 28–29 invoke Graphics2D method setPaint to set the Paint object that determines the color for the shape to display. A Paint object implements interface java.awt.Paint. It can be something as simple as one of the predeclared Color objects introduced in Section 13.3 (class Color implements Paint), or it can be an instance of the Java 2D API’s GradientPaint, SystemColor, TexturePaint, LinearGradientPaint or RadialGradientPaint classes. In this case, we use a GradientPaint object. Class GradientPaint helps draw a shape in gradually changing colors—called a gradient. The GradientPaint constructor used here requires seven arguments. The first two specify the starting coordinates for the gradient. The third specifies the starting Color for the gradient. The fourth and fifth specify the ending coordinates for the gradient. The sixth specifies the ending Color for the gradient. The last argument specifies whether the gradient is cyclic (true) or acyclic (false). The two sets of coordinates determine the direction of the gradient. Because the second coordinate (35, 100) is down and to the right of the first coordinate (5, 30), the gradient goes down and to the right at an angle. Because this gradient is cyclic (true), the color starts with blue, gradually becomes yellow, then gradually returns to blue. If the gradient is acyclic, the color transitions from the first color specified (e.g., blue) to the second color (e.g., yellow). Line 30 uses Graphics2D method fill to draw a filled Shape object—an object that implements interface Shape (package java.awt). In this case, we display an Ellipse2D.Double object. The Ellipse2D.Double constructor receives four arguments specifying the bounding rectangle for the ellipse to display. Rectangles, Strokes Next we draw a red rectangle with a thick border. Line 33 invokes setPaint to set the Paint object to Color.RED. Line 34 uses Graphics2D method setStroke to set the characteristics of the rectangle’s border (or the lines for any other shape). Method setStroke requires as its argument an object that implements interface Stroke (package java.awt). In this case, we use an instance of class BasicStroke. Class BasicStroke provides several constructors to specify the width of the line, how the line ends (called the end caps), how lines join together (called line joins) and the dash attributes of the line (if it’s a dashed line). The constructor here specifies that the line should be 10 pixels wide. Line 35 uses Graphics2D method draw to draw a Shape object—in this case, a Rectangle2D.Double. The Rectangle2D.Double constructor receives arguments specifying the rectangle’s upper-left x-coordinate, upper-left y-coordinate, width and height. 13.8 Java 2D API 585 Rounded Rectangles, BufferedImages and TexturePaint Objects Next we draw a rounded rectangle filled with a pattern created in a BufferedImage (package java.awt.image) object. Lines 38–39 create the BufferedImage object. Class BufferedImage can be used to produce images in color and grayscale. This particular BufferedImage is 10 pixels wide and 10 pixels tall (as specified by the first two arguments of the constructor). The third argument BufferedImage.TYPE_INT_RGB indicates that the image is stored in color using the RGB color scheme. To create the rounded rectangle’s fill pattern, we must first draw into the BufferedImage. Line 42 creates a Graphics2D object (by calling BufferedImage method createGraphics) that can be used to draw into the BufferedImage. Lines 43–50 use methods setColor, fillRect and drawRect to create the pattern. Lines 53–54 set the Paint object to a new TexturePaint (package java.awt) object. A TexturePaint object uses the image stored in its associated BufferedImage (the first constructor argument) as the fill texture for a filled-in shape. The second argument specifies the Rectangle area from the BufferedImage that will be replicated through the texture. In this case, the Rectangle is the same size as the BufferedImage. However, a smaller portion of the BufferedImage can be used. Lines 55–56 use Graphics2D method fill to draw a filled Shape object—in this case, a RoundRectangle2D.Double. The constructor for class RoundRectangle2D.Double receives six arguments specifying the rectangle dimensions and the arc width and arc height used to determine the rounding of the corners. Arcs Next we draw a pie-shaped arc with a thick white line. Line 59 sets the Paint object to Color.WHITE. Line 60 sets the Stroke object to a new BasicStroke for a line 6 pixels wide. Lines 61–62 use Graphics2D method draw to draw a Shape object—in this case, an Arc2D.Double. The Arc2D.Double constructor’s first four arguments specify the upperleft x-coordinate, upper-left y-coordinate, width and height of the bounding rectangle for the arc. The fifth argument specifies the start angle. The sixth argument specifies the arc angle. The last argument specifies how the arc is closed. Constant Arc2D.PIE indicates that the arc is closed by drawing two lines—one line from the arc’s starting point to the center of the bounding rectangle and one line from the center of the bounding rectangle to the ending point. Class Arc2D provides two other static constants for specifying how the arc is closed. Constant Arc2D.CHORD draws a line from the starting point to the ending point. Constant Arc2D.OPEN specifies that the arc should not be closed. Lines Finally, we draw two lines using Line2D objects—one solid and one dashed. Line 65 sets the Paint object to Color.GREEN. Line 66 uses Graphics2D method draw to draw a Shape object—in this case, an instance of class Line2D.Double. The Line2D.Double constructor’s arguments specify the starting coordinates and ending coordinates of the line. Line 69 declares a one-element float array containing the value 10. This array describes the dashes in the dashed line. In this case, each dash will be 10 pixels long. To create dashes of different lengths in a pattern, simply provide the length of each dash as an element in the array. Line 70 sets the Paint object to Color.YELLOW. Lines 71–72 set the Stroke object to a new BasicStroke. The line will be 4 pixels wide and will have rounded ends (BasicStroke.CAP_ROUND). If lines join together (as in a rectangle at the corners), 586 Chapter 13 Graphics and Java 2D their joining will be rounded (BasicStroke.JOIN_ROUND). The dashes argument specifies the dash lengths for the line. The last argument indicates the starting index in the dashes array for the first dash in the pattern. Line 73 then draws a line with the current Stroke. Creating Your Own Shapes with General Paths Next we present a general path—a shape constructed from straight lines and complex curves. A general path is represented with an object of class GeneralPath (package java.awt.geom). The application of Figs. 13.31 and 13.32 demonstrates drawing a general path in the shape of a five-pointed star. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 // Fig. 13.31: Shapes2JPanel.java // Demonstrating a general path. import java.awt.Color; import java.awt.Graphics; import java.awt.Graphics2D; import java.awt.geom.GeneralPath; import java.security.SecureRandom; import javax.swing.JPanel; public class Shapes2JPanel extends JPanel { // draw general paths @Override public void paintComponent(Graphics g) { super.paintComponent(g); SecureRandom random = new SecureRandom(); int[] xPoints = {55, 67, 109, 73, 83, 55, 27, 37, 1, 43}; int[] yPoints = {0, 36, 36, 54, 96, 72, 96, 54, 36, 36}; Graphics2D g2d = (Graphics2D) g; GeneralPath star = new GeneralPath(); // set the initial coordinate of the General Path star.moveTo(xPoints[0], yPoints[0]); // create the star--this does not draw the star for (int count = 1; count < xPoints.length; count++) star.lineTo(xPoints[count], yPoints[count]); star.closePath(); // close the shape g2d.translate(150, 150); // translate the origin to (150, 150) // rotate around origin and draw stars in random colors for (int count = 1; count <= 20; count++) { g2d.rotate(Math.PI / 10.0); // rotate coordinate system Fig. 13.31 | Java 2D general paths. (Part 1 of 2.) 13.8 Java 2D API 41 42 43 44 45 46 47 48 587 // set random drawing color g2d.setColor(new Color(random.nextInt(256), random.nextInt(256), random.nextInt(256))); g2d.fill(star); // draw filled star } } } // end class Shapes2JPanel Fig. 13.31 | Java 2D general paths. (Part 2 of 2.) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 // Fig. 13.32: Shapes2.java // Demonstrating a general path. import java.awt.Color; import javax.swing.JFrame; public class Shapes2 { // execute application public static void main(String[] args) { // create frame for Shapes2JPanel JFrame frame = new JFrame("Drawing 2D Shapes"); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); Shapes2JPanel shapes2JPanel = new Shapes2JPanel(); frame.add(shapes2JPanel); frame.setBackground(Color.WHITE); frame.setSize(315, 330); frame.setVisible(true); } } // end class Shapes2 Fig. 13.32 | Demonstrating a general path. Lines 19–20 (Fig. 13.31) declare two int arrays representing the x- and y-coordinates of the points in the star. Line 23 creates GeneralPath object star. Line 26 uses GeneralPath method moveTo to specify the first point in the star. The for statement in lines 29– 588 Chapter 13 Graphics and Java 2D 30 uses GeneralPath method lineTo to draw a line to the next point in the star. Each new call to lineTo draws a line from the previous point to the current point. Line 32 uses GeneralPath method closePath to draw a line from the last point to the point specified in the last call to moveTo. This completes the general path. Line 34 uses Graphics2D method translate to move the drawing origin to location (150, 150). All drawing operations now use location (150, 150) as (0, 0). The for statement in lines 37–46 draws the star 20 times by rotating it around the new origin point. Line 39 uses Graphics2D method rotate to rotate the next displayed shape. The argument specifies the rotation angle in radians (with 360° = 2π radians). Line 45 uses Graphics2D method fill to draw a filled version of the star. 13.9 Wrap-Up In this chapter, you learned how to use Java’s graphics capabilities to produce colorful drawings. You learned how to specify the location of an object using Java’s coordinate system, and how to draw on a window using the paintComponent method. You were introduced to class Color, and you learned how to use this class to specify different colors using their RGB components. You used the JColorChooser dialog to allow users to select colors in a program. You then learned how to work with fonts when drawing text on a window. You learned how to create a Font object from a font name, style and size, as well as how to access the metrics of a font. From there, you learned how to draw various shapes on a window, such as rectangles (regular, rounded and 3D), ovals and polygons, as well as lines and arcs. You then used the Java 2D API to create more complex shapes and to fill them with gradients or patterns. The chapter concluded with a discussion of general paths, used to construct shapes from straight lines and complex curves. In the next chapter, we discuss class String and its methods. We introduce regular expressions for pattern matching in strings and demonstrate how to validate user input with regular expressions. Summary Section 13.1 Introduction • • • • Java’s coordinate system (p. 556) is a scheme for identifying every point (p. 567) on the screen. A coordinate pair (p. 556) has an x-coordinate (horizontal) and a y-coordinate (vertical). Coordinates are used to indicate where graphics should be displayed on a screen. Coordinate units are measured in pixels (p. 556). A pixel is a display monitor’s smallest unit of resolution. Section 13.2 Graphics Contexts and Graphics Objects • A Java graphics context (p. 558) enables drawing on the screen. • Class Graphics (p. 558) contains methods for drawing strings, lines, rectangles and other shapes. Methods are also included for font manipulation and color manipulation. • A Graphics object manages a graphics context and draws pixels on the screen that represent text and other graphical objects, e.g., lines, ellipses, rectangles and other polygons (p. 558). • Class Graphics is an abstract class. Each Java implementation has a Graphics subclass that provides drawing capabilities. This implementation is hidden from us by class Graphics, which supplies the interface that enables us to use graphics in a platform-independent manner. Summary 589 • Method paintComponent can be used to draw graphics in any JComponent component. • Method paintComponent receives a Graphics object that is passed to the method by the system when a lightweight Swing component needs to be repainted. • When an application executes, the application container calls method paintComponent. For paintComponent to be called again, an event must occur. • When a JComponent is displayed, its paintComponent method is called. • Calling method repaint (p. 559) on a component updates the graphics drawn on that component. Section 13.3 Color Control • Class Color (p. 559) declares methods and constants for manipulating colors in a Java program. • Every color is created from a red, a green and a blue component. Together these components are called RGB values (p. 560). The RGB components specify the amount of red, green and blue in a color, respectively. The larger the value, the greater the amount of that particular color. • Color methods getRed, getGreen and getBlue (p. 560) return int values from 0 to 255 representing the amount of red, green and blue, respectively. • Graphics method getColor (p. 560) returns a Color object with the current drawing color. • Graphics method setColor (p. 560) sets the current drawing color. • Graphics method fillRect (p. 560) draws a rectangle filled by the Graphics object’s current color. • Graphics method drawString (p. 562) draws a String in the current color. • The JColorChooser GUI component (p. 563) enables application users to select colors. • JColorChooser static method showDialog (p. 564) displays a modal JColorChooser dialog. Section 13.4 Manipulating Fonts • Class Font (p. 566) contains methods and constants for manipulating fonts. • Class Font’s constructor takes three arguments—the font name (p. 567), font style and font size. • A Font’s font style can be Font.PLAIN, Font.ITALIC or Font.BOLD (each is a static field of class Font). Font styles can be used in combination (e.g., Font.ITALIC + Font.BOLD). • The font size is measured in points. A point is 1/72 of an inch. • Graphics method setFont (p. 567) sets the drawing font in which text will be displayed. • Font method getSize (p. 567) returns the font size in points. • Font method getName (p. 567) returns the current font name as a string. • Font method getStyle (p. 569) returns an integer value representing the current Font’s style. • Font method getFamily (p. 569) returns the name of the font family to which the current font belongs. The name of the font family is platform specific. • Class FontMetrics (p. 569) contains methods for obtaining font information. • Font metrics (p. 569) include height, descent and leading. Section 13.5 Drawing Lines, Rectangles and Ovals • methods fillRoundRect (p. 573) and drawRoundRect (p. 573) draw rectangles with rounded corners. • Graphics methods draw3DRect (p. 575) and fill3DRect (p. 575) draw three-dimensional rectangles. • Graphics methods drawOval (p. 575) and fillOval (p. 575) draw ovals. Graphics 590 Chapter 13 Graphics and Java 2D Section 13.6 Drawing Arcs • An arc (p. 575) is drawn as a portion of an oval. • Arcs sweep from a starting angle by the number of degrees specified by their arc angle (p. 575). • Graphics methods drawArc (p. 575) and fillArc (p. 575) are used for drawing arcs. Section 13.7 Drawing Polygons and Polylines • Class Polygon contains methods for creating polygons. • Polygons are closed multisided shapes composed of straight-line segments. • Polylines (p. 578) are sequences of connected points. • Graphics method drawPolyline (p. 580) displays a series of connected lines. • Graphics methods drawPolygon (p. 580) and fillPolygon (p. 581) are used to draw polygons. • Polygon method addPoint (p. 581) adds pairs of x- and y-coordinates to the Polygon. Section 13.8 Java 2D API • The Java 2D API (p. 581) provides advanced two-dimensional graphics capabilities. • Class Graphics2D (p. 581)—a subclass of Graphics—is used for drawing with the Java 2D API. • The Java 2D API’s classes for drawing shapes include Line2D.Double, Rectangle2D.Double, RoundRectangle2D.Double, Arc2D.Double and Ellipse2D.Double (p. 581). • Class GradientPaint (p. 584) helps draw a shape in gradually changing colors—called a gradient (p. 584). • Graphics2D method fill (p. 584) draws a filled object of any type that implements interface Shape (p. 584). • Class BasicStroke (p. 584) helps specify the drawing characteristics of lines. • Graphics2D method draw (p. 584) is used to draw a Shape object. • Classes GradientPaint (p. 584) and TexturePaint (p. 585) help specify the characteristics for filling shapes with colors or patterns. • A general path (p. 586) is a shape constructed from straight lines and complex curves and is represented with an object of class GeneralPath (p. 586). • GeneralPath method moveTo (p. 587) specifies the first point in a general path. • GeneralPath method lineTo (p. 588) draws a line to the next point in the path. Each new call to lineTo draws a line from the previous point to the current point. • GeneralPath method closePath (p. 588) draws a line from the last point to the point specified in the last call to moveTo. This completes the general path. • Graphics2D method translate (p. 588) is used to move the drawing origin to a new location. • Graphics2D method rotate (p. 588) is used to rotate the next displayed shape. Self-Review Exercises 13.1 Fill in the blanks in each of the following statements: a) In Java 2D, method of class sets the characteristics of a stroke used to draw a shape. b) Class helps specify the fill for a shape such that the fill gradually changes from one color to another. c) The method of class Graphics draws a line between two points. d) RGB is short for , and . Answers to Self-Review Exercises 591 e) Font sizes are measured in units called . helps specify the fill for a shape using a pattern drawn in a BufferedImage. f) Class 13.2 State whether each of the following is true or false. If false, explain why. a) The first two arguments of Graphics method drawOval specify the center coordinate of the oval. b) In the Java coordinate system, x-coordinates increase from left to right and y-coordinates from top to bottom. c) Graphics method fillPolygon draws a filled polygon in the current color. d) Graphics method drawArc allows negative angles. e) Graphics method getSize returns the size of the current font in centimeters. f) Pixel coordinate (0, 0) is located at the exact center of the monitor. 13.3 Find the error(s) in each of the following and explain how to correct them. Assume that g is a Graphics object. a) g.setFont("SansSerif"); b) g.erase(x, y, w, h); // clear rectangle at (x, y) c) Font f = new Font("Serif", Font.BOLDITALIC, 12); d) g.setColor(255, 255, 0); // change color to yellow Answers to Self-Review Exercises 13.1 a) setStroke, Graphics2D. b) GradientPaint. c) drawLine. d) red, green, blue. e) points. f) TexturePaint. 13.2 a) b) c) d) e) f) False. The first two arguments specify the upper-left corner of the bounding rectangle. True. True. True. False. Font sizes are measured in points. False. The coordinate (0,0) corresponds to the upper-left corner of a GUI component on which drawing occurs. 13.3 a) The setFont method takes a Font object as an argument—not a String. b) The Graphics class does not have an erase method. The clearRect method should be used. c) Font.BOLDITALIC is not a valid font style. To get a bold italic font, use Font.BOLD + Font.ITALIC. d) Method setColor takes a Color object as an argument, not three integers. Exercises 13.4 13.5 Fill in the blanks in each of the following statements: a) Class of the Java 2D API is used to draw ovals. b) Methods draw and fill of class Graphics2D require an object of type argument. c) The three constants that specify font style are , and sets the painting color for Java 2D shapes. d) Graphics2D method as their . State whether each of the following is true or false. If false, explain why. a) Graphics method drawPolygon automatically connects the endpoints of the polygon. b) Graphics method drawLine draws a line between two points. c) Graphics method fillArc uses degrees to specify the angle. d) In the Java coordinate system, values on the y-axis increase from left to right. e) Graphics inherits directly from class Object. 592 Chapter 13 Graphics and Java 2D f) Graphics is an abstract class. g) The Font class inherits directly from class Graphics. 13.6 (Concentric Circles Using Method drawArc) Write an application that draws a series of eight concentric circles. The circles should be separated by 10 pixels. Use Graphics method drawArc. 13.7 (Concentric Circles Using Class Ellipse2D.Double) Modify your solution to Exercise 13.6 to draw the ovals by using class Ellipse2D.Double and method draw of class Graphics2D. 13.8 (Random Lines Using Class Line2D.Double) Modify your solution to Exercise 13.7 to draw random lines in random colors and random thicknesses. Use class Line2D.Double and method draw of class Graphics2D to draw the lines. 13.9 (Random Triangles) Write an application that displays randomly generated triangles in different colors. Each triangle should be filled with a different color. Use class GeneralPath and method fill of class Graphics2D to draw the triangles. 13.10 (Random Characters) Write an application that randomly draws characters in different fonts, sizes and colors. 13.11 (Grid Using Method drawLine) Write an application that draws an 8-by-8 grid. Use method drawLine. Graphics 13.12 (Grid Using Class Line2D.Double) Modify your solution to Exercise 13.11 to draw the grid using instances of class Line2D.Double and method draw of class Graphics2D. 13.13 (Grid Using Method drawRect) Write an application that draws a 10-by-10 grid. Use the method drawRect. Graphics 13.14 (Grid Using Class Rectangle2D.Double) Modify your solution to Exercise 13.13 to draw the grid by using class Rectangle2D.Double and method draw of class Graphics2D. 13.15 (Drawing Tetrahedrons) Write an application that draws a tetrahedron (a three-dimensional shape with four triangular faces). Use class GeneralPath and method draw of class Graphics2D. 13.16 (Drawing Cubes) Write an application that draws a cube. Use class GeneralPath and method draw of class Graphics2D. 13.17 (Circles Using Class Ellipse2D.Double) Write an application that asks the user to input the radius of a circle as a floating-point number and draws the circle, as well as the values of the circle’s diameter, circumference and area. Use the value 3.14159 for π. [Note: You may also use the predefined constant Math.PI for the value of π. This constant is more precise than the value 3.14159. Class Math is declared in the java.lang package, so you need not import it.] Use the following formulas (r is the radius): diameter = 2r circumference = 2πr area = πr2 The user should also be prompted for a set of coordinates in addition to the radius. Then draw the circle and display its diameter, circumference and area, using an Ellipse2D.Double object to represent the circle and method draw of class Graphics2D to display it. 13.18 (Screen Saver) Write an application that simulates a screen saver. The application should randomly draw lines using method drawLine of class Graphics. After drawing 100 lines, the application should clear itself and start drawing lines again. To allow the program to draw continuously, place a call to repaint as the last line in method paintComponent. Do you notice any problems with this on your system? 13.19 (Screen Saver Using Timer) Package javax.swing contains a class called Timer that is capable of calling method actionPerformed of interface ActionListener at a fixed time interval (speci- Exercises 593 fied in milliseconds). Modify your solution to Exercise 13.18 to remove the call to repaint from method paintComponent. Declare your class to implement ActionListener. (The actionPerformed method should simply call repaint.) Declare an instance variable of type Timer called timer in your class. In the constructor for your class, write the following statements: timer = new Timer(1000, this); timer.start(); This creates an instance of class Timer that will call 1000 milliseconds (i.e., every second). this object’s actionPerformed method every 13.20 (Screen Saver for a Random Number of Lines) Modify your solution to Exercise 13.19 to enable the user to enter the number of random lines that should be drawn before the application clears itself and starts drawing lines again. Use a JTextField to obtain the value. The user should be able to type a new number into the JTextField at any time during the program’s execution. Use an inner class to perform event handling for the JTextField. 13.21 (Screen Saver with Shapes) Modify your solution to Exercise 13.20 such that it uses random-number generation to choose different shapes to display. Use methods of class Graphics. 13.22 (Screen Saver Using the Java 2D API) Modify your solution to Exercise 13.21 to use classes and drawing capabilities of the Java 2D API. Draw shapes like rectangles and ellipses, with randomly generated gradients. Use class GradientPaint to generate the gradient. 13.23 (Turtle Graphics) Modify your solution to Exercise 7.21—Turtle Graphics—to add a graphical user interface using JTextFields and JButtons. Draw lines rather than asterisks (*). When the turtle graphics program specifies a move, translate the number of positions into a number of pixels on the screen by multiplying the number of positions by 10 (or any value you choose). Implement the drawing with Java 2D API features. 13.24 (Knight’s Tour) Produce a graphical version of the Knight’s Tour problem (Exercise 7.22, Exercise 7.23 and Exercise 7.26). As each move is made, the appropriate cell of the chessboard should be updated with the proper move number. If the result of the program is a full tour or a closed tour, the program should display an appropriate message. If you like, use class Timer (see Exercise 13.19) to help animate the Knight’s Tour. 13.25 (Tortoise and Hare) Produce a graphical version of the Tortoise and Hare simulation (Exercise 7.28). Simulate the mountain by drawing an arc that extends from the bottom-left corner of the window to the top-right corner. The tortoise and the hare should race up the mountain. Implement the graphical output to actually print the tortoise and the hare on the arc for every move. [Hint: Extend the length of the race from 70 to 300 to allow yourself a larger graphics area.] 13.26 (Drawing Spirals) Write an application that uses Graphics method drawPolyline to draw a spiral similar to the one shown in Fig. 13.33. 13.27 (Pie Chart) Write a program that inputs four numbers and graphs them as a pie chart. Use class Arc2D.Double and method fill of class Graphics2D to perform the drawing. Draw each piece of the pie in a separate color. 13.28 (Selecting Shapes) Write an application that allows the user to select a shape from a JComboBox and draws it 20 times with random locations and dimensions in method paintComponent. The first item in the JComboBox should be the default shape that is displayed the first time paintComponent is called. 13.29 (Random Colors) Modify Exercise 13.28 to draw each of the 20 randomly sized shapes in a randomly selected color. Use all 13 predefined Color objects in an array of Colors. 13.30 (JColorChooser Dialog) Modify Exercise 13.28 to allow the user to select the color in which shapes should be drawn from a JColorChooser dialog. 594 Chapter 13 Graphics and Java 2D Fig. 13.33 | Spiral drawn using method drawPolyline. (Optional) GUI and Graphics Case Study Exercise: Adding Java 2D 13.31 Java 2D introduces many new capabilities for creating unique and impressive graphics. We’ll add a small subset of these features to the drawing application you created in Exercise 12.17. In this version, you’ll enable the user to specify gradients for filling shapes and to change stroke characteristics for drawing lines and outlines of shapes. The user will be able to choose which colors compose the gradient and set the width and dash length of the stroke. First, you must update the MyShape hierarchy to support Java 2D functionality. Make the following changes in class MyShape: a) Change abstract method draw’s parameter type from Graphics to Graphics2D. b) Change all variables of type Color to type Paint to enable support for gradients. [Note: Recall that class Color implements interface Paint.] c) Add an instance variable of type Stroke in class MyShape and a Stroke parameter in the constructor to initialize the new instance variable. The default stroke should be an instance of class BasicStroke. Classes MyLine, MyBoundedShape, MyOval and MyRectangle should each add a Stroke parameter to their constructors. In the draw methods, each shape should set the Paint and the Stroke before drawing or filling a shape. Since Graphics2D is a subclass of Graphics, we can continue to use Graphics methods drawLine, drawOval, fillOval, and so on to draw the shapes. When these methods are called, they’ll draw the appropriate shape using the specified Paint and Stroke settings. Next, you’ll update the DrawPanel to handle the Java 2D features. Change all Color variables to Paint variables. Declare an instance variable currentStroke of type Stroke and provide a set method for it. Update the calls to the individual shape constructors to include the Paint and Stroke arguments. In method paintComponent, cast the Graphics reference to type Graphics2D and use the Graphics2D reference in each call to MyShape method draw. Next, make the new Java 2D features accessible from the GUI. Create a JPanel of GUI components for setting the Java 2D options. Add these components at the top of the DrawFrame below the panel that currently contains the standard shape controls (see Fig. 13.34). These GUI components should include: a) A checkbox to specify whether to paint using a gradient. b) Two JButtons that each show a JColorChooser dialog to allow the user to choose the first and second color in the gradient. (These will replace the JComboBox used for choosing the color in Exercise 12.17.) c) A text field for entering the Stroke width. d) A text field for entering the Stroke dash length. e) A checkbox for selecting whether to draw a dashed or solid line. Making a Difference 595 Fig. 13.34 | Drawing with Java 2D. If the user selects to draw with a gradient, set the Paint on the DrawPanel to be a gradient of the two colors chosen by the user. The expression new GradientPaint(0, 0, color1, 50, 50, color2, true)) creates a GradientPaint that cycles diagonally from the upper-left to the bottom-right every 50 pixels. Variables color1 and color2 represent the colors chosen by the user. If the user does not select to use a gradient, then simply set the Paint on the DrawPanel to be the first Color chosen by the user. For strokes, if the user chooses a solid line, then create the Stroke with the expression new BasicStroke(width, BasicStroke.CAP_ROUND, BasicStroke.JOIN_ROUND) where variable width is the width specified by the user in the line-width text field. If the user chooses a dashed line, then create the Stroke with the expression new BasicStroke(width, BasicStroke.CAP_ROUND, BasicStroke.JOIN_ROUND, 10, dashes, 0) where width again is the width in the line-width field, and dashes is an array with one element whose value is the length specified in the dash-length field. The Panel and Stroke objects should be passed to the shape object’s constructor when the shape is created in DrawPanel. Making a Difference 13.32 (Large-Type Displays for People with Low Vision) The accessibility of computers and the Internet to all people, regardless of disabilities, is becoming more important as these tools play increasing roles in our personal and business lives. According to a recent estimate by the World Health Organization (www.who.int/mediacentre/factsheets/fs282/en/), 246 million people worldwide have low vision. To learn more about low vision, check out the GUI-based low-vision simulation at www.webaim.org/simulations/lowvision.php. People with low vision might prefer to choose a font and/or a larger font size when reading electronic documents and web pages. Java has five built-in “logical” fonts that are guaranteed to be available in any Java implementation, including Serif, Sans-serif and Monospaced. Write a GUI application that provides a JTextArea in which the user can type text. Allow the user to select Serif, Sans-serif or Monospaced from a JComboBox. Provide a Bold JCheckBox, which, if checked, makes the text bold. Include Increase Font Size and Decrease Font Size JButtons that allow the user to scale the size of the font up or down, respectively, by one point at a time. Start with a font size of 18 points. For the purposes of this exercise, set the font size on the JComboBox, JButtons and JCheckBox to 20 points so that a person with low vision will be able to read the text on them. 14 The chief defect of Henry King Was chewing little bits of string. —Hilaire Belloc Vigorous writing is concise. A sentence should contain no unnecessary words, a paragraph no unnecessary sentences. —William Strunk, Jr. I have made this letter longer than usual, because I lack the time to make it short. —Blaise Pascal Objectives In this chapter you’ll: ■ Create and manipulate immutable character-string objects of class String. ■ Create and manipulate mutable character-string objects of class StringBuilder. ■ Create and manipulate objects of class Character. ■ Break a String object into tokens using String method split. ■ Use regular expressions to validate String data entered into an application. Strings, Characters and Regular Expressions 14.1 Introduction 14.1 Introduction 14.2 Fundamentals of Characters and Strings 14.3 Class String 14.3.1 String Constructors 14.3.2 String Methods length, charAt and getChars 14.3.3 Comparing Strings 14.3.4 Locating Characters and Substrings in Strings 14.3.5 Extracting Substrings from Strings 14.3.6 Concatenating Strings 14.3.7 Miscellaneous String Methods 14.3.8 String Method valueOf 597 14.4.2 StringBuilder Methods length, capacity, setLength and ensureCapacity 14.4.3 StringBuilder Methods charAt, setCharAt, getChars and reverse 14.4.4 StringBuilder append Methods 14.4.5 StringBuilder Insertion and Deletion Methods 14.5 Class Character 14.6 Tokenizing Strings 14.7 Regular Expressions, Class Pattern and Class Matcher 14.8 Wrap-Up 14.4 Class StringBuilder 14.4.1 StringBuilder Constructors Summary | Self-Review Exercises | Answers to Self-Review Exercises | Exercises | Special Section: Advanced String-Manipulation Exercises | Special Section: Challenging String-Manipulation Projects | Making a Difference 14.1 Introduction This chapter introduces Java’s string- and character-processing capabilities. The techniques discussed here are appropriate for validating program input, displaying information to users and other text-based manipulations. They’re also appropriate for developing text editors, word processors, page-layout software, computerized typesetting systems and other kinds of text-processing software. We’ve presented several string-processing capabilities in earlier chapters. This chapter discusses in detail the capabilities of classes String, StringBuilder and Character from the java.lang package—these classes provide the foundation for string and character manipulation in Java. The chapter also discusses regular expressions that provide applications with the capability to validate input. The functionality is located in the String class along with classes Matcher and Pattern located in the java.util.regex package. 14.2 Fundamentals of Characters and Strings Characters are the fundamental building blocks of Java source programs. Every program is composed of a sequence of characters that—when grouped together meaningfully—are interpreted by the Java compiler as a series of instructions used to accomplish a task. A program may contain character literals. A character literal is an integer value represented as a character in single quotes. For example, 'z' represents the integer value of z, and '\t' represents the integer value of a tab character. The value of a character literal is the integer value of the character in the Unicode character set. Appendix B presents the integer equivalents of the characters in the ASCII character set, which is a subset of Unicode (discussed in online Appendix H). Recall from Section 2.2 that a string is a sequence of characters treated as a single unit. A string may include letters, digits and various special characters, such as +, -, *, / and $. A string is an object of class String. String literals (stored in memory as String objects) are written as a sequence of characters in double quotation marks, as in: 598 Chapter 14 Strings, Characters and Regular Expressions "John Q. Doe" "9999 Main Street" "Waltham, Massachusetts" "(201) 555-1212" (a name) (a street address) (a city and state) (a telephone number) A string may be assigned to a String reference. The declaration String color = "blue"; initializes String variable color to refer to a String object that contains the string "blue". Performance Tip 14.1 To conserve memory, Java treats all string literals with the same contents as a single String object that has many references to it. 14.3 Class String Class String is used to represent strings in Java. The next several subsections cover many of class String’s capabilities. 14.3.1 String Constructors Class String provides constructors for initializing String objects in a variety of ways. Four of the constructors are demonstrated in the main method of Fig. 14.1. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 s1 s2 s3 s4 // Fig. 14.1: StringConstructors.java // String class constructors. public class StringConstructors { public static void main(String[] args) { char[] charArray = {'b', 'i', 'r', 't', 'h', ' ', 'd', 'a', 'y'}; String s = new String("hello"); // use String String String String String constructors s1 = new String(); s2 = new String(s); s3 = new String(charArray); s4 = new String(charArray, 6, 3); System.out.printf( "s1 = %s%ns2 = %s%ns3 = %s%ns4 = %s%n", s1, s2, s3, s4); } } // end class StringConstructors = = hello = birth day = day Fig. 14.1 | String class constructors. 14.3 Class String 599 Line 12 instantiates a new String using class String’s no-argument constructor and assigns its reference to s1. The new String object contains no characters (i.e., the empty string, which can also be represented as "") and has a length of 0. Line 13 instantiates a new String object using class String’s constructor that takes a String object as an argument and assigns its reference to s2. The new String object contains the same sequence of characters as the String object s that’s passed as an argument to the constructor. Performance Tip 14.2 It’s not necessary to copy an existing String object. String objects are immutable, because class String does not provide methods that allow the contents of a String object to be modified after it is created. Line 14 instantiates a new String object and assigns its reference to s3 using class constructor that takes a char array as an argument. The new String object contains a copy of the characters in the array. Line 15 instantiates a new String object and assigns its reference to s4 using class String’s constructor that takes a char array and two integers as arguments. The second argument specifies the starting position (the offset) from which characters in the array are accessed. Remember that the first character is at position 0. The third argument specifies the number of characters (the count) to access in the array. The new String object is formed from the accessed characters. If the offset or the count specified as an argument results in accessing an element outside the bounds of the character array, a StringIndexOutOfBoundsException is thrown. String’s 14.3.2 String Methods length, charAt and getChars methods length, charAt and getChars return the length of a String, obtain the character at a specific location in a String and retrieve a set of characters from a String as a char array, respectively. Figure 14.2 demonstrates each of these methods. String 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 // Fig. 14.2: StringMiscellaneous.java // This application demonstrates the length, charAt and getChars // methods of the String class. public class StringMiscellaneous { public static void main(String[] args) { String s1 = "hello there"; char[] charArray = new char[5]; Fig. 14.2 | System.out.printf("s1: %s", s1); // test length method System.out.printf("%nLength of s1: %d", s1.length()); // loop through characters in s1 with charAt and display reversed System.out.printf("%nThe string reversed is: "); String methods length, charAt and getChars. (Part 1 of 2.) 600 19 20 21 22 23 24 25 26 27 28 29 30 31 32 Chapter 14 Strings, Characters and Regular Expressions for (int count = s1.length() - 1; count >= 0; count--) System.out.printf("%c ", s1.charAt(count)); // copy characters from string into charArray s1.getChars(0, 5, charArray, 0); System.out.printf("%nThe character array is: "); for (char character : charArray) System.out.print(character); System.out.println(); } } // end class StringMiscellaneous s1: hello there Length of s1: 11 The string reversed is: e r e h t The character array is: hello Fig. 14.2 | String o l l e h methods length, charAt and getChars. (Part 2 of 2.) Line 15 uses String method length to determine the number of characters in String Like arrays, strings know their own length. However, unlike arrays, you access a String’s length via class String’s length method. Lines 20–21 print the characters of the String s1 in reverse order (and separated by spaces). String method charAt (line 21) returns the character at a specific position in the String. Method charAt receives an integer argument that’s used as the index and returns the character at that position. Like arrays, the first element of a String is at position 0. Line 24 uses String method getChars to copy the characters of a String into a character array. The first argument is the starting index from which characters are to be copied. The second argument is the index that’s one past the last character to be copied from the String. The third argument is the character array into which the characters are to be copied. The last argument is the starting index where the copied characters are placed in the target character array. Next, lines 27–28 print the char array contents one character at a time. s1. 14.3.3 Comparing Strings Chapter 19 discusses sorting and searching arrays. Frequently, the information being sorted or searched consists of Strings that must be compared to place them into order or to determine whether a string appears in an array (or other collection). Class String provides methods for comparing strings, as demonstrated in the next two examples. To understand what it means for one string to be greater than or less than another, consider the process of alphabetizing a series of last names. No doubt, you’d place “Jones” before “Smith” because the first letter of “Jones” comes before the first letter of “Smith” in the alphabet. But the alphabet is more than just a list of 26 letters—it’s an ordered list of characters. Each letter occurs in a specific position within the list. Z is more than just a letter of the alphabet—it’s specifically the twenty-sixth letter of the alphabet. 14.3 Class String 601 How does the computer know that one letter “comes before” another? All characters are represented in the computer as numeric codes (see Appendix B). When the computer compares Strings, it actually compares the numeric codes of the characters in the Strings. Figure 14.3 demonstrates String methods equals, equalsIgnoreCase, compareTo and regionMatches and using the equality operator == to compare String objects. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 // Fig. 14.3: StringCompare.java // String methods equals, equalsIgnoreCase, compareTo and regionMatches. public class StringCompare { public static void main(String[] args) { String s1 = new String("hello"); // s1 is a copy of "hello" String s2 = "goodbye"; String s3 = "Happy Birthday"; String s4 = "happy birthday"; Fig. 14.3 | (Part 1 of 2.) System.out.printf( "s1 = %s%ns2 = %s%ns3 = %s%ns4 = %s%n%n", s1, s2, s3, s4); // test for equality if (s1.equals("hello")) // true System.out.println("s1 equals \"hello\""); else System.out.println("s1 does not equal \"hello\""); // test for equality with == if (s1 == "hello") // false; they are not the same object System.out.println("s1 is the same object as \"hello\""); else System.out.println("s1 is not the same object as \"hello\""); // test for equality (ignore case) if (s3.equalsIgnoreCase(s4)) // true System.out.printf("%s equals %s with case ignored%n", s3, s4); else System.out.println("s3 does not equal s4"); // test compareTo System.out.printf( "%ns1.compareTo(s2) System.out.printf( "%ns2.compareTo(s1) System.out.printf( "%ns1.compareTo(s1) System.out.printf( "%ns3.compareTo(s4) System.out.printf( "%ns4.compareTo(s3) String is %d", s1.compareTo(s2)); is %d", s2.compareTo(s1)); is %d", s1.compareTo(s1)); is %d", s3.compareTo(s4)); is %d%n%n", s4.compareTo(s3)); methods equals, equalsIgnoreCase, compareTo and regionMatches. 602 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 s1 s2 s3 s4 Chapter 14 Strings, Characters and Regular Expressions // test regionMatches (case sensitive) if (s3.regionMatches(0, s4, 0, 5)) System.out.println("First 5 characters of s3 and s4 match"); else System.out.println( "First 5 characters of s3 and s4 do not match"); // test regionMatches (ignore case) if (s3.regionMatches(true, 0, s4, 0, 5)) System.out.println( "First 5 characters of s3 and s4 match with case ignored"); else System.out.println( "First 5 characters of s3 and s4 do not match"); } } // end class StringCompare = = = = hello goodbye Happy Birthday happy birthday s1 equals "hello" s1 is not the same object as "hello" Happy Birthday equals happy birthday with case ignored s1.compareTo(s2) s2.compareTo(s1) s1.compareTo(s1) s3.compareTo(s4) s4.compareTo(s3) is is is is is 1 -1 0 -32 32 First 5 characters of s3 and s4 do not match First 5 characters of s3 and s4 match with case ignored Fig. 14.3 | String methods equals, equalsIgnoreCase, compareTo and regionMatches. (Part 2 of 2.) Method equals The condition at line 17 uses method equals to compare String s1 and the String literal "hello" for equality. Method equals (a method of class Object overridden in String) tests any two objects for equality—the strings contained in the two objects are identical. The method returns true if the contents of the objects are equal, and false otherwise. The preceding condition is true because String s1 was initialized with the string literal "hello". Method equals uses a lexicographical comparison—it compares the integer Unicode values (see online Appendix H for more information) that represent each character in each String. Thus, if the String "hello" is compared to the string "HELLO", the result is false, because the integer representation of a lowercase letter is different from that of the corresponding uppercase letter. String 14.3 Class String 603 Comparing Strings with the == Operator The condition at line 23 uses the equality operator == to compare String s1 for equality with the String literal "hello". When primitive-type values are compared with ==, the result is true if both values are identical. When references are compared with ==, the result is true if both references refer to the same object in memory. To compare the actual contents (or state information) of objects for equality, a method must be invoked. In the case of Strings, that method is equals. The preceding condition evaluates to false at line 23 because the reference s1 was initialized with the statement s1 = new String("hello"); which creates a new String object with a copy of string literal "hello" and assigns the new object to variable s1. If s1 had been initialized with the statement s1 = "hello"; which directly assigns the string literal "hello" to variable s1, the condition would be Remember that Java treats all string literal objects with the same contents as one String object to which there can be many references. Thus, lines 8, 17 and 23 all refer to the same String object "hello" in memory. true. Common Programming Error 14.1 Comparing references with == can lead to logic errors, because == compares the references to determine whether they refer to the same object, not whether two objects have the same contents. When two separate objects that contain the same values are compared with ==, the result will be false. When comparing objects to determine whether they have the same contents, use method equals. Method equalsIgnoreCase If you’re sorting Strings, you may compare them for equality with method equalsIgnoreCase, which ignores whether the letters in each String are uppercase or lowercase when performing the comparison. Thus, "hello" and "HELLO" compare as equal. Line 29 uses String method equalsIgnoreCase to compare String s3—Happy Birthday—for equality with String s4—happy birthday. The result of this comparison is true because the comparison ignores case. String Method compareTo Lines 35–44 use method compareTo to compare Strings. Method compareTo is declared in the Comparable interface and implemented in the String class. Line 36 compares String s1 to String s2. Method compareTo returns 0 if the Strings are equal, a negative number if the String that invokes compareTo is less than the String that’s passed as an argument and a positive number if the String that invokes compareTo is greater than the String that’s passed as an argument. Method compareTo uses a lexicographical comparison—it compares the numeric values of corresponding characters in each String. String Method regionMatches The condition at line 47 uses a version of String method regionMatches to compare portions of two Strings for equality. The first argument to this version of the method is the starting index in the String that invokes the method. The second argument is a comparString 604 Chapter 14 Strings, Characters and Regular Expressions ison String. The third argument is the starting index in the comparison String. The last argument is the number of characters to compare between the two Strings. The method returns true only if the specified number of characters are lexicographically equal. Finally, the condition at line 54 uses a five-argument version of String method regionMatches to compare portions of two Strings for equality. When the first argument is true, the method ignores the case of the characters being compared. The remaining arguments are identical to those described for the four-argument regionMatches method. Methods startsWith and endsWith The next example (Fig. 14.4) demonstrates String methods startsWith and endsWith. Method main creates array strings containing "started", "starting", "ended" and "ending". The remainder of method main consists of three for statements that test the elements of the array to determine whether they start with or end with a particular set of characters. String 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 // Fig. 14.4: StringStartEnd.java // String methods startsWith and endsWith. public class StringStartEnd { public static void main(String[] args) { String[] strings = {"started", "starting", "ended", "ending"}; // test method startsWith for (String string : strings) { if (string.startsWith("st")) System.out.printf("\"%s\" starts with \"st\"%n", string); } System.out.println(); // test method startsWith starting from position 2 of string for (String string : strings) { if (string.startsWith("art", 2)) System.out.printf( "\"%s\" starts with \"art\" at position 2%n", string); } System.out.println(); // test method endsWith for (String string : strings) { if (string.endsWith("ed")) System.out.printf("\"%s\" ends with \"ed\"%n", string); } } } // end class StringStartEnd Fig. 14.4 | String methods startsWith and endsWith. (Part 1 of 2.) 14.3 Class String 605 "started" starts with "st" "starting" starts with "st" "started" starts with "art" at position 2 "starting" starts with "art" at position 2 "started" ends with "ed" "ended" ends with "ed" Fig. 14.4 | String methods startsWith and endsWith. (Part 2 of 2.) Lines 11–15 use the version of method startsWith that takes a String argument. The condition in the if statement (line 13) determines whether each String in the array starts with the characters "st". If so, the method returns true and the application prints that String. Otherwise, the method returns false and nothing happens. Lines 20–25 use the startsWith method that takes a String and an integer as arguments. The integer specifies the index at which the comparison should begin in the String. The condition in the if statement (line 22) determines whether each String in the array has the characters "art" beginning with the third character in each String. If so, the method returns true and the application prints the String. The third for statement (lines 30–34) uses method endsWith, which takes a String argument. The condition at line 32 determines whether each String in the array ends with the characters "ed". If so, the method returns true and the application prints the String. 14.3.4 Locating Characters and Substrings in Strings Often it’s useful to search a string for a character or set of characters. For example, if you’re creating your own word processor, you might want to provide a capability for searching through documents. Figure 14.5 demonstrates the many versions of String methods indexOf and lastIndexOf that search for a specified character or substring in a String. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 // Fig. 14.5: StringIndexMethods.java // String searching methods indexOf and lastIndexOf. public class StringIndexMethods { public static void main(String[] args) { String letters = "abcdefghijklmabcdefghijklm"; // test indexOf to System.out.printf( "'c' is located System.out.printf( "'a' is located System.out.printf( "'$' is located locate a character in a string at index %d%n", letters.indexOf('c')); at index %d%n", letters.indexOf('a', 1)); at index %d%n%n", letters.indexOf('$')); Fig. 14.5 | String-searching methods indexOf and lastIndexOf. (Part 1 of 2.) 606 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 Chapter 14 Strings, Characters and Regular Expressions // test lastIndexOf to find a character in a string System.out.printf("Last 'c' is located at index %d%n", letters.lastIndexOf('c')); System.out.printf("Last 'a' is located at index %d%n", letters.lastIndexOf('a', 25)); System.out.printf("Last '$' is located at index %d%n%n", letters.lastIndexOf('$')); // test indexOf to locate a substring in a string System.out.printf("\"def\" is located at index %d%n", letters.indexOf("def")); System.out.printf("\"def\" is located at index %d%n", letters.indexOf("def", 7)); System.out.printf("\"hello\" is located at index %d%n%n", letters.indexOf("hello")); // test lastIndexOf to find a substring in a string System.out.printf("Last \"def\" is located at index %d%n", letters.lastIndexOf("def")); System.out.printf("Last \"def\" is located at index %d%n", letters.lastIndexOf("def", 25)); System.out.printf("Last \"hello\" is located at index %d%n", letters.lastIndexOf("hello")); } } // end class StringIndexMethods 'c' is located at index 2 'a' is located at index 13 '$' is located at index -1 Last 'c' is located at index 15 Last 'a' is located at index 13 Last '$' is located at index -1 "def" is located at index 3 "def" is located at index 16 "hello" is located at index -1 Last "def" is located at index 16 Last "def" is located at index 16 Last "hello" is located at index -1 Fig. 14.5 | String-searching methods indexOf and lastIndexOf. (Part 2 of 2.) All the searches in this example are performed on the String letters (initialized with Lines 11–16 use method indexOf to locate the first occurrence of a character in a String. If the method finds the character, it returns the character’s index in the String—otherwise, it returns –1. There are two versions of indexOf that search for characters in a String. The expression in line 12 uses the version of method indexOf that takes an integer representation of the character to find. The expression at line 14 uses another version of method indexOf, which takes two integer arguments—the character and the starting index at which the search of the String should begin. "abcdefghijklmabcdefghijklm"). 14.3 Class String 607 Lines 19–24 use method lastIndexOf to locate the last occurrence of a character in a String. The method searches from the end of the String toward the beginning. If it finds the character, it returns the character’s index in the String—otherwise, it returns –1. There are two versions of lastIndexOf that search for characters in a String. The expression at line 20 uses the version that takes the integer representation of the character. The expression at line 22 uses the version that takes two integer arguments—the integer representation of the character and the index from which to begin searching backward. Lines 27–40 demonstrate versions of methods indexOf and lastIndexOf that each take a String as the first argument. These versions perform identically to those described earlier except that they search for sequences of characters (or substrings) that are specified by their String arguments. If the substring is found, these methods return the index in the String of the first character in the substring. 14.3.5 Extracting Substrings from Strings Class String provides two substring methods to enable a new String object to be created by copying part of an existing String object. Each method returns a new String object. Both methods are demonstrated in Fig. 14.6. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 // Fig. 14.6: SubString.java // String class substring methods. public class SubString { public static void main(String[] args) { String letters = "abcdefghijklmabcdefghijklm"; // test substring methods System.out.printf("Substring from index 20 to end is \"%s\"%n", letters.substring(20)); System.out.printf("%s \"%s\"%n", "Substring from index 3 up to, but not including 6 is", letters.substring(3, 6)); } } // end class SubString Substring from index 20 to end is "hijklm" Substring from index 3 up to, but not including 6 is "def" Fig. 14.6 | String class substring methods. The expression letters.substring(20) at line 12 uses the substring method that takes one integer argument. The argument specifies the starting index in the original String letters from which characters are to be copied. The substring returned contains a copy of the characters from the starting index to the end of the String. Specifying an index outside the bounds of the String causes a StringIndexOutOfBoundsException. Line 15 uses the substring method that takes two integer arguments—the starting index from which to copy characters in the original String and the index one beyond the 608 Chapter 14 Strings, Characters and Regular Expressions last character to copy (i.e., copy up to, but not including, that index in the String). The substring returned contains a copy of the specified characters from the original String. An index outside the bounds of the String causes a StringIndexOutOfBoundsException. 14.3.6 Concatenating Strings method concat (Fig. 14.7) concatenates two String objects (similar to using the + operator) and returns a new String object containing the characters from both original Strings. The expression s1.concat(s2) at line 13 forms a String by appending the characters in s2 to the those in s1. The original Strings to which s1 and s2 refer are not modified. String 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 // Fig. 14.7: StringConcatenation.java // String method concat. public class StringConcatenation { public static void main(String[] args) { String s1 = "Happy "; String s2 = "Birthday"; System.out.printf("s1 = %s%ns2 = %s%n%n",s1, s2); System.out.printf( "Result of s1.concat(s2) = %s%n", s1.concat(s2)); System.out.printf("s1 after concatenation = %s%n", s1); } } // end class StringConcatenation s1 = Happy s2 = Birthday Result of s1.concat(s2) = Happy Birthday s1 after concatenation = Happy Fig. 14.7 | String method concat. 14.3.7 Miscellaneous String Methods Class String provides several methods that return Strings or character arrays containing copies of an original String’s contents which are then modified. These methods—none of which modify the String on which they’re called—are demonstrated in Fig. 14.8. 1 2 3 4 5 6 7 // Fig. 14.8: StringMiscellaneous2.java // String methods replace, toLowerCase, toUpperCase, trim and toCharArray. public class StringMiscellaneous2 { public static void main(String[] args) { Fig. 14.8 | (Part 1 of 2.) String methods replace, toLowerCase, toUpperCase, trim and toCharArray. 14.3 Class String 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 String s1 = "hello"; String s2 = "GOODBYE"; String s3 = " spaces 609 "; System.out.printf("s1 = %s%ns2 = %s%ns3 = %s%n%n", s1, s2, s3); // test method replace System.out.printf( "Replace 'l' with 'L' in s1: %s%n%n", s1.replace('l', 'L')); // test toLowerCase and toUpperCase System.out.printf("s1.toUpperCase() = %s%n", s1.toUpperCase()); System.out.printf("s2.toLowerCase() = %s%n%n", s2.toLowerCase()); // test trim method System.out.printf("s3 after trim = \"%s\"%n%n", s3.trim()); // test toCharArray method char[] charArray = s1.toCharArray(); System.out.print("s1 as a character array = "); for (char character : charArray) System.out.print(character); System.out.println(); } } // end class StringMiscellaneous2 s1 = hello s2 = GOODBYE s3 = spaces Replace 'l' with 'L' in s1: heLLo s1.toUpperCase() = HELLO s2.toLowerCase() = goodbye s3 after trim = "spaces" s1 as a character array = hello Fig. 14.8 | String methods replace, toLowerCase, toUpperCase, trim and toCharArray. (Part 2 of 2.) Line 16 uses String method replace to return a new String object in which every occurrence in s1 of character 'l' (lowercase el) is replaced with character 'L'. Method replace leaves the original String unchanged. If there are no occurrences of the first argument in the String, method replace returns the original String. An overloaded version of method replace enables you to replace substrings rather than individual characters. Line 19 uses String method toUpperCase to generate a new String with uppercase letters where corresponding lowercase letters exist in s1. The method returns a new String object containing the converted String and leaves the original String unchanged. If there are no characters to convert, method toUpperCase returns the original String. 610 Chapter 14 Strings, Characters and Regular Expressions Line 20 uses String method toLowerCase to return a new String object with lowercase letters where corresponding uppercase letters exist in s2. The original String remains unchanged. If there are no characters in the original String to convert, toLowerCase returns the original String. Line 23 uses String method trim to generate a new String object that removes all white-space characters that appear at the beginning and/or end of the String on which trim operates. The method returns a new String object containing the String without leading or trailing white space. The original String remains unchanged. If there are no white-space characters at the beginning and/or end, trim returns the original String. Line 26 uses String method toCharArray to create a new character array containing a copy of the characters in s1. Lines 29–30 output each char in the array. 14.3.8 String Method valueOf As we’ve seen, every object in Java has a toString method that enables a program to obtain the object’s string representation. Unfortunately, this technique cannot be used with primitive types because they do not have methods. Class String provides static methods that take an argument of any type and convert it to a String object. Figure 14.9 demonstrates the String class valueOf methods. The expression String.valueOf(charArray) at line 18 uses the character array charArray to create a new String object. The expression String.valueOf(charArray, 3, 3) at line 20 uses a portion of the character array charArray to create a new String object. The second argument specifies the starting index from which the characters are used. The third argument specifies the number of characters to be used. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 // Fig. 14.9: StringValueOf.java // String valueOf methods. public class StringValueOf { public static void main(String[] args) { char[] charArray = {'a', 'b', 'c', 'd', 'e', 'f'}; boolean booleanValue = true; char characterValue = 'Z'; int integerValue = 7; long longValue = 10000000000L; // L suffix indicates long float floatValue = 2.5f; // f indicates that 2.5 is a float double doubleValue = 33.333; // no suffix, double is default Object objectRef = "hello"; // assign string to an Object reference Fig. 14.9 | System.out.printf( "char array = %s%n", String.valueOf(charArray)); System.out.printf("part of char array = %s%n", String.valueOf(charArray, 3, 3)); System.out.printf( "boolean = %s%n", String.valueOf(booleanValue)); System.out.printf( "char = %s%n", String.valueOf(characterValue)); String valueOf methods. (Part 1 of 2.) 14.4 Class StringBuilder 25 26 27 28 29 30 31 32 611 System.out.printf("int = %s%n", String.valueOf(integerValue)); System.out.printf("long = %s%n", String.valueOf(longValue)); System.out.printf("float = %s%n", String.valueOf(floatValue)); System.out.printf( "double = %s%n", String.valueOf(doubleValue)); System.out.printf("Object = %s", String.valueOf(objectRef)); } } // end class StringValueOf char array = abcdef part of char array = def boolean = true char = Z int = 7 long = 10000000000 float = 2.5 double = 33.333 Object = hello Fig. 14.9 | String valueOf methods. (Part 2 of 2.) There are seven other versions of method valueOf, which take arguments of type and Object, respectively. These are demonstrated in lines 21–30. The version of valueOf that takes an Object as an argument can do so because all Objects can be converted to Strings with method toString. [Note: Lines 12–13 use literal values 10000000000L and 2.5f as the initial values of long variable longValue and float variable floatValue, respectively. By default, Java treats integer literals as type int and floating-point literals as type double. Appending the letter L to the literal 10000000000 and appending letter f to the literal 2.5 indicates to the compiler that 10000000000 should be treated as a long and 2.5 as a float. An uppercase L or lowercase l can be used to denote a variable of type long and an uppercase F or lowercase f can be used to denote a variable of type float.] boolean, char, int, long, float, double 14.4 Class StringBuilder We now discuss the features of class StringBuilder for creating and manipulating dynamic string information—that is, modifiable strings. Every StringBuilder is capable of storing a number of characters specified by its capacity. If a StringBuilder’s capacity is exceeded, the capacity expands to accommodate the additional characters. Performance Tip 14.3 Java can perform certain optimizations involving String objects (such as referring to one String object from multiple variables) because it knows these objects will not change. Strings (not StringBuilders) should be used if the data will not change. Performance Tip 14.4 In programs that frequently perform string concatenation, or other string modifications, it’s often more efficient to implement the modifications with class StringBuilder. 612 Chapter 14 Strings, Characters and Regular Expressions Software Engineering Observation 14.1 StringBuilders are not thread safe. If multiple threads require access to the same dynamic string information, use class StringBuffer in your code. Classes StringBuilder and StringBuffer provide identical capabilities, but class StringBuffer is thread safe. For more details on threading, see Chapter 23. 14.4.1 StringBuilder Constructors Class StringBuilder provides four constructors. We demonstrate three of these in Fig. 14.10. Line 8 uses the no-argument StringBuilder constructor to create a StringBuilder with no characters in it and an initial capacity of 16 characters (the default for a StringBuilder). Line 9 uses the StringBuilder constructor that takes an integer argument to create a StringBuilder with no characters in it and the initial capacity specified by the integer argument (i.e., 10). Line 10 uses the StringBuilder constructor that takes a String argument to create a StringBuilder containing the characters in the String argument. The initial capacity is the number of characters in the String argument plus 16. Lines 12–14 implicitly use the method toString of class StringBuilder to output the StringBuilders with the printf method. In Section 14.4.4, we discuss how Java uses StringBuilder objects to implement the + and += operators for string concatenation. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 // Fig. 14.10: StringBuilderConstructors.java // StringBuilder constructors. public class StringBuilderConstructors { public static void main(String[] args) { StringBuilder buffer1 = new StringBuilder(); StringBuilder buffer2 = new StringBuilder(10); StringBuilder buffer3 = new StringBuilder("hello"); System.out.printf("buffer1 = \"%s\"%n", buffer1); System.out.printf("buffer2 = \"%s\"%n", buffer2); System.out.printf("buffer3 = \"%s\"%n", buffer3); } } // end class StringBuilderConstructors buffer1 = "" buffer2 = "" buffer3 = "hello" Fig. 14.10 | StringBuilder constructors. 14.4.2 StringBuilder Methods length, capacity, setLength and ensureCapacity Class StringBuilder provides methods length and capacity to return the number of characters currently in a StringBuilder and the number of characters that can be stored 14.4 Class StringBuilder in a 613 without allocating more memory, respectively. Method ensureguarantees that a StringBuilder has at least the specified capacity. Method setLength increases or decreases the length of a StringBuilder. Figure 14.11 demonstrates these methods. StringBuilder Capacity 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 // Fig. 14.11: StringBuilderCapLen.java // StringBuilder length, setLength, capacity and ensureCapacity methods. public class StringBuilderCapLen { public static void main(String[] args) { StringBuilder buffer = new StringBuilder("Hello, how are you?"); System.out.printf("buffer = %s%nlength = %d%ncapacity = %d%n%n", buffer.toString(), buffer.length(), buffer.capacity()); buffer.ensureCapacity(75); System.out.printf("New capacity = %d%n%n", buffer.capacity()); buffer.setLength(10)); System.out.printf("New length = %d%nbuffer = %s%n", buffer.length(), buffer.toString()); } } // end class StringBuilderCapLen buffer = Hello, how are you? length = 19 capacity = 35 New capacity = 75 New length = 10 buffer = Hello, how Fig. 14.11 | StringBuilder length, setLength, capacity and ensureCapacity methods. The application contains one StringBuilder called buffer. Line 8 uses the Stringconstructor that takes a String argument to initialize the StringBuilder with "Hello, how are you?". Lines 10–11 print the contents, length and capacity of the StringBuilder. Note in the output window that the capacity of the StringBuilder is initially 35. Recall that the StringBuilder constructor that takes a String argument initializes the capacity to the length of the string passed as an argument plus 16. Line 13 uses method ensureCapacity to expand the capacity of the StringBuilder to a minimum of 75 characters. Actually, if the original capacity is less than the argument, the method ensures a capacity that’s the greater of the number specified as an argument and twice the original capacity plus 2. The StringBuilder’s current capacity remains unchanged if it’s more than the specified capacity. Builder 614 Chapter 14 Strings, Characters and Regular Expressions Performance Tip 14.5 Dynamically increasing the capacity of a StringBuilder can take a relatively long time. Executing a large number of these operations can degrade the performance of an application. If a StringBuilder is going to increase greatly in size, possibly multiple times, setting its capacity high at the beginning will increase performance. Line 16 uses method setLength to set the length of the StringBuilder to 10. If the specified length is less than the current number of characters in the StringBuilder, its contents are truncated to the specified length (i.e., the characters in the StringBuilder after the specified length are discarded). If the specified length is greater than the number of characters currently in the StringBuilder, null characters (characters with the numeric representation 0) are appended until the total number of characters in the StringBuilder is equal to the specified length. 14.4.3 StringBuilder Methods charAt, setCharAt, getChars and reverse Class StringBuilder provides methods charAt, setCharAt, getChars and reverse to manipulate the characters in a StringBuilder (Fig. 14.12). Method charAt (line 12) takes an integer argument and returns the character in the StringBuilder at that index. Method getChars (line 15) copies characters from a StringBuilder into the character array passed as an argument. This method takes four arguments—the starting index from which characters should be copied in the StringBuilder, the index one past the last character to be copied from the StringBuilder, the character array into which the characters are to be copied and the starting location in the character array where the first character should be placed. Method setCharAt (lines 21 and 22) takes an integer and a character argument and sets the character at the specified position in the StringBuilder to the character argument. Method reverse (line 25) reverses the contents of the StringBuilder. Attempting to access a character that’s outside the bounds of a StringBuilder results in a StringIndexOutOfBoundsException. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 // Fig. 14.12: StringBuilderChars.java // StringBuilder methods charAt, setCharAt, getChars and reverse. public class StringBuilderChars { public static void main(String[] args) { StringBuilder buffer = new StringBuilder("hello there"); System.out.printf("buffer = %s%n", buffer.toString()); System.out.printf("Character at 0: %s%nCharacter at 4: %s%n%n", buffer.charAt(0), buffer.charAt(4)); char[] charArray = new char[buffer.length()]; buffer.getChars(0, buffer.length(), charArray, 0); System.out.print("The characters are: "); Fig. 14.12 | of 2.) StringBuilder methods charAt, setCharAt, getChars and reverse. (Part 1 14.4 Class StringBuilder 17 18 19 20 21 22 23 24 25 26 27 28 615 for (char character : charArray) System.out.print(character); buffer.setCharAt(0, 'H'); buffer.setCharAt(6, 'T'); System.out.printf("%n%nbuffer = %s", buffer.toString()); buffer.reverse(); System.out.printf("%n%nbuffer = %s%n", buffer.toString()); } } // end class StringBuilderChars buffer = hello there Character at 0: h Character at 4: o The characters are: hello there buffer = Hello There buffer = erehT olleH Fig. 14.12 | StringBuilder methods charAt, setCharAt, getChars and reverse. (Part 2 of 2.) 14.4.4 StringBuilder append Methods Class StringBuilder provides overloaded append methods (Fig. 14.13) to allow values of various types to be appended to the end of a StringBuilder. Versions are provided for each of the primitive types and for character arrays, Strings, Objects, and more. (Remember that method toString produces a string representation of any Object.) Each method takes its argument, converts it to a string and appends it to the StringBuilder. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 // Fig. 14.13: StringBuilderAppend.java // StringBuilder append methods. public class StringBuilderAppend { public static void main(String[] args) { Object objectRef = "hello"; String string = "goodbye"; char[] charArray = {'a', 'b', 'c', 'd', 'e', 'f'}; boolean booleanValue = true; char characterValue = 'Z'; int integerValue = 7; long longValue = 10000000000L; float floatValue = 2.5f; double doubleValue = 33.333; Fig. 14.13 | StringBuilder append methods. (Part 1 of 2.) 616 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 Chapter 14 Strings, Characters and Regular Expressions StringBuilder lastBuffer = new StringBuilder("last buffer"); StringBuilder buffer = new StringBuilder(); buffer.append(objectRef) .append("%n") .append(string) .append("%n") .append(charArray) .append("%n") .append(charArray, 0, 3) .append("%n") .append(booleanValue) .append("%n") .append(characterValue); .append("%n") .append(integerValue) .append("%n") .append(longValue) .append("%n") .append(floatValue) .append("%n") .append(doubleValue) .append("%n") .append(lastBuffer); System.out.printf("buffer contains%n%s%n", buffer.toString()); } } // end StringBuilderAppend buffer contains hello goodbye abcdef abc true Z 7 10000000000 2.5 33.333 last buffer Fig. 14.13 | StringBuilder append methods. (Part 2 of 2.) The compiler can use StringBuilder and the append methods to implement the and += String concatenation operators. For example, assuming the declarations String string1 = "hello"; String string2 = "BC"; int value = 22; the statement String s = string1 + string2 + value; + 14.4 Class StringBuilder 617 concatenates "hello", "BC" and 22. The concatenation can be performed as follows: String s = new StringBuilder().append("hello").append("BC"). append(22).toString(); First, the preceding statement creates an empty StringBuilder, then appends to it the strings "hello" and "BC" and the integer 22. Next, StringBuilder’s toString method converts the StringBuilder to a String object to be assigned to String s. The statement s += "!"; can be performed as follows (this may differ by compiler): s = new StringBuilder().append(s).append("!").toString(); This creates an empty StringBuilder, then appends to it the current contents of s followed by "!". Next, StringBuilder’s method toString (which must be called explicitly here) returns the StringBuilder’s contents as a String, and the result is assigned to s. 14.4.5 StringBuilder Insertion and Deletion Methods provides overloaded insert methods to insert values of various types at any position in a StringBuilder. Versions are provided for the primitive types and for character arrays, Strings, Objects and CharSequences. Each method takes its second argument and inserts it at the index specified by the first argument. If the first argument is less than 0 or greater than the StringBuilder’s length, a StringIndexOutOfBoundsException occurs. Class StringBuilder also provides methods delete and deleteCharAt to delete characters at any position in a StringBuilder. Method delete takes two arguments—the starting index and the index one past the end of the characters to delete. All characters beginning at the starting index up to but not including the ending index are deleted. Method deleteCharAt takes one argument—the index of the character to delete. Invalid indices cause both methods to throw a StringIndexOutOfBoundsException. Figure 14.14 demonstrates methods insert, delete and deleteCharAt. StringBuilder 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 // Fig. 14.14: StringBuilderInsertDelete.java // StringBuilder methods insert, delete and deleteCharAt. public class StringBuilderInsertDelete { public static void main(String[] args) { Object objectRef = "hello"; String string = "goodbye"; char[] charArray = {'a', 'b', 'c', 'd', 'e', 'f'}; boolean booleanValue = true; char characterValue = 'K'; int integerValue = 7; long longValue = 10000000; float floatValue = 2.5f; // f suffix indicates that 2.5 is a float double doubleValue = 33.333; Fig. 14.14 | StringBuilder methods insert, delete and deleteCharAt. (Part 1 of 2.) 618 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 Chapter 14 Strings, Characters and Regular Expressions StringBuilder buffer = new StringBuilder(); buffer.insert(0, buffer.insert(0, buffer.insert(0, buffer.insert(0, buffer.insert(0, buffer.insert(0, buffer.insert(0, buffer.insert(0, buffer.insert(0, buffer.insert(0, buffer.insert(0, buffer.insert(0, buffer.insert(0, buffer.insert(0, buffer.insert(0, buffer.insert(0, buffer.insert(0, buffer.insert(0, buffer.insert(0, objectRef); " "); // each of these contains two spaces string); " "); charArray); " "); charArray, 3, 3); " "); booleanValue); " "); characterValue); " "); integerValue); " "); longValue); " "); floatValue); " "); doubleValue); System.out.printf( "buffer after inserts:%n%s%n%n", buffer.toString()); buffer.deleteCharAt(10); // delete 5 in 2.5 buffer.delete(2, 6); // delete .333 in 33.333 System.out.printf( "buffer after deletes:%n%s%n", buffer.toString()); } } // end class StringBuilderInsertDelete buffer after inserts: 33.333 2.5 10000000 buffer after deletes: 33 2. 10000000 7 K Fig. 14.14 | 7 K true StringBuilder true def def abcdef abcdef goodbye goodbye hello hello methods insert, delete and deleteCharAt. (Part 2 of 2.) 14.5 Class Character Java provides eight type-wrapper classes—Boolean, Character, Double, Float, Byte, Short, Integer and Long—that enable primitive-type values to be treated as objects. In this section, we present class Character—the type-wrapper class for primitive type char. Most Character methods are static methods designed for convenience in processing individual char values. These methods take at least a character argument and perform either a test or a manipulation of the character. Class Character also contains a constructor that receives a char argument to initialize a Character object. Most of the methods of class Character are presented in the next three examples. For more informa- 14.5 Class Character 619 tion on class Character (and all the type-wrapper classes), see the java.lang package in the Java API documentation. Figure 14.15 demonstrates static methods that test characters to determine whether they’re a specific character type and the static methods that perform case conversions on characters. You can enter any character and apply the methods to the character. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 // Fig. 14.15: StaticCharMethods.java // Character static methods for testing characters and converting case. import java.util.Scanner; public class StaticCharMethods { public static void main(String[] args) { Scanner scanner = new Scanner(System.in); // create scanner System.out.println("Enter a character and press Enter"); String input = scanner.next(); char c = input.charAt(0); // get input character // display character info System.out.printf("is defined: %b%n", Character.isDefined(c)); System.out.printf("is digit: %b%n", Character.isDigit(c)); System.out.printf("is first character in a Java identifier: %b%n", Character.isJavaIdentifierStart(c)); System.out.printf("is part of a Java identifier: %b%n", Character.isJavaIdentifierPart(c)); System.out.printf("is letter: %b%n", Character.isLetter(c)); System.out.printf( "is letter or digit: %b%n", Character.isLetterOrDigit(c)); System.out.printf( "is lower case: %b%n", Character.isLowerCase(c)); System.out.printf( "is upper case: %b%n", Character.isUpperCase(c)); System.out.printf( "to upper case: %s%n", Character.toUpperCase(c)); System.out.printf( "to lower case: %s%n", Character.toLowerCase(c)); } } // end class StaticCharMethods Enter a character and press Enter A is defined: true is digit: false is first character in a Java identifier: true is part of a Java identifier: true is letter: true is letter or digit: true is lower case: false is upper case: true to upper case: A to lower case: a Fig. 14.15 | Character static methods for testing characters and converting case. (Part 1 of 2.) 620 Chapter 14 Strings, Characters and Regular Expressions Enter a character and press Enter 8 is defined: true is digit: true is first character in a Java identifier: false is part of a Java identifier: true is letter: false is letter or digit: true is lower case: false is upper case: false to upper case: 8 to lower case: 8 Enter a character and press Enter $ is defined: true is digit: false is first character in a Java identifier: true is part of a Java identifier: true is letter: false is letter or digit: false is lower case: false is upper case: false to upper case: $ to lower case: $ Fig. 14.15 | Character static methods for testing characters and converting case. (Part 2 of 2.) Line 15 uses Character method isDefined to determine whether character c is defined in the Unicode character set. If so, the method returns true; otherwise, it returns false. Line 16 uses Character method isDigit to determine whether character c is a defined Unicode digit. If so, the method returns true, and otherwise, false. Line 18 uses Character method isJavaIdentifierStart to determine whether c is a character that can be the first character of an identifier in Java—that is, a letter, an underscore (_) or a dollar sign ($). If so, the method returns true, and otherwise, false. Line 20 uses Character method isJavaIdentifierPart to determine whether character c is a character that can be used in an identifier in Java—that is, a digit, a letter, an underscore (_) or a dollar sign ($). If so, the method returns true, and otherwise, false. Line 21 uses Character method isLetter to determine whether character c is a letter. If so, the method returns true, and otherwise, false. Line 23 uses Character method isLetterOrDigit to determine whether character c is a letter or a digit. If so, the method returns true, and otherwise, false. Line 25 uses Character method isLowerCase to determine whether character c is a lowercase letter. If so, the method returns true, and otherwise, false. Line 27 uses Character method isUpperCase to determine whether character c is an uppercase letter. If so, the method returns true, and otherwise, false. Line 29 uses Character method toUpperCase to convert the character c to its uppercase equivalent. The method returns the converted character if the character has an upper- 14.5 Class Character 621 case equivalent, and otherwise, the method returns its original argument. Line 31 uses method toLowerCase to convert the character c to its lowercase equivalent. The method returns the converted character if the character has a lowercase equivalent, and otherwise, the method returns its original argument. Figure 14.16 demonstrates static Character methods digit and forDigit, which convert characters to digits and digits to characters, respectively, in different number systems. Common number systems include decimal (base 10), octal (base 8), hexadecimal (base 16) and binary (base 2). The base of a number is also known as its radix. For more information on conversions between number systems, see online Appendix J. Character 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 // Fig. 14.16: StaticCharMethods2.java // Character class static conversion methods. import java.util.Scanner; public class StaticCharMethods2 { // executes application public static void main(String[] args) { Scanner scanner = new Scanner(System.in); // get radix System.out.println("Please enter a radix:"); int radix = scanner.nextInt(); // get user choice System.out.printf("Please choose one:%n1 -- %s%n2 -- %s%n", "Convert digit to character", "Convert character to digit"); int choice = scanner.nextInt(); // process request switch (choice) { case 1: // convert digit to character System.out.println("Enter a digit:"); int digit = scanner.nextInt(); System.out.printf("Convert digit to character: %s%n", Character.forDigit(digit, radix)); break; case 2: // convert character to digit System.out.println("Enter a character:"); char character = scanner.next().charAt(0); System.out.printf("Convert character to digit: %s%n", Character.digit(character, radix)); break; } } } // end class StaticCharMethods2 Fig. 14.16 | Character class static conversion methods. (Part 1 of 2.) 622 Chapter 14 Strings, Characters and Regular Expressions Please enter a radix: 16 Please choose one: 1 -- Convert digit to character 2 -- Convert character to digit 2 Enter a character: A Convert character to digit: 10 Please enter a radix: 16 Please choose one: 1 -- Convert digit to character 2 -- Convert character to digit 1 Enter a digit: 13 Convert digit to character: d Fig. 14.16 | Character class static conversion methods. (Part 2 of 2.) Line 28 uses method forDigit to convert the integer digit into a character in the number system specified by the integer radix (the base of the number). For example, the decimal integer 13 in base 16 (the radix) has the character value 'd'. Lowercase and uppercase letters represent the same value in number systems. Line 35 uses method digit to convert variable character into an integer in the number system specified by the integer radix (the base of the number). For example, the character 'A' is the base 16 (the radix) representation of the base 10 value 10. The radix must be between 2 and 36, inclusive. Figure 14.17 demonstrates the constructor and several instance methods of class Character—charValue, toString and equals. Lines 7–8 instantiate two Character objects by assigning the character constants 'A' and 'a', respectively, to the Character variables. Java automatically converts these char literals into Character objects—a process known as autoboxing that we discuss in more detail in Section 16.4. Line 11 uses Character method charValue to return the char value stored in Character object c1. Line 11 returns a string representation of Character object c2 using method toString. The condition in line 13 uses method equals to determine whether the object c1 has the same contents as the object c2 (i.e., the characters inside each object are equal). 1 2 3 4 5 6 7 8 // Fig. 14.17: OtherCharMethods.java // Character class instance methods. public class OtherCharMethods { public static void main(String[] args) { Character c1 = 'A'; Character c2 = 'a'; Fig. 14.17 | Character class instance methods. (Part 1 of 2.) 14.6 Tokenizing Strings 9 10 11 12 13 14 15 16 17 18 623 System.out.printf( "c1 = %s%nc2 = %s%n%n", c1.charValue(), c2.toString()); if (c1.equals(c2)) System.out.println("c1 and c2 are equal%n"); else System.out.println("c1 and c2 are not equal%n"); } } // end class OtherCharMethods c1 = A c2 = a c1 and c2 are not equal Fig. 14.17 | Character class instance methods. (Part 2 of 2.) 14.6 Tokenizing Strings When you read a sentence, your mind breaks it into tokens—individual words and punctuation marks that convey meaning to you. Compilers also perform tokenization. They break up statements into individual pieces like keywords, identifiers, operators and other programming-language elements. We now study class String’s split method, which breaks a String into its component tokens. Tokens are separated from one another by delimiters, typically white-space characters such as space, tab, newline and carriage return. Other characters can also be used as delimiters to separate tokens. The application in Fig. 14.18 demonstrates String’s split method. When the user presses the Enter key, the input sentence is stored in variable sentence. Line 17 invokes String method split with the String argument " ", which returns an array of Strings. The space character in the argument String is the delimiter that method split uses to locate the tokens in the String. As you’ll learn in the next section, the argument to method split can be a regular expression for more complex tokenizing. Line 19 displays the length of the array tokens—i.e., the number of tokens in sentence. Lines 21– 22 output each token on a separate line. 1 2 3 4 5 6 7 8 9 10 11 12 // Fig. 14.18: TokenTest.java // StringTokenizer object used to tokenize strings. import java.util.Scanner; import java.util.StringTokenizer; public class TokenTest { // execute application public static void main(String[] args) { // get sentence Scanner scanner = new Scanner(System.in); Fig. 14.18 | StringTokenizer object used to tokenize strings. (Part 1 of 2.) 624 13 14 15 16 17 18 19 20 21 22 23 24 Chapter 14 Strings, Characters and Regular Expressions System.out.println("Enter a sentence and press Enter"); String sentence = scanner.nextLine(); // process user sentence String[] tokens = sentence.split(" "); System.out.printf("Number of elements: %d%nThe tokens are:%n", tokens.length); for (String token : tokens) System.out.println(token); } } // end class TokenTest Enter a sentence and press Enter This is a sentence with seven tokens Number of elements: 7 The tokens are: This is a sentence with seven tokens Fig. 14.18 | StringTokenizer object used to tokenize strings. (Part 2 of 2.) 14.7 Regular Expressions, Class Pattern and Class Matcher A regular expression is a String that describes a search pattern for matching characters in other Strings. Such expressions are useful for validating input and ensuring that data is in a particular format. For example, a ZIP code must consist of five digits, and a last name must contain only letters, spaces, apostrophes and hyphens. One application of regular expressions is to facilitate the construction of a compiler. Often, a large and complex regular expression is used to validate the syntax of a program. If the program code does not match the regular expression, the compiler knows that there’s a syntax error in the code. Class String provides several methods for performing regular-expression operations, the simplest of which is the matching operation. String method matches receives a String that specifies the regular expression and matches the contents of the String object on which it’s called to the regular expression. The method returns a boolean indicating whether the match succeeded. A regular expression consists of literal characters and special symbols. Figure 14.19 specifies some predefined character classes that can be used with regular expressions. A character class is an escape sequence that represents a group of characters. A digit is any numeric character. A word character is any letter (uppercase or lowercase), any digit or the underscore character. A white-space character is a space, a tab, a carriage return, a newline or a form feed. Each character class matches a single character in the String we’re attempting to match with the regular expression. 14.7 Regular Expressions, Class Pattern and Class Matcher Character Matches Character Matches \d any digit any word character any white-space character \D any nondigit any nonword character any non-whitespace character \w \s \W \S 625 Fig. 14.19 | Predefined character classes. Regular expressions are not limited to these predefined character classes. The expressions employ various operators and other forms of notation to match complex patterns. We examine several of these techniques in the application in Figs. 14.20 and 14.21, which validates user input via regular expressions. [Note: This application is not designed to match all possible valid user input.] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 // Fig. 14.20: ValidateInput.java // Validating user information using regular expressions. public class ValidateInput { // validate first name public static boolean validateFirstName(String firstName) { return firstName.matches("[A-Z][a-zA-Z]*"); } // validate last name public static boolean validateLastName(String lastName) { return lastName.matches("[a-zA-z]+(['-][a-zA-Z]+)*"); } // validate address public static boolean validateAddress(String address) { return address.matches( "\\d+\\s+([a-zA-Z]+|[a-zA-Z]+\\s[a-zA-Z]+)"); } // validate city public static boolean validateCity(String city) { return city.matches("([a-zA-Z]+|[a-zA-Z]+\\s[a-zA-Z]+)"); } // validate state public static boolean validateState(String state) { Fig. 14.20 | Validating user information using regular expressions. (Part 1 of 2.) 626 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 Chapter 14 Strings, Characters and Regular Expressions return state.matches("([a-zA-Z]+|[a-zA-Z]+\\s[a-zA-Z]+)"); } // validate zip public static boolean validateZip(String zip) { return zip.matches("\\d{5}"); } // validate phone public static boolean validatePhone(String phone) { return phone.matches("[1-9]\\d{2}-[1-9]\\d{2}-\\d{4}"); } } // end class ValidateInput Fig. 14.20 | Validating user information using regular expressions. (Part 2 of 2.) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 // Fig. 14.21: Validate.java // Input and validate data from user using the ValidateInput class. import java.util.Scanner; public class Validate { public static void main(String[] args) { // get user input Scanner scanner = new Scanner(System.in); System.out.println("Please enter first name:"); String firstName = scanner.nextLine(); System.out.println("Please enter last name:"); String lastName = scanner.nextLine(); System.out.println("Please enter address:"); String address = scanner.nextLine(); System.out.println("Please enter city:"); String city = scanner.nextLine(); System.out.println("Please enter state:"); String state = scanner.nextLine(); System.out.println("Please enter zip:"); String zip = scanner.nextLine(); System.out.println("Please enter phone:"); String phone = scanner.nextLine(); // validate user input and display error message System.out.println("%nValidate Result:"); if (!ValidateInput.validateFirstName(firstName)) System.out.println("Invalid first name"); else if (!ValidateInput.validateLastName(lastName)) System.out.println("Invalid last name"); else if (!ValidateInput.validateAddress(address)) System.out.println("Invalid address"); Fig. 14.21 | Input and validate data from user using the ValidateInput class. (Part 1 of 2.) 14.7 Regular Expressions, Class Pattern and Class Matcher 35 36 37 38 39 40 41 42 43 44 45 46 627 else if (!ValidateInput.validateCity(city)) System.out.println("Invalid city"); else if (!ValidateInput.validateState(state)) System.out.println("Invalid state"); else if (!ValidateInput.validateZip(zip)) System.out.println("Invalid zip code"); else if (!ValidateInput.validatePhone(phone)) System.out.println("Invalid phone number"); else System.out.println("Valid input. Thank you."); } } // end class Validate Please enter first name: Jane Please enter last name: Doe Please enter address: 123 Some Street Please enter city: Some City Please enter state: SS Please enter zip: 123 Please enter phone: 123-456-7890 Validate Result: Invalid zip code Please enter first name: Jane Please enter last name: Doe Please enter address: 123 Some Street Please enter city: Some City Please enter state: SS Please enter zip: 12345 Please enter phone: 123-456-7890 Validate Result: Valid input. Thank you. Fig. 14.21 | Input and validate data from user using the ValidateInput class. (Part 2 of 2.) Figure 14.20 validates user input. Line 9 validates the first name. To match a set of characters that does not have a predefined character class, use square brackets, []. For example, the pattern "[aeiou]" matches a single character that’s a vowel. Character ranges 628 Chapter 14 Strings, Characters and Regular Expressions are represented by placing a dash (-) between two characters. In the example, "[A-Z]" matches a single uppercase letter. If the first character in the brackets is "^", the expression accepts any character other than those indicated. However, "[^Z]" is not the same as "[AY]", which matches uppercase letters A–Y—"[^Z]" matches any character other than capital Z, including lowercase letters and nonletters such as the newline character. Ranges in character classes are determined by the letters’ integer values. In this example, "[A-Za-z]" matches all uppercase and lowercase letters. The range "[A-z]" matches all letters and also matches those characters (such as [and \) with an integer value between uppercase Z and lowercase a (for more information on integer values of characters see Appendix B). Like predefined character classes, character classes delimited by square brackets match a single character in the search object. In line 9, the asterisk after the second character class indicates that any number of letters can be matched. In general, when the regular-expression operator "*" appears in a regular expression, the application attempts to match zero or more occurrences of the subexpression immediately preceding the "*". Operator "+" attempts to match one or more occurrences of the subexpression immediately preceding "+". So both "A*" and "A+" will match "AAA" or "A", but only "A*" will match an empty string. If method validateFirstName returns true (line 29 of Fig. 14.21), the application attempts to validate the last name (line 31) by calling validateLastName (lines 13–16 of Fig. 14.20). The regular expression to validate the last name matches any number of letters split by spaces, apostrophes or hyphens. Line 33 of Fig. 14.21 calls method validateAddress (lines 19–23 of Fig. 14.20) to validate the address. The first character class matches any digit one or more times (\\d+). Two \ characters are used, because \ normally starts an escape sequence in a string. So \\d in a String represents the regular-expression pattern \d. Then we match one or more white-space characters (\\s+). The character "|" matches the expression to its left or to its right. For example, "Hi (John|Jane)" matches both "Hi John" and "Hi Jane". The parentheses are used to group parts of the regular expression. In this example, the left side of | matches a single word, and the right side matches two words separated by any amount of white space. So the address must contain a number followed by one or two words. Therefore, "10 Broadway" and "10 Main Street" are both valid addresses in this example. The city (lines 26–29 of Fig. 14.20) and state (lines 32–35 of Fig. 14.20) methods also match any word of at least one character or, alternatively, any two words of at least one character if the words are separated by a single space, so both Waltham and West Newton would match. Quantifiers The asterisk (*) and plus (+) are formally called quantifiers. Figure 14.22 lists all the quantifiers. We’ve already discussed how the asterisk (*) and plus (+) quantifiers work. All quantifiers affect only the subexpression immediately preceding the quantifier. Quantifier question mark (?) matches zero or one occurrences of the expression that it quantifies. A set of braces containing one number ({n}) matches exactly n occurrences of the expression it quantifies. We demonstrate this quantifier to validate the zip code in Fig. 14.20 at line 40. Including a comma after the number enclosed in braces matches at least n occurrences of the quantified expression. The set of braces containing two numbers ({n,m}), matches between n and m occurrences of the expression that it qualifies. Quantifiers may be applied to patterns enclosed in parentheses to create more complex regular expressions. 14.7 Regular Expressions, Class Pattern and Class Matcher Quantifier Matches * Matches zero or more occurrences of the pattern. Matches one or more occurrences of the pattern. Matches zero or one occurrences of the pattern. Matches exactly n occurrences. Matches at least n occurrences. Matches between n and m (inclusive) occurrences. + ? {n} {n,} {n,m} 629 Fig. 14.22 | Quantifiers used in regular expressions. All of the quantifiers are greedy. This means that they’ll match as many occurrences as they can as long as the match is still successful. However, if any of these quantifiers is followed by a question mark (?), the quantifier becomes reluctant (sometimes called lazy). It then will match as few occurrences as possible as long as the match is still successful. The zip code (line 40 in Fig. 14.20) matches a digit five times. This regular expression uses the digit character class and a quantifier with the digit 5 between braces. The phone number (line 46 in Fig. 14.20) matches three digits (the first one cannot be zero) followed by a dash followed by three more digits (again the first one cannot be zero) followed by four more digits. String method matches checks whether an entire String conforms to a regular expression. For example, we want to accept "Smith" as a last name, but not "9@Smith#". If only a substring matches the regular expression, method matches returns false. Replacing Substrings and Splitting Strings Sometimes it’s useful to replace parts of a string or to split a string into pieces. For this purpose, class String provides methods replaceAll, replaceFirst and split. These methods are demonstrated in Fig. 14.23. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 // Fig. 14.23: RegexSubstitution.java // String methods replaceFirst, replaceAll and split. import java.util.Arrays; public class RegexSubstitution { public static void main(String[] args) { String firstString = "This sentence ends in 5 stars *****"; String secondString = "1, 2, 3, 4, 5, 6, 7, 8"; System.out.printf("Original String 1: %s%n", firstString); // replace '*' with '^' firstString = firstString.replaceAll("\\*", "^"); System.out.printf("^ substituted for *: %s%n", firstString); Fig. 14.23 | String methods replaceFirst, replaceAll and split. (Part 1 of 2.) 630 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 Chapter 14 Strings, Characters and Regular Expressions // replace 'stars' with 'carets' firstString = firstString.replaceAll("stars", "carets"); System.out.printf( "\"carets\" substituted for \"stars\": %s%n", firstString); // replace words with 'word' System.out.printf("Every word replaced by \"word\": %s%n%n", firstString.replaceAll("\\w+", "word")); System.out.printf("Original String 2: %s%n", secondString); // replace first three digits with 'digit' for (int i = 0; i < 3; i++) secondString = secondString.replaceFirst("\\d", "digit"); System.out.printf( "First 3 digits replaced by \"digit\" : %s%n", secondString); System.out.print("String split at commas: "); String[] results = secondString.split(",\\s*"); // split on commas System.out.println(Arrays.toString(results)); } } // end class RegexSubstitution Original String 1: This sentence ends in 5 stars ***** ^ substituted for *: This sentence ends in 5 stars ^^^^^ "carets" substituted for "stars": This sentence ends in 5 carets ^^^^^ Every word replaced by "word": word word word word word word ^^^^^ Original String 2: 1, 2, 3, 4, 5, 6, 7, 8 First 3 digits replaced by "digit" : digit, digit, digit, 4, 5, 6, 7, 8 String split at commas: ["digit", "digit", "digit", "4", "5", "6", "7", "8"] Fig. 14.23 | String methods replaceFirst, replaceAll and split. (Part 2 of 2.) Method replaceAll replaces text in a String with new text (the second argument) wherever the original String matches a regular expression (the first argument). Line 15 replaces every instance of "*" in firstString with "^". The regular expression ("\\*") precedes character * with two backslashes. Normally, * is a quantifier indicating that a regular expression should match any number of occurrences of a preceding pattern. However, in line 15, we want to find all occurrences of the literal character *—to do this, we must escape character * with character \. Escaping a special regular-expression character with \ instructs the matching engine to find the actual character. Since the expression is stored in a Java String and \ is a special character in Java Strings, we must include an additional \. So the Java String "\\*" represents the regular-expression pattern \* which matches a single * character in the search string. In line 20, every match for the regular expression "stars" in firstString is replaced with "carets". Line 27 uses replaceAll to replace all words in the string with "word". 14.7 Regular Expressions, Class Pattern and Class Matcher 631 Method replaceFirst (line 33) replaces the first occurrence of a pattern match. Java are immutable; therefore, method replaceFirst returns a new String in which the appropriate characters have been replaced. This line takes the original String and replaces it with the String returned by replaceFirst. By iterating three times we replace the first three instances of a digit (\d) in secondString with the text "digit". Method split divides a String into several substrings. The original is broken in any location that matches a specified regular expression. Method split returns an array of Strings containing the substrings between matches for the regular expression. In line 39, we use method split to tokenize a String of comma-separated integers. The argument is the regular expression that locates the delimiter. In this case, we use the regular expression ",\\s*" to separate the substrings wherever a comma occurs. By matching any whitespace characters, we eliminate extra spaces from the resulting substrings. The commas and white-space characters are not returned as part of the substrings. Again, the Java String ",\\s*" represents the regular expression ,\s*. Line 40 uses Arrays method toString to display the contents of array results in square brackets and separated by commas. Strings Classes Pattern and Matcher In addition to the regular-expression capabilities of class String, Java provides other classes in package java.util.regex that help developers manipulate regular expressions. Class Pattern represents a regular expression. Class Matcher contains both a regular-expression pattern and a CharSequence in which to search for the pattern. CharSequence (package java.lang) is an interface that allows read access to a sequence of characters. The interface requires that the methods charAt, length, subSequence and toString be declared. Both String and StringBuilder implement interface CharSequence, so an instance of either of these classes can be used with class Matcher. Common Programming Error 14.2 A regular expression can be tested against an object of any class that implements interface CharSequence, but the regular expression must be a String. Attempting to create a regular expression as a StringBuilder is an error. If a regular expression will be used only once, static Pattern method matches can be used. This method takes a String that specifies the regular expression and a CharSequence on which to perform the match. This method returns a boolean indicating whether the search object (the second argument) matches the regular expression. If a regular expression will be used more than once (in a loop, for example), it’s more efficient to use static Pattern method compile to create a specific Pattern object for that regular expression. This method receives a String representing the pattern and returns a new Pattern object, which can then be used to call method matcher. This method receives a CharSequence to search and returns a Matcher object. Matcher provides method matches, which performs the same task as Pattern method matches, but receives no arguments—the search pattern and search object are encapsulated in the Matcher object. Class Matcher provides other methods, including find, lookingAt, replaceFirst and replaceAll. Figure 14.24 presents a simple example that employs regular expressions. This program matches birthdays against a regular expression. The expression matches only birthdays that do not occur in April and that belong to people whose names begin with "J". 632 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Chapter 14 Strings, Characters and Regular Expressions // Fig. 14.24: RegexMatches.java // Classes Pattern and Matcher. import java.util.regex.Matcher; import java.util.regex.Pattern; public class RegexMatches { public static void main(String[] args) { // create regular expression Pattern expression = Pattern.compile("J.*\\d[0-35-9]-\\d\\d-\\d\\d"); String string1 = "Jane's Birthday is 05-12-75\n" + "Dave's Birthday is 11-04-68\n" + "John's Birthday is 04-28-73\n" + "Joe's Birthday is 12-17-77"; // match regular expression to string and print matches Matcher matcher = expression.matcher(string1); while (matcher.find()) System.out.println(matcher.group()); } } // end class RegexMatches Jane's Birthday is 05-12-75 Joe's Birthday is 12-17-77 Fig. 14.24 | Classes Pattern and Matcher. Lines 11–12 create a Pattern by invoking static Pattern method compile. The dot character "." in the regular expression (line 12) matches any single character except a newline character. Line 20 creates the Matcher object for the compiled regular expression and the matching sequence (string1). Lines 22–23 use a while loop to iterate through the String. Line 22 uses Matcher method find to attempt to match a piece of the search object to the search pattern. Each call to this method starts at the point where the last call ended, so multiple matches can be found. Matcher method lookingAt performs the same way, except that it always starts from the beginning of the search object and will always find the first match if there is one. Common Programming Error 14.3 Method matches (from class String, Pattern or Matcher) will return true only if the entire search object matches the regular expression. Methods find and lookingAt (from class Matcher) will return true if a portion of the search object matches the regular expression. Line 23 uses Matcher method group, which returns the String from the search object that matches the search pattern. The String that’s returned is the one that was last matched by a call to find or lookingAt. The output in Fig. 14.24 shows the two matches that were found in string1. 14.8 Wrap-Up 633 Java SE 8 As you’ll see in Section 17.7, you can combine regular-expression processing with Java SE 8 lambdas and streams to implement powerful String-and-file processing applications. 14.8 Wrap-Up In this chapter, you learned about more String methods for selecting portions of Strings and manipulating Strings. You learned about the Character class and some of the methods it declares to handle chars. The chapter also discussed the capabilities of the StringBuilder class for creating Strings. The end of the chapter discussed regular expressions, which provide a powerful capability to search and match portions of Strings that fit a particular pattern. In the next chapter, you’ll learn about file processing, including how persistent data is stored and and retrieved. Summary Section 14.2 Fundamentals of Characters and Strings • A character literal’s value (p. 597) is its integer value in Unicode (p. 597). Strings can include letters, digits and special characters such as +, -, *, / and $. A string in Java is an object of class String. String literals (p. 597) are often referred to as String objects and are written in a program in double quotes. Section 14.3 Class String • • • • • • • • • • • • • String objects are immutable (p. 599)—after they’re created, their character contents cannot be changed. String method length (p. 599) returns the number of characters in a String. String method charAt (p. 599) returns the character at a specific position. String method regionMatches (p. 601) compares portions of two strings for equality. String method equals tests for equality. The method returns true if the contents of the Strings are equal, false otherwise. Method equals uses a lexicographical comparison (p. 602) for Strings. When primitive-type values are compared with ==, the result is true if both values are identical. When references are compared with ==, the result is true if both refer to the same object. Java treats all string literals with the same contents as a single String object. String method equalsIgnoreCase performs a case-insensitive string comparison. String method compareTo uses a lexicographical comparison and returns 0 if the Strings are equal, a negative number if the string that calls compareTo is less than the argument String and a positive number if the string that calls compareTo is greater than than the argument String. String methods startsWith and endsWith (p. 604) determine whether a string starts with or ends with the specified characters, respectively. String method indexOf (p. 605) locates the first occurrence of a character or a substring in a string. String method lastIndexOf (p. 605) locates the last occurrence of a character or a substring in a string. String method substring copies and returns part of an existing string object. String method concat (p. 608) concatenates two string objects and returns a new string object. 634 • • • • • Chapter 14 Strings, Characters and Regular Expressions method replace returns a new string object that replaces every occurrence in a String of its first character argument with its second character argument. String method toUpperCase (p. 609) returns a new string with uppercase letters in the positions where the original string had lowercase letters. String method toLowerCase (p. 610) returns a new string with lowercase letters in the positions where the original string had uppercase letters. String method trim (p. 610) returns a new string object in which all white-space characters (e.g., spaces, newlines and tabs) have been removed from the beginning and end of a string. String method toCharArray (p. 610) returns a char array containing a copy of the string’s characters. String class static method valueOf returns its argument converted to a string. String Section 14.4 Class StringBuilder • Class StringBuilder provides constructors that enable StringBuilders to be initialized with no characters and an initial capacity of 16 characters, with no characters and an initial capacity specified in the integer argument, or with a copy of the characters of the String argument and an initial capacity that’s the number of characters in the String argument plus 16. • StringBuilder method length (p. 612) returns the number of characters currently stored in a StringBuilder. StringBuilder method capacity (p. 612) returns the number of characters that can be stored in a StringBuilder without allocating more memory. • StringBuilder method ensureCapacity (p. 613) ensures that a StringBuilder has at least the specified capacity. Method setLength increases or decreases the length of a StringBuilder. • StringBuilder method charAt (p. 614) returns the character at the specified index. Method setCharAt (p. 614) sets the character at the specified position. StringBuilder method getChars (p. 614) copies characters in the StringBuilder into the character array passed as an argument. • StringBuilder’s overloaded append methods (p. 615) add primitive-type, character-array, String, Object or CharSequence (p. 615) values to the end of a StringBuilder. • StringBuilder’s overloaded insert (p. 617) methods insert primitive-type, character-array, String, Object or CharSequence values at any position in a StringBuilder. Section 14.5 Class Character • Character method isDefined (p. 620) determines whether a character is in the Unicode charac- ter set. • • • • • • method isDigit (p. 620) determines whether a character is a defined Unicode digit. Character method isJavaIdentifierStart (p. 620) determines whether a character can be used as the first character of a Java identifier. Character method isJavaIdentifierPart (p. 620) determines if a character can be used in an identifier. Character method isLetter (p. 620) determines whether a character is a letter. Character method isLetterOrDigit (p. 620) determines if a character is a letter or a digit. Character method isLowerCase (p. 620) determines whether a character is a lowercase letter. Character method isUpperCase (p. 620) determines whether a character is an uppercase letter. Character method toUpperCase (p. 620) converts a character to its uppercase equivalent. Character method toLowerCase (p. 621) converts a character to its lowercase equivalent. Character method digit (p. 621) converts its character argument into an integer in the number system specified by its integer argument radix (p. 621). Character method forDigit (p. 621) converts its integer argument digit into a character in the number system specified by its integer argument radix. Character Summary • 635 Character method charValue (p. 622) returns the char stored in a Character object. Character method toString returns a String representation of a Character. Section 14.6 Tokenizing Strings • Class String’s split method (p. 623) tokenizes a String based on the delimiter (p. 623) specified as an argument and returns an array of Strings containing the tokens (p. 623). Section 14.7 Regular Expressions, Class Pattern and Class Matcher • Regular expressions (p. 624) are sequences of characters and symbols that define a set of strings. They’re useful for validating input and ensuring that data is in a particular format. • String method matches (p. 624) receives a string that specifies the regular expression and matches the contents of the String object on which it’s called to the regular expression. The method returns a boolean indicating whether the match succeeded. • A character class is an escape sequence that represents a group of characters. Each character class matches a single character in the string we’re attempting to match with the regular expression. • A word character (\w; p. 624) is any letter (uppercase or lowercase), any digit or the underscore character. • A white-space character (\s) is a space, a tab, a carriage return, a newline or a form feed. • A digit (\d) is any numeric character. • To match a set of characters that does not have a predefined character class (p. 624), use square brackets, []. Ranges can be represented by placing a dash (-) between two characters. If the first character in the brackets is "^", the expression accepts any character other than those indicated. • When the regular expression operator "*" appears in a regular expression, the program attempts to match zero or more occurrences of the subexpression immediately preceding the "*". • Operator "+" attempts to match one or more occurrences of the subexpression preceding it. • The character "|" allows a match of the expression to its left or to its right. • Parentheses () are used to group parts of the regular expression. • The asterisk (*) and plus (+) are formally called quantifiers (p. 628). • A quantifier affects only the subexpression immediately preceding it. • Quantifier question mark (?) matches zero or one occurrences of the expression that it quantifies. • A set of braces containing one number ({n}) matches exactly n occurrences of the expression it quantifies. Including a comma after the number enclosed in braces matches at least n occurrences. • A set of braces containing two numbers ({n,m}) matches between n and m occurrences of the expression that it qualifies. • Quantifiers are greedy (p. 629)—they’ll match as many occurrences as they can as long as the match is successful. If a quantifier is followed by a question mark (?), the quantifier becomes reluctant (p. 629), matching as few occurrences as possible as long as the match is successful. • String method replaceAll (p. 629) replaces text in a string with new text (the second argument) wherever the original string matches a regular expression (the first argument). • Escaping a special regular-expression character with a \ instructs the regular-expression matching engine to find the actual character, as opposed to what it represents in a regular expression. • String method replaceFirst (p. 629) replaces the first occurrence of a pattern match and returns a new string in which the appropriate characters have been replaced. • String method split (p. 629) divides a string into substrings at any location that matches a specified regular expression and returns an array of the substrings. 636 Chapter 14 Strings, Characters and Regular Expressions • Class Pattern (p. 631) represents a regular expression. • Class Matcher (p. 631) contains a regular-expression pattern and a CharSequence in which to search. • CharSequence is an interface (p. 631) that allows read access to a sequence of characters. Both String and StringBuilder implement this interface, so they can be used with class Matcher. • If a regular expression will be used only once, static Pattern method matches (p. 631) takes a string that specifies the regular expression and a CharSequence on which to perform the match. This method returns a boolean indicating whether the search object matches the regular expression. • If a regular expression will be used more than once, it’s more efficient to use static Pattern method compile (p. 631) to create a specific Pattern object for that regular expression. This method receives a string representing the pattern and returns a new Pattern object. • Pattern method matcher (p. 631) receives a CharSequence to search and returns a Matcher object. Matcher method matches (p. 631) performs the same task as Pattern method matches but without arguments. • Matcher method find (p. 631) attempts to match a piece of the search object to the search pattern. Each call to this method starts at the point where the last call ended, so multiple matches can be found. • Matcher method lookingAt (p. 631) performs the same as find, except that it always starts from the beginning of the search object and will always find the first match if there is one. • Matcher method group (p. 632) returns the string from the search object that matches the search pattern. The string returned is the one that was last matched by a call to find or lookingAt. Self-Review Exercises 14.1 State whether each of the following is true or false. If false, explain why. a) When String objects are compared using ==, the result is true if the Strings contain the same values. b) A String can be modified after it’s created. 14.2 For each of the following, write a single statement that performs the indicated task: a) Compare the string in s1 to the string in s2 for equality of contents. b) Append the string s2 to the string s1, using +=. c) Determine the length of the string in s1. Answers to Self-Review Exercises 14.1 a) False. String objects are compared using operator == to determine whether they’re the same object in memory. b) False. String objects are immutable and cannot be modified after they’re created. StringBuilder objects can be modified after they’re created. 14.2 a) b) c) s1.equals(s2) s1 += s2; s1.length() Exercises 14.3 (Comparing Strings) Write an application that uses String method compareTo to compare two strings input by the user. Output whether the first string is less than, equal to or greater than the second. Exercises 14.4 (Comparing Portions of Strings) Write an application that uses String method 637 region- Matches to compare two strings input by the user. The application should input the number of char- acters to be compared and the starting index of the comparison. The application should state whether the strings are equal. Ignore the case of the characters when performing the comparison. 14.5 (Random Sentences) Write an application that uses random-number generation to create sentences. Use four arrays of strings called article, noun, verb and preposition. Create a sentence by selecting a word at random from each array in the following order: article, noun, verb, preposition, article and noun. As each word is picked, concatenate it to the previous words in the sentence. The words should be separated by spaces. When the final sentence is output, it should start with a capital letter and end with a period. The application should generate and display 20 sentences. The article array should contain the articles "the", "a", "one", "some" and "any"; the noun array should contain the nouns "boy", "girl", "dog", "town" and "car"; the verb array should contain the verbs "drove", "jumped", "ran", "walked" and "skipped"; the preposition array should contain the prepositions "to", "from", "over", "under" and "on". 14.6 (Project: Limericks) A limerick is a humorous five-line verse in which the first and second lines rhyme with the fifth, and the third line rhymes with the fourth. Using techniques similar to those developed in Exercise 14.5, write a Java application that produces random limericks. Polishing this application to produce good limericks is a challenging problem, but the result will be worth the effort! 14.7 (Pig Latin) Write an application that encodes English-language phrases into pig Latin. Pig Latin is a form of coded language. There are many different ways to form pig Latin phrases. For simplicity, use the following algorithm: To form a pig Latin phrase from an English-language phrase, tokenize the phrase into words with String method split. To translate each English word into a pig Latin word, place the first letter of the English word at the end of the word and add the letters “ay.” Thus, the word “jump” becomes “umpjay,” the word “the” becomes “hetay,” and the word “computer” becomes “omputercay.” Blanks between words remain as blanks. Assume the following: The English phrase consists of words separated by blanks, there are no punctuation marks and all words have two or more letters. Method printLatinWord should display each word. Each token is passed to method printLatinWord to print the pig Latin word. Enable the user to input the sentence. Keep a running display of all the converted sentences in a text area. 14.8 (Tokenizing Telephone Numbers) Write an application that inputs a telephone number as a string in the form (555) 555-5555. The application should use String method split to extract the area code as a token, the first three digits of the phone number as a token and the last four digits of the phone number as a token. The seven digits of the phone number should be concatenated into one string. Both the area code and the phone number should be printed. Remember that you’ll have to change delimiter characters during the tokenization process. 14.9 (Displaying a Sentence with Its Words Reversed) Write an application that inputs a line of text, tokenizes the line with String method split and outputs the tokens in reverse order. Use space characters as delimiters. 14.10 (Displaying Strings in Uppercase and Lowercase) Write an application that inputs a line of text and outputs the text twice—once in all uppercase letters and once in all lowercase letters. 14.11 (Searching Strings) Write an application that inputs a line of text and a search character and uses String method indexOf to determine the number of occurrences of the character in the text. 14.12 (Searching Strings) Write an application based on the application in Exercise 14.11 that inputs a line of text and uses String method indexOf to determine the total number of occurrences of each letter of the alphabet in the text. Uppercase and lowercase letters should be counted together. Store the totals for each letter in an array, and print the values in tabular format after the totals have been determined. 638 Chapter 14 Strings, Characters and Regular Expressions 14.13 (Tokenizing and Comparing Strings) Write an application that reads a line of text, tokenizes the line using space characters as delimiters and outputs only those words beginning with the letter "b". 14.14 (Tokenizing and Comparing Strings) Write an application that reads a line of text, tokenizes it using space characters as delimiters and outputs only those words ending with the letters "ED". 14.15 (Converting int Values to Characters) Write an application that inputs an integer code for a character and displays the corresponding character. Modify this application so that it generates all possible three-digit codes in the range from 000 to 255 and attempts to print the corresponding characters. 14.16 (Defining Your Own String Methods) Write your own versions of String search methods and lastIndexOf. indexOf 14.17 (Creating Three-Letter Strings from a Five-Letter Word) Write an application that reads a five-letter word from the user and produces every possible three-letter string that can be derived from the letters of that word. For example, the three-letter words produced from the word “bathe” include “ate,” “bat,” “bet,” “tab,” “hat,” “the” and “tea.” Special Section: Advanced String-Manipulation Exercises The preceding exercises are keyed to the text and designed to test your understanding of fundamental string-manipulation concepts. This section includes a collection of intermediate and advanced string-manipulation exercises. You should find these problems challenging, yet entertaining. The problems vary considerably in difficulty. Some require an hour or two of application writing and implementation. Others are useful for lab assignments that might require two or three weeks of study and implementation. Some are challenging term projects. 14.18 (Text Analysis) The availability of computers with string-manipulation capabilities has resulted in some rather interesting approaches to analyzing the writings of great authors. Much attention has been focused on whether William Shakespeare ever lived. Some scholars believe there’s substantial evidence indicating that Christopher Marlowe actually penned the masterpieces attributed to Shakespeare. Researchers have used computers to find similarities in the writings of these two authors. This exercise examines three methods for analyzing texts with a computer. a) Write an application that reads a line of text from the keyboard and prints a table indicating the number of occurrences of each letter of the alphabet in the text. For example, the phrase To be, or not to be: that is the question: contains one “a,” two “b’s,” no “c’s,” and so on. b) Write an application that reads a line of text and prints a table indicating the number of one-letter words, two-letter words, three-letter words, and so on, appearing in the text. For example, Fig. 14.25 shows the counts for the phrase Whether 'tis nobler in the mind to suffer c) Write an application that reads a line of text and prints a table indicating the number of occurrences of each different word in the text. The application should include the words in the table in the same order in which they appear in the text. For example, the lines To be, or not to be: that is the question: Whether 'tis nobler in the mind to suffer contain the word “to” three times, the word “be” two times, the word “or” once, etc. Special Section: Advanced String-Manipulation Exercises Word length Occurrences 1 2 3 4 5 6 7 0 2 1 2 (including 'tis) 0 2 1 639 Fig. 14.25 | Word-length counts for the string "Whether 'tis nobler in the mind to suffer". 14.19 (Printing Dates in Various Formats) Dates are printed in several common formats. Two of the more common formats are 04/25/1955 and April 25, 1955 Write an application that reads a date in the first format and prints it in the second format. 14.20 (Check Protection) Computers are frequently employed in check-writing systems, such as payroll and accounts payable applications. There are many strange stories about weekly paychecks being printed (by mistake) for amounts in excess of $1 million. Incorrect amounts are printed by computerized check-writing systems because of human error or machine failure. Systems designers build controls into their systems to prevent such erroneous checks from being issued. Another serious problem is the intentional alteration of a check amount by someone who plans to cash a check fraudulently. To prevent a dollar amount from being altered, some computerized check-writing systems employ a technique called check protection. Checks designed for imprinting by computer contain a fixed number of spaces in which the computer may print an amount. Suppose a paycheck contains eight blank spaces in which the computer is supposed to print the amount of a weekly paycheck. If the amount is large, then all eight of the spaces will be filled. For example, 1,230.60 -------12345678 (check amount) (position numbers) On the other hand, if the amount is less than $1000, then several of the spaces would ordinarily be left blank. For example, 99.87 -------12345678 contains three blank spaces. If a check is printed with blank spaces, it’s easier for someone to alter the amount. To prevent alteration, many check-writing systems insert leading asterisks to protect the amount as follows: ***99.87 -------12345678 Write an application that inputs a dollar amount to be printed on a check, then prints the amount in check-protected format with leading asterisks if necessary. Assume that nine spaces are available for printing the amount. 640 Chapter 14 Strings, Characters and Regular Expressions 14.21 (Writing the Word Equivalent of a Check Amount) Continuing the discussion in Exercise 14.20, we reiterate the importance of designing check-writing systems to prevent alteration of check amounts. One common security method requires that the amount be written in numbers and spelled out in words as well. Even if someone is able to alter the numerical amount of the check, it’s extremely difficult to change the amount in words. Write an application that inputs a numeric check amount that’s less than $1000 and writes the word equivalent of the amount. For example, the amount 112.43 should be written as ONE hundred TWELVE and 43/100 14.22 (Morse Code) Perhaps the most famous of all coding schemes is the Morse code, developed by Samuel Morse in 1832 for use with the telegraph system. The Morse code assigns a series of dots and dashes to each letter of the alphabet, each digit, and a few special characters (e.g., period, comma, colon, semicolon). In sound-oriented systems, the dot represents a short sound and the dash a long sound. Other representations of dots and dashes are used with light-oriented systems and signal-flag systems. Separation between words is indicated by a space or, simply, the absence of a dot or dash. In a sound-oriented system, a space is indicated by a short time during which no sound is transmitted. The international version of the Morse code appears in Fig. 14.26. Write an application that reads an English-language phrase and encodes it into Morse code. Also write an application that reads a phrase in Morse code and converts it into the English-language equivalent. Use one blank between each Morse-coded letter and three blanks between each Morse-coded word. Character Code Character Code Character A B C D E F G H I J K L M .- N O P Q R S T U V W X Y Z -. Digits 1 2 3 4 5 6 7 8 9 0 -... -.-. -.. . ..-. --. .... .. .---..-.. -- --.--. --..-. ... ......--..- Code .---..--...-......... -.... --... ---.. ----. ----- -.---.. Fig. 14.26 | Letters and digits as expressed in international Morse code. 14.23 (Metric Conversions) Write an application that will assist the user with metric conversions. Your application should allow the user to specify the names of the units as strings (i.e., centimeters, liters, grams, and so on, for the metric system and inches, quarts, pounds, and so on, for the English system) and should respond to simple questions, such as "How many inches are in 2 meters?" "How many liters are in 10 quarts?" Special Section: Challenging String-Manipulation Projects 641 Your application should recognize invalid conversions. For example, the question "How many feet are in 5 kilograms?" is not meaningful because "feet" is a unit of length, whereas "kilograms" is a unit of mass. Special Section: Challenging String-Manipulation Projects 14.24 (Project: A Spelling Checker) Many popular word-processing software packages have builtin spell checkers. In this project, you’re asked to develop your own spell-checker utility. We make suggestions to help get you started. You should then consider adding more capabilities. Use a computerized dictionary (if you have access to one) as a source of words. Why do we type so many words with incorrect spellings? In some cases, it’s because we simply do not know the correct spelling, so we make a best guess. In some cases, it’s because we transpose two letters (e.g., “defualt” instead of “default”). Sometimes we double-type a letter accidentally (e.g., “hanndy” instead of “handy”). Sometimes we type a nearby key instead of the one we intended (e.g., “biryhday” instead of “birthday”), and so on. Design and implement a spell-checker application in Java. Your application should maintain an array wordList of strings. Enable the user to enter these strings. [Note: In Chapter 15, we introduce file processing. With this capability, you can obtain the words for the spell checker from a computerized dictionary stored in a file.] Your application should ask a user to enter a word. The application should then look up that word in the wordList array. If the word is in the array, your application should print "Word is spelled correctly." If the word is not in the array, your application should print "Word is not spelled correctly." Then your application should try to locate other words in wordList that might be the word the user intended to type. For example, you can try all possible single transpositions of adjacent letters to discover that the word “default” is a direct match to a word in wordList. Of course, this implies that your application will check all other single transpositions, such as “edfault,” “dfeault,” “deafult,” “defalut” and “defautl.” When you find a new word that matches one in wordList, print it in a message, such as Did you mean "default"? Implement other tests, such as replacing each double letter with a single letter, and any other tests you can develop to improve the value of your spell checker. 14.25 (Project: A Crossword Puzzle Generator) Most people have worked a crossword puzzle, but few have ever attempted to generate one. Generating a crossword puzzle is suggested here as a stringmanipulation project requiring substantial sophistication and effort. There are many issues the programmer must resolve to get even the simplest crossword-puzzle-generator application working. For example, how do you represent the grid of a crossword puzzle inside the computer? Should you use a series of strings or two-dimensional arrays? The programmer needs a source of words (i.e., a computerized dictionary) that can be directly referenced by the application. In what form should these words be stored to facilitate the complex manipulations required by the application? If you’re really ambitious, you’ll want to generate the clues portion of the puzzle, in which the brief hints for each across word and each down word are printed. Merely printing a version of the blank puzzle itself is not a simple problem. Making a Difference 14.26 (Cooking with Healthier Ingredients) Obesity in America is increasing at an alarming rate. Check the map from the Centers for Disease Control and Prevention (CDC) at www.cdc.gov/ nccdphp/dnpa/Obesity/trend/maps/index.htm, which shows obesity trends in the United States 642 Chapter 14 Strings, Characters and Regular Expressions over the last 20 years. As obesity increases, so do occurrences of related problems (e.g., heart disease, high blood pressure, high cholesterol, type 2 diabetes). Write a program that helps users choose healthier ingredients when cooking, and helps those allergic to certain foods (e.g., nuts, gluten) find substitutes. The program should read a recipe from a JTextArea and suggest healthier replacements for some of the ingredients. For simplicity, your program should assume the recipe has no abbreviations for measures such as teaspoons, cups, and tablespoons, and uses numerical digits for quantities (e.g., 1 egg, 2 cups) rather than spelling them out (one egg, two cups). Some common substitutions are shown in Fig. 14.27. Your program should display a warning such as, “Always consult your physician before making significant changes to your diet.” Your program should take into consideration that replacements are not always one-for-one. For example, if a cake recipe calls for three eggs, it might reasonably use six egg whites instead. Conversion data for measurements and substitutes can be obtained at websites such as: chinesefood.about.com/od/recipeconversionfaqs/f/usmetricrecipes.htm www.pioneerthinking.com/eggsub.html www.gourmetsleuth.com/conversions.htm Your program should consider the user’s health concerns, such as high cholesterol, high blood pressure, weight loss, gluten allergy, and so on. For high cholesterol, the program should suggest substitutes for eggs and dairy products; if the user wishes to lose weight, low-calorie substitutes for ingredients such as sugar should be suggested. Ingredient Substitution 1 cup sour cream 1 cup milk 1 teaspoon lemon juice 1 cup sugar 1 cup yogurt 1/2 cup evaporated milk and 1/2 cup water 1/2 teaspoon vinegar 1/2 cup honey, 1 cup molasses or 1/4 cup agave nectar 1 cup margarine or yogurt 1 cup rye or rice flour 1 cup cottage cheese or 1/8 cup mayonnaise and 7/8 cup yogurt 2 tablespoons cornstarch, arrowroot flour or potato starch or 2 egg whites or 1/2 of a large banana (mashed) 1 cup soy milk 1/4 cup applesauce whole-grain bread 1 cup butter 1 cup flour 1 cup mayonnaise 1 egg 1 cup milk 1/4 cup oil white bread Fig. 14.27 | Common food substitutions. 14.27 (Spam Scanner) Spam (or junk e-mail) costs U.S. organizations billions of dollars a year in spam-prevention software, equipment, network resources, bandwidth, and lost productivity. Research online some of the most common spam e-mail messages and words, and check your own junk e-mail folder. Create a list of 30 words and phrases commonly found in spam messages. Write an application in which the user enters an e-mail message in a JTextArea. Then, scan the message for each of the 30 keywords or phrases. For each occurrence of one of these within the message, add Making a Difference 643 a point to the message’s “spam score.” Next, rate the likelihood that the message is spam, based on the number of points it received. 14.28 (SMS Language) Short Message Service (SMS) is a communications service that allows sending text messages of 160 or fewer characters between mobile phones. With the proliferation of mobile phone use worldwide, SMS is being used in many developing nations for political purposes (e.g., voicing opinions and opposition), reporting news about natural disasters, and so on. For example, check out comunica.org/radio2.0/archives/87. Since the length of SMS messages is limited, SMS Language—abbreviations of common words and phrases in mobile text messages, emails, instant messages, etc.—is often used. For example, “in my opinion” is “imo” in SMS Language. Research SMS Language online. Write a GUI application in which the user can enter a message using SMS Language, then click a button to translate it into English (or your own language). Also provide a mechanism to translate text written in English (or your own language) into SMS Language. One potential problem is that one SMS abbreviation could expand into a variety of phrases. For example, IMO (as used above) could also stand for “International Maritime Organization,” “in memory of,” etc. 15 Consciousness … does not appear to itself chopped up in bits. … A “river” or a “stream” are the metaphors by which it is most naturally described. —William James Objectives In this chapter you’ll: ■ Create, read, write and update files. ■ Retrieve information about files and directories using features of the NIO.2 APIs. ■ Learn the differences between text files and binary files. ■ Use class Formatter to output text to a file. ■ Use class Scanner to input text from a file. ■ Write objects to and read objects from a file using object serialization, interface Serializable and classes ObjectOutputStream and ObjectInputStream. ■ Use a JFileChooser dialog to allow users to select files or directories on disk. Files, Streams and Object Serialization 15.1 Introduction 15.1 Introduction 15.2 Files and Streams 15.3 Using NIO Classes and Interfaces to Get File and Directory Information 15.4 Sequential-Access Text Files 15.4.1 Creating a Sequential-Access Text File 15.4.2 Reading Data from a SequentialAccess Text File 15.4.3 Case Study: A Credit-Inquiry Program 15.4.4 Updating Sequential-Access Files 15.5 Object Serialization 645 15.5.1 Creating a Sequential-Access File Using Object Serialization 15.5.2 Reading and Deserializing Data from a Sequential-Access File 15.6 Opening Files with JFileChooser 15.7 (Optional) Additional java.io Classes 15.7.1 Interfaces and Classes for Byte-Based Input and Output 15.7.2 Interfaces and Classes for CharacterBased Input and Output 15.8 Wrap-Up Summary | Self-Review Exercises | Answers to Self-Review Exercises | Exercises | Making a Difference 15.1 Introduction Data stored in variables and arrays is temporary—it’s lost when a local variable goes out of scope or when the program terminates. For long-term retention of data, even after the programs that create the data terminate, computers use files. You use files every day for tasks such as writing a document or creating a spreadsheet. Computers store files on secondary storage devices, including hard disks, flash drives, DVDs and more. Data maintained in files is persistent data—it exists beyond the duration of program execution. In this chapter, we explain how Java programs create, update and process files. We begin with a discussion of Java’s architecture for handling files programmatically. Next we explain that data can be stored in text files and binary files—and we cover the differences between them. We demonstrate retrieving information about files and directories using classes Paths and Files and interfaces Path and DirectoryStream (all from package java.nio.file), then consider the mechanisms for writing data to and reading data from files. We show how to create and manipulate sequential-access text files. Working with text files allows you to quickly and easily start manipulating files. As you’ll learn, however, it’s difficult to read data from text files back into object form. Fortunately, many objectoriented languages (including Java) provide ways to write objects to and read objects from files (known as object serialization and deserialization). To demonstrate this, we recreate some of our sequential-access programs that used text files, this time by storing objects in and retrieving objects from binary files. 15.2 Files and Streams Java views each file as a sequential stream of bytes (Fig. 15.1).1 Every operating system provides a mechanism to determine the end of a file, such as an end-of-file marker or a count of the total bytes in the file that’s recorded in a system-maintained administrative data structure. A Java program processing a stream of bytes simply receives an indication from the operating system when it reaches the end of the stream—the program does not need to know how the underlying platform represents files or streams. In some cases, the 1. Java’s NIO APIs also include classes and interfaces that implement so-called channel-based architecture for high-performance I/O. These topics are beyond the scope of this book. 646 Chapter 15 Files, Streams and Object Serialization end-of-file indication occurs as an exception. In others, the indication is a return value from a method invoked on a stream-processing object. 0 1 2 3 4 5 6 7 8 9 ... ... n-1 end-of-file marker Fig. 15.1 | Java’s view of a file of n bytes. Byte-Based and Character-Based Streams File streams can be used to input and output data as bytes or characters. • Byte-based streams output and input data in its binary format—a char is two bytes, an int is four bytes, a double is eight bytes, etc. • Character-based streams output and input data as a sequence of characters in which every character is two bytes—the number of bytes for a given value depends on the number of characters in that value. For example, the value 2000000000 requires 20 bytes (10 characters at two bytes per character) but the value 7 requires only two bytes (1 character at two bytes per character). Files created using byte-based streams are referred to as binary files, while files created using character-based streams are referred to as text files. Text files can be read by text editors, while binary files are read by programs that understand the file’s specific content and its ordering. A numeric value in a binary file can be used in calculations, whereas the character 5 is simply a character that can be used in a string of text, as in "Sarah Miller is 15 years old". Standard Input, Standard Output and Standard Error Streams A Java program opens a file by creating an object and associating a stream of bytes or characters with it. The object’s constructor interacts with the operating system to open the file. Java can also associate streams with different devices. When a Java program begins executing, it creates three stream objects that are associated with devices—System.in, System.out and System.err. The System.in (standard input stream) object normally enables a program to input bytes from the keyboard. Object System.out (the standard output stream object) normally enables a program to output character data to the screen. Object System.err (the standard error stream object) normally enables a program to output character-based error messages to the screen. Each stream can be redirected. For System.in, this capability enables the program to read bytes from a different source. For System.out and System.err, it enables the output to be sent to a different location, such as a file on disk. Class System provides methods setIn, setOut and setErr to redirected the standard input, output and error streams, respectively. The java.io and java.nio Packages Java programs perform stream-based processing with classes and interfaces from package java.io and the subpackages of java.nio—Java’s New I/O APIs that were first introduced in Java SE 6 and that have been enhanced since. There are also other packages throughout the Java APIs containing classes and interfaces based on those in the java.io and java.nio packages. 15.3 Using NIO Classes and Interfaces to Get File and Directory Information 647 Character-based input and output can be performed with classes Scanner and Foras you’ll see in Section 15.4. You’ve used class Scanner extensively to input data from the keyboard. Scanner also can read data from a file. Class Formatter enables formatted data to be output to any text-based stream in a manner similar to method System.out.printf. Appendix I presents the details of formatted output with printf. All these features can be used to format text files as well. In Chapter 28, we use stream classes to implement networking applications. matter, Java SE 8 Adds Another Type of Stream Chapter 17, Java SE 8 Lambdas and Streams, introduces a new type of stream that’s used to process collections of elements (like arrays and ArrayLists), rather than the streams of bytes we discuss in this chapter’s file-processing examples. 15.3 Using NIO Classes and Interfaces to Get File and Directory Information Interfaces Path and DirectoryStream and classes Paths and Files (all from package jaare useful for retrieving information about files and directories on disk: va.nio.file) • Path interface—Objects of classes that implement this interface represent the location of a file or directory. Path objects do not open files or provide any file-processing capabilities. • Paths • Files • DirectoryStream interface—Objects of class—Provides static methods used to get a Path object representing a file or directory location. class—Provides static methods for common file and directory manipulations, such as copying files; creating and deleting files and directories; getting information about files and directories; reading the contents of files; getting objects that allow you to manipulate the contents of files and directories; and more classes that implement this interface enable a program to iterate through the contents of a directory. Creating Path Objects You’ll use class static method get of class Paths to convert a String representing a file’s or directory’s location into a Path object. You can then use the methods of interface Path and class Files to determine information about the specified file or directory. We discuss several such methods momentarily. For complete lists of their methods, visit: http://docs.oracle.com/javase/7/docs/api/java/nio/file/Path.html http://docs.oracle.com/javase/7/docs/api/java/nio/file/Files.html Absolute vs. Relative Paths A file or directory’s path specifies its location on disk. The path includes some or all of the directories leading to the file or directory. An absolute path contains all directories, starting with the root directory, that lead to a specific file or directory. Every file or directory on a particular disk drive has the same root directory in its path. A relative path is “relative” to another directory—for example, a path relative to the directory in which the application began executing. 648 Chapter 15 Files, Streams and Object Serialization Getting Path Objects from URIs An overloaded version of Files static method get uses a URI object to locate the file or directory. A Uniform Resource Identifier (URI) is a more general form of the Uniform Resource Locators (URLs) that are used to locate websites. For example, the URL http:/ /www.deitel.com/ is the URL for the Deitel & Associates website. URIs for locating files vary across operating systems. On Windows platforms, the URI file://C:/data.txt identifies the file data.txt stored in the root directory of the C: drive. On UNIX/Linux platforms, the URI file:/home/student/data.txt identifies the file data.txt stored in the home directory of the user student. Example: Getting File and Directory Information Figure 15.2 prompts the user to enter a file or directory name, then uses classes Paths, Path, Files and DirectoryStream to output information about that file or directory.The program begins by prompting the user for a file or directory (line 16). Line 19 inputs the filename or directory name and passes it to Paths static method get, which converts the String to a Path. Line 21 invokes Files static method exists, which receives a Path and determines whether it exists (either as a file or as a directory) on disk. If the name does not exist, control proceeds to line 49, which displays a message containing the Path’s String representation followed by “does not exist.” Otherwise, lines 24–45 execute: • Path method getFileName (line 24) gets the String name of the file or directory without any location information. • Files static method isDirectory (line 26) receives a Path and returns a boolean indicating whether that Path represents a directory on disk. • Path method isAbsolute (line 28) returns a boolean indicating whether that Path represents an absolute path to a file or directory. • Files static method getLastModifiedTime (line 30) receives a Path and returns a FileTime (package java.nio.file.attribute) indicating when the file was last modified. The program outputs the FileTime’s default String representation. • Files static method size (line 31) receives a Path and returns a long representing the number of bytes in the file or directory. For directories, the value returned is platform specific. • Path method toString (called implicitly at line 32) returns a String representing the Path. • Path method toAbsolutePath (line 33) converts the Path on which it’s called to an absolute path. If the Path represents a directory (line 35), lines 40–41 use Files static method newDirectoryStream (lines 40–41) to get a DirectoryStream containing Path objects for the directory’s contents. Lines 43–44 display the String representation of each Path in the DirectoryStream. Note that DirectoryStream is a generic type like ArrayList (Section 7.16). 15.3 Using NIO Classes and Interfaces to Get File and Directory Information 649 The first output of this program demonstrates a Path for the folder containing this chapter’s examples. The second output demonstrates a Path for this example’s source code file. In both cases, we specified an absolute path. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 // Fig. 15.2: FileAndDirectoryInfo.java // File class used to obtain file and directory information. import java.io.IOException; import java.nio.file.DirectoryStream; import java.nio.file.Files; import java.nio.file.Path; import java.nio.file.Paths; import java.util.Scanner; public class FileAndDirectoryInfo { public static void main(String[] args) throws IOException { Scanner input = new Scanner(System.in); Fig. 15.2 | System.out.println("Enter file or directory name:"); // create Path object based on user input Path path = Paths.get(input.nextLine()); if (Files.exists(path)) // if path exists, output info about it { // display file (or directory) information System.out.printf("%n%s exists%n", path.getFileName()); System.out.printf("%s a directory%n", Files.isDirectory(path) ? "Is" : "Is not"); System.out.printf("%s an absolute path%n", path.isAbsolute() ? "Is" : "Is not"); System.out.printf("Last modified: %s%n", Files.getLastModifiedTime(path)); System.out.printf("Size: %s%n", Files.size(path)); System.out.printf("Path: %s%n", path); System.out.printf("Absolute path: %s%n", path.toAbsolutePath()); if (Files.isDirectory(path)) // output directory listing { System.out.printf("%nDirectory contents:%n"); // object for iterating through a directory's contents DirectoryStream directoryStream = Files.newDirectoryStream(path); for (Path p : directoryStream) System.out.println(p); } } else // not file or directory, output error message { File class used to obtain file and directory information. (Part 1 of 2.) 650 49 50 51 52 Chapter 15 Files, Streams and Object Serialization System.out.printf("%s does not exist%n", path); } } // end main } // end class FileAndDirectoryInfo Enter file or directory name: c:\examples\ch15 ch15 exists Is a directory Is an absolute path Last modified: 2013-11-08T19:50:00.838256Z Size: 4096 Path: c:\examples\ch15 Absolute path: c:\examples\ch15 Directory contents: C:\examples\ch15\fig15_02 C:\examples\ch15\fig15_12_13 C:\examples\ch15\SerializationApps C:\examples\ch15\TextFileApps Enter file or directory name: C:\examples\ch15\fig15_02\FileAndDirectoryInfo.java FileAndDirectoryInfo.java exists Is not a directory Is an absolute path Last modified: 2013-11-08T19:59:01.848255Z Size: 2952 Path: C:\examples\ch15\fig15_02\FileAndDirectoryInfo.java Absolute path: C:\examples\ch15\fig15_02\FileAndDirectoryInfo.java Fig. 15.2 | File class used to obtain file and directory information. (Part 2 of 2.) Error-Prevention Tip 15.1 Once you’ve confirmed that a Path exists, it’s still possible that the methods demonstrated in Fig. 15.2 will throw IOExceptions. For example, the file or directory represented by the Path could be deleted from the system after the call to Files method exists and before the other statements in lines 24–45 execute. Industrial strength file- and directory-processing programs require extensive exception handling to recover from such possibilities. Separator Characters A separator character is used to separate directories and files in a path. On a Windows computer, the separator character is a backslash (\). On a Linux or Mac OS X system, it’s a forward slash (/). Java processes both characters identically in a path name. For example, if we were to use the path c:\Program Files\Java\jdk1.6.0_11\demo/jfc which employs each separator character, Java would still process the path properly. 15.4 Sequential-Access Text Files 651 Good Programming Practice 15.1 When building Strings that represent path information, use File.separator to obtain the local computer’s proper separator character rather than explicitly using / or \. This constant is a String consisting of one character—the proper separator for the system. Common Programming Error 15.1 Using \ as a directory separator rather than \\ in a string literal is a logic error. A single \ indicates that the \ followed by the next character represents an escape sequence. Use \\ to insert a \ in a string literal. 15.4 Sequential-Access Text Files Next, we create and manipulate sequential-access files in which records are stored in order by the record-key field. We begin with text files, enabling the reader to quickly create and edit human-readable files. We discuss creating, writing data to, reading data from and updating sequential-access text files. We also include a credit-inquiry program that retrieves data from a file. The programs in Sections 15.4.1–15.4.3 are all in the chapter’s TextFileApps directory so that they can manipulate the same text file, which is also stored in that directory. 15.4.1 Creating a Sequential-Access Text File Java imposes no structure on a file—notions such as records do not exist as part of the Java language. Therefore, you must structure files to meet the requirements of your applications. In the following example, we see how to impose a keyed record structure on a file. The program in this section creates a simple sequential-access file that might be used in an accounts receivable system to keep track of the amounts owed to a company by its credit clients. For each client, the program obtains from the user an account number and the client’s name and balance (i.e., the amount the client owes the company for goods and services received). Each client’s data constitutes a “record” for that client. This application uses the account number as the record key—the file’s records will be created and maintained in account-number order. The program assumes that the user enters the records in account-number order. In a comprehensive accounts receivable system (based on sequential-access files), a sorting capability would be provided so that the user could enter the records in any order. The records would then be sorted and written to the file. Class CreateTextFile Class CreateTextFile (Fig. 15.3) uses a Formatter to output formatted Strings, using the same formatting capabilities as method System.out.printf. A Formatter object can output to various locations, such as to a command window or to a file, as we do in this example. The Formatter object is instantiated in line 26 in method openFile (lines 22– 38). The constructor used in line 26 takes one argument—a String containing the name of the file, including its path. If a path is not specified, as is the case here, the JVM assumes that the file is in the directory from which the program was executed. For text files, we use the .txt file extension. If the file does not exist, it will be created. If an existing file is opened, its contents are truncated—all the data in the file is discarded. If no exception occurs, the file is open for writing and the resulting Formatter object can be used to write data to the file. 652 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 Chapter 15 Files, Streams and Object Serialization // Fig. 15.3: CreateTextFile.java // Writing data to a sequential text file with class Formatter. import java.io.FileNotFoundException; import java.lang.SecurityException; import java.util.Formatter; import java.util.FormatterClosedException; import java.util.NoSuchElementException; import java.util.Scanner; public class CreateTextFile { private static Formatter output; // outputs text to a file public static void main(String[] args) { openFile(); addRecords(); closeFile(); } // open file clients.txt public static void openFile() { try { output = new Formatter("clients.txt"); // open the file } catch (SecurityException securityException) { System.err.println("Write permission denied. Terminating."); System.exit(1); // terminate the program } catch (FileNotFoundException fileNotFoundException) { System.err.println("Error opening file. Terminating."); System.exit(1); // terminate the program } } // add records to file public static void addRecords() { Scanner input = new Scanner(System.in); System.out.printf("%s%n%s%n? ", "Enter account number, first name, last name and balance.", "Enter end-of-file indicator to end input."); while (input.hasNext()) // loop until end-of-file indicator { try { Fig. 15.3 | Writing data to a sequential text file with class Formatter. (Part 1 of 2.) 15.4 Sequential-Access Text Files 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 653 // output new record to file; assumes valid input output.format("%d %s %s %.2f%n", input.nextInt(), input.next(), input.next(), input.nextDouble()); } catch (FormatterClosedException formatterClosedException) { System.err.println("Error writing to file. Terminating."); break; } catch (NoSuchElementException elementException) { System.err.println("Invalid input. Please try again."); input.nextLine(); // discard input so user can try again } System.out.print("? "); } // end while } // end method addRecords // close file public static void closeFile() { if (output != null) output.close(); } } // end class CreateTextFile Enter Enter ? 100 ? 200 ? 300 ? 400 ? 500 ? ^Z account number, first name, last name and balance. end-of-file indicator to end input. Bob Blue 24.98 Steve Green -345.67 Pam White 0.00 Sam Red -42.16 Sue Yellow 224.62 Fig. 15.3 | Writing data to a sequential text file with class Formatter. (Part 2 of 2.) Lines 28–32 handle the SecurityException, which occurs if the user does not have permission to write data to the file. Lines 33–37 handle the FileNotFoundException, which occurs if the file does not exist and a new file cannot be created. This exception may also occur if there’s an error opening the file. In both exception handlers we call static method System.exit and pass the value 1. This method terminates the application. An argument of 0 to method exit indicates successful program termination. A nonzero value, such as 1 in this example, normally indicates that an error has occurred. This value is passed to the command window that executed the program. The argument is useful if the program is executed from a batch file on Windows systems or a shell script on UNIX/ Linux/Mac OS X systems. Batch files and shell scripts offer a convenient way of executing several programs in sequence. When the first program ends, the next program begins execution. It’s possible to use the argument to method exit in a batch file or shell script to determine whether other programs should execute. For more information on batch files or shell scripts, see your operating system’s documentation. 654 Chapter 15 Files, Streams and Object Serialization Method addRecords (lines 41–69) prompts the user to enter the various fields for each record or the end-of-file key sequence when data entry is complete. Figure 15.4 lists the key combinations for entering end-of-file for various computer systems. Operating system Key combination UNIX/Linux/Mac OS X Windows d z Fig. 15.4 | End-of-file key combinations. Lines 44–46 prompt the user for input. Line 48 uses Scanner method hasNext to determine whether the end-of-file key combination has been entered. The loop executes until hasNext encounters end-of-file. Lines 53–54 use a Scanner to read data from the user, then output the data as a record using the Formatter. Each Scanner input method throws a NoSuchElementException (handled in lines 61–65) if the data is in the wrong format (e.g., a String when an int is expected) or if there’s no more data to input. The record’s information is output using method format, which can perform identical formatting to the System.out.printf method used extensively in earlier chapters. Method format outputs a formatted String to the output destination of the Formatter object—the file clients.txt. The format string "%d %s %s %.2f%n" indicates that the current record will be stored as an integer (the account number) followed by a String (the first name), another String (the last name) and a floating-point value (the balance). Each piece of information is separated from the next by a space, and the double value (the balance) is output with two digits to the right of the decimal point (as indicated by the .2 in %.2f). The data in the text file can be viewed with a text editor or retrieved later by a program designed to read the file (Section 15.4.2). When lines 66–68 execute, if the Formatter object is closed, a FormatterClosedException will be thrown. This exception is handled in lines 76–80. [Note: You can also output data to a text file using class java.io.PrintWriter, which provides format and printf methods for outputting formatted data.] Lines 93–97 declare method closeFile, which closes the Formatter and the underlying output file. Line 96 closes the object by simply calling method close. If method close is not called explicitly, the operating system normally will close the file when program execution terminates—this is an example of operating-system “housekeeping.” However, you should always explicitly close a file when it’s no longer needed. Sample Output The sample data for this application is shown in Fig. 15.5. In the sample output, the user enters information for five accounts, then enters end-of-file to signal that data entry is complete. The sample output does not show how the data records actually appear in the file. In the next section, to verify that the file was created successfully, we present a program that reads the file and prints its contents. Because this is a text file, you can also verify the information simply by opening the file in a text editor. 15.4 Sequential-Access Text Files 655 Sample data 100 Bob Blue 24.98 200 Steve Green -345.67 300 Pam White 0.00 400 Sam Red -42.16 500 Sue Yellow 224.62 Fig. 15.5 | Sample data for the program in Fig. 15.3. 15.4.2 Reading Data from a Sequential-Access Text File Data is stored in files so that it may be retrieved for processing when needed. Section 15.4.1 demonstrated how to create a file for sequential access. This section shows how to read data sequentially from a text file. We demonstrate how class Scanner can be used to input data from a file rather than the keyboard. The application (Fig. 15.6) reads records from the file "clients.txt" created by the application of Section 15.4.1 and displays the record contents. Line 13 declares a Scanner that will be used to retrieve input from the file. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 // Fig. 15.6: ReadTextFile.java // This program reads a text file and displays each record. import java.io.IOException; import java.lang.IllegalStateException; import java.nio.file.Files; import java.nio.file.Path; import java.nio.file.Paths; import java.util.NoSuchElementException; import java.util.Scanner; public class ReadTextFile { private static Scanner input; public static void main(String[] args) { openFile(); readRecords(); closeFile(); } // open file clients.txt public static void openFile() { try { input = new Scanner(Paths.get("clients.txt")); } Fig. 15.6 | Sequential file reading using a Scanner. (Part 1 of 2.) 656 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 Chapter 15 Files, Streams and Object Serialization catch (IOException ioException) { System.err.println("Error opening file. Terminating."); System.exit(1); } } // read record from file public static void readRecords() { System.out.printf("%-10s%-12s%-12s%10s%n", "Account", "First Name", "Last Name", "Balance"); try { while (input.hasNext()) // while there is more to read { // display record contents System.out.printf("%-10d%-12s%-12s%10.2f%n", input.nextInt(), input.next(), input.next(), input.nextDouble()); } } catch (NoSuchElementException elementException) { System.err.println("File improperly formed. Terminating."); } catch (IllegalStateException stateException) { System.err.println("Error reading from file. Terminating."); } } // end method readRecords // close file and terminate application public static void closeFile() { if (input != null) input.close(); } } // end class ReadTextFile Account 100 200 300 400 500 First Name Bob Steve Pam Sam Sue Last Name Blue Green White Red Yellow Balance 24.98 -345.67 0.00 -42.16 224.62 Fig. 15.6 | Sequential file reading using a Scanner. (Part 2 of 2.) Method openFile (lines 23–34) opens the file for reading by instantiating a Scanner object in line 27. We pass a Path object to the constructor, which specifies that the Scanner object will read from the file "clients.txt" located in the directory from which the application executes. If the file cannot be found, an IOException occurs. The exception is handled in lines 29–33. 15.4 Sequential-Access Text Files 657 Method readRecords (lines 37–59) reads and displays records from the file. Lines 39–40 display headers for the columns in the application’s output. Lines 44–49 read and display data from the file until the end-of-file marker is reached (in which case, method hasNext will return false at line 44). Lines 47–48 use Scanner methods nextInt, next and nextDouble to input an int (the account number), two Strings (the first and last names) and a double value (the balance). Each record is one line of data in the file. If the information in the file is not properly formed (e.g., there’s a last name where there should be a balance), a NoSuchElementException occurs when the record is input. This exception is handled in lines 51–54. If the Scanner was closed before the data was input, an IllegalStateException occurs (handled in lines 55–58). Note in the format string in line 47 that the account number, first name and last name are left justified, while the balance is right justified and output with two digits of precision. Each iteration of the loop inputs one line of text from the text file, which represents one record. Lines 62–66 define method closeFile, which closes the Scanner. 15.4.3 Case Study: A Credit-Inquiry Program To retrieve data sequentially from a file, programs start from the beginning of the file and read all the data consecutively until the desired information is found. It might be necessary to process the file sequentially several times (from the beginning of the file) during the execution of a program. Class Scanner does not allow repositioning to the beginning of the file. If it’s necessary to read the file again, the program must close the file and reopen it. The program in Figs. 15.7–15.8 allows a credit manager to obtain lists of customers with zero balances (i.e., customers who do not owe any money), customers with credit balances (i.e., customers to whom the company owes money) and customers with debit balances (i.e., customers who owe the company money for goods and services received). A credit balance is a negative amount, a debit balance a positive amount. MenuOption enum We begin by creating an enum type (Fig. 15.7) to define the different menu options the credit manager will have—this is required if you need to provide specific values for the enum constants. The options and their values are listed in lines 7–10. 1 2 3 4 5 6 7 8 9 10 11 12 13 // Fig. 15.7: MenuOption.java // enum type for the credit-inquiry program's options. public enum MenuOption { // declare contents of enum type ZERO_BALANCE(1), CREDIT_BALANCE(2), DEBIT_BALANCE(3), END(4); private final int value; // current menu option Fig. 15.7 | enum type for the credit-inquiry program’s menu options. (Part 1 of 2.) 658 14 15 16 17 18 19 Chapter 15 Files, Streams and Object Serialization // constructor private MenuOption(int value) { this.value = value; } } // end enum MenuOption Fig. 15.7 | enum type for the credit-inquiry program’s menu options. (Part 2 of 2.) Class Figure 15.8 contains the functionality for the credit-inquiry program. The program displays a text menu and allows the credit manager to enter one of three options to obtain credit information: CreditInquiry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 • Option 1 (ZERO_BALANCE) displays accounts with zero balances. • Option 2 (CREDIT_BALANCE) displays accounts with credit balances. • Option 3 (DEBIT_BALANCE) displays accounts with debit balances. • Option 4 (END) terminates program execution. // Fig. 15.8: CreditInquiry.java // This program reads a file sequentially and displays the // contents based on the type of account the user requests // (credit balance, debit balance or zero balance). import java.io.IOException; import java.lang.IllegalStateException; import java.nio.file.Paths; import java.util.NoSuchElementException; import java.util.Scanner; public class CreditInquiry { private final static MenuOption[] choices = MenuOption.values(); public static void main(String[] args) { // get user's request (e.g., zero, credit or debit balance) MenuOption accountType = getRequest(); while (accountType != MenuOption.END) { switch (accountType) { case ZERO_BALANCE: System.out.printf("%nAccounts with zero balances:%n"); break; case CREDIT_BALANCE: System.out.printf("%nAccounts with credit balances:%n"); break; Fig. 15.8 | Credit-inquiry program. (Part 1 of 4.) 15.4 Sequential-Access Text Files 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 659 case DEBIT_BALANCE: System.out.printf("%nAccounts with debit balances:%n"); break; } readRecords(accountType); accountType = getRequest(); // get user's request } } // obtain request from user private static MenuOption getRequest() { int request = 4; // display request options System.out.printf("%nEnter request%n%s%n%s%n%s%n%s%n", " 1 - List accounts with zero balances", " 2 - List accounts with credit balances", " 3 - List accounts with debit balances", " 4 - Terminate program"); try { Scanner input = new Scanner(System.in); do // input user request { System.out.printf("%n? "); request = input.nextInt(); } while ((request < 1) || (request > 4)); } catch (NoSuchElementException noSuchElementException) { System.err.println("Invalid input. Terminating."); } return choices[request - 1]; // return enum value for option } // read records from file and display only records of appropriate type private static void readRecords(MenuOption accountType) { // open file and process contents try (Scanner input = new Scanner(Paths.get("clients.txt"))) { while (input.hasNext()) // more data to read { int accountNumber = input.nextInt(); String firstName = input.next(); String lastName = input.next(); double balance = input.nextDouble(); Fig. 15.8 | Credit-inquiry program. (Part 2 of 4.) 660 Chapter 15 Files, Streams and Object Serialization 83 // if proper acount type, display record 84 if (shouldDisplay(accountType, balance)) 85 System.out.printf("%-10d%-12s%-12s%10.2f%n", accountNumber, 86 firstName, lastName, balance); 87 else 88 input.nextLine(); // discard the rest of the current record 89 } 90 } 91 catch (NoSuchElementException | 92 IllegalStateException | IOException e) 93 { 94 System.err.println("Error processing file. Terminating."); 95 System.exit(1); 96 } 97 } // end method readRecords 98 99 // use record type to determine if record should be displayed 100 private static boolean shouldDisplay( 101 MenuOption accountType, double balance) 102 { 103 if ((accountType == MenuOption.CREDIT_BALANCE) && (balance < 0)) 104 return true; 105 else if ((accountType == MenuOption.DEBIT_BALANCE) && (balance > 0)) 106 return true; 107 else if ((accountType == MenuOption.ZERO_BALANCE) && (balance == 0)) 108 return true; 109 110 return false; 111 } 112 } // end class CreditInquiry Enter request 1 - List accounts with zero balances 2 - List accounts with credit balances 3 - List accounts with debit balances 4 - Terminate program ? 1 Accounts with zero balances: 300 Pam White 0.00 Enter request 1 - List accounts with zero balances 2 - List accounts with credit balances 3 - List accounts with debit balances 4 - Terminate program ? 2 Accounts with credit balances: 200 Steve Green 400 Sam Red -345.67 -42.16 Fig. 15.8 | Credit-inquiry program. (Part 3 of 4.) 15.4 Sequential-Access Text Files 661 Enter request 1 - List accounts with zero balances 2 - List accounts with credit balances 3 - List accounts with debit balances 4 - Terminate program ? 3 Accounts with debit balances: 100 Bob Blue 500 Sue Yellow 24.98 224.62 Enter request 1 - List accounts with zero balances 2 - List accounts with credit balances 3 - List accounts with debit balances 4 - Terminate program ? 4 Fig. 15.8 | Credit-inquiry program. (Part 4 of 4.) The record information is collected by reading through the file and determining if each record satisfies the criteria for the selected account type. Line 18 in main calls method getRequest (lines 41–68) to display the menu options, translates the number typed by the user into a MenuOption and stores the result in MenuOption variable accountType. Lines 20–37 loop until the user specifies that the program should terminate. Lines 22–33 display a header for the current set of records to be output to the screen. Line 35 calls method readRecords (lines 71–97), which loops through the file and reads every record. Method readRecords uses a try-with-resources statement (introduced in Section 11.12) to create a Scanner that opens the file for reading (line 74)—recall that try-with-resources will close its resource(s) when the try block terminates successfully or due to an exception. The file will be opened for reading with a new Scanner object each time readRecords is called, so that we can again read from the beginning of the file. Lines 78–81 read a record. Line 84 calls method shouldDisplay (lines 100–111) to determine whether the current record satisfies the account type requested. If shouldDisplay returns true, the program displays the account information. When the end-of-file marker is reached, the loop terminates and the try-with-resources statement closes the Scanner and the file. Once all the records have been read, control returns to main and getRequest is again called (line 36) to retrieve the user’s next menu option. 15.4.4 Updating Sequential-Access Files The data in many sequential files cannot be modified without the risk of destroying other data in the file. For example, if the name “White” needs to be changed to “Worthington,” the old name cannot simply be overwritten, because the new name requires more space. The record for White was written to the file as 300 Pam White 0.00 662 Chapter 15 Files, Streams and Object Serialization If the record is rewritten beginning at the same location in the file using the new name, the record will be 300 Pam Worthington 0.00 The new record is larger (has more characters) than the original record. “Worthington” would overwrite the “0.00” in the current record and the characters beyond the second “o” in “Worthington” will overwrite the beginning of the next sequential record in the file. The problem here is that fields in a text file—and hence records—can vary in size. For example, 7, 14, –117, 2074 and 27383 are all ints stored in the same number of bytes (4) internally, but they’re different-sized fields when written to a file as text. Therefore, records in a sequential-access file are not usually updated in place—instead, the entire file is rewritten. To make the preceding name change, the records before 300 Pam White 0.00 would be copied to a new file, the new record (which can be of a different size than the one it replaces) would be written and the records after 300 Pam White 0.00 would be copied to the new file. Rewriting the entire file is uneconomical to update just one record, but reasonable if a substantial number of records need to be updated. 15.5 Object Serialization In Section 15.4, we demonstrated how to write the individual fields of a record into a file as text, and how to read those fields from a file. When the data was output to disk, certain information was lost, such as the type of each value. For instance, if the value "3" is read from a file, there’s no way to tell whether it came from an int, a String or a double. We have only data, not type information, on a disk. Sometimes we want to read an object from or write an object to a file or over a network connection. Java provides object serialization for this purposes. A serialized object is an object represented as a sequence of bytes that includes the object’s data as well as information about the object’s type and the types of data stored in the object. After a serialized object has been written into a file, it can be read from the file and deserialized—that is, the type information and bytes that represent the object and its data can be used to recreate the object in memory. Classes ObjectInputStream and ObjectOutputStream Classes ObjectInputStream and ObjectOutputStream (package java.io), which respectively implement the ObjectInput and ObjectOutput interfaces, enable entire objects to be read from or written to a stream (possibly a file). To use serialization with files, we initialize ObjectInputStream and ObjectOutputStream objects with stream objects that read from and write to files. Initializing stream objects with other stream objects in this manner is sometimes called wrapping—the new stream object being created wraps the stream object specified as a constructor argument. Classes ObjectInputStream and ObjectOutputStream simply read and write the byte-based representation of objects—they don’t know where to read the bytes from or write them to. The stream object that you pass to the ObjectInputStream constructor supplies the bytes that the ObjectInputStream converts into objects. Similarly, the stream object that you pass to the ObjectOutputStream constructor takes the byte-based representation of the object that the ObjectOutputStream produces and writes the bytes to the specified destination (e.g., a file, a network connection, etc.). 15.5 Object Serialization 663 Interfaces ObjectOutput and ObjectInput The ObjectOutput interface contains method writeObject, which takes an Object as an argument and writes its information to an OutputStream. A class that implements interface ObjectOutput (such as ObjectOutputStream) declares this method and ensures that the object being output implements interface Serializable (discussed shortly). Similarly, the ObjectInput interface contains method readObject, which reads and returns a reference to an Object from an InputStream. After an object has been read, its reference can be cast to the object’s actual type. As you’ll see in Chapter 28, applications that communicate via a network, such as the Internet, can also transmit objects across the network. 15.5.1 Creating a Sequential-Access File Using Object Serialization This section and Section 15.5.2 create and manipulate sequential-access files using object serialization. The object serialization we show here is performed with byte-based streams, so the sequential files created and manipulated will be binary files. Recall that binary files typically cannot be viewed in standard text editors. For this reason, we write a separate application that knows how to read and display serialized objects. We begin by creating and writing serialized objects to a sequential-access file. The example is similar to the one in Section 15.4, so we focus only on the new features. Defining Class Account We begin by defining class Account (Fig. 15.9), which encapsulates the client record information used by the serialization examples. These examples and class Account are all located in the SerializationApps directory with the chapter’s examples. This allows class Account to be used by both examples, because their files are defined in the same default package. Class Account contains private instance variables account, firstName, lastName and balance (lines 7–10) and set and get methods for accessing these instance variables. Though the set methods do not validate the data in this example, they should do so in an “industrial-strength” system. Class Account implements interface Serializable (line 5), which allows objects of this class to be serialized and deserialized with ObjectOutputStreams and ObjectInputStreams, respectively. Interface Serializable is a tagging interface. Such an interface does not contain methods. A class that implements Serializable is tagged as being a Serializable object. This is important, because an ObjectOutputStream will not output an object unless it is a Serializable object, which is the case for any object of a class that implements Serializable. 1 2 3 4 5 6 7 8 9 10 11 // Fig. 15.9: Account.java // Serializable Account class for storing records as objects. import java.io.Serializable; public class Account implements Serializable { private int account; private String firstName; private String lastName; private double balance; Fig. 15.9 | Account class for serializable objects. (Part 1 of 3.) 664 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 Chapter 15 Files, Streams and Object Serialization // initializes an Account with default values public Account() { this(0, "", "", 0.0); // call other constructor } // initializes an Account with provided values public Account(int account, String firstName, String lastName, double balance) { this.account = account; this.firstName = firstName; this.lastName = lastName; this.balance = balance; } // set account number public void setAccount(int acct) { this.account = account; } // get account number public int getAccount() { return account; } // set first name public void setFirstName(String firstName) { this.firstName = firstName; } // get first name public String getFirstName() { return firstName; } // set last name public void setLastName(String lastName) { this.lastName = lastName; } // get last name public String getLastName() { return lastName; } Fig. 15.9 | Account class for serializable objects. (Part 2 of 3.) 15.5 Object Serialization 64 65 66 67 68 69 70 71 72 73 74 75 665 // set balance public void setBalance(double balance) { this.balance = balance; } // get balance public double getBalance() { return balance; } } // end class Account Fig. 15.9 | Account class for serializable objects. (Part 3 of 3.) In a Serializable class, every instance variable must be Serializable. Non-Seriinstance variables must be declared transient to indicate that they should be ignored during the serialization process. By default, all primitive-type variables are serializable. For reference-type variables, you must check the class’s documentation (and possibly its superclasses) to ensure that the type is Serializable. For example, Strings are Serializable. By default, arrays are serializable; however, in a reference-type array, the referenced objects might not be. Class Account contains private data members account, firstName, lastName and balance—all of which are Serializable. This class also provides public get and set methods for accessing the private fields. alizable Writing Serialized Objects to a Sequential-Access File Now let’s discuss the code that creates the sequential-access file (Fig. 15.10). We concentrate only on new concepts here. To open the file, line 27 calls Files static method newOutputStream, which receives a Path specifying the file to open and, if the file exists, returns an OutputStream that can be used to write to the file. Existing files that are opened for output in this manner are truncated. There is no standard filename extension for files that store serialized objects, so we chose the .ser. 1 2 3 4 5 6 7 8 9 10 11 12 13 // Fig. 15.10: CreateSequentialFile.java // Writing objects sequentially to a file with class ObjectOutputStream. import java.io.IOException; import java.io.ObjectOutputStream; import java.nio.file.Files; import java.nio.file.Paths; import java.util.NoSuchElementException; import java.util.Scanner; public class CreateSequentialFile { private static ObjectOutputStream output; // outputs data to file Fig. 15.10 | Sequential file created using ObjectOutputStream. (Part 1 of 3.) 666 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Chapter 15 Files, Streams and Object Serialization public static void main(String[] args) { openFile(); addRecords(); closeFile(); } // open file clients.ser public static void openFile() { try { output = new ObjectOutputStream( Files.newOutputStream(Paths.get("clients.ser"))); } catch (IOException ioException) { System.err.println("Error opening file. Terminating."); System.exit(1); // terminate the program } } // add records to file public static void addRecords() { Scanner input = new Scanner(System.in); System.out.printf("%s%n%s%n? ", "Enter account number, first name, last name and balance.", "Enter end-of-file indicator to end input."); while (input.hasNext()) // loop until end-of-file indicator { try { // create new record; this example assumes valid input Account record = new Account(input.nextInt(), input.next(), input.next(), input.nextDouble()); // serialize record object into file output.writeObject(record); } catch (NoSuchElementException elementException) { System.err.println("Invalid input. Please try again."); input.nextLine(); // discard input so user can try again } catch (IOException ioException) { System.err.println("Error writing to file. Terminating."); break; } Fig. 15.10 | Sequential file created using ObjectOutputStream. (Part 2 of 3.) 15.5 Object Serialization 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 667 System.out.print("? "); } } // close file and terminate application public static void closeFile() { try { if (output != null) output.close(); } catch (IOException ioException) { System.err.println("Error closing file. Terminating."); } } } // end class CreateSequentialFile Enter Enter ? 100 ? 200 ? 300 ? 400 ? 500 ? ^Z account number, first name, last name and balance. end-of-file indicator to end input. Bob Blue 24.98 Steve Green -345.67 Pam White 0.00 Sam Red -42.16 Sue Yellow 224.62 Fig. 15.10 | Sequential file created using ObjectOutputStream. (Part 3 of 3.) Class OutputStream provides methods for outputting byte arrays and individual but we wish to write objects to a file. For this reason, lines 26–27 pass the InputStream to class ObjectInputStream’s constructor, which wrap the OutputStream in an ObjectOutputStream. The ObjectOutputStream object uses the OutputStream to write into the file the bytes that represent entire objects. Lines 26–27 might throw an IOException if a problem occurs while opening the file (e.g., when a file is opened for writing on a drive with insufficient space or when a read-only file is opened for writing). If so, the program displays an error message (lines 29–33). If no exception occurs, the file is open, and variable output can be used to write objects to it. This program assumes that data is input correctly and in the proper record-number order. Method addRecords (lines 37–69) performs the write operation. Lines 50–51 create an Account object from the data entered by the user. Line 54 calls ObjectOutputStream method writeObject to write the record object to the output file. Only one statement is required to write the entire object. Method closeFile (lines 72–83) calls ObjectOutputStream method close on output to close both the ObjectOutputStream and its underlying OutputStream. The call to method close is contained in a try block, because close throws an IOException if the file cannot be closed properly. When using wrapped streams, closing the outermost stream also closes the wrapped stream as well. bytes, 668 Chapter 15 Files, Streams and Object Serialization In the sample execution for the program in Fig. 15.10, we entered information for five accounts—the same information shown in Fig. 15.5. The program does not show how the data records actually appear in the file. Remember that now we’re using binary files, which are not humanly readable. To verify that the file has been created successfully, the next section presents a program to read the file’s contents. 15.5.2 Reading and Deserializing Data from a Sequential-Access File The preceding section showed how to create a file for sequential access using object serialization. In this section, we discuss how to read serialized data sequentially from a file. The program in Fig. 15.11 reads records from a file created by the program in Section 15.5.1 and displays the contents. The program opens the file for input by calling Files static method newInputStream, which receives a Path specifying the file to open and, if the file exists, returns an InputStream that can be used to read from the file. In Fig. 15.10, we wrote objects to the file, using an ObjectOutputStream object. Data must be read from the file in the same format in which it was written. Therefore, we use an ObjectInputStream wrapped around an InputStream (lines 26–27). If no exceptions occur when opening the file, variable input can be used to read objects from the file. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 // Fig. 15.11: ReadSequentialFile.java // Reading a file of objects sequentially with ObjectInputStream // and displaying each record. import java.io.EOFException; import java.io.IOException; import java.io.ObjectInputStream; import java.nio.file.Files; import java.nio.file.Paths; public class ReadSequentialFile { private static ObjectInputStream input; public static void main(String[] args) { openFile(); readRecords(); closeFile(); } // enable user to select file to open public static void openFile() { try // open file { input = new ObjectInputStream( Files.newInputStream(Paths.get("clients.ser"))); } catch (IOException ioException) { Fig. 15.11 | Reading a file of objects sequentially with ObjectInputStream and displaying each record. (Part 1 of 3.) 15.5 Object Serialization 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 669 System.err.println("Error opening file."); System.exit(1); } } // read record from file public static void readRecords() { System.out.printf("%-10s%-12s%-12s%10s%n", "Account", "First Name", "Last Name", "Balance"); try { while (true) // loop until there is an EOFException { Account record = (Account) input.readObject(); // display record contents System.out.printf("%-10d%-12s%-12s%10.2f%n", record.getAccount(), record.getFirstName(), record.getLastName(), record.getBalance()); } } catch (EOFException endOfFileException) { System.out.printf("%No more records%n"); } catch (ClassNotFoundException classNotFoundException) { System.err.println("Invalid object type. Terminating."); } catch (IOException ioException) { System.err.println("Error reading from file. Terminating."); } } // end method readRecords // close file and terminate application public static void closeFile() { try { if (input != null) input.close(); } catch (IOException ioException) { System.err.println("Error closing file. Terminating."); System.exit(1); } } } // end class ReadSequentialFile Fig. 15.11 | Reading a file of objects sequentially with ObjectInputStream and displaying each record. (Part 2 of 3.) 670 Account 100 200 300 400 500 Chapter 15 Files, Streams and Object Serialization First Name Bob Steve Pam Sam Sue Last Name Blue Green White Red Yellow Balance 24.98 -345.67 0.00 -42.16 224.62 No more records Fig. 15.11 | Reading a file of objects sequentially with ObjectInputStream and displaying each record. (Part 3 of 3.) The program reads records from the file in method readRecords (lines 37–66). Line 46 calls ObjectInputStream method readObject to read an Object from the file. To use Account-specific methods, we downcast the returned Object to type Account. Method readObject throws an EOFException (processed at lines 54–57) if an attempt is made to read beyond the end of the file. Method readObject throws a ClassNotFoundException if the class for the object being read cannot be located. This may occur if the file is accessed on a computer that does not have the class. Software Engineering Observation 15.1 This section introduced object serialization and demonstrated basic serialization techniques. Serialization is a deep subject with many traps and pitfalls. Before implementing object serialization in industrial-strength applications, carefully read the online Java documentation for object serialization. 15.6 Opening Files with JFileChooser Class JFileChooser displays a dialog that enables the user to easily select files or directories. To demonstrate JFileChooser, we enhance the example in Section 15.3, as shown in Figs. 15.12–15.13. The example now contains a graphical user interface, but still displays the same data as before. The constructor calls method analyzePath in line 24. This method then calls method getFileOrDirectoryPath in line 31 to retrieve a Path object representing the selected file or directory. Method getFileOrDirectoryPath (lines 71–85 of Fig. 15.12) creates a JFileChooser (line 74). Lines 75–76 call method setFileSelectionMode to specify what the user can select from the fileChooser. For this program, we use JFileChooser static constant FILES_AND_DIRECTORIES to indicate that files and directories can be selected. Other static constants include FILES_ONLY (the default) and DIRECTORIES_ONLY. Line 77 calls method showOpenDialog to display the JFileChooser dialog titled Open. Argument this specifies the JFileChooser dialog’s parent window, which determines the position of the dialog on the screen. If null is passed, the dialog is displayed in the center of the screen—otherwise, the dialog is centered over the application window (specified by the argument this). A JFileChooser dialog is a modal dialog that does not allow the user to interact with any other window in the program until the dialog is closed. The user selects the drive, directory or filename, then clicks Open. Method showOpenDialog returns an integer specifying which button (Open or Cancel) the user clicked to close the dialog. Line 15.6 Opening Files with JFileChooser 671 48 tests whether the user clicked Cancel by comparing the result with static constant If they’re equal, the program terminates. Line 84 calls JFileChooser method getSelectedFile to retrieve a File object (package java.io) representing the file or directory that the user selected, then calls File method toPath to return a Path object. The program then displays information about the selected file or directory. CANCEL_OPTION. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 // Fig. 15.12: JFileChooserDemo.java // Demonstrating JFileChooser. import java.io.IOException; import java.nio.file.DirectoryStream; import java.nio.file.Files; import java.nio.file.Path; import java.nio.file.Paths; import javax.swing.JFileChooser; import javax.swing.JFrame; import javax.swing.JOptionPane; import javax.swing.JScrollPane; import javax.swing.JTextArea; public class JFileChooserDemo extends JFrame { private final JTextArea outputArea; // displays file contents // set up GUI public JFileChooserDemo() throws IOException { super("JFileChooser Demo"); outputArea = new JTextArea(); add(new JScrollPane(outputArea)); // outputArea is scrollable analyzePath(); // get Path from user and display info } // display information about file or directory user specifies public void analyzePath() throws IOException { // get Path to user-selected file or directory Path path = getFileOrDirectoryPath(); if (path != null && Files.exists(path)) // if exists, display info { // gather file (or directory) information StringBuilder builder = new StringBuilder(); builder.append(String.format("%s:%n", path.getFileName())); builder.append(String.format("%s a directory%n", Files.isDirectory(path) ? "Is" : "Is not")); builder.append(String.format("%s an absolute path%n", path.isAbsolute() ? "Is" : "Is not")); builder.append(String.format("Last modified: %s%n", Files.getLastModifiedTime(path))); builder.append(String.format("Size: %s%n", Files.size(path))); builder.append(String.format("Path: %s%n", path)); Fig. 15.12 | Demonstrating JFileChooser. (Part 1 of 2.) 672 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 Chapter 15 Files, Streams and Object Serialization builder.append(String.format("Absolute path: %s%n", path.toAbsolutePath())); if (Files.isDirectory(path)) // output directory listing { builder.append(String.format("%nDirectory contents:%n")); // object for iterating through a directory's contents DirectoryStream directoryStream = Files.newDirectoryStream(path); for (Path p : directoryStream) builder.append(String.format("%s%n", p)); } outputArea.setText(builder.toString()); // display String content } else // Path does not exist { JOptionPane.showMessageDialog(this, path.getFileName() + " does not exist.", "ERROR", JOptionPane.ERROR_MESSAGE); } } // end method analyzePath // allow user to specify file or directory name private Path getFileOrDirectoryPath() { // configure dialog allowing selection of a file or directory JFileChooser fileChooser = new JFileChooser(); fileChooser.setFileSelectionMode( JFileChooser.FILES_AND_DIRECTORIES); int result = fileChooser.showOpenDialog(this); // if user clicked Cancel button on dialog, return if (result == JFileChooser.CANCEL_OPTION) System.exit(1); // return Path representing the selected file return fileChooser.getSelectedFile().toPath(); } } // end class JFileChooserDemo Fig. 15.12 | Demonstrating JFileChooser. (Part 2 of 2.) 1 2 3 4 5 6 7 // Fig. 15.13: JFileChooserTest.java // Tests class JFileChooserDemo. import java.io.IOException; import javax.swing.JFrame; public class JFileChooserTest { Fig. 15.13 | Testing class FileDemonstration. (Part 1 of 2.) 15.7 (Optional) Additional java.io Classes 8 9 10 11 12 13 14 15 673 public static void main(String[] args) throws IOException { JFileChooserDemo application = new JFileChooserDemo(); application.setSize(400, 400); application.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); application.setVisible(true); } } // end class JFileChooserTest a) Use this dialog to locate and select a file or directory Click Open to submit file or directory name to program Files and directories are displayed here b) Selected file’s or directory’s information; if it’s a directory, the contents of that directory are displayed Fig. 15.13 | Testing class FileDemonstration. (Part 2 of 2.) 15.7 (Optional) Additional java.io Classes This section overviews additional interfaces and classes (from package java.io). 15.7.1 Interfaces and Classes for Byte-Based Input and Output InputStream and OutputStream are abstract classes that declare methods for performing byte-based input and output, respectively. Pipe Streams Pipes are synchronized communication channels between threads. We discuss threads in Chapter 23. Java provides PipedOutputStream (a subclass of OutputStream) and Piped- 674 Chapter 15 Files, Streams and Object Serialization (a subclass of InputStream) to establish pipes between two threads in a program. One thread sends data to another by writing to a PipedOutputStream. The target thread reads information from the pipe via a PipedInputStream. InputStream Filter Streams A FilterInputStream filters an InputStream, and a FilterOutputStream filters an OutputStream. Filtering means simply that the filter stream provides additional functionality, such as aggregating bytes into meaningful primitive-type units. FilterInputStream and FilterOutputStream are typically used as superclasses, so some of their filtering capabilities are provided by their subclasses. A PrintStream (a subclass of FilterOutputStream) performs text output to the specified stream. Actually, we’ve been using PrintStream output throughout the text to this point—System.out and System.err are PrintStream objects. Data Streams Reading data as raw bytes is fast, but crude. Usually, programs read data as aggregates of bytes that form ints, floats, doubles and so on. Java programs can use several classes to input and output data in aggregate form. Interface DataInput describes methods for reading primitive types from an input stream. Classes DataInputStream and RandomAccessFile each implement this interface to read sets of bytes and view them as primitive-type values. Interface DataInput includes methods such as readBoolean, readByte, readChar, readDouble, readFloat, readFully (for byte arrays), readInt, readLong, readShort, readUnsignedByte, readUnsignedShort, readUTF (for reading Unicode characters encoded by Java—we discuss UTF encoding in Appendix H) and skipBytes. Interface DataOutput describes a set of methods for writing primitive types to an output stream. Classes DataOutputStream (a subclass of FilterOutputStream) and RandomAccessFile each implement this interface to write primitive-type values as bytes. Interface DataOutput includes overloaded versions of method write (for a byte or for a byte array) and methods writeBoolean, writeByte, writeBytes, writeChar, writeChars (for Unicode Strings), writeDouble, writeFloat, writeInt, writeLong, writeShort and writeUTF (to output text modified for Unicode). Buffered Streams Buffering is an I/O-performance-enhancement technique. With a BufferedOutputStream (a subclass of class FilterOutputStream), each output statement does not necessarily result in an actual physical transfer of data to the output device (which is a slow operation compared to processor and main memory speeds). Rather, each output operation is directed to a region in memory called a buffer that’s large enough to hold the data of many output operations. Then, actual transfer to the output device is performed in one large physical output operation each time the buffer fills. The output operations directed to the output buffer in memory are often called logical output operations. With a BufferedOutputStream, a partially filled buffer can be forced out to the device at any time by invoking the stream object’s flush method. Using buffering can greatly increase the performance of an application. Typical I/O operations are extremely slow compared with the speed of accessing data in computer 15.7 (Optional) Additional java.io Classes 675 memory. Buffering reduces the number of I/O operations by first combining smaller outputs together in memory. The number of actual physical I/O operations is small compared with the number of I/O requests issued by the program. Thus, the program that’s using buffering is more efficient. Performance Tip 15.1 Buffered I/O can yield significant performance improvements over unbuffered I/O. With a BufferedInputStream (a subclass of class FilterInputStream), many “logical” chunks of data from a file are read as one large physical input operation into a memory buffer. As a program requests each new chunk of data, it’s taken from the buffer. (This procedure is sometimes referred to as a logical input operation.) When the buffer is empty, the next actual physical input operation from the input device is performed to read in the next group of “logical” chunks of data. Thus, the number of actual physical input operations is small compared with the number of read requests issued by the program. Memory-Based byte Array Steams Java stream I/O includes capabilities for inputting from byte arrays in memory and outputting to byte arrays in memory. A ByteArrayInputStream (a subclass of InputStream) reads from a byte array in memory. A ByteArrayOutputStream (a subclass of OutputStream) outputs to a byte array in memory. One use of byte-array I/O is data validation. A program can input an entire line at a time from the input stream into a byte array. Then a validation routine can scrutinize the contents of the byte array and correct the data if necessary. Finally, the program can proceed to input from the byte array, “knowing” that the input data is in the proper format. Outputting to a byte array is a nice way to take advantage of the powerful output-formatting capabilities of Java streams. For example, data can be stored in a byte array, using the same formatting that will be displayed at a later time, and the byte array can then be output to a file to preserve the formatting. Sequencing Input from Multiple Streams A SequenceInputStream (a subclass of InputStream) logically concatenates several InputStreams—the program sees the group as one continuous InputStream. When the program reaches the end of one input stream, that stream closes, and the next stream in the sequence opens. 15.7.2 Interfaces and Classes for Character-Based Input and Output In addition to the byte-based streams, Java provides the Reader and Writer abstract classes, which are character-based streams like those you used for text-file processing in Section 15.4. Most of the byte-based streams have corresponding character-based concrete Reader or Writer classes. Character-Based Buffering Readers and Writers Classes BufferedReader (a subclass of abstract class Reader) and BufferedWriter (a subclass of abstract class Writer) enable buffering for character-based streams. Remember that character-based streams use Unicode characters—such streams can process data in any language that the Unicode character set represents. 676 Chapter 15 Files, Streams and Object Serialization Memory-Based char Array Readers and Writers Classes CharArrayReader and CharArrayWriter read and write, respectively, a stream of characters to a char array. A LineNumberReader (a subclass of BufferedReader) is a buffered character stream that keeps track of the number of lines read—newlines, returns and carriage-return–line-feed combinations increment the line count. Keeping track of line numbers can be useful if the program needs to inform the reader of an error on a specific line. Character-Based File, Pipe and String Readers and Writers An InputStream can be converted to a Reader via class InputStreamReader. Similarly, an OutputStream can be converted to a Writer via class OutputStreamWriter. Class FileReader (a subclass of InputStreamReader) and class FileWriter (a subclass of OutputStreamWriter) read characters from and write characters to a file, respectively. Class PipedReader and class PipedWriter implement piped-character streams for transferring data between threads. Class StringReader and StringWriter read characters from and write characters to Strings, respectively. A PrintWriter writes characters to a stream. 15.8 Wrap-Up In this chapter, you learned how to manipulate persistent data. We compared byte-based and character-based streams, and introduced several classes from packages java.io and java.nio.file. You used classes Files and Paths and interfaces Path and DirectoryStream to retrieve information about files and directories. You used sequential-access file processing to manipulate records that are stored in order by the record-key field. You learned the differences between text-file processing and object serialization, and used serialization to store and retrieve entire objects. The chapter concluded with a small example of using a JFileChooser dialog to allow users to easily select files from a GUI. The next chapter discusses Java’s classes for manipulating collections of data—such as class ArrayList, which we introduced in Section 7.16. Summary Section 15.1 Introduction • Computers use files for long-term retention of large amounts of persistent data (p. 645), even after the programs that created the data terminate. • Computers store files on secondary storage devices (p. 645) such as hard disks. Section 15.2 Files and Streams • Java views each file as a sequential stream of bytes (p. 645). • Every operating system provides a mechanism to determine the end of a file, such as an end-offile marker (p. 645) or a count of the total bytes in the file. • Byte-based streams (p. 646) represent data in binary format. • Character-based streams (p. 646) represent data as sequences of characters. • Files created using byte-based streams are binary files (p. 646). Files created using characterbased streams are text files (p. 646). Text files can be read by text editors, whereas binary files are read by a program that converts the data to a human-readable format. Summary 677 • Java also can associate streams with different devices. Three stream objects are associated with devices when a Java program begins executing—System.in, System.out and System.err. Section 15.3 Using NIO Classes and Interfaces to Get File and Directory Information • A Path (p. 647) represents the location of a file or directory. Path objects do not open files or provide any file-processing capabilities. • Class Paths (p. 647) is used to get a Path object representing a file or directory location. • Class Files (p. 647) provides static methods for common file and directory manipulations, including methods for copying files; creating and deleting files and directories; getting information about files and directories; reading the contents of files; getting objects that allow you to manipulate the contents of files and directories; and more. • A DirectoryStream (p. 647) enables a program to iterate through the contents of a directory. • The static method get (p. 647) of class Paths converts a String representing a file’s or directory’s location into a Path object. • Character-based input and output can be performed with classes Scanner and Formatter. • Class Formatter (p. 647) enables formatted data to be output to the screen or to a file in a manner similar to System.out.printf. • An absolute path (p. 647) contains all the directories, starting with the root directory (p. 647), that lead to a specific file or directory. Every file or directory on a disk drive has the same root directory in its path. • A relative path (p. 647) starts from the directory in which the application began executing. • Files static method exists (p. 648) receives a Path and determines whether it exists (either as a file or as a directory) on disk. • Path method getFileName (p. 648) gets the String name of a file or directory without any location information. • Files static method isDirectory (p. 648) receives a Path and returns a boolean indicating whether that Path represents a directory on disk. • Path method isAbsolute (p. 648) returns a boolean indicating whether a Path represents an absolute path to a file or directory. • Files static method getLastModifiedTime (p. 648) receives a Path and returns a FileTime (package java.nio.file.attribute) indicating when the file was last modified. • Files static method size (p. 648) receives a Path and returns a long representing the number of bytes in the file or directory. For directories, the value returned is platform specific. • Path method toString (p. 648) returns a String representation of the Path. • Path method toAbsolutePath (p. 648) converts the Path on which it’s called to an absolute path. • Files static method newDirectoryStream (p. 648) returns a DirectoryStream containing Path objects for a directory’s contents. • A separator character (p. 650) is used to separate directories and files in the path. Section 15.4 Sequential-Access Text Files • Java imposes no structure on a file. You must structure files to meet your application’s needs. • To retrieve data sequentially from a file, programs normally start from the beginning of the file and read all the data consecutively until the desired information is found. • Data in many sequential files cannot be modified without the risk of destroying other data in the file. Records in a sequential-access file are usually updated by rewriting the entire file. 678 Chapter 15 Files, Streams and Object Serialization Section 15.5 Object Serialization • Java provides a mechanism called object serialization (p. 662) that enables entire objects to be written to or read from a stream. • A serialized object (p. 662) is represented as a sequence of bytes that includes the object’s data as well as information about the object’s type and the types of data it stores. • After a serialized object has been written into a file, it can be read from the file and deserialized (p. 662) to recreate the object in memory. • Classes ObjectInputStream (p. 662) and ObjectOutputStream (p. 662) enable entire objects to be read from or written to a stream (possibly a file). • Only classes that implement interface Serializable (p. 663) can be serialized and deserialized. • The ObjectOutput interface (p. 662) contains method writeObject (p. 663), which takes an Object as an argument and writes its information to an OutputStream. A class that implements this interface, such as ObjectOutputStream, would ensure that the Object is Serializable. • The ObjectInput interface (p. 662) contains method readObject (p. 663), which reads and returns a reference to an Object from an InputStream. After an object has been read, its reference can be cast to the object’s actual type. Section 15.6 Opening Files with JFileChooser • Class JFileChooser (p. 670) is used to display a dialog that enables users of a program to easily select files or directories from a GUI. Section 15.7 (Optional) Additional java.io Classes • InputStream and OutputStream are abstract classes for performing byte-based I/O. • Pipes (p. 673) are synchronized communication channels between threads. One thread sends data via a PipedOutputStream (p. 673). The target thread reads information from the pipe via a PipedInputStream (p. 673). • A filter stream (p. 674) provides additional functionality, such as aggregating data bytes into meaningful primitive-type units. FilterInputStream (p. 674) and FilterOutputStream are typically extended, so some of their filtering capabilities are provided by their concrete subclasses. • A PrintStream (p. 674) performs text output. System.out and System.err are PrintStreams. • Interface DataInput describes methods for reading primitive types from an input stream. Classes DataInputStream (p. 674) and RandomAccessFile each implement this interface. • Interface DataOutput describes methods for writing primitive types to an output stream. Classes DataOutputStream (p. 674) and RandomAccessFile each implement this interface. • Buffering is an I/O-performance-enhancement technique. Buffering reduces the number of I/O operations by combining smaller outputs together in memory. The number of physical I/O operations is much smaller than the number of I/O requests issued by the program. • With a BufferedOutputStream (p. 674) each output operation is directed to a buffer (p. 674) large enough to hold the data of many output operations. Transfer to the output device is performed in one large physical output operation (p. 674) when the buffer fills. A partially filled buffer can be forced out to the device at any time by invoking the stream object’s flush method (p. 674). • With a BufferedInputStream (p. 675), many “logical” chunks of data from a file are read as one large physical input operation (p. 675) into a memory buffer. As a program requests data, it’s taken from the buffer. When the buffer is empty, the next actual physical input operation is performed. • A ByteArrayInputStream reads from a byte array in memory. A ByteArrayOutputStream outputs to a byte array in memory. Self-Review Exercises 679 • A SequenceInputStream concatenates several InputStreams. When the program reaches the end of an input stream, that stream closes, and the next stream in the sequence opens. • The Reader (p. 675) and Writer (p. 675) abstract classes are Unicode character-based streams. Most byte-based streams have corresponding character-based concrete Reader or Writer classes. • Classes BufferedReader (p. 675) and BufferedWriter (p. 675) buffer character-based streams. • Classes CharArrayReader (p. 676) and CharArrayWriter (p. 676) manipulate char arrays. • A LineNumberReader (p. 676) is a buffered character stream that tracks the number of lines read. • Classes FileReader (p. 676) and FileWriter (p. 676) perform character-based file I/O. • Class PipedReader (p. 676) and class PipedWriter (p. 676) implement piped-character streams for transferring data between threads. • Class StringReader (p. 676) and StringWriter (p. 676) read characters from and write characters to Strings, respectively. A PrintWriter (p. 654) writes characters to a stream. Self-Review Exercises 15.1 Determine whether each of the following statements is true or false. If false, explain why. a) You must explicitly create the stream objects System.in, System.out and System.err. b) When reading data from a file using class Scanner, if you wish to read data in the file multiple times, the file must be closed and reopened to read from the beginning of the file. c) Files static method exists receives a Path and determines whether it exists (either as a file or as a directory) on disk d) Binary files are human readable in a text editor. e) An absolute path contains all the directories, starting with the root directory, that lead to a specific file or directory. f) Class Formatter contains method printf, which enables formatted data to be output to the screen or to a file. 15.2 Complete the following tasks, assuming that each applies to the same program: a) Write a statement that opens file "oldmast.txt" for input—use Scanner variable inOldMaster. b) Write a statement that opens file "trans.txt" for input—use Scanner variable inTransaction. c) Write a statement that opens file "newmast.txt" for output (and creation)—use formatter variable outNewMaster. d) Write the statements needed to read a record from the file "oldmast.txt". Use the data to create an object of class Account—use Scanner variable inOldMaster. Assume that class Account is the same as the Account class in Fig. 15.9. e) Write the statements needed to read a record from the file "trans.txt". The record is an object of class TransactionRecord—use Scanner variable inTransaction. Assume that class TransactionRecord contains method setAccount (which takes an int) to set the account number and method setAmount (which takes a double) to set the amount of the transaction. f) Write a statement that outputs a record to the file "newmast.txt". The record is an object of type Account—use Formatter variable outNewMaster. 15.3 Complete the following tasks, assuming that each applies to the same program: a) Write a statement that opens file "oldmast.ser" for input—use ObjectInputStream variable inOldMaster to wrap an InputStream object. b) Write a statement that opens file "trans.ser" for input—use ObjectInputStream variable inTransaction to wrap an InputStream object. 680 Chapter 15 Files, Streams and Object Serialization c) Write a statement that opens file "newmast.ser" for output (and creation)—use ObjectOutputStream variable outNewMaster to wrap an OutputStream. d) Write a statement that reads a record from the file "oldmast.ser". The record is an object of class Account—use ObjectInputStream variable inOldMaster. Assume class Account is the same as the Account class in Fig. 15.9 e) Write a statement that reads a record from the file "trans.ser". The record is an object of class TransactionRecord—use ObjectInputStream variable inTransaction. f) Write a statement that outputs a record of type Account to the file "newmast.ser"—use ObjectOutputStream variable outNewMaster. Answers to Self-Review Exercises 15.1 a) False. These three streams are created for you when a Java application begins executing. b) True. c) True. d) False. Text files are human readable in a text editor. Binary files might be human readable, but only if the bytes in the file represent ASCII characters. e) True. f) False. Class Formatter contains method format, which enables formatted data to be output to the screen or to a file. 15.2 a) b) c) d) Scanner inOldMaster = new Scanner(Paths.get("oldmast.txt")); Scanner inTransaction = new Scanner(Paths.get("trans.txt")); Formatter outNewMaster = new Formatter("newmast.txt"); Account account = new Account(); account.setAccount(inOldMaster.nextInt()); account.setFirstName(inOldMaster.next()); account.setLastName(inOldMaster.next()); account.setBalance(inOldMaster.nextDouble()); e) TransactionRecord transaction = new Transaction(); transaction.setAccount(inTransaction.nextInt()); transaction.setAmount(inTransaction.nextDouble()); f) outNewMaster.format("%d %s %s %.2f%n", account.getAccount(), account.getFirstName(), account.getLastName(), account.getBalance()); 15.3 a) ObjectInputStream inOldMaster = new ObjectInputStream( b) ObjectInputStream inTransaction = new ObjectInputStream( c) ObjectOutputStream outNewMaster = new ObjectOutputStream( d) e) f) Account = (Account) inOldMaster.readObject(); Files.newInputStream(Paths.get("oldmast.ser"))); Files.newOutputStream(Paths.get("trans.ser"))); Files.newOutputStream(Paths.get("newmast.ser"))); transactionRecord = (TransactionRecord) inTransaction.readObject(); outNewMaster.writeObject(newAccount); Exercises 15.4 (File Matching) Self-Review Exercise 15.2 asked you to write a series of single statements. Actually, these statements form the core of an important type of file-processing program—namely, a file-matching program. In commercial data processing, it’s common to have several files in each application system. In an accounts receivable system, for example, there’s generally a master file containing detailed information about each customer, such as the customer’s name, address, telephone number, outstanding balance, credit limit, discount terms, contract arrangements and possibly a condensed history of recent purchases and cash payments. Exercises 681 As transactions occur (i.e., sales are made and payments arrive in the mail), information about them is entered into a file. At the end of each business period (a month for some companies, a week for others, and a day in some cases), the file of transactions (called "trans.txt") is applied to the master file (called "oldmast.txt") to update each account’s purchase and payment record. During an update, the master file is rewritten as the file "newmast.txt", which is then used at the end of the next business period to begin the updating process again. File-matching programs must deal with certain problems that do not arise in single-file programs. For example, a match does not always occur. If a customer on the master file has not made any purchases or cash payments in the current business period, no record for this customer will appear on the transaction file. Similarly, a customer who did make some purchases or cash payments could have just moved to this community, and if so, the company may not have had a chance to create a master record for this customer. Write a complete file-matching accounts receivable program. Use the account number on each file as the record key for matching purposes. Assume that each file is a sequential text file with records stored in increasing account-number order. a) Define class TransactionRecord. Objects of this class contain an account number and amount for the transaction. Provide methods to modify and retrieve these values. b) Modify class Account in Fig. 15.9 to include method combine, which takes a TransactionRecord object and combines the balance of the Account object and the amount value of the TransactionRecord object. c) Write a program to create data for testing the program. Use the sample account data in Figs. 15.14 and 15.15. Run the program to create the files trans.txt and oldmast.txt to be used by your file-matching program. Master file account number Name Balance 100 Alan Jones 348.17 300 Mary Smith 27.19 500 Sam Sharp 0.00 700 Suzy Green –14.22 Fig. 15.14 | Sample data for master file. Transaction file account number Transaction amount 100 27.14 300 62.11 400 100.56 900 82.17 Fig. 15.15 | Sample data for transaction file. d) Create class FileMatch to perform the file-matching functionality. The class should contain methods that read oldmast.txt and trans.txt. When a match occurs (i.e., records with the same account number appear in both the master file and the transaction 682 Chapter 15 Files, Streams and Object Serialization file), add the dollar amount in the transaction record to the current balance in the master record, and write the "newmast.txt" record. (Assume that purchases are indicated by positive amounts in the transaction file and payments by negative amounts.) When there’s a master record for a particular account, but no corresponding transaction record, merely write the master record to "newmast.txt". When there’s a transaction record, but no corresponding master record, print to a log file the message "Unmatched transaction record for account number…" (fill in the account number from the transaction record). The log file should be a text file named "log.txt". 15.5 (File Matching with Multiple Transactions) It’s possible (and actually common) to have several transaction records with the same record key. This situation occurs, for example, when a customer makes several purchases and cash payments during a business period. Rewrite your accounts receivable file-matching program from Exercise 15.4 to provide for the possibility of handling several transaction records with the same record key. Modify the test data of CreateData.java to include the additional transaction records in Fig. 15.16. Account number Dollar amount 300 83.89 700 80.78 700 1.53 Fig. 15.16 | Additional transaction records. 15.6 (File Matching with Object Serialization) Recreate your solution for Exercise 15.5 using object serialization. Use the statements from Exercise 15.3 as your basis for this program. You may want to create applications to read the data stored in the .ser files—the code in Section 15.5.2 can be modified for this purpose. 15.7 (Telephone-Number Word Generator) Standard telephone keypads contain the digits zero through nine. The numbers two through nine each have three letters associated with them (Fig. 15.17). Many people find it difficult to memorize phone numbers, so they use the correspondence between digits and letters to develop seven-letter words that correspond to their phone numbers. For example, a person whose telephone number is 686-2377 might use the correspondence indicated in Fig. 15.17 to develop the seven-letter word “NUMBERS.” Every seven-letter word corresponds to exactly one seven-digit telephone number. A restaurant wishing to increase its takeout business could surely do so with the number 825-3688 (i.e., “TAKEOUT”). Digit Letters Digit Letters Digit Letters 2 A B C 5 J K L 8 T U V 3 D E F 6 M N O 9 W X Y 4 G H I 7 P R S Fig. 15.17 | Telephone keypad digits and letters. Every seven-letter phone number corresponds to many different seven-letter words, but most of these words represent unrecognizable juxtapositions of letters. It’s possible, however, that the Making a Difference 683 owner of a barbershop would be pleased to know that the shop’s telephone number, 424-7288, corresponds to “HAIRCUT.” A veterinarian with the phone number 738-2273 would be pleased to know that the number corresponds to the letters “PETCARE.” An automotive dealership would be pleased to know that the dealership number, 639-2277, corresponds to “NEWCARS.” Write a program that, given a seven-digit number, uses a PrintStream object to write to a file every possible seven-letter word combination corresponding to that number. There are 2,187 (37) such combinations. Avoid phone numbers with the digits 0 and 1. 15.8 (Student Poll) Figure 7.8 contains an array of survey responses that’s hard coded into the program. Suppose we wish to process survey results that are stored in a file. This exercise requires two separate programs. First, create an application that prompts the user for survey responses and outputs each response to a file. Use a Formatter to create a file called numbers.txt. Each integer should be written using method format. Then modify the program in Fig. 7.8 to read the survey responses from numbers.txt. The responses should be read from the file by using a Scanner. Use method nextInt to input one integer at a time from the file. The program should continue to read responses until it reaches the end of the file. The results should be output to the text file "output.txt". 15.9 (Adding Object Serialization to the MyShape Drawing Application) Modify Exercise 12.17 to allow the user to save a drawing into a file or load a prior drawing from a file using object serialization. Add buttons Load (to read objects from a file) and Save (to write objects to a file). Use an ObjectOutputStream to write to the file and an ObjectInputStream to read from the file. Write the array of MyShape objects using method writeObject (class ObjectOutputStream), and read the array using method readObject (ObjectInputStream). The object-serialization mechanism can read or write entire arrays—it’s not necessary to manipulate each element of the array of MyShape objects individually. It’s simply required that all the shapes be Serializable. For both the Load and Save buttons, use a JFileChooser to allow the user to select the file in which the shapes will be stored or from which they’ll be read. When the user first runs the program, no shapes should be displayed on the screen. The user can display shapes by opening a previously saved file or by drawing new shapes. Once there are shapes on the screen, users can save them to a file using the Save button. Making a Difference 15.10 (Phishing Scanner) Phishing is a form of identity theft in which, in an e-mail, a sender posing as a trustworthy source attempts to acquire private information, such as your user names, passwords, credit-card numbers and social security number. Phishing e-mails claiming to be from popular banks, credit-card companies, auction sites, social networks and online payment services may look quite legitimate. These fraudulent messages often provide links to spoofed (fake) websites where you’re asked to enter sensitive information. Search online for phishing scams. Also check out the Anti-Phishing Working Group (www.antiphishing.org), and the FBI’s Cyber Investigations website (www.fbi.gov/about-us/ investigate/cyber/cyber), where you’ll find information about the latest scams and how to protect yourself. Create a list of 30 words, phrases and company names commonly found in phishing messages. Assign a point value to each based on your estimate of its likeliness to be in a phishing message (e.g., one point if it’s somewhat likely, two points if moderately likely, or three points if highly likely). Write an application that scans a file of text for these terms and phrases. For each occurrence of a keyword or phrase within the text file, add the assigned point value to the total points for that word or phrase. For each keyword or phrase found, output one line with the word or phrase, the number of occurrences and the point total. Then show the point total for the entire message. Does your program assign a high point total to some actual phishing e-mails you’ve received? Does it assign a high point total to some legitimate e-mails you’ve received? 16 I think this is the most extraordinary collection of talent, of human knowledge, that has ever been gathered together at the White House— with the possible exception of when Thomas Jefferson dined alone. —John F. Kennedy Objectives In this chapter you’ll: ■ Learn what collections are. ■ Use class Arrays for array manipulations. ■ Learn the type-wrapper classes that enable programs to process primitive data values as objects. ■ Use prebuilt generic data structures from the collections framework. ■ Use iterators to “walk through” a collection. ■ Use persistent hash tables manipulated with objects of class Properties. ■ Learn about synchronization and modifiability wrappers. Generic Collections 16.1 Introduction 16.1 16.2 16.3 16.4 16.5 Introduction Collections Overview Type-Wrapper Classes Autoboxing and Auto-Unboxing Interface Collection and Class Collections 16.6 Lists 16.6.1 ArrayList and Iterator 16.6.2 LinkedList 16.7 Collections Methods 16.7.1 Method sort 16.7.2 Method shuffle 16.7.3 Methods reverse, fill, copy, max and min 685 16.7.4 Method binarySearch 16.7.5 Methods addAll, frequency and disjoint 16.8 Stack Class of Package java.util 16.9 Class PriorityQueue and Interface Queue 16.10 16.11 16.12 16.13 16.14 16.15 16.16 Sets Maps Properties Class Synchronized Collections Unmodifiable Collections Abstract Implementations Wrap-Up Summary | Self-Review Exercises | Answers to Self-Review Exercises | Exercises 16.1 Introduction In Section 7.16, we introduced the generic ArrayList collection—a dynamically resizable array-like data structure that stores references to objects of a type that you specify when you create the ArrayList. In this chapter, we continue our discussion of the Java collections framework, which contains many other prebuilt generic data-structures. Some examples of collections are your favorite songs stored on your smartphone or media player, your contacts list, the cards you hold in a card game, the members of your favorite sports team and the courses you take at once in school. We discuss the collections-framework interfaces that declare the capabilities of each collection type, various classes that implement these interfaces, methods that process collection objects, and iterators that “walk through” collections. Java SE 8 After reading Chapter 17, Java SE 8 Lambdas and Streams, you’ll be able to reimplement many of Chapter 16’s examples in a more concise and elegant manner, and in a way that makes them easier to parallelize to improve performance on today’s multi-core systems. In Chapter 23, Concurrency, you’ll learn how to improve performance on multi-core systems using Java’s concurrent collections and parallel stream operations. 16.2 Collections Overview A collection is a data structure—actually, an object—that can hold references to other objects. Usually, collections contain references to objects of any type that has the is-a relationship with the type stored in the collection. The collections-framework interfaces declare the operations to be performed generically on various types of collections. Figure 16.1 lists some of the collections framework interfaces. Several implementations of these interfaces are provided within the framework. You may also provide your own implementations. 686 Chapter 16 Generic Collections Interface Description Collection The root interface in the collections hierarchy from which interfaces Set, Queue and List are derived. A collection that does not contain duplicates. An ordered collection that can contain duplicate elements. A collection that associates keys to values and cannot contain duplicate keys. Map does not derive from Collection. Typically a first-in, first-out collection that models a waiting line; other orders can be specified. Set List Map Queue Fig. 16.1 | Some collections-framework interfaces. Object-Based Collections The collections framework classes and interfaces are members of package java.util. In early Java versions, the collections framework classes stored and manipulated only Object references, enabling you to store any object in a collection, because all classes directly or indirectly derive from class Object. Programs normally need to process specific types of objects. As a result, the Object references obtained from a collection need to be downcast to an appropriate type to allow the program to process the objects correctly. As we discussed in Chapter 10, downcasting generally should be avoided. Generic Collections To elminate this problem, the collections framework was enhanced with the generics capabilities that we introduced with generic ArrayLists in Chapter 7 and that we discuss in more detail in Chapter 20, Generic Classes and Methods. Generics enable you to specify the exact type that will be stored in a collection and give you the benefits of compile-time type checking—the compiler issues error messages if you use inappropriate types in your collections. Once you specify the type stored in a generic collection, any reference you retrieve from the collection will have that type. This eliminates the need for explicit type casts that can throw ClassCastExceptions if the referenced object is not of the appropriate type. In addition, the generic collections are backward compatible with Java code that was written before generics were introduced. Good Programming Practice 16.1 Avoid reinventing the wheel—rather than building your own data structures, use the interfaces and collections from the Java collections framework, which have been carefully tested and tuned to meet most application requirements. Choosing a Collection The documentation for each collection discusses its memory requirements and its methods’ performance characteristics for operations such as adding and removing elements, searching for elements, sorting elements and more. Before choosing a collection, review the online documentation for the collection category you’re considering (Set, List, Map, Queue, etc.), then choose the implementation that best meets your application’s needs. Chapter 19, Searching, Sorting and Big O, discusses a means for describing how hard an 16.3 Type-Wrapper Classes 687 algorithm works to perform its task—based on the number of data items to be processed. After reading Chapter 19, you’ll better understand each collection’s performance characteristics as described in the online documentation. 16.3 Type-Wrapper Classes Each primitive type (listed in Appendix D) has a corresponding type-wrapper class (in package java.lang). These classes are called Boolean, Byte, Character, Double, Float, Integer, Long and Short. These enable you to manipulate primitive-type values as objects. This is important because the data structures that we reuse or develop in Chapters 16–21 manipulate and share objects—they cannot manipulate variables of primitive types. However, they can manipulate objects of the type-wrapper classes, because every class ultimately derives from Object. Each of the numeric type-wrapper classes—Byte, Short, Integer, Long, Float and Double—extends class Number. Also, the type-wrapper classes are final classes, so you cannot extend them. Primitive types do not have methods, so the methods related to a primitive type are located in the corresponding type-wrapper class (e.g., method parseInt, which converts a String to an int value, is located in class Integer). 16.4 Autoboxing and Auto-Unboxing Java provides boxing and unboxing conversions that automatically convert between primitive-type values and type-wrapper objects. A boxing conversion converts a value of a primitive type to an object of the corresponding type-wrapper class. An unboxing conversion converts an object of a type-wrapper class to a value of the corresponding primitive type. These conversions are performed automatically—called autoboxing and auto-unboxing. Consider the following statements: Integer[] integerArray = new Integer[5]; // create integerArray integerArray[0] = 10; // assign Integer 10 to integerArray[0] int value = integerArray[0]; // get int value of Integer In this case, autoboxing occurs when assigning an int value (10) to integerArray[0], because integerArray stores references to Integer objects, not int values. Auto-unboxing occurs when assigning integerArray[0] to int variable value, because variable value stores an int value, not a reference to an Integer object. Boxing conversions also occur in conditions, which can evaluate to primitive boolean values or Boolean objects. Many of the examples in Chapters 16–21 use these conversions to store primitive values in and retrieve them from data structures. 16.5 Interface Collection and Class Collections Interface Collection contains bulk operations (i.e., operations performed on an entire collection) for operations such as adding, clearing and comparing objects (or elements) in a collection. A Collection can also be converted to an array. In addition, interface Collection provides a method that returns an Iterator object, which allows a program to walk through the collection and remove elements from it during the iteration. We discuss class Iterator in Section 16.6.1. Other methods of interface Collection enable a program to determine a collection’s size and whether a collection is empty. 688 Chapter 16 Generic Collections Software Engineering Observation 16.1 Collection is used commonly as a parameter type in methods to allow polymorphic processing of all objects that implement interface Collection. Software Engineering Observation 16.2 Most collection implementations provide a constructor that takes a Collection argument, thereby allowing a new collection to be constructed containing the elements of the specified collection. Class Collections provides static methods that search, sort and perform other operations on collections. Section 16.7 discusses more about Collections methods. We also cover Collections’ wrapper methods that enable you to treat a collection as a synchronized collection (Section 16.13) or an unmodifiable collection (Section 16.14). Synchronized collections are for use with multithreading (discussed in Chapter 23), which enables programs to perform operations in parallel. When two or more threads of a program share a collection, problems might occur. As an analogy, consider a traffic intersection. If all cars were allowed to access the intersection at the same time, collisions might occur. For this reason, traffic lights are provided to control access to the intersection. Similarly, we can synchronize access to a collection to ensure that only one thread manipulates the collection at a time. The synchronization wrapper methods of class Collections return synchronized versions of collections that can be shared among threads in a program. Unmodifiable collections are useful when clients of a class need to view a collection’s elements, but they should not be allowed to modify the collection by adding and removing elements. 16.6 Lists A List (sometimes called a sequence) is an ordered Collection that can contain duplicate elements. Like array indices, List indices are zero based (i.e., the first element’s index is zero). In addition to the methods inherited from Collection, List provides methods for manipulating elements via their indices, manipulating a specified range of elements, searching for elements and obtaining a ListIterator to access the elements. Interface List is implemented by several classes, including ArrayList, LinkedList and Vector. Autoboxing occurs when you add primitive-type values to objects of these classes, because they store only references to objects. Classes ArrayList and Vector are resizable-array implementations of List. Inserting an element between existing elements of an ArrayList or Vector is an inefficient operation—all elements after the new one must be moved out of the way, which could be an expensive operation in a collection with a large number of elements. A LinkedList enables efficient insertion (or removal) of elements in the middle of a collection, but is much less efficient than an ArrayList for jumping to a specific element in the collection. We discuss the architecture of linked lists in Chapter 21. ArrayList and Vector have nearly identical behaviors. Operations on Vectors are synchronized by default, whereas those on ArrayLists are not. Also, class Vector is from Java 1.0, before the collections framework was added to Java. As such, Vector has some methods that are not part of interface List and are not implemented in class ArrayList. For example, Vector methods addElement and add both append an element to a Vector, but only method add is specified in interface List and implemented by ArrayList. Unsynchro- 16.6 Lists 689 nized collections provide better performance than synchronized ones. For this reason, Arrayis typically preferred over Vector in programs that do not share a collection among threads. Separately, the Java collections API provides synchronization wrappers (Section 16.13) that can be used to add synchronization to the unsynchronized collections, and several powerful synchronized collections are available in the Java concurrency APIs. List Performance Tip 16.1 ArrayLists behave like Vectors without synchronization and therefore execute faster than Vectors, because ArrayLists do not have the overhead of thread synchronization. Software Engineering Observation 16.3 LinkedLists can be used to create stacks, queues and deques (double-ended queues, pronounced “decks”). The collections framework provides implementations of some of these data structures. The following three subsections demonstrate the List and Collection capabilities. Section 16.6.1 removes elements from an ArrayList with an Iterator. Section 16.6.2 uses ListIterator and several List- and LinkedList-specific methods. 16.6.1 ArrayList and Iterator Figure 16.2 uses an ArrayList (introduced in Section 7.16) to demonstrate several capabilities of interface Collection. The program places two Color arrays in ArrayLists and uses an Iterator to remove elements in the second ArrayList collection from the first. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 // Fig. 16.2: CollectionTest.java // Collection interface demonstrated via an ArrayList object. import java.util.List; import java.util.ArrayList; import java.util.Collection; import java.util.Iterator; public class CollectionTest { public static void main(String[] args) { // add elements in colors array to list String[] colors = {"MAGENTA", "RED", "WHITE", "BLUE", "CYAN"}; List list = new ArrayList(); Fig. 16.2 | for (String color : colors) list.add(color); // adds color to end of list // add elements in removeColors array to removeList String[] removeColors = {"RED", "WHITE", "BLUE"}; List removeList = new ArrayList(); for (String color : removeColors) removeList.add(color); Collection interface demonstrated via an ArrayList object. (Part 1 of 2.) 690 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 Chapter 16 Generic Collections // output list contents System.out.println("ArrayList: "); for (int count = 0; count < list.size(); count++) System.out.printf("%s ", list.get(count)); // remove from list the colors contained in removeList removeColors(list, removeList); // output list contents System.out.printf("%n%nArrayList after calling removeColors:%n"); for (String color : list) System.out.printf("%s ", color); } // remove colors specified in collection2 from collection1 private static void removeColors(Collection collection1, Collection collection2) { // get iterator Iterator iterator = collection1.iterator(); // loop while collection has items while (iterator.hasNext()) { if (collection2.contains(iterator.next())) iterator.remove(); // remove current element } } } // end class CollectionTest ArrayList: MAGENTA RED WHITE BLUE CYAN ArrayList after calling removeColors: MAGENTA CYAN Fig. 16.2 | Collection interface demonstrated via an ArrayList object. (Part 2 of 2.) Lines 13 and 20 declare and initialize String arrays colors and removeColors. Lines 14 and 21 create ArrayList objects and assign their references to List variables list and removeList, respectively. Recall that ArrayList is a generic class, so we can specify a type argument (String in this case) to indicate the type of the elements in each list. Because you specify the type to store in a collection at compile time, generic collections provide compile-time type safety that allows the compiler to catch attempts to use invalid types. For example, you cannot store Employees in a collection of Strings. Lines 16–17 populate list with Strings stored in array colors, and lines 23–24 populate removeList with Strings stored in array removeColors using List method add. Lines 29–30 output each element of list. Line 29 calls List method size to get the number of elements in the ArrayList. Line 30 uses List method get to retrieve indi- 16.6 Lists 691 vidual element values. Lines 29–30 also could have used the enhanced for statement (which we’ll demonstrate with collections in other examples). Line 33 calls method removeColors (lines 43–55), passing list and removeList as arguments. Method removeColors deletes the Strings in removeList from the Strings in list. Lines 38–39 print list’s elements after removeColors completes its task. Method removeColors declares two Collection parameters (lines 43– 44)—any two Collections containing Strings can be passed as arguments. The method accesses the elements of the first Collection (collection1) via an Iterator. Line 47 calls Collection method iterator to get an Iterator for the Collection. Interfaces Collection and Iterator are generic types. The loop-continuation condition (line 50) calls Iterator method hasNext to determine whether there are more elements to iterate through. Method hasNext returns true if another element exists and false otherwise. The if condition in line 52 calls Iterator method next to obtain a reference to the next element, then uses method contains of the second Collection (collection2) to determine whether collection2 contains the element returned by next. If so, line 53 calls Iterator method remove to remove the element from the Collection collection1. Common Programming Error 16.1 If a collection is modified by one of its methods after an iterator is created for that collection, the iterator immediately becomes invalid—any operation performed with the iterator fails immediate and throws a ConcurrentModificationException. For this reason, iterators are said to be “fail fast.” Fail-fast iterators help ensure that a modifiable collection is not manipulated by two or more threads at the same time, which could corrupt the collection. In Chapter 23, Concurrency, you’ll learn about concurrent collections (package java.util.concurrent) that can be safely manipulated by multiple concurrent threads. Software Engineering Observation 16.4 We refer to the ArrayLists in this example via List variables. This makes our code more flexible and easier to modify—if we later determine that LinkedLists would be more appropriate, only the lines where we created the ArrayList objects (lines 14 and 21) need to be modified. In general, when you create a collection object, refer to that object with a variable of the corresponding collection interface type. Type Inference with the <> Notation Lines 14 and 21 specify the type stored in the ArrayList (that is, String) on the left and right sides of the initialization statements. Java SE 7 introduced type inferencing with the <> notation—known as the diamond notation—in statements that declare and create generic type variables and objects. For example, line 14 can be written as: List list = new ArrayList<>(); In this case, Java uses the type in angle brackets on the left of the declaration (that is, as the type stored in the ArrayList created on the right side of the declaration. We’ll use this syntax for the remaining examples in this chapter. String) 16.6.2 LinkedList Figure 16.3 demonstrates various operations on LinkedLists. The program creates two LinkedLists of Strings. The elements of one List are added to the other. Then all the Strings are converted to uppercase, and a range of elements is deleted. 692 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 Chapter 16 Generic Collections // Fig. 16.3: ListTest.java // Lists, LinkedLists and ListIterators. import java.util.List; import java.util.LinkedList; import java.util.ListIterator; public class ListTest { public static void main(String[] args) { // add colors elements to list1 String[] colors = {"black", "yellow", "green", "blue", "violet", "silver"}; List list1 = new LinkedList<>(); for (String color : colors) list1.add(color); // add colors2 elements to list2 String[] colors2 = {"gold", "white", "brown", "blue", "gray", "silver"}; List list2 = new LinkedList<>(); for (String color : colors2) list2.add(color); list1.addAll(list2); // concatenate lists list2 = null; // release resources printList(list1); // print list1 elements convertToUppercaseStrings(list1); // convert to uppercase string printList(list1); // print list1 elements System.out.printf("%nDeleting elements 4 to 6..."); removeItems(list1, 4, 7); // remove items 4-6 from list printList(list1); // print list1 elements printReversedList(list1); // print list in reverse order } // output List contents private static void printList(List list) { System.out.printf("%nlist:%n"); for (String color : list) System.out.printf("%s ", color); System.out.println(); } // locate String objects and convert to uppercase private static void convertToUppercaseStrings(List list) { Fig. 16.3 | Lists, LinkedLists and ListIterators. (Part 1 of 2.) 16.6 Lists 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 693 ListIterator iterator = list.listIterator(); while (iterator.hasNext()) { String color = iterator.next(); // get item iterator.set(color.toUpperCase()); // convert to upper case } } // obtain sublist and use clear method to delete sublist items private static void removeItems(List list, int start, int end) { list.subList(start, end).clear(); // remove items } // print reversed list private static void printReversedList(List list) { ListIterator iterator = list.listIterator(list.size()); System.out.printf("%nReversed List:%n"); // print list in reverse order while (iterator.hasPrevious()) System.out.printf("%s ", iterator.previous()); } } // end class ListTest list: black yellow green blue violet silver gold white brown blue gray silver list: BLACK YELLOW GREEN BLUE VIOLET SILVER GOLD WHITE BROWN BLUE GRAY SILVER Deleting elements 4 to 6... list: BLACK YELLOW GREEN BLUE WHITE BROWN BLUE GRAY SILVER Reversed List: SILVER GRAY BLUE BROWN WHITE BLUE GREEN YELLOW BLACK Fig. 16.3 | Lists, LinkedLists and ListIterators. (Part 2 of 2.) Lines 14 and 22 create LinkedLists list1 and list2 of type String. LinkedList is a generic class that has one type parameter for which we specify the type argument String in this example. Lines 16–17 and 24–25 call List method add to append elements from arrays colors and colors2 to the ends of list1 and list2, respectively. Line 27 calls List method addAll to append all elements of list2 to the end of list1. Line 28 sets list2 to null, because list2 is no longer needed. Line 29 calls method printList (lines 41–49) to output list1’s contents. Line 31 calls method convertToUppercaseStrings (lines 52–61) to convert each String element to uppercase, then line 32 calls printList again to display the modified Strings. Line 35 calls method 694 Chapter 16 Generic Collections (lines 64–68) to remove the range of elements starting at index 4 up to, but not including, index 7 of the list. Line 37 calls method printReversedList (lines 71–80) to print the list in reverse order. removeItems Method convertToUppercaseStrings Method convertToUppercaseStrings (lines 52–61) changes lowercase String elements in its List argument to uppercase Strings. Line 54 calls List method listIterator to get the List’s bidirectional iterator (i.e., one that can traverse a List backward or forward). ListIterator is also a generic class. In this example, the ListIterator references String objects, because method listIterator is called on a List of Strings. Line 56 calls method hasNext to determine whether the List contains another element. Line 58 gets the next String in the List. Line 59 calls String method toUpperCase to get an uppercase version of the String and calls ListIterator method set to replace the current String to which iterator refers with the String returned by method toUpperCase. Like method toUpperCase, String method toLowerCase returns a lowercase version of the String. Method removeItems Method removeItems (lines 64–68) removes a range of items from the list. Line 67 calls List method subList to obtain a portion of the List (called a sublist). This is called a range-view method, which enables the program to view a portion of the list. The sublist is simply a view into the List on which subList is called. Method subList takes as arguments the beginning and ending index for the sublist. The ending index is not part of the range of the sublist. In this example, line 35 passes 4 for the beginning index and 7 for the ending index to subList. The sublist returned is the set of elements with indices 4 through 6. Next, the program calls List method clear on the sublist to remove the elements of the sublist from the List. Any changes made to a sublist are also made to the original List. Method printReversedList Method printReversedList (lines 71–80) prints the list backward. Line 73 calls List method listIterator with the starting position as an argument (in our case, the last element in the list) to get a bidirectional iterator for the list. List method size returns the number of items in the List. The while condition (line 78) calls ListIterator’s hasPrevious method to determine whether there are more elements while traversing the list backward. Line 79 calls ListIterator’s previous method to get the previous element from the list and outputs it to the standard output stream. Views into Collections and Arrays Method asList Class Arrays provides static method asList to view an array (sometimes called the backing array) as a List collection. A List view allows you to manipulate the array as if it were a list. This is useful for adding the elements in an array to a collection and for sorting array elements. The next example demonstrates how to create a LinkedList with a List view of an array, because we cannot pass the array to a LinkedList constructor. Sorting array elements with a List view is demonstrated in Fig. 16.7. Any modifications made through the List view change the array, and any modifications made to the array change the List view. The only operation permitted on the view returned by asList is set, which changes the value of the view and the backing array. Any other attempts to change the view (such as adding or removing elements) result in an UnsupportedOperationException. 16.6 Lists 695 Viewing Arrays as Lists and Converting Lists to Arrays Figure 16.4 uses Arrays method asList to view an array as a List and uses List method toArray to get an array from a LinkedList collection. The program calls method asList to create a List view of an array, which is used to initialize a LinkedList object, then adds a series of Strings to the LinkedList and calls method toArray to obtain an array containing references to the Strings. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 // Fig. 16.4: UsingToArray.java // Viewing arrays as Lists and converting Lists to arrays. import java.util.LinkedList; import java.util.Arrays; public class UsingToArray { // creates a LinkedList, adds elements and converts to array public static void main(String[] args) { String[] colors = {"black", "blue", "yellow"}; LinkedList links = new LinkedList<>(Arrays.asList(colors)); links.addLast("red"); // add as last item links.add("pink"); // add to the end links.add(3, "green"); // add at 3rd index links.addFirst("cyan"); // add as first item // get LinkedList elements as an array colors = links.toArray(new String[links.size()]); System.out.println("colors: "); for (String color : colors) System.out.println(color); } } // end class UsingToArray colors: cyan black blue yellow green red pink Fig. 16.4 | Viewing arrays as Lists and converting Lists to arrays. Line 12 constructs a LinkedList of Strings containing the elements of array colors. method asList returns a List view of the array, then uses that to initialize the LinkedList with its constructor that receives a Collection as an argument (a List is a Collection). Line 14 calls LinkedList method addLast to add "red" to the end of links. Lines 15–16 call LinkedList method add to add "pink" as the last element and "green" as the element at index 3 (i.e., the fourth element). Method addLast (line 14) functions Arrays 696 Chapter 16 Generic Collections identically to method add (line 15). Line 17 calls LinkedList method addFirst to add "cyan" as the new first item in the LinkedList. The add operations are permitted because they operate on the LinkedList object, not the view returned by asList. [Note: When "cyan" is added as the first element, "green" becomes the fifth element in the LinkedList.] Line 20 calls the List interface’s toArray method to get a String array from links. The array is a copy of the list’s elements—modifying the array’s contents does not modify the list. The array passed to method toArray is of the same type that you’d like method toArray to return. If the number of elements in that array is greater than or equal to the number of elements in the LinkedList, toArray copies the list’s elements into its array argument and returns that array. If the LinkedList has more elements than the number of elements in the array passed to toArray, toArray allocates a new array of the same type it receives as an argument, copies the list’s elements into the new array and returns the new array. Common Programming Error 16.2 Passing an array that contains data to toArray can cause logic errors. If the number of elements in the array is smaller than the number of elements in the list on which toArray is called, a new array is allocated to store the list’s elements—without preserving the array argument’s elements. If the number of elements in the array is greater than the number of elements in the list, the elements of the array (starting at index zero) are overwritten with the list’s elements. Array elements that are not overwritten retain their values. 16.7 Collections Methods Class Collections provides several high-performance algorithms for manipulating collection elements. The algorithms (Fig. 16.5) are implemented as static methods. The methods sort, binarySearch, reverse, shuffle, fill and copy operate on Lists. Methods min, max, addAll, frequency and disjoint operate on Collections. Method Description sort Sorts the elements of a List. Locates an object in a List, using the high-performance binary search algorithm which we introduced in Section 7.15 and discuss in detail in Section 19.4. Reverses the elements of a List. Randomly orders a List’s elements. Sets every List element to refer to a specified object. Copies references from one List into another. Returns the smallest element in a Collection. Returns the largest element in a Collection. Appends all elements in an array to a Collection. Calculates how many collection elements are equal to the specified element. Determines whether two collections have no elements in common. binarySearch reverse shuffle fill copy min max addAll frequency disjoint Fig. 16.5 | Collections methods. 16.7 Collections Methods 697 Software Engineering Observation 16.5 The collections framework methods are polymorphic. That is, each can operate on objects that implement specific interfaces, regardless of the underlying implementations. 16.7.1 Method sort Method sort sorts the elements of a List, which must implement the Comparable interface. The order is determined by the natural order of the elements’ type as implemented by a compareTo method. For example, the natural order for numeric values is ascending order, and the natural order for Strings is based on their lexicographical ordering (Section 14.3). Method compareTo is declared in interface Comparable and is sometimes called the natural comparison method. The sort call may specify as a second argument a Comparator object that determines an alternative ordering of the elements. Sorting in Ascending Order Figure 16.6 uses Collections method sort to order the elements of a List in ascending order (line 17). Method sort performs an iterative merge sort (we demonstrated a recursive merge sort in Section 19.8). Line 14 creates list as a List of Strings. Lines 15 and 18 each use an implicit call to the list’s toString method to output the list contents in the format shown in the output. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 // Fig. 16.6: Sort1.java // Collections method sort. import java.util.List; import java.util.Arrays; import java.util.Collections; public class Sort1 { public static void main(String[] args) { String[] suits = {"Hearts", "Diamonds", "Clubs", "Spades"}; // Create and display a list containing the suits array elements List list = Arrays.asList(suits); System.out.printf("Unsorted array elements: %s%n", list); Collections.sort(list); // sort ArrayList System.out.printf("Sorted array elements: %s%n", list); } } // end class Sort1 Unsorted array elements: [Hearts, Diamonds, Clubs, Spades] Sorted array elements: [Clubs, Diamonds, Hearts, Spades] Fig. 16.6 | Collections method sort. Sorting in Descending Order Figure 16.7 sorts the same list of strings used in Fig. 16.6 in descending order. The example introduces the Comparator interface, which is used for sorting a Collection’s elements in 698 Chapter 16 Generic Collections a different order. Line 18 calls Collections’s method sort to order the List in descending order. The static Collections method reverseOrder returns a Comparator object that orders the collection’s elements in reverse order. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 // Fig. 16.7: Sort2.java // Using a Comparator object with method sort. import java.util.List; import java.util.Arrays; import java.util.Collections; public class Sort2 { public static void main(String[] args) { String[] suits = {"Hearts", "Diamonds", "Clubs", "Spades"}; // Create and display a list containing the suits array elements List list = Arrays.asList(suits); // create List System.out.printf("Unsorted array elements: %s%n", list); // sort in descending order using a comparator Collections.sort(list, Collections.reverseOrder()); System.out.printf("Sorted list elements: %s%n", list); } } // end class Sort2 Unsorted array elements: [Hearts, Diamonds, Clubs, Spades] Sorted list elements: [Spades, Hearts, Diamonds, Clubs] Fig. 16.7 | Collections method sort with a Comparator object. Sorting with a Comparator Figure 16.8 creates a custom Comparator class, named TimeComparator, that implements interface Comparator to compare two Time2 objects. Class Time2, declared in Fig. 8.5, represents times with hours, minutes and seconds. 1 2 3 4 5 6 7 8 9 10 11 12 13 // Fig. 16.8: TimeComparator.java // Custom Comparator class that compares two Time2 objects. import java.util.Comparator; public class TimeComparator implements Comparator { @Override public int compare(Time2 time1, Time2 time2) { int hourDifference = time1.getHour() - time2.getHour(); if (hourDifference != 0) // test the hour first return hourCompare; Fig. 16.8 | Custom Comparator class that compares two Time2 objects. (Part 1 of 2.) 16.7 Collections Methods 14 15 16 17 18 19 20 21 22 23 699 int minuteDifference = time1.getMinute() - time2.getMinute(); if (minuteDifference != 0) // then test the minute return minuteDifference; int secondDifference = time1.getSecond() - time2.getSecond(); return secondDifference; } } // end class TimeComparator Fig. 16.8 | Custom Comparator class that compares two Time2 objects. (Part 2 of 2.) Class TimeComparator implements interface Comparator, a generic type that takes one type argument (in this case Time2). A class that implements Comparator must declare a compare method that receives two arguments and returns a negative integer if the first argument is less than the second, 0 if the arguments are equal or a positive integer if the first argument is greater than the second. Method compare (lines 7–22) performs comparisons between Time2 objects. Line 10 calculates the difference between the hours of the two Time2 objects. If the hours are different (line 12), then we return this value. If this value is positive, then the first hour is greater than the second and the first time is greater than the second. If this value is negative, then the first hour is less than the second and the first time is less than the second. If this value is zero, the hours are the same and we must test the minutes (and maybe the seconds) to determine which time is greater. Figure 16.9 sorts a list using the custom Comparator class TimeComparator. Line 11 creates an ArrayList of Time2 objects. Recall that both ArrayList and List are generic types and accept a type argument that specifies the element type of the collection. Lines 13–17 create five Time2 objects and add them to this list. Line 23 calls method sort, passing it an object of our TimeComparator class (Fig. 16.8). 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 // Fig. 16.9: Sort3.java // Collections method sort with a custom Comparator object. import java.util.List; import java.util.ArrayList; import java.util.Collections; public class Sort3 { public static void main(String[] args) { List list = new ArrayList<>(); // create List Fig. 16.9 | list.add(new list.add(new list.add(new list.add(new list.add(new Collections Time2(6, 24, 34)); Time2(18, 14, 58)); Time2(6, 05, 34)); Time2(12, 14, 58)); Time2(6, 24, 22)); method sort with a custom Comparator object. (Part 1 of 2.) 700 19 20 21 22 23 24 25 26 27 28 Chapter 16 Generic Collections // output List elements System.out.printf("Unsorted array elements:%n%s%n", list); // sort in order using a comparator Collections.sort(list, new TimeComparator()); // output List elements System.out.printf("Sorted list elements:%n%s%n", list); } } // end class Sort3 Unsorted array elements: [6:24:34 AM, 6:14:58 PM, 6:05:34 AM, 12:14:58 PM, 6:24:22 AM] Sorted list elements: [6:05:34 AM, 6:24:22 AM, 6:24:34 AM, 12:14:58 PM, 6:14:58 PM] Fig. 16.9 | Collections method sort with a custom Comparator object. (Part 2 of 2.) 16.7.2 Method shuffle Method shuffle randomly orders a List’s elements. Chapter 7 presented a card shuffling and dealing simulation that shuffled a deck of cards with a loop. Figure 16.10 uses method shuffle to shuffle a deck of Card objects that might be used in a card-game simulator. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 // Fig. 16.10: DeckOfCards.java // Card shuffling and dealing with Collections method shuffle. import java.util.List; import java.util.Arrays; import java.util.Collections; // class to represent a Card in a deck of cards class Card { public static enum Face {Ace, Deuce, Three, Four, Five, Six, Seven, Eight, Nine, Ten, Jack, Queen, King }; public static enum Suit {Clubs, Diamonds, Hearts, Spades}; private final Face face; private final Suit suit; // constructor public Card(Face face, Suit suit) { this.face = face; this.suit = suit; } // return face of the card public Face getFace() { Fig. 16.10 | Card shuffling and dealing with Collections method shuffle. (Part 1 of 3.) 16.7 Collections Methods 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 701 return face; } // return suit of Card public Suit getSuit() { return suit; } // return String representation of Card public String toString() { return String.format("%s of %s", face, suit); } } // end class Card // class DeckOfCards declaration public class DeckOfCards { private List list; // declare List that will store Cards // set up deck of Cards and shuffle public DeckOfCards() { Card[] deck = new Card[52]; int count = 0; // number of cards // populate deck with Card objects for (Card.Suit suit: Card.Suit.values()) { for (Card.Face face: Card.Face.values()) { deck[count] = new Card(face, suit); ++count; } } list = Arrays.asList(deck); // get List Collections.shuffle(list); // shuffle deck } // end DeckOfCards constructor // output deck public void printCards() { // display 52 cards in two columns for (int i = 0; i < list.size(); i++) System.out.printf("%-19s%s", list.get(i), ((i + 1) % 4 == 0) ? "%n" : ""); } public static void main(String[] args) { Fig. 16.10 | Card shuffling and dealing with Collections method shuffle. (Part 2 of 3.) 702 79 80 81 82 Chapter 16 Generic Collections DeckOfCards cards = new DeckOfCards(); cards.printCards(); } } // end class DeckOfCards Deuce of Clubs Three of Diamonds Three of Spades Ten of Spades Nine of Clubs Ten of Clubs Queen of Diamonds Ace of Spades Seven of Diamonds Seven of Spades Eight of Clubs Six of Clubs Five of Spades Six of Spades Five of Clubs Six of Diamonds King of Diamonds Ten of Diamonds Five of Hearts Ace of Diamonds Deuce of Spades Three of Hearts King of Hearts Three of Clubs Nine of Spades King of Spades Nine of Diamonds Deuce of Diamonds King of Clubs Eight of Spades Eight of Diamonds Ace of Clubs Four of Clubs Ace of Hearts Four of Spades Seven of Hearts Queen of Clubs Four of Hearts Jack of Spades Ten of Hearts Seven of Clubs Jack of Hearts Six of Hearts Eight of Hearts Deuce of Hearts Nine of Hearts Jack of Diamonds Four of Diamonds Five of Diamonds Queen of Spades Jack of Clubs Queen of Hearts Fig. 16.10 | Card shuffling and dealing with Collections method shuffle. (Part 3 of 3.) Class Card (lines 8–41) represents a card in a deck of cards. Each Card has a face and a suit. Lines 10–12 declare two enum types—Face and Suit—which represent the face and the suit of the card, respectively. Method toString (lines 37–40) returns a String containing the face and suit of the Card separated by the string " of ". When an enum constant is converted to a String, the constant’s identifier is used as the String representation. Normally we would use all uppercase letters for enum constants. In this example, we chose to use capital letters for only the first letter of each enum constant because we want the card to be displayed with initial capital letters for the face and the suit (e.g., "Ace of Spades"). Lines 55–62 populate the deck array with cards that have unique face and suit combinations. Both Face and Suit are public static enum types of class Card. To use these enum types outside of class Card, you must qualify each enum’s type name with the name of the class in which it resides (i.e., Card) and a dot (.) separator. Hence, lines 55 and 57 use Card.Suit and Card.Face to declare the control variables of the for statements. Recall that method values of an enum type returns an array that contains all the constants of the enum type. Lines 55–62 use enhanced for statements to construct 52 new Cards. The shuffling occurs in line 65, which calls static method shuffle of class Collections to shuffle the elements of the array. Method shuffle requires a List argument, so we must obtain a List view of the array before we can shuffle it. Line 64 invokes static method asList of class Arrays to get a List view of the deck array. Method printCards (lines 69–75) displays the deck of cards in four columns. In each iteration of the loop, lines 73–74 output a card left justified in a 19-character field followed by either a newline or an empty string based on the number of cards output so far. If the number of cards is divisible by 4, a newline is output; otherwise, the empty string is output. 16.7.3 Methods reverse, fill, copy, max and min Class Collections provides methods for reversing, filling and copying Lists. Collections method reverse reverses the order of the elements in a List, and method fill overwrites 16.7 Collections Methods 703 elements in a List with a specified value. The fill operation is useful for reinitializing a Method copy takes two arguments—a destination List and a source List. Each source List element is copied to the destination List. The destination List must be at least as long as the source List; otherwise, an IndexOutOfBoundsException occurs. If the destination List is longer, the elements not overwritten are unchanged. Each method we’ve seen so far operates on Lists. Methods min and max each operate on any Collection. Method min returns the smallest element in a Collection, and method max returns the largest element in a Collection. Both of these methods can be called with a Comparator object as a second argument to perform custom comparisons of objects, such as the TimeComparator in Fig. 16.9. Figure 16.11 demonstrates methods reverse, fill, copy, max and min. List. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 // Fig. 16.11: Algorithms1.java // Collections methods reverse, fill, copy, max and min. import java.util.List; import java.util.Arrays; import java.util.Collections; public class Algorithms1 { public static void main(String[] args) { // create and display a List Character[] letters = {'P', 'C', 'M'}; List list = Arrays.asList(letters); // get List System.out.println("list contains: "); output(list); // reverse and display the List Collections.reverse(list); // reverse order the elements System.out.printf("%nAfter calling reverse, list contains:%n"); output(list); // create copyList from an array of 3 Characters Character[] lettersCopy = new Character[3]; List copyList = Arrays.asList(lettersCopy); // copy the contents of list into copyList Collections.copy(copyList, list); System.out.printf("%nAfter copying, copyList contains:%n"); output(copyList); // fill list with Rs Collections.fill(list, 'R'); System.out.printf("%nAfter calling fill, list contains:%n"); output(list); } // output List information private static void output(List listRef) { Fig. 16.11 | Collections methods reverse, fill, copy, max and min. (Part 1 of 2.) 704 40 41 42 43 44 45 46 47 48 Chapter 16 Generic Collections System.out.print("The list is: "); for (Character element : listRef) System.out.printf("%s ", element); System.out.printf("%nMax: %s", Collections.max(listRef)); System.out.printf(" Min: %s%n", Collections.min(listRef)); } } // end class Algorithms1 list contains: The list is: P C M Max: P Min: C After calling reverse, list contains: The list is: M C P Max: P Min: C After copying, copyList contains: The list is: M C P Max: P Min: C After calling fill, list contains: The list is: R R R Max: R Min: R Fig. 16.11 | Collections methods reverse, fill, copy, max and min. (Part 2 of 2.) Line 13 creates List variable list and initializes it with a List view of the Character array letters. Lines 14–15 output the current contents of the List. Line 18 calls Collections method reverse to reverse the order of list. Method reverse takes one List argument. Since list is a List view of array letters, the array’s elements are now in reverse order. The reversed contents are output in lines 19–20. Line 27 uses Collections method copy to copy list’s elements into copyList. Changes to copyList do not change letters, because copyList is a separate List that’s not a List view of the array letters. Method copy requires two List arguments—the destination List and the source List. Line 32 calls Collections method fill to place the character 'R' in each list element. Because list is a List view of the array letters, this operation changes each element in letters to 'R'. Method fill requires a List for the first argument and an Object for the second argument—in this case, the Object is the boxed version of the character 'R'. Lines 45–46 call Collections methods max and min to find the largest and the smallest element of a Collection, respectively. Recall that interface List extends interface Collection, so a List is a Collection. 16.7.4 Method binarySearch The high-speed binary search algorithm—which we discuss in detail in Section 19.4—is built into the Java collections framework as a static Collections method binarySearch. This method locates an object in a List (e.g., a LinkedList or an ArrayList). If the object is found, its index is returned. If the object is not found, binarySearch returns a negative 16.7 Collections Methods 705 value. Method binarySearch determines this negative value by first calculating the insertion point and making its sign negative. Then, binarySearch subtracts 1 from the insertion point to obtain the return value, which guarantees that method binarySearch returns positive numbers (>= 0) if and only if the object is found. If multiple elements in the list match the search key, there’s no guarantee which one will be located first. Figure 16.12 uses method binarySearch to search for a series of strings in an ArrayList. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 // Fig. 16.12: BinarySearchTest.java // Collections method binarySearch. import java.util.List; import java.util.Arrays; import java.util.Collections; import java.util.ArrayList; public class BinarySearchTest { public static void main(String[] args) { // create an ArrayList from the contents of colors array String[] colors = {"red", "white", "blue", "black", "yellow", "purple", "tan", "pink"}; List list = new ArrayList<>(Arrays.asList(colors)); Collections.sort(list); // sort the ArrayList System.out.printf("Sorted ArrayList: %s%n", list); // search list for various values printSearchResults(list, "black"); // first item printSearchResults(list, "red"); // middle item printSearchResults(list, "pink"); // last item printSearchResults(list, "aqua"); // below lowest printSearchResults(list, "gray"); // does not exist printSearchResults(list, "teal"); // does not exist } // perform search and display result private static void printSearchResults( List list, String key) { int result = 0; System.out.printf("%nSearching for: %s%n", key); result = Collections.binarySearch(list, key); if (result >= 0) System.out.printf("Found at index %d%n", result); else System.out.printf("Not Found (%d)%n",result); } } // end class BinarySearchTest Fig. 16.12 | Collections method binarySearch. (Part 1 of 2.) 706 Chapter 16 Generic Collections Sorted ArrayList: [black, blue, pink, purple, red, tan, white, yellow] Searching for: black Found at index 0 Searching for: red Found at index 4 Searching for: pink Found at index 2 Searching for: aqua Not Found (-1) Searching for: gray Not Found (-3) Searching for: teal Not Found (-7) Fig. 16.12 | Collections method binarySearch. (Part 2 of 2.) Lines 15–16 initialize list with an ArrayList containing a copy of the elements in array colors. Collections method binarySearch expects its List argument’s elements to be sorted in ascending order, so line 18 uses Collections method sort to sort the list. If the List argument’s elements are not sorted, the result of using binarySearch is undefined. Line 19 outputs the sorted list. Lines 22–27 call method printSearchResults (lines 31–43) to perform searches and output the results. Line 37 calls Collections method binarySearch to search list for the specified key. Method binarySearch takes a List as the first argument and an Object as the second argument. Lines 39–42 output the results of the search. An overloaded version of binarySearch takes a Comparator object as its third argument, which specifies how binarySearch should compare the search key to the List’s elements. 16.7.5 Methods addAll, frequency and disjoint Class Collections also provides the methods addAll, frequency and disjoint. Collecmethod addAll takes two arguments—a Collection into which to insert the new element(s) and an array that provides elements to be inserted. Collections method frequency takes two arguments—a Collection to be searched and an Object to be searched for in the collection. Method frequency returns the number of times that the second argument appears in the collection. Collections method disjoint takes two Collections and returns true if they have no elements in common. Figure 16.13 demonstrates the use of methods addAll, frequency and disjoint. tions 1 2 3 4 5 6 // Fig. Fig. 16.13: Algorithms2.java // Collections methods addAll, frequency and disjoint. import java.util.ArrayList; import java.util.List; import java.util.Arrays; import java.util.Collections; Fig. 16.13 | Collections methods addAll, frequency and disjoint. (Part 1 of 2.) 16.7 Collections Methods 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 707 public class Algorithms2 { public static void main(String[] args) { // initialize list1 and list2 String[] colors = {"red", "white", "yellow", "blue"}; List list1 = Arrays.asList(colors); ArrayList list2 = new ArrayList<>(); list2.add("black"); // add "black" to the end of list2 list2.add("red"); // add "red" to the end of list2 list2.add("green"); // add "green" to the end of list2 System.out.print("Before addAll, list2 contains: "); // display elements in list2 for (String s : list2) System.out.printf("%s ", s); Collections.addAll(list2, colors); // add colors Strings to list2 System.out.printf("%nAfter addAll, list2 contains: "); // display elements in list2 for (String s : list2) System.out.printf("%s ", s); // get frequency of "red" int frequency = Collections.frequency(list2, "red"); System.out.printf( "%nFrequency of red in list2: %d%n", frequency); // check whether list1 and list2 have elements in common boolean disjoint = Collections.disjoint(list1, list2); System.out.printf("list1 and list2 %s elements in common%n", (disjoint ? "do not have" : "have")); } } // end class Algorithms2 Before addAll, list2 contains: black red green After addAll, list2 contains: black red green red white yellow blue Frequency of red in list2: 2 list1 and list2 have elements in common Fig. 16.13 | Collections methods addAll, frequency and disjoint. (Part 2 of 2.) Line 14 initializes list1 with elements in array colors, and lines 17–19 add Strings and "green" to list2. Line 27 invokes method addAll to add elements in array colors to list2. Line 36 gets the frequency of String "red" in list2 using method frequency. Line 41 invokes method disjoint to test whether Collections list1 and list2 have elements in common, which they do in this example. "black", "red" 708 Chapter 16 Generic Collections 16.8 Stack Class of Package java.util We introduced the concept of a stack in Section 6.6 when we discussed the method-call stack. In Chapter 21, Custom Generic Data Structures, we’ll learn how to build data structures, including linked lists, stacks, queues and trees. In a world of software reuse, rather than building data structures as we need them, we can often take advantage of existing data structures. In this section, we investigate class Stack in the Java utilities package (java.util). Class Stack extends class Vector to implement a stack data structure. Figure 16.14 demonstrates several Stack methods. For the details of class Stack, visit http:// docs.oracle.com/javase/7/docs/api/java/util/Stack.html. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 // Fig. 16.14: StackTest.java // Stack class of package java.util. import java.util.Stack; import java.util.EmptyStackException; public class StackTest { public static void main(String[] args) { Stack stack = new Stack<>(); // create a Stack // use push method stack.push(12L); // push long value 12L System.out.println("Pushed 12L"); printStack(stack); stack.push(34567); // push int value 34567 System.out.println("Pushed 34567"); printStack(stack); stack.push(1.0F); // push float value 1.0F System.out.println("Pushed 1.0F"); printStack(stack); stack.push(1234.5678); // push double value 1234.5678 System.out.println("Pushed 1234.5678 "); printStack(stack); // remove items from stack try { Number removedObject = null; // pop elements from stack while (true) { removedObject = stack.pop(); // use pop method System.out.printf("Popped %s%n", removedObject); printStack(stack); } } catch (EmptyStackException emptyStackException) { Fig. 16.14 | Stack class of package java.util. (Part 1 of 2.) 16.8 Stack Class of Package java.util 41 42 43 44 45 46 47 48 49 50 51 52 53 709 emptyStackException.printStackTrace(); } } // display Stack contents private static void printStack(Stack stack) { if (stack.isEmpty()) System.out.printf("stack is empty%n%n"); // the stack is empty else // stack is not empty System.out.printf("stack contains: %s (top)%n", stack); } } // end class StackTest Pushed 12L stack contains: [12] Pushed 34567 stack contains: [12, Pushed 1.0F stack contains: [12, Pushed 1234.5678 stack contains: [12, Popped 1234.5678 stack contains: [12, Popped 1.0 stack contains: [12, Popped 34567 stack contains: [12] Popped 12 stack is empty (top) 34567] (top) 34567, 1.0] (top) 34567, 1.0, 1234.5678] (top) 34567, 1.0] (top) 34567] (top) (top) java.util.EmptyStackException at java.util.Stack.peek(Unknown Source) at java.util.Stack.pop(Unknown Source) at StackTest.main(StackTest.java:34) Fig. 16.14 | Stack class of package java.util. (Part 2 of 2.) Error-Prevention Tip 16.1 Because Stack extends Vector, all public Vector methods can be called on Stack objects, even if the methods do not represent conventional stack operations. For example, Vector method add can be used to insert an element anywhere in a stack—an operation that could “corrupt” the stack. When manipulating a Stack, only methods push and pop should be used to add elements to and remove elements from the Stack, respectively. In Section 21.5, we create a Stack class using composition so that the Stack provides in its public interface only the capabilities that should be allowed by a Stack. Line 10 creates an empty Stack of Numbers. Class Number (in package java.lang) is the superclass of the type-wrapper classes for the primitive numeric types (e.g., Integer, Double). By creating a Stack of Numbers, objects of any class that extends Number can be pushed onto the Stack. Lines 13, 16, 19 and 22 each call Stack method push to add a Number object to the top of the stack. Note the literals 12L (line 13) and 1.0F (line 19). Any integer literal that has the suffix L is a long value. An integer literal without a suffix 710 Chapter 16 Generic Collections is an int value. Similarly, any floating-point literal that has the suffix F is a float value. A floating-point literal without a suffix is a double value. You can learn more about numeric literals in the Java Language Specification at http://docs.oracle.com/javase/ specs/jls/se7/html/jls-15.html#jls-15.8.1. An infinite loop (lines 32–37) calls Stack method pop to remove the top element of the stack. The method returns a Number reference to the removed element. If there are no elements in the Stack, method pop throws an EmptyStackException, which terminates the loop. Class Stack also declares method peek. This method returns the top element of the stack without popping the element off the stack. Method printStack (lines 46–52) displays the stack’s contents. The current top of the stack (the last value pushed onto the stack) is the first value printed. Line 48 calls Stack method isEmpty (inherited by Stack from class Vector) to determine whether the stack is empty. If it’s empty, the method returns true; otherwise, false. 16.9 Class PriorityQueue and Interface Queue Recall that a queue is a collection that represents a waiting line—typically, insertions are made at the back of a queue and deletions are made from the front. In Section 21.6, we’ll discuss and implement a queue data structure. In this section, we investigate Java’s Queue interface and PriorityQueue class from package java.util. Interface Queue extends interface Collection and provides additional operations for inserting, removing and inspecting elements in a queue. PriorityQueue, which implements the Queue interface, orders elements by their natural ordering as specified by Comparable elements’ compareTo method or by a Comparator object that’s supplied to the constructor. Class PriorityQueue provides functionality that enables insertions in sorted order into the underlying data structure and deletions from the front of the underlying data structure. When adding elements to a PriorityQueue, the elements are inserted in priority order such that the highest-priority element (i.e., the largest value) will be the first element removed from the PriorityQueue. The common PriorityQueue operations are offer to insert an element at the appropriate location based on priority order, poll to remove the highest-priority element of the priority queue (i.e., the head of the queue), peek to get a reference to the highest-priority element of the priority queue (without removing that element), clear to remove all elements in the priority queue and size to get the number of elements in the priority queue. Figure 16.15 demonstrates the PriorityQueue class. Line 10 creates a PriorityQueue that stores Doubles with an initial capacity of 11 elements and orders the elements according to the object’s natural ordering (the defaults for a PriorityQueue). PriorityQueue is a generic class. Line 10 instantiates a PriorityQueue with a type argument Double. Class PriorityQueue provides five additional constructors. One of these takes an int and a Comparator object to create a PriorityQueue with the initial capacity specified by the int and the ordering by the Comparator. Lines 13–15 use method offer to add elements to the priority queue. Method offer throws a NullPointerException if the program attempts to add a null object to the queue. The loop in lines 20–24 uses method size to determine whether the priority queue is empty (line 20). While there are more elements, line 22 uses PriorityQueue method peek to retrieve the highest-priority element in the queue for output (without actually removing it from the queue). Line 23 removes the highest-priority element in the queue with method poll, which returns the removed element. 16.10 Sets 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 711 // Fig. 16.15: PriorityQueueTest.java // PriorityQueue test program. import java.util.PriorityQueue; public class PriorityQueueTest { public static void main(String[] args) { // queue of capacity 11 PriorityQueue queue = new PriorityQueue<>(); // insert elements to queue queue.offer(3.2); queue.offer(9.8); queue.offer(5.4); System.out.print("Polling from queue: "); // display elements in queue while (queue.size() > 0) { System.out.printf("%.1f ", queue.peek()); // view top element queue.poll(); // remove top element } } } // end class PriorityQueueTest Polling from queue: 3.2 5.4 9.8 Fig. 16.15 | PriorityQueue test program. 16.10 Sets A Set is an unordered Collection of unique elements (i.e., no duplicates). The collections framework contains several Set implementations, including HashSet and TreeSet. HashSet stores its elements in a hash table, and TreeSet stores its elements in a tree. Hash tables are presented in Section 16.11. Trees are discussed in Section 21.7. Figure 16.16 uses a HashSet to remove duplicate strings from a List. Recall that both List and Collection are generic types, so line 16 creates a List that contains String objects, and line 20 passes a Collection of Strings to method printNonDuplicates. Method printNonDuplicates (lines 24–35) takes a Collection argument. Line 27 constructs a HashSet from the Collection argument. By definition, Sets do not contain duplicates, so when the HashSet is constructed, it removes any duplicates in the Collection. Lines 31–32 output elements in the Set. 1 2 3 4 // Fig. 16.16: SetTest.java // HashSet used to remove duplicate values from array of strings. import java.util.List; import java.util.Arrays; Fig. 16.16 | HashSet used to remove duplicate values from an array of strings. (Part 1 of 2.) 712 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 Chapter 16 Generic Collections import java.util.HashSet; import java.util.Set; import java.util.Collection; public class SetTest { public static void main(String[] args) { // create and display a List String[] colors = {"red", "white", "blue", "green", "gray", "orange", "tan", "white", "cyan", "peach", "gray", "orange"}; List list = Arrays.asList(colors); System.out.printf("List: %s%n", list); // eliminate duplicates then print the unique values printNonDuplicates(list); } // create a Set from a Collection to eliminate duplicates private static void printNonDuplicates(Collection values) { // create a HashSet Set set = new HashSet<>(values); System.out.printf("%nNonduplicates are: "); for (String value : set) System.out.printf("%s ", value); System.out.println(); } } // end class SetTest List: [red, white, blue, green, gray, orange, tan, white, cyan, peach, gray, orange] Nonduplicates are: orange green white peach gray cyan red blue tan Fig. 16.16 | HashSet used to remove duplicate values from an array of strings. (Part 2 of 2.) Sorted Sets The collections framework also includes the SortedSet interface (which extends Set) for sets that maintain their elements in sorted order—either the elements’ natural order (e.g., numbers are in ascending order) or an order specified by a Comparator. Class TreeSet implements SortedSet. The program in Fig. 16.17 places Strings into a TreeSet. The Strings are sorted as they’re added to the TreeSet. This example also demonstrates rangeview methods, which enable a program to view a portion of a collection. Line 14 creates a TreeSet that contains the elements of array colors, then assigns the new TreeSet to SortedSet variable tree. Line 17 outputs the initial set of strings using method printSet (lines 33–39), which we discuss momentarily. Line 31 calls TreeSet method headSet to get a subset of the TreeSet in which every 16.10 Sets 713 element is less than "orange". The view returned from headSet is then output with If any changes are made to the subset, they’ll also be made to the original TreeSet, because the subset returned by headSet is a view of the TreeSet. Line 25 calls TreeSet method tailSet to get a subset in which each element is greater than or equal to "orange", then outputs the result. Any changes made through the tailSet view are made to the original TreeSet. Lines 28–29 call SortedSet methods first and last to get the smallest and largest elements of the set, respectively. Method printSet (lines 33–39) accepts a SortedSet as an argument and prints it. Lines 35–36 print each element of the SortedSet using the enhanced for statement. printSet. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 // Fig. 16.17: SortedSetTest.java // Using SortedSets and TreeSets. import java.util.Arrays; import java.util.SortedSet; import java.util.TreeSet; public class SortedSetTest { public static void main(String[] args) { // create TreeSet from array colors String[] colors = {"yellow", "green", "black", "tan", "grey", "white", "orange", "red", "green"}; SortedSet tree = new TreeSet<>(Arrays.asList(colors)); System.out.print("sorted set: "); printSet(tree); // get headSet based on "orange" System.out.print("headSet (\"orange\"): printSet(tree.headSet("orange") ); "); // get tailSet based upon "orange" System.out.print("tailSet (\"orange\"): printSet(tree.tailSet("orange") ); "); // get first and last elements System.out.printf("first: %s%n", tree.first()); System.out.printf("last : %s%n", tree.last()); } // output SortedSet using enhanced for statement private static void printSet(SortedSet set) { for (String s : set) System.out.printf("%s ", s); System.out.println(); } } // end class SortedSetTest Fig. 16.17 | Using SortedSets and TreeSets. (Part 1 of 2.) 714 Chapter 16 Generic Collections sorted set: black green grey orange red tan white yellow headSet ("orange"): black green grey tailSet ("orange"): orange red tan white yellow first: black last : yellow Fig. 16.17 | Using SortedSets and TreeSets. (Part 2 of 2.) 16.11 Maps Maps associate keys to values. The keys in a Map must be unique, but the associated values need not be. If a Map contains both unique keys and unique values, it’s said to implement a one-to-one mapping. If only the keys are unique, the Map is said to implement a manyto-one mapping—many keys can map to one value. Maps differ from Sets in that Maps contain keys and values, whereas Sets contain only values. Three of the several classes that implement interface Map are Hashtable, HashMap and TreeMap. Hashtables and HashMaps store elements in hash tables, and TreeMaps store elements in trees. This section discusses hash tables and provides an example that uses a HashMap to store key–value pairs. Interface SortedMap extends Map and maintains its keys in sorted order—either the elements’ natural order or an order specified by a Comparator. Class TreeMap implements SortedMap. Implementation with Hash Tables When a program creates objects, it may need to store and retrieve them efficiently. Storing and retrieving information with arrays is efficient if some aspect of your data directly matches a numerical key value and if the keys are unique and tightly packed. If you have 100 employees with nine-digit social security numbers and you want to store and retrieve employee data by using the social security number as a key, the task will require an array with over 800 million elements, because nine-digit Social Security numbers must begin with 001–899 (excluding 666) as per the Social Security Administration’s website Map http://www.socialsecurity.gov/employer/randomization.html This is impractical for virtually all applications that use social security numbers as keys. A program having an array that large could achieve high performance for both storing and retrieving employee records by simply using the social security number as the array index. Numerous applications have this problem—namely, that either the keys are of the wrong type (e.g., not positive integers that correspond to array subscripts) or they’re of the right type, but sparsely spread over a huge range. What is needed is a high-speed scheme for converting keys such as social security numbers, inventory part numbers and the like into unique array indices. Then, when an application needs to store something, the scheme could convert the application’s key rapidly into an index, and the record could be stored at that slot in the array. Retrieval is accomplished the same way: Once the application has a key for which it wants to retrieve a data record, the application simply applies the conversion to the key—this produces the array index where the data is stored and retrieved. The scheme we describe here is the basis of a technique called hashing. Why the name? When we convert a key into an array index, we literally scramble the bits, forming 16.11 Maps 715 a kind of “mishmashed,” or hashed, number. The number actually has no real significance beyond its usefulness in storing and retrieving a particular data record. A glitch in the scheme is called a collision—this occurs when two different keys “hash into” the same cell (or element) in the array. We cannot store two values in the same space, so we need to find an alternative home for all values beyond the first that hash to a particular array index. There are many schemes for doing this. One is to “hash again” (i.e., to apply another hashing transformation to the key to provide a next candidate cell in the array). The hashing process is designed to distribute the values throughout the table, so the assumption is that an available cell will be found with just a few hashes. Another scheme uses one hash to locate the first candidate cell. If that cell is occupied, successive cells are searched in order until an available cell is found. Retrieval works the same way: The key is hashed once to determine the initial location and check whether it contains the desired data. If it does, the search is finished. If it does not, successive cells are searched linearly until the desired data is found. The most popular solution to hash-table collisions is to have each cell of the table be a hash “bucket,” typically a linked list of all the key–value pairs that hash to that cell. This is the solution that Java’s Hashtable and HashMap classes (from package java.util) implement. Both Hashtable and HashMap implement the Map interface. The primary differences between them are that HashMap is unsynchronized (multiple threads should not modify a HashMap concurrently) and allows null keys and null values. A hash table’s load factor affects the performance of hashing schemes. The load factor is the ratio of the number of occupied cells in the hash table to the total number of cells in the hash table. The closer this ratio gets to 1.0, the greater the chance of collisions. Performance Tip 16.2 The load factor in a hash table is a classic example of a memory-space/execution-time trade-off: By increasing the load factor, we get better memory utilization, but the program runs slower, due to increased hashing collisions. By decreasing the load factor, we get better program speed, because of reduced hashing collisions, but we get poorer memory utilization, because a larger portion of the hash table remains empty. Computer science students study hashing schemes in courses called “Data Structures” and “Algorithms.” Classes Hashtable and HashMap enable you to use hashing without having to implement hash-table mechanisms—a classic example of reuse. This concept is profoundly important in our study of object-oriented programming. As discussed in earlier chapters, classes encapsulate and hide complexity (i.e., implementation details) and offer user-friendly interfaces. Properly crafting classes to exhibit such behavior is one of the most valued skills in the field of object-oriented programming. Figure 16.18 uses a HashMap to count the number of occurrences of each word in a string. 1 2 3 4 5 6 // Fig. 16.18: WordTypeCount.java // Program counts the number of occurrences of each word in a String. import java.util.Map; import java.util.HashMap; import java.util.Set; import java.util.TreeSet; Fig. 16.18 | Program counts the number of occurrences of each word in a String. (Part 1 of 3.) 716 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 Chapter 16 Generic Collections import java.util.Scanner; public class WordTypeCount { public static void main(String[] args) { // create HashMap to store String keys and Integer values Map myMap = new HashMap<>(); createMap(myMap); // create map based on user input displayMap(myMap); // display map content } // create map from user input private static void createMap(Map map) { Scanner scanner = new Scanner(System.in); // create scanner System.out.println("Enter a string:"); // prompt for user input String input = scanner.nextLine(); // tokenize the input String[] tokens = input.split(" "); // processing input text for (String token : tokens) { String word = token.toLowerCase(); // get lowercase word // if the map contains the word if (map.containsKey(word)) // is word in map { int count = map.get(word); // get current count map.put(word, count + 1); // increment count } else map.put(word, 1); // add new word with a count of 1 to map } } // display map content private static void displayMap(Map map) { Set keys = map.keySet(); // get keys // sort keys TreeSet sortedKeys = new TreeSet<>(keys); System.out.printf("%nMap contains:%nKey\t\tValue%n"); // generate output for each key in map for (String key : sortedKeys) System.out.printf("%-10s%10s%n", key, map.get(key)); Fig. 16.18 | Program counts the number of occurrences of each word in a String. (Part 2 of 3.) 16.11 Maps 60 61 62 63 717 System.out.printf( "%nsize: %d%nisEmpty: %b%n", map.size(), map.isEmpty()); } } // end class WordTypeCount Enter a string: this is a sample sentence with several words this is another sample sentence with several different words Map contains: Key a another different is sample sentence several this with words Value 1 1 1 2 2 2 2 2 2 2 size: 10 isEmpty: false Fig. 16.18 | Program counts the number of occurrences of each word in a String. (Part 3 of 3.) Line 14 creates an empty HashMap with a default initial capacity (16 elements) and a default load factor (0.75)—these defaults are built into the implementation of HashMap. When the number of occupied slots in the HashMap becomes greater than the capacity times the load factor, the capacity is doubled automatically. HashMap is a generic class that takes two type arguments—the type of key (i.e., String) and the type of value (i.e., Integer). Recall that the type arguments passed to a generic class must be reference types, hence the second type argument is Integer, not int. Line 16 calls method createMap (lines 21–44), which uses a Map to store the number of occurrences of each word in the sentence. Line 25 obtains the user input, and line 28 tokenizes it. Lines 31–43 convert the next token to lowercase letters (line 33), then call Map method containsKey (line 36) to determine whether the word is in the map (and thus has occurred previously in the string). If the Map does not contain the word, line 42 uses Map method put to create a new entry, with the word as the key and an Integer object containing 1 as the value. Autoboxing occurs when the program passes integer 1 to method put, because the map stores the number of occurrences as an Integer. If the word does exist in the map, line 38 uses Map method get to obtain the key’s associated value (the count) in the map. Line 39 increments that value and uses put to replace the key’s associated value. Method put returns the key’s prior associated value, or null if the key was not in the map. Error-Prevention Tip 16.2 Always use immutable keys with a Map. The key determines where the corresponding value is placed. If the key has changed since the insert operation, when you subsequently attempt to retrieve that value, it might not be found. In this chapter’s examples, we use Strings as keys and Strings are immutable. 718 Chapter 16 Generic Collections Method displayMap (lines 47–62) displays all the entries in the map. It uses HashMap method keySet (line 49) to get a set of the keys. The keys have type String in the map, so method keySet returns a generic type Set with type parameter specified to be String. Line 52 creates a TreeSet of the keys, in which the keys are sorted. The loop in lines 57– 58 accesses each key and its value in the map. Line 58 displays each key and its value using format specifier %-10s to left align each key and format specifier %10s to right align each value. The keys are displayed in ascending order. Line 61 calls Map method size to get the number of key–value pairs in the Map. Line 61 also calls Map method isEmpty, which returns a boolean indicating whether the Map is empty. 16.12 Properties Class A Properties object is a persistent Hashtable that stores key–value pairs of Strings—assuming that you use methods setProperty and getProperty to manipulate the table rather than inherited Hashtable methods put and get. By “persistent,” we mean that the Properties object can be written to an output stream (possibly a file) and read back in through an input stream. A common use of Properties objects in prior versions of Java was to maintain application-configuration data or user preferences for applications. [Note: The Preferences API (package java.util.prefs) is meant to replace this particular use of class Properties but is beyond the scope of this book. To learn more, visit http://bit.ly/JavaPreferences.] Class Properties extends class Hashtable. Figure 16.19 demonstrates several methods of class Properties. Line 13 creates an empty Properties table with no default properties. Class Properties also provides an overloaded constructor that receives a reference to a Properties object containing default property values. Lines 16 and 17 each call Properties method setProperty to store a value for the specified key. If the key does not exist in the table, setProperty returns null; otherwise, it returns the previous value for that key. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 // Fig. 16.19: PropertiesTest.java // Demonstrates class Properties of the java.util package. import java.io.FileOutputStream; import java.io.FileInputStream; import java.io.IOException; import java.util.Properties; import java.util.Set; public class PropertiesTest { public static void main(String[] args) { Properties table = new Properties(); // set properties table.setProperty("color", "blue"); table.setProperty("width", "200"); System.out.println("After setting properties"); listProperties(table); Fig. 16.19 | Properties class of package java.util. (Part 1 of 3.) 16.12 Properties Class 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 719 // replace property value table.setProperty("color", "red"); System.out.println("After replacing properties"); listProperties(table); saveProperties(table); table.clear(); // empty table System.out.println("After clearing properties"); listProperties(table); loadProperties(table); // get value of property color Object value = table.getProperty("color"); // check if value is in table if (value != null) System.out.printf("Property color's value is %s%n", value); else System.out.println("Property color is not in table"); } // save properties to a file private static void saveProperties(Properties props) { // save contents of table try { FileOutputStream output = new FileOutputStream("props.dat"); props.store(output, "Sample Properties"); // save properties output.close(); System.out.println("After saving properties"); listProperties(props); } catch (IOException ioException) { ioException.printStackTrace(); } } // load properties from a file private static void loadProperties(Properties props) { // load contents of table try { FileInputStream input = new FileInputStream("props.dat"); props.load(input); // load properties input.close(); Fig. 16.19 | Properties class of package java.util. (Part 2 of 3.) 720 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 Chapter 16 Generic Collections System.out.println("After loading properties"); listProperties(props); } catch (IOException ioException) { ioException.printStackTrace(); } } // output property values private static void listProperties(Properties props) { Set