Transcript
INTRODUCTION TO
JAVA ®
PROGRAMMING COMPREHENSIVE VERSION Tenth Edition
Y. Daniel Liang Armstrong Atlantic State University
Boston Columbus Indianapolis New York San Francisco Upper Saddle River Amsterdam Cape Town Dubai London Madrid Milan Munich Paris Montreal Toronto Delhi Mexico City Sao Paulo Sydney Hong Kong Seoul Singapore Taipei Tokyo
To Samantha, Michael, and Michelle
Editorial Director, ECS: Marcia Horton Executive Editor: Tracy Johnson (Dunkelberger) Editorial Assistant: Jenah Blitz-Stoehr Director of Marketing: Christy Lesko Marketing Manager: Yez Alayan Marketing Assistant: Jon Bryant Director of Program Management: Erin Gregg Program Management-Team Lead: Scott Disanno Program Manager: Carole Snyder Project Management-Team Lead: Laura Burgess Project Manager: Robert Engelhardt Procurement Specialist: Linda Sager
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Credits and acknowledgments borrowed from other sources and reproduced, with permission, in this textbook appear on the appropriate page within text. Microsoft® and Windows® are registered trademarks of the Microsoft Corporation in the U.S.A. and other countries. Screen shots and icons reprinted with permission from the Microsoft Corporation. This book is not sponsored or endorsed by or affiliated with the Microsoft Corporation.
Copyright © 2015, 2013, 2011 Pearson Education, Inc., publishing as Prentice Hall, 1 Lake Street, Upper Saddle River, New Jersey, 07458. All rights reserved. Printed in the United States of America. This publication is protected by Copyright, and permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. To obtain permission(s) to use material from this work, please submit a written request to Pearson Education, Inc., Permissions Department, One Lake Street, Upper Saddle River, New Jersey 07458, or you may fax your request to 201-236-3290. Many of the designations by manufacturers and sellers to distinguish their products are claimed as trademarks. Where those designations appear in this book, and the publisher was aware of a trademark claim, the designations have been printed in initial caps or all caps. Library of Congress Cataloging-in-Publication Data available upon request.
10 9 8 7 6 5 4 3 2 1 ISBN 10: 0-13-376131-2 ISBN 13: 978-0-13-376131-3
PREFACE Dear Reader, Many of you have provided feedback on earlier editions of this book, and your comments and suggestions have greatly improved the book. This edition has been substantially enhanced in presentation, organization, examples, exercises, and supplements. The new edition: ■
Replaces Swing with JavaFX. JavaFX is a new framework for developing Java GUI programs. JavaFX greatly simplifies GUI programming and is easier to learn than Swing.
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Introduces exception handling, abstract classes, and interfaces before GUI programming to enable the GUI chapters to be skipped completely if the instructor chooses not to cover GUI.
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Covers introductions to objects and strings earlier in Chapter 4 to enable students to use objects and strings to develop interesting programs early.
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Includes many new interesting examples and exercises to stimulate student interests. More than 100 additional programming exercises are provided to instructors only on the Companion Website.
Please visit www.pearsonhighered.com/liang for a complete list of new features as well as correlations to the previous edition. The book is fundamentals first by introducing basic programming concepts and techniques before designing custom classes. The fundamental concepts and techniques of selection statements, loops, methods, and arrays are the foundation for programming. Building this strong foundation prepares students to learn object-oriented programming and advanced Java programming. This book teaches programming in a problem-driven way that focuses on problem solving rather than syntax. We make introductory programming interesting by using thoughtprovoking problems in a broad context. The central thread of early chapters is on problem solving. Appropriate syntax and library are introduced to enable readers to write programs for solving the problems. To support the teaching of programming in a problem-driven way, the book provides a wide variety of problems at various levels of difficulty to motivate students. To appeal to students in all majors, the problems cover many application areas, including math, science, business, financial, gaming, animation, and multimedia. The book seamlessly integrates programming, data structures, and algorithms into one text. It employs a practical approach to teach data structures. We first introduce how to use various data structures to develop efficient algorithms, and then show how to implement these data structures. Through implementation, students gain a deep understanding on the efficiency of data structures and on how and when to use certain data structures. Finally we design and implement custom data structures for trees and graphs. The book is widely used in the introductory programming, data structures, and algorithms courses in the universities around the world. This comprehensive version covers fundamentals of programming, object-oriented programming, GUI programming, data structures, algorithms, concurrency, networking, database, and Web programming. It is designed to prepare students to become proficient Java programmers. A brief version (Introduction to Java Programming, Brief Version, Tenth Edition) is available for a first course on programming, commonly known as CS1. The brief version contains the first 18 chapters of the comprehensive version. The first 13 chapters are appropriate for preparing the AP Computer Science exam. The best way to teach programming is by example, and the only way to learn programming is by doing. Basic concepts are explained by example and a large number of exercises
what is new?
fundamentals-first
problem-driven
data structures
comprehensive version
brief version AP Computer Science
examples and exercises
iii
iv Preface with various levels of difficulty are provided for students to practice. For our programming courses, we assign programming exercises after each lecture. Our goal is to produce a text that teaches problem solving and programming in a broad context using a wide variety of interesting examples. If you have any comments on and suggestions for improving the book, please email me. Sincerely, Y. Daniel Liang
[email protected] www.cs.armstrong.edu/liang www.pearsonhighered.com/liang
ACM/IEEE Curricular 2013 and ABET Course Assessment The new ACM/IEEE Computer Science Curricular 2013 defines the Body of Knowledge organized into 18 Knowledge Areas. To help instructors design the courses based on this book, we provide sample syllabi to identify the Knowledge Areas and Knowledge Units. The sample syllabi are for a three semester course sequence and serve as an example for institutional customization. The sample syllabi are available to instructors at www.pearsonhighered.com/liang. Many of our users are from the ABET-accredited programs. A key component of the ABET accreditation is to identify the weakness through continuous course assessment against the course outcomes. We provide sample course outcomes for the courses and sample exams for measuring course outcomes on the instructor Website accessible from www.pearsonhighered.com/liang.
What’s New in This Edition? This edition is completely revised in every detail to enhance clarity, presentation, content, examples, and exercises. The major improvements are as follows: ■
Updated to Java 8.
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Since Swing is replaced by JavaFX, all GUI examples and exercises are revised using JavaFX.
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Lambda expressions are used to simplify coding in JavaFX and threads.
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More than 100 additional programming exercises with solutions are provided to the instructor on the Companion Website. These exercises are not printed in the text.
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Math methods are introduced earlier in Chapter 4 to enable students to write code using math functions.
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Strings are introduced earlier in Chapter 4 to enable students to use objects and strings to develop interesting programs early.
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The GUI chapters are moved to after abstract classes and interfaces so that these chapters can be easily skipped if the instructor chooses not to cover GUI.
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Chapters 4, 14, 15, and 16 are brand new chapters.
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Chapters 28 and 29 have been substantially revised with simpler implementations for minimum spanning trees and shortest paths.
Preface v
Pedagogical Features The book uses the following elements to help students get the most from the material: ■
The Objectives at the beginning of each chapter list what students should learn from the chapter. This will help them determine whether they have met the objectives after completing the chapter.
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The Introduction opens the discussion with representative problems to give the reader an overview of what to expect from the chapter.
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Key Points highlight the important concepts covered in each section.
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Check Points provide review questions to help students track their progress as they read through the chapter and evaluate their learning.
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Problems and Case Studies, carefully chosen and presented in an easy-to-follow style, teach problem solving and programming concepts. The book uses many small, simple, and stimulating examples to demonstrate important ideas.
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The Chapter Summary reviews the important subjects that students should understand and remember. It helps them reinforce the key concepts they have learned in the chapter.
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Quizzes are accessible online, grouped by sections, for students to do self-test on programming concepts and techniques.
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Programming Exercises are grouped by sections to provide students with opportunities to apply the new skills they have learned on their own. The level of difficulty is rated as easy (no asterisk), moderate (*), hard (**), or challenging (***). The trick of learning programming is practice, practice, and practice. To that end, the book provides a great many exercises. Additionally, more than 100 programming exercises with solutions are provided to the instructors on the Companion Website. These exercises are not printed in the text.
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Notes, Tips, Cautions, and Design Guides are inserted throughout the text to offer valuable advice and insight on important aspects of program development.
Note Provides additional information on the subject and reinforces important concepts.
Tip Teaches good programming style and practice.
Caution Helps students steer away from the pitfalls of programming errors.
Design Guide Provides guidelines for designing programs.
Flexible Chapter Orderings The book is designed to provide flexible chapter orderings to enable GUI, exception handling, recursion, generics, and the Java Collections Framework to be covered earlier or later. The diagram on the next page shows the chapter dependencies.
vi Preface
Part I: Fundamentals of Programming Chapter 1 Introduction to Computers, Programs, and Java Chapter 2 Elementary Programming Chapter 3 Selections
Part II: Object-Oriented Programming
Part IV: Data Structures and Algorithms
Part III: GUI Programming
Chapter 9 Objects and Classes
Chapter 14 JavaFX Basics
Ch 7
Chapter 18 Recursion
Chapter 10 Thinking in Objects
Chapter 15 Event-Driven Programming and Animations
Ch 13
Chapter 19 Generics
Chapter 11 Inheritance and Polymorphism Chapter 12 Exception Handling and Text I/O
Chapter 4 Mathematical Functions, Characters, and Strings
Chapter 13 Abstract Classes and Interfaces
Chapter 5 Loops
Chapter 17 Binary I/O
Chapter 16 JavaFX Controls and Multimedia
Ch 16
Chapter 30 Multithreading and Parallel Programming Chapter 31 Networking
Chapter 20 Lists, Stacks, Queues, and Priority Queues
Chapter 32 Java Database Programming
Chapter 21 Sets and Maps Chapter 33 JavaServer Faces
Chapter 34 Advanced GUI Programming
Chapter 22 Developping Efficient Algorithms Chapter 35 Advanced Database Programming
Chapter 23 Sorting
Chapter 6 Methods
Chapter 24 Implementing Lists, Stacks, Queues, and Priority Queues
Chapter 7 Single-Dimensional Arrays
Chapter 25 Binary Search Trees
Chapter 8 Multidimensional Arrays
Part V: Advanced Java Programming
Chapter 36 Internationalization
Chapter 37 Servlets
Note: Chapters 1–18 are in the brief version of this book.
Chapter 26 AVL Trees
Note: Chapters 1–33 are in the comprehensive version.
Chapter 27 Hashing
Note: Chapters 34–42 are bonus chapters available from the Companion Website.
Chapter 28 Graphs and Applications
Chapter 38 JavaServer Pages
Chapter 39 Web Services
Chapter 29 Weighted Graphs and Applications Chapter 40 2-4 Trees and BTrees Chapter 41 Red-Black Trees
Ch 9
Chapter 42 Testing Using JUnit
Preface vii
Organization of the Book The chapters can be grouped into five parts that, taken together, form a comprehensive introduction to Java programming, data structures and algorithms, and database and Web programming. Because knowledge is cumulative, the early chapters provide the conceptual basis for understanding programming and guide students through simple examples and exercises; subsequent chapters progressively present Java programming in detail, culminating with the development of comprehensive Java applications. The appendixes contain a mixed bag of topics, including an introduction to number systems, bitwise operations, regular expressions, and enumerated types. Part I: Fundamentals of Programming (Chapters 1–8) The first part of the book is a stepping stone, preparing you to embark on the journey of learning Java. You will begin to learn about Java (Chapter 1) and fundamental programming techniques with primitive data types, variables, constants, assignments, expressions, and operators (Chapter 2), selection statements (Chapter 3), mathematical functions, characters, and strings (Chapter 4), loops (Chapter 5), methods (Chapter 6), and arrays (Chapters 7–8). After Chapter 7, you can jump to Chapter 18 to learn how to write recursive methods for solving inherently recursive problems. Part II: Object-Oriented Programming (Chapters 9–13, and 17) This part introduces object-oriented programming. Java is an object-oriented programming language that uses abstraction, encapsulation, inheritance, and polymorphism to provide great flexibility, modularity, and reusability in developing software. You will learn programming with objects and classes (Chapters 9–10), class inheritance (Chapter 11), polymorphism (Chapter 11), exception handling (Chapter 12), abstract classes (Chapter 13), and interfaces (Chapter 13). Text I/O is introduced in Chapter 12 and binary I/O is discussed in Chapter 17. Part III: GUI Programming (Chapters 14–16 and Bonus Chapter 34) JavaFX is a new framework for developing Java GUI programs. It is not only useful for developing GUI programs, but also an excellent pedagogical tool for learning object-oriented programming. This part introduces Java GUI programming using JavaFX in Chapters 14–16. Major topics include GUI basics (Chapter 14), container panes (Chapter 14), drawing shapes (Chapter 14), event-driven programming (Chapter 15), animations (Chapter 15), and GUI controls (Chapter 16), and playing audio and video (Chapter 16). You will learn the architecture of JavaFX GUI programming and use the controls, shapes, panes, image, and video to develop useful applications. Chapter 34 covers advanced features in JavaFX. Part IV: Data Structures and Algorithms (Chapters 18–29 and Bonus Chapters 40–41) This part covers the main subjects in a typical data structures and algorithms course. Chapter 18 introduces recursion to write methods for solving inherently recursive problems. Chapter 19 presents how generics can improve software reliability. Chapters 20 and 21 introduce the Java Collection Framework, which defines a set of useful API for data structures. Chapter 22 discusses measuring algorithm efficiency in order to choose an appropriate algorithm for applications. Chapter 23 describes classic sorting algorithms. You will learn how to implement several classic data structures lists, queues, and priority queues in Chapter 24. Chapters 25 and 26 introduce binary search trees and AVL trees. Chapter 27 presents hashing and implementing maps and sets using hashing. Chapters 28 and 29 introduce graph applications. The 2-4 trees, B-trees, and red-black trees are covered in Bonus Chapters 40–41. Part V: Advanced Java Programming (Chapters 30–33 and Bonus Chapters 35–39, 42) This part of the book is devoted to advanced Java programming. Chapter 30 treats the use of multithreading to make programs more responsive and interactive and introduces parallel programming. Chapter 31 discusses how to write programs that talk with each other from different hosts over the Internet. Chapter 32 introduces the use of Java to develop database
viii Preface projects. Chapter 33 introduces modern Web application development using JavaServer Faces. Chapter 35 delves into advanced Java database programming. Chapter 36 covers the use of internationalization support to develop projects for international audiences. Chapters 37 and 38 introduce how to use Java servlets and JavaServer Pages to generate dynamic content from Web servers. Chapter 39 discusses Web services. Chapter 42 introduces testing Java programs using JUnit. Appendixes This part of the book covers a mixed bag of topics. Appendix A lists Java keywords. Appendix B gives tables of ASCII characters and their associated codes in decimal and in hex. Appendix C shows the operator precedence. Appendix D summarizes Java modifiers and their usage. Appendix E discusses special floating-point values. Appendix F introduces number systems and conversions among binary, decimal, and hex numbers. Finally, Appendix G introduces bitwise operations. Appendix H introduces regular expressions. Appendix I covers enumerated types.
Java Development Tools
IDE tutorials
You can use a text editor, such as the Windows Notepad or WordPad, to create Java programs and to compile and run the programs from the command window. You can also use a Java development tool, such as NetBeans or Eclipse. These tools support an integrated development environment (IDE) for developing Java programs quickly. Editing, compiling, building, executing, and debugging programs are integrated in one graphical user interface. Using these tools effectively can greatly increase your programming productivity. NetBeans and Eclipse are easy to use if you follow the tutorials. Tutorials on NetBeans and Eclipse can be found under Tutorials on the Student Companion Website at www.pearsonhighered.com/liang.
Student Resource Website The Student Resource Website www.pearsonhighered.com/liang provides access to some of the following resources. Other resources are available using the student access code printed on the inside front cover of this book. (For students with a used copy of this book, you can purchase access to the premium student resources through www.pearsonhighered.com/liang.) ■
Answers to review questions
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Solutions to even-numbered programming exercises
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Source code for the examples in the book
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Interactive quiz (organized by sections for each chapter)
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Supplements
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Debugging tips
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Algorithm animations
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Errata
Instructor Resource Website The Instructor Resource Website, accessible from www.pearsonhighered.com/liang, provides access to the following resources: ■
Microsoft PowerPoint slides with interactive buttons to view full-color, syntax-highlighted source code and to run programs without leaving the slides.
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Solutions to all programming exercises. Students will have access to the solutions of evennumbered programming exercises.
Preface ix ■
More than 100 additional programming exercises organized by chapters. These exercises are available only to the instructors. Solutions to these exercises are provided.
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Web-based quiz generator. (Instructors can choose chapters to generate quizzes from a large database of more than two thousand questions.)
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Sample exams. Most exams have four parts: ■
Multiple-choice questions or short-answer questions
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Correct programming errors
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Trace programs
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Write programs
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ACM/IEEE Curricula 2013. The new ACM/IEEE Computer Science Curricula 2013 defines the Body of Knowledge organized into 18 Knowledge Areas. To help instructors design the courses based on this book, we provide sample syllabi to identify the Knowledge Areas and Knowledge Units. The sample syllabi are for a three semester course sequence and serve as an example for institutional customization. Instructors can access the syllabi at www.pearsonhighered.com/liang.
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Sample exams with ABET course assessment.
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Projects. In general, each project gives a description and asks students to analyze, design, and implement the project.
Some readers have requested the materials from the Instructor Resource Website. Please understand that these are for instructors only. Such requests will not be answered.
Online Practice and Assessment with MyProgrammingLab MyProgrammingLab helps students fully grasp the logic, semantics, and syntax of programming. Through practice exercises and immediate, personalized feedback, MyProgrammingLab improves the programming competence of beginning students who often struggle with the basic concepts and paradigms of popular high-level programming languages. A self-study and homework tool, a MyProgrammingLab course consists of hundreds of small practice problems organized around the structure of this textbook. For students, the system automatically detects errors in the logic and syntax of their code submissions and offers targeted hints that enable students to figure out what went wrong—and why. For instructors, a comprehensive gradebook tracks correct and incorrect answers and stores the code inputted by students for review. MyProgrammingLab is offered to users of this book in partnership with Turing’s Craft, the makers of the CodeLab interactive programming exercise system. For a full demonstration, to see feedback from instructors and students, or to get started using MyProgrammingLab in your course, visit www.myprogramminglab.com.
VideoNotes We are excited about the new VideoNotes feature that is found in this new edition. These videos provide additional help by presenting examples of key topics and showing how to solve problems completely, from design through coding. VideoNotes are available from www.pearsonhighered.com/liang.
VideoNote
x Preface
Algorithm Animations Animation
We have provided numerous animations for algorithms. These are valuable pedagogical tools to demonstrate how algorithms work. Algorithm animations can be accessed from the Companion Website.
Acknowledgments I would like to thank Armstrong Atlantic State University for enabling me to teach what I write and for supporting me in writing what I teach. Teaching is the source of inspiration for continuing to improve the book. I am grateful to the instructors and students who have offered comments, suggestions, bug reports, and praise. This book has been greatly enhanced thanks to outstanding reviews for this and previous editions. The reviewers are: Elizabeth Adams (James Madison University), Syed Ahmed (North Georgia College and State University), Omar Aldawud (Illinois Institute of Technology), Stefan Andrei (Lamar University), Yang Ang (University of Wollongong, Australia), Kevin Bierre (Rochester Institute of Technology), David Champion (DeVry Institute), James Chegwidden (Tarrant County College), Anup Dargar (University of North Dakota), Charles Dierbach (Towson University), Frank Ducrest (University of Louisiana at Lafayette), Erica Eddy (University of Wisconsin at Parkside), Deena Engel (NewYork University), Henry A. Etlinger (Rochester Institute of Technology), James Ten Eyck (Marist College), Myers Foreman (Lamar University), Olac Fuentes (University of Texas at El Paso), Edward F. Gehringer (North Carolina State University), Harold Grossman (Clemson University), Barbara Guillot (Louisiana State University), Stuart Hansen (University of Wisconsin, Parkside), Dan Harvey (Southern Oregon University), Ron Hofman (Red River College, Canada), Stephen Hughes (Roanoke College), Vladan Jovanovic (Georgia Southern University), Edwin Kay (Lehigh University), Larry King (University of Texas at Dallas), Nana Kofi (Langara College, Canada), George Koutsogiannakis (Illinois Institute of Technology), Roger Kraft (Purdue University at Calumet), Norman Krumpe (Miami University), Hong Lin (DeVry Institute), Dan Lipsa (Armstrong Atlantic State University), James Madison (Rensselaer Polytechnic Institute), Frank Malinowski (Darton College), Tim Margush (University of Akron), Debbie Masada (Sun Microsystems), Blayne Mayfield (Oklahoma State University), John McGrath (J.P. McGrath Consulting), Hugh McGuire (Grand Valley State), Shyamal Mitra (University of Texas at Austin), Michel Mitri (James Madison University), Kenrick Mock (University of Alaska Anchorage), Frank Murgolo (California State University, Long Beach), Jun Ni (University of Iowa), Benjamin Nystuen (University of Colorado at Colorado Springs), Maureen Opkins (CA State University, Long Beach), Gavin Osborne (University of Saskatchewan), Kevin Parker (Idaho State University), Dale Parson (Kutztown University), Mark Pendergast (Florida Gulf Coast University), Richard Povinelli (Marquette University), Roger Priebe (University of Texas at Austin), Mary Ann Pumphrey (De Anza Junior College), Pat Roth (Southern Polytechnic State University), Amr Sabry (Indiana University), Ben Setzer (Kennesaw State University), Carolyn Schauble (Colorado State University), David Scuse (University of Manitoba), Ashraf Shirani (San Jose State University), Daniel Spiegel (Kutztown University), Joslyn A. Smith (Florida Atlantic University), Lixin Tao (Pace University), Ronald F. Taylor (Wright State University), Russ Tront (Simon Fraser University), Deborah Trytten (University of Oklahoma), Michael Verdicchio (Citadel), Kent Vidrine (George Washington University), and Bahram Zartoshty (California State University at Northridge). It is a great pleasure, honor, and privilege to work with Pearson. I would like to thank Tracy Johnson and her colleagues Marcia Horton, Yez Alayan, Carole Snyder, Scott Disanno, Bob Engelhardt, Haseen Khan, and their colleagues for organizing, producing, and promoting this project. As always, I am indebted to my wife, Samantha, for her love, support, and encouragement.
BRIEF CONTENTS 1 Introduction to Computers, Programs, 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
and Java Elementary Programming Selections Mathematical Functions, Characters, and Strings Loops Methods Single-Dimensional Arrays Multidimensional Arrays Objects and Classes Object-Oriented Thinking Inheritance and Polymorphism Exception Handling and Text I/O Abstract Classes and Interfaces JavaFX Basics Event-Driven Programming and Animations JavaFX UI Controls and Multimedia Binary I/O Recursion Generics Lists, Stacks, Queues, and Priority Queues Sets and Maps Developing Efficient Algorithms Sorting Implementing Lists, Stacks, Queues, and Priority Queues Binary Search Trees AVL Trees Hashing Graphs and Applications
1 33 75 119 157 203 245 287 321 365 409 449 495 535 585 629 677 705 737 761 797 821 861 895 929 965 985 1015
29 30 31 32 33
Weighted Graphs and Applications 1061 Multithreading and Parallel Programming 1097 Networking 1139 Java Database Programming 1173 JavaServer Faces 1213
Chapters 34–42 are bonus Web chapters 34-1 34 Advanced JavaFX 35 Advanced Database Programming 35-1 36 Internationalization 36-1 37 Servlets 37-1 38 JavaServer Pages 38-1 39 Web Services 39-1 40 2-4 Trees and B-Trees 40-1 41 Red-Black Trees 41-1 42 Testing Using JUnit 42-1
Appendixes A B C D E F G H I
Java Keywords The ASCII Character Set Operator Precedence Chart Java Modifiers Special Floating-Point Values Number Systems Bitwise Operatoirns Regular Expressions Enumerated Types
Index
1263 1266 1268 1270 1272 1273 1277 1278 1283 1289
xi
CONTENTS Chapter 1 Introduction to Computers, Programs, and Java 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 1.10 1.11 1.12
Introduction What Is a Computer? Programming Languages Operating Systems Java, the World Wide Web, and Beyond The Java Language Specification, API, JDK, and IDE A Simple Java Program Creating, Compiling, and Executing a Java Program Programming Style and Documentation Programming Errors Developing Java Programs Using NetBeans Developing Java Programs Using Eclipse
Chapter 2 Elementary Programming 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10 2.11 2.12 2.13 2.14 2.15 2.16 2.17 2.18
Introduction Writing a Simple Program Reading Input from the Console Identifiers Variables Assignment Statements and Assignment Expressions Named Constants Naming Conventions Numeric Data Types and Operations Numeric Literals Evaluating Expressions and Operator Precedence Case Study: Displaying the Current Time Augmented Assignment Operators Increment and Decrement Operators Numeric Type Conversions Software Development Process Case Study: Counting Monetary Units Common Errors and Pitfalls
Chapter 3 Selections 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11 3.12 3.13 3.14
xii
Introduction boolean Data Type if Statements Two-Way if-else Statements Nested if and Multi-Way if-else Statements
Common Errors and Pitfalls Generating Random Numbers Case Study: Computing Body Mass Index Case Study: Computing Taxes Logical Operators Case Study: Determining Leap Year Case Study: Lottery switch Statements Conditional Expressions
1 2 2 7 9 10 11 12 15 18 20 23 25
33 34 34 37 39 40 41 43 44 44 48 50 52 54 55 56 59 63 65
75 76 76 78 80 81 83 87 89 90 93 97 98 100 103
xiii Operator Precedence and Associativity Debugging
104 106
Chapter 4 Mathematical Functions, Characters, and Strings
119
3.15 3.16
4.1 4.2 4.3 4.4 4.5 4.6
Introduction Common Mathematical Functions Character Data Type and Operations The String Type Case Studies Formatting Console Output
Chapter 5 Loops 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 5.10 5.11
Introduction The while Loop The do-while Loop The for Loop Which Loop to Use? Nested Loops Minimizing Numeric Errors Case Studies Keywords break and continue Case Study: Checking Palindromes Case Study: Displaying Prime Numbers
Chapter 6 Methods 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 6.10 6.11
Introduction Defining a Method Calling a Method void Method Example Passing Arguments by Values Modularizing Code Case Study: Converting Hexadecimals to Decimals Overloading Methods The Scope of Variables Case Study: Generating Random Characters Method Abstraction and Stepwise Refinement
Chapter 7 Single-Dimensional Arrays 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 7.10 7.11 7.12 7.13
Introduction Array Basics Case Study: Analyzing Numbers Case Study: Deck of Cards Copying Arrays Passing Arrays to Methods Returning an Array from a Method Case Study: Counting the Occurrences of Each Letter Variable-Length Argument Lists Searching Arrays Sorting Arrays The Arrays Class Command-Line Arguments
Chapter 8 Multidimensional Arrays 8.1 8.2
Introduction Two-Dimensional Array Basics
120 120 125 130 139 145
157 158 158 168 170 174 176 178 179 184 187 188
203 204 204 206 209 212 215 217 219 222 223 225
245 246 246 253 254 256 257 260 261 264 265 269 270 272
287 288 288
xiv Contents 8.3 8.4 8.5 8.6 8.7 8.8
Processing Two-Dimensional Arrays Passing Two-Dimensional Arrays to Methods Case Study: Grading a Multiple-Choice Test Case Study: Finding the Closest Pair Case Study: Sudoku Multidimensional Arrays
Chapter 9 Objects and Classes 9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 9.9 9.10 9.11 9.12 9.13 9.14
Introduction Defining Classes for Objects Example: Defining Classes and Creating Objects Constructing Objects Using Constructors Accessing Objects via Reference Variables Using Classes from the Java Library Static Variables, Constants, and Methods Visibility Modifiers Data Field Encapsulation Passing Objects to Methods Array of Objects Immutable Objects and Classes The Scope of Variables The this Reference
Chapter 10 Object-Oriented Thinking 10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8 10.9 10.10 10.11
Introduction Class Abstraction and Encapsulation Thinking in Objects Class Relationships Case Study: Designing the Course Class Case Study: Designing a Class for Stacks Processing Primitive Data Type Values as Objects Automatic Conversion between Primitive Types and Wrapper Class Types The BigInteger and BigDecimal Classes The String Class The StringBuilder and StringBuffer Classes
Chapter 11 Inheritance and Polymorphism 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 11.14 11.15
Introduction Superclasses and Subclasses Using the super Keyword Overriding Methods Overriding vs. Overloading The Object Class and Its toString() Method Polymorphism Dynamic Binding Casting Objects and the instanceof Operator The Object’s equals Method The ArrayList Class Useful Methods for Lists Case Study: A Custom Stack Class The protected Data and Methods Preventing Extending and Overriding
Chapter 12 Exception Handling and Text I/O 12.1 12.2
Introduction Exception-Handling Overview
291 293 294 296 298 301
321 322 322 324 329 330 334 337 342 344 347 351 353 355 356
365 366 366 370 373 376 378 380 383 384 386 392
409 410 410 416 419 420 422 423 424 427 431 432 438 439 440 442
449 450 450
Contents xv 12.3 12.4 12.5 12.6 12.7 12.8 12.9 12.10 12.11 12.12 12.13
Exception Types More on Exception Handling The finally Clause When to Use Exceptions Rethrowing Exceptions Chained Exceptions Defining Custom Exception Classes The File Class File Input and Output Reading Data from the Web Case Study: Web Crawler
Chapter 13 Abstract Classes and Interfaces 13.1 13.2 13.3 13.4 13.5 13.6 13.7 13.8 13.9 13.10
Introduction Abstract Classes Case Study: the Abstract Number Class Case Study: Calendar and GregorianCalendar Interfaces The Comparable Interface The Cloneable Interface Interfaces vs. Abstract Classes Case Study: The Rational Class Class Design Guidelines
Chapter 14 JavaFX Basics 14.1 14.2 14.3 14.4 14.5 14.6 14.7 14.8 14.9 14.10 14.11 14.12
Introduction JavaFX vs Swing and AWT The Basic Structure of a JavaFX Program Panes, UI Controls, and Shapes Property Binding Common Properties and Methods for Nodes The Color Class The Font Class The Image and ImageView Classes Layout Panes Shapes Case Study: The ClockPane Class
Chapter 15 Event-Driven Programming and Animations 15.1 15.2 15.3 15.4 15.5 15.6 15.7 15.8 15.9 15.10 15.11 15.12
Introduction Events and Event Sources Registering Handlers and Handling Events Inner Classes Anonymous Inner Class Handlers Simplifying Event Handling Using Lambda Expressions Case Study: Loan Calculator Mouse Events Key Events Listeners for Observable Objects Animation Case Study: Bouncing Ball
Chapter 16 JavaFX UI Controls and Multimedia 16.1 16.2
Introduction Labeled and Label
455 458 466 467 468 469 470 473 476 482 484
495 496 496 501 503 506 509 513 517 520 525
535 536 536 536 539 542 545 546 547 549 552 560 572
585 586 588 589 593 594 597 600 602 603 606 608 616
629 630 630
xvi Contents 16.3 16.4 16.5 16.6 16.7 16.8 16.9 16.10 16.11 16.12 16.13 16.14
Button CheckBox RadioButton TextField TextArea ComboBox ListView ScrollBar Slider
Case Study: Developing a Tic-Tac-Toe Game Video and Audio Case Study: National Flags and Anthems
Chapter 17 Binary I/O 17.1 17.2 17.3 17.4 17.5 17.6 17.7
Introduction How Is Text I/O Handled in Java? Text I/O vs. Binary I/O Binary I/O Classes Case Study: Copying Files Object I/O Random-Access Files
Chapter 18 Recursion 18.1 18.2 18.3 18.4 18.5 18.6 18.7 18.8 18.9 18.10
Introduction Case Study: Computing Factorials Case Study: Computing Fibonacci Numbers Problem Solving Using Recursion Recursive Helper Methods Case Study: Finding the Directory Size Case Study: Tower of Hanoi Case Study: Fractals Recursion vs. Iteration Tail Recursion
Chapter 19 Generics 19.1 19.2 19.3 19.4 19.5 19.6 19.7 19.8 19.9
Introduction Motivations and Benefits Defining Generic Classes and Interfaces Generic Methods Case Study: Sorting an Array of Objects Raw Types and Backward Compatibility Wildcard Generic Types Erasure and Restrictions on Generics Case Study: Generic Matrix Class
Chapter 20 Lists, Stacks, Queues, and Priority Queues 20.1 20.2 20.3 20.4 20.5 20.6 20.7 20.8
Introduction Collections Iterators Lists The Comparator Interface Static Methods for Lists and Collections Case Study: Bouncing Balls Vector and Stack Classes
632 634 637 639 641 644 647 651 654 657 662 665
677 678 678 679 680 691 692 697
705 706 706 709 712 714 717 719 722 726 727
737 738 738 740 742 744 746 747 750 752
761 762 762 766 767 772 773 777 781
Contents xvii 20.9 20.10
Queues and Priority Queues Case Study: Evaluating Expressions
Chapter 21 Sets and Maps
783 786
797
Introduction Sets Comparing the Performance of Sets and Lists Case Study: Counting Keywords Maps Case Study: Occurrences of Words Singleton and Unmodifiable Collections and Maps
798 798 806 809 810 815 816
Chapter 22 Developing Efficient Algorithms
821
21.1 21.2 21.3 21.4 21.5 21.6 21.7
22.1 22.2 22.3 22.4 22.5 22.6 22.7 22.8 22.9 22.10
Introduction Measuring Algorithm Efficiency Using Big O Notation Examples: Determining Big O Analyzing Algorithm Time Complexity Finding Fibonacci Numbers Using Dynamic Programming Finding Greatest Common Divisors Using Euclid’s Algorithm Efficient Algorithms for Finding Prime Numbers Finding the Closest Pair of Points Using Divide-and-Conquer Solving the Eight Queens Problem Using Backtracking Computational Geometry: Finding a Convex Hull
Chapter 23 Sorting 23.1 23.2 23.3 23.4 23.5 23.6 23.7 23.8
Introduction Insertion Sort Bubble Sort Merge Sort Quick Sort Heap Sort Bucket Sort and Radix Sort External Sort
Chapter 24 Implementing Lists, Stacks, Queues, and Priority Queues 24.1 24.2 24.3 24.4 24.5 24.6
Introduction Common Features for Lists Array Lists Linked Lists Stacks and Queues Priority Queues
Chapter 25 Binary Search Trees 25.1 25.2 25.3 25.4 25.5 25.6
Introduction Binary Search Trees Deleting Elements from a BST Tree Visualization and MVC Iterators Case Study: Data Compression
Chapter 26 AVL Trees 26.1 26.2 26.3
Introduction Rebalancing Trees Designing Classes for AVL Trees
822 822 824 828 831 833 837 843 846 849
861 862 862 864 867 870 874 881 883
895 896 896 900 906 920 924
929 930 930 943 949 952 954
965 966 966 969
xviii Contents 26.4 26.5 26.6 26.7 26.8 26.9
Overriding the insert Method Implementing Rotations Implementing the delete Method The AVLTree Class Testing the AVLTree Class AVL Tree Time Complexity Analysis
Chapter 27 Hashing 27.1 27.2 27.3 27.4 27.5 27.6 27.7 27.8
Introduction What Is Hashing? Hash Functions and Hash Codes Handling Collisions Using Open Addressing Handling Collisions Using Separate Chaining Load Factor and Rehashing Implementing a Map Using Hashing Implementing Set Using Hashing
Chapter 28 Graphs and Applications 28.1 28.2 28.3 28.4 28.5 28.6 28.7 28.8 28.9 28.10
Introduction Basic Graph Terminologies Representing Graphs Modeling Graphs Graph Visualization Graph Traversals Depth-First Search (DFS) Case Study: The Connected Circles Problem Breadth-First Search (BFS) Case Study: The Nine Tails Problem
Chapter 29 Weighted Graphs and Applications 29.1 29.2 29.3 29.4 29.5 29.6
Introduction Representing Weighted Graphs The WeightedGraph Class Minimum Spanning Trees Finding Shortest Paths Case Study: The Weighted Nine Tails Problem
Chapter 30 Multithreading and Parallel Programming 30.1 30.2 30.3 30.4 30.5 30.6 30.7 30.8 30.9 30.10 30.11 30.12 30.13 30.14 30.15 30.16
Introduction Thread Concepts Creating Tasks and Threads The Thread Class Case Study: Flashing Text Thread Pools Thread Synchronization Synchronization Using Locks Cooperation among Threads Case Study: Producer/Consumer Blocking Queues Semaphores Avoiding Deadlocks Thread States Synchronized Collections Parallel Programming
970 971 972 972 978 981
985 986 986 987 989 993 993 995 1004
1015 1016 1017 1019 1024 1034 1037 1038 1042 1045 1048
1061 1062 1063 1065 1072 1078 1086
1097 1098 1098 1098 1102 1105 1106 1108 1112 1114 1119 1122 1124 1126 1126 1127 1128
Contents xix
Chapter 31 Networking
1139
Introduction Client/Server Computing The InetAddress Class Serving Multiple Clients Sending and Receiving Objects Case Study: Distributed Tic-Tac-Toe Games
1140 1140 1147 1148 1151 1156
Chapter 32 Java Database Programming
1173
31.1 31.2 31.3 31.4 31.5 31.6
32.1 32.2 32.3 32.4 32.5 32.6 32.7
Introduction Relational Database Systems SQL JDBC PreparedStatement CallableStatement
Retrieving Metadata
Chapter 33 JavaServer Faces 33.1 33.2 33.3 33.4 33.5 33.6 33.7 33.8 33.9
Introduction Getting Started with JSF JSF GUI Components Processing the Form Case Study: Calculator Session Tracking Validating Input Binding Database with Facelets Opening New JSF Pages
1174 1174 1178 1189 1197 1199 1202
1213 1214 1214 1222 1226 1230 1233 1235 1239 1245
Bonus Chapters 34–42 are available from the Companion Website at www.pearsonhighered.com/liang:
Chapter 34 Advanced JavaFX
34-1
Chapter 35 Advanced Database Programming
35-1
Chapter 36 Internationalization
36-1
Chapter 37 Servlets
37-1
Chapter 38 JavaServer Pages
38-1
Chapter 39 Web Services
39-1
Chapter 40 2-4 Trees and B-Trees
40-1
Chapter 41 Red-Black Trees
41-1
Chapter 42 Testing Using JUnit
42-1
xx Contents
APPENDIXES Appendix A Appendix B Appendix C Appendix D Appendix E Appendix F Appendix G Appendix H Appendix I
INDEX
Java Keywords
1263
The ASCII Character Set
1266
Operator Precedence Chart
1268
Java Modifiers
1270
Special Floating-Point Values
1272
Number Systems
1273
Bitwise Operations
1277
Regular Expressions
1278
Enumerated Types
1283 1289
VideoNotes VideoNote
Locations of VideoNotes http://www.pearsonhighered.com/liang
Chapter 1
Chapter 2
Chapter 3
Chapter 4
Chapter 5
Chapter 6
Chapter 7
Introduction to Computers, Programs, and Java
1
Your first Java program Compile and run a Java program NetBeans brief tutorial Eclipse brief tutorial
12 17 23 25
Elementary Programming
33
Obtain input Use operators / and % Software development process Compute loan payments Compute BMI
37 52 59 60 72
Selections
75
Program addition quiz Program subtraction quiz Use multi-way if-else statements Sort three integers Check point location
77 87 90 110 112
Mathematical Functions, Characters, and Strings
119
Introduce math functions Introduce strings and objects Convert hex to decimal Compute great circle distance Convert hex to binary
120 130 143 151 153
Loops
157
Guess a number Multiple subtraction quiz Minimize numeric errors Display loan schedule Sum a series
161 164 178 194 195
Methods
203
Define/invoke max method Use void method Modularize code Stepwise refinement Reverse an integer Estimate p
206 209 215 225 234 237
Single-Dimensional Arrays
245
Random shuffling Deck of cards Selection sort
250 254 269
Chapter 8
Chapter 9
Command-line arguments Coupon collector’s problem Consecutive four
272 281 283
Multidimensional Arrays
287
Find the row with the largest sum Grade multiple-choice test Sudoku Multiply two matrices Even number of 1s
292 294 298 307 314
Objects and Classes
321
Define classes and objects Use classes Static vs. instance Data field encapsulation The Fan class
322 334 337 344 362
Chapter 10 Object-Oriented Thinking The Loan class The BMI class The StackOfIntegers class Process large numbers The String class The MyPoint class
Chapter 11 Inheritance and Polymorphism Geometric class hierarchy Polymorphism and dynamic binding demo The ArrayList class The MyStack class New Account class
Chapter 12 Exception Handling and Text I/O Exception-handling advantages Create custom exception classes Write and read data HexFormatException
Chapter 13 Abstract Classes and Interfaces Abstract GeometricObject class Calendar and GregorianCalendar classes The concept of interface Redesign the Rectangle class
Chapter 14 JavaFX Basics Understand property binding Use Image and ImageView Use layout panes
365 367 370 378 384 386 400
409 410 424 432 439 446
449 450 470 476 489
495 496 503 506 530
535 542 549 552
xxi
xxii VideoNotes Use shapes Display a tictactoe board Display a bar chart
Chapter 15 Event-Driven Programming and Animations Handler and its registration Anonymous handler Move message using the mouse Animate a rising flag Flashing text Simple calculator Check mouse point location Display a running fan
Chapter 16 JavaFX UI Controls and Multimedia Use ListView Use Slider
560 578 580
TicTacToe Use Media, MediaPlayer, and MediaView Audio and image Use radio buttons and text fields Set fonts
657 662 666 669 671
585 592 595 602 608 614 621 622 625
629 647 654
Chapter 17 Binary I/O Copy file Object I/O Split a large file
Chapter 18 Recursion Binary search Directory size Fractal (Sierpinski triangle) Search a string in a directory Recursive tree
677 691 693 702
705 716 717 722 733 736
CHAPTER
1 INTRODUCTION TO COMPUTERS, PROGRAMS, AND JAVA Objectives ■
To understand computer basics, programs, and operating systems (§§1.2–1.4).
■
To describe the relationship between Java and the World Wide Web (§1.5).
■
To understand the meaning of Java language specification, API, JDK, and IDE (§1.6).
■
To write a simple Java program (§1.7).
■
To display output on the console (§1.7).
■
To explain the basic syntax of a Java program (§1.7).
■
To create, compile, and run Java programs (§1.8).
■
To use sound Java programming style and document programs properly (§1.9).
■
To explain the differences between syntax errors, runtime errors, and logic errors (§1.10).
■
To develop Java programs using NetBeans (§1.11).
■
To develop Java programs using Eclipse (§1.12).
2 Chapter 1
Introduction to Computers, Programs, and Java
1.1 Introduction Key Point what is programming? programming program
The central theme of this book is to learn how to solve problems by writing a program. This book is about programming. So, what is programming? The term programming means to create (or develop) software, which is also called a program. In basic terms, software contains the instructions that tell a computer—or a computerized device—what to do. Software is all around you, even in devices that you might not think would need it. Of course, you expect to find and use software on a personal computer, but software also plays a role in running airplanes, cars, cell phones, and even toasters. On a personal computer, you use word processors to write documents, Web browsers to explore the Internet, and e-mail programs to send and receive messages. These programs are all examples of software. Software developers create software with the help of powerful tools called programming languages. This book teaches you how to create programs by using the Java programming language. There are many programming languages, some of which are decades old. Each language was invented for a specific purpose—to build on the strengths of a previous language, for example, or to give the programmer a new and unique set of tools. Knowing that there are so many programming languages available, it would be natural for you to wonder which one is best. But, in truth, there is no “best” language. Each one has its own strengths and weaknesses. Experienced programmers know that one language might work well in some situations, whereas a different language may be more appropriate in other situations. For this reason, seasoned programmers try to master as many different programming languages as they can, giving them access to a vast arsenal of software-development tools. If you learn to program using one language, you should find it easy to pick up other languages. The key is to learn how to solve problems using a programming approach. That is the main theme of this book. You are about to begin an exciting journey: learning how to program. At the outset, it is helpful to review computer basics, programs, and operating systems. If you are already familiar with such terms as CPU, memory, disks, operating systems, and programming languages, you may skip Sections 1.2–1.4.
1.2 What Is a Computer? Key Point hardware software
bus
A computer is an electronic device that stores and processes data. A computer includes both hardware and software. In general, hardware comprises the visible, physical elements of the computer, and software provides the invisible instructions that control the hardware and make it perform specific tasks. Knowing computer hardware isn’t essential to learning a programming language, but it can help you better understand the effects that a program’s instructions have on the computer and its components. This section introduces computer hardware components and their functions. A computer consists of the following major hardware components (Figure 1.1): ■
A central processing unit (CPU)
■
Memory (main memory)
■
Storage devices (such as disks and CDs)
■
Input devices (such as the mouse and keyboard)
■
Output devices (such as monitors and printers)
■
Communication devices (such as modems and network interface cards)
A computer’s components are interconnected by a subsystem called a bus. You can think of a bus as a sort of system of roads running among the computer’s components; data and power travel along the bus from one part of the computer to another. In personal computers,
1.2 What Is a Computer? 3 Bus
Storage Devices
Memory
CPU
e.g., Disk, CD, and Tape
Communication Devices
Input Devices
Output Devices
e.g., Modem and NIC
e.g., Keyboard, Mouse
e.g., Monitor, Printer
FIGURE 1.1 A computer consists of a CPU, memory, storage devices, input devices, output devices, and communication devices.
the bus is built into the computer’s motherboard, which is a circuit case that connects all of the parts of a computer together.
1.2.1
Central Processing Unit
The central processing unit (CPU) is the computer’s brain. It retrieves instructions from memory and executes them. The CPU usually has two components: a control unit and an arithmetic/logic unit. The control unit controls and coordinates the actions of the other components. The arithmetic/logic unit performs numeric operations (addition, subtraction, multiplication, division) and logical operations (comparisons). Today’s CPUs are built on small silicon semiconductor chips that contain millions of tiny electric switches, called transistors, for processing information. Every computer has an internal clock, which emits electronic pulses at a constant rate. These pulses are used to control and synchronize the pace of operations. A higher clock speed enables more instructions to be executed in a given period of time. The unit of measurement of clock speed is the hertz (Hz), with 1 hertz equaling 1 pulse per second. In the 1990s, computers measured clocked speed in megahertz (MHz), but CPU speed has been improving continuously; the clock speed of a computer is now usually stated in gigahertz (GHz). Intel’s newest processors run at about 3 GHz. CPUs were originally developed with only one core. The core is the part of the processor that performs the reading and executing of instructions. In order to increase CPU processing power, chip manufacturers are now producing CPUs that contain multiple cores. A multicore CPU is a single component with two or more independent cores. Today’s consumer computers typically have two, three, and even four separate cores. Soon, CPUs with dozens or even hundreds of cores will be affordable.
1.2.2
motherboard
CPU
speed hertz megahertz gigahertz core
Bits and Bytes
Before we discuss memory, let’s look at how information (data and programs) are stored in a computer. A computer is really nothing more than a series of switches. Each switch exists in two states: on or off. Storing information in a computer is simply a matter of setting a sequence of switches on or off. If the switch is on, its value is 1. If the switch is off, its value is 0. These 0s and 1s are interpreted as digits in the binary number system and are called bits (binary digits). The minimum storage unit in a computer is a byte. A byte is composed of eight bits. A small number such as 3 can be stored as a single byte. To store a number that cannot fit into a single byte, the computer uses several bytes. Data of various kinds, such as numbers and characters, are encoded as a series of bytes. As a programmer, you don’t need to worry about the encoding and decoding of data, which the computer system performs automatically, based on the encoding scheme. An encoding scheme is a set of rules that govern how a computer translates characters, numbers, and symbols into data the computer can actually work with. Most schemes translate each character
bits byte
encoding scheme
4 Chapter 1
Introduction to Computers, Programs, and Java into a predetermined string of bits. In the popular ASCII encoding scheme, for example, the character C is represented as 01000011 in one byte. A computer’s storage capacity is measured in bytes and multiples of the byte, as follows:
kilobyte (KB)
■
A kilobyte (KB) is about 1,000 bytes.
megabyte (MB)
■
A megabyte (MB) is about 1 million bytes.
gigabyte (GB)
■
A gigabyte (GB) is about 1 billion bytes.
terabyte (TB)
■
A terabyte (TB) is about 1 trillion bytes.
A typical one-page word document might take 20 KB. Therefore, 1 MB can store 50 pages of documents and 1 GB can store 50,000 pages of documents. A typical two-hour highresolution movie might take 8 GB, so it would require 160 GB to store 20 movies.
1.2.3 memory
unique address RAM
Memory
A computer’s memory consists of an ordered sequence of bytes for storing programs as well as data that the program is working with. You can think of memory as the computer’s work area for executing a program. A program and its data must be moved into the computer’s memory before they can be executed by the CPU. Every byte in the memory has a unique address, as shown in Figure 1.2. The address is used to locate the byte for storing and retrieving the data. Since the bytes in the memory can be accessed in any order, the memory is also referred to as random-access memory (RAM). Memory address
2000 2001 2002 2003 2004
FIGURE 1.2 locations.
Memory content
01000011 01110010 01100101 01110111 00000011
Encoding for character ‘C’ Encoding for character ‘r’ Encoding for character ‘e’ Encoding for character ‘w’ Encoding for number 3
Memory stores data and program instructions in uniquely addressed memory
Today’s personal computers usually have at least 4 gigabyte of RAM, but they more commonly have 6 to 8 GB installed. Generally speaking, the more RAM a computer has, the faster it can operate, but there are limits to this simple rule of thumb. A memory byte is never empty, but its initial content may be meaningless to your program. The current content of a memory byte is lost whenever new information is placed in it. Like the CPU, memory is built on silicon semiconductor chips that have millions of transistors embedded on their surface. Compared to CPU chips, memory chips are less complicated, slower, and less expensive.
1.2.4 storage devices
Storage Devices
A computer’s memory (RAM) is a volatile form of data storage: any information that has been stored in memory (i.e., saved) is lost when the system’s power is turned off. Programs and data are permanently stored on storage devices and are moved, when the computer
1.2 What Is a Computer? 5 actually uses them, to memory, which operates at much faster speeds than permanent storage devices can. There are three main types of storage devices: ■
Magnetic disk drives
■
Optical disc drives (CD and DVD)
■
USB flash drives
Drives are devices for operating a medium, such as disks and CDs. A storage medium physically stores data and program instructions. The drive reads data from the medium and writes data onto the medium.
drive
Disks A computer usually has at least one hard disk drive. Hard disks are used for permanently storing data and programs. Newer computers have hard disks that can store from 500 gigabytes to 1 terabytes of data. Hard disk drives are usually encased inside the computer, but removable hard disks are also available.
hard disk
CDs and DVDs CD stands for compact disc. There are two types of CD drives: CD-R and CD-RW. A CD-R is for read-only permanent storage; the user cannot modify its contents once they are recorded. A CD-RW can be used like a hard disk; that is, you can write data onto the disc, and then overwrite that data with new data. A single CD can hold up to 700 MB. Most new PCs are equipped with a CD-RW drive that can work with both CD-R and CD-RW discs. DVD stands for digital versatile disc or digital video disc. DVDs and CDs look alike, and you can use either to store data. A DVD can hold more information than a CD; a standard DVD’s storage capacity is 4.7 GB. Like CDs, there are two types of DVDs: DVD-R (readonly) and DVD-RW (rewritable).
CD-R CD-RW
DVD
USB Flash Drives Universal serial bus (USB) connectors allow the user to attach many kinds of peripheral devices to the computer. You can use a USB to connect a printer, digital camera, mouse, external hard disk drive, and other devices to the computer. A USB flash drive is a device for storing and transporting data. A flash drive is small— about the size of a pack of gum. It acts like a portable hard drive that can be plugged into your computer’s USB port. USB flash drives are currently available with up to 256 GB storage capacity.
1.2.5
Input and Output Devices
Input and output devices let the user communicate with the computer. The most common input devices are keyboards and mice. The most common output devices are monitors and printers.
The Keyboard A keyboard is a device for entering input. Compact keyboards are available without a numeric keypad. Function keys are located across the top of the keyboard and are prefaced with the letter F. Their functions depend on the software currently being used. A modifier key is a special key (such as the Shift, Alt, and Ctrl keys) that modifies the normal action of another key when the two are pressed simultaneously. The numeric keypad, located on the right side of most keyboards, is a separate set of keys styled like a calculator to use for entering numbers quickly. Arrow keys, located between the main keypad and the numeric keypad, are used to move the mouse pointer up, down, left, and right on the screen in many kinds of programs.
function key modifier key numeric keypad arrow keys
6 Chapter 1
Introduction to Computers, Programs, and Java The Insert, Delete, Page Up, and Page Down keys are used in word processing and other programs for inserting text and objects, deleting text and objects, and moving up or down through a document one screen at a time.
Insert key Delete key Page Up key Page Down key
The Mouse A mouse is a pointing device. It is used to move a graphical pointer (usually in the shape of an arrow) called a cursor around the screen or to click on-screen objects (such as a button) to trigger them to perform an action.
The Monitor The monitor displays information (text and graphics). The screen resolution and dot pitch determine the quality of the display. The screen resolution specifies the number of pixels in horizontal and vertical dimensions of the display device. Pixels (short for “picture elements”) are tiny dots that form an image on the screen. A common resolution for a 17-inch screen, for example, is 1,024 pixels wide and 768 pixels high. The resolution can be set manually. The higher the resolution, the sharper and clearer the image is. The dot pitch is the amount of space between pixels, measured in millimeters. The smaller the dot pitch, the sharper the display.
screen resolution pixels
dot pitch
1.2.6
Communication Devices
Computers can be networked through communication devices, such as a dial-up modem (modulator/demodulator), a DSL or cable modem, a wired network interface card, or a wireless adapter. dial-up modem
■
A dial-up modem uses a phone line and can transfer data at a speed up to 56,000 bps (bits per second).
digital subscriber line (DSL)
■
A digital subscriber line (DSL) connection also uses a standard phone line, but it can transfer data 20 times faster than a standard dial-up modem.
cable modem
■
A cable modem uses the cable TV line maintained by the cable company and is generally faster than DSL.
network interface card (NIC) local area network (LAN) million bits per second (mbps)
■
A network interface card (NIC) is a device that connects a computer to a local area network (LAN). LANs are commonly used in universities, businesses, and government agencies. A high-speed NIC called 1000BaseT can transfer data at 1,000 million bits per second (mbps).
■
Wireless networking is now extremely popular in homes, businesses, and schools. Every laptop computer sold today is equipped with a wireless adapter that enables the computer to connect to a local area network and the Internet.
Note Answers to checkpoint questions are on the Companion Website.
✓
Check Point
1.1 1.2 1.3 1.4 1.5 1.6 1.7
What are hardware and software? List five major hardware components of a computer. What does the acronym “CPU” stand for? What unit is used to measure CPU speed? What is a bit? What is a byte? What is memory for? What does RAM stand for? Why is memory called RAM? What unit is used to measure memory size?
1.3 Programming Languages 7 1.8 1.9
What unit is used to measure disk size? What is the primary difference between memory and a storage device?
1.3 Programming Languages Computer programs, known as software, are instructions that tell a computer what to do. Computers do not understand human languages, so programs must be written in a language a computer can use. There are hundreds of programming languages, and they were developed to make the programming process easier for people. However, all programs must be converted into the instructions the computer can execute.
1.3.1
Key Point
Machine Language
A computer’s native language, which differs among different types of computers, is its machine language—a set of built-in primitive instructions. These instructions are in the form of binary code, so if you want to give a computer an instruction in its native language, you have to enter the instruction as binary code. For example, to add two numbers, you might have to write an instruction in binary code, like this:
machine language
1101101010011010
1.3.2
Assembly Language
Programming in machine language is a tedious process. Moreover, programs written in machine language are very difficult to read and modify. For this reason, assembly language was created in the early days of computing as an alternative to machine languages. Assembly language uses a short descriptive word, known as a mnemonic, to represent each of the machine-language instructions. For example, the mnemonic add typically means to add numbers and sub means to subtract numbers. To add the numbers 2 and 3 and get the result, you might write an instruction in assembly code like this:
assembly language
add 2, 3, result
Assembly languages were developed to make programming easier. However, because the computer cannot execute assembly language, another program—called an assembler—is used to translate assembly-language programs into machine code, as shown in Figure 1.3.
Assembly Source File ... add 2, 3, result ...
assembler
Machine-Code File
Assembler
... 1101101010011010 ...
FIGURE 1.3 An assembler translates assembly-language instructions into machine code. Writing code in assembly language is easier than in machine language. However, it is still tedious to write code in assembly language. An instruction in assembly language essentially corresponds to an instruction in machine code. Writing in assembly requires that you know how the CPU works. Assembly language is referred to as a low-level language, because assembly language is close in nature to machine language and is machine dependent.
low-level language
8 Chapter 1
Introduction to Computers, Programs, and Java
1.3.3
High-Level Language
In the 1950s, a new generation of programming languages known as high-level languages emerged. They are platform independent, which means that you can write a program in a highlevel language and run it in different types of machines. High-level languages are English-like and easy to learn and use. The instructions in a high-level programming language are called statements. Here, for example, is a high-level language statement that computes the area of a circle with a radius of 5:
high-level language
statement
area = 5 * 5 * 3.14159;
There are many high-level programming languages, and each was designed for a specific purpose. Table 1.1 lists some popular ones.
TABLE 1.1 Popular High-Level Programming Languages Language
Description
Ada
Named for Ada Lovelace, who worked on mechanical general-purpose computers. The Ada language was developed for the Department of Defense and is used mainly in defense projects.
BASIC
Beginner’s All-purpose Symbolic Instruction Code. It was designed to be learned and used easily by beginners.
C
Developed at Bell Laboratories. C combines the power of an assembly language with the ease of use and portability of a high-level language.
C++
C++ is an object-oriented language, based on C.
C#
Pronounced “C Sharp.” It is a hybrid of Java and C++ and was developed by Microsoft.
COBOL
COmmon Business Oriented Language. Used for business applications.
FORTRAN
FORmula TRANslation. Popular for scientific and mathematical applications.
Java
Developed by Sun Microsystems, now part of Oracle. It is widely used for developing platform-independent Internet applications.
Pascal
Named for Blaise Pascal, who pioneered calculating machines in the seventeenth century. It is a simple, structured, general-purpose language primarily for teaching programming.
Python
A simple general-purpose scripting language good for writing short programs.
Visual Basic
Visual Basic was developed by Microsoft and it enables the programmers to rapidly develop graphical user interfaces.
A program written in a high-level language is called a source program or source code. Because a computer cannot execute a source program, a source program must be translated into machine code for execution. The translation can be done using another programming tool called an interpreter or a compiler.
source program source code interpreter compiler
✓
Check Point
1.10 1.11 1.12 1.13 1.14
■
An interpreter reads one statement from the source code, translates it to the machine code or virtual machine code, and then executes it right away, as shown in Figure 1.4a. Note that a statement from the source code may be translated into several machine instructions.
■
A compiler translates the entire source code into a machine-code file, and the machine-code file is then executed, as shown in Figure 1.4b. What language does the CPU understand? What is an assembly language? What is an assembler? What is a high-level programming language? What is a source program?
1.4 Operating Systems 9 1.15 1.16 1.17
What is an interpreter? What is a compiler? What is the difference between an interpreted language and a compiled language?
High-Level Source File Output
... area = 5 * 5 * 3.1415; ...
Interpreter
(a) High-Level Source File ... area = 5 * 5 * 3.1415; ...
Machine-Code File
Compiler
... 0101100011011100 1111100011000100 ...
Output Executor
(b)
FIGURE 1.4 (a) An interpreter translates and executes a program one statement at a time. (b) A compiler translates the entire source program into a machine-language file for execution.
1.4 Operating Systems The operating system (OS) is the most important program that runs on a computer. The OS manages and controls a computer’s activities. The popular operating systems for general-purpose computers are Microsoft Windows, Mac OS, and Linux. Application programs, such as a Web browser or a word processor, cannot run unless an operating system is installed and running on the computer. Figure 1.5 shows the interrelationship of hardware, operating system, application software, and the user. User
Application Programs
Operating System
Hardware
FIGURE 1.5 Users and applications access the computer’s hardware via the operating system. The major tasks of an operating system are as follows: ■
Controlling and monitoring system activities
■
Allocating and assigning system resources
■
Scheduling operations
Key Point operating system (OS)
10 Chapter 1
Introduction to Computers, Programs, and Java
1.4.1
Controlling and Monitoring System Activities
Operating systems perform basic tasks, such as recognizing input from the keyboard, sending output to the monitor, keeping track of files and folders on storage devices, and controlling peripheral devices, such as disk drives and printers. An operating system must also ensure that different programs and users working at the same time do not interfere with each other. In addition, the OS is responsible for security, ensuring that unauthorized users and programs are not allowed to access the system.
1.4.2
Allocating and Assigning System Resources
The operating system is responsible for determining what computer resources a program needs (such as CPU time, memory space, disks, input and output devices) and for allocating and assigning them to run the program.
1.4.3
Scheduling Operations
The OS is responsible for scheduling programs’ activities to make efficient use of system resources. Many of today’s operating systems support techniques such as multiprogramming, multithreading, and multiprocessing to increase system performance. Multiprogramming allows multiple programs to run simultaneously by sharing the same CPU. The CPU is much faster than the computer’s other components. As a result, it is idle most of the time—for example, while waiting for data to be transferred from a disk or waiting for other system resources to respond. A multiprogramming OS takes advantage of this situation by allowing multiple programs to use the CPU when it would otherwise be idle. For example, multiprogramming enables you to use a word processor to edit a file at the same time as your Web browser is downloading a file. Multithreading allows a single program to execute multiple tasks at the same time. For instance, a word-processing program allows users to simultaneously edit text and save it to a disk. In this example, editing and saving are two tasks within the same application. These two tasks may run concurrently. Multiprocessing, or parallel processing, uses two or more processors together to perform subtasks concurrently and then combine solutions of the subtasks to obtain a solution for the entire task. It is like a surgical operation where several doctors work together on one patient.
multiprogramming
multithreading
multiprocessing
✓
Check Point
1.18 1.19 1.20
What is an operating system? List some popular operating systems. What are the major responsibilities of an operating system? What are multiprogramming, multithreading, and multiprocessing?
1.5 Java, the World Wide Web, and Beyond Key Point
Java is a powerful and versatile programming language for developing software running on mobile devices, desktop computers, and servers. This book introduces Java programming. Java was developed by a team led by James Gosling at Sun Microsystems. Sun Microsystems was purchased by Oracle in 2010. Originally called Oak, Java was designed in 1991 for use in embedded chips in consumer electronic appliances. In 1995, renamed Java, it was redesigned for developing Web applications. For the history of Java, see www.java.com/en/javahistory/index.jsp. Java has become enormously popular. Its rapid rise and wide acceptance can be traced to its design characteristics, particularly its promise that you can write a program once and run it anywhere. As stated by its designer, Java is simple, object oriented, distributed,
1.6 The Java Language Specification, API, JDK, and IDE 11 interpreted, robust, secure, architecture neutral, portable, high performance, multithreaded, and dynamic. For the anatomy of Java characteristics, see www.cs.armstrong.edu/ liang/JavaCharacteristics.pdf. Java is a full-featured, general-purpose programming language that can be used to develop robust mission-critical applications. Today, it is employed not only for Web programming but also for developing standalone applications across platforms on servers, desktop computers, and mobile devices. It was used to develop the code to communicate with and control the robotic rover on Mars. Many companies that once considered Java to be more hype than substance are now using it to create distributed applications accessed by customers and partners across the Internet. For every new project being developed today, companies are asking how they can use Java to make their work easier. The World Wide Web is an electronic information repository that can be accessed on the Internet from anywhere in the world. The Internet, the Web’s infrastructure, has been around for more than forty years. The colorful World Wide Web and sophisticated Web browsers are the major reason for the Internet’s popularity. Java initially became attractive because Java programs can be run from a Web browser. Such programs are called applets. Applets employ a modern graphical interface with buttons, text fields, text areas, radio buttons, and so on, to interact with users on the Web and process their requests. Applets make the Web responsive, interactive, and fun to use. Applets are embedded in an HTML file. HTML (Hypertext Markup Language) is a simple scripting language for laying out documents, linking documents on the Internet, and bringing images, sound, and video alive on the Web. Today, you can use Java to develop rich Internet applications. A rich Internet application (RIA) is a Web application designed to deliver the same features and functions normally associated with deskop applications. Java is now very popular for developing applications on Web servers. These applications process data, perform computations, and generate dynamic Web pages. Many commercial Websites are developed using Java on the backend. Java is a versatile programming language: you can use it to develop applications for desktop computers, servers, and small handheld devices. The software for Android cell phones is developed using Java.
1.21 1.22 1.23
Who invented Java? Which company owns Java now? What is a Java applet? What programming language does Android use?
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1.6 The Java Language Specification, API, JDK, and IDE Java syntax is defined in the Java language specification, and the Java library is defined in the Java API. The JDK is the software for developing and running Java programs. An IDE is an integrated development environment for rapidly developing programs. Computer languages have strict rules of usage. If you do not follow the rules when writing a program, the computer will not be able to understand it. The Java language specification and the Java API define the Java standards. The Java language specification is a technical definition of the Java programming language’s syntax and semantics. You can find the complete Java language specification at http://docs.oracle.com/javase/specs/. The application program interface (API), also known as library, contains predefined classes and interfaces for developing Java programs. The API is still expanding. You can view and download the latest version of the Java API at http://download.java.net/jdk8/docs/api/.
Key Point
Java language specification
API library
12 Chapter 1
Introduction to Computers, Programs, and Java Java is a full-fledged and powerful language that can be used in many ways. It comes in three editions:
Java SE, EE, and ME
■
Java Standard Edition (Java SE) to develop client-side applications. The applications can run standalone or as applets running from a Web browser.
■
Java Enterprise Edition (Java EE) to develop server-side applications, such as Java servlets, JavaServer Pages (JSP), and JavaServer Faces (JSF).
■
Java Micro Edition (Java ME) to develop applications for mobile devices, such as cell phones.
This book uses Java SE to introduce Java programming. Java SE is the foundation upon which all other Java technology is based. There are many versions of Java SE. The latest, Java SE 8, is used in this book. Oracle releases each version with a Java Development Toolkit (JDK). For Java SE 8, the Java Development Toolkit is called JDK 1.8 (also known as Java 8 or JDK 8). The JDK consists of a set of separate programs, each invoked from a command line, for developing and testing Java programs. Instead of using the JDK, you can use a Java development tool (e.g., NetBeans, Eclipse, and TextPad)—software that provides an integrated development environment (IDE) for developing Java programs quickly. Editing, compiling, building, debugging, and online help are integrated in one graphical user interface. You simply enter source code in one window or open an existing file in a window, and then click a button or menu item or press a function key to compile and run the program.
Java Development Toolkit (JDK) JDK 1.8 = JDK 8
Integrated development environment
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Check Point
1.24 1.25 1.26 1.27
What is the Java language specification? What does JDK stand for? What does IDE stand for? Are tools like NetBeans and Eclipse different languages from Java, or are they dialects or extensions of Java?
1.7 A Simple Java Program Key Point what is a console? console input console output
A Java program is executed from the main method in the class. Let’s begin with a simple Java program that displays the message Welcome to Java! on the console. (The word console is an old computer term that refers to the text entry and display device of a computer. Console input means to receive input from the keyboard, and console output means to display output on the monitor.) The program is shown in Listing 1.1.
LISTING 1.1 Welcome.java class main method
display message
VideoNote
1 2 3 4 5 6
public class Welcome { public static void main(String[] args) { // Display message Welcome to Java! on the console System.out.println("Welcome to Java!"); } }
Your first Java program Welcome to Java!
line numbers
Note that the line numbers are for reference purposes only; they are not part of the program. So, don’t type line numbers in your program.
1.7 A Simple Java Program 13 Line 1 defines a class. Every Java program must have at least one class. Each class has a name. By convention, class names start with an uppercase letter. In this example, the class name is Welcome. Line 2 defines the main method. The program is executed from the main method. A class may contain several methods. The main method is the entry point where the program begins execution. A method is a construct that contains statements. The main method in this program contains the System.out.println statement. This statement displays the string Welcome to Java! on the console (line 4). String is a programming term meaning a sequence of characters. A string must be enclosed in double quotation marks. Every statement in Java ends with a semicolon (;), known as the statement terminator. Reserved words, or keywords, have a specific meaning to the compiler and cannot be used for other purposes in the program. For example, when the compiler sees the word class, it understands that the word after class is the name for the class. Other reserved words in this program are public, static, and void. Line 3 is a comment that documents what the program is and how it is constructed. Comments help programmers to communicate and understand the program. They are not programming statements and thus are ignored by the compiler. In Java, comments are preceded by two slashes (//) on a line, called a line comment, or enclosed between /* and */ on one or several lines, called a block comment or paragraph comment. When the compiler sees //, it ignores all text after // on the same line. When it sees /*, it scans for the next */ and ignores any text between /* and */. Here are examples of comments:
class name main method
string statement terminator reserved word keyword
comment
line comment block comment
// This application program displays Welcome to Java! /* This application program displays Welcome to Java! */ /* This application program displays Welcome to Java! */
A pair of curly braces in a program forms a block that groups the program’s components. In Java, each block begins with an opening brace ({) and ends with a closing brace (}). Every class has a class block that groups the data and methods of the class. Similarly, every method has a method block that groups the statements in the method. Blocks can be nested, meaning that one block can be placed within another, as shown in the following code.
block
public class Welcome { public static void main(String[] args) { Class block System.out.println("Welcome to Java!"); Method block } }
Tip An opening brace must be matched by a closing brace. Whenever you type an opening brace, immediately type a closing brace to prevent the missing-brace error. Most Java IDEs automatically insert the closing brace for each opening brace.
match braces
Caution Java source programs are case sensitive. It would be wrong, for example, to replace main in the program with Main.
You have seen several special characters (e.g., { }, //, ;) in the program. They are used in almost every program. Table 1.2 summarizes their uses. The most common errors you will make as you learn to program will be syntax errors. Like any programming language, Java has its own syntax, and you need to write code that
case sensitive
special characters common errors
14 Chapter 1
Introduction to Computers, Programs, and Java TABLE 1.2 Special Characters
syntax rules
Character
Name
Description
{}
Opening and closing braces
Denote a block to enclose statements.
()
Opening and closing parentheses
Used with methods.
[]
Opening and closing brackets
Denote an array.
//
Double slashes
Precede a comment line.
""
Opening and closing quotation marks
Enclose a string (i.e., sequence of characters).
;
Semicolon
Mark the end of a statement.
conforms to the syntax rules. If your program violates a rule—for example, if the semicolon is missing, a brace is missing, a quotation mark is missing, or a word is misspelled—the Java compiler will report syntax errors. Try to compile the program with these errors and see what the compiler reports.
Note You are probably wondering why the main method is defined this way and why System.out.println(...) is used to display a message on the console. For the time being, simply accept that this is how things are done. Your questions will be fully answered in subsequent chapters.
The program in Listing 1.1 displays one message. Once you understand the program, it is easy to extend it to display more messages. For example, you can rewrite the program to display three messages, as shown in Listing 1.2.
LISTING 1.2 WelcomeWithThreeMessages.java class main method
display message
1 2 3 4 5 6 7
public class WelcomeWithThreeMessages { public static void main(String[] args) { System.out.println("Programming is fun!"); System.out.println("Fundamentals First"); System.out.println("Problem Driven"); } }
Programming is fun! Fundamentals First Problem Driven
Further, you can perform mathematical computations and display the result on the console. 10.5 + 2 * 3 Listing 1.3 gives an example of evaluating . 45 - 3.5
LISTING 1.3 ComputeExpression.java class main method
compute expression
1 2 3 4 5
public class ComputeExpression { public static void main(String[] args) { System.out.println((10.5 + 2 * 3) / (45 – 3.5)); } }
0.39759036144578314
1.8 Creating, Compiling, and Executing a Java Program 15 The multiplication operator in Java is *. As you can see, it is a straightforward process to translate an arithmetic expression to a Java expression. We will discuss Java expressions further in Chapter 2.
1.28 1.29 1.30 1.31 1.32
What is a keyword? List some Java keywords. Is Java case sensitive? What is the case for Java keywords? What is a comment? Is the comment ignored by the compiler? How do you denote a comment line and a comment paragraph? What is the statement to display a string on the console? Show the output of the following code:
✓
Check Point
public class Test { public static void main(String[] args) { System.out.println("3.5 * 4 / 2 – 2.5 is "); System.out.println(3.5 * 4 / 2 – 2.5); } }
1.8 Creating, Compiling, and Executing a Java Program You save a Java program in a .java file and compile it into a .class file. The .class file is executed by the Java Virtual Machine. You have to create your program and compile it before it can be executed. This process is repetitive, as shown in Figure 1.6. If your program has compile errors, you have to modify the program to fix them, and then recompile it. If your program has runtime errors or does not produce the correct result, you have to modify the program, recompile it, and execute it again. You can use any text editor or IDE to create and edit a Java source-code file. This section demonstrates how to create, compile, and run Java programs from a command window. Sections 1.10 and 1.11 will introduce developing Java programs using NetBeans and Eclipse. From the command window, you can use a text editor such as Notepad to create the Java source-code file, as shown in Figure 1.7.
Key Point
command window
Note The source file must end with the extension .java and must have the same exact name as the public class name. For example, the file for the source code in Listing 1.1 should be named Welcome.java, since the public class name is Welcome.
A Java compiler translates a Java source file into a Java bytecode file. The following command compiles Welcome.java:
file name Welcome.java, compile
javac Welcome.java
Note You must first install and configure the JDK before you can compile and run programs. See Supplement I.B, Installing and Configuring JDK 8, for how to install the JDK and set up the environment to compile and run Java programs. If you have trouble compiling and running programs, see Supplement I.C, Compiling and Running Java from the Command Window. This supplement also explains how to use basic DOS commands and how to use Windows Notepad to create and edit files. All the supplements are accessible from the Companion Website at www.cs.armstrong.edu/liang/intro10e/ supplement.html.
If there aren’t any syntax errors, the compiler generates a bytecode file with a .class extension. Thus, the preceding command generates a file named Welcome.class, as shown
Supplement I.B Supplement I.C
.class bytecode file
16 Chapter 1
Introduction to Computers, Programs, and Java Create/Modify Source Code
Source code (developed by the programmer) Saved on the disk public class Welcome { public static void main(String[] args) { System.out.println("Welcome to Java!"); Source Code } }
Bytecode (generated by the compiler for JVM to read and interpret) … Method Welcome() 0 aload_0 …
Compile Source Code e.g., javac Welcome.java If compile errors occur Stored on the disk
Method void main(java.lang.String[]) 0 getstatic #2 … 3 ldc #3
5 invokevirtual #4 … 8 return
Bytecode
Run Bytecode e.g., java Welcome
“Welcome to Java ” is displayed on the console Welcome to Java!
Result If runtime errors or incorrect result
FIGURE 1.6 The Java program-development process consists of repeatedly creating/modifying source code, compiling, and executing programs.
FIGURE 1.7
bytecode Java Virtual Machine (JVM)
interpret bytecode
run
You can create a Java source file using Windows Notepad.
in Figure 1.8a. The Java language is a high-level language, but Java bytecode is a low-level language. The bytecode is similar to machine instructions but is architecture neutral and can run on any platform that has a Java Virtual Machine (JVM), as shown in Figure 1.8b. Rather than a physical machine, the virtual machine is a program that interprets Java bytecode. This is one of Java’s primary advantages: Java bytecode can run on a variety of hardware platforms and operating systems. Java source code is compiled into Java bytecode and Java bytecode is interpreted by the JVM. Your Java code may use the code in the Java library. The JVM executes your code along with the code in the library. To execute a Java program is to run the program’s bytecode. You can execute the bytecode on any platform with a JVM, which is an interpreter. It translates the individual instructions in the bytecode into the target machine language code one at a time rather than the whole program as a single unit. Each step is executed immediately after it is translated. The following command runs the bytecode for Listing 1.1: java Welcome
1.8 Creating, Compiling, and Executing a Java Program 17 Java Bytecode
Java Compiler
Welcome.class (Java bytecode executable file)
Ja
generates
executed by JVM
tual Mac Vir h va
e in
Welcome.java (Java sourcecode file)
compiled by
Any Computer
Library Code
(a)
(b)
FIGURE 1.8 (a) Java source code is translated into bytecode. (b) Java bytecode can be executed on any computer with a Java Virtual Machine. Figure 1.9 shows the javac command for compiling Welcome.java. The compiler generates the Welcome.class file, and this file is executed using the java command.
javac command java command
Note For simplicity and consistency, all source-code and class files used in this book are placed under c:\book unless specified otherwise.
c:\book
Compile Show files VideoNote
Compile and run a Java program Run
FIGURE 1.9
The output of Listing 1.1 displays the message “Welcome to Java!”
Caution Do not use the extension .class in the command line when executing the program. Use java ClassName to run the program. If you use java ClassName.class in the command line, the system will attempt to fetch ClassName.class.class.
java ClassName
Tip If you execute a class file that does not exist, a NoClassDefFoundError will occur. If you execute a class file that does not have a main method or you mistype the main method (e.g., by typing Main instead of main), a NoSuchMethodError will occur.
NoClassDefFoundError NoSuchMethodError
Note When executing a Java program, the JVM first loads the bytecode of the class to memory using a program called the class loader. If your program uses other classes, the class loader dynamically loads them just before they are needed. After a class is loaded, the JVM uses a program called the bytecode verifier to check the validity of the bytecode and
class loader bytecode verifier
18 Chapter 1
Introduction to Computers, Programs, and Java to ensure that the bytecode does not violate Java’s security restrictions. Java enforces strict security to make sure that Java class files are not tampered with and do not harm your computer.
Pedagogical Note Your instructor may require you to use packages for organizing programs. For example, you may place all programs in this chapter in a package named chapter1. For instructions on how to use packages, see Supplement I.F, Using Packages to Organize the Classes in the Text.
use package
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Check Point
1.33 1.34 1.35 1.36 1.37 1.38 1.39 1.40
What is the Java source filename extension, and what is the Java bytecode filename extension? What are the input and output of a Java compiler? What is the command to compile a Java program? What is the command to run a Java program? What is the JVM? Can Java run on any machine? What is needed to run Java on a computer? If a NoClassDefFoundError occurs when you run a program, what is the cause of the error? If a NoSuchMethodError occurs when you run a program, what is the cause of the error?
1.9 Programming Style and Documentation Key Point programming style documentation
Good programming style and proper documentation make a program easy to read and help programmers prevent errors. Programming style deals with what programs look like. A program can compile and run properly even if written on only one line, but writing it all on one line would be bad programming style because it would be hard to read. Documentation is the body of explanatory remarks and comments pertaining to a program. Programming style and documentation are as important as coding. Good programming style and appropriate documentation reduce the chance of errors and make programs easy to read. This section gives several guidelines. For more detailed guidelines, see Supplement I.D, Java Coding Style Guidelines, on the Companion Website.
1.9.1
javadoc comment
Appropriate Comments and Comment Styles
Include a summary at the beginning of the program that explains what the program does, its key features, and any unique techniques it uses. In a long program, you should also include comments that introduce each major step and explain anything that is difficult to read. It is important to make comments concise so that they do not crowd the program or make it difficult to read. In addition to line comments (beginning with //) and block comments (beginning with /*), Java supports comments of a special type, referred to as javadoc comments. javadoc comments begin with /** and end with */. They can be extracted into an HTML file using the JDK’s javadoc command. For more information, see Supplement III.Y, javadoc Comments, on the companion Website. Use javadoc comments (/** ... */) for commenting on an entire class or an entire method. These comments must precede the class or the method header in order to be extracted into a javadoc HTML file. For commenting on steps inside a method, use line comments (//).
1.9 Programming Style and Documentation 19 To see an example of a javadoc HTML file, check out www.cs.armstrong.edu/liang/javadoc/ Exercise1.html. Its corresponding Java code is shown in www.cs.armstrong.edu/liang/javadoc/ Exercise1.java.
1.9.2
Proper Indentation and Spacing
A consistent indentation style makes programs clear and easy to read, debug, and maintain. Indentation is used to illustrate the structural relationships between a program’s components or statements. Java can read the program even if all of the statements are on the same long line, but humans find it easier to read and maintain code that is aligned properly. Indent each subcomponent or statement at least two spaces more than the construct within which it is nested. A single space should be added on both sides of a binary operator, as shown in the following statement:
1.9.3
System.out.println(3+4*4);
Bad style
System.out.println(3 + 4 * 4);
Good style
indent code
Block Styles
A block is a group of statements surrounded by braces. There are two popular styles, next-line style and end-of-line style, as shown below.
public class Test { public static void main(String[] args) { System.out.println("Block Styles"); } }
public class Test { public static void main(String[] args) { System.out.println("Block Styles"); } }
Next-line style
End-of-line style
The next-line style aligns braces vertically and makes programs easy to read, whereas the end-of-line style saves space and may help avoid some subtle programming errors. Both are acceptable block styles. The choice depends on personal or organizational preference. You should use a block style consistently—mixing styles is not recommended. This book uses the end-of-line style to be consistent with the Java API source code.
1.41
Reformat the following program according to the programming style and documentation guidelines. Use the end-of-line brace style. public class Test { // Main method public static void main(String[] args) { /** Display output */ System.out.println("Welcome to Java"); } }
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20 Chapter 1
Introduction to Computers, Programs, and Java
1.10 Programming Errors Key Point
Programming errors can be categorized into three types: syntax errors, runtime errors, and logic errors.
1.10.1
Syntax Errors
Errors that are detected by the compiler are called syntax errors or compile errors. Syntax errors result from errors in code construction, such as mistyping a keyword, omitting some necessary punctuation, or using an opening brace without a corresponding closing brace. These errors are usually easy to detect because the compiler tells you where they are and what caused them. For example, the program in Listing 1.4 has a syntax error, as shown in Figure 1.10.
syntax errors compile errors
LISTING 1.4 ShowSyntaxErrors.java 1 2 3 4 5
public class ShowSyntaxErrors { public static main(String[] args) { System.out.println("Welcome to Java); } }
Four errors are reported, but the program actually has two errors: ■
The keyword void is missing before main in line 2.
■
The string Welcome to Java should be closed with a closing quotation mark in line 3.
Since a single error will often display many lines of compile errors, it is a good practice to fix errors from the top line and work downward. Fixing errors that occur earlier in the program may also fix additional errors that occur later.
Compile
FIGURE 1.10
The compiler reports syntax errors.
Tip If you don’t know how to correct it, compare your program closely, character by character, with similar examples in the text. In the first few weeks of this course, you will probably spend a lot of time fixing syntax errors. Soon you will be familiar with Java syntax and can quickly fix syntax errors.
fix syntax errors
1.10.2 runtime errors
Runtime Errors
Runtime errors are errors that cause a program to terminate abnormally. They occur while a program is running if the environment detects an operation that is impossible to carry out. Input mistakes typically cause runtime errors. An input error occurs when the program is
1.10 Programming Errors 21 waiting for the user to enter a value, but the user enters a value that the program cannot handle. For instance, if the program expects to read in a number, but instead the user enters a string, this causes data-type errors to occur in the program. Another example of runtime errors is division by zero. This happens when the divisor is zero for integer divisions. For instance, the program in Listing 1.5 would cause a runtime error, as shown in Figure 1.11.
LISTING 1.5 ShowRuntimeErrors.java 1 2 3 4 5
public class ShowRuntimeErrors { public static void main(String[] args) { System.out.println(1 / 0); } }
runtime error
Run
FIGURE 1.11
1.10.3
The runtime error causes the program to terminate abnormally.
Logic Errors
Logic errors occur when a program does not perform the way it was intended to. Errors of this kind occur for many different reasons. For example, suppose you wrote the program in Listing 1.6 to convert Celsius 35 degrees to a Fahrenheit degree:
LISTING 1.6 ShowLogicErrors.java 1 2 3 4 5 6
public class ShowLogicErrors { public static void main(String[] args) { System.out.println("Celsius 35 is Fahrenheit degree "); System.out.println((9 / 5) * 35 + 32); } }
Celsius 35 is Fahrenheit degree 67
You will get Fahrenheit 67 degrees, which is wrong. It should be 95.0. In Java, the division for integers is the quotient—the fractional part is truncated—so in Java 9 / 5 is 1. To get the correct result, you need to use 9.0 / 5, which results in 1.8. In general, syntax errors are easy to find and easy to correct because the compiler gives indications as to where the errors came from and why they are wrong. Runtime errors are not difficult to find, either, since the reasons and locations for the errors are displayed on the console when the program aborts. Finding logic errors, on the other hand, can be very challenging. In the upcoming chapters, you will learn the techniques of tracing programs and finding logic errors.
1.10.4
Common Errors
Missing a closing brace, missing a semicolon, missing quotation marks for strings, and misspelling names are common errors for new programmers.
logic errors
22 Chapter 1
Introduction to Computers, Programs, and Java Common Error 1: Missing Braces The braces are used to denote a block in the program. Each opening brace must be matched by a closing brace. A common error is missing the closing brace. To avoid this error, type a closing brace whenever an opening brace is typed, as shown in the following example. public class Welcome { }
Type this closing brace right away to match the opening brace
If you use an IDE such as NetBeans and Eclipse, the IDE automatically inserts a closing brace for each opening brace typed. Common Error 2: Missing Semicolons Each statement ends with a statement terminator (;). Often, a new programmer forgets to place a statement terminator for the last statement in a block, as shown in the following example. public static void main(String[] args) { System.out.println("Programming is fun!"); System.out.println("Fundamentals First"); System.out.println("Problem Driven") }
Missing a semicolon Common Error 3: Missing Quotation Marks A string must be placed inside the quotation marks. Often, a new programmer forgets to place a quotation mark at the end of a string, as shown in the following example. System.out.println("Problem Driven );
Missing a quotation mark If you use an IDE such as NetBeans and Eclipse, the IDE automatically inserts a closing quotation mark for each opening quotation mark typed. Common Error 4: Misspelling Names Java is case sensitive. Misspelling names is a common error for new programmers. For example, the word main is misspelled as Main and String is misspelled as string in the following code. 1 2 3 4 5
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Check Point
1.42 1.43 1.44 1.45 1.46
public class Test { public static void Main(string[] args) { System.out.println((10.5 + 2 * 3) / (45 – 3.5)); } }
What are syntax errors (compile errors), runtime errors, and logic errors? Give examples of syntax errors, runtime errors, and logic errors. If you forget to put a closing quotation mark on a string, what kind error will be raised? If your program needs to read integers, but the user entered strings, an error would occur when running this program. What kind of error is this? Suppose you write a program for computing the perimeter of a rectangle and you mistakenly write your program so that it computes the area of a rectangle. What kind of error is this?
1.11 Developing Java Programs Using NetBeans 23 1.47
Identify and fix the errors in the following code: 1 2 3 4 5
public class Welcome { public void Main(String[] args) { System.out.println('Welcome to Java!); } }
1.11 Developing Java Programs Using NetBeans You can edit, compile, run, and debug Java Programs using NetBeans. NetBeans and Eclipse are two free popular integrated development environments for developing Java programs. They are easy to learn if you follow simple instructions. We recommend that you use either one for developing Java programs. This section gives the essential instructions to guide new users to create a project, create a class, compile, and run a class in NetBeans. The use of Eclipse will be introduced in the next section. For instructions on downloading and installing latest version of NetBeans, see Supplement II.B.
1.11.1
Creating a Java Project
Before you can create Java programs, you need to first create a project. A project is like a folder to hold Java programs and all supporting files. You need to create a project only once. Here are the steps to create a Java project: 1. Choose File, New Project to display the New Project dialog box, as shown in Figure 1.12. 2. Select Java in the Categories section and Java Application in the Projects section and click Next to display the New Java Application dialog box, as shown in Figure 1.13. 3. Type demo in the Project Name field and c:\michael in Project Location field. Uncheck Use Dedicated Folder for Storing Libraries and uncheck Create Main Class. 4. Click Finish to create the project, as shown in Figure 1.14.
1.11.2
Creating a Java Class
After a project is created, you can create Java programs in the project using the following steps: 1. Right-click the demo node in the project pane to display a context menu. Choose New, Java Class to display the New Java Class dialog box, as shown in Figure 1.15.
FIGURE 1.12 The New Project dialog is used to create a new project and specify a project type.
Key Point
VideoNote
NetBeans brief tutorial
24 Chapter 1
Introduction to Computers, Programs, and Java
FIGURE 1.13 The New Java Application dialog is for specifying a project name and location.
FIGURE 1.14
A New Java project named demo is created.
FIGURE 1.15
The New Java Class dialog box is used to create a new Java class.
2. Type Welcome in the Class Name field and select the Source Packages in the Location field. Leave the Package field blank. This will create a class in the default package. 3. Click Finish to create the Welcome class. The source code file Welcome.java is placed under the node. 4. Modify the code in the Welcome class to match Listing 1.1 in the text, as shown in Figure 1.16.
1.12 Developing Java Programs Using Eclipse 25
Edit pane
Output pane
FIGURE 1.16 You can edit a program and run it in NetBeans.
1.11.3
Compiling and Running a Class
To run Welcome.java, right-click Welcome.java to display a context menu and choose Run File, or simply press Shift + F6. The output is displayed in the Output pane, as shown in Figure 1.16. The Run File command automatically compiles the program if the program has been changed.
1.12 Developing Java Programs Using Eclipse You can edit, compile, run, and debug Java Programs using Eclipse. The preceding section introduced developing Java programs using NetBeans. You can also use Eclipse to develop Java programs. This section gives the essential instructions to guide new users to create a project, create a class, and compile/run a class in Eclipse. For instructions on downloading and installing latest version of Eclipse, see Supplement II.D.
1.12.1
Key Point
Creating a Java Project
Before creating Java programs in Eclipse, you need to first create a project to hold all files.
VideoNote
Here are the steps to create a Java project in Eclipse:
Eclipse brief tutorial
1. Choose File, New, Java Project to display the New Project wizard, as shown in Figure 1.17. 2. Type demo in the Project name field. As you type, the Location field is automatically set by default. You may customize the location for your project. 3. Make sure that you selected the options Use project folder as root for sources and class files so that the .java and .class files are in the same folder for easy access. 4. Click Finish to create the project, as shown in Figure 1.18.
1.12.2
Creating a Java Class
After a project is created, you can create Java programs in the project using the following steps: 1. Choose File, New, Class to display the New Java Class wizard. 2. Type Welcome in the Name field.
26 Chapter 1
Introduction to Computers, Programs, and Java
FIGURE 1.17
The New Java Project dialog is for specifying a project name and properties.
FIGURE 1.18
A New Java project named demo is created.
3. Check the option public static void main(String[] args). 4. Click Finish to generate the template for the source code Welcome.java, as shown in Figure 1.19.
1.12 Developing Java Programs Using Eclipse 27
FIGURE 1.19
1.12.3
The New Java Class dialog box is used to create a new Java class.
Compiling and Running a Class
To run the program, right-click the class in the project to display a context menu. Choose Run, Java Application in the context menu to run the class. The output is displayed in the Console pane, as shown in Figure 1.20.
Edit pane
Output pane
FIGURE 1.20
You can edit a program and run it in Eclipse.
28 Chapter 1
Introduction to Computers, Programs, and Java
KEY TERMS Application Program Interface (API) 11 assembler 7 assembly language 7 bit 3 block 13 block comment 13 bus 2 byte 3 bytecode 16 bytecode verifier 17 cable modem 6 central processing unit (CPU) 3 class loader 17 comment 13 compiler 8 console 12 dot pitch 6 DSL (digital subscriber line) 6 encoding scheme 3 hardware 2 high-level language 8 integrated development environment (IDE) 12 interpreter 8 java command 17 Java Development Toolkit (JDK) 12 Java language specification 11
Java Virtual Machine (JVM) 16 17 keyword (or reserved word) 13 library 11 line comment 13 logic error 21 low-level language 7 machine language 7 main method 13 memory 4 modem 00 motherboard 3 network interface card (NIC) 6 operating system (OS) 9 pixel 6 program 2 programming 2 runtime error 20 screen resolution 6 software 2 source code 8 source program 8 statement 8 statement terminator 13 storage devices 4 syntax error 20 javac command
Note Supplement I.A
The above terms are defined in this chapter. Supplement I.A, Glossary, lists all the key terms and descriptions in the book, organized by chapters.
CHAPTER SUMMARY 1. A computer is an electronic device that stores and processes data. 2. A computer includes both hardware and software. 3. Hardware is the physical aspect of the computer that can be touched. 4. Computer programs, known as software, are the invisible instructions that control the hardware and make it perform tasks.
5. Computer programming is the writing of instructions (i.e., code) for computers to perform. 6. The central processing unit (CPU) is a computer’s brain. It retrieves instructions from memory and executes them.
7. Computers use zeros and ones because digital devices have two stable states, referred to by convention as zero and one.
Chapter Summary 29 8. A bit is a binary digit 0 or 1. 9. A byte is a sequence of 8 bits. 10. A kilobyte is about 1,000 bytes, a megabyte about 1 million bytes, a gigabyte about 1 billion bytes, and a terabyte about 1,000 gigabytes.
11. Memory stores data and program instructions for the CPU to execute. 12. A memory unit is an ordered sequence of bytes. 13. Memory is volatile, because information is lost when the power is turned off. 14. Programs and data are permanently stored on storage devices and are moved to memory when the computer actually uses them.
15. The machine language is a set of primitive instructions built into every computer. 16. Assembly language is a low-level programming language in which a mnemonic is used to represent each machine-language instruction.
17. High-level languages are English-like and easy to learn and program. 18. A program written in a high-level language is called a source program. 19. A compiler is a software program that translates the source program into a machinelanguage program.
20. The operating system (OS) is a program that manages and controls a computer’s activities. 21. Java is platform independent, meaning that you can write a program once and run it on any computer.
22. Java programs can be embedded in HTML pages and downloaded by Web browsers to bring live animation and interaction to Web clients.
23. The Java source file name must match the public class name in the program. Java source code files must end with the .java extension.
24. Every class is compiled into a separate bytecode file that has the same name as the class and ends with the .class extension.
25. To compile a Java source-code file from the command line, use the javac command. 26. To run a Java class from the command line, use the java command. 27. Every Java program is a set of class definitions. The keyword class introduces a class definition. The contents of the class are included in a block.
28. A block begins with an opening brace ({) and ends with a closing brace (}). 29. Methods are contained in a class. To run a Java program, the program must have a main method. The main method is the entry point where the program starts when it is executed.
30 Chapter 1
Introduction to Computers, Programs, and Java 30. Every statement in Java ends with a semicolon (;), known as the statement terminator. 31. Reserved words, or keywords, have a specific meaning to the compiler and cannot be used for other purposes in the program.
32. In Java, comments are preceded by two slashes (//) on a line, called a line comment, or enclosed between /* and */ on one or several lines, called a block comment or paragraph comment. Comments are ignored by the compiler.
33. Java source programs are case sensitive. 34. Programming errors can be categorized into three types: syntax errors, runtime errors, and logic errors. Errors reported by a compiler are called syntax errors or compile errors. Runtime errors are errors that cause a program to terminate abnormally. Logic errors occur when a program does not perform the way it was intended to.
QUIZ Answer the quiz for this chapter online at www.cs.armstrong.edu/liang/intro10e/quiz.html.
PROGRAMMING EXERCISES Note Solutions to even-numbered programming exercises are on the Companion Website. Solutions to all programming exercises are on the Instructor Resource Website. Additional programming exercises with solutions are provided to the instructors on the Instructor Resource Website. The level of difficulty is rated easy (no star), moderate (*), hard (**), or challenging (***).
level of difficulty
1.1 1.2 *1.3
(Display three messages) Write a program that displays Welcome to Java, Welcome to Computer Science, and Programming is fun. (Display five messages) Write a program that displays Welcome to Java five times. (Display a pattern) Write a program that displays the following pattern: J J J
J J J
1.4
1.6
A
V
V V V V V
A
A A A AAAAA
V A
A
(Print a table) Write a program that displays the following table: a 1 2 3 4
1.5
A A A AAAAA
a^2 1 4 9 16
a^3 1 8 27 64
(Compute expressions) Write a program that displays the result of 9.5 * 4.5 - 2.5 * 3 . 45.5 - 3.5 (Summation of a series) Write a program that displays the result of 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9.
Programming Exercises 31 1.7
(Approximatep) p can be computed using the following formula: p = 4 * ¢1 -
1 1 1 1 1 + - + + c≤ 3 5 7 9 11
Write a program that displays the result of 4 * ¢ 1 and 4 * ¢ 1 -
1 1 1 1 1 + - + ≤ 3 5 7 9 11
1 1 1 1 1 1 + - + + ≤. Use 1.0 instead of 1 in your 3 5 7 9 11 13
program.
1.8
(Area and perimeter of a circle) Write a program that displays the area and perimeter of a circle that has a radius of 5.5 using the following formula: perimeter = 2 * radius * p area = radius * radius * p
1.9
(Area and perimeter of a rectangle) Write a program that displays the area and perimeter of a rectangle with the width of 4.5 and height of 7.9 using the following formula: area = width * height
1.10
(Average speed in miles) Assume a runner runs 14 kilometers in 45 minutes and 30 seconds. Write a program that displays the average speed in miles per hour. (Note that 1 mile is 1.6 kilometers.) (Population projection) The U.S. Census Bureau projects population based on the following assumptions:
*1.11
■ ■ ■
1.12 *1.13
One birth every 7 seconds One death every 13 seconds One new immigrant every 45 seconds
Write a program to display the population for each of the next five years. Assume the current population is 312,032,486 and one year has 365 days. Hint: In Java, if two integers perform division, the result is an integer. The fractional part is truncated. For example, 5 / 4 is 1 (not 1.25) and 10 / 4 is 2 (not 2.5). To get an accurate result with the fractional part, one of the values involved in the division must be a number with a decimal point. For example, 5.0 / 4 is 1.25 and 10 / 4.0 is 2.5. (Average speed in kilometers) Assume a runner runs 24 miles in 1 hour, 40 minutes, and 35 seconds. Write a program that displays the average speed in kilometers per hour. (Note that 1 mile is 1.6 kilometers.) (Algebra: solve 2 * 2 linear equations) You can use Cramer’s rule to solve the following 2 * 2 system of linear equation: ed - bf af - ec ax + by = e x = y = cx + dy = f ad - bc ad - bc Write a program that solves the following equation and displays the value for x and y : 3.4x + 50.2y = 44.5 2.1x + .55y = 5.9
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CHAPTER
ELEMENTARY PROGRAMMING Objectives ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■
To write Java programs to perform simple computations (§2.2). To obtain input from the console using the Scanner class (§2.3). To use identifiers to name variables, constants, methods, and classes (§2.4). To use variables to store data (§§2.5–2.6). To program with assignment statements and assignment expressions (§2.6). To use constants to store permanent data (§2.7). To name classes, methods, variables, and constants by following their naming conventions (§2.8). To explore Java numeric primitive data types: byte, short, int, long, float, and double (§2.9.1). To read a byte, short, int, long, float, or double value from the keyboard (§2.9.2). To perform operations using operators +, -, *, /, and % (§2.9.3). To perform exponent operations using Math.pow(a, b) (§2.9.4). To write integer literals, floating-point literals, and literals in scientific notation (§2.10). To write and evaluate numeric expressions (§2.11). To obtain the current system time using System.currentTimeMillis() (§2.12). To use augmented assignment operators (§2.13). To distinguish between postincrement and preincrement and between postdecrement and predecrement (§2.14). To cast the value of one type to another type (§2.15). To describe the software development process and apply it to develop the loan payment program (§2.16). To write a program that converts a large amount of money into smaller units (§2.17). To avoid common errors and pitfalls in elementary programming (§2.18).
2
34 Chapter 2
Elementary Programming
2.1 Introduction Key Point
The focus of this chapter is on learning elementary programming techniques to solve problems. In Chapter 1 you learned how to create, compile, and run very basic Java programs. Now you will learn how to solve problems by writing programs. Through these problems, you will learn elementary programming using primitive data types, variables, constants, operators, expressions, and input and output. Suppose, for example, that you need to take out a student loan. Given the loan amount, loan term, and annual interest rate, can you write a program to compute the monthly payment and total payment? This chapter shows you how to write programs like this. Along the way, you learn the basic steps that go into analyzing a problem, designing a solution, and implementing the solution by creating a program.
2.2 Writing a Simple Program Key Point problem
algorithm
pseudocode
Writing a program involves designing a strategy for solving the problem and then using a programming language to implement that strategy. Let’s first consider the simple problem of computing the area of a circle. How do we write a program for solving this problem? Writing a program involves designing algorithms and translating algorithms into programming instructions, or code. An algorithm describes how a problem is solved by listing the actions that need to be taken and the order of their execution. Algorithms can help the programmer plan a program before writing it in a programming language. Algorithms can be described in natural languages or in pseudocode (natural language mixed with some programming code). The algorithm for calculating the area of a circle can be described as follows: 1. Read in the circle’s radius. 2. Compute the area using the following formula: area = radius * radius * p 3. Display the result.
Tip It’s always good practice to outline your program (or its underlying problem) in the form of an algorithm before you begin coding.
When you code—that is, when you write a program—you translate an algorithm into a program. You already know that every Java program begins with a class definition in which the keyword class is followed by the class name. Assume that you have chosen ComputeArea as the class name. The outline of the program would look like this: public class ComputeArea { // Details to be given later }
As you know, every Java program must have a main method where program execution begins. The program is then expanded as follows: public class ComputeArea { public static void main(String[] args) { // Step 1: Read in radius // Step 2: Compute area
2.2 Writing a Simple Program 35 // Step 3: Display the area } }
The program needs to read the radius entered by the user from the keyboard. This raises two important issues: ■
Reading the radius.
■
Storing the radius in the program.
Let’s address the second issue first. In order to store the radius, the program needs to declare a symbol called a variable. A variable represents a value stored in the computer’s memory. Rather than using x and y as variable names, choose descriptive names: in this case, radius for radius, and area for area. To let the compiler know what radius and area are, specify their data types. That is the kind of data stored in a variable, whether integer, real number, or something else. This is known as declaring variables. Java provides simple data types for representing integers, real numbers, characters, and Boolean types. These types are known as primitive data types or fundamental types. Real numbers (i.e., numbers with a decimal point) are represented using a method known as floating-point in computers. So, the real numbers are also called floating-point numbers. In Java, you can use the keyword double to declare a floating-point variable. Declare radius and area as double. The program can be expanded as follows: public class ComputeArea { public static void main(String[] args) { double radius; double area; // Step 1: Read in radius // Step 2: Compute area // Step 3: Display the area } }
The program declares radius and area as variables. The reserved word double indicates that radius and area are floating-point values stored in the computer. The first step is to prompt the user to designate the circle’s radius. You will soon learn how to prompt the user for information. For now, to learn how variables work, you can assign a fixed value to radius in the program as you write the code; later, you’ll modify the program to prompt the user for this value. The second step is to compute area by assigning the result of the expression radius * radius * 3.14159 to area. In the final step, the program will display the value of area on the console by using the System.out.println method. Listing 2.1 shows the complete program, and a sample run of the program is shown in Figure 2.1.
LISTING 2.1 ComputeArea.java 1 2 3 4 5 6
public class ComputeArea { public static void main(String[] args) { double radius; // Declare radius double area; // Declare area // Assign a radius
variable descriptive names data type declare variables primitive data types
floating-point number
36 Chapter 2
Elementary Programming 7 8 9 10 11 12 13 14 15 16
radius = 20; // radius is now 20 // Compute area area = radius * radius * 3.14159; // Display results System.out.println("The area for the circle of radius " + radius + " is " + area); } }
Compile Run
FIGURE 2.1
declare variable assign value
tracing program
The program displays the area of a circle.
Variables such as radius and area correspond to memory locations. Every variable has a name, a type, a size, and a value. Line 3 declares that radius can store a double value. The value is not defined until you assign a value. Line 7 assigns 20 into variable radius. Similarly, line 4 declares variable area, and line 10 assigns a value into area. The following table shows the value in the memory for area and radius as the program is executed. Each row in the table shows the values of variables after the statement in the corresponding line in the program is executed. This method of reviewing how a program works is called tracing a program. Tracing programs are helpful for understanding how programs work, and they are useful tools for finding errors in programs. line#
radius
no value
4 7
20
10
concatenate strings concatenate strings with numbers
area
no value
3
1256.636
The plus sign (+) has two meanings: one for addition and the other for concatenating (combining) strings. The plus sign (+) in lines 13–14 is called a string concatenation operator. It combines two strings into one. If a string is combined with a number, the number is converted into a string and concatenated with the other string. Therefore, the plus signs (+) in lines 13–14 concatenate strings into a longer string, which is then displayed in the output. Strings and string concatenation will be discussed further in Chapter 4.
Caution A string cannot cross lines in the source code. Thus, the following statement would result in a compile error: System.out.println("Introduction to Java Programming, by Y. Daniel Liang"); break a long string
To fix the error, break the string into separate substrings, and use the concatenation operator (+) to combine them: System.out.println("Introduction to Java Programming, " + "by Y. Daniel Liang");
2.3 Reading Input from the Console 37 2.1
Identify and fix the errors in the following code: 1 2 3 4 5 6 7 8 9 10
public class Test { public void main(string[] args) { double i = 50.0; double k = i + 50.0; double j = k + 1;
✓
Check Point
System.out.println("j is " + j + " and k is " + k); } }
2.3 Reading Input from the Console Reading input from the console enables the program to accept input from the user. In Listing 2.1, the radius is fixed in the source code. To use a different radius, you have to modify the source code and recompile it. Obviously, this is not convenient, so instead you can use the Scanner class for console input. Java uses System.out to refer to the standard output device and System.in to the standard input device. By default, the output device is the display monitor and the input device is the keyboard. To perform console output, you simply use the println method to display a primitive value or a string to the console. Console input is not directly supported in Java, but you can use the Scanner class to create an object to read input from System.in, as follows:
Key Point
VideoNote
Obtain input
Scanner input = new Scanner(System.in);
The syntax new Scanner(System.in) creates an object of the Scanner type. The syntax Scanner input declares that input is a variable whose type is Scanner. The whole line Scanner input = new Scanner(System.in) creates a Scanner object and assigns its reference to the variable input. An object may invoke its methods. To invoke a method on an object is to ask the object to perform a task. You can invoke the nextDouble() method to read a double value as follows: double radius = input.nextDouble();
This statement reads a number from the keyboard and assigns the number to radius. Listing 2.2 rewrites Listing 2.1 to prompt the user to enter a radius.
LISTING 2.2 ComputeAreaWithConsoleInput.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
import java.util.Scanner; // Scanner is in the java.util package
import class
public class ComputeAreaWithConsoleInput { public static void main(String[] args) { // Create a Scanner object Scanner input = new Scanner(System.in);
create a Scanner
// Prompt the user to enter a radius System.out.print("Enter a number for radius: "); double radius = input.nextDouble(); // Compute area double area = radius * radius * 3.14159; // Display results
read a double
38 Chapter 2
Elementary Programming 16 17 18 19
System.out.println("The area for the circle of radius " + radius + " is " + area); } }
Enter a number for radius: 2.5 The area for the circle of radius 2.5 is 19.6349375
Enter a number for radius: 23 The area for the circle of radius 23.0 is 1661.90111
prompt
Line 9 displays a string "Enter a number for radius: " to the console. This is known as a prompt, because it directs the user to enter an input. Your program should always tell the user what to enter when expecting input from the keyboard. The print method in line 9 System.out.print("Enter a number for radius: ");
print vs. println
is identical to the println method except that println moves to the beginning of the next line after displaying the string, but print does not advance to the next line when completed. Line 6 creates a Scanner object. The statement in line 10 reads input from the keyboard. double radius = input.nextDouble();
specific import
After the user enters a number and presses the Enter key, the program reads the number and assigns it to radius. More details on objects will be introduced in Chapter 9. For the time being, simply accept that this is how to obtain input from the console. The Scanner class is in the java.util package. It is imported in line 1. There are two types of import statements: specific import and wildcard import. The specific import specifies a single class in the import statement. For example, the following statement imports Scanner from the package java.util. import java.util.Scanner;
wildcard import
The wildcard import imports all the classes in a package by using the asterisk as the wildcard. For example, the following statement imports all the classes from the package java.util. import java.uitl.*;
no performance difference
The information for the classes in an imported package is not read in at compile time or runtime unless the class is used in the program. The import statement simply tells the compiler where to locate the classes. There is no performance difference between a specific import and a wildcard import declaration. Listing 2.3 gives an example of reading multiple input from the keyboard. The program reads three numbers and displays their average.
LISTING 2.3 ComputeAverage.java import class
create a Scanner
1 2 3 4 5 6 7
import java.util.Scanner; // Scanner is in the java.util package public class ComputeAverage { public static void main(String[] args) { // Create a Scanner object Scanner input = new Scanner(System.in);
2.4 Identifiers 39 8 9 10 11 12 13 14 15 16 17 18 19 20 21
// Prompt the user to enter three numbers System.out.print("Enter three numbers: "); double number1 = input.nextDouble(); double number2 = input.nextDouble(); double number3 = input.nextDouble();
read a double
// Compute average double average = (number1 + number2 + number3) / 3; // Display results System.out.println("The average of " + number1 + " " + number2 + " " + number3 + " is " + average); } }
Enter three numbers: 1 2 3 The average of 1.0 2.0 3.0 is 2.0
Enter three numbers: 10.5 11 11.5 The average of 10.5 11.0 11.5 is 11.0
The code for importing the Scanner class (line 1) and creating a Scanner object (line 6) are the same as in the preceding example as well as in all new programs you will write for reading input from the keyboard. Line 9 prompts the user to enter three numbers. The numbers are read in lines 10–12. You may enter three numbers separated by spaces, then press the Enter key, or enter each number followed by a press of the Enter key, as shown in the sample runs of this program. If you entered an input other than a numeric value, a runtime error would occur. In Chapter 12, you will learn how to handle the exception so that the program can continue to run.
enter input in one line
enter input in multiple lines
runtime error
Note Most of the programs in the early chapters of this book perform three steps—input, process, and output—called IPO. Input is receiving input from the user; process is producing results using the input; and output is displaying the results.
2.2
How do you write a statement to let the user enter a double value from the keyboard? What happens if you entered 5a when executing the following code? double radius = input.nextDouble();
2.3
IPO
✓
Check Point
Are there any performance differences between the following two import statements? import java.util.Scanner; import java.util.*;
2.4 Identifiers Identifiers are the names that identify the elements such as classes, methods, and variables in a program. As you see in Listing 2.3, ComputeAverage, main, input, number1, number2, number3, and so on are the names of things that appear in the program. In programming terminology, such names are called identifiers. All identifiers must obey the following rules: ■
An identifier is a sequence of characters that consists of letters, digits, underscores (_), and dollar signs ($).
Key Point
identifiers identifier naming rules
40 Chapter 2
Elementary Programming ■
An identifier must start with a letter, an underscore (_), or a dollar sign ($). It cannot start with a digit.
■
An identifier cannot be a reserved word. (See Appendix A for a list of reserved words.)
■
An identifier cannot be true, false, or null.
■
An identifier can be of any length.
For example, $2, ComputeArea, area, radius, and print are legal identifiers, whereas 2A and d+4 are not because they do not follow the rules. The Java compiler detects illegal identifiers and reports syntax errors.
Note Since Java is case sensitive, area, Area, and AREA are all different identifiers.
case sensitive
Tip Identifiers are for naming variables, methods, classes, and other items in a program. Descriptive identifiers make programs easy to read. Avoid using abbreviations for identifiers. Using complete words is more descriptive. For example, numberOfStudents is better than numStuds, numOfStuds, or numOfStudents. We use descriptive names for complete programs in the text. However, we will occasionally use variable names such as i, j, k, x, and y in the code snippets for brevity. These names also provide a generic tone to the code snippets.
descriptive names
Tip Do not name identifiers with the $ character. By convention, the $ character should be used only in mechanically generated source code.
the $ character
✓
Check Point
2.4
Which of the following identifiers are valid? Which are Java keywords? miles, Test, a++, ––a, 4#R, $4, #44, apps class, public, int, x, y, radius
2.5 Variables Key Point why called variables?
Variables are used to represent values that may be changed in the program. As you see from the programs in the preceding sections, variables are used to store values to be used later in a program. They are called variables because their values can be changed. In the program in Listing 2.2, radius and area are variables of the double type. You can assign any numerical value to radius and area, and the values of radius and area can be reassigned. For example, in the following code, radius is initially 1.0 (line 2) and then changed to 2.0 (line 7), and area is set to 3.14159 (line 3) and then reset to 12.56636 (line 8). 1 2 3 4 5 6 7 8 9
// Compute the first area radius = 1.0; radius: 1.0 area = radius * radius * 3.14159; area: 3.14159 System.out.println("The area is " + area + " for radius " + radius); // Compute the second area radius = 2.0; radius: 2.0 area = radius * radius * 3.14159; area: 12.56636 System.out.println("The area is " + area + " for radius " + radius);
Variables are for representing data of a certain type. To use a variable, you declare it by telling the compiler its name as well as what type of data it can store. The variable declaration
2.6 Assignment Statements and Assignment Expressions 41 tells the compiler to allocate appropriate memory space for the variable based on its data type. The syntax for declaring a variable is datatype variableName;
Here are some examples of variable declarations:
declare variable
int count; // Declare count to be an integer variable double radius; // Declare radius to be a double variable double interestRate; // Declare interestRate to be a double variable
These examples use the data types int and double. Later you will be introduced to additional data types, such as byte, short, long, float, char, and boolean. If variables are of the same type, they can be declared together, as follows: datatype variable1, variable2, ..., variablen;
The variables are separated by commas. For example, int i, j, k; // Declare i, j, and k as int variables
Variables often have initial values. You can declare a variable and initialize it in one step. Consider, for instance, the following code:
initialize variables
int count = 1;
This is equivalent to the next two statements: int count; count = 1;
You can also use a shorthand form to declare and initialize variables of the same type together. For example, int i = 1, j = 2;
Tip A variable must be declared before it can be assigned a value. A variable declared in a method must be assigned a value before it can be used. Whenever possible, declare a variable and assign its initial value in one step. This will make the program easy to read and avoid programming errors.
Every variable has a scope. The scope of a variable is the part of the program where the variable can be referenced. The rules that define the scope of a variable will be introduced gradually later in the book. For now, all you need to know is that a variable must be declared and initialized before it can be used.
2.5
Identify and fix the errors in the following code: 1 2 3 4 5 6
public class Test { public static void main(String[] args) { int i = k + 2; System.out.println(i); } }
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Check Point
2.6 Assignment Statements and Assignment Expressions An assignment statement designates a value for a variable. An assignment statement can be used as an expression in Java.
Key Point
42 Chapter 2 assignment statement assignment operator
Elementary Programming After a variable is declared, you can assign a value to it by using an assignment statement. In Java, the equal sign (=) is used as the assignment operator. The syntax for assignment statements is as follows: variable = expression;
expression
An expression represents a computation involving values, variables, and operators that, taking them together, evaluates to a value. For example, consider the following code: int y = 1; // double radius = 1.0; // int x = 5 * (3 / 2); // x = y + 1; // double area = radius * radius *
Assign 1 to variable y Assign 1.0 to variable radius Assign the value of the expression to x Assign the addition of y and 1 to x 3.14159; // Compute area
You can use a variable in an expression. A variable can also be used in both sides of the = operator. For example, x = x + 1;
In this assignment statement, the result of x + 1 is assigned to x. If x is 1 before the statement is executed, then it becomes 2 after the statement is executed. To assign a value to a variable, you must place the variable name to the left of the assignment operator. Thus, the following statement is wrong: 1 = x;
// Wrong
Note In mathematics, x = 2 * x + 1 denotes an equation. However, in Java, x = 2 * x + 1 is an assignment statement that evaluates the expression 2 * x + 1 and assigns the result to x.
assignment expression
In Java, an assignment statement is essentially an expression that evaluates to the value to be assigned to the variable on the left side of the assignment operator. For this reason, an assignment statement is also known as an assignment expression. For example, the following statement is correct: System.out.println(x = 1);
which is equivalent to x = 1; System.out.println(x);
If a value is assigned to multiple variables, you can use this syntax: i = j = k = 1;
which is equivalent to k = 1; j = k; i = j;
Note In an assignment statement, the data type of the variable on the left must be compatible with the data type of the value on the right. For example, int x = 1.0 would be
2.7 Named Constants 43 illegal, because the data type of x is int. You cannot assign a double value (1.0) to an int variable without using type casting. Type casting is introduced in Section 2.15.
2.6
Identify and fix the errors in the following code: 1 2 3 4 5 6
public class Test { public static void main(String[] args) { int i = j = k = 2; System.out.println(i + " " + j + " " + k); } }
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Check Point
2.7 Named Constants A named constant is an identifier that represents a permanent value. The value of a variable may change during the execution of a program, but a named constant, or simply constant, represents permanent data that never changes. In our ComputeArea program, p is a constant. If you use it frequently, you don’t want to keep typing 3.14159; instead, you can declare a constant for p. Here is the syntax for declaring a constant:
Key Point constant
final datatype CONSTANTNAME = value;
A constant must be declared and initialized in the same statement. The word final is a Java keyword for declaring a constant. For example, you can declare p as a constant and rewrite Listing 2.1 as in Listing 2.4.
final keyword
LISTING 2.4 ComputeAreaWithConstant.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
import java.util.Scanner; // Scanner is in the java.util package public class ComputeAreaWithConstant { public static void main(String[] args) { final double PI = 3.14159; // Declare a constant // Create a Scanner object Scanner input = new Scanner(System.in); // Prompt the user to enter a radius System.out.print("Enter a number for radius: "); double radius = input.nextDouble(); // Compute area double area = radius * radius * PI; // Display result System.out.println("The area for the circle of radius " + radius + " is " + area); } }
There are three benefits of using constants: (1) you don’t have to repeatedly type the same value if it is used multiple times; (2) if you have to change the constant value (e.g., from 3.14 to 3.14159 for PI), you need to change it only in a single location in the source code; and (3) a descriptive name for a constant makes the program easy to read.
benefits of constants
44 Chapter 2
Elementary Programming
2.8 Naming Conventions Key Point
Sticking with the Java naming conventions makes your programs easy to read and avoids errors. Make sure that you choose descriptive names with straightforward meanings for the variables, constants, classes, and methods in your program. As mentioned earlier, names are case sensitive. Listed below are the conventions for naming variables, methods, and classes.
name variables and methods
■
Use lowercase for variables and methods. If a name consists of several words, concatenate them into one, making the first word lowercase and capitalizing the first letter of each subsequent word—for example, the variables radius and area and the method print.
name classes
■
Capitalize the first letter of each word in a class name—for example, the class names ComputeArea and System.
name constants
■
Capitalize every letter in a constant, and use underscores between words—for example, the constants PI and MAX_VALUE.
It is important to follow the naming conventions to make your programs easy to read.
Caution Do not choose class names that are already used in the Java library. For example, since the System class is defined in Java, you should not name your class System.
name classes
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Check Point
2.7 2.8
2.9
What are the benefits of using constants? Declare an int constant SIZE with value 20. What are the naming conventions for class names, method names, constants, and variables? Which of the following items can be a constant, a method, a variable, or a class according to the Java naming conventions? MAX_VALUE, Test, read, readDouble Translate the following algorithm into Java code: Step 1: Declare a double variable named miles with initial value 100. Step 2: Declare a double constant named KILOMETERS_PER_MILE with value 1.609. Step 3: Declare a double variable named kilometers, multiply miles and KILOMETERS_PER_MILE, and assign the result to kilometers. Step 4: Display kilometers to the console. What is kilometers after Step 4?
2.9 Numeric Data Types and Operations Key Point
Java has six numeric types for integers and floating-point numbers with operators +, -, *, /, and %.
2.9.1
Numeric Types
Every data type has a range of values. The compiler allocates memory space for each variable or constant according to its data type. Java provides eight primitive data types for numeric values, characters, and Boolean values. This section introduces numeric data types and operators. Table 2.1 lists the six numeric data types, their ranges, and their storage sizes.
2.9 Numeric Data Types and Operations 45 TABLE 2.1 Numeric Data Types Name
Range
byte
-2 to 2 - 1 ( -128 to 127)
8-bit signed
byte type
short
-215 to 215 - 1 ( - 32768 to 32767)
16-bit signed
short type
int
-231 to 231 - 1 ( - 2147483648 to 2147483647)
32-bit signed
int type
long
-263 to 263 - 1
64-bit signed
long type
32-bit IEEE 754
float type
64-bit IEEE 754
double type
7
Storage Size 7
(i.e., - 9223372036854775808 to 9223372036854775807) Negative range: - 3.4028235E + 38 to - 1.4E - 45
float
Positive range: 1.4E - 45 to 3.4028235E + 38 Negative range: - 1.7976931348623157E + 308 to -4.9E - 324
double
Positive range: 4.9E - 324 to 1.7976931348623157E + 308
Note IEEE 754 is a standard approved by the Institute of Electrical and Electronics Engineers for representing floating-point numbers on computers. The standard has been widely adopted. Java uses the 32-bit IEEE 754 for the float type and the 64-bit IEEE 754 for the double type. The IEEE 754 standard also defines special floating-point values, which are listed in Appendix E.
Java uses four types for integers: byte, short, int, and long. Choose the type that is most appropriate for your variable. For example, if you know an integer stored in a variable is within a range of a byte, declare the variable as a byte. For simplicity and consistency, we will use int for integers most of the time in this book. Java uses two types for floating-point numbers: float and double. The double type is twice as big as float, so the double is known as double precision and float as single precision. Normally, you should use the double type, because it is more accurate than the float type.
2.9.2
Reading Numbers from the Keyboard
You know how to use the nextDouble() method in the Scanner class to read a double value from the keyboard. You can also use the methods listed in Table 2.2 to read a number of the byte, short, int, long, and float type.
TABLE 2.2 Methods for Scanner Objects Method
Description
nextByte()
reads an integer of the byte type.
nextShort()
reads an integer of the short type.
nextInt()
reads an integer of the int type.
nextLong()
reads an integer of the long type.
nextFloat()
reads a number of the float type.
nextDouble()
reads a number of the double type.
integer types
floating-point types
46 Chapter 2
Elementary Programming Here are examples for reading values of various types from the keyboard: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Scanner input = new Scanner(System.in); System.out.print("Enter a byte value: "); byte byteValue = input.nextByte(); System.out.print("Enter a short value: "); short shortValue = input.nextShort(); System.out.print("Enter an int value: "); int intValue = input.nextInt(); System.out.print("Enter a long value: "); long longValue = input.nextLong(); System.out.print("Enter a float value: "); float floatValue = input.nextFloat();
If you enter a value with an incorrect range or format, a runtime error would occur. For example, you enter a value 128 for line 3, an error would occur because 128 is out of range for a byte type integer.
2.9.3 operators +, -, *, /, % operands
Numeric Operators
The operators for numeric data types include the standard arithmetic operators: addition (+), subtraction (–), multiplication (*), division (/), and remainder (%), as shown in Table 2.3. The operands are the values operated by an operator.
TABLE 2.3 Numeric Operators
integer division
Name
Meaning
Example
Result
+
Addition
34 + 1
35
-
Subtraction
34.0 – 0.1
33.9
*
Multiplication
300 * 30
9000
/
Division
1.0 / 2.0
0.5
%
Remainder
20 % 3
2
When both operands of a division are integers, the result of the division is the quotient and the fractional part is truncated. For example, 5 / 2 yields 2, not 2.5, and –5 / 2 yields -2, not –2.5. To perform a float-point division, one of the operands must be a floating-point number. For example, 5.0 / 2 yields 2.5. The % operator, known as remainder or modulo operator, yields the remainder after division. The operand on the left is the dividend and the operand on the right is the divisor. Therefore, 7 % 3 yields 1, 3 % 7 yields 3, 12 % 4 yields 0, 26 % 8 yields 2, and 20 % 13 yields 7. 2 3
7 6 1
0 7
3 0 3
3 4
12 12 0
3 8
26 24 2
Divisor
13
1
Quotient
20
Dividend
13 7
Remainder
The % operator is often used for positive integers, but it can also be used with negative integers and floating-point values. The remainder is negative only if the dividend is negative. For example, -7 % 3 yields -1, -12 % 4 yields 0, -26 % -8 yields -2, and 20 % -13 yields 7.
2.9 Numeric Data Types and Operations 47 Remainder is very useful in programming. For example, an even number % 2 is always 0 and an odd number % 2 is always 1. Thus, you can use this property to determine whether a number is even or odd. If today is Saturday, it will be Saturday again in 7 days. Suppose you and your friends are going to meet in 10 days. What day is in 10 days? You can find that the day is Tuesday using the following expression: Day 6 in a week is Saturday A week has 7 days (6 + 10) % 7 is 2
After 10 days
Day 2 in a week is Tuesday Note: Day 0 in a week is Sunday
The program in Listing 2.5 obtains minutes and remaining seconds from an amount of time in seconds. For example, 500 seconds contains 8 minutes and 20 seconds.
LISTING 2.5 DisplayTime.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
import java.util.Scanner;
import Scanner
public class DisplayTime { public static void main(String[] args) { Scanner input = new Scanner(System.in); // Prompt the user for input System.out.print("Enter an integer for seconds: "); int seconds = input.nextInt();
create a Scanner
int minutes = seconds / 60; // Find minutes in seconds int remainingSeconds = seconds % 60; // Seconds remaining System.out.println(seconds + " seconds is " + minutes + " minutes and " + remainingSeconds + " seconds");
read an integer divide remainder
} }
Enter an integer for seconds: 500 500 seconds is 8 minutes and 20 seconds
line# 8 10 11
seconds
minutes
remainingSeconds
500 8 20
The nextInt() method (line 8) reads an integer for seconds. Line 10 obtains the minutes using seconds / 60. Line 11 (seconds % 60) obtains the remaining seconds after taking away the minutes. The + and - operators can be both unary and binary. A unary operator has only one operand; a binary operator has two. For example, the - operator in -5 is a unary operator to negate number 5, whereas the - operator in 4 - 5 is a binary operator for subtracting 5 from 4.
unary operator binary operator
48 Chapter 2
Elementary Programming
2.9.4 Math.pow(a, b) method
Exponent Operations
The Math.pow(a, b) method can be used to compute ab. The pow method is defined in the Math class in the Java API. You invoke the method using the syntax Math.pow(a, b) (e.g., Math.pow(2, 3)), which returns the result of ab (23). Here, a and b are parameters for the pow method and the numbers 2 and 3 are actual values used to invoke the method. For example, System.out.println(Math.pow(2, 3)); // Displays 8.0 System.out.println(Math.pow(4, 0.5)); // Displays 2.0 System.out.println(Math.pow(2.5, 2)); // Displays 6.25 System.out.println(Math.pow(2.5, -2)); // Displays 0.16
Chapter 5 introduces more details on methods. For now, all you need to know is how to invoke the pow method to perform the exponent operation.
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Check Point
2.10
Find the largest and smallest byte, short, int, long, float, and double. Which of these data types requires the least amount of memory?
2.11
Show the result of the following remainders. 56 78 -34 -34 5 1
% 6 % -4 % 5 % -5 % 1 % 5
2.12
If today is Tuesday, what will be the day in 100 days?
2.13
What is the result of 25 / 4? How would you rewrite the expression if you wished the result to be a floating-point number?
2.14
Show the result of the following code: System.out.println(2 System.out.println(2 System.out.println(2 System.out.println(2
2.15
* * * *
(5 / 2 + 5 / 2)); 5 / 2 + 2 * 5 / 2); (5 / 2)); 5 / 2);
Are the following statements correct? If so, show the output. System.out.println("25 / 4 is " + 25 / System.out.println("25 / 4.0 is " + 25 System.out.println("3 * 2 / 4 is " + 3 System.out.println("3.0 * 2 / 4 is " +
4); / 4.0); * 2 / 4); 3.0 * 2 / 4);
2.16
Write a statement to display the result of 23.5.
2.17
Suppose m and r are integers. Write a Java expression for mr2 to obtain a floatingpoint result.
2.10 Numeric Literals Key Point literal
A literal is a constant value that appears directly in a program. For example, 34 and 0.305 are literals in the following statements: int numberOfYears = 34; double weight = 0.305;
2.10 Numeric Literals 49
2.10.1
Integer Literals
An integer literal can be assigned to an integer variable as long as it can fit into the variable. A compile error will occur if the literal is too large for the variable to hold. The statement byte b = 128, for example, will cause a compile error, because 128 cannot be stored in a variable of the byte type. (Note that the range for a byte value is from –128 to 127.) An integer literal is assumed to be of the int type, whose value is between -231 (-2147483648) and 231 - 1 (2147483647). To denote an integer literal of the long type, append the letter L or l to it. For example, to write integer 2147483648 in a Java program, you have to write it as 2147483648L or 2147483648l, because 2147483648 exceeds the range for the int value. L is preferred because l (lowercase L) can easily be confused with 1 (the digit one).
Note By default, an integer literal is a decimal integer number. To denote a binary integer literal, use a leading 0b or 0B (zero B), to denote an octal integer literal, use a leading 0 (zero), and to denote a hexadecimal integer literal, use a leading 0x or 0X (zero X). For example,
binary, octal, and hex literals
System.out.println(0B1111); // Displays 15 System.out.println(07777); // Displays 4095 System.out.println(0XFFFF); // Displays 65535
Hexadecimal numbers, binary numbers, and octal numbers are introduced in Appendix F.
2.10.2
Floating-Point Literals
Floating-point literals are written with a decimal point. By default, a floating-point literal is treated as a double type value. For example, 5.0 is considered a double value, not a float value. You can make a number a float by appending the letter f or F, and you can make a number a double by appending the letter d or D. For example, you can use 100.2f or 100.2F for a float number, and 100.2d or 100.2D for a double number.
suffix f or F suffix d or D
Note The double type values are more accurate than the float type values. For example, System.out.println("1.0 / 3.0 is " + 1.0 / 3.0); x
displays 1.0 / 3.0 is 0.3333333333333333 16 digits System.out.println("1.0F / 3.0F is " + 1.0F / 3.0F);
s
displays 1.0F / 3.0F is 0.33333334 8 digits
A float value has 7 to 8 number of significant digits and a double value has 15 to 17 number of significant digits.
2.10.3
Scientific Notation
Floating-point literals can be written in scientific notation in the form of a * 10b. For example, the scientific notation for 123.456 is 1.23456 * 102 and for 0.0123456 is 1.23456 * 10-2. A special syntax is used to write scientific notation numbers. For example, 1.23456 * 102 is written as 1.23456E2 or 1.23456E+2 and 1.23456 * 10-2 as 1.23456E-2. E (or e) represents an exponent and can be in either lowercase or uppercase.
double vs. float
50 Chapter 2
Elementary Programming Note The float and double types are used to represent numbers with a decimal point. Why are they called floating-point numbers? These numbers are stored in scientific notation internally. When a number such as 50.534 is converted into scientific notation, such as 5.0534E+1, its decimal point is moved (i.e., floated) to a new position.
why called floating-point?
Note To improve readability, Java allows you to use underscores between two digits in a number literal. For example, the following literals are correct. long ssn = 232_45_4519; long creditCardNumber = 2324_4545_4519_3415L;
However, 45_ or _45 is incorrect. The underscore must be placed between two digits.
underscores in numbers
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Check Point
2.18
How many accurate digits are stored in a float or double type variable?
2.19
Which of the following are correct literals for floating-point numbers? 12.3, 12.3e+2, 23.4e-2, –334.4, 20.5, 39F, 40D
2.20
Which of the following are the same as 52.534? 5.2534e+1, 0.52534e+2, 525.34e-1, 5.2534e+0
2.21
Which of the following are correct literals? 5_2534e+1, _2534, 5_2, 5_
2.11 Evaluating Expressions and Operator Precedence Key Point
Java expressions are evaluated in the same way as arithmetic expressions. Writing a numeric expression in Java involves a straightforward translation of an arithmetic expression using Java operators. For example, the arithmetic expression 10(y - 5)(a + b + c) 3 + 4x 4 9 + x ≤ + 9¢ + y x x 5 can be translated into a Java expression as: (3 + 4 * x) / 5 – 10 * (y - 5) * (a + b + c) / x + 9 * (4 / x + (9 + x) / y)
evaluating an expression
operator precedence rule
Though Java has its own way to evaluate an expression behind the scene, the result of a Java expression and its corresponding arithmetic expression is the same. Therefore, you can safely apply the arithmetic rule for evaluating a Java expression. Operators contained within pairs of parentheses are evaluated first. Parentheses can be nested, in which case the expression in the inner parentheses is evaluated first. When more than one operator is used in an expression, the following operator precedence rule is used to determine the order of evaluation. ■
Multiplication, division, and remainder operators are applied first. If an expression contains several multiplication, division, and remainder operators, they are applied from left to right.
■
Addition and subtraction operators are applied last. If an expression contains several addition and subtraction operators, they are applied from left to right.
2.11 Evaluating Expressions and Operator Precedence 51 Here is an example of how an expression is evaluated: 3 + 4 * 4 + 5 * (4 + 3) - 1 (1) inside parentheses first 3 + 4 * 4 + 5 * 7 – 1 (2) multiplication 3 + 16 + 5 * 7 – 1 (3) multiplication 3 + 16 + 35 – 1 (4) addition 19 + 35 – 1 (5) addition 54 – 1 (6) subtraction 53
Listing 2.6 gives a program that converts a Fahrenheit degree to Celsius using the formula celsius = ( 59 ) (fahrenheit - 32).
LISTING 2.6 FahrenheitToCelsius.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
import java.util.Scanner; public class FahrenheitToCelsius { public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.print("Enter a degree in Fahrenheit: "); double fahrenheit = input.nextDouble(); // Convert Fahrenheit to Celsius double celsius = (5.0 / 9) * (fahrenheit - 32); System.out.println("Fahrenheit " + fahrenheit + " is " + celsius + " in Celsius");
divide
} }
Enter a degree in Fahrenheit: 100 Fahrenheit 100.0 is 37.77777777777778 in Celsius
line# 8
fahrenheit
celsius
100
11
37.77777777777778
Be careful when applying division. Division of two integers yields an integer in Java. 59 is translated to 5.0 / 9 instead of 5 / 9 in line 11, because 5 / 9 yields 0 in Java.
2.22
How would you write the following arithmetic expression in Java? 3 + d(2 + a) 4 a. - 9(a + bc) + 3(r + 34) a + bd b. 5.5 * (r + 2.5)2.5 + t
integer vs. floating-point division
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Check Point
52 Chapter 2
Elementary Programming
2.12 Case Study: Displaying the Current Time Key Point
VideoNote
Use operators / and % currentTimeMillis
You can invoke System.currentTimeMillis() to return the current time. The problem is to develop a program that displays the current time in GMT (Greenwich Mean Time) in the format hour:minute:second, such as 13:19:8. The currentTimeMillis method in the System class returns the current time in milliseconds elapsed since midnight, January 1, 1970 GMT, as shown in Figure 2.2. This time is known as the UNIX epoch. The epoch is the point when time starts, and 1970 was the year when the UNIX operating system was formally introduced.
UNIX epoch Elapsed time UNIX epoch 01-01-1970 00:00:00 GMT
Time Current time System.currentTimeMillis()
FIGURE 2.2 The System.currentTimeMillis() returns the number of milliseconds since the UNIX epoch. You can use this method to obtain the current time, and then compute the current second, minute, and hour as follows. 1. Obtain the total milliseconds since midnight, January 1, 1970, in totalMilliseconds by invoking System.currentTimeMillis() (e.g., 1203183068328 milliseconds). 2. Obtain the total seconds totalSeconds by dividing totalMilliseconds by 1000 (e.g., 1203183068328 milliseconds / 1000 = 1203183068 seconds). 3. Compute the current second from totalSeconds % 60 (e.g., 1203183068 seconds % 60 = 8, which is the current second). 4. Obtain the total minutes totalMinutes by dividing totalSeconds by 60 (e.g., 1203183068 seconds / 60 = 20053051 minutes). 5. Compute the current minute from totalMinutes % 60 (e.g., 20053051 minutes % 60 = 31, which is the current minute). 6. Obtain the total hours totalHours by dividing totalMinutes by 60 (e.g., 20053051 minutes / 60 = 334217 hours). 7. Compute the current hour from totalHours % 24 (e.g., 334217 hours % 24 = 17, which is the current hour). Listing 2.7 gives the complete program.
LISTING 2.7 ShowCurrentTime.java
totalMilliseconds
totalSeconds
currentSecond
1 2 3 4 5 6 7 8 9 10
public class ShowCurrentTime { public static void main(String[] args) { // Obtain the total milliseconds since midnight, Jan 1, 1970 long totalMilliseconds = System.currentTimeMillis(); // Obtain the total seconds since midnight, Jan 1, 1970 long totalSeconds = totalMilliseconds / 1000; // Compute the current second in the minute in the hour long currentSecond = totalSeconds % 60;
2.12 Case Study: Displaying the Current Time 53 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
// Obtain the total minutes long totalMinutes = totalSeconds / 60;
totalMinutes
// Compute the current minute in the hour long currentMinute = totalMinutes % 60;
currentMinute
// Obtain the total hours long totalHours = totalMinutes / 60;
totalHours
// Compute the current hour long currentHour = totalHours % 24;
currentHour
// Display results System.out.println("Current time is " + currentHour + ":" + currentMinute + ":" + currentSecond + " GMT");
preparing output
} }
Current time is 17:31:8 GMT
Line 4 invokes System.currentTimeMillis() to obtain the current time in milliseconds as a long value. Thus, all the variables are declared as the long type in this program. The seconds, minutes, and hours are extracted from the current time using the / and % operators (lines 6–22).
line#
4
7
10
13
16
19
22
variables totalMilliseconds totalSeconds currentSecond
1203183068328 1203183068 8
totalMinutes currentMinute
20053051 31
totalHours
334217
currentHour
17
In the sample run, a single digit 8 is displayed for the second. The desirable output would be 08. This can be fixed by using a method that formats a single digit with a prefix 0 (see Exercise 6.37).
2.23
How do you obtain the current second, minute, and hour?
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Check Point
54 Chapter 2
Elementary Programming
2.13 Augmented Assignment Operators Key Point
The operators +, -, *, /, and % can be combined with the assignment operator to form augmented operators. Very often the current value of a variable is used, modified, and then reassigned back to the same variable. For example, the following statement increases the variable count by 1: count = count + 1;
Java allows you to combine assignment and addition operators using an augmented (or compound) assignment operator. For example, the preceding statement can be written as count += 1; addition assignment operator
The += is called the addition assignment operator. Table 2.4 shows other augmented assignment operators.
TABLE 2.4 Augmented Assignment Operators Operator
Name
Example
Equivalent
+=
Addition assignment
i += 8
i = i + 8
-=
Subtraction assignment
i -= 8
i = i – 8
*=
Multiplication assignment
i *= 8
i = i * 8
/=
Division assignment
i /=
i = i / 8
%=
Remainder assignment
i %= 8
8
i = i % 8
The augmented assignment operator is performed last after all the other operators in the expression are evaluated. For example, x /= 4 + 5.5 * 1.5;
is same as x = x / (4 + 5.5 * 1.5);
Caution There are no spaces in the augmented assignment operators. For example, + = should be +=.
Note Like the assignment operator (=), the operators (+=, -=, *=, /=, %=) can be used to form an assignment statement as well as an expression. For example, in the following code, x += 2 is a statement in the first line and an expression in the second line. x += 2; // Statement System.out.println(x += 2); // Expression
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Check Point
2.24
Show the output of the following code: double a = 6.5; a += a + 1;
2.14 Increment and Decrement Operators 55 System.out.println(a); a = 6; a /= 2; System.out.println(a);
2.14 Increment and Decrement Operators The increment operator (++) and decrement operator (– –) are for incrementing and decrementing a variable by 1. The ++ and —— are two shorthand operators for incrementing and decrementing a variable by 1. These are handy because that’s often how much the value needs to be changed in many programming tasks. For example, the following code increments i by 1 and decrements j by 1.
Key Point increment operator (++) decrement operator (−−)
int i = 3, j = 3; i++; // i becomes 4 j——; // j becomes 2
i++ is pronounced as i plus plus and i—— as i minus minus. These operators are known as postfix increment (or postincrement) and postfix decrement (or postdecrement), because the operators ++ and —— are placed after the variable. These operators can also be placed before the variable. For example,
postincrement postdecrement
int i = 3, j = 3; ++i; // i becomes 4 ——j; // j becomes 2
++i increments i by 1 and ——j decrements j by 1. These operators are known as prefix increment (or preincrement) and prefix decrement (or predecrement). As you see, the effect of i++ and ++i or i—— and ——i are the same in the preceding examples. However, their effects are different when they are used in statements that do more than just increment and decrement. Table 2.5 describes their differences and gives examples.
TABLE 2.5 Increment and Decrement Operators Operator
Name
Description
Example (assume i = 1)
++var
preincrement
Increment var by 1, and use the new var value in the statement
int j = ++i; // j is 2, i is 2
var++
postincrement
Increment var by 1, but use the original var value in the statement
int j = i++; // j is 1, i is 2
——var
predecrement
Decrement var by 1, and use the new var value in the statement
int j = ——i; // j is 0, i is 0
var——
postdecrement
Decrement var by 1, and use the original var value in the statement
int j = i——; // j is 1, i is 0
Here are additional examples to illustrate the differences between the prefix form of ++ (or ——) and the postfix form of ++ (or −−). Consider the following code: int i = 10; int newNum = 10 * i++;
Same effect as
System.out.print("i is " + i + ", newNum is " + newNum); i is 11, newNum is 100
int newNum = 10 * i; i = i + 1;
preincrement predecrement
56 Chapter 2
Elementary Programming In this case, i is incremented by 1, then the old value of i is used in the multiplication. So newNum becomes 100. If i++ is replaced by ++i as follows, int i = 10; int newNum = 10 * (++i);
Same effect as
System.out.print("i is " + i + ", newNum is " + newNum);
i = i + 1; int newNum = 10 * i;
i is 11, newNum is 110
i is incremented by 1, and the new value of i is used in the multiplication. Thus newNum becomes 110. Here is another example: double x = 1.0; double y = 5.0; double z = x–– + (++y);
After all three lines are executed, y becomes 6.0, z becomes 7.0, and x becomes 0.0.
Tip Using increment and decrement operators makes expressions short, but it also makes them complex and difficult to read. Avoid using these operators in expressions that modify multiple variables or the same variable multiple times, such as this one: int k = ++i + i.
✓
Check Point
2.25
Which of these statements are true? a. Any expression can be used as a statement. b. The expression x++ can be used as a statement. c. The statement x = x + 5 is also an expression. d. The statement x = y = x = 0 is illegal.
2.26
Show the output of the following code: int a = 6; int b = a++; System.out.println(a); System.out.println(b); a = 6; b = ++a; System.out.println(a); System.out.println(b);
2.15 Numeric Type Conversions Key Point
Floating-point numbers can be converted into integers using explicit casting. Can you perform binary operations with two operands of different types? Yes. If an integer and a floating-point number are involved in a binary operation, Java automatically converts the integer to a floating-point value. So, 3 * 4.5 is same as 3.0 * 4.5.
2.15 Numeric Type Conversions 57 You can always assign a value to a numeric variable whose type supports a larger range of values; thus, for instance, you can assign a long value to a float variable. You cannot, however, assign a value to a variable of a type with a smaller range unless you use type casting. Casting is an operation that converts a value of one data type into a value of another data type. Casting a type with a small range to a type with a larger range is known as widening a type. Casting a type with a large range to a type with a smaller range is known as narrowing a type. Java will automatically widen a type, but you must narrow a type explicitly. The syntax for casting a type is to specify the target type in parentheses, followed by the variable’s name or the value to be cast. For example, the following statement
casting widening a type narrowing a type
System.out.println((int)1.7);
displays 1. When a double value is cast into an int value, the fractional part is truncated. The following statement System.out.println((double)1 / 2);
displays 0.5, because 1 is cast to 1.0 first, then 1.0 is divided by 2. However, the statement System.out.println(1 / 2);
displays 0, because 1 and 2 are both integers and the resulting value should also be an integer.
Caution Casting is necessary if you are assigning a value to a variable of a smaller type range, such as assigning a double value to an int variable. A compile error will occur if casting is not used in situations of this kind. However, be careful when using casting, as loss of information might lead to inaccurate results.
possible loss of precision
Note Casting does not change the variable being cast. For example, d is not changed after casting in the following code: double d = 4.5; int i = (int)d;
// i becomes 4, but d is still 4.5
Note In Java, an augmented expression of the form x1 op= x2 is implemented as x1 = (T)(x1 op x2), where T is the type for x1. Therefore, the following code is correct. int sum = 0; sum += 4.5; // sum becomes 4 after this statement sum += 4.5 is equivalent to sum = (int)(sum + 4.5).
Note To assign a variable of the int type to a variable of the short or byte type, explicit casting must be used. For example, the following statements have a compile error: int i = 1; byte b = i; // Error because explicit casting is required
However, so long as the integer literal is within the permissible range of the target variable, explicit casting is not needed to assign an integer literal to a variable of the short or byte type (see Section 2.10, Numeric Literals).
The program in Listing 2.8 displays the sales tax with two digits after the decimal point.
casting in an augmented expression
58 Chapter 2
Elementary Programming
LISTING 2.8 SalesTax.java 1 2 3 4 5 6 7 8 9 10 11 12 13
casting
import java.util.Scanner; public class SalesTax { public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.print("Enter purchase amount: "); double purchaseAmount = input.nextDouble(); double tax = purchaseAmount * 0.06; System.out.println("Sales tax is $" + (int)(tax * 100) / 100.0); } }
Enter purchase amount: 197.55 Sales tax is $11.85
line# 8
purchaseAmount
tax
output
197.55
10
11.853
11
11.85
The variable purchaseAmount is 197.55 (line 8). The sales tax is 6% of the purchase, so the tax is evaluated as 11.853 (line 10). Note that
formatting numbers
tax * 100 is 1185.3 (int)(tax * 100) is 1185 (int)(tax * 100) / 100.0 is 11.85
So, the statement in line 11 displays the tax 11.85 with two digits after the decimal point.
✓
Check Point
2.27 2.28 2.29
Can different types of numeric values be used together in a computation? What does an explicit casting from a double to an int do with the fractional part of the double value? Does casting change the variable being cast? Show the following output: float f = 12.5F; int i = (int)f; System.out.println("f is " + f); System.out.println("i is " + i);
2.30 2.31
If you change (int)(tax * 100) / 100.0 to (int)(tax * 100) / 100 in line 11 in Listing 2.8, what will be the output for the input purchase amount of 197.55? Show the output of the following code: double amount = 5; System.out.println(amount / 2); System.out.println(5 / 2);
2.16 Software Development Process 59
2.16 Software Development Process The software development life cycle is a multistage process that includes requirements specification, analysis, design, implementation, testing, deployment, and maintenance. Developing a software product is an engineering process. Software products, no matter how large or how small, have the same life cycle: requirements specification, analysis, design, implementation, testing, deployment, and maintenance, as shown in Figure 2.3.
Key Point
VideoNote
Software development process
Requirements Specification Input, Process, Output IPO
System Analysis System Design
Implementation
Testing
Deployment
Maintenance
FIGURE 2.3 At any stage of the software development life cycle, it may be necessary to go back to a previous stage to correct errors or deal with other issues that might prevent the software from functioning as expected.
Requirements specification is a formal process that seeks to understand the problem that the software will address and to document in detail what the software system needs to do. This phase involves close interaction between users and developers. Most of the examples in this book are simple, and their requirements are clearly stated. In the real world, however, problems are not always well defined. Developers need to work closely with their customers (the individuals or organizations that will use the software) and study the problem carefully to identify what the software needs to do. System analysis seeks to analyze the data flow and to identify the system’s input and output. When you do analysis, it helps to identify what the output is first, and then figure out what input data you need in order to produce the output. System design is to design a process for obtaining the output from the input. This phase involves the use of many levels of abstraction to break down the problem into manageable components and design strategies for implementing each component. You can view each component as a subsystem that performs a specific function of the system. The essence of system analysis and design is input, process, and output (IPO). Implementation involves translating the system design into programs. Separate programs are written for each component and then integrated to work together. This phase requires the use of a programming language such as Java. The implementation involves coding, selftesting, and debugging (that is, finding errors, called bugs, in the code).
requirements specification
system analysis
system design
IPO implementation
60 Chapter 2 testing
deployment
maintenance
VideoNote
Compute loan payments
Elementary Programming Testing ensures that the code meets the requirements specification and weeds out bugs. An independent team of software engineers not involved in the design and implementation of the product usually conducts such testing. Deployment makes the software available for use. Depending on the type of software, it may be installed on each user’s machine or installed on a server accessible on the Internet. Maintenance is concerned with updating and improving the product. A software product must continue to perform and improve in an ever-evolving environment. This requires periodic upgrades of the product to fix newly discovered bugs and incorporate changes. To see the software development process in action, we will now create a program that computes loan payments. The loan can be a car loan, a student loan, or a home mortgage loan. For an introductory programming course, we focus on requirements specification, analysis, design, implementation, and testing. Stage 1: Requirements Specification The program must satisfy the following requirements: ■
It must let the user enter the interest rate, the loan amount, and the number of years for which payments will be made.
■
It must compute and display the monthly payment and total payment amounts.
Stage 2: System Analysis The output is the monthly payment and total payment, which can be obtained using the following formulas: monthlyPayment =
loanAmount * monthlyInterestRate 1 1 (1 + monthlyInterestRate)numberOfYears * 12
totalPayment = monthlyPayment * numberOfYears * 12 So, the input needed for the program is the monthly interest rate, the length of the loan in years, and the loan amount.
Note The requirements specification says that the user must enter the annual interest rate, the loan amount, and the number of years for which payments will be made. During analysis, however, it is possible that you may discover that input is not sufficient or that some values are unnecessary for the output. If this happens, you can go back and modify the requirements specification.
Note In the real world, you will work with customers from all walks of life. You may develop software for chemists, physicists, engineers, economists, and psychologists, and of course you will not have (or need) complete knowledge of all these fields. Therefore, you don’t have to know how formulas are derived, but given the monthly interest rate, the number of years, and the loan amount, you can compute the monthly payment in this program. You will, however, need to communicate with customers and understand how a mathematical model works for the system.
2.16 Software Development Process 61 Stage 3: System Design During system design, you identify the steps in the program. Step 1.
Prompt the user to enter the annual interest rate, the number of years, and the loan amount.
(The interest rate is commonly expressed as a percentage of the principal for a period of one year. This is known as the annual interest rate.) Step 2.
The input for the annual interest rate is a number in percent format, such as 4.5%. The program needs to convert it into a decimal by dividing it by 100. To obtain the monthly interest rate from the annual interest rate, divide it by 12, since a year has 12 months. So, to obtain the monthly interest rate in decimal format, you need to divide the annual interest rate in percentage by 1200. For example, if the annual interest rate is 4.5%, then the monthly interest rate is 4.5/1200 = 0.00375.
Step 3.
Compute the monthly payment using the preceding formula.
Step 4.
Compute the total payment, which is the monthly payment multiplied by 12 and multiplied by the number of years.
Step 5.
Display the monthly payment and total payment.
Stage 4: Implementation Implementation is also known as coding (writing the code). In the formula, you have to compute (1 + monthlyInterestRate)numberOfYears * 12, which can be obtained using Math.pow(1 + monthlyInterestRate, numberOfYears * 12). Listing 2.9 gives the complete program.
Math.pow(a, b) method
LISTING 2.9 ComputeLoan.java 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
import java.util.Scanner;
import class
public class ComputeLoan { public static void main(String[] args) { // Create a Scanner Scanner input = new Scanner(System.in);
create a Scanner
// Enter annual interest rate in percentage, e.g., 7.25% System.out.print("Enter annual interest rate, e.g., 7.25%: "); double annualInterestRate = input.nextDouble();
enter interest rate
// Obtain monthly interest rate double monthlyInterestRate = annualInterestRate / 1200; // Enter number of years System.out.print( "Enter number of years as an integer, e.g., 5: "); int numberOfYears = input.nextInt();
enter years
// Enter loan amount System.out.print("Enter loan amount, e.g., 120000.95: "); double loanAmount = input.nextDouble();
enter loan amount
// Calculate payment double monthlyPayment = loanAmount * monthlyInterestRate / (1 - 1 / Math.pow(1 + monthlyInterestRate, numberOfYears * 12));
monthlyPayment
62 Chapter 2
Elementary Programming 27 28 29 30 31 32 33 34 35
totalPayment
casting casting
double totalPayment = monthlyPayment * numberOfYears * 12; // Display results System.out.println("The monthly payment is $" + (int)(monthlyPayment * 100) / 100.0); System.out.println("The total payment is $" + (int)(totalPayment * 100) / 100.0); } }
Enter annual interest rate, e.g., 5.75%: 5.75 Enter number of years as an integer, e.g., 5: 15 Enter loan amount, e.g., 120000.95: 250000 The monthly payment is $2076.02 The total payment is $373684.53
line#
10
13
18
22
25
27
variables annualInterestRate
5.75
monthlyInterestRate
0.0047916666666
numberOfYears
15
loanAmount
250000
monthlyPayment
2076.0252175
totalPayment
java.lang package
373684.539
Line 10 reads the annual interest rate, which is converted into the monthly interest rate in line 13. Choose the most appropriate data type for the variable. For example, numberOfYears is best declared as an int (line 18), although it could be declared as a long, float, or double. Note that byte might be the most appropriate for numberOfYears. For simplicity, however, the examples in this book will use int for integer and double for floating-point values. The formula for computing the monthly payment is translated into Java code in lines 25–27. Casting is used in lines 31 and 33 to obtain a new monthlyPayment and totalPayment with two digits after the decimal points. The program uses the Scanner class, imported in line 1. The program also uses the Math class, and you might be wondering why that class isn’t imported into the program. The Math class is in the java.lang package, and all classes in the java.lang package are implicitly imported. Therefore, you don’t need to explicitly import the Math class. Stage 5: Testing After the program is implemented, test it with some sample input data and verify whether the output is correct. Some of the problems may involve many cases, as you will see in later chapters. For these types of problems, you need to design test data that cover all cases.
Tip incremental code and test
The system design phase in this example identified several steps. It is a good approach to code and test these steps incrementally by adding them one at a time. This approach makes it much easier to pinpoint problems and debug the program.
2.17 Case Study: Counting Monetary Units 63 2.32
How would you write the following arithmetic expression? -b + 2b2 - 4ac 2a
✓
Check Point
2.17 Case Study: Counting Monetary Units This section presents a program that breaks a large amount of money into smaller units.
Key Point
Suppose you want to develop a program that changes a given amount of money into smaller monetary units. The program lets the user enter an amount as a double value representing a total in dollars and cents, and outputs a report listing the monetary equivalent in the maximum number of dollars, quarters, dimes, nickels, and pennies, in this order, to result in the minimum number of coins. Here are the steps in developing the program: 1. Prompt the user to enter the amount as a decimal number, such as 11.56. 2. Convert the amount (e.g., 11.56) into cents (1156). 3. Divide the cents by 100 to find the number of dollars. Obtain the remaining cents using the cents remainder 100. 4. Divide the remaining cents by 25 to find the number of quarters. Obtain the remaining cents using the remaining cents remainder 25. 5. Divide the remaining cents by 10 to find the number of dimes. Obtain the remaining cents using the remaining cents remainder 10. 6. Divide the remaining cents by 5 to find the number of nickels. Obtain the remaining cents using the remaining cents remainder 5. 7. The remaining cents are the pennies. 8. Display the result. The complete program is given in Listing 2.10.
LISTING 2.10 ComputeChange.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
import java.util.Scanner;
import class
public class ComputeChange { public static void main(String[] args) { // Create a Scanner Scanner input = new Scanner(System.in); // Receive the amount System.out.print( "Enter an amount in double, for example 11.56: "); double amount = input.nextDouble();
enter input
int remainingAmount = (int)(amount * 100); // Find the number of one dollars int numberOfOneDollars = remainingAmount / 100; remainingAmount = remainingAmount % 100; // Find the number of quarters in the remaining amount int numberOfQuarters = remainingAmount / 25;
dollars
quarters
64 Chapter 2
dimes
nickels
pennies
output
Elementary Programming 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42
remainingAmount = remainingAmount % 25; // Find the number of dimes in the remaining amount int numberOfDimes = remainingAmount / 10; remainingAmount = remainingAmount % 10; // Find the number of nickels in the remaining amount int numberOfNickels = remainingAmount / 5; remainingAmount = remainingAmount % 5; // Find the number of pennies in the remaining amount int numberOfPennies = remainingAmount; // Display results System.out.println("Your amount " + amount + " consists of"); System.out.println(" " + numberOfOneDollars + " dollars"); System.out.println(" " + numberOfQuarters + " quarters "); System.out.println(" " + numberOfDimes + " dimes"); System.out.println(" " + numberOfNickels + " nickels"); System.out.println(" " + numberOfPennies + " pennies"); } }
Enter an amount, for example, 11.56: 11.56 Your amount 11.56 consists of 11 dollars 2 quarters 0 dimes 1 nickels 1 pennies
line#
11
13
16
17
20
21
24
25
28
29
32
variables amount remainingAmount numberOfOneDollars numberOfQuarters numberOfDimes numberOfNickels numberOfPennies
11.56 1156
56
6
6
1
11 2 0 1 1
The variable amount stores the amount entered from the console (line 11). This variable is not changed, because the amount has to be used at the end of the program to display the results. The program introduces the variable remainingAmount (line 13) to store the changing remaining amount. The variable amount is a double decimal representing dollars and cents. It is converted to an int variable remainingAmount, which represents all the cents. For instance, if amount
2.18 Common Errors and Pitfalls 65 is 11.56, then the initial remainingAmount is 1156. The division operator yields the integer part of the division, so 1156 / 100 is 11. The remainder operator obtains the remainder of the division, so 1156 % 100 is 56. The program extracts the maximum number of singles from the remaining amount and obtains a new remaining amount in the variable remainingAmount (lines 16–17). It then extracts the maximum number of quarters from remainingAmount and obtains a new remainingAmount (lines 20–21). Continuing the same process, the program finds the maximum number of dimes, nickels, and pennies in the remaining amount. One serious problem with this example is the possible loss of precision when casting a double amount to an int remainingAmount. This could lead to an inaccurate result. If you try to enter the amount 10.03, 10.03 * 100 becomes 1002.9999999999999. You will find that the program displays 10 dollars and 2 pennies. To fix the problem, enter the amount as an integer value representing cents (see Programming Exercise 2.22).
2.33
Show the output with the input value 1.99.
2.18 Common Errors and Pitfalls Common elementary programming errors often involve undeclared variables, uninitialized variables, integer overflow, unintended integer division, and round-off errors.
loss of precision
✓
Check Point
Key Point
Common Error 1: Undeclared/Uninitialized Variables and Unused Variables A variable must be declared with a type and assigned a value before using it. A common error is not declaring a variable or initializing a variable. Consider the following code: double interestRate = 0.05; double interest = interestrate * 45;
This code is wrong, because interestRate is assigned a value 0.05; but interestrate has not been declared and initialized. Java is case sensitive, so it considers interestRate and interestrate to be two different variables. If a variable is declared, but not used in the program, it might be a potential programming error. So, you should remove the unused variable from your program. For example, in the following code, taxRate is never used. It should be removed from the code. double interestRate = 0.05; double taxRate = 0.05; double interest = interestRate * 45; System.out.println("Interest is " + interest);
If you use an IDE such as Eclipse and NetBeans, you will receive a warning on unused variables. Common Error 2: Integer Overflow Numbers are stored with a limited numbers of digits. When a variable is assigned a value that is too large (in size) to be stored, it causes overflow. For example, executing the following statement causes overflow, because the largest value that can be stored in a variable of the int type is 2147483647. 2147483648 will be too large for an int value. int value = 2147483647 + 1; // value will actually be -2147483648
Likewise, executing the following statement causes overflow, because the smallest value that can be stored in a variable of the int type is -2147483648. -2147483649 is too large in size to be stored in an int variable.
what is overflow?
66 Chapter 2
Elementary Programming int value = -2147483648 - 1; // value will actually be 2147483647
what is underflow?
Java does not report warnings or errors on overflow, so be careful when working with numbers close to the maximum or minimum range of a given type. When a floating-point number is too small (i.e., too close to zero) to be stored, it causes underflow. Java approximates it to zero, so normally you don’t need to be concerned about underflow. Common Error 3: Round-off Errors
floating-point approximation
A round-off error, also called a rounding error, is the difference between the calculated approximation of a number and its exact mathematical value. For example, 1/3 is approximately 0.333 if you keep three decimal places, and is 0.3333333 if you keep seven decimal places. Since the number of digits that can be stored in a variable is limited, round-off errors are inevitable. Calculations involving floating-point numbers are approximated because these numbers are not stored with complete accuracy. For example, System.out.println(1.0 - 0.1 - 0.1 - 0.1 - 0.1 - 0.1);
displays 0.5000000000000001, not 0.5, and System.out.println(1.0 - 0.9);
displays 0.09999999999999998, not 0.1. Integers are stored precisely. Therefore, calculations with integers yield a precise integer result. Common Error 4: Unintended Integer Division Java uses the same divide operator, namely /, to perform both integer and floating-point division. When two operands are integers, the / operator performs an integer division. The result of the operation is an integer. The fractional part is truncated. To force two integers to perform a floating-point division, make one of the integers into a floating-point number. For example, the code in (a) displays that average is 1 and the code in (b) displays that average is 1.5.
int number1 = 1; int number2 = 2; double average = (number1 + number2) / 2; System.out.println(average);
int number1 = 1; int number2 = 2; double average = (number1 + number2) / 2.0; System.out.println(average);
(a)
(b)
Common Pitfall 1: Redundant Input Objects New programmers often write the code to create multiple input objects for each input. For example, the following code reads an integer and a double value. Scanner input = new Scanner(System.in); System.out.print("Enter an integer: "); int v1 = input.nextInt(); Scanner input1 = new Scanner(System.in); System.out.print("Enter a double value: "); double v2 = input1.nextDouble();
BAD CODE
Chapter Summary 67 The code is not wrong, but inefficient. It creates two input objects unnecessarily and may lead to some subtle errors. You should rewrite the code as follows: GOOD CODE Scanner input = new Scanner(System.in); System.out.print("Enter an integer: "); int v1 = input.nextInt(); System.out.print("Enter a double value: "); double v2 = input.nextDouble();
2.34 2.35 2.36 2.37
Can you declare a variable as int and later redeclare it as double? What is an integer overflow? Can floating-point operations cause overflow? Will overflow cause a runtime error? What is a round-off error? Can integer operations cause round-off errors? Can floating-point operations cause round-off errors?
KEY TERMS algorithm 34 assignment operator (=) 42 assignment statement 42 byte type 45 casting 57 constant 43 data type 35 declare variables 35 decrement operator (– –) 55 double type 45 expression 42 final keyword 43 float type 45 floating-point number 35 identifier 39 increment operator (++) 55 incremental code and testing 62 int type 45 IPO 39 literal 48 long type 45
narrowing (of types) 57 operands 46 operator 46 overflow 65 postdecrement 55 postincrement 55 predecrement 55 preincrement 55 primitive data type 35 pseudocode 34 requirements specification scope of a variable 41 short type 45 specific import 38 system analysis 59 system design 59 underflow 66 UNIX epoch 52 variable 35 widening (of types) 57 wildcard import 00
59
CHAPTER SUMMARY 1. Identifiers are names for naming elements such as variables, constants, methods, classes, packages in a program.
2. An identifier is a sequence of characters that consists of letters, digits, underscores (_), and dollar signs ($). An identifier must start with a letter or an underscore. It cannot start with a digit. An identifier cannot be a reserved word. An identifier can be of any length.
3. Variables are used to store data in a program. To declare a variable is to tell the compiler what type of data a variable can hold.
✓
Check Point
68 Chapter 2
Elementary Programming 4. There are two types of import statements: specific import and wildcard import. The specific import specifies a single class in the import statement; the wildcard import imports all the classes in a package.
5. In Java, the equal sign (=) is used as the assignment operator. 6. A variable declared in a method must be assigned a value before it can be used. 7. A named constant (or simply a constant) represents permanent data that never changes. 8. A named constant is declared by using the keyword final. 9. Java provides four integer types (byte, short, int, and long) that represent integers of four different sizes.
10. Java provides two floating-point types (float and
double) that represent floating-
point numbers of two different precisions.
11. Java provides operators that perform numeric operations: + (addition), – (subtraction), * (multiplication), / (division), and % (remainder).
12. Integer arithmetic (/) yields an integer result. 13. The numeric operators in a Java expression are applied the same way as in an arithmetic expression.
14. Java provides the augmented assignment operators += (addition assignment), –= (subtraction assignment), *= (multiplication assignment), /= (division assignment), and %= (remainder assignment).
15. The increment operator (++) and the decrement operator (––) increment or decrement a variable by 1.
16. When evaluating an expression with values of mixed types, Java automatically converts the operands to appropriate types.
17. You can explicitly convert a value from one type to another using the (type)value notation.
18. Casting a variable of a type with a small range to a variable of a type with a larger range is known as widening a type.
19. Casting a variable of a type with a large range to a variable of a type with a smaller range is known as narrowing a type.
20. Widening a type can be performed automatically without explicit casting. Narrowing a type must be performed explicitly.
21. In computer science, midnight of January 1, 1970, is known as the UNIX epoch.
QUIZ Answer the quiz for this chapter online at www.cs.armstrong.edu/liang/intro10e/quiz.html.
Programming Exercises 69
PROGRAMMING EXERCISES Debugging TIP The compiler usually gives a reason for a syntax error. If you don’t know how to correct it, compare your program closely, character by character, with similar examples in the text.
learn from examples
Pedagogical Note Instructors may ask you to document your analysis and design for selected exercises. Use your own words to analyze the problem, including the input, output, and what needs to be computed, and describe how to solve the problem in pseudocode.
Sections 2.2–2.12
2.1
(Convert Celsius to Fahrenheit) Write a program that reads a Celsius degree in a double value from the console, then converts it to Fahrenheit and displays the result. The formula for the conversion is as follows: fahrenheit = (9 / 5) * celsius + 32
Hint: In Java, 9 / 5 is 1, but 9.0 / 5 is 1.8. Here is a sample run:
Enter a degree in Celsius: 43 43 Celsius is 109.4 Fahrenheit
2.2
(Compute the volume of a cylinder) Write a program that reads in the radius and length of a cylinder and computes the area and volume using the following formulas: area = radius * radius * p volume = area * length
Here is a sample run:
Enter the radius and length of a cylinder: 5.5 12 The area is 95.0331 The volume is 1140.4
2.3
(Convert feet into meters) Write a program that reads a number in feet, converts it to meters, and displays the result. One foot is 0.305 meter. Here is a sample run:
Enter a value for feet: 16.5 16.5 feet is 5.0325 meters
document analysis and design
70 Chapter 2
Elementary Programming 2.4
(Convert pounds into kilograms) Write a program that converts pounds into kilograms. The program prompts the user to enter a number in pounds, converts it to kilograms, and displays the result. One pound is 0.454 kilograms. Here is a sample run:
Enter a number in pounds: 55.5 55.5 pounds is 25.197 kilograms
*2.5
(Financial application: calculate tips) Write a program that reads the subtotal and the gratuity rate, then computes the gratuity and total. For example, if the user enters 10 for subtotal and 15% for gratuity rate, the program displays $1.5 as gratuity and $11.5 as total. Here is a sample run:
Enter the subtotal and a gratuity rate: 10 15 The gratuity is $1.5 and total is $11.5
**2.6
(Sum the digits in an integer) Write a program that reads an integer between 0 and 1000 and adds all the digits in the integer. For example, if an integer is 932, the sum of all its digits is 14. Hint: Use the % operator to extract digits, and use the / operator to remove the extracted digit. For instance, 932 % 10 = 2 and 932 / 10 = 93. Here is a sample run:
Enter a number between 0 and 1000: 999 The sum of the digits is 27
*2.7
(Find the number of years) Write a program that prompts the user to enter the minutes (e.g., 1 billion), and displays the number of years and days for the minutes. For simplicity, assume a year has 365 days. Here is a sample run:
Enter the number of minutes: 1000000000 1000000000 minutes is approximately 1902 years and 214 days
*2.8
(Current time) Listing 2.7, ShowCurrentTime.java, gives a program that displays the current time in GMT. Revise the program so that it prompts the user to enter the time zone offset to GMT and displays the time in the specified time zone. Here is a sample run:
Enter the time zone offset to GMT: −5 The current time is 4:50:34
Programming Exercises 71 2.9
(Physics: acceleration) Average acceleration is defined as the change of velocity divided by the time taken to make the change, as shown in the following formula: v1 - v0 t Write a program that prompts the user to enter the starting velocity v0 in meters/ second, the ending velocity v1 in meters/second, and the time span t in seconds, and displays the average acceleration. Here is a sample run: a =
Enter v0, v1, and t: 5.5 50.9 4.5 The average acceleration is 10.0889
2.10
(Science: calculating energy) Write a program that calculates the energy needed to heat water from an initial temperature to a final temperature. Your program should prompt the user to enter the amount of water in kilograms and the initial and final temperatures of the water. The formula to compute the energy is Q = M * (finalTemperature – initialTemperature) * 4184
where M is the weight of water in kilograms, temperatures are in degrees Celsius, and energy Q is measured in joules. Here is a sample run:
Enter the amount of water in kilograms: 55.5 Enter the initial temperature: 3.5 Enter the final temperature: 10.5 The energy needed is 1625484.0
2.11
(Population projection) Rewrite Programming Exercise 1.11 to prompt the user to enter the number of years and displays the population after the number of years. Use the hint in Programming Exercise 1.11 for this program. The population should be cast into an integer. Here is a sample run of the program:
Enter the number of years: 5 The population in 5 years is 325932970
2.12
(Physics: finding runway length) Given an airplane’s acceleration a and take-off speed v, you can compute the minimum runway length needed for an airplane to take off using the following formula: v2 2a Write a program that prompts the user to enter v in meters/second (m/s) and the acceleration a in meters/second squared (m/s2), and displays the minimum runway length. Here is a sample run: length =
Enter speed and acceleration: 60 3.5 The minimum runway length for this airplane is 514.286
72 Chapter 2
Elementary Programming **2.13
(Financial application: compound value) Suppose you save $100 each month into a savings account with the annual interest rate 5%. Thus, the monthly interest rate is 0.05/12 = 0.00417. After the first month, the value in the account becomes 100 * (1 + 0.00417) = 100.417
After the second month, the value in the account becomes (100 + 100.417) * (1 + 0.00417) = 201.252
After the third month, the value in the account becomes (100 + 201.252) * (1 + 0.00417) = 302.507
and so on. Write a program that prompts the user to enter a monthly saving amount and displays the account value after the sixth month. (In Exercise 5.30, you will use a loop to simplify the code and display the account value for any month.)
Enter the monthly saving amount: 100 After the sixth month, the account value is $608.81
*2.14 (Health application: computing BMI) Body Mass Index (BMI) is a measure of health on weight. It can be calculated by taking your weight in kilograms and dividing by the square of your height in meters. Write a program that prompts the user to enter a weight in pounds and height in inches and displays the BMI. Note that one pound is 0.45359237 kilograms and one inch is 0.0254 meters. Here is a sample run:
VideoNote
Compute BMI
Enter weight in pounds: 95.5 Enter height in inches: 50 BMI is 26.8573
2.15
(Geometry: distance of two points) Write a program that prompts the user to enter two points (x1, y1) and (x2, y2) and displays their distance between them. The formula for computing the distance is 2(x2 - x1)2 + (y2 - y1)2. Note that you can use Math.pow(a, 0.5) to compute 2a. Here is a sample run: Enter x1 and y1: 1.5 -3.4 Enter x2 and y2: 4 5 The distance between the two points is 8.764131445842194
2.16
(Geometry: area of a hexagon) Write a program that prompts the user to enter the side of a hexagon and displays its area. The formula for computing the area of a hexagon is Area =
323 2 s, 2
Programming Exercises 73 where s is the length of a side. Here is a sample run:
Enter the side: 5.5 The area of the hexagon is 78.5895
*2.17
(Science: wind-chill temperature) How cold is it outside? The temperature alone is not enough to provide the answer. Other factors including wind speed, relative humidity, and sunshine play important roles in determining coldness outside. In 2001, the National Weather Service (NWS) implemented the new wind-chill temperature to measure the coldness using temperature and wind speed. The formula is twc = 35.74 + 0.6215ta - 35.75v 0.16 + 0.4275tav 0.16 where ta is the outside temperature measured in degrees Fahrenheit and v is the speed measured in miles per hour. twc is the wind-chill temperature. The formula cannot be used for wind speeds below 2 mph or temperatures below -58 ºF or above 41ºF. Write a program that prompts the user to enter a temperature between -58 ºF and 41ºF and a wind speed greater than or equal to 2 and displays the wind-chill temperature. Use Math.pow(a, b) to compute v 0.16. Here is a sample run:
Enter the temperature in Fahrenheit between -58°F and 41°F: 5.3 Enter the wind speed (>=2) in miles per hour: 6 The wind chill index is -5.56707
2.18
(Print a table) Write a program that displays the following table. Cast floatingpoint numbers into integers. a 1 2 3 4 5
*2.19
b 2 3 4 5 6
pow(a, b) 1 8 81 1024 15625
(Geometry: area of a triangle) Write a program that prompts the user to enter three points (x1, y1), (x2, y2), (x3, y3) of a triangle and displays its area. The formula for computing the area of a triangle is s = (side1 + side2 + side3)/2; area = 2s(s - side1)(s - side2)(s - side3) Here is a sample run:
Enter three points for a triangle: 1.5 -3.4 4.6 5 9.5 -3.4 The area of the triangle is 33.6
74 Chapter 2
Elementary Programming Sections 2.13–2.17
*2.20
(Financial application: calculate interest) If you know the balance and the annual percentage interest rate, you can compute the interest on the next monthly payment using the following formula: interest = balance * (annualInterestRate/1200) Write a program that reads the balance and the annual percentage interest rate and displays the interest for the next month. Here is a sample run:
Enter balance and interest rate (e.g., 3 for 3%): 1000 3.5 The interest is 2.91667
*2.21
(Financial application: calculate future investment value) Write a program that reads in investment amount, annual interest rate, and number of years, and displays the future investment value using the following formula: futureInvestmentValue = investmentAmount * (1 + monthlyInterestRate)numberOfYears*12 For example, if you enter amount 1000, annual interest rate 3.25%, and number of years 1, the future investment value is 1032.98. Here is a sample run:
Enter investment amount: 1000.56 Enter annual interest rate in percentage: 4.25 Enter number of years: 1 Accumulated value is $1043.92
*2.22
*2.23
(Financial application: monetary units) Rewrite Listing 2.10, ComputeChange .java, to fix the possible loss of accuracy when converting a double value to an int value. Enter the input as an integer whose last two digits represent the cents. For example, the input 1156 represents 11 dollars and 56 cents. (Cost of driving) Write a program that prompts the user to enter the distance to drive, the fuel efficiency of the car in miles per gallon, and the price per gallon, and displays the cost of the trip. Here is a sample run:
Enter the driving distance: 900.5 Enter miles per gallon: 25.5 Enter price per gallon: 3.55 The cost of driving is $125.36
CHAPTER
3 SELECTIONS Objectives ■
To declare boolean variables and write Boolean expressions using relational operators (§3.2).
■
To implement selection control using one-way if statements (§3.3).
■
To implement selection control using two-way if-else statements (§3.4).
■
To implement selection control using nested if and multi-way if statements (§3.5).
■
To avoid common errors and pitfalls in if statements (§3.6).
■
To generate random numbers using the Math.random() method (§3.7).
■
To program using selection statements for a variety of examples (SubtractionQuiz, BMI, ComputeTax) (§§3.7–3.9).
■
To combine conditions using logical operators (!, &&, ||, and ^) (§3.10).
■
To program using selection statements with combined conditions (LeapYear, Lottery) (§§3.11–3.12).
■
To implement selection control using switch statements (§3.13).
■
To write expressions using the conditional expression (§3.14).
■
To examine the rules governing operator precedence and associativity (§3.15).
■
To apply common techniques to debug errors (§3.16).
76 Chapter 3
Selections
3.1 Introduction problem
Key Point
selection statements
The program can decide which statements to execute based on a condition. If you enter a negative value for radius in Listing 2.2, ComputeAreaWithConsoleInput.java, the program displays an invalid result. If the radius is negative, you don’t want the program to compute the area. How can you deal with this situation? Like all high-level programming languages, Java provides selection statements: statements that let you choose actions with alternative courses. You can use the following selection statement to replace lines 12–17 in Listing 2.2: if (radius < 0) { System.out.println("Incorrect input"); } else { area = radius * radius * 3.14159; System.out.println("Area is " + area); }
Selection statements use conditions that are Boolean expressions. A Boolean expression is an expression that evaluates to a Boolean value: true or false. We now introduce Boolean types and relational operators.
Boolean expression Boolean value
3.2 boolean Data Type Key Point boolean data type
relational operators
The boolean data type declares a variable with the value either true or false. How do you compare two values, such as whether a radius is greater than 0, equal to 0, or less than 0? Java provides six relational operators (also known as comparison operators), shown in Table 3.1, which can be used to compare two values (assume radius is 5 in the table).
TABLE 3.1 Relational Operators Java Operator
Mathematics Symbol
Name
Example (radius is 5)
Result
<
<
less than
radius < 0
false
<=
≤
less than or equal to
radius <= 0
false
>
>
greater than
radius > 0
true
>=
≥
greater than or equal to
radius >= 0
true
==
=
equal to
radius == 0
false
!=
≠
not equal to
radius != 0
true
Caution == vs. =
The equality testing operator is two equal signs (==), not a single equal sign (=). The latter symbol is for assignment.
The result of the comparison is a Boolean value: true or false. For example, the following statement displays true: double radius = 1; System.out.println(radius > 0); Boolean variable
A variable that holds a Boolean value is known as a Boolean variable. The boolean data type is used to declare Boolean variables. A boolean variable can hold one of the
3.2 boolean Data Type 77 two values: true or false. For example, the following statement assigns true to the variable lightsOn: boolean lightsOn = true;
true and false are literals, just like a number such as 10. They are treated as reserved words and cannot be used as identifiers in the program. Suppose you want to develop a program to let a first-grader practice addition. The program randomly generates two single-digit integers, number1 and number2, and displays to the student a question such as “What is 1 + 7?,” as shown in the sample run in Listing 3.1. After the student types the answer, the program displays a message to indicate whether it is true or false. There are several ways to generate random numbers. For now, generate the first integer using System.currentTimeMillis() % 10 and the second using System.currentTimeMillis() / 7 % 10. Listing 3.1 gives the program. Lines 5–6 generate two numbers, number1 and number2. Line 14 obtains an answer from the user. The answer is graded in line 18 using a Boolean expression number1 + number2 == answer.
Boolean literals
VideoNote
Program addition quiz
LISTING 3.1 AdditionQuiz.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
import java.util.Scanner; public class AdditionQuiz { public static void main(String[] args) { int number1 = (int)(System.currentTimeMillis() % 10); int number2 = (int)(System.currentTimeMillis() / 7 % 10);
generate number1 generate number2
// Create a Scanner Scanner input = new Scanner(System.in); System.out.print( "What is " + number1 + " + " + number2 + "? ");
show question
int number = input.nextInt(); System.out.println( number1 + " + " + number2 + " = " + answer + " is " + (number1 + number2 == answer)); } }
What is 1 + 7? 8 1 + 7 = 8 is true
What is 4 + 8? 9 4 + 8 = 9 is false
line# 5 6 14 16
number1
number2
answer
output
4 8 9 4 + 8 = 9 is false
display result
78 Chapter 3
Selections
✓
Check Point
3.1 3.2
List six relational operators. Assuming that x is 1, show the result of the following Boolean expressions: (x (x (x (x (x
3.3
> 0) < 0) != 0) >= 0) != 1)
Can the following conversions involving casting be allowed? Write a test program to verify your answer. boolean b = true; i = (int)b; int i = 1; boolean b = (boolean)i;
3.3 if Statements Key Point why if statement?
An if statement is a construct that enables a program to specify alternative paths of execution. The preceding program displays a message such as “6 + 2 = 7 is false.” If you wish the message to be “6 + 2 = 7 is incorrect,” you have to use a selection statement to make this minor change. Java has several types of selection statements: one-way if statements, two-way if-else statements, nested if statements, multi-way if-else statements, switch statements, and conditional expressions. A one-way if statement executes an action if and only if the condition is true. The syntax for a one-way if statement is: if (boolean-expression) { statement(s); }
if statement
The flowchart in Figure 3.1a illustrates how Java executes the syntax of an if statement. A flowchart is a diagram that describes an algorithm or process, showing the steps as boxes of various kinds, and their order by connecting these with arrows. Process operations are represented in these boxes, and arrows connecting them represent the flow of control. A diamond box denotes a Boolean condition and a rectangle box represents statements.
flowchart
booleanexpression
true
false
(radius >= 0)
false
true
Statement(s)
area = radius * radius * PI; System.out.println("The area for the circle of" + " radius " + radius + " is " + area);
(a)
(b)
FIGURE 3.1 An if statement executes statements if the boolean-expression evaluates to true.
3.3 if Statements 79 If the boolean-expression evaluates to true, the statements in the block are executed. As an example, see the following code: if (radius >= 0) { area = radius * radius * PI; System.out.println("The area for the circle of radius " + radius + " is " + area); }
The flowchart of the preceding statement is shown in Figure 3.1b. If the value of radius is greater than or equal to 0, then the area is computed and the result is displayed; otherwise, the two statements in the block will not be executed. The boolean-expression is enclosed in parentheses. For example, the code in (a) is wrong. It should be corrected, as shown in (b).
if i > 0 { System.out.println("i is positive"); }
if (i > 0) { System.out.println("i is positive"); }
(a) Wrong
(b) Correct
The block braces can be omitted if they enclose a single statement. For example, the following statements are equivalent.
if (i > 0) { System.out.println("i is positive"); }
Equivalent
if (i > 0) System.out.println("i is positive");
(a)
(b)
Note Omitting braces makes the code shorter, but it is prone to errors. It is a common mistake to forget the braces when you go back to modify the code that omits the braces.
Omitting braces or not
Listing 3.2 gives a program that prompts the user to enter an integer. If the number is a multiple of 5, the program displays HiFive. If the number is divisible by 2, it displays HiEven.
LISTING 3.2 SimpleIfDemo.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
import java.util.Scanner; public class SimpleIfDemo { public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.println("Enter an integer: "); int number = input.nextInt();
} }
enter input
if (number % 5 == 0) System.out.println("HiFive");
check 5
if (number % 2 == 0) System.out.println("HiEven");
check even
80 Chapter 3
Selections Enter an integer: HiEven
Enter an integer: HiFive HiEven
4
30
The program prompts the user to enter an integer (lines 6–7) and displays HiFive if it is divisible by 5 (lines 9–10) and HiEven if it is divisible by 2 (lines 12–13).
✓
Check Point
3.4 3.5
Write an if statement that assigns 1 to x if y is greater than 0. Write an if statement that increases pay by 3% if score is greater than 90.
3.4 Two-Way if-else Statements Key Point
An if-else statement decides the execution path based on whether the condition is true or false. A one-way if statement performs an action if the specified condition is true. If the condition is false, nothing is done. But what if you want to take alternative actions when the condition is false? You can use a two-way if-else statement. The actions that a two-way if-else statement specifies differ based on whether the condition is true or false. Here is the syntax for a two-way if-else statement: if (boolean-expression) { statement(s)-for-the-true-case; } else { statement(s)-for-the-false-case; }
The flowchart of the statement is shown in Figure 3.2.
true
Statement(s) for the true case
booleanexpression
false
Statement(s) for the false case
FIGURE 3.2 An if-else statement executes statements for the true case if the Booleanexpression evaluates to true; otherwise, statements for the false case are executed.
3.5 Nested if and Multi-Way if-else Statements 81 If the boolean-expression evaluates to true, the statement(s) for the true case are executed; otherwise, the statement(s) for the false case are executed. For example, consider the following code: if (radius >= 0) { area = radius * radius * PI; System.out.println("The area for the circle of radius " + radius + " is " + area); } else { System.out.println("Negative input"); }
two-way if-else statement
If radius >= 0 is true, area is computed and displayed; if it is false, the message "Negative input" is displayed. As usual, the braces can be omitted if there is only one statement within them. The braces enclosing the System.out.println("Negative input") statement can therefore be omitted in the preceding example. Here is another example of using the if-else statement. The example checks whether a number is even or odd, as follows: if (number % 2 == 0) System.out.println(number + " is even."); else System.out.println(number + " is odd.");
3.6 3.7
Write an if statement that increases pay by 3% if score is greater than 90, otherwise increases pay by 1%. What is the output of the code in (a) and (b) if number is 30? What if number is 35?
if (number % 2 == 0) System.out.println(number + " is even."); System.out.println(number + " is odd.");
✓
Check Point
if (number % 2 == 0) System.out.println(number + " is even."); else System.out.println(number + " is odd.");
(a)
(b)
3.5 Nested if and Multi-Way if-else Statements An if statement can be inside another if statement to form a nested if statement. The statement in an if or if-else statement can be any legal Java statement, including another if or if-else statement. The inner if statement is said to be nested inside the outer if statement. The inner if statement can contain another if statement; in fact, there is no limit to the depth of the nesting. For example, the following is a nested if statement: if (i > k) { if (j > k) System.out.println("i and j are greater than k"); } else System.out.println("i is less than or equal to k");
The if (j > k) statement is nested inside the if (i > k) statement. The nested if statement can be used to implement multiple alternatives. The statement given in Figure 3.3a, for instance, prints a letter grade according to the score, with multiple alternatives.
Key Point nested if statement
82 Chapter 3
Selections if (score >= 90.0) System.out.print("A"); else if (score >= 80.0) System.out.print("B"); else if (score >= 70.0) System.out.print("C"); else if (score >= 60.0) System.out.print("D"); else System.out.print("F");
Equivalent
if (score >= 90.0) System.out.print("A"); else if (score >= 80.0) System.out.print("B"); else if (score >= 70.0) System.out.print("C"); else if (score >= 60.0) System.out.print("D"); else System.out.print("F");
This is better
(a)
(b)
FIGURE 3.3 A preferred format for multiple alternatives is shown in (b) using a multi-way if-else statement.
The execution of this if statement proceeds as shown in Figure 3.4. The first condition (score >= 90.0) is tested. If it is true, the grade is A. If it is false, the second condition (score >= 80.0) is tested. If the second condition is true, the grade is B. If that condition is false, the third condition and the rest of the conditions (if necessary) are tested until a condition is met or all of the conditions prove to be false. If all of the conditions are false, the grade is F. Note that a condition is tested only when all of the conditions that come before it are false.
score >= 90
true
false
score >= 80
false
grade is A true
score >= 70
false
grade is B true
score >= 60
false
grade is C true
grade is D grade is F
FIGURE 3.4 You can use a multi-way if-else statement to assign a grade.
3.6 Common Errors and Pitfalls 83 The if statement in Figure 3.3a is equivalent to the if statement in Figure 3.3b. In fact, Figure 3.3b is the preferred coding style for multiple alternative if statements. This style, called multi-way if-else statements, avoids deep indentation and makes the program easy to read.
3.8
Suppose x = 3 and y = 2; show the output, if any, of the following code. What is the output if x = 3 and y = 4? What is the output if x = 2 and y = 2? Draw a flowchart of the code.
multi-way if statement
✓
Check Point
if (x > 2) { if (y > 2) { z = x + y; System.out.println("z is " + z); } } else System.out.println("x is " + x);
3.9
Suppose x = 2 and y = 3. Show the output, if any, of the following code. What is the output if x = 3 and y = 2? What is the output if x = 3 and y = 3? if (x > 2) if (y > 2) { int z = x + y; System.out.println("z is " + z); } else System.out.println("x is " + x);
3.10
What is wrong in the following code? if (score >= 60.0) System.out.println("D"); else if (score >= 70.0) System.out.println("C"); else if (score >= 80.0) System.out.println("B"); else if (score >= 90.0) System.out.println("A"); else System.out.println("F");
3.6 Common Errors and Pitfalls Forgetting necessary braces, ending an if statement in the wrong place, mistaking == for =, and dangling else clauses are common errors in selection statements. Duplicated statements in if-else statements and testing equality of double values are common pitfalls. The following errors are common among new programmers. Common Error 1: Forgetting Necessary Braces The braces can be omitted if the block contains a single statement. However, forgetting the braces when they are needed for grouping multiple statements is a common programming error. If you modify the code by adding new statements in an if statement without braces, you will have to insert the braces. For example, the following code in (a) is wrong. It should be written with braces to group multiple statements, as shown in (b).
Key Point
84 Chapter 3
Selections if (radius >= 0) area = radius * radius * PI; System.out.println("The area " + " is " + area);
if (radius >= 0) { area = radius * radius * PI; System.out.println("The area " + " is " + area); }
(a) Wrong
(b) Correct
Common Error 2: Wrong Semicolon at the if Line Adding a semicolon at the end of an if line, as shown in (a) below, is a common mistake. Logic error
Empty block
if (radius >= 0); { area = radius * radius * PI; System.out.println("The area " + " is " + area); }
Equivalent
if (radius >= 0) { }; { area = radius * radius * PI; System.out.println("The area " + " is " + area); }
(a)
(b)
This mistake is hard to find, because it is neither a compile error nor a runtime error; it is a logic error. The code in (a) is equivalent to that in (b) with an empty block. This error often occurs when you use the next-line block style. Using the end-of-line block style can help prevent this error. Common Error 3: Redundant Testing of Boolean Values To test whether a boolean variable is true or false in a test condition, it is redundant to use the equality testing operator like the code in (a):
if (even == true) System.out.println( "It is even."); (a)
Equivalent This is better
if (even) System.out.println( "It is even."); (b)
Instead, it is better to test the boolean variable directly, as shown in (b). Another good reason for doing this is to avoid errors that are difficult to detect. Using the = operator instead of the == operator to compare the equality of two items in a test condition is a common error. It could lead to the following erroneous statement: if (even = true) System.out.println("It is even.");
This statement does not have compile errors. It assigns true to even, so that even is always true. Common Error 4: Dangling else Ambiguity The code in (a) below has two if clauses and one else clause. Which if clause is matched by the else clause? The indentation indicates that the else clause matches the first if clause.
3.6 Common Errors and Pitfalls 85 However, the else clause actually matches the second if clause. This situation is known as the dangling else ambiguity. The else clause always matches the most recent unmatched if clause in the same block. So, the statement in (a) is equivalent to the code in (b).
int i = 1, j = 2, k = 3; if (i > j) if (i > k) System.out.println("A"); else System.out.println("B");
Equivalent
This is better with correct indentation
int i = 1, j = 2, k = 3; if (i > j) if (i > k) System.out.println("A"); else System.out.println("B");
(a)
(b)
Since (i > j) is false, nothing is displayed from the statements in (a) and (b). To force the else clause to match the first if clause, you must add a pair of braces: int i = 1, j = 2, k = 3; if (i > j) { if (i > k) System.out.println("A"); } else System.out.println("B");
This statement displays B. Common Error 5: Equality Test of Two Floating-Point Values As discussed in Common Error 3 in Section 2.18, floating-point numbers have a limited precision and calculations; involving floating-point numbers can introduce round-off errors. So, equality test of two floating-point values is not reliable. For example, you expect the following code to display true, but surprisingly it displays false. double x = 1.0 - 0.1 - 0.1 - 0.1 - 0.1 - 0.1; System.out.println(x == 0.5);
Here, x is not exactly 0.5, but is 0.5000000000000001. You cannot reliably test equality of two floating-point values. However, you can compare whether they are close enough by testing whether the difference of the two numbers is less than some threshold. That is, two numbers x and y are very close if |x−y| < e for a very small value, e. e, a Greek letter pronounced epsilon, is commonly used to denote a very small value. Normally, you set e to 10-14 for comparing two values of the double type and to 10-7 for comparing two values of the float type. For example, the following code final double EPSILON = 1E-14; double x = 1.0 - 0.1 - 0.1 - 0.1 - 0.1 - 0.1; if (Math.abs(x - 0.5) < EPSILON) System.out.println(x + " is approximately 0.5");
will display that 0.5000000000000001 is approximately 0.5
The Math.abs(a) method can be used to return the absolute value of a.
dangling else ambiguity
86 Chapter 3
Selections Common Pitfall 1: Simplifying Boolean Variable Assignment Often, new programmers write the code that assigns a test condition to a boolean variable like the code in (a): if (number % 2 == 0) even = true; else even = false;
Equivalent
boolean even = number % 2 == 0;
This is shorter
(a)
(b)
This is not an error, but it should be better written as shown in (b). Common Pitfall 2: Avoiding Duplicate Code in Different Cases Often, new programmers write the duplicate code in different cases that should be combined in one place. For example, the highlighted code in the following statement is duplicated. if (inState) { tuition = 5000; System.out.println("The tuition is " + tuition); } else { tuition = 15000; System.out.println("The tuition is " + tuition); }
This is not an error, but it should be better written as follows: if (inState) { tuition = 5000; } else { tuition = 15000; } System.out.println("The tuition is " + tuition);
The new code removes the duplication and makes the code easy to maintain, because you only need to change in one place if the print statement is modified.
✓
Check Point
if (i > 0) if (j > 0) x = 0; else if (k > 0) y = 0; else z = 0;
3.11
Which of the following statements are equivalent? Which ones are correctly indented?
if (i > 0) { if (j > 0) x = 0; else if (k > 0) y = 0; } else z = 0; (b)
(a)
3.12
if (i > 0) if (j > 0) x = 0; else if (k > 0) y = 0; else z = 0;
if (i > 0) if (j > 0) x = 0; else if (k > 0) y = 0; else z = 0;
(c)
Rewrite the following statement using a Boolean expression: if (count % 10 == 0) newLine = true; else newLine = false;
(d)
3.7 Generating Random Numbers 87 3.13
Are the following statements correct? Which one is better?
if (age < 16) System.out.println ("Cannot get a driver's license"); if (age >= 16) System.out.println ("Can get a driver's license");
if (age < 16) System.out.println ("Cannot get a driver's license"); else System.out.println ("Can get a driver's license"); (b)
(a)
3.14
What is the output of the following code if number is 14, 15, or 30?
if (number % 2 == 0) System.out.println (number + " is even"); if (number % 5 == 0) System.out.println (number + " is multiple of 5");
if (number % 2 == 0) System.out.println (number + " is even"); else if (number % 5 == 0) System.out.println (number + " is multiple of 5");
(a)
(b)
3.7 Generating Random Numbers You can use Math.random() to obtain a random double value between 0.0 and 1.0, excluding 1.0. Suppose you want to develop a program for a first-grader to practice subtraction. The program randomly generates two single-digit integers, number1 and number2, with number1 >= number2, and it displays to the student a question such as “What is 9 - 2?” After the student enters the answer, the program displays a message indicating whether it is correct. The previous programs generate random numbers using System.currentTimeMillis(). A better approach is to use the random() method in the Math class. Invoking this method returns a random double value d such that 0.0 … d 6 1.0. Thus, (int)(Math.random() * 10) returns a random single-digit integer (i.e., a number between 0 and 9). The program can work as follows:
Key Point
VideoNote
Program subtraction quiz random() method
1. Generate two single-digit integers into number1 and number2. 2. If number1 < number2, swap number1 with number2. 3. Prompt the student to answer, "What is number1 – number2?" 4. Check the student’s answer and display whether the answer is correct. The complete program is shown in Listing 3.3.
LISTING 3.3 SubtractionQuiz.java 1 2 3 4 5 6 7 8 9 10 11
import java.util.Scanner; public class SubtractionQuiz { public static void main(String[] args) { // 1. Generate two random single-digit integers int number1 = (int)(Math.random() * 10); int number2 = (int)(Math.random() * 10); // 2. If number1 < number2, swap number1 with number2 if (number1 < number2) { int temp = number1;
random number
88 Chapter 3
Selections 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
get answer
check the answer
number1 = number2; number2 = temp; } // 3. Prompt the student to answer ”What is number1 – number2?” System.out.print ("What is " + number1 + " - " + number2 + "? "); Scanner input = new Scanner(System.in); int answer = input.nextInt(); // 4. Grade the answer and display the result if (number1 - number2 == answer) System.out.println("You are correct!"); else { System.out.println("Your answer is wrong."); System.out.println(number1 + " - " + number2 + " should be " + (number1 - number2)); } } }
What is 6 - 6? 0 You are correct!
What is 9 - 2? 5 Your answer is wrong 9 - 2 is 7
line# 6
number1
number2
7
13
answer
output
2 9
11 12
temp
2 9 2
20
5
26
Your answer is wrong 9 – 2 should be 7
To swap two variables number1 and number2, a temporary variable temp (line 11) is used to first hold the value in number1. The value in number2 is assigned to number1 (line 12), and the value in temp is assigned to number2 (line 13).
✓
Check Point
3.15
Which of the following is a possible output from invoking Math.random()? 323.4, 0.5, 34, 1.0, 0.0, 0.234
3.16
a. How do you generate a random integer i such that 0 … i 6 20? b. How do you generate a random integer i such that 10 … i 6 20? c. How do you generate a random integer i such that 10 … i … 50? d. Write an expression that returns 0 or 1 randomly.
3.8 Case Study: Computing Body Mass Index 89
3.8 Case Study: Computing Body Mass Index You can use nested if statements to write a program that interprets body mass index. Body Mass Index (BMI) is a measure of health based on height and weight. It can be calculated by taking your weight in kilograms and dividing it by the square of your height in meters. The interpretation of BMI for people 20 years or older is as follows:
BMI
Interpretation
BMI < 18.5 18.5 ≤ BMI < 25.0 25.0 ≤ BMI < 30.0 30.0 ≤ BMI
Underweight Normal Overweight Obese
Key Point
Write a program that prompts the user to enter a weight in pounds and height in inches and displays the BMI. Note that one pound is 0.45359237 kilograms and one inch is 0.0254 meters. Listing 3.4 gives the program.
LISTING 3.4 ComputeAndInterpretBMI.java 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
import java.util.Scanner; public class ComputeAndInterpretBMI { public static void main(String[] args) { Scanner input = new Scanner(System.in); // Prompt the user to enter weight in pounds System.out.print("Enter weight in pounds: "); double weight = input.nextDouble();
input weight
// Prompt the user to enter height in inches System.out.print("Enter height in inches: "); double height = input.nextDouble();
input height
final double KILOGRAMS_PER_POUND = 0.45359237; // Constant final double METERS_PER_INCH = 0.0254; // Constant // Compute BMI double weightInKilograms = weight * KILOGRAMS_PER_POUND; double heightInMeters = height * METERS_PER_INCH; double bmi = weightInKilograms / (heightInMeters * heightInMeters); // Display result System.out.println("BMI is " + bmi); if (bmi < 18.5) System.out.println("Underweight"); else if (bmi < 25) System.out.println("Normal"); else if (bmi < 30) System.out.println("Overweight"); else System.out.println("Obese"); } }
compute bmi
display output
90 Chapter 3
Selections Enter weight in pounds: 146 Enter height in inches: 70 BMI is 20.948603801493316 Normal
line# 9 13
weight
height
weightInKilograms
heightInMeters
bmi
output
146 70
19
66.22448602
20
1.778
21
20.9486
25
BMI is 20.95
31
Normal
The constants KILOGRAMS_PER_POUND and METERS_PER_INCH are defined in lines 15–16. Using constants here makes programs easy to read. You should test the input that covers all possible cases for BMI to ensure that the program works for all cases.
test all cases
3.9 Case Study: Computing Taxes Key Point
VideoNote
Use multi-way if-else statements
You can use nested if statements to write a program for computing taxes. The United States federal personal income tax is calculated based on filing status and taxable income. There are four filing statuses: single filers, married filing jointly or qualified widow(er), married filing separately, and head of household. The tax rates vary every year. Table 3.2 shows the rates for 2009. If you are, say, single with a taxable income of $10,000, the first $8,350 is taxed at 10% and the other $1,650 is taxed at 15%, so, your total tax is $1,082.50.
TABLE 3.2 2009 U.S. Federal Personal Tax Rates Marginal Tax Rate 10%
Married Filing Jointly or Qualifying Widow(er)
Single $0 – $8,350
$0 – $16,700
Married Filing Separately $0 – $8,350
Head of Household $0 – $11,950
15%
$8,351 – $33,950
$16,701 – $67,900
$8,351 – $33,950
$11,951 – $45,500
25%
$33,951 – $82,250
$67,901 – $137,050
$33,951 – $68,525
$45,501 – $117,450
28%
$82,251 – $171,550
$137,051 – $208,850
$68,526 – $104,425
$117,451 – $190,200
33%
$171,551 – $372,950
$208,851 – $372,950
$104,426 – $186,475
$190,201 – $372,950
35%
$372,951+
$372,951+
$186,476+
$372,951+
You are to write a program to compute personal income tax. Your program should prompt the user to enter the filing status and taxable income and compute the tax. Enter 0 for single filers, 1 for married filing jointly or qualified widow(er), 2 for married filing separately, and 3 for head of household.
3.9 Case Study: Computing Taxes 91 Your program computes the tax for the taxable income based on the filing status. The filing status can be determined using if statements outlined as follows: if (status == 0) { // Compute tax for single filers } else if (status == 1) { // Compute tax for married filing jointly or qualifying widow(er) } else if (status == 2) { // Compute tax for married filing separately } else if (status == 3) { // Compute tax for head of household } else { // Display wrong status }
For each filing status there are six tax rates. Each rate is applied to a certain amount of taxable income. For example, of a taxable income of $400,000 for single filers, $8,350 is taxed at 10%, (33,950 – 8,350) at 15%, (82,250 – 33,950) at 25%, (171,550 – 82,250) at 28%, (372,950 – 171,550) at 33%, and (400,000 – 372,950) at 35%. Listing 3.5 gives the solution for computing taxes for single filers. The complete solution is left as an exercise.
LISTING 3.5 ComputeTax.java 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
import java.util.Scanner; public class ComputeTax { public static void main(String[] args) { // Create a Scanner Scanner input = new Scanner(System.in); // Prompt the user to enter filing status System.out.print("(0-single filer, 1-married jointly or " + "qualifying widow(er), 2-married separately, 3-head of " + "household) Enter the filing status: "); int status = input.nextInt();
input status
// Prompt the user to enter taxable income System.out.print("Enter the taxable income: "); double income = input.nextDouble();
input income
// Compute tax double tax = 0;
compute tax
if (status == 0) { // Compute tax for single filers if (income <= 8350) tax = income * 0.10; else if (income <= 33950) tax = 8350 * 0.10 + (income - 8350) * 0.15; else if (income <= 82250) tax = 8350 * 0.10 + (33950 - 8350) * 0.15 + (income - 33950) * 0.25; else if (income <= 171550) tax = 8350 * 0.10 + (33950 - 8350) * 0.15 + (82250 - 33950) * 0.25 + (income - 82250) * 0.28;
92 Chapter 3
Selections
exit program
display output
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
else if (income <= 372950) tax = 8350 * 0.10 + (33950 - 8350) * (82250 - 33950) * 0.25 + (171550 (income - 171550) * 0.33; else tax = 8350 * 0.10 + (33950 - 8350) * (82250 - 33950) * 0.25 + (171550 (372950 - 171550) * 0.33 + (income
0.15 + 82250) * 0.28 +
0.15 + 82250) * 0.28 + - 372950) * 0.35;
} else if (status == 1) { // Left as an exercise // Compute tax for married file jointly or qualifying widow(er) } else if (status == 2) { // Compute tax for married separately // Left as an exercise } else if (status == 3) { // Compute tax for head of household // Left as an exercise } else { System.out.println("Error: invalid status"); System.exit(1); } // Display the result System.out.println("Tax is " + (int)(tax * 100) / 100.0); } }
(0-single filer, 1-married jointly or qualifying widow(er), 2-married separately, 3-head of household) Enter the filing status: 0 Enter the taxable income: 400000 Tax is 117683.5
line# 13 17
income
tax
output
0 400000
20
0
38
117683.5
57
System.exit(status)
status
Tax is 117683.5
The program receives the filing status and taxable income. The multi-way if-else statements (lines 22, 42, 45, 48, 51) check the filing status and compute the tax based on the filing status. System.exit(status) (line 53) is defined in the System class. Invoking this method terminates the program. The status 0 indicates that the program is terminated normally. A nonzero status code indicates abnormal termination. An initial value of 0 is assigned to tax (line 20). A compile error would occur if it had no initial value, because all of the other statements that assign values to tax are within the if statement. The compiler thinks that these statements may not be executed and therefore reports a compile error.
3.10 Logical Operators 93 To test a program, you should provide the input that covers all cases. For this program, your input should cover all statuses (0, 1, 2, 3). For each status, test the tax for each of the six brackets. So, there are a total of 24 cases.
test all cases
Tip For all programs, you should write a small amount of code and test it before moving on to add more code. This is called incremental development and testing. This approach makes testing easier, because the errors are likely in the new code you just added.
3.17
✓
Are the following two statements equivalent?
if (income <= 10000) tax = income * 0.1; else if (income <= 20000) tax = 1000 + (income – 10000) * 0.15;
incremental development and testing
Check Point
if (income <= 10000) tax = income * 0.1; else if (income > 10000 && income <= 20000) tax = 1000 + (income – 10000) * 0.15;
3.10 Logical Operators The logical operators !, &&, ||, and ^ can be used to create a compound Boolean expression. Sometimes, whether a statement is executed is determined by a combination of several conditions. You can use logical operators to combine these conditions to form a compound Boolean expression. Logical operators, also known as Boolean operators, operate on Boolean values to create a new Boolean value. Table 3.3 lists the Boolean operators. Table 3.4 defines the not (!) operator, which negates true to false and false to true. Table 3.5 defines the and (&&) operator. The and (&&) of two Boolean operands is true if and only if both operands are true. Table 3.6 defines the or (||) operator. The or (||) of two Boolean operands is true if at least one of the operands is true. Table 3.7 defines the exclusive or (^) operator. The exclusive or (^) of two Boolean operands is true if and only if the two operands have different Boolean values. Note that p1 ^ p2 is the same as p1 != p2.
TABLE 3.3 Boolean Operators Operator
Name
Description
!
not
logical negation
&&
and
logical conjunction
||
or
logical disjunction
^
exclusive or
logical exclusion
TABLE 3.4 Truth Table for Operator ! p
!p
Example (assume age = 24, weight = 140)
true
false
!(age > 18) is false, because (age > 18) is true.
false
true
!(weight == 150) is true, because (weight == 150) is false.
Key Point
94 Chapter 3
Selections TABLE 3.5 Truth Table for Operator && p1
p2
p1 && p2
false
false
false
false
true
false
true
false
false
true
true
true
Example (assume age = 24, weight = 140)
(age > 28) && (weight <= 140) is true, because (age > 28) is false.
(age > 18) && (weight >= 140) is true, because (age > 18) and (weight >= 140) are both true.
TABLE 3.6 Truth Table for Operator || p1
p2
p1 || p2
Example (assume age = 24, weight = 140)
false
false
false
(age > 34) || (weight >= 150) is false, because (age > 34) and (weight >= 150) are both false.
false
true
true
true
false
true
true
true
true
(age > 18) || (weight < 140) is true, because (age > 18) is true.
TABLE 3.7 Truth Table for Operator ^ p1
p2
p1 ^ p2
Example (assume age = 24, weight = 140)
false
false
false
(age > 34) ^ (weight > 140) is false, because (age > 34) and (weight > 140) are both false.
false
true
true
(age > 34) ^ (weight >= 140) is true, because (age > 34) is false but (weight >= 140) is true.
true
false
true
true
true
false
Listing 3.6 gives a program that checks whether a number is divisible by 2 and 3, by 2 or 3, and by 2 or 3 but not both:
LISTING 3.6 TestBooleanOperators.java import class
input and
1 2 3 4 5 6 7 8 9 10 11 12 13 14
import java.util.Scanner; public class TestBooleanOperators { public static void main(String[] args) { // Create a Scanner Scanner input = new Scanner(System.in); // Receive an input System.out.print("Enter an integer: "); int number = input.nextInt(); if (number % 2 == 0 && number % 3 == 0) System.out.println(number + " is divisible by 2 and 3.");
3.10 Logical Operators 95 15 16 17 18 19 20 21 22
if (number % 2 == 0 || number % 3 == 0) System.out.println(number + " is divisible by 2 or 3.");
or
if (number % 2 == 0 ^ number % 3 == 0) System.out.println(number + " is divisible by 2 or 3, but not both.");
exclusive or
} }
Enter an integer: 4 4 is divisible by 2 or 3. 4 is divisible by 2 or 3, but not both.
Enter an integer: 18 18 is divisible by 2 and 3. 18 is divisible by 2 or 3.
(number % 2 == 0 && number % 3 == 0) (line 12) checks whether the number is divisible by both 2 and 3. (number % 2 == 0 || number % 3 == 0) (line 15) checks whether the number is divisible by 2 or by 3. (number % 2 == 0 ^ number % 3 == 0) (line 18) checks whether the number is divisible by 2 or 3, but not both.
Caution In mathematics, the expression 1 <= numberOfDaysInAMonth <= 31
is correct. However, it is incorrect in Java, because 1 <= numberOfDaysInAMonth is evaluated to a boolean value, which cannot be compared with 31. Here, two operands (a boolean value and a numeric value) are incompatible. The correct expression in Java is
incompatible operands
(1 <= numberOfDaysInAMonth) && (numberOfDaysInAMonth <= 31)
Note De Morgan’s law, named after Indian-born British mathematician and logician Augustus De Morgan (1806–1871), can be used to simplify Boolean expressions. The law states: !(condition1 && condition2) is the same as !condition1 || !condition2 !(condition1 || condition2) is the same as !condition1 && !condition2
For example, ! (number % 2 == 0 && number % 3 == 0)
can be simplified using an equivalent expression: (number % 2 != 0 || number % 3 != 0)
As another example, !(number == 2 || number == 3)
is better written as number != 2 && number != 3
De Morgan’s law
96 Chapter 3
Selections If one of the operands of an && operator is false, the expression is false; if one of the operands of an || operator is true, the expression is true. Java uses these properties to improve the performance of these operators. When evaluating p1 && p2, Java first evaluates p1 and then, if p1 is true, evaluates p2; if p1 is false, it does not evaluate p2. When evaluating p1 || p2, Java first evaluates p1 and then, if p1 is false, evaluates p2; if p1 is true, it does not evaluate p2. In programming language terminology, && and || are known as the short-circuit or lazy operators. Java also provides the unconditional AND (&) and OR (|) operators, which are covered in Supplement III.C for advanced readers.
short-circuit operator lazy operator
✓
Check Point
3.18
Assuming that x is 1, show the result of the following Boolean expressions. (true) && (3 > 4) !(x > 0) && (x > 0) (x > 0) || (x < 0) (x != 0) || (x == 0) (x >= 0) || (x < 0) (x != 1) == !(x == 1)
3.19
(a) Write a Boolean expression that evaluates to true if a number stored in variable num is between 1 and 100. (b) Write a Boolean expression that evaluates to true if a number stored in variable num is between 1 and 100 or the number is negative.
3.20
(a) Write a Boolean expression for 0 x - 5 0 6 4.5. (b) Write a Boolean expression for 0 x - 5 0 7 4.5.
3.21
Assume that x and y are int type. Which of the following are legal Java expressions? x > y > 0 x = y && y x /= y x or y x and y (x != 0) || (x = 0)
3.22
Are the following two expressions the same? a. x % 2 == 0 && x % 3 == 0 b. x % 6 == 0
3.23 3.24
What is the value of the expression x >= 50 && x <= 100 if x is 45, 67, or 101? Suppose, when you run the following program, you enter the input 2 3 6 from the console. What is the output? public class Test { public static void main(String[] args) { java.util.Scanner input = new java.util.Scanner(System.in); double x = input.nextDouble(); double y = input.nextDouble(); double z = input.nextDouble(); System.out.println("(x < y && y System.out.println("(x < y || y System.out.println("!(x < y) is System.out.println("(x + y < z) System.out.println("(x + y > z)
< z) is " < z) is " " + !(x < is " + (x is " + (x
+ (x < y && y < z)); + (x < y || y < z)); y)); + y < z)); + y > z));
} }
3.25
Write a Boolean expression that evaluates to true if age is greater than 13 and less than 18.
3.11 Case Study: Determining Leap Year 97 3.26
Write a Boolean expression that evaluates to true if weight is greater than 50 pounds or height is greater than 60 inches.
3.27
Write a Boolean expression that evaluates to true if weight is greater than 50 pounds and height is greater than 60 inches. Write a Boolean expression that evaluates to true if either weight is greater than 50 pounds or height is greater than 60 inches, but not both.
3.28
3.11 Case Study: Determining Leap Year A year is a leap year if it is divisible by 4 but not by 100, or if it is divisible by 400. You can use the following Boolean expressions to check whether a year is a leap year:
Key Point
// A leap year is divisible by 4 boolean isLeapYear = (year % 4 == 0); // A leap year is divisible by 4 but not by 100 isLeapYear = isLeapYear && (year % 100 != 0); // A leap year is divisible by 4 but not by 100 or divisible by 400 isLeapYear = isLeapYear || (year % 400 == 0);
Or you can combine all these expressions into one like this: isLeapYear = (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0);
Listing 3.7 gives the program that lets the user enter a year and checks whether it is a leap year.
LISTING 3.7 LeapYear.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
import java.util.Scanner; public class LeapYear { public static void main(String[] args) { // Create a Scanner Scanner input = new Scanner(System.in); System.out.print("Enter a year: "); int year = input.nextInt(); // Check if the year is a leap year boolean isLeapYear = (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0); // Display the result System.out.println(year + " is a leap year? " + isLeapYear); } }
Enter a year: 2008 2008 is a leap year? true
Enter a year: 1900 1900 is a leap year? false
input
leap year?
display result
98 Chapter 3
Selections Enter a year: 2002 2002 is a leap year? false
3.12 Case Study: Lottery Key Point
The lottery program involves generating random numbers, comparing digits, and using Boolean operators. Suppose you want to develop a program to play lottery. The program randomly generates a lottery of a two-digit number, prompts the user to enter a two-digit number, and determines whether the user wins according to the following rules: 1. If the user input matches the lottery number in the exact order, the award is $10,000. 2. If all digits in the user input match all digits in the lottery number, the award is $3,000. 3. If one digit in the user input matches a digit in the lottery number, the award is $1,000. Note that the digits of a two-digit number may be 0. If a number is less than 10, we assume the number is preceded by a 0 to form a two-digit number. For example, number 8 is treated as 08 and number 0 is treated as 00 in the program. Listing 3.8 gives the complete program.
LISTING 3.8 Lottery.java
generate a lottery number
enter a guess
exact match? match all digits?
match one digit?
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
import java.util.Scanner; public class Lottery { public static void main(String[] args) { // Generate a lottery number int lottery = (int)(Math.random() * 100); // Prompt the user to enter a guess Scanner input = new Scanner(System.in); System.out.print("Enter your lottery pick (two digits): "); int guess = input.nextInt(); // Get digits from lottery int lotteryDigit1 = lottery / 10; int lotteryDigit2 = lottery % 10; // Get digits from guess int guessDigit1 = guess / 10; int guessDigit2 = guess % 10; System.out.println("The lottery number is " + lottery); // Check the guess if (guess == lottery) System.out.println("Exact match: you win $10,000"); else if (guessDigit2 == lotteryDigit1 && guessDigit1 == lotteryDigit2) System.out.println("Match all digits: you win $3,000"); else if (guessDigit1 == lotteryDigit1 || guessDigit1 == lotteryDigit2 || guessDigit2 == lotteryDigit1 || guessDigit2 == lotteryDigit2) System.out.println("Match one digit: you win $1,000");
3.12 Case Study: Lottery 99 34 35 36 37
else System.out.println("Sorry, no match"); } }
Enter your lottery pick (two digits): 15 The lottery number is 15 Exact match: you win $10,000
Enter your lottery pick (two digits): 45 The lottery number is 54 Match all digits: you win $3,000
Enter your lottery pick: 23 The lottery number is 34 Match one digit: you win $1,000
Enter your lottery pick: 23 The lottery number is 14 Sorry: no match
line#
6
11
14
15
18
19
33
variable lottery guess lotteryDigit1 lotteryDigit2 guessDigit1 guessDigit2
34 23 3 4 2 3
Output
Match one digit: you win $1,000
The program generates a lottery using the random() method (line 6) and prompts the user to enter a guess (line 11). Note that guess % 10 obtains the last digit from guess and guess / 10 obtains the first digit from guess, since guess is a two-digit number (lines 18–19). The program checks the guess against the lottery number in this order: 1. First, check whether the guess matches the lottery exactly (line 24). 2. If not, check whether the reversal of the guess matches the lottery (lines 26–27). 3. If not, check whether one digit is in the lottery (lines 29–32). 4. If not, nothing matches and display "Sorry, no match" (lines 34–35).
100 Chapter 3
Selections
3.13 switch Statements Key Point
A switch statement executes statements based on the value of a variable or an expression. The if statement in Listing 3.5, ComputeTax.java, makes selections based on a single true or false condition. There are four cases for computing taxes, which depend on the value of status. To fully account for all the cases, nested if statements were used. Overuse of nested if statements makes a program difficult to read. Java provides a switch statement to simplify coding for multiple conditions. You can write the following switch statement to replace the nested if statement in Listing 3.5: switch (status) { case 0: compute tax for single filers; break; case 1: compute tax for married jointly or qualifying widow(er); break; case 2: compute tax for married filing separately; break; case 3: compute tax for head of household; break; default: System.out.println("Error: invalid status"); System.exit(1); }
The flowchart of the preceding switch statement is shown in Figure 3.5.
status is 0
status is 1
status is 2
status is 3
default
Compute tax for single filers
Compute tax for married jointly or qualifying widow(er)
break
break
Compute tax for married filing separately
break
Compute tax for head of household
break
Default actions
FIGURE 3.5 The switch statement checks all cases and executes the statements in the matched case.
switch statement
This statement checks to see whether the status matches the value 0, 1, 2, or 3, in that order. If matched, the corresponding tax is computed; if not matched, a message is displayed. Here is the full syntax for the switch statement: switch (switch-expression) { case value1: statement(s)1; break;
3.13 switch Statements 101 case value2: statement(s)2; break; ... case valueN: statement(s)N; break; default: statement(s)-for-default; }
The switch statement observes the following rules: ■
The switch-expression must yield a value of char, byte, short, int, or String type and must always be enclosed in parentheses. (The char and String types will be introduced in the next chapter.)
■
The value1, . . ., and valueN must have the same data type as the value of the switchexpression. Note that value1, . . ., and valueN are constant expressions, meaning that they cannot contain variables, such as 1 + x.
■
When the value in a case statement matches the value of the switch-expression, the statements starting from this case are executed until either a break statement or the end of the switch statement is reached.
■
The default case, which is optional, can be used to perform actions when none of the specified cases matches the switch-expression.
■
The keyword break is optional. The break statement immediately ends the switch statement.
Caution Do not forget to use a break statement when one is needed. Once a case is matched, the statements starting from the matched case are executed until a break statement or the end of the switch statement is reached. This is referred to as fall-through behavior. For example, the following code displays Weekdays for day of 1 to 5 and Weekends for day 0 and 6. switch case case case case case case case }
(day) { 1: 2: 3: 4: 5: System.out.println("Weekday"); break; 0: 6: System.out.println("Weekend");
Tip To avoid programming errors and improve code maintainability, it is a good idea to put a comment in a case clause if break is purposely omitted.
Now let us write a program to find out the Chinese Zodiac sign for a given year. The Chinese Zodiac is based on a twelve-year cycle, with each year represented by an animal— monkey, rooster, dog, pig, rat, ox, tiger, rabbit, dragon, snake, horse, or sheep—in this cycle, as shown in Figure 3.6. Note that year % 12 determines the Zodiac sign. 1900 is the year of the rat because 1900 % 12 is 4. Listing 3.9 gives a program that prompts the user to enter a year and displays the animal for the year.
without break fall-through behavior
102 Chapter 3
Selections pig
rat ox
dog
tiger
rooster
year % 12 = rabbit
monkey
dragon
sheep horse
FIGURE 3.6
snake
0: monkey 1: rooster 2: dog 3: pig 4: rat 5: ox 6: tiger 7: rabbit 8: dragon 9: snake 10: horse 11: sheep
The Chinese Zodiac is based on a twelve-year cycle.
LISTING 3.9 ChineseZodiac.java 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
enter year determine Zodiac sign
✓
Check Point
import java.util.Scanner; public class ChineseZodiac { public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.print("Enter a year: "); int year = input.nextInt(); switch case case case case case case case case case case case case }
(year % 12) { 0: System.out.println("monkey"); break; 1: System.out.println("rooster"); break; 2: System.out.println("dog"); break; 3: System.out.println("pig"); break; 4: System.out.println("rat"); break; 5: System.out.println("ox"); break; 6: System.out.println("tiger"); break; 7: System.out.println("rabbit"); break; 8: System.out.println("dragon"); break; 9: System.out.println("snake"); break; 10: System.out.println("horse"); break; 11: System.out.println("sheep");
} }
Enter a year: rabbit
1963
Enter a year: ox
1877
3.29
What data types are required for a switch variable? If the keyword break is not used after a case is processed, what is the next statement to be executed? Can you convert a switch statement to an equivalent if statement, or vice versa? What are the advantages of using a switch statement?
3.14 Conditional Expressions 103 3.30
What is y after the following switch statement is executed? Rewrite the code using an if-else statement. x = 3; y = 3; switch (x + 3) { case 6: y = 1; default: y += 1; }
3.31
What is x after the following if-else statement is executed? Use a switch statement to rewrite it and draw the flowchart for the new switch statement. int x = 1, a = 3; if (a == 1) x += 5; else if (a == 2) x += 10; else if (a == 3) x += 16; else if (a == 4) x += 34;
3.32
Write a switch statement that displays Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, if day is 0, 1, 2, 3, 4, 5, 6, accordingly.
3.14 Conditional Expressions A conditional expression evaluates an expression based on a condition. You might want to assign a value to a variable that is restricted by certain conditions. For example, the following statement assigns 1 to y if x is greater than 0, and -1 to y if x is less than or equal to 0.
Key Point
if (x > 0) y = 1; else y = -1;
Alternatively, as in the following example, you can use a conditional expression to achieve the same result. y = (x > 0) ? 1 : -1;
Conditional expressions are in a completely different style, with no explicit if in the statement. The syntax is: boolean-expression ? expression1 : expression2;
The result of this conditional expression is expression1 if boolean-expression is true; otherwise the result is expression2. Suppose you want to assign the larger number of variable num1 and num2 to max. You can simply write a statement using the conditional expression: max = (num1 > num2) ? num1 : num2;
For another example, the following statement displays the message “num is even” if num is even, and otherwise displays “num is odd.” System.out.println((num % 2 == 0) ? "num is even" : "num is odd");
conditional expression
104 Chapter 3
Selections As you can see from these examples, conditional expressions enable you to write short and concise code.
Note The symbols ? and : appear together in a conditional expression. They form a conditional operator and also called a ternary operator because it uses three operands. It is the only ternary operator in Java.
conditional operator ternary operator
✓
Check Point
3.33
Suppose that, when you run the following program, you enter the input 2 3 6 from the console. What is the output? public class Test { public static void main(String[] args) { java.util.Scanner input = new java.util.Scanner(System.in); double x = input.nextDouble(); double y = input.nextDouble(); double z = input.nextDouble(); System.out.println((x < y && y < z) ? "sorted" : "not sorted"); } }
3.34
Rewrite the following if statements using the conditional operator. if (ages >= 16) ticketPrice = 20; else ticketPrice = 10;
3.35
Rewrite the following conditional expressions using if-else statements. a. score = (x > 10) ? 3 * scale : 4 * scale; b. tax = (income > 10000) ? income * 0.2 : income * 0.17 + 1000; c. System.out.println((number % 3 == 0) ? i : j);
3.36
Write conditional expression that returns -1 or 1 randomly.
3.15 Operator Precedence and Associativity Key Point
Operator precedence and associativity determine the order in which operators are evaluated. Section 2.11 introduced operator precedence involving arithmetic operators. This section discusses operator precedence in more detail. Suppose that you have this expression: 3 + 4 * 4 > 5 * (4 + 3) – 1 && (4 - 3 > 5)
operator precedence
What is its value? What is the execution order of the operators? The expression within parentheses is evaluated first. (Parentheses can be nested, in which case the expression within the inner parentheses is executed first.) When evaluating an expression without parentheses, the operators are applied according to the precedence rule and the associativity rule. The precedence rule defines precedence for operators, as shown in Table 3.8, which contains the operators you have learned so far. Operators are listed in decreasing order of precedence from top to bottom. The logical operators have lower precedence than the relational operators and the relational operators have lower precedence than the arithmetic operators. Operators with the same precedence appear in the same group. (See Appendix C, Operator Precedence Chart, for a complete list of Java operators and their precedence.)
3.15 Operator Precedence and Associativity 105 TABLE 3.8 Operator Precedence Chart Precedence
Operator var++ and var–– (Postfix) +, – (Unary plus and minus), ++var and ––var (Prefix)
(type) (Casting) !(Not) *, /, % (Multiplication, division, and remainder) +, – (Binary addition and subtraction) <, <=, >, >= (Relational) ==, != (Equality) ^ (Exclusive OR) && (AND) || (OR) =, +=, –=, *=, /=, %= (Assignment operator)
If operators with the same precedence are next to each other, their associativity determines the order of evaluation. All binary operators except assignment operators are left associative. For example, since + and – are of the same precedence and are left associative, the expression
operator associativity
is equivalent to
a - b + c – d
((a - b) + c) - d
Assignment operators are right associative. Therefore, the expression is equivalent to
a = b += c = 5
a = (b += (c = 5))
Suppose a, b, and c are 1 before the assignment; after the whole expression is evaluated, a becomes 6, b becomes 6, and c becomes 5. Note that left associativity for the assignment operator would not make sense.
Note Java has its own way to evaluate an expression internally. The result of a Java evaluation is the same as that of its corresponding arithmetic evaluation. Advanced readers may refer to Supplement III.B for more discussions on how an expression is evaluated in Java behind the scenes.
3.37
List the precedence order of the Boolean operators. Evaluate the following expressions: true || true && false true && true || false
3.38 3.39
True or false? All the binary operators except = are left associative. Evaluate the following expressions: 2 * 2 - 3 > 2 && 4 – 2 > 5 2 * 2 - 3 > 2 || 4 – 2 > 5
behind the scenes
✓
Check Point
106 Chapter 3
Selections 3.40
Is (x > 0 && x < 10) the same as ((x > 0) && (x < 10))? Is (x > 0 || x < 10) the same as ((x > 0) || (x < 10))? Is (x > 0 || x < 10 && y < 0) the same as (x > 0 || (x < 10 && y < 0))?
3.16 Debugging Key Point
bugs debugging hand-traces
Debugging is the process of finding and fixing errors in a program. As mentioned in Section 1.10.1, syntax errors are easy to find and easy to correct because the compiler gives indications as to where the errors came from and why they are there. Runtime errors are not difficult to find either, because the Java interpreter displays them on the console when the program aborts. Finding logic errors, on the other hand, can be very challenging. Logic errors are called bugs. The process of finding and correcting errors is called debugging. A common approach to debugging is to use a combination of methods to help pinpoint the part of the program where the bug is located. You can hand-trace the program (i.e., catch errors by reading the program), or you can insert print statements in order to show the values of the variables or the execution flow of the program. These approaches might work for debugging a short, simple program, but for a large, complex program, the most effective approach is to use a debugger utility. JDK includes a command-line debugger, jdb, which is invoked with a class name. jdb is itself a Java program, running its own copy of Java interpreter. All the Java IDE tools, such as Eclipse and NetBeans, include integrated debuggers. The debugger utilities let you follow the execution of a program. They vary from one system to another, but they all support most of the following helpful features. ■
Executing a single statement at a time: The debugger allows you to execute one statement at a time so that you can see the effect of each statement.
■
Tracing into or stepping over a method: If a method is being executed, you can ask the debugger to enter the method and execute one statement at a time in the method, or you can ask it to step over the entire method. You should step over the entire method if you know that the method works. For example, always step over system-supplied methods, such as System.out.println.
■
Setting breakpoints: You can also set a breakpoint at a specific statement. Your program pauses when it reaches a breakpoint. You can set as many breakpoints as you want. Breakpoints are particularly useful when you know where your programming error starts. You can set a breakpoint at that statement and have the program execute until it reaches the breakpoint.
■
Displaying variables: The debugger lets you select several variables and display their values. As you trace through a program, the content of a variable is continuously updated.
■
Displaying call stacks: The debugger lets you trace all of the method calls. This feature is helpful when you need to see a large picture of the program-execution flow.
■
Modifying variables: Some debuggers enable you to modify the value of a variable when debugging. This is convenient when you want to test a program with different samples but do not want to leave the debugger.
Tip debugging in IDE
If you use an IDE such as Eclipse or NetBeans, please refer to Learning Java Effectively with Eclipse/NetBeans in Supplements II.C and II.E on the Companion Website. The supplement shows you how to use a debugger to trace programs and how debugging can help in learning Java effectively.
Chapter Summary 107
KEY TERMS Boolean expression 76 boolean data type 76 Boolean value 76 conditional operator 104 dangling else ambiguity 85 debugging 106 fall-through behavior 101
flowchart 78 lazy operator 96 operator associativity 105 operator precedence 104 selection statement 76 short-circuit operator 96
CHAPTER SUMMARY 1. A boolean type variable can store a true or false value. 2. The relational operators (<, <=, ==, !=, >, >=) yield a Boolean value. 3. Selection statements are used for programming with alternative courses of actions. There are several types of selection statements: one-way if statements, two-way if-else statements, nested if statements, multi-way if-else statements, switch statements, and conditional expressions.
4. The various if statements all make control decisions based on a Boolean expression. Based on the true or false evaluation of the expression, these statements take one of two possible courses.
5. The Boolean operators &&, ||, !, and ^ operate with Boolean values and variables. 6. When evaluating p1
&& p2, Java first evaluates p1 and then evaluates p2 if p1 is true; if p1 is false, it does not evaluate p2. When evaluating p1 || p2, Java first evaluates p1 and then evaluates p2 if p1 is false; if p1 is true, it does not evaluate p2. Therefore, && is referred to as the conditional or short-circuit AND operator, and || is referred to as the conditional or short-circuit OR operator.
7. The switch statement makes control decisions based on a switch expression of type char, byte, short, int, or String.
8. The keyword break is optional in a switch statement, but it is normally used at the end of each case in order to skip the remainder of the switch statement. If the break statement is not present, the next case statement will be executed.
9. The operators in expressions are evaluated in the order determined by the rules of parentheses, operator precedence, and operator associativity.
10. Parentheses can be used to force the order of evaluation to occur in any sequence. 11. Operators with higher precedence are evaluated earlier. For operators of the same precedence, their associativity determines the order of evaluation.
12. All binary operators except assignment operators are left-associative; assignment operators are right-associative.
108 Chapter 3
Selections
TEST QUESTIONS Answer the quiz for this chapter online at www.cs.armstrong.edu/liang/intro10e/quiz.html.
PROGRAMMING EXERCISES Pedagogical Note think before coding
For each exercise, carefully analyze the problem requirements and design strategies for solving the problem before coding.
Debugging Tip Before you ask for help, read and explain the program to yourself, and trace it using several representative inputs by hand or using an IDE debugger. You learn how to program by debugging your own mistakes.
learn from mistakes
Section 3.2
*3.1
(Algebra: solve quadratic equations) The two roots of a quadratic equation ax2 + bx + c = 0 can be obtained using the following formula: r1 =
-b + 2b2 - 4ac 2a
and r2 =
-b - 2b2 - 4ac 2a
b2 - 4ac is called the discriminant of the quadratic equation. If it is positive, the equation has two real roots. If it is zero, the equation has one root. If it is negative, the equation has no real roots. Write a program that prompts the user to enter values for a, b, and c and displays the result based on the discriminant. If the discriminant is positive, display two roots. If the discriminant is 0, display one root. Otherwise, display “The equation has no real roots”. Note that you can use Math.pow(x, 0.5) to compute 2x. Here are some sample runs.
Enter a, b, c: 1.0 3 1 The equation has two roots -0.381966 and -2.61803
Enter a, b, c: 1 2.0 1 The equation has one root -1
Enter a, b, c: 1 2 3 The equation has no real roots
3.2
(Game: add three numbers) The program in Listing 3.1, AdditionQuiz.java, generates two integers and prompts the user to enter the sum of these two integers. Revise the program to generate three single-digit integers and prompt the user to enter the sum of these three integers.
Programming Exercises 109 Sections 3.3–3.7
*3.3
(Algebra: solve 2 * 2 linear equations) A linear equation can be solved using Cramer’s rule given in Programming Exercise 1.13. Write a program that prompts the user to enter a, b, c, d, e, and f and displays the result. If ad - bc is 0, report that “The equation has no solution.”
Enter a, b, c, d, e, f: 9.0 4.0 3.0 -5.0 -6.0 -21.0 x is -2.0 and y is 3.0
Enter a, b, c, d, e, f: 1.0 2.0 2.0 4.0 4.0 5.0 The equation has no solution
**3.4 *3.5
(Random month) Write a program that randomly generates an integer between 1 and 12 and displays the English month name January, February, …, December for the number 1, 2, …, 12, accordingly. (Find future dates) Write a program that prompts the user to enter an integer for today’s day of the week (Sunday is 0, Monday is 1, …, and Saturday is 6). Also prompt the user to enter the number of days after today for a future day and display the future day of the week. Here is a sample run:
Enter today's day: 1 Enter the number of days elapsed since today: 3 Today is Monday and the future day is Thursday
Enter today's day: 0 Enter the number of days elapsed since today: 31 Today is Sunday and the future day is Wednesday
*3.6
(Health application: BMI) Revise Listing 3.4, ComputeAndInterpretBMI.java, to let the user enter weight, feet, and inches. For example, if a person is 5 feet and 10 inches, you will enter 5 for feet and 10 for inches. Here is a sample run:
Enter weight in pounds: 140 Enter feet: 5 Enter inches: 10 BMI is 20.087702275404553 Normal
3.7
(Financial application: monetary units) Modify Listing 2.10, ComputeChange .java, to display the nonzero denominations only, using singular words for single units such as 1 dollar and 1 penny, and plural words for more than one unit such as 2 dollars and 3 pennies.
110 Chapter 3
VideoNote
Sort three integers
Selections *3.8
(Sort three integers) Write a program that prompts the user to enter three integers and display the integers in non-decreasing order.
**3.9
(Business: check ISBN-10) An ISBN-10 (International Standard Book Number) consists of 10 digits: d1d2d3d4d5d6d7d8d9d10. The last digit, d10, is a checksum, which is calculated from the other nine digits using the following formula: (d1 * 1 + d2 * 2 + d3 * 3 + d4 * 4 + d5 * 5 + d6 * 6 + d7 * 7 + d8 * 8 + d9 * 9) % 11 If the checksum is 10, the last digit is denoted as X according to the ISBN-10 convention. Write a program that prompts the user to enter the first 9 digits and displays the 10-digit ISBN (including leading zeros). Your program should read the input as an integer. Here are sample runs: Enter the first 9 digits of an ISBN as integer: 013601267 The ISBN-10 number is 0136012671
Enter the first 9 digits of an ISBN as integer: 013031997 The ISBN-10 number is 013031997X
3.10
(Game: addition quiz) Listing 3.3, SubtractionQuiz.java, randomly generates a subtraction question. Revise the program to randomly generate an addition question with two integers less than 100.
Sections 3.8–3.16
*3.11
(Find the number of days in a month) Write a program that prompts the user to enter the month and year and displays the number of days in the month. For example, if the user entered month 2 and year 2012, the program should display that February 2012 had 29 days. If the user entered month 3 and year 2015, the program should display that March 2015 had 31 days.
3.12
(Palindrome number) Write a program that prompts the user to enter a three-digit integer and determines whether it is a palindrome number. A number is palindrome if it reads the same from right to left and from left to right. Here is a sample run of this program: Enter a three-digit integer: 121 121 is a palindrome
Enter a three-digit integer: 123 123 is not a palindrome
*3.13 3.14
(Financial application: compute taxes) Listing 3.5, ComputeTax.java, gives the source code to compute taxes for single filers. Complete Listing 3.5 to compute the taxes for all filing statuses. (Game: heads or tails) Write a program that lets the user guess whether the flip of a coin results in heads or tails. The program randomly generates an integer 0 or 1, which represents head or tail. The program prompts the user to enter a guess and reports whether the guess is correct or incorrect.
Programming Exercises 111 **3.15
3.16 *3.17
(Game: lottery) Revise Listing 3.8, Lottery.java, to generate a lottery of a threedigit number. The program prompts the user to enter a three-digit number and determines whether the user wins according to the following rules: 1. If the user input matches the lottery number in the exact order, the award is $10,000. 2. If all digits in the user input match all digits in the lottery number, the award is $3,000. 3. If one digit in the user input matches a digit in the lottery number, the award is $1,000. (Random point) Write a program that displays a random coordinate in a rectangle. The rectangle is centered at (0, 0) with width 100 and height 200. (Game: scissor, rock, paper) Write a program that plays the popular scissor-rockpaper game. (A scissor can cut a paper, a rock can knock a scissor, and a paper can wrap a rock.) The program randomly generates a number 0, 1, or 2 representing scissor, rock, and paper. The program prompts the user to enter a number 0, 1, or 2 and displays a message indicating whether the user or the computer wins, loses, or draws. Here are sample runs:
scissor (0), rock (1), paper (2): 1 The computer is scissor. You are rock. You won
scissor (0), rock (1), paper (2): 2 The computer is paper. You are paper too. It is a draw
*3.18
(Cost of shipping) A shipping company uses the following function to calculate the cost (in dollars) of shipping based on the weight of the package (in pounds). 3.5, if 0 6 5.5, if 1 6 c(w) = d 8.5, if 3 6 10.5, if 10
**3.19
*3.20
w w w 6
6= 1 6= 3 6 = 10 w 6 = 20
Write a program that prompts the user to enter the weight of the package and display the shipping cost. If the weight is greater than 50, display a message “the package cannot be shipped.” (Compute the perimeter of a triangle) Write a program that reads three edges for a triangle and computes the perimeter if the input is valid. Otherwise, display that the input is invalid. The input is valid if the sum of every pair of two edges is greater than the remaining edge. (Science: wind-chill temperature) Programming Exercise 2.17 gives a formula to compute the wind-chill temperature. The formula is valid for temperatures in the range between −58ºF and 41ºF and wind speed greater than or equal to 2. Write a program that prompts the user to enter a temperature and a wind speed. The program displays the wind-chill temperature if the input is valid; otherwise, it displays a message indicating whether the temperature and/or wind speed is invalid.
112 Chapter 3
Selections Comprehensive
**3.21
(Science: day of the week) Zeller’s congruence is an algorithm developed by Christian Zeller to calculate the day of the week. The formula is h = ¢q +
j 26(m + 1) k + k + + + 5j≤ % 7 10 4 4
where ■ h
is the day of the week (0: Saturday, 1: Sunday, 2: Monday, 3: Tuesday, 4: Wednesday, 5: Thursday, 6: Friday).
■ q
is the day of the month.
■ m
is the month (3: March, 4: April, …, 12: December). January and February are counted as months 13 and 14 of the previous year. year ■ j is the century (i.e., ). 100 ■ k is the year of the century (i.e., year % 100). Note that the division in the formula performs an integer division. Write a program that prompts the user to enter a year, month, and day of the month, and displays the name of the day of the week. Here are some sample runs:
Enter year: (e.g., 2012): 2015 Enter month: 1-12: 1 Enter the day of the month: 1-31: 25 Day of the week is Sunday
Enter year: (e.g., 2012): 2012 Enter month: 1-12: 5 Enter the day of the month: 1-31: 12 Day of the week is Saturday
**3.22 VideoNote
Check point location
(Hint: January and February are counted as 13 and 14 in the formula, so you need to convert the user input 1 to 13 and 2 to 14 for the month and change the year to the previous year.) (Geometry: point in a circle?) Write a program that prompts the user to enter a point (x, y) and checks whether the point is within the circle centered at (0, 0) with radius 10. For example, (4, 5) is inside the circle and (9, 9) is outside the circle, as shown in Figure 3.7a. (Hint: A point is in the circle if its distance to (0, 0) is less than or equal to 10. The formula for computing the distance is 2(x2 - x1)2 + (y2 - y1)2. Test your program to cover all cases.) Two sample runs are shown below.
Enter a point with two coordinates: 4 5 Point (4.0, 5.0) is in the circle
Enter a point with two coordinates: 9 9 Point (9.0, 9.0) is not in the circle
Programming Exercises 113 y-axis
y-axis (9, 9) (4, 5)
(6, 4) (2, 2)
(0, 0)
x-axis
(a)
(0, 0)
x-axis
(b)
FIGURE 3.7 (a) Points inside and outside of the circle. (b) Points inside and outside of the rectangle.
**3.23
(Geometry: point in a rectangle?) Write a program that prompts the user to enter a point (x, y) and checks whether the point is within the rectangle centered at (0, 0) with width 10 and height 5. For example, (2, 2) is inside the rectangle and (6, 4) is outside the rectangle, as shown in Figure 3.7b. (Hint: A point is in the rectangle if its horizontal distance to (0, 0) is less than or equal to 10 / 2 and its vertical distance to (0, 0) is less than or equal to 5.0 / 2. Test your program to cover all cases.) Here are two sample runs.
Enter a point with two coordinates: 2 2 Point (2.0, 2.0) is in the rectangle
Enter a point with two coordinates: 6 4 Point (6.0, 4.0) is not in the rectangle
**3.24
(Game: pick a card) Write a program that simulates picking a card from a deck of 52 cards. Your program should display the rank (Ace, 2, 3, 4, 5, 6, 7, 8, 9, 10, Jack, Queen, King) and suit (Clubs, Diamonds, Hearts, Spades) of the card. Here is a sample run of the program:
The card you picked is Jack of Hearts
*3.25
(Geometry: intersecting point) Two points on line 1 are given as (x1, y1) and (x2, y2) and on line 2 as (x3, y3) and (x4, y4), as shown in Figure 3.8a–b. The intersecting point of the two lines can be found by solving the following linear equation: (y1 - y2)x - (x1 - x2)y = (y1 - y2)x1 - (x1 - x2)y1 (y3 - y4)x - (x3 - x4)y = (y3 - y4)x3 - (x3 - x4)y3 This linear equation can be solved using Cramer’s rule (see Programming Exercise 3.3). If the equation has no solutions, the two lines are parallel (Figure 3.8c).
114 Chapter 3
Selections Write a program that prompts the user to enter four points and displays the intersecting point. Here are sample runs: (x2, y2)
(x2, y2)
(x2, y2) (x3, y3)
(x3, y3) (x3, y3) (x4, y4) (x1, y1)
FIGURE 3.8
(x4, y4)
(x1, y1) (a)
(x1, y1)
(b)
(x4, y4) (c)
Two lines intersect in (a and b) and two lines are parallel in (c).
Enter x1, y1, x2, y2, x3, y3, x4, y4: 2 2 5 -1.0 4.0 2.0 -1.0 -2.0 The intersecting point is at (2.88889, 1.1111)
Enter x1, y1, x2, y2, x3, y3, x4, y4: 2 2 7 6.0 4.0 2.0 -1.0 -2.0 The two lines are parallel
3.26
(Use the &&, || and ^ operators) Write a program that prompts the user to enter an integer and determines whether it is divisible by 5 and 6, whether it is divisible by 5 or 6, and whether it is divisible by 5 or 6, but not both. Here is a sample run of this program:
Enter Is 10 Is 10 Is 10
**3.27
an integer: 10 divisible by 5 and 6? false divisible by 5 or 6? true divisible by 5 or 6, but not both? true
(Geometry: points in triangle?) Suppose a right triangle is placed in a plane as shown below. The right-angle point is placed at (0, 0), and the other two points are placed at (200, 0), and (0, 100). Write a program that prompts the user to enter a point with x- and y-coordinates and determines whether the point is inside the triangle. Here are the sample runs:
(0, 100) p2 p1 (0, 0)
(200, 0)
Enter a point's x- and y-coordinates: 100.5 25.5 The point is in the triangle
Programming Exercises 115 Enter a point's x- and y-coordinates: 100.5 50.5 The point is not in the triangle
**3.28
(Geometry: two rectangles) Write a program that prompts the user to enter the center x-, y-coordinates, width, and height of two rectangles and determines whether the second rectangle is inside the first or overlaps with the first, as shown in Figure 3.9. Test your program to cover all cases. w1
w1
w2 h1 h2
(x1, y1)
h1
w2
(x1, y1)
(x2, y2)
h2
(a)
(x2, y2)
(b)
FIGURE 3.9 (a) A rectangle is inside another one. (b) A rectangle overlaps another one. Here are the sample runs:
Enter r1's center x-, y-coordinates, width, and height: 2.5 4 2.5 43 Enter r2's center x-, y-coordinates, width, and height: 1.5 5 0.5 3 r2 is inside r1
Enter r1's center x-, y-coordinates, width, and height: 1 2 3 5.5 Enter r2's center x-, y-coordinates, width, and height: 3 4 4.5 5 r2 overlaps r1
Enter r1's center x-, y-coordinates, width, and height: 1 2 3 3 Enter r2's center x-, y-coordinates, width, and height: 40 45 3 2 r2 does not overlap r1
**3.29
(Geometry: two circles) Write a program that prompts the user to enter the center coordinates and radii of two circles and determines whether the second circle is inside the first or overlaps with the first, as shown in Figure 3.10. (Hint: circle2 is inside circle1 if the distance between the two centers 6 = |r1 - r2| and circle2 overlaps circle1 if the distance between the two centers <= r1 + r2. Test your program to cover all cases.) Here are the sample runs:
Enter circle1's center x-, y-coordinates, and radius: 0.5 5.1 13 Enter circle2's center x-, y-coordinates, and radius: 1 1.7 4.5 circle2 is inside circle1
116 Chapter 3
Selections
r1
r1 (x1, y1)
(x1, y1) r2
r2
(x2, y2)
(a)
(x2, y2) (b)
FIGURE 3.10 (a) A circle is inside another circle. (b) A circle overlaps another circle.
Enter circle1's center x-, y-coordinates, and radius: 3.4 5.7 5.5 Enter circle2's center x-, y-coordinates, and radius: 6.7 3.5 3 circle2 overlaps circle1
Enter circle1's center x-, y-coordinates, and radius: 3.4 5.5 1 Enter circle2's center x-, y-coordinates, and radius: 5.5 7.2 1 circle2 does not overlap circle1
*3.30
(Current time) Revise Programming Exercise 2.8 to display the hour using a 12-hour clock. Here is a sample run:
Enter the time zone offset to GMT: -5 The current time is 4:50:34 AM
*3.31
(Financials: currency exchange) Write a program that prompts the user to enter the exchange rate from currency in U.S. dollars to Chinese RMB. Prompt the user to enter 0 to convert from U.S. dollars to Chinese RMB and 1 to convert from Chinese RMB and U.S. dollars. Prompt the user to enter the amount in U.S. dollars or Chinese RMB to convert it to Chinese RMB or U.S. dollars, respectively. Here are the sample runs:
Enter the exchange rate from dollars to RMB: 6.81 Enter 0 to convert dollars to RMB and 1 vice versa: 0 Enter the dollar amount: 100 $100.0 is 681.0 yuan
Enter the exchange rate from dollars to RMB: 6.81 Enter 0 to convert dollars to RMB and 1 vice versa: 5 Enter the RMB amount: 10000 10000.0 yuan is $1468.43
Programming Exercises 117 Enter the exchange rate from dollars to RMB: 6.81 Enter 0 to convert dollars to RMB and 1 vice versa: 5 Incorrect input
*3.32
(Geometry: point position) Given a directed line from point p0(x0, y0) to p1(x1, y1), you can use the following condition to decide whether a point p2(x2, y2) is on the left of the line, on the right, or on the same line (see Figure 3.11):
7 0 p2 is on the left side of the line (x1 - x0)*(y2 - y0) - (x2 - x0)*(y1 - y0) c =0 p2 is on the same line 6 0 p2 is on the right side of the line p1
p1
p2
p1
p2
p2 p0 (a)
p0
p0 (b)
(c)
FIGURE 3.11 (a) p2 is on the left of the line. (b) p2 is on the right of the line. (c) p2 is on the same line. Write a program that prompts the user to enter the three points for p0, p1, and p2 and displays whether p2 is on the left of the line from p0 to p1, on the right, or on the same line. Here are some sample runs:
Enter three points for p0, p1, and p2: 4.4 2 6.5 9.5 -5 4 (-5.0, 4.0) is on the left side of the line from (4.4, 2.0) to (6.5, 9.5)
Enter three points for p0, p1, and p2: 1 1 5 5 2 2 (2.0, 2.0) is on the line from (1.0, 1.0) to (5.0, 5.0)
Enter three points for p0, p1, and p2: 3.4 2 6.5 9.5 5 2.5 (5.0, 2.5) is on the right side of the line from (3.4, 2.0) to (6.5, 9.5)
*3.33
(Financial: compare costs) Suppose you shop for rice in two different packages. You would like to write a program to compare the cost. The program prompts the user to enter the weight and price of the each package and displays the one with the better price. Here is a sample run:
Enter weight and price for package 1: 50 24.59 Enter weight and price for package 2: 25 11.99 Package 2 has a better price.
118 Chapter 3
Selections Enter weight and price for package 1: 50 25 Enter weight and price for package 2: 25 12.5 Two packages have the same price.
*3.34
(Geometry: point on line segment) Programming Exercise 3.32 shows how to test whether a point is on an unbounded line. Revise Programming Exercise 3.32 to test whether a point is on a line segment. Write a program that prompts the user to enter the three points for p0, p1, and p2 and displays whether p2 is on the line segment from p0 to p1. Here are some sample runs:
Enter three points for p0, p1, and p2: 1 1 2.5 2.5 1.5 1.5 (1.5, 1.5) is on the line segment from (1.0, 1.0) to (2.5, 2.5)
Enter three points for p0, p1, and p2: 1 1 2 2 3.5 3.5 (3.5, 3.5) is not on the line segment from (1.0, 1.0) to (2.0, 2.0)
CHAPTER
MATHEMATICAL FUNCTIONS, CHARACTERS, AND STRINGS Objectives ■ ■ ■ ■ ■
To solve mathematical problems by using the methods in the Math class (§4.2). To represent characters using the char type (§4.3). To encode characters using ASCII and Unicode (§4.3.1). To represent special characters using the escape sequences (§4.4.2). To cast a numeric value to a character and cast a character to an integer (§4.3.3).
■
To compare and test characters using the static methods in the Character class (§4.3.4).
■
To introduce objects and instance methods (§4.4).
■
To represent strings using the String object (§4.4). To return the string length using the length() method (§4.4.1). To return a character in the string using the charAt(i) method (§4.4.2). To use the + operator to concatenate strings (§4.4.3).
■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■
To return an uppercase string or a lowercase string and to trim a string (§4.4.4). To read strings from the console (§4.4.5). To read a character from the console (§4.4.6). To compare strings using the equals method and the compareTo methods (§4.4.7). To obtain substrings (§4.4.8). To find a character or a substring in a string using the indexOf method (§4.4.9). To program using characters and strings (GuessBirthday) (§4.5.1). To convert a hexadecimal character to a decimal value (HexDigit2Dec) (§4.5.2). To revise the lottery program using strings (LotteryUsingStrings) (§4.5.3). To format output using the System.out.printf method (§4.6).
4
120 Chapter 4
Mathematical Functions, Characters, and Strings
4.1 Introduction Key Point
The focus of this chapter is to introduce mathematical functions, characters, string objects, and use them to develop programs. The preceding chapters introduced fundamental programming techniques and taught you how to write simple programs to solve basic problems using selection statements. This chapter introduces methods for performing common mathematical operations. You will learn how to create custom methods in Chapter 6. Suppose you need to estimate the area enclosed by four cities, given the GPS locations (latitude and longitude) of these cities, as shown in the following diagram. How would you write a program to solve this problem? You will be able to write such a program after completing this chapter.
problem
Charlotte (35.2270869, –80.8431267)
Atlanta (33.7489954, –84.3879824)
Savannah (32.0835407, –81.0998342)
Orlando (28.5383355, –81.3792365)
Because strings are frequently used in programming, it is beneficial to introduce strings early so that you can begin to use them to develop useful programs. This chapter gives a brief introduction to string objects; you will learn more on objects and strings in Chapters 9 and 10.
4.2 Common Mathematical Functions Key Point
Java provides many useful methods in the Math class for performing common mathematical functions. A method is a group of statements that performs a specific task. You have already used the pow(a, b) method to compute ab in Section 2.9.4, Exponent Operations and the random() method for generating a random number in Section 3.7. This section introduces other useful methods in the Math class. They can be categorized as trigonometric methods, exponent methods, and service methods. Service methods include the rounding, min, max, absolute, and random methods. In addition to methods, the Math class provides two useful double constants, PI and E (the base of natural logarithms). You can use these constants as Math.PI and Math.E in any program.
4.2.1 VideoNote
Introduce math functions
Trigonometric Methods
The Math class contains the following methods as shown in Table 4.1 for performing trigonometric functions:
TABLE 4.1 Trigonometric Methods in the Math Class Method
Description
sin(radians)
Returns the trigonometric sine of an angle in radians.
cos(radians)
Returns the trigonometric cosine of an angle in radians.
tan(radians)
Returns the trigonometric tangent of an angle in radians.
toRadians(degree)
Returns the angle in radians for the angle in degree.
toDegree(radians)
Returns the angle in degrees for the angle in radians.
asin(a)
Returns the angle in radians for the inverse of sine.
acos(a)
Returns the angle in radians for the inverse of cosine.
atan(a)
Returns the angle in radians for the inverse of tangent.
4.2 Common Mathematical Functions 121 The parameter for sin, cos, and tan is an angle in radians. The return value for asin, acos, and atan is a degree in radians in the range between -p/2 and p/2. One degree is
equal to p/180 in radians, 90 degrees is equal to p/2 in radians, and 30 degrees is equal to p/6 in radians. For example, Math.toDegrees(Math.PI / 2) returns 90.0 Math.toRadians(30) returns 0.5236 (same as π/6) Math.sin(0) returns 0.0 Math.sin(Math.toRadians(270)) returns -1.0 Math.sin(Math.PI / 6) returns 0.5 Math.sin(Math.PI / 2) returns 1.0 Math.cos(0) returns 1.0 Math.cos(Math.PI / 6) returns 0.866 Math.cos(Math.PI / 2) returns 0 Math.asin(0.5) returns 0.523598333 (same as π/6) Math.acos(0.5) returns 1.0472 (same as π/3) Math.atan(1.0) returns 0.785398 (same as π/4)
4.2.2
Exponent Methods
There are five methods related to exponents in the Math class as shown in Table 4.2.
TABLE 4.2 Exponent Methods in the Math Class Method
Description
exp(x)
Returns e raised to power of x (ex).
log(x)
Returns the natural logarithm of x (ln(x) = loge(x)).
log10(x)
Returns the base 10 logarithm of x (log10(x)).
pow(a, b)
Returns a raised to the power of b (ab).
sqrt(x)
Returns the square root of x ( 2x) for x 7 = 0.
For example, Math.exp(1) returns 2.71828 Math.log(Math.E) returns 1.0 Math.log10(10) returns 1.0 Math.pow(2, 3) returns 8.0 Math.pow(3, 2) returns 9.0 Math.pow(4.5, 2.5) returns 22.91765 Math.sqrt(4) returns 2.0 Math.sqrt(10.5) returns 4.24
4.2.3
The Rounding Methods
The Math class contains five rounding methods as shown in Table 4.3.
TABLE 4.3 Rounding Methods in the Math Class Method
Description
ceil(x)
x is rounded up to its nearest integer. This integer is returned as a double value.
floor(x)
x is rounded down to its nearest integer. This integer is returned as a double value.
rint(x)
x is rounded up to its nearest integer. If x is equally close to two integers, the even one is returned as a double value.
round(x)
Returns (int)Math.floor(x + 0.5) if x is a float and returns (long)Math.floor(x + 0.5) if x is a double.
122 Chapter 4
Mathematical Functions, Characters, and Strings For example, Math.ceil(2.1) returns 4.0 Math.ceil(2.0) returns 2.0 Math.ceil(-2.0) returns -2.0 Math.ceil(-2.1) returns -2.0 Math.floor(2.1) returns 2.0 Math.floor(2.0) returns 2.0 Math.floor(-2.0) returns –2.0 Math.floor(-2.1) returns -4.0 Math.rint(2.1) returns 2.0 Math.rint(-2.0) returns –2.0 Math.rint(-2.1) returns -2.0 Math.rint(2.5) returns 2.0 Math.rint(4.5) returns 4.0 Math.rint(-2.5) returns -2.0 Math.round(2.6f) returns 3 // Returns int Math.round(2.0) returns 2 // Returns long Math.round(-2.0f) returns -2 // Returns int Math.round(-2.6) returns -3 // Returns long Math.round(-2.4) returns -2 // Returns long
4.2.4
The min, max, and abs Methods
The min and max methods return the minimum and maximum numbers of two numbers (int, long, float, or double). For example, max(4.4, 5.0) returns 5.0, and min(3, 2) returns 2. The abs method returns the absolute value of the number (int, long, float, or double). For example, Math.max(2, 3) returns 3 Math.max(2.5, 3) returns 4.0 Math.min(2.5, 4.6) returns 2.5 Math.abs(-2) returns 2 Math.abs(-2.1) returns 2.1
4.2.5
The random Method
You have used the random() method in the preceding chapter. This method generates a random double value greater than or equal to 0.0 and less than 1.0 (0 <= Math.random() < 1.0). You can use it to write a simple expression to generate random numbers in any range. For example,
(int)(Math.random() * 10) 50 + (int)(Math.random() * 50)
Returns a random integer between 0 and 9. Returns a random integer between 50 and 99.
In general,
a + Math.random() * b
Returns a random number between a and a + b, excluding a + b.
4.2 Common Mathematical Functions 123
4.2.6
Case Study: Computing Angles of a Triangle
You can use the math methods to solve many computational problems. Given the three sides of a triangle, for example, you can compute the angles by using the following formula: A = acos((a * a - b * b - c * c) / (-2 * b * c)) B = acos((b * b - a * a - c * c) / (-2 * a * c)) C = acos((c * c - b * b - a * a) / (-2 * a * b))
x2, y2
c
a
B
C A
x3, y3
b
x1, y1
Don’t be intimidated by the mathematic formula. As we discussed early in Listing 2.9, ComuteLoan.java, you don’t have to know how the mathematical formula is derived in order to write a program for computing the loan payments. Here in this example, given the length of three sides, you can use this formula to write a program to compute the angles without having to know how the formula is derived. In order to compute the lengths of the sides, we need to know the coordinates of three corner points and compute the distances between the points. Listing 4.1 is an example of a program that prompts the user to enter the x- and y-coordinates of the three corner points in a triangle and then displays the three angles.
LISTING 4.1 ComputeAngles.java 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
import java.util.Scanner; public class ComputeAngles { public static void main(String[] args) { Scanner input = new Scanner(System.in); // Prompt the user to enter three points System.out.print("Enter three points: "); double x1 = input.nextDouble(); double y1 = input.nextDouble(); double x2 = input.nextDouble(); double y2 = input.nextDouble(); double x3 = input.nextDouble(); double y3 = input.nextDouble(); // Compute three sides double a = Math.sqrt((x2 - x3) * (x2 - x3) + (y2 - y3) * (y2 - y3)); double b = Math.sqrt((x1 - x3) * (x1 - x3) + (y1 - y3) * (y1 - y3)); double c = Math.sqrt((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2));
enter three points
compute sides
// Compute three angles double A = Math.toDegrees(Math.acos((a * a - b * b - c * c) / (-2 * b * c))); double B = Math.toDegrees(Math.acos((b * b - a * a - c * c) / (-2 * a * c))); double C = Math.toDegrees(Math.acos((c * c - b * b - a * a) / (-2 * a * b))); // Display results System.out.println("The three angles are " + Math.round(A * 100) / 100.0 + " " +
display result
124 Chapter 4
Mathematical Functions, Characters, and Strings 35 36 37 38
Math.round(B * 100) / 100.0 + " " + Math.round(C * 100) / 100.0); } }
Enter three points: 1 1 6.5 1 6.5 2.5 The three angles are 15.26 90.0 74.74
The program prompts the user to enter three points (line 8). This prompting message is not clear. You should give the user explicit instructions on how to enter these points as follows: System.out.print("Enter the coordinates of three points separated " + "by spaces like x1 y1 x2 y2 x3 y3: ");
Note that the distance between two points (x1, y1) and (x2, y2) can be computed using the formula 2(x2 - x1)2 + (y2 - y1)2. The program computes the distances between two points (lines 17–22), and applies the formula to compute the angles (lines 25–30). The angles are rounded to display up to two digits after the decimal point (lines 34–36). The Math class is used in the program, but not imported, because it is in the java. lang package. All the classes in the java.lang package are implicitly imported in a Java program.
✓
Check Point
4.1
Evaluate the following method calls: (a) Math.sqrt(4) (b) Math.sin(2 * Math.PI)
(j) Math.floor(-2.5) (k) Math.round(-2.5f)
(c) Math.cos(2 * Math.PI) (d) Math.pow(2, 2)
(l) Math.round(-2.5) (m) Math.rint(2.5)
(e) (f) (g) (h) (i)
(n) (o) (p) (q) (r)
Math.log(Math.E) Math.exp(1) Math.max(2, Math.min(3, 4)) Math.rint(-2.5) Math.ceil(-2.5)
Math.ceil(2.5) Math.floor(2.5) Math.round(2.5f) Math.round(2.5) Math.round(Math.abs(-2.5))
4.2 4.3
True or false? The argument for trigonometric methods is an angle in radians. Write a statement that converts 47 degrees to radians and assigns the result to a variable.
4.4
Write a statement that converts π / 7 to an angle in degrees and assigns the result to a variable. Write an expression that obtains a random integer between 34 and 55. Write an expression that obtains a random integer between 0 and 999. Write an expression that obtains a random number between 5.5 and 55.5.
4.5 4.6 4.7
Why does the Math class not need to be imported? What is Math.log(Math.exp(5.5))? What is Math.exp(Math.log(5.5))? What is Math.asin(Math.sin(Math.PI / 6))? What is Math.sin(Math. asin(Math.PI / 6))?
4.3 Character Data Type and Operations 125
4.3 Character Data Type and Operations A character data type represents a single character. In addition to processing numeric values, you can process characters in Java. The character data type, char, is used to represent a single character. A character literal is enclosed in single quotation marks. Consider the following code:
Key Point char type
char letter = 'A'; char numChar = '4';
The first statement assigns character A to the char variable letter. The second statement assigns digit character 4 to the char variable numChar.
Caution A string literal must be enclosed in quotation marks (" "). A character literal is a single character enclosed in single quotation marks (' '). Therefore, "A" is a string, but 'A' is a character.
4.3.1
char literal
Unicode and ASCII code
Computers use binary numbers internally. A character is stored in a computer as a sequence of 0s and 1s. Mapping a character to its binary representation is called encoding. There are different ways to encode a character. How characters are encoded is defined by an encoding scheme. Java supports Unicode, an encoding scheme established by the Unicode Consortium to support the interchange, processing, and display of written texts in the world’s diverse languages. Unicode was originally designed as a 16-bit character encoding. The primitive data type char was intended to take advantage of this design by providing a simple data type that could hold any character. However, it turned out that the 65,536 characters possible in a 16-bit encoding are not sufficient to represent all the characters in the world. The Unicode standard therefore has been extended to allow up to 1,112,064 characters. Those characters that go beyond the original 16-bit limit are called supplementary characters. Java supports the supplementary characters. The processing and representing of supplementary characters are beyond the scope of this book. For simplicity, this book considers only the original 16-bit Unicode characters. These characters can be stored in a char type variable. A 16-bit Unicode takes two bytes, preceded by \u, expressed in four hexadecimal digits that run from \u0000 to \uFFFF. Hexadecimal numbers are introduced in Appendix F, Number Systems. For example, the English word welcome is translated into Chinese using two characters, . The Unicodes of these two characters are \u6B22\u8FCE. The Unicodes for the Greek letters a b g are \u03b1 \u03b2 \u03b4. Most computers use ASCII (American Standard Code for Information Interchange), an 8-bit encoding scheme for representing all uppercase and lowercase letters, digits, punctuation marks, and control characters. Unicode includes ASCII code, with \u0000 to \u007F corresponding to the 128 ASCII characters. Table 4.4 shows the ASCII code for some commonly used characters. Appendix B, ‘The ASCII Character Set,’ gives a complete list of ASCII characters and their decimal and hexadecimal codes.
TABLE 4.4 ASCII Code for Commonly Used Characters Characters
Code Value in Decimal
Unicode Value
'0' to '9'
48 to 57
\u0030 to \u0039
'A' to 'Z'
65 to 90
\u0041 to \u005A
'a' to 'z'
97 to 122
\u0061 to \u007A
encoding
Unicode original Unicode
supplementary Unicode
126 Chapter 4 ASCII
Mathematical Functions, Characters, and Strings You can use ASCII characters such as 'X', '1', and '$' in a Java program as well as Unicodes. Thus, for example, the following statements are equivalent: char letter = 'A'; char letter = '\u0041'; // Character A's Unicode is 0041
Both statements assign character A to the char variable letter.
Note The increment and decrement operators can also be used on char variables to get the next or preceding Unicode character. For example, the following statements display character b.
char increment and
decrement
char ch = 'a'; System.out.println(++ch);
4.3.2
Escape Sequences for Special Characters
Suppose you want to print a message with quotation marks in the output. Can you write a statement like this? System.out.println("He said "Java is fun"");
escape sequence
No, this statement has a compile error. The compiler thinks the second quotation character is the end of the string and does not know what to do with the rest of characters. To overcome this problem, Java uses a special notation to represent special characters, as shown in Table 4.5. This special notation, called an escape sequence, consists of a backslash (\) followed by a character or a combination of digits. For example, \t is an escape sequence for the Tab character and an escape sequence such as \u03b1 is used to represent a Unicode. The symbols in an escape sequence are interpreted as a whole rather than individually. An escape sequence is considered as a single character. So, now you can print the quoted message using the following statement: System.out.println("He said \"Java is fun\"");
The output is He said "Java is fun"
Note that the symbols \ and " together represent one character.
TABLE 4.5 Escape Sequences
escape character
Escape Sequence
Name
Unicode Code
Decimal Value
\b
Backspace
\u0008
\t
Tab
\u0009
9
\n
Linefeed
\u000A
10
\f
Formfeed
\u000C
12
\r
Carriage Return
\u000D
13
\\
Backslash
\u005C
92
\"
Double Quote
\u0022
34
8
The backslash \ is called an escape character. It is a special character. To display this character, you have to use an escape sequence \\. For example, the following code System.out.println("\\t is a tab character");
displays \t is a tab character
4.3 Character Data Type and Operations 127
4.3.3
Casting between char and Numeric Types
A char can be cast into any numeric type, and vice versa. When an integer is cast into a char, only its lower 16 bits of data are used; the other part is ignored. For example: char ch = (char)0XAB0041; // The lower 16 bits hex code 0041 is // assigned to ch System.out.println(ch); // ch is character A
When a floating-point value is cast into a char, the floating-point value is first cast into an int, which is then cast into a char. char ch = (char)65.25; System.out.println(ch);
// Decimal 65 is assigned to ch // ch is character A
When a char is cast into a numeric type, the character’s Unicode is cast into the specified numeric type. int i = (int)'A'; // The Unicode of character A is assigned to i System.out.println(i); // i is 65
Implicit casting can be used if the result of a casting fits into the target variable. Otherwise, explicit casting must be used. For example, since the Unicode of 'a' is 97, which is within the range of a byte, these implicit castings are fine: byte b = 'a'; int i = 'a';
But the following casting is incorrect, because the Unicode \uFFF4 cannot fit into a byte: byte b = '\uFFF4';
To force this assignment, use explicit casting, as follows: byte b = (byte)'\uFFF4';
Any positive integer between 0 and FFFF in hexadecimal can be cast into a character implicitly. Any number not in this range must be cast into a char explicitly. All numeric operators can be applied to char operands. A char operand is automatically cast into a number if the other operand is a number or a character. If the other operand is a string, the character is concatenated with the string. For example, the following statements int i = '2' + '3'; // (int)'2' is 50 and (int)'3' is 51 System.out.println("i is " + i); // i is 101 int j = 2 + 'a'; // (int)'a' is 97 System.out.println("j is " + j); // j is 99 System.out.println(j + " is the Unicode for character " + (char)j); // 99 is the Unicode for character c System.out.println("Chapter " + '2');
display i is 101 j is 99 99 is the Unicode for character c Chapter 2
numeric operators on characters
128 Chapter 4
Mathematical Functions, Characters, and Strings
4.3.4
Comparing and Testing Characters
Two characters can be compared using the relational operators just like comparing two numbers. This is done by comparing the Unicodes of the two characters. For example, 'a' < 'b' is true because the Unicode for 'a' (97) is less than the Unicode for 'b' (98). 'a' < 'A' is false because the Unicode for 'a' (97) is greater than the Unicode for 'A' (65). '1' < '8' is true because the Unicode for '1' (49) is less than the Unicode for '8' (56).
Often in the program, you need to test whether a character is a number, a letter, an uppercase letter, or a lowercase letter. As shown in Appendix B, the ASCII character set, that the Unicodes for lowercase letters are consecutive integers starting from the Unicode for 'a', then for 'b', 'c', . . ., and 'z'. The same is true for the uppercase letters and for numeric characters. This property can be used to write the code to test characters. For example, the following code tests whether a character ch is an uppercase letter, a lowercase letter, or a digital character. if (ch >= 'A' && ch <= 'Z') System.out.println(ch + " else if (ch >= 'a' && ch <= System.out.println(ch + " else if (ch >= '0' && ch <= System.out.println(ch + "
is an uppercase letter"); 'z') is a lowercase letter"); '9') is a numeric character");
For convenience, Java provides the following methods in the Character class for testing characters as shown in Table 4.6.
TABLE 4.6 Methods in the Character Class Method
Description
isDigit(ch)
Returns true if the specified character is a digit.
isLetter(ch)
Returns true if the specified character is a letter.
isLetterOfDigit(ch)
Returns true if the specified character is a letter or digit.
isLowerCase(ch)
Returns true if the specified character is a lowercase letter.
isUpperCase(ch)
Returns true if the specified character is an uppercase letter.
toLowerCase(ch)
Returns the lowercase of the specified character.
toUpperCase(ch)
Returns the uppercase of the specified character.
For example, System.out.println("isDigit('a') is " + Character.isDigit('a')); System.out.println("isLetter('a') is " + Character.isLetter('a')); System.out.println("isLowerCase('a') is " + Character.isLowerCase('a')); System.out.println("isUpperCase('a') is " + Character.isUpperCase('a')); System.out.println("toLowerCase('T') is " + Character.toLowerCase('T')); System.out.println("toUpperCase('q') is " + Character.toUpperCase('q'));
displays isDigit('a') is false isLetter('a') is true
4.3 Character Data Type and Operations 129 isLowerCase('a') isUpperCase('a') toLowerCase('T') toUpperCase('q')
is is is is
true false t Q
4.8
Use print statements to find out the ASCII code for '1', 'A', 'B', 'a', and 'b'. Use print statements to find out the character for the decimal codes 40, 59, 79, 85, and 90. Use print statements to find out the character for the hexadecimal code 40, 5A, 71, 72, and 7A.
4.9
Which of the following are correct literals for characters? '1', '\u345dE', '\u3fFa', '\b', '\t'
4.10 4.11
How do you display the characters \ and "? Evaluate the following: int i = '1'; int j = '1' + '2' * ('4' - '3') + 'b' / 'a'; int k = 'a'; char c = 90;
4.12
Can the following conversions involving casting be allowed? If so, find the converted result. char c = 'A'; int i = (int)c; float f = 1000.34f; int i = (int)f; double d = 1000.34; int i = (int)d; int i = 97; char c = (char)i;
4.13
Show the output of the following program: public class Test { public static void main(String[] args) { char x = 'a'; char y = 'c'; System.out.println(++x); System.out.println(y++); System.out.println(x - y); } }
4.14 4.15
Write the code that generates a random lowercase letter. Show the output of the following statements: System.out.println('a' System.out.println('a' System.out.println('a' System.out.println('a' System.out.println('a' System.out.println('a'
< 'b'); <= 'A'); > 'b'); >= 'A'); == 'a'); != 'b');
✓
Check Point
130 Chapter 4
Mathematical Functions, Characters, and Strings
4.4 The String Type Key Point
VideoNote
Introduce strings and objects
A string is a sequence of characters. The char type represents only one character. To represent a string of characters, use the data type called String. For example, the following code declares message to be a string with the value "Welcome to Java". String message = "Welcome to Java";
String is a predefined class in the Java library, just like the classes System and Scanner. The String type is not a primitive type. It is known as a reference type. Any Java class can be used as a reference type for a variable. The variable declared by a reference type is known as a reference variable that references an object. Here, message is a reference variable that references a string object with contents Welcome to Java. Reference data types will be discussed in detail in Chapter 9, Objects and Classes. For the time being, you need to know only how to declare a String variable, how to assign a string to the variable, and how to use the methods in the String class. More details on using strings will be covered in Chapter 10. Table 4.7 lists the String methods for obtaining string length, for accessing characters in the string, for concatenating strings, for converting a string to upper or lowercases, and for trimming a string.
TABLE 4.7 Simple Methods for String Objects Method
Description
length()
Returns the number of characters in this string.
charAt(index)
Returns the character at the specified index from this string.
concat(s1)
Returns a new string that concatenates this string with string s1.
toUpperCase()
Returns a new string with all letters in uppercase.
toLowerCase()
Returns a new string with all letters in lowercase
trim()
Returns a new string with whitespace characters trimmed on both sides.
instance method static method
Strings are objects in Java. The methods in Table 4.7 can only be invoked from a specific string instance. For this reason, these methods are called instance methods. A noninstance method is called a static method. A static method can be invoked without using an object. All the methods defined in the Math class are static methods. They are not tied to a specific object instance. The syntax to invoke an instance method is referenceVariable.methodName(arguments). A method may have many arguments or no arguments. For example, the charAt(index) method has one argument, but the length() method has no arguments. Recall that the syntax to invoke a static method is ClassName .methodName(arguments). For example, the pow method in the Math class can be invoked using Math.pow(2, 2.5).
4.4.1
Getting String Length
You can use the length() method to return the number of characters in a string. For example, the following code String message = "Welcome to Java"; System.out.println("The length of " + message + " is " + message.length());
4.4 The String Type 131 displays The length of Welcome to Java is 15
Note When you use a string, you often know its literal value. For convenience, Java allows you to use the string literal to refer directly to strings without creating new variables. Thus, "Welcome to Java".length() is correct and returns 15. Note that "" denotes an empty string and "".length() is 0.
4.4.2
string literal empty string
Getting Characters from a String
The s.charAt(index) method can be used to retrieve a specific character in a string s, where the index is between 0 and s.length()–1. For example, message.charAt(0) returns the character W, as shown in Figure 4.1. Note that the index for the first character in the string is 0.
Indices message
0
1
2
3
4
5
6
W
e
l
c
o
m
e
message.charAt(0)
FIGURE 4.1
7
8
9
t
o
charAt(index)
10 11 12 13 14
message.length() is 15
J
a
v
a
message.charAt(14)
The characters in a String object can be accessed using its index.
Caution Attempting to access characters in a string s out of bounds is a common programming error. To avoid it, make sure that you do not use an index beyond s.length() – 1. For example, s.charAt(s.length()) would cause a StringIndexOutOfBoundsException.
4.4.3
string index range
Concatenating Strings
You can use the concat method to concatenate two strings. The statement shown below, for example, concatenates strings s1 and s2 into s3: String s3 = s1.concat(s2);
s1.concat(s2)
Because string concatenation is heavily used in programming, Java provides a convenient way to accomplish it. You can use the plus (+) operator to concatenate two strings, so the previous statement is equivalent to String s3 = s1 + s2;
s1 + s2
The following code combines the strings message, " and ", and "HTML" into one string: String myString = message + " and " + "HTML";
Recall that the + operator can also concatenate a number with a string. In this case, the number is converted into a string and then concatenated. Note that at least one of the operands must be a string in order for concatenation to take place. If one of the operands is a nonstring
concatenate strings and numbers
132 Chapter 4
Mathematical Functions, Characters, and Strings (e.g., a number), the nonstring value is converted into a string and concatenated with the other string. Here are some examples: // Three strings are concatenated String message = "Welcome " + "to " + "Java"; // String Chapter is concatenated with number 2 String s = "Chapter" + 2; // s becomes Chapter2 // String Supplement is concatenated with character B String s1 = "Supplement" + 'B'; // s1 becomes SupplementB
If neither of the operands is a string, the plus sign (+) is the addition operator that adds two numbers. The augmented += operator can also be used for string concatenation. For example, the following code appends the string "and Java is fun" with the string "Welcome to Java" in message. message += " and Java is fun";
So the new message is "Welcome to Java and Java is fun". If i = 1 and j = 2, what is the output of the following statement? System.out.println("i + j is " + i + j);
The output is "i + j is 12" because "i + j is " is concatenated with the value of i first. To force i + j to be executed first, enclose i + j in the parentheses, as follows: System.out.println("i + j is " + (i + j));
4.4.4
Converting Strings
The toLowerCase() method returns a new string with all lowercase letters and the toUpperCase() method returns a new string with all uppercase letters. For example, toLowerCase() toUpperCase()
whitespace character
trim()
"Welcome".toLowerCase() returns a new string welcome. "Welcome".toUpperCase() returns a new string WELCOME.
The trim() method returns a new string by eliminating whitespace characters from both ends of the string. The characters ' ', \t, \f, \r, or \n are known as whitespace characters. For example, "\t Good Night \n".trim() returns a new string Good Night.
4.4.5 read strings
Reading a String from the Console
To read a string from the console, invoke the next() method on a Scanner object. For example, the following code reads three strings from the keyboard: Scanner input = new Scanner(System.in); System.out.print("Enter three words separated by spaces: "); String s1 = input.next(); String s2 = input.next(); String s3 = input.next(); System.out.println("s1 is " + s1); System.out.println("s2 is " + s2); System.out.println("s3 is " + s3);
4.4 The String Type 133 Enter s1 is s2 is s3 is
three words separated by spaces: Welcome to Java Welcome to Java
The next() method reads a string that ends with a whitespace character. You can use the nextLine() method to read an entire line of text. The nextLine() method reads a string that ends with the Enter key pressed. For example, the following statements read a line of text.
whitespace character
Scanner input = new Scanner(System.in); System.out.println("Enter a line: "); String s = input.nextLine(); System.out.println("The line entered is " + s);
Enter a line: Welcome to Java The line entered is Welcome to Java
Important Caution To avoid input errors, do not use nextLine() after nextByte(), nextShort(), nextInt(), nextLong(), nextFloat(), nextDouble(), or next(). The reasons will be explained in Section 12.11.4, ‘How Does Scanner Work?’
4.4.6
avoid input errors
Reading a Character from the Console
To read a character from the console, use the nextLine() method to read a string and then invoke the charAt(0) method on the string to return a character. For example, the following code reads a character from the keyboard: Scanner input = new Scanner(System.in); System.out.print("Enter a character: "); String s = input.nextLine(); char ch = s.charAt(0); System.out.println("The character entered is " + ch);
4.4.7
Comparing Strings
The String class contains the methods as shown in Table 4.8 for comparing two strings.
TABLE 4.8 Comparison Methods for String Objects Method
Description
equals(s1)
Returns true if this string is equal to string s1.
equalsIgnoreCase(s1)
Returns true if this string is equal to string s1; it is case insensitive.
compareTo(s1)
Returns an integer greater than 0, equal to 0, or less than 0 to indicate whether this string is greater than, equal to, or less than s1.
compareToIgnoreCase(s1)
Same as compareTo except that the comparison is case insensitive.
startsWith(prefix)
Returns true if this string starts with the specified prefix.
endsWith(suffix)
Returns true if this string ends with the specified suffix.
contains(s1)
Returns true if s1 is a substring in this string.
134 Chapter 4
Mathematical Functions, Characters, and Strings How do you compare the contents of two strings? You might attempt to use the == operator, as follows:
==
if (string1 == string2) System.out.println("string1 and string2 are the same object"); else System.out.println("string1 and string2 are different objects");
However, the == operator checks only whether string1 and string2 refer to the same object; it does not tell you whether they have the same contents. Therefore, you cannot use the == operator to find out whether two string variables have the same contents. Instead, you should use the equals method. The following code, for instance, can be used to compare two strings: string1.equals(string2)
if (string1.equals(string2)) System.out.println("string1 and string2 have the same contents"); else System.out.println("string1 and string2 are not equal");
For example, the following statements display true and then false. String s1 = "Welcome to Java"; String s2 = "Welcome to Java"; String s3 = "Welcome to C++"; System.out.println(s1.equals(s2)); // true System.out.println(s1.equals(s3)); // false
The compareTo method can also be used to compare two strings. For example, consider the following code: s1.compareTo(s2)
s1.compareTo(s2)
The method returns the value 0 if s1 is equal to s2, a value less than 0 if s1 is lexicographically (i.e., in terms of Unicode ordering) less than s2, and a value greater than 0 if s1 is lexicographically greater than s2. The actual value returned from the compareTo method depends on the offset of the first two distinct characters in s1 and s2 from left to right. For example, suppose s1 is abc and s2 is abg, and s1.compareTo(s2) returns -4. The first two characters (a vs. a) from s1 and s2 are compared. Because they are equal, the second two characters (b vs. b) are compared. Because they are also equal, the third two characters (c vs. g) are compared. Since the character c is 4 less than g, the comparison returns -4.
Caution Syntax errors will occur if you compare strings by using relational operators >, >=, <, or <=. Instead, you have to use s1.compareTo(s2).
Note The equals method returns true if two strings are equal and false if they are not. The compareTo method returns 0, a positive integer, or a negative integer, depending on whether one string is equal to, greater than, or less than the other string.
The String class also provides the equalsIgnoreCase and compareToIgnoreCase methods for comparing strings. The equalsIgnoreCase and compareToIgnoreCase methods ignore the case of the letters when comparing two strings. You can also use str.startsWith(prefix) to check whether string str starts with a specified prefix, str.endsWith(suffix) to check whether string str ends with a specified suffix, and str .contains(s1) to check whether string str contains string s1 . For example, "Welcome to Java".startsWith("We") returns true. "Welcome to Java".startsWith("we") returns false. "Welcome to Java".endsWith("va") returns true.
4.4 The String Type 135 "Welcome to Java".endsWith("v") returns false. "Welcome to Java".contains("to") returns true. "Welcome to Java".contains("To") returns false.
Listing 4.2 gives a program that prompts the user to enter two cities and displays them in alphabetical order.
LISTING 4.2 OrderTwoCities.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
import java.util.Scanner; public class OrderTwoCities { public static void main(String[] args) { Scanner input = new Scanner(System.in); // Prompt the user to enter two cities System.out.print("Enter the first city: "); String city1 = input.nextLine(); System.out.print("Enter the second city: "); String city2 = input.nextLine(); if (city1.compareTo(city2) < 0) System.out.println("The cities in alphabetical order are " + city1 + " " + city2); else System.out.println("The cities in alphabetical order are " + city2 + " " + city1);
input city1 input city2 compare two cities
} }
Enter the first city: New York Enter the second city: Boston The cities in alphabetical order are Boston New York
The program reads two strings for two cities (lines 9, 11). If input.nextLine() is replaced by input.next() (line 9), you cannot enter a string with spaces for city1. Since a city name may contain multiple words separated by spaces, the program uses the nextLine method to read a string (lines 9, 11). Invoking city1.compareTo(city2) compares two strings city1 with city2 (line 13). A negative return value indicates that city1 is less than city2.
4.4.8
Obtaining Substrings
You can obtain a single character from a string using the charAt method. You can also obtain a substring from a string using the substring method in the String class, as shown in Table 4.9. For example, String message = "Welcome to Java"; String message = message.substring(0, 11) + "HTML"; The string message now becomes Welcome to HTML.
TABLE 4.9 The String class contains the methods for obtaining substrings. Method
Description
substring(beginIndex)
Returns this string’s substring that begins with the character at the specified beginIndex and extends to the end of the string, as shown in Figure 4.2.
substring(beginIndex,
Returns this string’s substring that begins at the specified beginIndex and extends to the character at index endIndex – 1, as shown in Figure 4.2. Note that the character at endIndex is not part of the substring.
endIndex)
136 Chapter 4
Mathematical Functions, Characters, and Strings Indices Message
0
1
2
3
4
5
6
W
e
l
c
o
m
e
7
8
9
t
o
message.substring(0, 11)
FIGURE 4.2
10 11 12 13 14 J
a
v
a
message.substring(11)
The substring method obtains a substring from a string.
Note beginIndex <= endIndex
If beginIndex is endIndex, substring(beginIndex, endIndex) returns an empty string with length 0. If beginIndex > endIndex, it would be a runtime error.
4.4.9
Finding a Character or a Substring in a String
The String class provides several versions of indexOf and lastIndexOf methods to find a character or a substring in a string, as shown in Table 4.10.
TABLE 4.10 The String class contains the methods for finding substrings. Method
Description
index(ch)
Returns the index of the first occurrence of ch in the string. Returns -1 if not matched.
indexOf(ch, fromIndex)
Returns the index of the first occurrence of ch after fromIndex in the string. Returns -1 if not matched.
indexOf(s)
Returns the index of the first occurrence of string s in this string. Returns -1 if not matched.
indexOf(s, fromIndex)
Returns the index of the first occurrence of string s in this string after fromIndex. Returns -1 if not matched.
lastIndexOf(ch)
Returns the index of the last occurrence of ch in the string. Returns -1 if not matched.
lastIndexOf(ch, fromIndex)
Returns the index of the last occurrence of ch before fromIndex in this string. Returns -1 if not matched.
lastIndexOf(s)
Returns the index of the last occurrence of string s. Returns -1 if not matched.
lastIndexOf(s, fromIndex)
Returns the index of the last occurrence of string s before fromIndex. Returns -1 if not matched.
For example, indexOf
"Welcome "Welcome "Welcome "Welcome "Welcome "Welcome
to to to to to to
Java".indexOf('W') returns 0. Java".indexOf('o') returns 4. Java".indexOf('o', 5) returns 9. Java".indexOf("come") returns 3. Java".indexOf("Java", 5) returns 11. Java".indexOf("java", 5) returns -1.
lastIndexOf
"Welcome "Welcome "Welcome "Welcome "Welcome "Welcome
to to to to to to
Java".lastIndexOf('W') returns 0. Java".lastIndexOf('o') returns 9. Java".lastIndexOf('o', 5) returns 4. Java".lastIndexOf("come") returns 3. Java".lastIndexOf("Java", 5) returns -1. Java".lastIndexOf("Java") returns 11.
Suppose a string s contains the first name and last name separated by a space. You can use the following code to extract the first name and last name from the string: int k = s.indexOf(' '); String firstName = s.substring(0, k); String lastName = s.substring(k + 1);
4.4 The String Type 137 For example, if s is Kim Jones, the following diagram illustrates how the first name and last name are extracted. Indices Message
0
1
2
K
i
m
3
4
5
6
7
8
J
o
n
e
s
k is 3 s.substring (0, k) is Kim
4.4.10
s.substring (k + 1) is Jones
Conversion between Strings and Numbers
You can convert a numeric string into a number. To convert a string into an int value, use the Integer.parseInt method, as follows:
Integer.parseInt method
int intValue = Integer.parseInt(intString);
where intString is a numeric string such as "123". To convert a string into a double value, use the Double.parseDouble method, as follows:
Double.parseDouble
method
double doubleValue = Double.parseDouble(doubleString);
where doubleString is a numeric string such as "123.45". If the string is not a numeric string, the conversion would cause a runtime error. The Integer and Double classes are both included in the java.lang package, and thus they are automatically imported. You can convert a number into a string, simply use the string concatenating operator as follows: String s = number + "";
4.16
number to string
Suppose that s1, s2, and s3 are three strings, given as follows: String s1 = "Welcome to Java"; String s2 = "Programming is fun"; String s3 = "Welcome to Java";
What are the results of the following expressions? (a) (b) (c) (d) (e) (f) (g) (h) (i) (j)
s1 == s2 s2 == s3 s1.equals(s2) s1.equals(s3) s1.compareTo(s2) s2.compareTo(s3) s2.compareTo(s2) s1.charAt(0) s1.indexOf('j') s1.indexOf("to")
(k) s1.lastIndexOf('a')
(l) (m) (n) (o) (p) (q) (r) (s) (t) (u)
s1.lastIndexOf("o", 15) s1.length() s1.substring(5) s1.substring(5, 11) s1.startsWith("Wel") s1.endsWith("Java") s1.toLowerCase() s1.toUpperCase() s1.concat(s2) s1.contains(s2)
(v) "\t Wel \t".trim()
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Mathematical Functions, Characters, and Strings 4.17
Suppose that s1 and s2 are two strings. Which of the following statements or expressions are incorrect? String s = "Welcome to Java"; String s3 = s1 + s2; String s3 = s1 - s2; s1 == s2; s1 >= s2; s1.compareTo(s2); int i = s1.length(); char c = s1(0); char c = s1.charAt(s1.length());
4.18
Show the output of the following statements (write a program to verify your results): System.out.println("1" System.out.println('1' System.out.println("1" System.out.println("1" System.out.println('1'
4.19
1); 1); 1 + 1); (1 + 1)); 1 + 1);
Evaluate the following expressions (write a program to verify your results): 1 1 1 1
4.20
+ + + + +
+ + + +
"Welcome "Welcome "Welcome "Welcome
" " " "
+ + + +
1 + 1 (1 + 1) ('\u0001' + 1) 'a' + 1
Let s1 be " Welcome " and s2 be " welcome ". Write the code for the following statements: (a) Check whether s1 is equal to s2 and assign the result to a Boolean variable isEqual. (b) Check whether s1 is equal to s2, ignoring case, and assign the result to a Boolean variable isEqual. (c) Compare s1 with s2 and assign the result to an int variable x. (d) Compare s1 with s2, ignoring case, and assign the result to an int variable x. (e) Check whether s1 has the prefix AAA and assign the result to a Boolean variable b. (f) Check whether s1 has the suffix AAA and assign the result to a Boolean variable b. (g) Assign the length of s1 to an int variable x. (h) Assign the first character of s1 to a char variable x. (i) Create a new string s3 that combines s1 with s2. (j) Create a substring of s1 starting from index 1. (k) Create a substring of s1 from index 1 to index 4. (l) Create a new string s3 that converts s1 to lowercase. (m) Create a new string s3 that converts s1 to uppercase. (n) Create a new string s3 that trims whitespace characters on both ends of s1.
4.5 Case Studies 139 (o) Assign the index of the first occurrence of the character e in s1 to an int variable x. (p) Assign the index of the last occurrence of the string abc in s1 to an int variable x.
4.5 Case Studies Strings are fundamental in programming. The ability to write programs using strings is essential in learning Java programming.
Key Point
You will frequently use strings to write useful programs. This section presents three examples of solving problems using strings.
4.5.1
Case Study: Guessing Birthdays
You can find out the date of the month when your friend was born by asking five questions. Each question asks whether the day is in one of the five sets of numbers.
= 19 +
3 5 7 1 9 11 13 15 17 19 21 23 25 27 29 31
2 3 6 7 10 11 14 15 18 19 22 23 26 27 30 31
4 5 6 7 12 13 14 15 20 21 22 23 28 29 30 31
8 9 10 11 12 13 14 15 24 25 26 27 28 29 30 31
Set1
Set2
Set3
Set4
16 20 24 28
17 21 25 29
18 22 26 30
19 23 27 31
Set5
The birthday is the sum of the first numbers in the sets where the day appears. For example, if the birthday is 19, it appears in Set1, Set2, and Set5. The first numbers in these three sets are 1, 2, and 16. Their sum is 19. Listing 4.3 gives a program that prompts the user to answer whether the day is in Set1 (lines 41–44), in Set2 (lines 50–53), in Set3 (lines 59–62), in Set4 (lines 68–71), and in Set5 (lines 77–80). If the number is in the set, the program adds the first number in the set to day (lines 47, 56, 65, 74, 83).
LISTING 4.3 GuessBirthday.java 1 2 3 4 5 6 7 8 9 10 11 12
import java.util.Scanner; public class GuessBirthday { public static void main(String[] args) { String set1 = " 1 3 5 7\n" + " 9 11 13 15\n" + "17 19 21 23\n" + "25 27 29 31"; String set2 = " 2 3 6 7\n" +
140 Chapter 4
day to be determined
in Set1?
in Set2?
in Set3?
Mathematical Functions, Characters, and Strings 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 64 65 66 67 68 69 70 71 72
"10 11 14 15\n" + "18 19 22 23\n" + "26 27 30 31"; String set3 " 4 5 6 "12 13 14 "20 21 22 "28 29 30
=
String set4 " 8 9 10 "12 13 14 "24 25 26 "28 29 30
= 11\n" + 15\n" + 27\n" + 31";
String set5 "16 17 18 "20 21 22 "24 25 26 "28 29 30
= 19\n" + 23\n" + 27\n" + 31";
7\n" + 15\n" + 23\n" + 31";
int day = 0; // Create a Scanner Scanner input = new Scanner(System.in); // Prompt the user to answer questions System.out.print("Is your birthday in Set1?\n"); System.out.print(set1); System.out.print("\nEnter 0 for No and 1 for Yes: "); int answer = input.nextInt(); if (answer == 1) day += 1; // Prompt the user to answer questions System.out.print("\nIs your birthday in Set2?\n"); System.out.print(set2); System.out.print("\nEnter 0 for No and 1 for Yes: "); answer = input.nextInt(); if (answer == 1) day += 2; // Prompt the user to answer questions System.out.print("Is your birthday in Set3?\n"); System.out.print(set3); System.out.print("\nEnter 0 for No and 1 for Yes: "); answer = input.nextInt(); if (answer == 1) day += 4; // Prompt the user to answer questions System.out.print("\nIs your birthday in Set4?\n"); System.out.print(set4); System.out.print("\nEnter 0 for No and 1 for Yes: "); answer = input.nextInt();
4.5 Case Studies 141 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87
if (answer == 1) day += 8;
in Set4?
// Prompt the user to answer questions System.out.print("\nIs your birthday in Set5?\n"); System.out.print(set5); System.out.print("\nEnter 0 for No and 1 for Yes: "); answer = input.nextInt(); if (answer == 1) day += 16; System.out.println("\nYour birthday is " + day + "!"); } }
Is your birthday in Set1? 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 Enter 0 for No and 1 for Yes: 1 Is your birthday in Set2? 2 3 6 7 10 11 14 15 18 19 22 23 26 27 30 31 Enter 0 for No and 1 for Yes: 1 Is your birthday in Set3? 4 5 6 7 12 13 14 15 20 21 22 23 28 29 30 31 Enter 0 for No and 1 for Yes: 0 Is your birthday in Set4? 8 9 10 11 12 13 14 15 24 25 26 27 28 29 30 31 Enter 0 for No and 1 for Yes: 0 Is your birthday in Set5? 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 Enter 0 for No and 1 for Yes: 1 Your birthday is 19!
in Set5?
142 Chapter 4
Mathematical Functions, Characters, and Strings line# 35
day
answer
0
44 47
1 1
53 56
1 3
62
0
71
0
80
1
83
output
19
85
mathematics behind the game
Your birthday is 19!
This game is easy to program. You may wonder how the game was created. The mathematics behind the game is actually quite simple. The numbers are not grouped together by accident— the way they are placed in the five sets is deliberate. The starting numbers in the five sets are 1, 2, 4, 8, and 16, which correspond to 1, 10, 100, 1000, and 10000 in binary (binary numbers are introduced in Appendix F, Number Systems). A binary number for decimal integers between 1 and 31 has at most five digits, as shown in Figure 4.3a. Let it be b5b4b3b2b1. Thus, b5b4b3b2b1 = b50000 + b4000 + b300 + b20 + b1,as shown in Figure 4.3b. If a day’s binary number has a digit 1 in bk, the number should appear in Setk. For example, number 19 is binary 10011, so it appears in Set1, Set2, and Set5. It is binary 1 + 10 + 10000 = 10011 or decimal 1 + 2 + 16 = 19. Number 31 is binary 11111, so it appears in Set1, Set2, Set3, Set4, and Set5. It is binary 1 + 10 + 100 + 1000 + 10000 = 11111 or decimal 1 + 2 + 4 + 8 + 16 = 31.
Decimal 1 2 3 ... 19 ... 31
Binary 00001 00010 00011 10011 11111 (a)
b5 0 0 0 b4 0 0 b3 0 b2 +
0 0 0 0 b1
10000 10 + 1 10011
b5 b 4 b 3 b 2 b 1
19
10000 1000 100 10 + 1 11111 31
(b)
FIGURE 4.3 (a) A number between 1 and 31 can be represented using a five-digit binary number. (b) A five-digit binary number can be obtained by adding binary numbers 1, 10, 100, 1000, or 10000.
✓
Check Point
4.21
If you run Listing 4.3 GuessBirthday.java with input 1 for Set1, Set3, and Set4 and 0 for Set2 and Set5, what will be the birthday?
4.5.2 Case Study: Converting a Hexadecimal Digit to a Decimal Value The hexadecimal number system has 16 digits: 0–9, A–F. The letters A, B, C, D, E, and F correspond to the decimal numbers 10, 11, 12, 13, 14, and 15. We now write a program that prompts the user to enter a hex digit and display its corresponding decimal value, as shown in Listing 4.4.
4.5 Case Studies 143
LISTING 4.4 HexDigit2Dec.java 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
import java.util.Scanner;
VideoNote
Convert hex to decimal public class HexDigit2Dec { public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.print("Enter a hex digit: "); String hexString = input.nextLine(); // Check if the hex string has exactly one character if (hexString.length() != 1) { System.out.println("You must enter exactly one character"); System.exit(1); } // Display decimal value for the hex digit char ch = hexString.charAt(0); if (ch <= 'F' && ch >= 'A') { int value = ch - 'A' + 10; System.out.println("The decimal value for hex digit " + ch + " is " + value); } else if (Character.isDigit(ch)) { System.out.println("The decimal value for hex digit " + ch + " is " + ch); } else { System.out.println(ch + " is an invalid input"); } } }
Enter a hex digit: AB7C You must enter exactly one character
Enter a hex digit: B The decimal value for hex digit B is 11
Enter a hex digit: 8 The decimal value for hex digit 8 is 8
Enter a hex digit: T T is an invalid input
The program reads a string from the console (line 7) and checks if the string contains a single character (line 10). If not, report an error and exit the program (line 12). The program invokes the Character.toUpperCase method to obtain the character ch as an uppercase letter (line 16). If ch is between 'A' and 'F' (line 17), the corresponding decimal value is ch – 'A' + 10 (line 18). Note that ch – 'A' is 0 if ch is 'A', ch – 'A' is 1
input string
check length
is A-F?
is 0-9?
144 Chapter 4
Mathematical Functions, Characters, and Strings if ch is 'B', and so on. When two characters perform a numerical operation, the characters’ Unicodes are used in the computation. The program invokes the Character.isDigit(ch) method to check if ch is between '0' and '9' (line 22). If so, the corresponding decimal digit is the same as ch (lines 23–24). If ch is not between 'A' and 'F' nor a digit character, the program displays an error message (line 27).
4.5.3
Case Study: Revising the Lottery Program Using Strings
The lottery program in Listing 3.8, Lottery.java, generates a random two-digit number, prompts the user to enter a two-digit number, and determines whether the user wins according to the following rule: 1. If the user input matches the lottery number in the exact order, the award is $10,000. 2. If all the digits in the user input match all the digits in the lottery number, the award is $3,000. 3. If one digit in the user input matches a digit in the lottery number, the award is $1,000. The program in Listing 3.8 uses an integer to store the number. Listing 4.5 gives a new program that generates a random two-digit string instead of a number and receives the user input as a string instead of a number.
LISTING 4.5 LotteryUsingStrings.java
generate a lottery
enter a guess
exact match? match all digits?
match one digit?
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
import java.util.Scanner; public class LotteryUsingStrings { public static void main(String[] args) { // Generate a lottery as a two-digit string String lottery = "" + (int)(Math.random() * 10) + (int)(Math.random() * 10); // Prompt the user to enter a guess Scanner input = new Scanner(System.in); System.out.print("Enter your lottery pick (two digits): "); String guess = input.nextLine(); // Get digits from lottery char lotteryDigit1 = lottery.charAt(0); char lotteryDigit2 = lottery.charAt(1); // Get digits from guess char guessDigit1 = guess.charAt(0); char guessDigit2 = guess.charAt(1); System.out.println("The lottery number is " + lottery); // Check the guess if (guess.equals(lottery)) System.out.println("Exact match: you win $10,000"); else if (guessDigit2 == lotteryDigit1 && guessDigit1 == lotteryDigit2) System.out.println("Match all digits: you win $3,000"); else if (guessDigit1 == lotteryDigit1 || guessDigit1 == lotteryDigit2 || guessDigit2 == lotteryDigit1 || guessDigit2 == lotteryDigit2) System.out.println("Match one digit: you win $1,000");
4.6 Formatting Console Output 145 35 36 37 38
else System.out.println("Sorry, no match"); } }
Enter your lottery pick (two digits): 00 The lottery number is 00 Exact match: you win $10,000
Enter your lottery pick (two digits): 45 The lottery number is 54 Match all digits: you win $3,000
Enter your lottery pick: 23 The lottery number is 34 Match one digit: you win $1,000
Enter your lottery pick: 23 The lottery number is 14 Sorry: no match
The program generates two random digits and concatenates them into the string lottery (lines 6–7). After this, lottery contains two random digits. The program prompts the user to enter a guess as a two-digit string (line 12) and checks the guess against the lottery number in this order: ■
First check whether the guess matches the lottery exactly (line 25).
■
If not, check whether the reversal of the guess matches the lottery (line 27).
■
If not, check whether one digit is in the lottery (lines 30–33).
■
If not, nothing matches and display “Sorry, no match” (line 36).
4.6 Formatting Console Output You can use the System.out.printf method to display formatted output on the console. Often, it is desirable to display numbers in a certain format. For example, the following code computes interest, given the amount and the annual interest rate. double amount = 12618.98; double interestRate = 0.0013; double interest = amount * interestRate; System.out.println("Interest is $" + interest);
Interest is $16.404674
Key Point
146 Chapter 4
Mathematical Functions, Characters, and Strings Because the interest amount is currency, it is desirable to display only two digits after the decimal point. To do this, you can write the code as follows: double amount = 12618.98; double interestRate = 0.0013; double interest = amount * interestRate; System.out.println("Interest is $" + (int)(interest * 100) / 100.0);
Interest is $16.4
However, the format is still not correct. There should be two digits after the decimal point: printf
16.40 rather than 16.4. You can fix it by using the printf method, like this: double amount = 12618.98; double interestRate = 0.0013; double interest = amount * interestRate; System.out.printf("Interest is $%4.2f", interest);
% 4 . 2 f field width
format specifier
conversion code
precision
Interest is $16.40
The syntax to invoke this method is System.out.printf(format, item1, item2, ..., itemk)
format specifier
where format is a string that may consist of substrings and format specifiers. A format specifier specifies how an item should be displayed. An item may be a numeric value, a character, a Boolean value, or a string. A simple format specifier consists of a percent sign (%) followed by a conversion code. Table 4.11 lists some frequently used simple format specifiers.
TABLE 4.11 Frequently Used Format Specifiers Format Specifier
Output
Example
%b
a Boolean value
true or false
%c
a character
‘a’
%d
a decimal integer
200
%f
a floating-point number
45.460000
%e
a number in standard scientific notation
4.556000e + 01
%s
a string
“Java is cool”
Here is an example: items int count = 5; double amount = 45.56; System.out.printf("count is %d and amount is %f", count, amount);
display
count is 5 and amount is 45.560000
4.6 Formatting Console Output 147 Items must match the format specifiers in order, in number, and in exact type. For example, the format specifier for count is %d and for amount is %f. By default, a floating-point value is displayed with six digits after the decimal point. You can specify the width and precision in a format specifier, as shown in the examples in Table 4.12.
TABLE 4.12 Examples of Specifying Width and Precision Example
Output
%5c
Output the character and add four spaces before the character item, because the width is 5.
%6b
Output the Boolean value and add one space before the false value and two spaces before the true value.
%5d
Output the integer item with width at least 5. If the number of digits in the item is 6 5, add spaces before the number. If the number of digits in the item is 7 5, the width is automatically increased.
%10.2f
Output the floating-point item with width at least 10 including a decimal point and two digits after the point. Thus, there are 7 digits allocated before the decimal point. If the number of digits before the decimal point in the item is 6 7, add spaces before the number. If the number of digits before the decimal point in the item is 7 7, the width is automatically increased.
%10.2e
Output the floating-point item with width at least 10 including a decimal point, two digits after the point and the exponent part. If the displayed number in scientific notation has width less than 10, add spaces before the number.
%12s
Output the string with width at least 12 characters. If the string item has fewer than 12 characters, add spaces before the string. If the string item has more than 12 characters, the width is automatically increased.
If an item requires more spaces than the specified width, the width is automatically increased. For example, the following code System.out.printf("%3d#%2s#%4.2f\n", 1234, "Java", 51.6653);
displays 1234#Java#51.67
The specified width for int item 1234 is 3, which is smaller than its actual size 4. The width is automatically increased to 4. The specified width for string item Java is 2, which is smaller than its actual size 4. The width is automatically increased to 4. The specified width for double item 51.6653 is 4, but it needs width 5 to display 51.67, so the width is automatically increased to 5. By default, the output is right justified. You can put the minus sign (-) in the format specifier to specify that the item is left justified in the output within the specified field. For example, the following statements System.out.printf("%8d%8s%8.1f\n", 1234, "Java", 5.63); System.out.printf("%-8d%-8s%-8.1f \n", 1234, "Java", 5.63);
display 8 8 8 1234 Java 5.6 1234 Java 5.6
where the square box (n) denotes a blank space.
right justify left justify
148 Chapter 4
Mathematical Functions, Characters, and Strings Caution The items must match the format specifiers in exact type. The item for the format specifier %f or %e must be a floating-point type value such as 40.0, not 40. Thus, an int variable cannot match %f or %e.
Tip The % sign denotes a format specifier. To output a literal % in the format string, use %%.
Listing 4.6 gives a program that uses printf to display a table.
LISTING 4.6 FormatDemo.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
display table header
values for 30 degrees
values for 60 degrees
public class FormatDemo { public static void main(String[] args) { // Display the header of the table System.out.printf("%-10s%-10s%-10s%-10s%-10s\n", "Degrees", "Radians", "Sine", "Cosine", "Tangent"); // Display values for 30 degrees int degrees = 30; double radians = Math.toRadians(degrees); System.out.printf("%-10d%-10.4f%-10.4f%-10.4f%-10.4f\n", degrees, radians, Math.sin(radians), Math.cos(radians), Math.tan(radians)); // Display values for 60 degrees degrees = 60; radians = Math.toRadians(degrees); System.out.printf("%-10d%-10.4f%-10.4f%-10.4f%-10.4f\n", degrees, radians, Math.sin(radians), Math.cos(radians), Math.tan(radians)); } }
Degrees 30 60
Radians 0.5236 1.0472
Sine 0.5000 0.8660
Cosine 0.8660 0.5000
Tangent 0.5773 1.7320
The statement in lines 4–5 displays the column names of the table. The column names are strings. Each string is displayed using the specifier %-10s, which left-justifies the string. The statement in lines 10–12 displays the degrees as an integer and four float values. The integer is displayed using the specifier %-10d and each float is displayed using the specifier %-10.4f, which specifies four digits after the decimal point.
✓
Check Point
4.22 4.23
What are the format specifiers for outputting a Boolean value, a character, a decimal integer, a floating-point number, and a string? What is wrong in the following statements? (a) System.out.printf("%5d %d", 1, 2, 3); (b) System.out.printf("%5d %f", 1); (c) System.out.printf("%5d %f", 1, 2);
Chapter Summary 149 4.24
Show the output of the following statements. (a) System.out.printf("amount is %f %e\n", 32.32, 32.32); (b) System.out.printf("amount is %5.2%% %5.4e\n", 32.327, 32.32); (c) System.out.printf("%6b\n", (1 > 2)); (d) System.out.printf("%6s\n", "Java"); (e) System.out.printf("%-6b%s\n", (1 > 2), "Java"); (f) System.out.printf("%6b%-8s\n", (1 > 2), "Java");
KEY TERMS char type
125 encoding 125 escape character 127 escape sequence 126 format specifier 146
instance method 130 static method 130 supplementary Unicode 125 Unicode 125 whitespace character 133
CHAPTER SUMMARY 1. Java provides the mathematical methods sin, cos, tan, asin, acos,
atan, toRadians, toDegree, exp, log, log10, pow, sqrt, cell, floor, rint, round, min, max, abs, and random in the Math class for performing mathematical functions.
2. The character type char represents a single character. 3. An escape sequence consists of a backslash (\) followed by a character or a combination of digits.
4. The character \ is called the escape character. 5. The characters ' ', \t, \f, \r, and \n are known as the whitespace characters. 6. Characters can be compared based on their Unicode using the relational operators. 7. The Character class contains the methods isDigit, isLetter, isLetterOrDigit, isLowerCase, isUpperCase for testing whether a character is a digit, letter, lowercase, and uppercase. It also contains the toLowerCase and toUpperCase methods for returning a lowercase or uppercase letter.
8. A string is a sequence of characters. A string value is enclosed in matching double quotes ("). A character value is enclosed in matching single quotes (').
9. Strings are objects in Java. A method that can only be invoked from a specific object is called an instance method. A non-instance method is called a static method, which can be invoked without using an object.
150 Chapter 4
Mathematical Functions, Characters, and Strings 10. You can get the length of a string by invoking its length() method, retrieve a character at the specified index in the string using the charAt(index) method, and use the indexOf and lastIndexOf methods to find a character or a substring in a string.
11. You can use the concat method to concatenate two strings, or the plus (+) operator to concatenate two or more strings.
12. You can use the substring method to obtain a substring from the string. 13. You can use the equals and compareTo methods to compare strings. The equals method returns true if two strings are equal, and false if they are not equal. The compareTo method returns 0, a positive integer, or a negative integer, depending on whether one string is equal to, greater than, or less than the other string.
14. The
printf method can be used to display a formatted output using format
specifiers.
QUIZ Answer the quiz for this chapter online at www.cs.armstrong.edu/liang/intro10e/quiz.html.
PROGRAMMING EXERCISES Section 4.2
4.1
(Geometry: area of a pentagon) Write a program that prompts the user to enter the length from the center of a pentagon to a vertex and computes the area of the pentagon, as shown in the following figure.
r
The formula for computing the area of a pentagon is Area =
5 * s2
, where p 4 * tan¢ ≤ 5 p s is the length of a side. The side can be computed using the formula s = 2r sin , 5 where r is the length from the center of a pentagon to a vertex. Round up two digits after the decimal point. Here is a sample run:
Enter the length from the center to a vertex: 5.5 The area of the pentagon is 71.92
Programming Exercises 151 *4.2
(Geometry: great circle distance) The great circle distance is the distance between two points on the surface of a sphere. Let (x1, y1) and (x2, y2) be the geographical latitude and longitude of two points. The great circle distance between the two points can be computed using the following formula: d = radius * arccos(sin(x1) * sin(x2) + cos(x1) * cos(x2) * cos(y1 - y2)) Write a program that prompts the user to enter the latitude and longitude of two points on the earth in degrees and displays its great circle distance. The average earth radius is 6,371.01 km. Note that you need to convert the degrees into radians using the Math.toRadians method since the Java trigonometric methods use radians. The latitude and longitude degrees in the formula are for north and west. Use negative to indicate south and east degrees. Here is a sample run:
Enter point 1 (latitude and longitude) in degrees: 39.55, -116.25 Enter point 2 (latitude and longitude) in degrees: 41.5, 87.37 The distance between the two points is 10691.79183231593 km
*4.3
4.4
(Geography: estimate areas) Find the GPS locations for Atlanta, Georgia; Orlando, Florida; Savannah, Georgia; and Charlotte, North Carolina from www.gps-data-team.com/map/ and compute the estimated area enclosed by these four cities. (Hint: Use the formula in Programming Exercise 4.2 to compute the distance between two cities. Divide the polygon into two triangles and use the formula in Programming Exercise 2.19 to compute the area of a triangle.) (Geometry: area of a hexagon) The area of a hexagon can be computed using the following formula (s is the length of a side): Area =
6 * s2 p 4 * tan¢ ≤ 6
Write a program that prompts the user to enter the side of a hexagon and displays its area. Here is a sample run:
Enter the side: 5.5 The area of the hexagon is 78.59
*4.5
(Geometry: area of a regular polygon) A regular polygon is an n-sided polygon in which all sides are of the same length and all angles have the same degree (i.e., the polygon is both equilateral and equiangular). The formula for computing the area of a regular polygon is Area =
n * s2 p 4 * tan¢ ≤ n
VideoNote
Compute great circle distance
152 Chapter 4
Mathematical Functions, Characters, and Strings Here, s is the length of a side. Write a program that prompts the user to enter the number of sides and their length of a regular polygon and displays its area. Here is a sample run: Enter the number of sides: 5 Enter the side: 6.5 The area of the polygon is 74.69017017488385
*4.6
(Random points on a circle) Write a program that generates three random points on a circle centered at (0, 0) with radius 40 and display three angles in a triangle formed by these three points, as shown in Figure 4.7a. (Hint: Generate a random angle a in radians between 0 and 2p, as shown in Figure 4.7b and the point determined by this angle is (r*cos(a), r*sin(a)).) x = r × cos(α) and y = r ×sin(α)
0 o’clock position p2
(x, y)
60 r 65
p3
p1
r
α
(0, 0) 55 p4 (a)
(b)
p5 (c)
FIGURE 4.7 (a) A triangle is formed from three random points on the circle. (b) A random point on the circle can be generated using a random angle a. (c) A pentagon is centered at (0, 0) with one point at the 0 o’clock position.
*4.7
(Corner point coordinates) Suppose a pentagon is centered at (0, 0) with one point at the 0 o’clock position, as shown in Figure 4.7c. Write a program that prompts the user to enter the radius of the bounding circle of a pentagon and displays the coordinates of the five corner points on the pentagon. Here is a sample run:
Enter the radius of the bounding circle: 100 The coordinates of five points on the pentagon are (95.1057, 30.9017) (0.000132679, 100) (-95.1056, 30.9019) (-58.7788, -80.9015) (58.7782, -80.902)
Sections 4.3–4.6
*4.8
(Find the character of an ASCII code) Write a program that receives an ASCII code (an integer between 0 and 127) and displays its character. Here is a sample run:
Enter an ASCII code: 69 The character for ASCII code 69 is E
Programming Exercises 153 *4.9
(Find the Unicode of a character) Write a program that receives a character and displays its Unicode. Here is a sample run:
Enter a character: E The Unicode for the character E is 69
*4.10
(Guess birthday) Rewrite Listing 4.3, GuessBirthday.java, to prompt the user to enter the character Y for Yes and N for No rather than entering 1 for Yes and 0 for No.
*4.11
(Decimal to hex) Write a program that prompts the user to enter an integer between 0 and 15 and displays its corresponding hex number. Here are some sample runs:
Enter a decimal value (0 to 15): 11 The hex value is B
Enter a decimal value (0 to 15): 5 The hex value is 5
Enter a decimal value (0 to 15): 31 31 is an invalid input
4.12
(Hex to binary) Write a program that prompts the user to enter a hex digit and displays its corresponding binary number. Here is a sample run:
VideoNote
Convert hex to binary Enter a hex digit: B The binary value is 1011
Enter a hex digit: G G is an invalid input
*4.13
(Vowel or consonant?) Write a program that prompts the user to enter a letter and check whether the letter is a vowel or consonant. Here is a sample run:
Enter a letter: B B is a consonant
Enter a letter grade: a a is a vowel
Enter a letter grade: # # is an invalid input
154 Chapter 4
Mathematical Functions, Characters, and Strings *4.14
(Convert letter grade to number) Write a program that prompts the user to enter a letter grade A, B, C, D, or F and displays its corresponding numeric value 4, 3, 2, 1, or 0. Here is a sample run:
Enter a letter grade: B The numeric value for grade B is 3
Enter a letter grade: T T is an invalid grade
*4.15
(Phone key pads) The international standard letter/number mapping found on the telephone is shown below:
Write a program that prompts the user to enter a letter and displays its corresponding number.
Enter a letter: A The corresponding number is 2
Enter a letter: a The corresponding number is 2
Enter a letter: + + is an invalid input
4.16 *4.17
(Random character) Write a program that displays a random uppercase letter using the Math.random() method. (Days of a month) Write a program that prompts the user to enter a year and the first three letters of a month name (with the first letter in uppercase) and displays the number of days in the month. Here is a sample run:
Enter a year: 2001 Enter a month: Jan Jan 2001 has 31 days
Programming Exercises 155 Enter a year: 2016 Enter a month: Feb Jan 2016 has 29 days
*4.18
(Student major and status) Write a program that prompts the user to enter two characters and displays the major and status represented in the characters. The first character indicates the major and the second is number character 1, 2, 3, 4, which indicates whether a student is a freshman, sophomore, junior, or senior. Suppose the following chracters are used to denote the majors: M: Mathematics C: Computer Science I: Information Technology
Here is a sample run: Enter two characters: M1 Mathematics Freshman
Enter two characters: C3 Computer Science Junior
Enter two characters: T3 Invalid input
4.19 4.20 *4.21
(Business: check ISBN-10) Rewrite the Programming Exercise 3.9 by entering the ISBN number as a string. (Process a string) Write a program that prompts the user to enter a string and displays its length and its first character. (Check SSN) Write a program that prompts the user to enter a Social Security number in the format DDD-DD-DDDD, where D is a digit. Your program should check whether the input is valid. Here are sample runs:
Enter a SSN: 232-23-5435 232-23-5435 is a valid social security number
Enter a SSN: 23-23-5435 23-23-5435 is an invalid social security number
4.22
(Check substring) Write a program that prompts the user to enter two strings and reports whether the second string is a substring of the first string.
Enter string s1: ABCD Enter string s2: BC BC is a substring of ABCD
156 Chapter 4
Mathematical Functions, Characters, and Strings Enter string s1: ABCD Enter string s2: BDC BDC is not a substring of ABCD
*4.23
(Financial application: payroll) Write a program that reads the following information and prints a payroll statement: Employee’s name (e.g., Smith) Number of hours worked in a week (e.g., 10) Hourly pay rate (e.g., 9.75) Federal tax withholding rate (e.g., 20%) State tax withholding rate (e.g., 9%) A sample run is shown below:
Enter Enter Enter Enter Enter
employee's name: Smith number of hours worked in a week: 10 hourly pay rate: 9.75 federal tax withholding rate: 0.20 state tax withholding rate: 0.09
Employee Name: Smith Hours Worked: 10.0 Pay Rate: $9.75 Gross Pay: $97.5 Deductions: Federal Withholding (20.0%): $19.5 State Withholding (9.0%): $8.77 Total Deduction: $28.27 Net Pay: $69.22
*4.24
(Order three cities) Write a program that prompts the user to enter three cities and displays them in ascending order. Here is a sample run:
Enter the Enter the Enter the The three
*4.25 *4.26
first city: Chicago second city: Los Angeles third city: Atlanta cities in alphabetical order are Atlanta Chicago Los Angeles
(Generate vehicle plate numbers) Assume a vehicle plate number consists of three uppercase letters followed by four digits. Write a program to generate a plate number. (Financial application: monetary units) Rewrite Listing 2.10, ComputeChange. java, to fix the possible loss of accuracy when converting a float value to an int value. Read the input as a string such as "11.56". Your program should extract the dollar amount before the decimal point and the cents after the decimal amount using the indexOf and substring methods.
CHAPTER
5 LOOPS Objectives ■
To write programs for executing statements repeatedly using a while loop (§5.2).
■
To follow the loop design strategy to develop loops (§§5.2.1–5.2.3).
■
To control a loop with a sentinel value (§5.2.4).
■
To obtain large input from a file using input redirection rather than typing from the keyboard (§5.2.5).
■
To write loops using do-while statements (§5.3).
■
To write loops using for statements (§5.4).
■
To discover the similarities and differences of three types of loop statements (§5.5).
■
To write nested loops (§5.6).
■
To learn the techniques for minimizing numerical errors (§5.7).
■
To learn loops from a variety of examples (GCD, FutureTuition, Dec2Hex) (§5.8).
■
To implement program control with break and continue (§5.9).
■
To process characters in a string using a loop in a case study for checking palindrome (§5.10).
■
To write a program that displays prime numbers (§5.11).
158 Chapter 5
Loops
5.1 Introduction problem
Key Point
A loop can be used to tell a program to execute statements repeatedly. Suppose that you need to display a string (e.g., Welcome to Java!) a hundred times. It would be tedious to have to write the following statement a hundred times:
100 times
System.out.println("Welcome to Java!"); System.out.println("Welcome to Java!"); ... System.out.println("Welcome to Java!");
So, how do you solve this problem? Java provides a powerful construct called a loop that controls how many times an operation or a sequence of operations is performed in succession. Using a loop statement, you simply tell the computer to display a string a hundred times without having to code the print statement a hundred times, as follows:
loop
int count = 0; while (count < 100) { System.out.println("Welcome to Java!"); count++; }
The variable count is initially 0. The loop checks whether count < 100 is true. If so, it executes the loop body to display the message Welcome to Java! and increments count by 1. It repeatedly executes the loop body until count < 100 becomes false. When count < 100 is false (i.e., when count reaches 100), the loop terminates and the next statement after the loop statement is executed. Loops are constructs that control repeated executions of a block of statements. The concept of looping is fundamental to programming. Java provides three types of loop statements: while loops, do-while loops, and for loops.
5.2 The while Loop Key Point while loop
loop body iteration loop-continuationcondition
A while loop executes statements repeatedly while the condition is true. The syntax for the while loop is: while (loop-continuation-condition) { // Loop body Statement(s); }
Figure 5.1a shows the while-loop flowchart. The part of the loop that contains the statements to be repeated is called the loop body. A one-time execution of a loop body is referred to as an iteration (or repetition) of the loop. Each loop contains a loop-continuation-condition, a Boolean expression that controls the execution of the body. It is evaluated each time to determine if the loop body is executed. If its evaluation is true, the loop body is executed; if its evaluation is false, the entire loop terminates and the program control turns to the statement that follows the while loop. The loop for displaying Welcome to Java! a hundred times introduced in the preceding section is an example of a while loop. Its flowchart is shown in Figure 5.1b. The
5.2 The while Loop 159 count = 0;
loopcontinuationcondition?
false
(count < 100)?
true
false
true
Statement(s) (loop body)
System.out.println("Welcome to Java!"); count++;
(a)
(b)
FIGURE 5.1
The while loop repeatedly executes the statements in the loop body when the loop-continuation-condition evaluates to true. loop-continuation-condition is count < 100 and the loop body contains the follow-
ing two statements: loop-continuation-condition int count = 0; while (count < 100) { System.out.printIn("Welcome to Java!"); count++; }
loop body
In this example, you know exactly how many times the loop body needs to be executed because the control variable count is used to count the number of executions. This type of loop is known as a counter-controlled loop.
Note The loop-continuation-condition must always appear inside the parentheses. The braces enclosing the loop body can be omitted only if the loop body contains one or no statement.
Here is another example to help understand how a loop works. int sum = 0, i = 1; while (i < 10) { sum = sum + i; i++; } System.out.println("sum is " + sum); // sum is 45
If i < 10 is true, the program adds i to sum. Variable i is initially set to 1, then is incremented to 2, 3, and up to 10. When i is 10, i < 10 is false, so the loop exits. Therefore, the sum is 1 + 2 + 3 + ... + 9 = 45. What happens if the loop is mistakenly written as follows? int sum = 0, i = 1; while (i < 10) { sum = sum + i; }
This loop is infinite, because i is always 1 and i < 10 will always be true.
counter-controlled loop
160 Chapter 5
Loops Note Make sure that the loop-continuation-condition eventually becomes false so that the loop will terminate. A common programming error involves infinite loops (i. e., the loop runs forever). If your program takes an unusually long time to run and does not stop, it may have an infinite loop. If you are running the program from the command window, press CTRL+C to stop it.
infinite loop
Caution Programmers often make the mistake of executing a loop one more or less time. This is commonly known as the off-by-one error. For example, the following loop displays Welcome to Java 101 times rather than 100 times. The error lies in the condition, which should be count < 100 rather than count <= 100.
off-by-one error
int count = 0; while (count <= 100) { System.out.println("Welcome to Java!"); count++; }
Recall that Listing 3.1, AdditionQuiz.java, gives a program that prompts the user to enter an answer for a question on addition of two single digits. Using a loop, you can now rewrite the program to let the user repeatedly enter a new answer until it is correct, as shown in Listing 5.1.
LISTING 5.1 RepeatAdditionQuiz.java
generate number1 generate number2
show question get first answer check answer
read an answer
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
import java.util.Scanner; public class RepeatAdditionQuiz { public static void main(String[] args) { int number1 = (int)(Math.random() * 10); int number2 = (int)(Math.random() * 10); // Create a Scanner Scanner input = new Scanner(System.in); System.out.print( "What is " + number1 + " + " + number2 + "? "); int answer = input.nextInt(); while (number1 + number2 != answer) { System.out.print("Wrong answer. Try again. What is " + number1 + " + " + number2 + "? "); answer = input.nextInt(); } System.out.println("You got it!"); } }
What is 5 + 9? 12 Wrong answer. Try again. What is 5 + 9? 34 Wrong answer. Try again. What is 5 + 9? 14 You got it!
5.2 The while Loop 161 The loop in lines 15–19 repeatedly prompts the user to enter an answer when number1 + number2 != answer is true. Once number1 + number2 != answer is false, the loop exits.
5.2.1
Case Study: Guessing Numbers
The problem is to guess what number a computer has in mind. You will write a program that randomly generates an integer between 0 and 100, inclusive. The program prompts the user to enter a number continuously until the number matches the randomly generated number. For each user input, the program tells the user whether the input is too low or too high, so the user can make the next guess intelligently. Here is a sample run:
VideoNote
Guess a number
Guess a magic number between 0 and 100 Enter your guess: 50 Your guess is too high Enter your guess: 25 Your guess is too low Enter your guess: 42 Your guess is too high Enter your guess: 39 Yes, the number is 39
The magic number is between 0 and 100. To minimize the number of guesses, enter 50 first. If your guess is too high, the magic number is between 0 and 49. If your guess is too low, the magic number is between 51 and 100. So, you can eliminate half of the numbers from further consideration after one guess. How do you write this program? Do you immediately begin coding? No. It is important to think before coding. Think how you would solve the problem without writing a program. You need first to generate a random number between 0 and 100, inclusive, then to prompt the user to enter a guess, and then to compare the guess with the random number. It is a good practice to code incrementally one step at a time. For programs involving loops, if you don’t know how to write a loop right away, you may first write the code for executing the loop one time, and then figure out how to repeatedly execute the code in a loop. For this program, you may create an initial draft, as shown in Listing 5.2.
intelligent guess
think before coding
code incrementally
LISTING 5.2 GuessNumberOneTime.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
import java.util.Scanner; public class GuessNumberOneTime { public static void main(String[] args) { // Generate a random number to be guessed int number = (int)(Math.random() * 101);
generate a number
Scanner input = new Scanner(System.in); System.out.println("Guess a magic number between 0 and 100"); // Prompt the user to guess the number System.out.print("\nEnter your guess: "); int guess = input.nextInt();
enter a guess
if (guess == number) System.out.println("Yes, the number is " + number);
correct guess?
162 Chapter 5 too high? too low?
Loops 17 18 19 20 21 22
else if (guess > number) System.out.println("Your guess is too high"); else System.out.println("Your guess is too low"); } }
When you run this program, it prompts the user to enter a guess only once. To let the user enter a guess repeatedly, you may wrap the code in lines 11–20 in a loop as follows: while (true) { // Prompt the user to guess the number System.out.print("\nEnter your guess: "); guess = input.nextInt(); if (guess == number) System.out.println("Yes, the number is " + number); else if (guess > number) System.out.println("Your guess is too high"); else System.out.println("Your guess is too low"); } // End of loop
This loop repeatedly prompts the user to enter a guess. However, this loop is not correct, because it never terminates. When guess matches number, the loop should end. So, the loop can be revised as follows: while (guess != number) { // Prompt the user to guess the number System.out.print("\nEnter your guess: "); guess = input.nextInt(); if (guess == number) System.out.println("Yes, the number is " + number); else if (guess > number) System.out.println("Your guess is too high"); else System.out.println("Your guess is too low"); } // End of loop
The complete code is given in Listing 5.3.
LISTING 5.3 GuessNumber.java
generate a number
1 2 3 4 5 6 7 8 9 10 11 12 13 14
import java.util.Scanner; public class GuessNumber { public static void main(String[] args) { // Generate a random number to be guessed int number = (int)(Math.random() * 101); Scanner input = new Scanner(System.in); System.out.println("Guess a magic number between 0 and 100"); int guess = -1; while (guess != number) { // Prompt the user to guess the number System.out.print("\nEnter your guess: ");
5.2 The while Loop 163 15 16 17 18 19 20 21 22 23 24 25
guess = input.nextInt();
enter a guess
if (guess == number) System.out.println("Yes, the number is " + number); else if (guess > number) System.out.println("Your guess is too high"); else System.out.println("Your guess is too low"); } // End of loop } }
line# 6
iteration 1 iteration 2 iteration 3 iteration 4
number
guess
output
39
11
–1
15
50
20 15
Your guess is too high 25
22 15
Your guess is too low 42
20 15
Your guess is too high 39
18
Yes, the number is 39
The program generates the magic number in line 6 and prompts the user to enter a guess continuously in a loop (lines 12–23). For each guess, the program checks whether the guess is correct, too high, or too low (lines 17–22). When the guess is correct, the program exits the loop (line 12). Note that guess is initialized to -1. Initializing it to a value between 0 and 100 would be wrong, because that could be the number to be guessed.
5.2.2
Loop Design Strategies
Writing a correct loop is not an easy task for novice programmers. Consider three steps when writing a loop. Step 1: Identify the statements that need to be repeated. Step 2: Wrap these statements in a loop like this: while (true) { Statements; }
Step 3: Code the loop-continuation-condition and add appropriate statements for controlling the loop. while (loop-continuation-condition) { Statements; Additional statements for controlling the loop; }
too high? too low?
164 Chapter 5
Loops
5.2.3 VideoNote
Multiple subtraction quiz
Case Study: Multiple Subtraction Quiz
The Math subtraction learning tool program in Listing 3.3, SubtractionQuiz.java, generates just one question for each run. You can use a loop to generate questions repeatedly. How do you write the code to generate five questions? Follow the loop design strategy. First identify the statements that need to be repeated. These are the statements for obtaining two random numbers, prompting the user with a subtraction question, and grading the question. Second, wrap the statements in a loop. Third, add a loop control variable and the loop-continuation-condition to execute the loop five times. Listing 5.4 gives a program that generates five questions and, after a student answers all five, reports the number of correct answers. The program also displays the time spent on the test and lists all the questions.
LISTING 5.4 SubtractionQuizLoop.java
get start time
loop
display a question
grade an answer increase correct count
increase control variable prepare 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 40 41 42
import java.util.Scanner; public class SubtractionQuizLoop { public static void main(String[] args) { final int NUMBER_OF_QUESTIONS = 5; // Number of questions int correctCount = 0; // Count the number of correct answers int count = 0; // Count the number of questions long startTime = System.currentTimeMillis(); String output = " "; // output string is initially empty Scanner input = new Scanner(System.in); while (count < NUMBER_OF_QUESTIONS) { // 1. Generate two random single-digit integers int number1 = (int)(Math.random() * 10); int number2 = (int)(Math.random() * 10); // 2. If number1 < number2, swap number1 with number2 if (number1 < number2) { int temp = number1; number1 = number2; number2 = temp; } // 3. Prompt the student to answer "What is number1 – number2?" System.out.print( "What is " + number1 + " - " + number2 + "? "); int answer = input.nextInt(); // 4. Grade the answer and display the result if (number1 - number2 == answer) { System.out.println("You are correct!"); correctCount++; // Increase the correct answer count } else System.out.println("Your answer is wrong.\n" + number1 + " - " + number2 + " should be " + (number1 - number2)); // Increase the question count count++; output += "\n" + number1 + "-" + number2 + "=" + answer + ((number1 - number2 == answer) ? " correct" : " wrong");
5.2 The while Loop 165 43 44 45 46 47 48 49 50 51
}
end loop
long endTime = System.currentTimeMillis(); long testTime = endTime - startTime;
get end time test time
System.out.println("Correct count is " + correctCount + "\nTest time is " + testTime / 1000 + " seconds\n" + output);
display result
} }
What is 9 – 2? 7 You are correct! What is 3 – 0? 3 You are correct! What is 3 – 2? 1 You are correct! What is 7 – 4? 4 Your answer is wrong. 7 – 4 should be 3 What is 7 – 5? 4 Your answer is wrong. 7 – 5 should be 2 Correct count is 3 Test time is 1021 seconds 9–2=7 3–0=3 3–2=1 7–4=4 7–5=4
correct correct correct wrong wrong
The program uses the control variable count to control the execution of the loop. count is initially 0 (line 7) and is increased by 1 in each iteration (line 39). A subtraction question is displayed and processed in each iteration. The program obtains the time before the test starts in line 8 and the time after the test ends in line 45, and computes the test time in line 46. The test time is in milliseconds and is converted to seconds in line 49.
5.2.4
Controlling a Loop with a Sentinel Value
Another common technique for controlling a loop is to designate a special value when reading and processing a set of values. This special input value, known as a sentinel value, signifies the end of the input. A loop that uses a sentinel value to control its execution is called a sentinel-controlled loop. Listing 5.5 writes a program that reads and calculates the sum of an unspecified number of integers. The input 0 signifies the end of the input. Do you need to declare a new variable for each input value? No. Just use one variable named data (line 12) to store the input value and use a variable named sum (line 15) to store the total. Whenever a value is read, assign it to data and, if it is not zero, add it to sum (line 17).
sentinel value sentinel-controlled loop
166 Chapter 5
Loops
LISTING 5.5 SentinelValue.java
input
loop
end of loop display result
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
import java.util.Scanner; public class SentinelValue { /** Main method */ public static void main(String[] args) { // Create a Scanner Scanner input = new Scanner(System.in); // Read an initial data System.out.print( "Enter an integer (the input ends if it is 0): "); int data = input.nextInt(); // Keep reading data until the input is 0 int sum = 0; while (data != 0) { sum += data; // Read the next data System.out.print( "Enter an integer (the input ends if it is 0): "); data = input.nextInt(); } System.out.println("The sum is " + sum); } }
Enter an integer (the input ends if it is 0): 2 Enter an integer (the input ends if it is 0): 3 Enter an integer (the input ends if it is 0): 4 Enter an integer (the input ends if it is 0): 0 The sum is 9
line# 12
iteration 1 iteration 2 iteration 3
Data
sum
2
15
0
17
2
22
3
17 22
5 4
17 22 25
output
9 0 The sum is 9
If data is not 0, it is added to sum (line 17) and the next item of input data is read (lines 20–22). If data is 0, the loop body is no longer executed and the while loop terminates. The input value 0 is the sentinel value for this loop. Note that if the first input read is 0, the loop body never executes, and the resulting sum is 0.
5.2 The while Loop 167 Caution Don’t use floating-point values for equality checking in a loop control. Because floatingpoint values are approximations for some values, using them could result in imprecise counter values and inaccurate results. Consider the following code for computing 1 + 0.9 + 0.8 + ... + 0.1: double item = 1; double sum = 0; while (item != 0) { // No guarantee item will be 0 sum += item; item -= 0.1; } System.out.println(sum);
Variable item starts with 1 and is reduced by 0.1 every time the loop body is executed. The loop should terminate when item becomes 0. However, there is no guarantee that item will be exactly 0, because the floating-point arithmetic is approximated. This loop seems okay on the surface, but it is actually an infinite loop.
5.2.5
numeric error
Input and Output Redirections
In the preceding example, if you have a large number of data to enter, it would be cumbersome to type from the keyboard. You can store the data separated by whitespaces in a text file, say input.txt, and run the program using the following command: java SentinelValue < input.txt
This command is called input redirection. The program takes the input from the file input .txt rather than having the user type the data from the keyboard at runtime. Suppose the contents of the file are
input redirection
2 3 4 5 6 7 8 9 12 23 32 23 45 67 89 92 12 34 35 3 1 2 4 0
The program should get sum to be 518. Similarly, there is output redirection, which sends the output to a file rather than displaying it on the console. The command for output redirection is:
output redirection
java ClassName > output.txt
Input and output redirection can be used in the same command. For example, the following command gets input from input.txt and sends output to output.txt: java SentinelValue
output.txt
Try running the program to see what contents are in output.txt.
5.1
Analyze the following code. Is count < 100 always true, always false, or sometimes true or sometimes false at Point A, Point B, and Point C? int count = 0; while (count < 100) { // Point A System.out.println("Welcome to Java!"); count++; // Point B } // Point C
✓
Check Point
168 Chapter 5
Loops 5.2 5.3
What is wrong if guess is initialized to 0 in line 11 in Listing 5.3? How many times are the following loop bodies repeated? What is the output of each loop?
int i = 1; while (i < 10) if (i % 2 == 0) System.out.println(i);
int i = 1; while (i < 10) if (i % 2 == 0) System.out.println(i++);
(a)
int i = 1; while (i < 10) if ((i++) % 2 == 0) System.out.println(i);
(b)
5.4
(c)
Suppose the input is 2 3 4 5 0. What is the output of the following code? import java.util.Scanner; public class Test { public static void main(String[] args) { Scanner input = new Scanner(System.in); int number, max; number = input.nextInt(); max = number; while (number != 0) { number = input.nextInt(); if (number > max) max = number; } System.out.println("max is " + max); System.out.println("number " + number); } }
5.5
What is the output of the following code? Explain the reason. int x = 80000000; while (x > 0) x++; System.out.println("x is " + x);
5.3 The do-while Loop Key Point
A do-while loop is the same as a while loop except that it executes the loop body first and then checks the loop continuation condition. The do-while loop is a variation of the while loop. Its syntax is:
do-while loop
do { // Loop body; Statement(s); } while (loop-continuation-condition);
Its execution flowchart is shown in Figure 5.2. The loop body is executed first, and then the loop-continuation-condition is evaluated. If the evaluation is true, the loop body is executed again; if it is false, the do-while
5.3 The do-while Loop 169
Statement(s) (loop body)
true
loopcontinuationcondition?
false
FIGURE 5.2 The do-while loop executes the loop body first, then checks the loopcontinuation-condition to determine whether to continue or terminate the loop. loop terminates. The difference between a while loop and a do-while loop is the order in which the loop-continuation-condition is evaluated and the loop body executed. You can write a loop using either the while loop or the do-while loop. Sometimes one is a more convenient choice than the other. For example, you can rewrite the while loop in Listing 5.5 using a do-while loop, as shown in Listing 5.6.
LISTING 5.6 TestDoWhile.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
import java.util.Scanner; public class TestDoWhile { /** Main method */ public static void main(String[] args) { int data; int sum = 0; // Create a Scanner Scanner input = new Scanner(System.in); // Keep reading data until the input is 0 do { // Read the next data System.out.print( "Enter an integer (the input ends if it is 0): "); data = input.nextInt(); sum += data; } while (data != 0); System.out.println("The sum is " + sum); } }
Enter an integer (the input ends if it is 0): 3 Enter an integer (the input ends if it is 0): 5 Enter an integer (the input ends if it is 0): 6 Enter an integer (the input ends if it is 0): 0 The sum is 14
loop
end loop
170 Chapter 5
Loops Tip Use a do-while loop if you have statements inside the loop that must be executed at least once, as in the case of the do-while loop in the preceding TestDoWhile program. These statements must appear before the loop as well as inside it if you use a while loop.
✓
Check Point
5.6
Suppose the input is 2 3 4 5 0. What is the output of the following code? import java.util.Scanner; public class Test { public static void main(String[] args) { Scanner input = new Scanner(System.in); int number, max; number = input.nextInt(); max = number; do { number = input.nextInt(); if (number > max) max = number; } while (number != 0); System.out.println("max is " + max); System.out.println("number " + number); } }
5.7
What are the differences between a while loop and a do-while loop? Convert the following while loop into a do-while loop. Scanner input = new Scanner(System.in); int sum = 0; System.out.println("Enter an integer " + "(the input ends if it is 0)"); int number = input.nextInt(); while (number != 0) { sum += number; System.out.println("Enter an integer " + "(the input ends if it is 0)"); number = input.nextInt(); }
5.4 The for Loop Key Point
A for loop has a concise syntax for writing loops. Often you write a loop in the following common form: i = initialValue; // Initialize loop control variable while (i < endValue) // Loop body ... i++; // Adjust loop control variable }
5.4 The for Loop 171 A for loop can be used to simplify the preceding loop as: for (i = initialValue; i < endValue; i++) // Loop body ... }
In general, the syntax of a for loop is: for (initial-action; loop-continuation-condition; action-after-each-iteration) { // Loop body; Statement(s); }
for loop
The flowchart of the for loop is shown in Figure 5.3a.
i = 0
initial-action
loopcontinuationcondition?
false
true
(i < 100)?
false
true
Statement(s) (loop body)
System.out.println( "Welcome to Java");
action-after-each-iteration
i++
(a)
(b)
FIGURE 5.3 A for loop performs an initial action once, then repeatedly executes the statements in the loop body, and performs an action after an iteration when the loop-continuation-condition evaluates to true. The for loop statement starts with the keyword for, followed by a pair of parentheses enclosing the control structure of the loop. This structure consists of initial-action, loop-continuation-condition, and action-after-each-iteration. The control structure is followed by the loop body enclosed inside braces. The initial-action, loopcontinuation-condition, and action-after-each-iteration are separated by semicolons. A for loop generally uses a variable to control how many times the loop body is executed and when the loop terminates. This variable is referred to as a control variable. The initialaction often initializes a control variable, the action-after-each-iteration usually increments or decrements the control variable, and the loop-continuation-condition
control variable
172 Chapter 5
Loops tests whether the control variable has reached a termination value. For example, the following for loop prints Welcome to Java! a hundred times: int i; for (i = 0; i < 100; i++) { System.out.println("Welcome to Java!"); }
initial-action
action-after-each-iteration
The flowchart of the statement is shown in Figure 5.3b. The for loop initializes i to 0, then repeatedly executes the println statement and evaluates i++ while i is less than 100. The initial-action, i = 0, initializes the control variable, i. The loopcontinuation-condition, i < 100, is a Boolean expression. The expression is evaluated right after the initialization and at the beginning of each iteration. If this condition is true, the loop body is executed. If it is false, the loop terminates and the program control turns to the line following the loop. The action-after-each-iteration, i++, is a statement that adjusts the control variable. This statement is executed after each iteration and increments the control variable. Eventually, the value of the control variable should force the loop-continuation-condition to become false; otherwise, the loop is infinite. The loop control variable can be declared and initialized in the for loop. Here is an example: for (int i = 0; i < 100; i++) { System.out.println("Welcome to Java!"); }
omitting braces
If there is only one statement in the loop body, as in this example, the braces can be omitted.
Tip declare control variable
The control variable must be declared inside the control structure of the loop or before the loop. If the loop control variable is used only in the loop, and not elsewhere, it is a good programming practice to declare it in the initial-action of the for loop. If the variable is declared inside the loop control structure, it cannot be referenced outside the loop. In the preceding code, for example, you cannot reference i outside the for loop, because it is declared inside the for loop.
Note for loop variations
The initial-action in a for loop can be a list of zero or more comma-separated variable declaration statements or assignment expressions. For example: for (int i = 0, j = 0; i + j < 10; i++, j++) { // Do something }
The action-after-each-iteration in a for loop can be a list of zero or more comma-separated statements. For example: for (int i = 1; i < 100; System.out.println(i), i++);
This example is correct, but it is a bad example, because it makes the code difficult to read. Normally, you declare and initialize a control variable as an initial action and increment or decrement the control variable as an action after each iteration.
Note If the loop-continuation-condition in a for loop is omitted, it is implicitly true. Thus the statement given below in (a), which is an infinite loop, is the same as in (b). To avoid confusion, though, it is better to use the equivalent loop in (c).
5.4 The for Loop 173 for ( ; ; ) { // Do something }
Equivalent
for ( ; true; ) { // Do something }
(a)
5.8
(b)
Equivalent
This is better
for (int i = 0; i < 10; i++) { sum += i; } (b)
(a)
5.10
(c)
Do the following two loops result in the same value in sum?
for (int i = 0; i < 10; ++i) { sum += i; }
5.9
while (true) { // Do something }
What are the three parts of a for loop control? Write a for loop that prints the numbers from 1 to 100. Suppose the input is 2 3 4 5 0. What is the output of the following code? import java.util.Scanner; public class Test { public static void main(String[] args) { Scanner input = new Scanner(System.in); int number, sum = 0, count; for (count = 0; count < 5; count++) { number = input.nextInt(); sum += number; } System.out.println("sum is " + sum); System.out.println("count is " + count); } }
5.11
What does the following statement do? for ( ; ; ) { // Do something }
5.12 5.13
If a variable is declared in a for loop control, can it be used after the loop exits? Convert the following for loop statement to a while loop and to a do-while loop: long sum = 0; for (int i = 0; i <= 1000; i++) sum = sum + i;
5.14
Count the number of iterations in the following loops. int count = 0; while (count < n) { count++; } (a)
for (int count = 0; count <= n; count++) { }
(b)
✓
Check Point
174 Chapter 5
Loops int count = 5; while (count < n) { count++; }
int count = 5; while (count < n) { count = count + 3; } (d)
(c)
5.5 Which Loop to Use? pretest loop posttest loop
Key Point
You can use a for loop, a while loop, or a do-while loop, whichever is convenient. The while loop and for loop are called pretest loops because the continuation condition is checked before the loop body is executed. The do-while loop is called a posttest loop because the condition is checked after the loop body is executed. The three forms of loop statements—while, do-while, and for—are expressively equivalent; that is, you can write a loop in any of these three forms. For example, a while loop in (a) in the following figure can always be converted into the for loop in (b).
while (loop-continuation-condition) { // Loop body }
for ( ; loop-continuation-condition; ) { // Loop body }
Equivalent
(a)
(b)
A for loop in (a) in the next figure can generally be converted into the while loop in (b) except in certain special cases (see Checkpoint Question 5.25 for such a case). for (initial-action; loop-continuation-condition; action-after-each-iteration) { // Loop body; }
Equivalent
initial-action; while (loop-continuation-condition) { // Loop body; action-after-each-iteration; } (b)
(a)
Use the loop statement that is most intuitive and comfortable for you. In general, a for loop may be used if the number of repetitions is known in advance, as, for example, when you need to display a message a hundred times. A while loop may be used if the number of repetitions is not fixed, as in the case of reading the numbers until the input is 0. A do-while loop can be used to replace a while loop if the loop body has to be executed before the continuation condition is tested.
Caution Adding a semicolon at the end of the for clause before the loop body is a common mistake, as shown below in (a). In (a), the semicolon signifies the end of the loop prematurely. The loop body is actually empty, as shown in (b). (a) and (b) are equivalent. Both are incorrect. Empty body
Error for (int i = 0; i < 10; i++); { System.out.println("i is " + i); }
for (int i = 0; i < 10; i++) { }; { System.out.println("i is " + i); }
(a)
(b)
5.5 Which Loop to Use? 175 Similarly, the loop in (c) is also wrong. (c) is equivalent to (d). Both are incorrect. Empty body
Error int i = 0; while (i < 10); { System.out.println("i is " + i); i++; }
int i = 0; while (i < 10) { }; { System.out.println("i is " + i); i++; }
(c)
(d)
These errors often occur when you use the next-line block style. Using the end-of-line block style can avoid errors of this type. In the case of the do-while loop, the semicolon is needed to end the loop. int i = 0; do { System.out.println("i is " + i); i++; } while (i < 10); Correct
5.15 5.16
Can you convert a for loop to a while loop? List the advantages of using for loops. Can you always convert a while loop into a for loop? Convert the following while loop into a for loop. int i = 1; int sum = 0; while (sum < 10000) { sum = sum + i; i++; }
5.17
Identify and fix the errors in the following code: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
public class Test { public void main(String[] args) { for (int i = 0; i < 10; i++); sum += i; if (i < j); System.out.println(i) else System.out.println(j); while (j < 10); { j++; } do { j++; } while (j < 10) } }
✓
Check Point
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Loops 5.18
What is wrong with the following programs?
1 public class ShowErrors { 2 public static void main(String[] args) { 3 int i = 0; 4 do { 5 System.out.println(i + 4); 6 i++; 7 } 8 while (i < 10) 9 } 10 }
1 public class ShowErrors { 2 public static void main(String[] args) { 3 for (int i = 0; i < 10; i++); 4 System.out.println(i + 4); 5 } 6 }
(a)
(b)
5.6 Nested Loops nested loop
Key Point
A loop can be nested inside another loop. Nested loops consist of an outer loop and one or more inner loops. Each time the outer loop is repeated, the inner loops are reentered, and started anew. Listing 5.7 presents a program that uses nested for loops to display a multiplication table.
LISTING 5.7 MultiplicationTable.java
table title
outer loop inner loop
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
public class MultiplicationTable { /** Main method */ public static void main(String[] args) { // Display the table heading System.out.println(" Multiplication Table"); // Display the number title System.out.print(" "); for (int j = 1; j <= 9; j++) System.out.print(" " + j); System.out.println("\n———————————————————————————————————————"); // Display table body for (int i = 1; i <= 9; i++) { System.out.print(i + " | "); for (int j = 1; j <= 9; j++) { // Display the product and align properly System.out.printf("%4d", i * j); } System.out.println(); } } }
Multiplication Table 1 2 3 4 5 6 7 8 9 ———————————————————————————————————————1 | 1 2 3 4 5 6 7 8 9 2 | 2 4 6 8 10 12 14 16 18 3 | 3 6 9 12 15 18 21 24 27 4 | 4 8 12 16 20 24 28 32 36 5 | 5 10 15 20 25 30 35 40 45 6 | 6 12 18 24 30 36 42 48 54 7 | 7 14 21 28 35 42 49 56 63 8 | 8 16 24 32 40 48 56 64 72 9 | 9 18 27 36 45 54 63 72 81
5.6 Nested Loops 177 The program displays a title (line 5) on the first line in the output. The first for loop (lines 9–10) displays the numbers 1 through 9 on the second line. A dashed (-) line is displayed on the third line (line 12). The next loop (lines 15–22) is a nested for loop with the control variable i in the outer loop and j in the inner loop. For each i, the product i * j is displayed on a line in the inner loop, with j being 1, 2, 3, . . ., 9.
Note Be aware that a nested loop may take a long time to run. Consider the following loop nested in three levels: for (int i = 0; i < 10000; i++) for (int j = 0; j < 10000; j++) for (int k = 0; k < 10000; k++) Perform an action
The action is performed one trillion times. If it takes 1 microsecond to perform the action, the total time to run the loop would be more than 277 hours. Note that 1 microsecond is one millionth (10– 6) of a second.
5.19
✓
How many times is the println statement executed?
Check Point
for (int i = 0; i < 10; i++) for (int j = 0; j < i; j++) System.out.println(i * j)
5.20
Show the output of the following programs. (Hint: Draw a table and list the variables in the columns to trace these programs.)
public class Test { public static void main(String[] args) { for (int i = 1; i < 5; i++) { int j = 0; while (j < i) { System.out.print(j + " "); j++; } } } }
public class Test { public static void main(String[] args) { int i = 0; while (i < 5) { for (int j = i; j > 1; j--) System.out.print(j + " "); System.out.println("****"); i++; } } }
(a)
public class Test { public static void main(String[] args) { int i = 5; while (i >= 1) { int num = 1; for (int j = 1; j <= i; j++) { System.out.print(num + "xxx"); num *= 2; }
(b)
public class Test { public static void main(String[] args) { int i = 1; do { int num = 1; for (int j = 1; j <= i; j++) { System.out.print(num + "G"); num += 2; }
System.out.println(); i--;
System.out.println(); i++; } while (i <= 5);
} }
}
}
} (c)
(d)
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5.7 Minimizing Numeric Errors Key Point
VideoNote
Minimize numeric errors
Using floating-point numbers in the loop continuation condition may cause numeric errors. Numeric errors involving floating-point numbers are inevitable, because floating-point numbers are represented in approximation in computers by nature. This section discusses how to minimize such errors through an example. Listing 5.8 presents an example summing a series that starts with 0.01 and ends with 1.0. The numbers in the series will increment by 0.01, as follows: 0.01 + 0.02 + 0.03, and so on.
LISTING 5.8 TestSum.java
loop
1 2 3 4 5 6 7 8 9 10 11 12 13
public class TestSum { public static void main(String[] args) { // Initialize sum float sum = 0; // Add 0.01, 0.02, ..., 0.99, 1 to sum for (float i = 0.01f; i <= 1.0f; i = i + 0.01f) sum += i; // Display result System.out.println("The sum is " + sum); } }
The sum is 50.499985
double precision
The for loop (lines 7–8) repeatedly adds the control variable i to sum. This variable, which begins with 0.01, is incremented by 0.01 after each iteration. The loop terminates when i exceeds 1.0. The for loop initial action can be any statement, but it is often used to initialize a control variable. From this example, you can see that a control variable can be a float type. In fact, it can be any data type. The exact sum should be 50.50, but the answer is 50.499985. The result is imprecise because computers use a fixed number of bits to represent floating-point numbers, and thus they cannot represent some floating-point numbers exactly. If you change float in the program to double, as follows, you should see a slight improvement in precision, because a double variable holds 64 bits, whereas a float variable holds 32 bits. // Initialize sum double sum = 0; // Add 0.01, 0.02, ..., 0.99, 1 to sum for (double i = 0.01; i <= 1.0; i = i + 0.01) sum += i;
numeric error
However, you will be stunned to see that the result is actually 49.50000000000003. What went wrong? If you display i for each iteration in the loop, you will see that the last i is slightly larger than 1 (not exactly 1). This causes the last i not to be added into sum. The fundamental problem is that the floating-point numbers are represented by approximation. To fix the problem, use an integer count to ensure that all the numbers are added to sum. Here is the new loop: double currentValue = 0.01; for (int count = 0; count < 100; count++) {
5.8 Case Studies 179 sum += currentValue; currentValue += 0.01; }
After this loop, sum is 50.50000000000003. This loop adds the numbers from smallest to biggest. What happens if you add numbers from biggest to smallest (i.e., 1.0, 0.99, 0.98, . . . , 0.02, 0.01 in this order) as follows: double currentValue = 1.0; for (int count = 0; count < 100; count++) { sum += currentValue; currentValue -= 0.01; }
After this loop, sum is 50.49999999999995. Adding from biggest to smallest is less accurate than adding from smallest to biggest. This phenomenon is an artifact of the finite-precision arithmetic. Adding a very small number to a very big number can have no effect if the result requires more precision than the variable can store. For example, the inaccurate result of 100000000.0 + 0.000000001 is 100000000.0. To obtain more accurate results, carefully select the order of computation. Adding smaller numbers before bigger numbers is one way to minimize errors.
avoiding numeric error
5.8 Case Studies Loops are fundamental in programming. The ability to write loops is essential in learning Java programming.
Key Point
If you can write programs using loops, you know how to program! For this reason, this section presents four additional examples of solving problems using loops.
5.8.1
Case Study: Finding the Greatest Common Divisor
The greatest common divisor (gcd) of the two integers 4 and 2 is 2. The greatest common divisor of the two integers 16 and 24 is 8. How would you write this program to find the greatest common divisor? Would you immediately begin to write the code? No. It is important to think before you code. Thinking enables you to generate a logical solution for the problem without concern about how to write the code. Let the two input integers be n1 and n2. You know that number 1 is a common divisor, but it may not be the greatest common divisor. So, you can check whether k (for k = 2, 3, 4, and so on) is a common divisor for n1 and n2, until k is greater than n1 or n2. Store the common divisor in a variable named gcd. Initially, gcd is 1. Whenever a new common divisor is found, it becomes the new gcd. When you have checked all the possible common divisors from 2 up to n1 or n2, the value in variable gcd is the greatest common divisor. Once you have a logical solution, type the code to translate the solution into a Java program as follows: int gcd = 1; // Initial gcd is 1 int k = 2; // Possible gcd while (k <= n1 && k <= n2) { if (n1 % k == 0 && n2 % k == 0) gcd = k; // Update gcd k++; // Next possible gcd } // After the loop, gcd is the greatest common divisor for n1 and n2
Listing 5.9 presents the program that prompts the user to enter two positive integers and finds their greatest common divisor.
gcd
think before you code
logical solution
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LISTING 5.9 GreatestCommonDivisor.java
input input gcd
check divisor
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
import java.util.Scanner; public class GreatestCommonDivisor { /** Main method */ public static void main(String[] args) { // Create a Scanner Scanner input = new Scanner(System.in); // Prompt the user to enter two integers System.out.print("Enter first integer: "); int n1 = input.nextInt(); System.out.print("Enter second integer: "); int n2 = input.nextInt(); int gcd = 1; // Initial gcd is 1 int k = 2; // Possible gcd while (k <= n1 && k <= n2) { if (n1 % k == 0 && n2 % k == 0) gcd = k; // Update gcd k++; } System.out.println("The greatest common divisor for " + n1 + " and " + n2 + " is " + gcd); } }
Enter first integer: 125 Enter second integer: 2525 The greatest common divisor for 125 and 2525 is 25
Translating a logical solution to Java code is not unique. For example, you could use a for loop to rewrite the code as follows: for (int k = 2; k <= n1 && k <= n2; k++) { if (n1 % k == 0 && n2 % k == 0) gcd = k; } multiple solutions
erroneous solutions
A problem often has multiple solutions, and the gcd problem can be solved in many ways. Programming Exercise 5.14 suggests another solution. A more efficient solution is to use the classic Euclidean algorithm (see Section 22.6). You might think that a divisor for a number n1 cannot be greater than n1 / 2 and would attempt to improve the program using the following loop: for (int k = 2; k <= n1 / 2 && k <= n2 / 2; k++) { if (n1 % k == 0 && n2 % k == 0) gcd = k; }
5.8 Case Studies 181 This revision is wrong. Can you find the reason? See Checkpoint Question 5.21 for the answer.
5.8.2
Case Study: Predicting the Future Tuition
Suppose that the tuition for a university is $10,000 this year and tuition increases 7% every year. In how many years will the tuition be doubled? Before you can write a program to solve this problem, first consider how to solve it by hand. The tuition for the second year is the tuition for the first year * 1.07. The tuition for a future year is the tuition of its preceding year * 1.07. Thus, the tuition for each year can be computed as follows: double tuition = 10000; tuition = tuition * 1.07; tuition = tuition * 1.07; tuition = tuition * 1.07; ...
int year = 0; year++; year++; year++;
// // // //
Year Year Year Year
think before you code
0 1 2 3
Keep computing the tuition for a new year until it is at least 20000. By then you will know how many years it will take for the tuition to be doubled. You can now translate the logic into the following loop: double tuition = 10000; // Year 0 int year = 0; while (tuition < 20000) { tuition = tuition * 1.07; year++; }
The complete program is shown in Listing 5.10.
LISTING 5.10 FutureTuition.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
public class FutureTuition { public static void main(String[] args) { double tuition = 10000; // Year 0 int year = 0; while (tuition < 20000) { tuition = tuition * 1.07; year++; } System.out.println("Tuition will be doubled in " + year + " years"); System.out.printf("Tuition will be $%.2f in %1d years", tuition, year); } }
Tuition will be doubled in 11 years Tuition will be $21048.52 in 11 years
The while loop (lines 5–8) is used to repeatedly compute the tuition for a new year. The loop terminates when the tuition is greater than or equal to 20000.
loop next year’s tuition
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5.8.3
Case Study: Converting Decimals to Hexadecimals
Hexadecimals are often used in computer systems programming (see Appendix F for an introduction to number systems). How do you convert a decimal number to a hexadecimal number? To convert a decimal number d to a hexadecimal number is to find the hexadecimal digits hn, hn - 1, hn - 2, c , h2, h1, and h0 such that d = hn * 16n + hn - 1 * 16n - 1 + hn - 2 * 16n - 2 + g + h2 * 162 + h1 * 161 + h0 * 160 These hexadecimal digits can be found by successively dividing d by 16 until the quotient is 0. The remainders are h0, h1, h2, c , hn - 2, hn - 1, and hn. The hexadecimal digits include the decimal digits 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9, plus A, which is the decimal value 10; B, which is the decimal value 11; C, which is 12; D, which is 13; E, which is 14; and F, which is 15. For example, the decimal number 123 is 7B in hexadecimal. The conversion is done as follows. Divide 123 by 16. The remainder is 11 (B in hexadecimal) and the quotient is 7. Continue divide 7 by 16. The remainder is 7 and the quotient is 0. Therefore 7B is the hexadecimal number for 123.
16
0
7
7
16 123
0 7
112 11
h1
h0
Quotient
Remainder
Listing 5.11 gives a program that prompts the user to enter a decimal number and converts it into a hex number as a string.
LISTING 5.11 Dec2Hex.java
input decimal
decimal to hex
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
import java.util.Scanner; public class Dec2Hex { /** Main method */ public static void main(String[] args) { // Create a Scanner Scanner input = new Scanner(System.in); // Prompt the user to enter a decimal integer System.out.print("Enter a decimal number: "); int decimal = input.nextInt(); // Convert decimal to hex String hex = ""; while (decimal != 0) { int hexValue = decimal % 16; // Convert a decimal value to a hex digit char hexDigit = (hexValue <= 9 && hexValue >= 0) ? (char)(hexValue + '0') : (char)(hexValue - 10 + 'A'); hex = hexDigit + hex;
5.8 Case Studies 183 24 25 26 27 28 29
decimal = decimal / 16; } System.out.println("The hex number is " + hex);
get a hex char
} }
get a letter
Enter a decimal number: 1234 The hex number is 4D2
line# 14
decimal
hex
1234
""
17
iteration 1
"2"
13
23 24
"D2"
4
23 24
D
4
17
iteration 3
2
77
17
iteration 2
hexDigit
2
23 24
hexValue
"4D2"
4
0
The program prompts the user to enter a decimal integer (line 11), converts it to a hex number as a string (lines 14–25), and displays the result (line 27). To convert a decimal to a hex number, the program uses a loop to successively divide the decimal number by 16 and obtain its remainder (line 17). The remainder is converted into a hex character (lines 20–21). The character is then appended to the hex string (line 23). The hex string is initially empty (line 14). Divide the decimal number by 16 to remove a hex digit from the number (line 24). The loop ends when the remaining decimal number becomes 0. The program converts a hexValue between 0 and 15 into a hex character. If hexValue is between 0 and 9, it is converted to (char)(hexValue + '0') (line 21). Recall that when adding a character with an integer, the character’s Unicode is used in the evaluation. For example, if hexValue is 5, (char)(hexValue + '0') returns 5. Similarly, if hexValue is between 10 and 15, it is converted to (char)(hexValue - 10 + 'A') (line 21). For instance, if hexValue is 11, (char)(hexValue - 10 + 'A') returns B.
5.21 5.22 5.23
Will the program work if n1 and n2 are replaced by n1 / 2 and n2 / 2 in line 17 in Listing 5.9? In Listing 5.11, why is it wrong if you change the code (char)(hexValue + '0') to hexValue + '0' in line 21? In Listing 5.11, how many times the loop body is executed for a decimal number 245 and how many times the loop body is executed for a decimal number 3245?
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Check Point
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5.9 Keywords break and continue Key Point
The break and continue keywords provide additional controls in a loop.
Pedagogical Note Two keywords, break and continue, can be used in loop statements to provide additional controls. Using break and continue can simplify programming in some cases. Overusing or improperly using them, however, can make programs difficult to read and debug. (Note to instructors: You may skip this section without affecting students’ understanding of the rest of the book.) break statement
You have used the keyword break in a switch statement. You can also use break in a loop to immediately terminate the loop. Listing 5.12 presents a program to demonstrate the effect of using break in a loop.
LISTING 5.12 TestBreak.java
break
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
public class TestBreak { public static void main(String[] args) { int sum = 0; int number = 0; while (number < 20) { number++; sum += number; if (sum >= 100) break; } System.out.println("The number is " + number); System.out.println("The sum is " + sum); } }
The number is 14 The sum is 105
The program in Listing 5.12 adds integers from 1 to 20 in this order to sum until sum is greater than or equal to 100. Without the if statement (line 9), the program calculates the sum of the numbers from 1 to 20. But with the if statement, the loop terminates when sum becomes greater than or equal to 100. Without the if statement, the output would be: The number is 20 The sum is 210 continue statement
You can also use the continue keyword in a loop. When it is encountered, it ends the current iteration and program control goes to the end of the loop body. In other words, continue breaks out of an iteration while the break keyword breaks out of a loop. Listing 5.13 presents a program to demonstrate the effect of using continue in a loop.
LISTING 5.13 TestContinue.java 1 2 3
public class TestContinue { public static void main(String[] args) { int sum = 0;
5.9 Keywords break and continue 185 4 5 6 7 8 9 10 11 12 13 14 15
int number = 0; while (number < 20) { number++; if (number ==10 || number == 11) continue; sum += number; } System.out.println("The sum is " + sum); } }
The sum is 189
The program in Listing 5.13 adds integers from 1 to 20 except 10 and 11 to sum. With the if statement in the program (line 8), the continue statement is executed when number becomes 10 or 11. The continue statement ends the current iteration so that the rest of the statement in the loop body is not executed; therefore, number is not added to sum when it is 10 or 11. Without the if statement in the program, the output would be as follows: The sum is 210
In this case, all of the numbers are added to sum, even when number is 10 or 11. Therefore, the result is 210, which is 21 more than it was with the if statement.
Note The continue statement is always inside a loop. In the while and do-while loops, the loop-continuation-condition is evaluated immediately after the continue statement. In the for loop, the action-after-each-iteration is performed, then the loop-continuation-condition is evaluated, immediately after the continue statement.
You can always write a program without using break or continue in a loop (see Checkpoint Question 5.26). In general, though, using break and continue is appropriate if it simplifies coding and makes programs easier to read. Suppose you need to write a program to find the smallest factor other than 1 for an integer n (assume n >= 2). You can write a simple and intuitive code using the break statement as follows: int factor = 2; while (factor <= n) { if (n % factor == 0) break; factor++; } System.out.println("The smallest factor other than 1 for " + n + " is " + factor);
You may rewrite the code without using break as follows: boolean found = false; int factor = 2; while (factor <= n && !found) { if (n % factor == 0)
continue
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Loops found = true; else factor++; } System.out.println("The smallest factor other than 1 for " + n + " is " + factor);
Obviously, the break statement makes this program simpler and easier to read in this case. However, you should use break and continue with caution. Too many break and continue statements will produce a loop with many exit points and make the program difficult to read.
Note Some programming languages have a goto statement. The goto statement indiscriminately transfers control to any statement in the program and executes it. This makes your program vulnerable to errors. The break and continue statements in Java are different from goto statements. They operate only in a loop or a switch statement. The break statement breaks out of the loop, and the continue statement breaks out of the current iteration in the loop.
goto
Note Programming is a creative endeavor. There are many different ways to write code. In fact, you can find a smallest factor using a rather simple code as follows: int factor = 2; while (factor <= n && n % factor != 0) factor++;
✓
Check Point
5.24
What is the keyword break for? What is the keyword continue for? Will the following programs terminate? If so, give the output.
int balance = 10; while (true) { if (balance < 9) break; balance = balance - 9; }
int balance = 10; while (true) { if (balance < 9) continue; balance = balance - 9; }
System.out.println("Balance is " + balance);
System.out.println("Balance is " + balance);
(a)
5.25
The for loop on the left is converted into the while loop on the right. What is wrong? Correct it.
int sum = 0; for (int i = 0; i < 4; i++) { if (i % 3 == 0) continue; sum += i; }
5.26
(b)
Converted Wrong conversion
int i = 0, sum = 0; while (i < 4) { if (i % 3 == 0) continue; sum += i; i++; }
Rewrite the programs TestBreak and TestContinue in Listings 5.12 and 5.13 without using break and continue.
5.10 Case Study: Checking Palindromes 187 5.27
After the break statement in (a) is executed in the following loop, which statement is executed? Show the output. After the continue statement in (b) is executed in the following loop, which statement is executed? Show the output.
for (int i = 1; i < 4; i++) { for (int j = 1; j < 4; j++) { if (i * j > 2) break;
for (int i = 1; i < 4; i++) { for (int j = 1; j < 4; j++) { if (i * j > 2) continue;
System.out.println(i * j);
System.out.println(i * j);
}
}
System.out.println(i); }
System.out.println(i); }
(a)
(b)
5.10 Case Study: Checking Palindromes This section presents a program that checks whether a string is a palindrome. A string is a palindrome if it reads the same forward and backward. The words “mom,” “dad,” and “noon,” for instance, are all palindromes. The problem is to write a program that prompts the user to enter a string and reports whether the string is a palindrome. One solution is to check whether the first character in the string is the same as the last character. If so, check whether the second character is the same as the second-to-last character. This process continues until a mismatch is found or all the characters in the string are checked, except for the middle character if the string has an odd number of characters. Listing 5.14 gives the program.
Key Point
think before you code
LISTING 5.14 Palindrome.java 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
import java.util.Scanner; public class Palindrome { /** Main method */ public static void main(String[] args) { // Create a Scanner Scanner input = new Scanner(System.in); // Prompt the user to enter a string System.out.print("Enter a string: "); String s = input.nextLine();
input string
// The index of the first character in the string int low = 0;
low index
// The index of the last character in the string int high = s.length() - 1;
high index
boolean isPalindrome = true; while (low < high) { if (s.charAt(low) != s.charAt(high)) { isPalindrome = false; break; }
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Loops 26 27 28 29 30 31 32 33 34 35
update indices
low++; high--; } if (isPalindrome) System.out.println(s + " is a palindrome"); else System.out.println(s + " is not a palindrome"); } }
Enter a string: noon noon is a palindrome
Enter a string: moon moon is not a palindrome
The program uses two variables, low and high, to denote the position of the two characters at the beginning and the end in a string s (lines 14, 17). Initially, low is 0 and high is s. length() – 1. If the two characters at these positions match, increment low by 1 and decrement high by 1 (lines 26–27). This process continues until (low >= high) or a mismatch is found (line 21). The program uses a boolean variable isPalindrome to denote whether the string s is palindrome. Initially, it is set to true (line 19). When a mismatch is discovered (line 21), isPalindrome is to false (line 22) and the loop is terminated with a break statement (line 23).
5.11 Case Study: Displaying Prime Numbers Key Point
This section presents a program that displays the first fifty prime numbers in five lines, each containing ten numbers. An integer greater than 1 is prime if its only positive divisor is 1 or itself. For example, 2, 3, 5, and 7 are prime numbers, but 4, 6, 8, and 9 are not. The problem is to display the first 50 prime numbers in five lines, each of which contains ten numbers. The problem can be broken into the following tasks: ■
Determine whether a given number is prime.
■
For number = 2, 3, 4, 5, 6, . . ., test whether it is prime.
■
Count the prime numbers.
■
Display each prime number, and display ten numbers per line.
Obviously, you need to write a loop and repeatedly test whether a new number is prime. If the number is prime, increase the count by 1. The count is 0 initially. When it reaches 50, the loop terminates. Here is the algorithm for the problem: Set the number of prime numbers to be printed as a constant NUMBER_OF_PRIMES; Use count to track the number of prime numbers and set an initial count to 0; Set an initial number to 2;
5.11 Case Study: Displaying Prime Numbers 189 while (count < NUMBER_OF_PRIMES) { Test whether number is prime; if number is prime { Display the prime number and increase the count; } Increment number by 1; }
To test whether a number is prime, check whether it is divisible by 2, 3, 4, and so on up to number/2. If a divisor is found, the number is not a prime. The algorithm can be described as follows: Use a boolean variable isPrime to denote whether the number is prime; Set isPrime to true initially; for (int divisor = 2; divisor <= number / 2; divisor++) { if (number % divisor == 0) { Set isPrime to false Exit the loop; } }
The complete program is given in Listing 5.15.
LISTING 5.15 PrimeNumber.java 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
public class PrimeNumber { public static void main(String[] args) { final int NUMBER_OF_PRIMES = 50; // Number of primes to display final int NUMBER_OF_PRIMES_PER_LINE = 10; // Display 10 per line int count = 0; // Count the number of prime numbers int number = 2; // A number to be tested for primeness System.out.println("The first 50 prime numbers are \n"); // Repeatedly find prime numbers while (count < NUMBER_OF_PRIMES) { // Assume the number is prime boolean isPrime = true; // Is the current number prime? // Test whether number is prime for (int divisor = 2; divisor <= number / 2; divisor++) { if (number % divisor == 0) { // If true, number is not prime isPrime = false; // Set isPrime to false break; // Exit the for loop } } // Display the prime number and increase the count if (isPrime) { count++; // Increase the count if (count % NUMBER_OF_PRIMES_PER_LINE == 0) { // Display the number and advance to the new line System.out.println(number); } else System.out.print(number + " ");
count prime numbers
check primeness
exit loop
display if prime
190 Chapter 5
Loops 33 34 35 36 37 38 39
} // Check if the next number is prime number++; } } }
The first 50 prime numbers are 2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97 101 103 107 109 113 127 131 137 139 149 151 157 163 167 173 179 181 191 193 197 199 211 223 227 229
subproblem
This is a complex program for novice programmers. The key to developing a programmatic solution for this problem, and for many other problems, is to break it into subproblems and develop solutions for each of them in turn. Do not attempt to develop a complete solution in the first trial. Instead, begin by writing the code to determine whether a given number is prime, then expand the program to test whether other numbers are prime in a loop. To determine whether a number is prime, check whether it is divisible by a number between 2 and number/2 inclusive (lines 16–21). If so, it is not a prime number (line 18); otherwise, it is a prime number. For a prime number, display it. If the count is divisible by 10 (lines 27–30), advance to a new line. The program ends when the count reaches 50. The program uses the break statement in line 19 to exit the for loop as soon as the number is found to be a nonprime. You can rewrite the loop (lines 16–21) without using the break statement, as follows: for (int divisor = 2; divisor <= number / 2 && isPrime; divisor++) { // If true, the number is not prime if (number % divisor == 0) { // Set isPrime to false, if the number is not prime isPrime = false; } }
However, using the break statement makes the program simpler and easier to read in this case.
KEY TERMS break statement 184 continue statement 184 do-while loop 168 for loop 171
infinite loop 160 input redirection 167 iteration 158 loop 158
loop body 158 nested loop 176 off-by-one error 160 output redirection 167 posttest loop 174 pretest loop 174 sentinel value 165 while loop 158
Programming Exercises 191
CHAPTER SUMMARY 1. There are three types of repetition statements: the while loop, the do-while loop, and the for loop.
2. The part of the loop that contains the statements to be repeated is called the loop body. 3. A one-time execution of a loop body is referred to as an iteration of the loop. 4. An infinite loop is a loop statement that executes infinitely. 5. In designing loops, you need to consider both the loop control structure and the loop body.
6. The while loop checks the loop-continuation-condition first. If the condition is true, the loop body is executed; if it is false, the loop terminates.
7. The do-while loop is similar to the while loop, except that the do-while loop executes the loop body first and then checks the loop-continuation-condition to decide whether to continue or to terminate.
8. The while loop and the do-while loop often are used when the number of repetitions is not predetermined.
9. A sentinel value is a special value that signifies the end of the loop. 10. The for loop generally is used to execute a loop body a fixed number of times. 11. The for loop control has three parts. The first part is an initial action that often initializes a control variable. The second part, the loop-continuation-condition, determines whether the loop body is to be executed. The third part is executed after each iteration and is often used to adjust the control variable. Usually, the loop control variables are initialized and changed in the control structure.
12. The while loop and for loop are called pretest loops because the continuation condition is checked before the loop body is executed.
13. The do-while loop is called a posttest loop because the condition is checked after the loop body is executed.
14. Two keywords, break and continue, can be used in a loop. 15. The break keyword immediately ends the innermost loop, which contains the break. 16. The continue keyword only ends the current iteration.
QUIZ Answer the quiz for this chapter online at www.cs.armstrong.edu/liang/intro10e/quiz.html.
PROGRAMMING EXERCISES Pedagogical Note Read each problem several times until you understand it. Think how to solve the problem before starting to write code. Translate your logic into a program. A problem often can be solved in many different ways. Students are encouraged to explore various solutions.
read and think before coding
explore solutions
192 Chapter 5
Loops Sections 5.2–5.7
*5.1 (Count positive and negative numbers and compute the average of numbers) Write a program that reads an unspecified number of integers, determines how many positive and negative values have been read, and computes the total and average of the input values (not counting zeros). Your program ends with the input 0. Display the average as a floating-point number. Here is a sample run:
Enter an integer, the input ends if it is 0: 1 2 -1 3 0 The number of positives is 3 The number of negatives is 1 The total is 5.0 The average is 1.25
Enter an integer, the input ends if it is 0: 0 No numbers are entered except 0
5.2 (Repeat additions) Listing 5.4, SubtractionQuizLoop.java, generates five random 5.3
subtraction questions. Revise the program to generate ten random addition questions for two integers between 1 and 15. Display the correct count and test time. (Conversion from kilograms to pounds) Write a program that displays the following table (note that 1 kilogram is 2.2 pounds): Kilograms 1 3 ... 197 199
Pounds 2.2 6.6 433.4 437.8
5.4 (Conversion from miles to kilometers) Write a program that displays the following table (note that 1 mile is 1.609 kilometers): Miles 1 2 ... 9 10
Kilometers 1.609 3.218 14.481 16.090
5.5 (Conversion from kilograms to pounds and pounds to kilograms) Write a program that displays the following two tables side by side: Kilograms 1 3 ... 197 199
Pounds 2.2 6.6
| | |
Pounds 20 25
433.4 437.8
| |
510 515
Kilograms 9.09 11.36 231.82 234.09
5.6 (Conversion from miles to kilometers) Write a program that displays the following two tables side by side:
Programming Exercises 193 Miles 1 2 ... 9 10
Kilometers 1.609 3.218
| | |
Kilometers 20 25
Miles 12.430 15.538
14.481 16.090
| |
60 65
37.290 40.398
**5.7 (Financial application: compute future tuition) Suppose that the tuition for a uni-
5.8 *5.9 5.10 5.11 5.12
versity is $10,000 this year and increases 5% every year. In one year, the tuition will be $10,500. Write a program that computes the tuition in ten years and the total cost of four years’ worth of tuition after the tenth year. (Find the highest score) Write a program that prompts the user to enter the number of students and each student’s name and score, and finally displays the name of the student with the highest score. (Find the two highest scores) Write a program that prompts the user to enter the number of students and each student’s name and score, and finally displays the student with the highest score and the student with the second-highest score. (Find numbers divisible by 5 and 6) Write a program that displays all the numbers from 100 to 1,000, ten per line, that are divisible by 5 and 6. Numbers are separated by exactly one space. (Find numbers divisible by 5 or 6, but not both) Write a program that displays all the numbers from 100 to 200, ten per line, that are divisible by 5 or 6, but not both. Numbers are separated by exactly one space. (Find the smallest n such that n2 7 12,000) Use a while loop to find the smallest integer n such that n2 is greater than 12,000.
5.13 (Find the largest n such that n3 6 12,000) Use a while loop to find the largest integer n such that n3 is less than 12,000.
Sections 5.8–5.10
*5.14 (Compute the greatest common divisor) Another solution for Listing 5.9 to find the greatest common divisor of two integers n1 and n2 is as follows: First find d to be the minimum of n1 and n2, then check whether d, d-1, d-2, . . . , 2, or 1 is a divisor for both n1 and n2 in this order. The first such common divisor is the greatest common divisor for n1 and n2. Write a program that prompts the user to enter two positive integers and displays the gcd.
*5.15 (Display the ASCII character table) Write a program that prints the characters in the ASCII character table from ! to ~. Display ten characters per line. The ASCII table is shown in Appendix B. Characters are separated by exactly one space.
*5.16 (Find the factors of an integer) Write a program that reads an integer and displays **5.17
all its smallest factors in increasing order. For example, if the input integer is 120, the output should be as follows: 2, 2, 2, 3, 5. (Display pyramid) Write a program that prompts the user to enter an integer from 1 to 15 and displays a pyramid, as shown in the following sample run: Enter the number of lines: 7
7
6 6
5 5 5
4 4 4 4
3 3 3 3 3
2 2 2 2 2 2
1 1 1 1 1 1 1
2 2 2 2 2 2
3 3 3 3 3
4 4 4 4
5 5 5
6 6
7
194 Chapter 5
Loops *5.18 (Display four patterns using loops) Use nested loops that display the following patterns in four separate programs: Pattern A
Pattern B
1
1 2 3 4 5 6
Pattern C
1 2
1 2 3 4 5
1 2 3
1 2 3 4
1 2 3 4
1 2 3
1 2 3 4 5
1 2
1 2 3 4 5 6
1
Pattern D 1
1 2 3 4 5 6
2 1
1 2 3 4 5
3 2 1
1 2 3 4
4 3 2 1
1 2 3
5 4 3 2 1
1 2
6 5 4 3 2 1
1
**5.19 (Display numbers in a pyramid pattern) Write a nested for loop that prints the following output: 1
1
1
2
1
1
2
4
2
1
1
2
4
8
4
2
1
1
2
4
8
16
8
4
2
1
1
2
4
8
16
32
16
8
4
2
1
1
2
4
8
16
32
64
32
16
8
4
2
1
2
4
8
16
32
64 128
64
32
16
8
4
2
1
*5.20 (Display prime numbers between 2 and 1,000) Modify Listing 5.15 to display all the prime numbers between 2 and 1,000, inclusive. Display eight prime numbers per line. Numbers are separated by exactly one space.
Comprehensive
**5.21 (Financial application: compare loans with various interest rates) Write a program that lets the user enter the loan amount and loan period in number of years and displays the monthly and total payments for each interest rate starting from 5% to 8%, with an increment of 1/8. Here is a sample run:
Loan Amount: 10000 Number of Years: 5 Interest Rate Monthly Payment 5.000% 5.125% 5.250% ... 7.875% 8.000%
**5.22 VideoNote
Display loan schedule
Total Payment
188.71 189.29 189.86
11322.74 11357.13 11391.59
202.17 202.76
12129.97 12165.84
For the formula to compute monthly payment, see Listing 2.9, ComputeLoan.java. (Financial application: loan amortization schedule) The monthly payment for a given loan pays the principal and the interest. The monthly interest is computed by multiplying the monthly interest rate and the balance (the remaining principal). The principal paid for the month is therefore the monthly payment minus the monthly interest. Write a program that lets the user enter the loan amount,
Programming Exercises 195 number of years, and interest rate and displays the amortization schedule for the loan. Here is a sample run:
Loan Amount: 10000 Number of Years: 1 Annual Interest Rate: 7 Monthly Payment: 865.26 Total Payment: 10383.21 Payment# 1 2 ... 11 12
Interest 58.33 53.62
Principal 806.93 811.64
10.0 5.01
855.26 860.25
Balance 9193.07 8381.43 860.27 0.01
Note The balance after the last payment may not be zero. If so, the last payment should be the normal monthly payment plus the final balance.
Hint: Write a loop to display the table. Since the monthly payment is the same for each month, it should be computed before the loop. The balance is initially the loan amount. For each iteration in the loop, compute the interest and principal, and update the balance. The loop may look like this: for (i = 1; i <= numberOfYears * 12; i++) { interest = monthlyInterestRate * balance; principal = monthlyPayment - interest; balance = balance - principal; System.out.println(i + "\t\t" + interest + "\t\t" + principal + "\t\t" + balance); }
*5.23 (Demonstrate cancellation errors) A cancellation error occurs when you are manipulating a very large number with a very small number. The large number may cancel out the smaller number. For example, the result of 100000000.0 + 0.000000001 is equal to 100000000.0. To avoid cancellation errors and obtain more accurate results, carefully select the order of computation. For example, in computing the following series, you will obtain more accurate results by computing from right to left rather than from left to right: 1 +
*5.24
1 1 1 + + c + n 2 3
Write a program that compares the results of the summation of the preceding series, computing from left to right and from right to left with n = 50000. (Sum a series) Write a program to sum the following series: 1 3 5 7 9 11 95 97 + + + + + + g + + 3 5 7 9 11 13 97 99
VideoNote
Sum a series
196 Chapter 5
Loops **5.25 (Compute p) You can approximate p by using the following series: p = 4¢ 1 -
**5.26
(-1)i + 1 1 1 1 1 1 + - + + g + ≤ 3 5 7 9 11 2i - 1
Write a program that displays the p value for i = 10000, 20000, …, and 100000. (Compute e) You can approximate e using the following series: e = 1 +
1 1 1 1 1 + + + + g + 1! 2! 3! 4! i!
Write a program that displays the e value for i = 10000, 20000, …, and 100000. (Hint: Because i! = i * (i - 1) * c * 2 * 1, then 1 1 is i! i(i - 1)!
**5.27 **5.28
Initialize e and item to be 1 and keep adding a new item to e. The new item is the previous item divided by i for i = 2, 3, 4, . . . .) (Display leap years) Write a program that displays all the leap years, ten per line, from 101 to 2100, separated by exactly one space. Also display the number of leap years in this period. (Display the first days of each month) Write a program that prompts the user to enter the year and first day of the year, and displays the first day of each month in the year. For example, if the user entered the year 2013, and 2 for Tuesday, January 1, 2013, your program should display the following output: January 1, 2013 is Tuesday ... December 1, 2013 is Sunday
**5.29 (Display calendars) Write a program that prompts the user to enter the year and first day of the year and displays the calendar table for the year on the console. For example, if the user entered the year 2013, and 2 for Tuesday, January 1, 2013, your program should display the calendar for each month in the year, as follows: January 2013 Sun
Mon
Tue
Wed
Thu
Fri
Sat
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
Programming Exercises 197 December 2013 Sun
Mon
Tue
Wed
Thu
Fri
Sat
1
2
3
4
5
6
7
8
9
10
11
12
13
14
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*5.30 (Financial application: compound value) Suppose you save $100 each month into a savings account with the annual interest rate 5%. So, the monthly interest rate is 0.05 / 12 = 0.00417. After the first month, the value in the account becomes 100 * (1 + 0.00417) = 100.417
After the second month, the value in the account becomes (100 + 100.417) * (1 + 0.00417) = 201.252
After the third month, the value in the account becomes (100 + 201.252) * (1 + 0.00417) = 302.507
*5.31
and so on. Write a program that prompts the user to enter an amount (e.g., 100), the annual interest rate (e.g., 5), and the number of months (e.g., 6) and displays the amount in the savings account after the given month. (Financial application: compute CD value) Suppose you put $10,000 into a CD with an annual percentage yield of 5.75%. After one month, the CD is worth 10000 + 10000 * 5.75 / 1200 = 10047.92
After two months, the CD is worth 10047.91 + 10047.91 * 5.75 / 1200 = 10096.06
After three months, the CD is worth 10096.06 + 10096.06 * 5.75 / 1200 = 10144.44
and so on. Write a program that prompts the user to enter an amount (e.g., 10000), the annual percentage yield (e.g., 5.75), and the number of months (e.g., 18) and displays a table as shown in the sample run.
198 Chapter 5
Loops Enter the initial deposit amount: 10000 Enter annual percentage yield: 5.75 Enter maturity period (number of months): 18 Month 1 2 ... 17 18
CD Value 10047.92 10096.06 10846.57 10898.54
**5.32 (Game: lottery) Revise Listing 3.8, Lottery.java, to generate a lottery of a twodigit number. The two digits in the number are distinct. (Hint: Generate the first digit. Use a loop to continuously generate the second digit until it is different from the first digit.)
**5.33 (Perfect number) A positive integer is called a perfect number if it is equal to
***5.34
the sum of all of its positive divisors, excluding itself. For example, 6 is the first perfect number because 6 = 3 + 2 + 1. The next is 28 = 14 + 7 + 4 + 2 + 1. There are four perfect numbers less than 10,000. Write a program to find all these four numbers. (Game: scissor, rock, paper) Programming Exercise 3.17 gives a program that plays the scissor-rock-paper game. Revise the program to let the user continuously play until either the user or the computer wins more than two times than its opponent.
*5.35 (Summation) Write a program to compute the following summation. 1 1 + 22
+
1 22 + 23
+
1 23 + 24
+ c +
1 2624 + 2625
**5.36 (Business application: checking ISBN ) Use loops to simplify Programming Exercise 3.9.
**5.37 (Decimal to binary) Write a program that prompts the user to enter a decimal integer and displays its corresponding binary value. Don’t use Java’s Integer .toBinaryString(int) in this program.
**5.38 (Decimal to octal) Write a program that prompts the user to enter a decimal integer and displays its corresponding octal value. Don’t use Java’s Integer .toOctalString(int) in this program.
*5.39 (Financial application: find the sales amount) You have just started a sales job in a department store. Your pay consists of a base salary and a commission. The base salary is $5,000. The scheme shown below is used to determine the commission rate. Sales Amount $0.01–$5,000 $5,000.01–$10,000 $10,000.01 and above
Commission Rate 8 percent 10 percent 12 percent
Note that this is a graduated rate. The rate for the first $5,000 is at 8%, the next $5000 is at 10%, and the rest is at 12%. If the sales amount is 25,000, the commission is 5,000 * 8% + 5,000 * 10% + 15,000 * 12% = 2,700.
Programming Exercises 199
5.40
Your goal is to earn $30,000 a year. Write a program that finds the minimum sales you have to generate in order to make $30,000. (Simulation: heads or tails) Write a program that simulates flipping a coin one million times and displays the number of heads and tails.
*5.41 (Occurrence of max numbers) Write a program that reads integers, finds the largest of them, and counts its occurrences. Assume that the input ends with number 0. Suppose that you entered 3 5 2 5 5 5 0; the program finds that the largest is 5 and the occurrence count for 5 is 4. (Hint: Maintain two variables, max and count. max stores the current max number, and count stores its occurrences. Initially, assign the first number to max and 1 to count. Compare each subsequent number with max. If the number is greater than max, assign it to max and reset count to 1. If the number is equal to max, increment count by 1.) Enter numbers: 3 5 2 5 5 5 0 The largest number is 5 The occurrence count of the largest number is 4
*5.42 (Financial application: find the sales amount) Rewrite Programming Exercise 5.39 as follows: ■ ■
Use a for loop instead of a do-while loop. Let the user enter COMMISSION_SOUGHT instead of fixing it as a constant.
*5.43 (Math: combinations) Write a program that displays all possible combinations for picking two numbers from integers 1 to 7. Also display the total number of all combinations. 1 2 1 3 ... ... The total number of all combinations is 21
*5.44 (Computer architecture: bit-level operations) A short value is stored in 16 bits. Write a program that prompts the user to enter a short integer and displays the 16 bits for the integer. Here are sample runs: Enter an integer: 5 The bits are 0000000000000101
Enter an integer: -5 The bits are 1111111111111011
(Hint: You need to use the bitwise right shift operator (>>) and the bitwise AND operator (&), which are covered in Appendix G, Bitwise Operations.)
**5.45 (Statistics: compute mean and standard deviation) In business applications, you are often asked to compute the mean and standard deviation of data. The mean is simply the average of the numbers. The standard deviation is a statistic that tells
200 Chapter 5
Loops you how tightly all the various data are clustered around the mean in a set of data. For example, what is the average age of the students in a class? How close are the ages? If all the students are the same age, the deviation is 0. Write a program that prompts the user to enter ten numbers, and displays the mean and standard deviations of these numbers using the following formula: n
n
mean =
a xi
i=1
n
n
x1 + x2 + g + xn = n
deviation =
2 a xi -
c
¢ a xi ≤
2
i=1
i=1
n
n - 1
Here is a sample run: Enter ten numbers: 1 2 3 4.5 5.6 6 7 8 9 10 The mean is 5.61 The standard deviation is 2.99794
*5.46 (Reverse a string) Write a program that prompts the user to enter a string and displays the string in reverse order. Enter a string: ABCD The reversed string is DCBA
*5.47 (Business: check ISBN-13) ISBN-13 is a new standard for indentifying books. It uses 13 digits d1d2d3d4d5d6d7d8d9d10d11d12d13. The last digit d13 is a checksum, which is calculated from the other digits using the following formula: 10 - (d1 + 3d2 + d3 + 3d4 + d5 + 3d6 + d7 + 3d8 + d9 + 3d10 + d11 + 3d12)%10 If the checksum is 10, replace it with 0. Your program should read the input as a string. Here are sample runs: Enter the first 12 digits of an ISBN-13 as a string: 978013213080 The ISBN-13 number is 9780132130806
Enter the first 12 digits of an ISBN-13 as a string: 978013213079 The ISBN-13 number is 9780132130790
Enter the first 12 digits of an ISBN-13 as a string: 97801320 97801320 is an invalid input
*5.48 (Process string) Write a program that prompts the user to enter a string and displays the characters at odd positions. Here is a sample run: Enter a string: Beijing Chicago BiigCiao
Programming Exercises 201 *5.49 (Count vowels and consonants) Assume letters A, E, I, O, and U as the vowels. Write a program that prompts the user to enter a string and displays the number of vowels and consonants in the string. Enter a string: Programming is fun The number of vowels is 5 The number of consonants is 11
*5.50 (Count uppercase letters) Write a program that prompts the user to enter a string and displays the number of the uppercase letters in the string. Enter a string: Welcome to Java The number of uppercase letters is 2
*5.51 (Longest common prefix) Write a program that prompts the user to enter two strings and displays the largest common prefix of the two strings. Here are some sample runs: Enter the first string: Welcome to C++ Enter the second string: Welcome to programming The common prefix is Welcome to
Enter the first string: Atlanta Enter the second string: Macon Atlanta and Macon have no common prefix
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CHAPTER
6 METHODS Objectives ■
To define methods with formal parameters (§6.2).
■
To invoke methods with actual parameters (i.e., arguments) (§6.2).
■
To define methods with a return value (§6.3).
■
To define methods without a return value (§6.4).
■
To pass arguments by value (§6.5).
■
To develop reusable code that is modular, easy to read, easy to debug, and easy to maintain (§6.6).
■
To write a method that converts hexadecimals to decimals (§6.7).
■
To use method overloading and understand ambiguous overloading (§6.8).
■
To determine the scope of variables (§6.9).
■
To apply the concept of method abstraction in software development (§6.10).
■
To design and implement methods using stepwise refinement (§6.10).
204 Chapter 6
Methods
6.1 Introduction Key Point problem
Methods can be used to define reusable code and organize and simplify coding. Suppose that you need to find the sum of integers from 1 to 10, from 20 to 37, and from 35 to 49, respectively. You may write the code as follows: int sum = 0; for (int i = 1; i <= 10; i++) sum += i; System.out.println("Sum from 1 to 10 is " + sum); sum = 0; for (int i = 20; i <= 37; i++) sum += i; System.out.println("Sum from 20 to 37 is " + sum); sum = 0; for (int i = 35; i <= 49; i++) sum += i; System.out.println("Sum from 35 to 49 is " + sum);
You may have observed that computing these sums from 1 to 10, from 20 to 37, and from 35 to 49 are very similar except that the starting and ending integers are different. Wouldn’t it be nice if we could write the common code once and reuse it? We can do so by defining a method and invoking it. The preceding code can be simplified as follows:
why methods?
1 2 3 4 5 6 7 8 9 10 11 12 13
define sum method
main method invoke sum
public static int sum(int i1, int i2) { int result = 0; for (int i = i1; i <= i2; i++) result += i; return result; } public static void main(String[] args) { System.out.println("Sum from 1 to 10 is " + sum(1, 10)); System.out.println("Sum from 20 to 37 is " + sum(20, 37)); System.out.println("Sum from 35 to 49 is " + sum(35, 49)); }
Lines 1–7 define the method named sum with two parameters i1 and i2. The statements in the main method invoke sum(1, 10) to compute the sum from 1 to 10, sum(20, 37) to compute the sum from 20 to 37, and sum(35, 49) to compute the sum from 35 to 49. A method is a collection of statements grouped together to perform an operation. In earlier chapters you have used predefined methods such as System.out.println, System.exit, Math .pow, and Math.random. These methods are defined in the Java library. In this chapter, you will learn how to define your own methods and apply method abstraction to solve complex problems.
method
6.2 Defining a Method Key Point
A method definition consists of its method name, parameters, return value type, and body. The syntax for defining a method is as follows: modifier returnValueType methodName(list of parameters) { // Method body; }
6.2 Defining a Method 205 Let’s look at a method defined to find the larger between two integers. This method, named max, has two int parameters, num1 and num2, the larger of which is returned by the method. Figure 6.1 illustrates the components of this method.
Define a method modifier method header
return value type
Invoke a method
method formal name parameters
public static int max(int num1, int num2) {
int z = max(x, y);
int result;
method body
if (num1 > num2) result = num1; else result = num2; return result;
parameter list
method signature
actual parameters (arguments)
return value
}
FIGURE 6.1 A method definition consists of a method header and a method body.
The method header specifies the modifiers, return value type, method name, and parameters of the method. The static modifier is used for all the methods in this chapter. The reason for using it will be discussed in Chapter 8, Objects and Classes. A method may return a value. The returnValueType is the data type of the value the method returns. Some methods perform desired operations without returning a value. In this case, the returnValueType is the keyword void. For example, the returnValueType is void in the main method, as well as in System.exit, and System.out.println. If a method returns a value, it is called a value-returning method; otherwise it is called a void method. The variables defined in the method header are known as formal parameters or simply parameters. A parameter is like a placeholder: when a method is invoked, you pass a value to the parameter. This value is referred to as an actual parameter or argument. The parameter list refers to the method’s type, order, and number of the parameters. The method name and the parameter list together constitute the method signature. Parameters are optional; that is, a method may contain no parameters. For example, the Math.random() method has no parameters. The method body contains a collection of statements that implement the method. The method body of the max method uses an if statement to determine which number is larger and return the value of that number. In order for a value-returning method to return a result, a return statement using the keyword return is required. The method terminates when a return statement is executed.
Note Some programming languages refer to methods as procedures and functions. In those languages, a value-returning method is called a function and a void method is called a procedure.
Caution In the method header, you need to declare each parameter separately. For instance, max(int num1, int num2) is correct, but max(int num1, num2) is wrong.
method header modifier
value-returning method void method formal parameter parameter actual parameter argument parameter list method signature
206 Chapter 6
Methods Note We say “define a method” and “declare a variable.” We are making a subtle distinction here. A definition defines what the defined item is, but a declaration usually involves allocating memory to store data for the declared item.
define vs. declare
6.3 Calling a Method Key Point
Calling a method executes the code in the method. In a method definition, you define what the method is to do. To execute the method, you have to call or invoke it. There are two ways to call a method, depending on whether the method returns a value or not. If a method returns a value, a call to the method is usually treated as a value. For example, int larger = max(3, 4);
calls max(3, 4) and assigns the result of the method to the variable larger. Another example of a call that is treated as a value is System.out.println(max(3, 4));
which prints the return value of the method call max(3, 4). If a method returns void, a call to the method must be a statement. For example, the method println returns void. The following call is a statement: System.out.println("Welcome to Java!");
Note A value-returning method can also be invoked as a statement in Java. In this case, the caller simply ignores the return value. This is not often done, but it is permissible if the caller is not interested in the return value.
When a program calls a method, program control is transferred to the called method. A called method returns control to the caller when its return statement is executed or when its methodending closing brace is reached. Listing 6.1 shows a complete program that is used to test the max method. VideoNote
Define/invoke max method main method
invoke max
define method
LISTING 6.1 TestMax.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
public class TestMax { /** Main method */ public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); System.out.println("The maximum of " + i + " and " + j + " is " + k); } /** Return the max of two numbers */ public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; return result; } }
6.3 Calling a Method 207 The maximum of 5 and 2 is 5
line# 4 5
i
j
k
num2
5
2
result
5 2
12
Invoking max
num1
13
undefined
16
5
6
5
This program contains the main method and the max method. The main method is just like any other method except that it is invoked by the JVM to start the program. The main method’s header is always the same. Like the one in this example, it includes the modifiers public and static, return value type void, method name main, and a parameter of the String[] type. String[] indicates that the parameter is an array of String, a subject addressed in Chapter 7. The statements in main may invoke other methods that are defined in the class that contains the main method or in other classes. In this example, the main method invokes max(i, j), which is defined in the same class with the main method. When the max method is invoked (line 6), variable i’s value 5 is passed to num1, and variable j’s value 2 is passed to num2 in the max method. The flow of control transfers to the max method, and the max method is executed. When the return statement in the max method is executed, the max method returns the control to its caller (in this case the caller is the main method). This process is illustrated in Figure 6.2.
main method
max method
pass the value i pass the value j
public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j);
public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2;
System.out.println( "The maximum of " + i + " and " + j + " is " + k); }
return result; }
FIGURE 6.2 When the max method is invoked, the flow of control transfers to it. Once the max method is finished, it returns control back to the caller.
Caution A return statement is required for a value-returning method. The method shown below in (a) is logically correct, but it has a compile error because the Java compiler thinks that this method might not return a value.
208 Chapter 6
Methods public static int sign(int n) { if (n > 0) return 1; else if (n == 0) return 0; else if (n < 0) return –1;
Should be
}
public static int sign(int n) { if (n > 0) return 1; else if (n == 0) return 0; else return –1;
} (a)
(b)
To fix this problem, delete if (n < 0) in (a), so the compiler will see a return statement to be reached regardless of how the if statement is evaluated.
Note Methods enable code sharing and reuse. The max method can be invoked from any class, not just TestMax. If you create a new class, you can invoke the max method using ClassName.methodName (i.e., TestMax.max).
reusing method
activation record call stack
Activation record for the main method k: j: 2 i: 5 (a) The main method is invoked.
Each time a method is invoked, the system creates an activation record (also called an activation frame) that stores parameters and variables for the method and places the activation record in an area of memory known as a call stack. A call stack is also known as an execution stack, runtime stack, or machine stack, and it is often shortened to just “the stack.” When a method calls another method, the caller’s activation record is kept intact, and a new activation record is created for the new method called. When a method finishes its work and returns to its caller, its activation record is removed from the call stack. A call stack stores the activation records in a last-in, first-out fashion: The activation record for the method that is invoked last is removed first from the stack. For example, suppose method m1 calls method m2, and m2 calls method m3. The runtime system pushes m1’s activation record into the stack, then m2’s, and then m3’s. After m3 is finished, its activation record is removed from the stack. After m2 is finished, its activation record is removed from the stack. After m1 is finished, its activation record is removed from the stack. Understanding call stacks helps you to comprehend how methods are invoked. The variables defined in the main method in Listing 6.1 are i, j, and k. The variables defined in the max method are num1, num2, and result. The variables num1 and num2 are defined in the method signature and are parameters of the max method. Their values are passed through method invocation. Figure 6.3 illustrates the activation records for method calls in the stack.
Activation record for the max method result: num2: 2 num1: 5
Activation record for the max method result: 5 num2: 2 num1: 5
Activation record for the main method k: j: 2 i: 5
Activation record for the main method k: j: 2 i: 5
(b) The max method is invoked.
(c) The max method is being executed.
Activation record for the main method k: 5 j: 2 i: 5 (d) The max method is finished and the return value is sent to k.
Stack is empty
(e) The main method is finished.
FIGURE 6.3 When the max method is invoked, the flow of control transfers to the max method. Once the max method is finished, it returns control back to the caller.
6.4 void Method Example 209
6.4 void Method Example A void method does not return a value. The preceding section gives an example of a value-returning method. This section shows how to define and invoke a void method. Listing 6.2 gives a program that defines a method named printGrade and invokes it to print the grade for a given score.
Key Point
VideoNote
LISTING 6.2 TestVoidMethod.java 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
public class TestVoidMethod { public static void main(String[] args) { System.out.print("The grade is "); printGrade(78.5); System.out.print("The grade is "); printGrade(59.5);
Use void method main method
invoke printGrade
printGrade method
} public static void printGrade(double score) { if (score >= 90.0) { System.out.println('A'); } else if (score >= 80.0) { System.out.println('B'); } else if (score >= 70.0) { System.out.println('C'); } else if (score >= 60.0) { System.out.println('D'); } else { System.out.println('F'); } } }
The grade is C The grade is F
The printGrade method is a void method because it does not return any value. A call to a void method must be a statement. Therefore, it is invoked as a statement in line 4 in the main method. Like any Java statement, it is terminated with a semicolon. To see the differences between a void and value-returning method, let’s redesign the printGrade method to return a value. The new method, which we call getGrade, returns the grade as shown in Listing 6.3.
invoke void method
void vs. value-returned
LISTING 6.3 TestReturnGradeMethod.java 1 2 3 4 5 6
public class TestReturnGradeMethod { public static void main(String[] args) { System.out.print("The grade is " + getGrade(78.5)); System.out.print("\nThe grade is " + getGrade(59.5)); }
main method
invoke getGrade
210 Chapter 6 getGrade method
Methods 7 8 9 10 11 12 13 14 15 16 17 18 19
public static char getGrade(double score) { if (score >= 90.0) return 'A'; else if (score >= 80.0) return 'B'; else if (score >= 70.0) return 'C'; else if (score >= 60.0) return 'D'; else return 'F'; } }
The grade is C The grade is F
The getGrade method defined in lines 7–18 returns a character grade based on the numeric score value. The caller invokes this method in lines 3–4. The getGrade method can be invoked by a caller wherever a character may appear. The printGrade method does not return any value, so it must be invoked as a statement.
Note return in void method
A return statement is not needed for a void method, but it can be used for terminating the method and returning to the method’s caller. The syntax is simply return;
This is not often done, but sometimes it is useful for circumventing the normal flow of control in a void method. For example, the following code has a return statement to terminate the method when the score is invalid. public static void printGrade(double score) { if (score < 0 || score > 100) { System.out.println("Invalid score"); return; } if (score >= 90.0) { System.out.println('A'); } else if (score >= 80.0) { System.out.println('B'); } else if (score >= 70.0) { System.out.println('C'); } else if (score >= 60.0) { System.out.println('D'); } else { System.out.println('F'); } }
6.4 void Method Example 211 6.1 6.2 6.3 6.4 6.5 6.6
What are the benefits of using a method? How do you define a method? How do you invoke a method? How do you simplify the max method in Listing 6.1 using the conditional operator? True or false? A call to a method with a void return type is always a statement itself, but a call to a value-returning method cannot be a statement by itself. What is the return type of a main method? What would be wrong with not writing a return statement in a value-returning method? Can you have a return statement in a void method? Does the return statement in the following method cause syntax errors? public static void xMethod(double x, double y) { System.out.println(x + y); return x + y; }
6.7 6.8
Define the terms parameter, argument, and method signature. Write method headers (not the bodies) for the following methods: a. Return a sales commission, given the sales amount and the commission rate. b. Display the calendar for a month, given the month and year. c. Return a square root of a number. d. Test whether a number is even, and returning true if it is. e. Display a message a specified number of times. f. Return the monthly payment, given the loan amount, number of years, and annual interest rate. g. Return the corresponding uppercase letter, given a lowercase letter.
6.9
Identify and correct the errors in the following program: 1 2 3 4 5 6 7 8 9 10 11 12
6.10
public class Test { public static method1(int n, m) { n += m; method2(3.4); } public static int method2(int n) { if (n > 0) return 1; else if (n == 0) return 0; else if (n < 0) return –1; } }
Reformat the following program according to the programming style and documentation guidelines proposed in Section 1.9, Programming Style and Documentation. Use the next-line brace style. public class Test { public static double method(double i, double j) { while (i < j) { j--; } return j; } }
✓
Check Point
212 Chapter 6
Methods
6.5 Passing Arguments by Values Key Point
parameter order association
The arguments are passed by value to parameters when invoking a method. The power of a method is its ability to work with parameters. You can use println to print any string and max to find the maximum of any two int values. When calling a method, you need to provide arguments, which must be given in the same order as their respective parameters in the method signature. This is known as parameter order association. For example, the following method prints a message n times: public static void nPrintln(String message, int n) { for (int i = 0; i < n; i++) System.out.println(message); }
You can use nPrintln("Hello", 3) to print Hello three times. The nPrintln("Hello", 3) statement passes the actual string parameter Hello to the parameter message, passes 3 to n, and prints Hello three times. However, the statement nPrintln(3, "Hello") would be wrong. The data type of 3 does not match the data type for the first parameter, message, nor does the second argument, Hello, match the second parameter, n.
Caution The arguments must match the parameters in order, number, and compatible type, as defined in the method signature. Compatible type means that you can pass an argument to a parameter without explicit casting, such as passing an int value argument to a double value parameter.
pass-by-value
When you invoke a method with an argument, the value of the argument is passed to the parameter. This is referred to as pass-by-value. If the argument is a variable rather than a literal value, the value of the variable is passed to the parameter. The variable is not affected, regardless of the changes made to the parameter inside the method. As shown in Listing 6.4, the value of x (1) is passed to the parameter n to invoke the increment method (line 5). The parameter n is incremented by 1 in the method (line 10), but x is not changed no matter what the method does.
LISTING 6.4 Increment.java
invoke increment
increment n
1 2 3 4 5 6 7 8 9 10 11 12 13
public class Increment { public static void main(String[] args) { int x = 1; System.out.println("Before the call, x is " + x); increment(x); System.out.println("After the call, x is " + x); } public static void increment(int n) { n++; System.out.println("n inside the method is " + n); } }
Before the call, x is 1 n inside the method is 2 After the call, x is 1
6.5 Passing Arguments by Values 213 Listing 6.5 gives another program that demonstrates the effect of passing by value. The program creates a method for swapping two variables. The swap method is invoked by passing two arguments. Interestingly, the values of the arguments are not changed after the method is invoked.
LISTING 6.5 TestPassByValue.java 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
public class TestPassByValue { /** Main method */ public static void main(String[] args) { // Declare and initialize variables int num1 = 1; int num2 = 2; System.out.println("Before invoking the swap method, num1 is " + num1 + " and num2 is " + num2); // Invoke the swap method to attempt to swap two variables swap(num1, num2); System.out.println("After invoking the swap method, num1 is " + num1 + " and num2 is " + num2); } /** Swap two variables */ public static void swap(int n1, int n2) { System.out.println("\tInside the swap method"); System.out.println("\t\tBefore swapping, n1 is " + n1 + " and n2 is " + n2); // Swap n1 with n2 int temp = n1; n1 = n2; n2 = temp; System.out.println("\t\tAfter swapping, n1 is " + n1 + " and n2 is " + n2); } }
Before invoking the swap method, num1 is 1 and num2 is 2 Inside the swap method Before swapping, n1 is 1 and n2 is 2 After swapping, n1 is 2 and n2 is 1 After invoking the swap method, num1 is 1 and num2 is 2
Before the swap method is invoked (line 12), num1 is 1 and num2 is 2. After the swap method is invoked, num1 is still 1 and num2 is still 2. Their values have not been swapped. As shown in Figure 6.4, the values of the arguments num1 and num2 are passed to n1 and n2, but n1 and n2 have their own memory locations independent of num1 and num2. Therefore, changes in n1 and n2 do not affect the contents of num1 and num2. Another twist is to change the parameter name n1 in swap to num1. What effect does this have? No change occurs, because it makes no difference whether the parameter and the argument have the same name. The parameter is a variable in the method with its own memory space. The variable is allocated when the method is invoked, and it disappears when the method is returned to its caller.
false swap
214 Chapter 6
Methods The values for n1 and n2 are swapped, but it does not affect num1 and num2.
The values of num1 and num2 are passed to n1 and n2.
Activation record for the main method
Activation record for the swap method temp: n2: 2 n1: 1
Activation record for the swap method temp: 1 n2: 1 n1: 2
Activation record for the main method
Activation record for the main method
num2: 2 num1: 1
num2: 2 num1: 1 The main method is invoked.
num2: 2 num1: 1
The swap method is invoked.
The swap method is executed.
Activation record for the main method
Stack is empty
num2: 2 num1: 1 The swap method is finished.
The main method is finished.
FIGURE 6.4 The values of the variables are passed to the method’s parameters.
Note For simplicity, Java programmers often say passing x to y, which actually means passing the value of argument x to parameter y.
✓
Check Point
6.11 6.12
How is an argument passed to a method? Can the argument have the same name as its parameter? Identify and correct the errors in the following program: 1 2 3 4 5 6 7 8 9 10 11
6.13
public class Test { public static void main(String[] args) { nPrintln(5, "Welcome to Java!"); } public static void nPrintln(String message, int n) { int n = 1; for (int i = 0; i < n; i++) System.out.println(message); } }
What is pass-by-value? Show the result of the following programs.
public class Test { public static void main(String[] args) { int max = 0; max(1, 2, max); System.out.println(max); } public static void max( int value1, int value2, int max) { if (value1 > value2) max = value1; else max = value2; }
public class Test { public static void main(String[] args) { int i = 1; while (i <= 6) { method1(i, 2); i++; } } public static void method1( int i, int num) { for (int j = 1; j <= i; j++) { System.out.print(num + " "); num *= 2; }
}
System.out.println(); } } (a)
(b)
6.6 Modularizing Code 215 public class Test { public static void main(String[] args) { // Initialize times int times = 3; System.out.println("Before the call," + " variable times is " + times);
public class Test { public static void main(String[] args) { int i = 0; while (i <= 4) { method1(i); i++; }
// Invoke nPrintln and display times nPrintln("Welcome to Java!", times); System.out.println("After the call," + " variable times is " + times);
System.out.println("i is " + i); }
} // Print the message n times public static void nPrintln( String message, int n) { while (n > 0) { System.out.println("n = " + n); System.out.println(message); n--; } }
public static void method1(int i) { do { if (i % 3 != 0) System.out.print(i + " "); i--; } while (i >= 1); System.out.println(); } }
} (c)
6.14
(d)
For (a) in the preceding question, show the contents of the activation records in the call stack just before the method max is invoked, just as max is entered, just before max is returned, and right after max is returned.
6.6 Modularizing Code Modularizing makes the code easy to maintain and debug and enables the code to be reused. Methods can be used to reduce redundant code and enable code reuse. Methods can also be used to modularize code and improve the quality of the program. Listing 5.9 gives a program that prompts the user to enter two integers and displays their greatest common divisor. You can rewrite the program using a method, as shown in Listing 6.6.
LISTING 6.6 GreatestCommonDivisorMethod.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14
import java.util.Scanner; public class GreatestCommonDivisorMethod { /** Main method */ public static void main(String[] args) { // Create a Scanner Scanner input = new Scanner(System.in); // Prompt the user to enter two integers System.out.print("Enter first integer: "); int n1 = input.nextInt(); System.out.print("Enter second integer: "); int n2 = input.nextInt();
Key Point
VideoNote
Modularize code
216 Chapter 6
Methods
invoke gcd
compute gcd
return gcd
15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
System.out.println("The greatest common divisor for " + n1 + " and " + n2 + " is " + gcd(n1, n2)); } /** Return the gcd of two public static int gcd(int int gcd = 1; // Initial int k = 2; // Possible
integers */ n1, int n2) { gcd is 1 gcd
while (k <= n1 && k <= n2) { if (n1 % k == 0 && n2 % k == 0) gcd = k; // Update gcd k++; } return gcd; // Return gcd } }
Enter first integer: 45 Enter second integer: 75 The greatest common divisor for 45 and 75 is 15
By encapsulating the code for obtaining the gcd in a method, this program has several advantages: 1. It isolates the problem for computing the gcd from the rest of the code in the main method. Thus, the logic becomes clear and the program is easier to read. 2. The errors on computing the gcd are confined in the gcd method, which narrows the scope of debugging. 3. The gcd method now can be reused by other programs. Listing 6.7 applies the concept of code modularization to improve Listing 5.15, PrimeNumber.java.
LISTING 6.7 PrimeNumberMethod.java
invoke printPrimeNumbers
printPrimeNumbers
method
invoke isPrime
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
public class PrimeNumberMethod { public static void main(String[] args) { System.out.println("The first 50 prime numbers are \n"); printPrimeNumbers(50); } public static void printPrimeNumbers(int numberOfPrimes) { final int NUMBER_OF_PRIMES_PER_LINE = 10; // Display 10 per line int count = 0; // Count the number of prime numbers int number = 2; // A number to be tested for primeness // Repeatedly find prime numbers while (count < numberOfPrimes) { // Print the prime number and increase the count if (isPrime(number)) { count++; // Increase the count
6.7 Case Study: Converting Hexadecimals to Decimals 217 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41
if (count % NUMBER_OF_PRIMES_PER_LINE == 0) { // Print the number and advance to the new line System.out.printf("%-5s\n", number); } else System.out.printf("%-5s", number); } // Check whether the next number is prime number++; } } /** Check whether number is prime */ public static boolean isPrime(int number) { for (int divisor = 2; divisor <= number / 2; divisor++) { if (number % divisor == 0) { // If true, number is not prime return false; // Number is not a prime } }
isPrime method
return true; // Number is prime } }
The first 50 prime numbers are 2 31 73 127 179
3 37 79 131 181
5 41 83 137 191
7 43 89 139 193
11 47 97 149 197
13 53 101 151 199
17 59 103 157 211
19 61 107 163 223
23 67 109 167 227
29 71 113 173 229
We divided a large problem into two subproblems: determining whether a number is a prime and printing the prime numbers. As a result, the new program is easier to read and easier to debug. Moreover, the methods printPrimeNumbers and isPrime can be reused by other programs.
6.7 Case Study: Converting Hexadecimals to Decimals This section presents a program that converts a hexadecimal number into a decimal number. Listing 5.11, Dec2Hex.java, gives a program that converts a decimal to a hexadecimal. How would you convert a hex number into a decimal? Given a hexadecimal number hnhn - 1hn - 2 c h2h1h0, the equivalent decimal value is hn * 16n + hn - 1 * 16n - 1 + hn - 2 * 16n - 2 + c + h2 * 162 + h1 * 161 + h0 * 160 For example, the hex number AB8C is 10 * 163 + 11 * 162 + 8 * 161 + 12 * 160 = 43916 Our program will prompt the user to enter a hex number as a string and convert it into a decimal using the following method: public static int hexToDecimal(String hex)
Key Point
218 Chapter 6
Methods A brute-force approach is to convert each hex character into a decimal number, multiply it by 16i for a hex digit at the i’s position, and then add all the items together to obtain the equivalent decimal value for the hex number. Note that hn * 16n + hn - 1 * 16n - 1 + hn - 2 * 16n - 2 + c + h1 * 161 + h0 * 160 = ( c ((hn * 16 + hn - 1) * 16 + hn - 2) * 16 + c + h1) * 16 + h0 This observation, known as the Horner’s algorithm, leads to the following efficient code for converting a hex string to a decimal number: int decimalValue = 0; for (int i = 0; i < hex.length(); i++) { char hexChar = hex.charAt(i); decimalValue = decimalValue * 16 + hexCharToDecimal(hexChar); }
Here is a trace of the algorithm for hex number AB8C:
i
hexChar
hexCharToDecimal (hexChar)
before the loop
decimalValue 0
after the 1st iteration
0
A
10
10
after the 2nd iteration
1
B
11
10 * 16 + 11
after the 3rd iteration
2
8
8
(10 * 16 + 11) * 16 + 8
after the 4th iteration
3
C
12
((10 * 16 + 11) * 16 + 8) * 16 + 12
Listing 6.8 gives the complete program.
LISTING 6.8
input string
hex to decimal
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
Hex2Dec.java
import java.util.Scanner; public class Hex2Dec { /** Main method */ public static void main(String[] args) { // Create a Scanner Scanner input = new Scanner(System.in); // Prompt the user to enter a string System.out.print("Enter a hex number: "); String hex = input.nextLine(); System.out.println("The decimal value for hex number " + hex + " is " + hexToDecimal(hex.toUpperCase())); } public static int hexToDecimal(String hex) { int decimalValue = 0; for (int i = 0; i < hex.length(); i++) { char hexChar = hex.charAt(i); decimalValue = decimalValue * 16 + hexCharToDecimal(hexChar);
6.8 Overloading Methods 219 22 23 24 25 26 27 28 29 30 31 32 33
} return decimalValue; } public static int hexCharToDecimal(char ch) { if (ch >= 'A' && ch <= 'F') return 10 + ch - 'A'; else // ch is '0', '1', ..., or '9' return ch - '0'; }
hex char to decimal check uppercase
}
Enter a hex number: AB8C The decimal value for hex number AB8C is 43916
Enter a hex number: af71 The decimal value for hex number af71 is 44913
The program reads a string from the console (line 11), and invokes the hexToDecimal method to convert a hex string to decimal number (line 14). The characters can be in either lowercase or uppercase. They are converted to uppercase before invoking the hexToDecimal method. The hexToDecimal method is defined in lines 17–25 to return an integer. The length of the string is determined by invoking hex.length() in line 19. The hexCharToDecimal method is defined in lines 27–32 to return a decimal value for a hex character. The character can be in either lowercase or uppercase. Recall that to subtract two characters is to subtract their Unicodes. For example, '5' – '0' is 5.
6.8 Overloading Methods Overloading methods enables you to define the methods with the same name as long as their signatures are different.
Key Point
The max method that was used earlier works only with the int data type. But what if you need to determine which of two floating-point numbers has the maximum value? The solution is to create another method with the same name but different parameters, as shown in the following code: public static double max(double num1, double num2) { if (num1 > num2) return num1; else return num2; }
If you call max with int parameters, the max method that expects int parameters will be invoked; if you call max with double parameters, the max method that expects double parameters will be invoked. This is referred to as method overloading; that is, two methods have the same name but different parameter lists within one class. The Java compiler determines which method to use based on the method signature.
method overloading
220 Chapter 6
Methods Listing 6.9 is a program that creates three methods. The first finds the maximum integer, the second finds the maximum double, and the third finds the maximum among three double values. All three methods are named max.
LISTING 6.9 TestMethodOverloading.java
overloaded max
overloaded max
overloaded max
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public class TestMethodOverloading { /** Main method */ public static void main(String[] args) { // Invoke the max method with int parameters System.out.println("The maximum of 3 and 4 is " + max(3, 4)); // Invoke the max method with the double parameters System.out.println("The maximum of 3.0 and 5.4 is " + max(3.0, 5.4)); // Invoke the max method with three double parameters System.out.println("The maximum of 3.0, 5.4, and 10.14 is " + max(3.0, 5.4, 10.14)); } /** Return the max of two int values */ public static int max(int num1, int num2) { if (num1 > num2) return num1; else return num2; } /** Find the max of two double values */ public static double max(double num1, double num2) { if (num1 > num2) return num1; else return num2; } /** Return the max of three double values */ public static double max(double num1, double num2, double num3) { return max(max(num1, num2), num3); } }
The maximum of 3 and 4 is 4 The maximum of 3.0 and 5.4 is 5.4 The maximum of 3.0, 5.4, and 10.14 is 10.14
When calling max(3, 4) (line 6), the max method for finding the maximum of two integers is invoked. When calling max(3.0, 5.4) (line 10), the max method for finding the maximum of two doubles is invoked. When calling max(3.0, 5.4, 10.14) (line 14), the max method for finding the maximum of three double values is invoked. Can you invoke the max method with an int value and a double value, such as max(2, 2.5)? If so, which of the max methods is invoked? The answer to the first question is yes. The answer to the second question is that the max method for finding the maximum of two double values is invoked. The argument value 2 is automatically converted into a double value and passed to this method.
6.8 Overloading Methods 221 You may be wondering why the method max(double, double) is not invoked for the call max(3, 4). Both max(double, double) and max(int, int) are possible matches for max(3, 4). The Java compiler finds the method that best matches a method invocation. Since the method max(int, int) is a better matches for max(3, 4) than max(double, double), max(int, int) is used to invoke max(3, 4).
Tip Overloading methods can make programs clearer and more readable. Methods that perform the same function with different types of parameters should be given the same name.
Note Overloaded methods must have different parameter lists. You cannot overload methods based on different modifiers or return types.
Note Sometimes there are two or more possible matches for the invocation of a method, but the compiler cannot determine the best match. This is referred to as ambiguous invocation. Ambiguous invocation causes a compile error. Consider the following code:
ambiguous invocation
public class AmbiguousOverloading { public static void main(String[] args) { System.out.println(max(1, 2)); } public static double max(int num1, double num2) { if (num1 > num2) return num1; else return num2; } public static double max(double num1, int num2) { if (num1 > num2) return num1; else return num2; } }
Both max(int, double) and max(double, int) are possible candidates to match max(1, 2). Because neither is better than the other, the invocation is ambiguous, resulting in a compile error.
6.15
6.16
What is method overloading? Is it permissible to define two methods that have the same name but different parameter types? Is it permissible to define two methods in a class that have identical method names and parameter lists but different return value types or different modifiers? What is wrong in the following program? public class Test { public static void method(int x) { } public static int method(int y) {
✓
Check Point
222 Chapter 6
Methods return y; } }
6.17
Given two method definitions, public static double m(double x, double y) public static double m(int x, double y)
tell which of the two methods is invoked for: a. double z = m(4, 5); b. double z = m(4, 5.4); c. double z = m(4.5, 5.4);
6.9 The Scope of Variables Key Point scope of variables local variable
The scope of a variable is the part of the program where the variable can be referenced. Section 2.5 introduced the scope of a variable. This section discusses the scope of variables in detail. A variable defined inside a method is referred to as a local variable. The scope of a local variable starts from its declaration and continues to the end of the block that contains the variable. A local variable must be declared and assigned a value before it can be used. A parameter is actually a local variable. The scope of a method parameter covers the entire method. A variable declared in the initial-action part of a for-loop header has its scope in the entire loop. However, a variable declared inside a for-loop body has its scope limited in the loop body from its declaration to the end of the block that contains the variable, as shown in Figure 6.5.
The scope of i
The scope of j
public static void method1() { . . for (int i = 1; i < 10; i++) { . . int j; . . . } }
FIGURE 6.5 A variable declared in the initial action part of a for-loop header has its scope in the entire loop.
You can declare a local variable with the same name in different blocks in a method, but you cannot declare a local variable twice in the same block or in nested blocks, as shown in Figure 6.6.
6.10 Case Study: Generating Random Characters 223 It is fine to declare i in two nonnested blocks. public static void method1() { int x = 1; int y = 1;
It is wrong to declare i in two nested blocks. public static void method2() { int i = 1; int sum = 0;
for (int i = 1; i < 10; i++) { x += i; } for (int i = 1; i < 10; i++) { y += i; }
for (int i = 1; i < 10; i++) sum += i; } }
}
FIGURE 6.6 A variable can be declared multiple times in nonnested blocks, but only once in nested blocks.
Caution Do not declare a variable inside a block and then attempt to use it outside the block. Here is an example of a common mistake: for (int i = 0; i < 10; i++) { } System.out.println(i);
The last statement would cause a syntax error, because variable i is not defined outside of the for loop.
6.18 6.19
What is a local variable? What is the scope of a local variable?
✓
Check Point
6.10 Case Study: Generating Random Characters A character is coded using an integer. Generating a random character is to generate an integer. Computer programs process numerical data and characters. You have seen many examples that involve numerical data. It is also important to understand characters and how to process them. This section presents an example of generating random characters. As introduced in Section 4.3, every character has a unique Unicode between 0 and FFFF in hexadecimal (65535 in decimal). To generate a random character is to generate a random integer between 0 and 65535 using the following expression (note that since 0 <= Math.random() < 1.0, you have to add 1 to 65535): (int)(Math.random() * (65535 + 1))
Now let’s consider how to generate a random lowercase letter. The Unicodes for lowercase letters are consecutive integers starting from the Unicode for a, then that for b, c, . . . , and z. The Unicode for a is (int)'a'
Thus, a random integer between (int)'a' and (int)'z' is (int)((int)'a' + Math.random() * ((int)'z' - (int)'a' + 1))
Key Point
224 Chapter 6
Methods As discussed in Section 4.3.3, all numeric operators can be applied to the char operands. The char operand is cast into a number if the other operand is a number or a character. Therefore, the preceding expression can be simplified as follows: 'a' + Math.random() * ('z' - 'a' + 1)
and a random lowercase letter is (char)('a' + Math.random() * ('z' - 'a' + 1))
Hence, a random character between any two characters ch1 and ch2 with ch1 < ch2 can be generated as follows: (char)(ch1 + Math.random() * (ch2 – ch1 + 1))
This is a simple but useful discovery. Listing 6.10 defines a class named RandomCharacter with five overloaded methods to get a certain type of character randomly. You can use these methods in your future projects.
LISTING 6.10 RandomCharacter.java getRandomCharacter
getRandomLower CaseLetter()
getRandomUpper CaseLetter()
getRandomDigit Character()
getRandomCharacter()
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public class RandomCharacter { /** Generate a random character between ch1 and ch2 */ public static char getRandomCharacter(char ch1, char ch2) { return (char)(ch1 + Math.random() * (ch2 - ch1 + 1)); } /** Generate a random lowercase letter */ public static char getRandomLowerCaseLetter() { return getRandomCharacter('a', 'z'); } /** Generate a random uppercase letter */ public static char getRandomUpperCaseLetter() { return getRandomCharacter('A', 'Z'); } /** Generate a random digit character */ public static char getRandomDigitCharacter() { return getRandomCharacter('0', '9'); } /** Generate a random character */ public static char getRandomCharacter() { return getRandomCharacter('\u0000', '\uFFFF'); } }
Listing 6.11 gives a test program that displays 175 random lowercase letters.
LISTING 6.11 TestRandomCharacter.java
constants
1 2 3 4 5 6 7 8
public class TestRandomCharacter { /** Main method */ public static void main(String[] args) { final int NUMBER_OF_CHARS = 175; final int CHARS_PER_LINE = 25; // Print random characters between 'a' and 'z', 25 chars per line for (int i = 0; i < NUMBER_OF_CHARS; i++) {
6.11 Method Abstraction and Stepwise Refinement 225 9 10 11 12 13 14 15 16
char ch = RandomCharacter.getRandomLowerCaseLetter(); if ((i + 1) % CHARS_PER_LINE == 0) System.out.println(ch); else System.out.print(ch);
lower-case letter
} } }
gmjsohezfkgtazqgmswfclrao pnrunulnwmaztlfjedmpchcif lalqdgivxkxpbzulrmqmbhikr lbnrjlsopfxahssqhwuuljvbe xbhdotzhpehbqmuwsfktwsoli cbuwkzgxpmtzihgatdslvbwbz bfesoklwbhnooygiigzdxuqni
Line 9 invokes getRandomLowerCaseLetter() defined in the RandomCharacter class. Note that getRandomLowerCaseLetter() does not have any parameters, but you still have to use the parentheses when defining and invoking the method.
parentheses required
6.11 Method Abstraction and Stepwise Refinement The key to developing software is to apply the concept of abstraction. You will learn many levels of abstraction from this book. Method abstraction is achieved by separating the use of a method from its implementation. The client can use a method without knowing how it is implemented. The details of the implementation are encapsulated in the method and hidden from the client who invokes the method. This is also known as information hiding or encapsulation. If you decide to change the implementation, the client program will not be affected, provided that you do not change the method signature. The implementation of the method is hidden from the client in a “black box,” as shown in Figure 6.7. Optional arguments for input
Key Point
VideoNote
Stepwise refinement method abstraction information hiding
Optional return value
Method Header Black box Method Body
FIGURE 6.7 The method body can be thought of as a black box that contains the detailed implementation for the method. You have already used the System.out.print method to display a string and the max method to find the maximum number. You know how to write the code to invoke these methods in your program, but as a user of these methods, you are not required to know how they are implemented. The concept of method abstraction can be applied to the process of developing programs. When writing a large program, you can use the divide-and-conquer strategy, also known as stepwise refinement, to decompose it into subproblems. The subproblems can be further decomposed into smaller, more manageable problems. Suppose you write a program that displays the calendar for a given month of the year. The program prompts the user to enter the year and the month, then displays the entire calendar for the month, as shown in the following sample run.
divide and conquer stepwise refinement
226 Chapter 6
Methods Enter full year (e.g., 2012): 2012 Enter month as number between 1 and 12: 3 March 2012 ----------------------------Sun Mon Tue Wed Thu Fri Sat 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
Let us use this example to demonstrate the divide-and-conquer approach.
6.11.1
Top-Down Design
How would you get started on such a program? Would you immediately start coding? Beginning programmers often start by trying to work out the solution to every detail. Although details are important in the final program, concern for detail in the early stages may block the problem-solving process. To make problem solving flow as smoothly as possible, this example begins by using method abstraction to isolate details from design and only later implements the details. For this example, the problem is first broken into two subproblems: get input from the user and print the calendar for the month. At this stage, you should be concerned with what the subproblems will achieve, not with how to get input and print the calendar for the month. You can draw a structure chart to help visualize the decomposition of the problem (see Figure 6.8a).
printCalendar (main)
readInput
printMonth
printMonth (a)
printMonthTitle
printMonthBody (b)
FIGURE 6.8 The structure chart shows that the printCalendar problem is divided into two subproblems, readInput and printMonth in (a), and that printMonth is divided into two smaller subproblems, printMonthTitle and printMonthBody in (b).
You can use Scanner to read input for the year and the month. The problem of printing the calendar for a given month can be broken into two subproblems: print the month title and print the month body, as shown in Figure 6.8b. The month title consists of three lines: month and year, a dashed line, and the names of the seven days of the week. You need to get the month name (e.g., January) from the numeric month (e.g., 1). This is accomplished in getMonthName (see Figure 6.9a). In order to print the month body, you need to know which day of the week is the first day of the month (getStartDay) and how many days the month has (getNumberOfDaysInMonth),
6.11 Method Abstraction and Stepwise Refinement 227 printMonthBody printMonthTitle getMonthName (a)
getNumberOfDaysInMonth
getStartDay
(b)
FIGURE 6.9 (a) To printMonthTitle, you need getMonthName. (b) The printMonthBody problem is refined into several smaller problems.
as shown in Figure 6.9b. For example, December 2013 has 31 days, and December 1, 2013, is a Sunday. How would you get the start day for the first date in a month? There are several ways to do so. For now, we’ll use an alternative approach. Assume you know that the start day for January 1, 1800, was a Wednesday (START_DAY_FOR_JAN_1_1800 = 3). You could compute the total number of days (totalNumberOfDays) between January 1, 1800, and the first date of the calendar month. The start day for the calendar month is (totalNumberOfDays + START_ DAY_FOR_JAN_1_1800) % 7, since every week has seven days. Thus, the getStartDay problem can be further refined as getTotalNumberOfDays, as shown in Figure 6.10a.
getStartDay
getTotalNumberOfDays getNumberOfDaysInMonth
getTotalNumberOfDays (a)
isLeapYear (b)
FIGURE 6.10 (a) To getStartDay, you need getTotalNumberOfDays. (b) The getTotalNumberOfDays problem is refined into two smaller problems.
To get the total number of days, you need to know whether the year is a leap year and the number of days in each month. Thus, getTotalNumberOfDays can be further refined into two subproblems: isLeapYear and getNumberOfDaysInMonth, as shown in Figure 6.10b. The complete structure chart is shown in Figure 6.11.
6.11.2
Top-Down and/or Bottom-Up Implementation
Now we turn our attention to implementation. In general, a subproblem corresponds to a method in the implementation, although some are so simple that this is unnecessary. You would need to decide which modules to implement as methods and which to combine with other methods. Decisions of this kind should be based on whether the overall program will be easier to read as a result of your choice. In this example, the subproblem readInput can be simply implemented in the main method. You can use either a “top-down” or a “bottom-up” approach. The top-down approach implements one method in the structure chart at a time from the top to the bottom. Stubs— a simple but incomplete version of a method—can be used for the methods waiting to be implemented. The use of stubs enables you to quickly build the framework of the program. Implement the main method first, and then use a stub for the printMonth method. For example,
top-down approach stub
228 Chapter 6
Methods printCalendar (main)
readInput
printMonth
printMonthTitle getMonthName
printMonthBody getStartDay getTotalNumberOfDays
getNumberOfDaysInMonth
isLeapYear
FIGURE 6.11 The structure chart shows the hierarchical relationship of the subproblems in the program.
let printMonth display the year and the month in the stub. Thus, your program may begin like this: public class PrintCalendar { /** Main method */ public static void main(String[] args) { Scanner input = new Scanner(System.in); // Prompt the user to enter year System.out.print("Enter full year (e.g., 2012): "); int year = input.nextInt(); // Prompt the user to enter month System.out.print("Enter month as a number between 1 and 12: "); int month = input.nextInt(); // Print calendar for the month of the year printMonth(year, month); } /** A stub for printMonth may look like this */ public static void printMonth(int year, int month){ System.out.print(month + " " + year); } /** A stub for printMonthTitle may look like this */ public static void printMonthTitle(int year, int month){ } /** A stub for getMonthBody may look like this */ public static void printMonthBody(int year, int month){ }
6.11 Method Abstraction and Stepwise Refinement 229 /** A stub for getMonthName may look like this */ public static String getMonthName(int month) { return "January"; // A dummy value } /** A stub for getStartDay may look like this */ public static int getStartDay(int year, int month) { return 1; // A dummy value } /** A stub for getTotalNumberOfDays may look like this */ public static int getTotalNumberOfDays(int year, int month) { return 10000; // A dummy value } /** A stub for getNumberOfDaysInMonth may look like this */ public static int getNumberOfDaysInMonth(int year, int month) { return 31; // A dummy value } /** A stub for isLeapYear may look like this */ public static Boolean isLeapYear(int year) { return true; // A dummy value } }
Compile and test the program, and fix any errors. You can now implement the printMonth method. For methods invoked from the printMonth method, you can again use stubs. The bottom-up approach implements one method in the structure chart at a time from the bottom to the top. For each method implemented, write a test program, known as the driver, to test it. The top-down and bottom-up approaches are equally good: Both approaches implement methods incrementally, help to isolate programming errors, and make debugging easy. They can be used together.
6.11.3
Implementation Details
The isLeapYear(int year) method can be implemented using the following code from Section 3.11: return year % 400 == 0 || (year % 4 == 0 && year % 100 != 0);
Use the following facts to implement getTotalNumberOfDaysInMonth(int year, int month): ■
January, March, May, July, August, October, and December have 31 days.
■
April, June, September, and November have 30 days.
■
February has 28 days during a regular year and 29 days during a leap year. A regular year, therefore, has 365 days, a leap year 366 days.
To implement getTotalNumberOfDays(int year, int month), you need to compute the total number of days (totalNumberOfDays) between January 1, 1800, and the first day of the calendar month. You could find the total number of days between the year 1800 and the calendar year and then figure out the total number of days prior to the calendar month in the calendar year. The sum of these two totals is totalNumberOfDays. To print a body, first pad some space before the start day and then print the lines for every week. The complete program is given in Listing 6.12.
bottom-up approach driver
230 Chapter 6
Methods
LISTING 6.12 PrintCalendar.java
printMonth
printMonthTitle
getMonthName
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import java.util.Scanner; public class PrintCalendar { /** Main method */ public static void main(String[] args) { Scanner input = new Scanner(System.in); // Prompt the user to enter year System.out.print("Enter full year (e.g., 2012): "); int year = input.nextInt(); // Prompt the user to enter month System.out.print("Enter month as a number between 1 and 12: "); int month = input.nextInt(); // Print calendar for the month of the year printMonth(year, month); } /** Print the calendar for a month in a year */ public static void printMonth(int year, int month) { // Print the headings of the calendar printMonthTitle(year, month); // Print the body of the calendar printMonthBody(year, month); } /** Print the month title, e.g., March 2012 */ public static void printMonthTitle(int year, int month) { System.out.println(" " + getMonthName(month) + " " + year); System.out.println("-----------------------------"); System.out.println(" Sun Mon Tue Wed Thu Fri Sat"); } /** Get the English name for the month */ public static String getMonthName(int month) { String monthName = ""; switch (month) { case 1: monthName = "January"; break; case 2: monthName = "February"; break; case 3: monthName = "March"; break; case 4: monthName = "April"; break; case 5: monthName = "May"; break; case 6: monthName = "June"; break; case 7: monthName = "July"; break; case 8: monthName = "August"; break; case 9: monthName = "September"; break; case 10: monthName = "October"; break; case 11: monthName = "November"; break; case 12: monthName = "December"; } return monthName; } /** Print month body */
6.11 Method Abstraction and Stepwise Refinement 231 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 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118
public static void printMonthBody(int year, int month) { // Get start day of the week for the first date in the month int startDay = getStartDay(year, month)
printMonthBody
// Get number of days in the month int numberOfDaysInMonth = getNumberOfDaysInMonth(year, month); // Pad space before the first day of the month int i = 0; for (i = 0; i < startDay; i++) System.out.print(" "); for (i = 1; i <= numberOfDaysInMonth; i++) { System.out.printf("%4d", i); if ((i + startDay) % 7 == 0) System.out.println(); } System.out.println(); } /** Get the start day of month/1/year */ public static int getStartDay(int year, int month) { final int START_DAY_FOR_JAN_1_1800 = 3; // Get total number of days from 1/1/1800 to month/1/year int totalNumberOfDays = getTotalNumberOfDays(year, month);
getStartDay
// Return the start day for month/1/year return (totalNumberOfDays + START_DAY_FOR_JAN_1_1800) % 7; } /** Get the total number of days since January 1, 1800 */ public static int getTotalNumberOfDays(int year, int month) { int total = 0;
getTotalNumberOfDays
// Get the total days from 1800 to 1/1/year for (int i = 1800; i < year; i++) if (isLeapYear(i)) total = total + 366; else total = total + 365; // Add days from Jan to the month prior to the calendar month for (int i = 1; i < month; i++) total = total + getNumberOfDaysInMonth(year, i); return total; } /** Get the number of days in a month */ public static int getNumberOfDaysInMonth(int year, int month) { if (month == 1 || month == 3 || month == 5 || month == 7 || month == 8 || month == 10 || month == 12) return 31; if (month == 4 || month == 6 || month == 9 || month == 11) return 30; if (month == 2) return isLeapYear(year) ? 29 : 28;
getNumberOfDaysInMonth
232 Chapter 6
Methods
isLeapYear
119 120 121 122 123 124 125 126 127
return 0; // If month is incorrect } /** Determine if it is a leap year */ public static boolean isLeapYear(int year) { return year % 400 == 0 || (year % 4 == 0 && year % 100 != 0); } }
The program does not validate user input. For instance, if the user enters either a month not in the range between 1 and 12 or a year before 1800, the program displays an erroneous calendar. To avoid this error, add an if statement to check the input before printing the calendar. This program prints calendars for a month but could easily be modified to print calendars for a whole year. Although it can print months only after January 1800, it could be modified to print months before 1800.
6.11.4
Benefits of Stepwise Refinement
Stepwise refinement breaks a large problem into smaller manageable subproblems. Each subproblem can be implemented using a method. This approach makes the program easier to write, reuse, debug, test, modify, and maintain.
Simpler Program The print calendar program is long. Rather than writing a long sequence of statements in one method, stepwise refinement breaks it into smaller methods. This simplifies the program and makes the whole program easier to read and understand.
Reusing Methods Stepwise refinement promotes code reuse within a program. The isLeapYear method is defined once and invoked from the getTotalNumberOfDays and getNumberOfDayInMonth methods. This reduces redundant code.
Easier Developing, Debugging, and Testing
incremental development and testing
Since each subproblem is solved in a method, a method can be developed, debugged, and tested individually. This isolates the errors and makes developing, debugging, and testing easier. When implementing a large program, use the top-down and/or bottom-up approach. Do not write the entire program at once. Using these approaches seems to take more development time (because you repeatedly compile and run the program), but it actually saves time and makes debugging easier.
Better Facilitating Teamwork When a large problem is divided into subprograms, subproblems can be assigned to different programmers. This makes it easier for programmers to work in teams.
KEY TERMS actual parameter 205 ambiguous invocation 221 argument 205 divide and conquer 225 formal parameter (i.e., parameter) information hiding 225 method 204 method abstraction 225
205
method overloading 219 method signature 205 modifier 205 parameter 205 pass-by-value 212 scope of a variable 222 stepwise refinement 225 stub 227
Chapter Summary 233
CHAPTER SUMMARY 1. Making programs modular and reusable is one of the central goals in software engineering. Java provides many powerful constructs that help to achieve this goal. Methods are one such construct.
2. The method header specifies the modifiers, return value type, method name, and parameters of the method. The static modifier is used for all the methods in this chapter.
3. A method may return a value. The returnValueType is the data type of the value the method returns. If the method does not return a value, the returnValueType is the keyword void.
4. The parameter list refers to the type, order, and number of a method’s parameters. The method name and the parameter list together constitute the method signature. Parameters are optional; that is, a method doesn’t need to contain any parameters.
5. A return statement can also be used in a void method for terminating the method and returning to the method’s caller. This is useful occasionally for circumventing the normal flow of control in a method.
6. The arguments that are passed to a method should have the same number, type, and order as the parameters in the method signature.
7. When a program calls a method, program control is transferred to the called method. A called method returns control to the caller when its return statement is executed or when its method-ending closing brace is reached.
8. A value-returning method can also be invoked as a statement in Java. In this case, the caller simply ignores the return value.
9. A method can be overloaded. This means that two methods can have the same name, as long as their method parameter lists differ.
10. A variable declared in a method is called a local variable. The scope of a local variable starts from its declaration and continues to the end of the block that contains the variable. A local variable must be declared and initialized before it is used.
11. Method abstraction is achieved by separating the use of a method from its implementation. The client can use a method without knowing how it is implemented. The details of the implementation are encapsulated in the method and hidden from the client who invokes the method. This is known as information hiding or encapsulation.
12. Method abstraction modularizes programs in a neat, hierarchical manner. Programs written as collections of concise methods are easier to write, debug, maintain, and modify than would otherwise be the case. This writing style also promotes method reusability.
13. When implementing a large program, use the top-down and/or bottom-up coding approach. Do not write the entire program at once. This approach may seem to take more time for coding (because you are repeatedly compiling and running the program), but it actually saves time and makes debugging easier.
234 Chapter 6
Methods
QUIZ Answer the quiz for this chapter online at www.cs.armstrong.edu/liang/intro10e/quiz.html.
PROGRAMMING EXERCISES Note A common error for the exercises in this chapter is that students don’t implement the methods to meet the requirements even though the output from the main program is correct. For an example of this type of error see www.cs.armstrong.edu/liang/ CommonMethodErrorJava.pdf.
Sections 6.2–6.9
6.1
(Math: pentagonal numbers) A pentagonal number is defined as n(3n–1)/2 for n = 1, 2, . . ., and so on. Therefore, the first few numbers are 1, 5, 12, 22, . . . . Write a method with the following header that returns a pentagonal number: public static int getPentagonalNumber(int n)
*6.2
Write a test program that uses this method to display the first 100 pentagonal numbers with 10 numbers on each line. (Sum the digits in an integer) Write a method that computes the sum of the digits in an integer. Use the following method header: public static int sumDigits(long n)
**6.3
For example, sumDigits(234) returns 9 (2 + 3 + 4). (Hint: Use the % operator to extract digits, and the / operator to remove the extracted digit. For instance, to extract 4 from 234, use 234 % 10 ( = 4). To remove 4 from 234, use 234 / 10 (= 23). Use a loop to repeatedly extract and remove the digit until all the digits are extracted. Write a test program that prompts the user to enter an integer and displays the sum of all its digits. (Palindrome integer) Write the methods with the following headers // Return the reversal of an integer, i.e., reverse(456) returns 654 public static int reverse(int number) // Return true if number is a palindrome public static boolean isPalindrome(int number)
*6.4 VideoNote
Reverse an integer
Use the reverse method to implement isPalindrome. A number is a palindrome if its reversal is the same as itself. Write a test program that prompts the user to enter an integer and reports whether the integer is a palindrome. (Display an integer reversed) Write a method with the following header to display an integer in reverse order: public static void reverse(int number)
For example, reverse(3456) displays 6543. Write a test program that prompts the user to enter an integer and displays its reversal.
*6.5
(Sort three numbers) Write a method with the following header to display three numbers in increasing order: public static void displaySortedNumbers( double num1, double num2, double num3)
Programming Exercises 235
*6.6
Write a test program that prompts the user to enter three numbers and invokes the method to display them in increasing order. (Display patterns) Write a method to display a pattern as follows: 1 2 1 3 2 1 ... n n-1 ... 3 2 1
The method header is public static void displayPattern(int n)
*6.7
(Financial application: compute the future investment value) Write a method that computes future investment value at a given interest rate for a specified number of years. The future investment is determined using the formula in Programming Exercise 2.21. Use the following method header: public static double futureInvestmentValue( double investmentAmount, double monthlyInterestRate, int years)
For example, futureInvestmentValue(10000, 0.05/12, 5) returns 12833.59. Write a test program that prompts the user to enter the investment amount (e.g., 1000) and the interest rate (e.g., 9%) and prints a table that displays future value for the years from 1 to 30, as shown below:
The amount invested: 1000 Annual interest rate: 9 Years Future Value 1 1093.80 2 1196.41 ... 29 13467.25 30 14730.57
6.8
(Conversions between Celsius and Fahrenheit) Write a class that contains the following two methods: /** Convert from Celsius to Fahrenheit */ public static double celsiusToFahrenheit(double celsius) /** Convert from Fahrenheit to Celsius */ public static double fahrenheitToCelsius(double fahrenheit)
The formula for the conversion is: fahrenheit = (9.0 / 5) * celsius + 32 celsius = (5.0 / 9) * (fahrenheit – 32)
236 Chapter 6
Methods Write a test program that invokes these methods to display the following tables: Celsius
Fahrenheit
|
Fahrenheit
Celsius
40.0
104.0
|
120.0
48.89
39.0
102.2
|
110.0
43.33
32.0
89.6
|
40.0
4.44
31.0
87.8
|
30.0
-1.11
...
6.9
(Conversions between feet and meters) Write a class that contains the following two methods: /** Convert from feet to meters */ public static double footToMeter(double foot) /** Convert from meters to feet */ public static double meterToFoot(double meter)
The formula for the conversion is: meter = 0.305 * foot foot = 3.279 * meter
Write a test program that invokes these methods to display the following tables:
6.10 6.11
Feet
Meters
|
Meters
Feet
1.0 2.0 ... 9.0 10.0
0.305 0.610
| |
20.0 25.0
65.574 81.967
2.745 3.050
| |
60.0 65.0
196.721 213.115
(Use the isPrime Method) Listing 6.7, PrimeNumberMethod.java, provides the isPrime(int number) method for testing whether a number is prime. Use this method to find the number of prime numbers less than 10000. (Financial application: compute commissions) Write a method that computes the commission, using the scheme in Programming Exercise 5.39. The header of the method is as follows: public static double computeCommission(double salesAmount)
Write a test program that displays the following table: Sales Amount
Commission
10000
900.0
15000
1500.0
... 95000
11100.0
100000
11700.0
Programming Exercises 237 6.12
(Display characters) Write a method that prints characters using the following header: public static void printChars(char ch1, char ch2, int numberPerLine)
This method prints the characters between ch1 and ch2 with the specified numbers per line. Write a test program that prints ten characters per line from 1 to Z. Characters are separated by exactly one space.
*6.13
(Sum series) Write a method to compute the following series: m(i) =
2 i 1 + + c + 2 3 i + 1
Write a test program that displays the following table:
i 1 2 ... 19 20
*6.14
m(i) 0.5000 1.1667 16.4023 17.3546
(Estimate p) p can be computed using the following series: m(i) = 4¢ 1 -
( -1)i + 1 1 1 1 1 1 + - + + g+ ≤ 3 5 7 9 11 2i - 1
Write a method that returns m(i) for a given i and write a test program that displays the following table:
*6.15
i
m(i)
1 101 201 301 401 501 601 701 801 901
4.0000 3.1515 3.1466 3.1449 3.1441 3.1436 3.1433 3.1430 3.1428 3.1427
(Financial application: print a tax table) Listing 3.5 gives a program to compute tax. Write a method for computing tax using the following header: public static double computeTax(int status, double taxableIncome)
VideoNote
Estimate p
238 Chapter 6
Methods Use this method to write a program that prints a tax table for taxable income from $50,000 to $60,000 with intervals of $50 for all the following statuses: Taxable Income
Single
Married Joint or Qualifying Widow(er)
Married Separate
Head of a House
50000 50050 ... 59950 60000
8688 8700
6665 6673
8688 8700
7353 7365
11175 11188
8158 8165
11175 11188
9840 9853
Hint:
round the tax into integers using Math.round .round(computeTax(status, taxableIncome)).
*6.16
(i.e.,
Math
(Number of days in a year) Write a method that returns the number of days in a year using the following header: public static int numberOfDaysInAYear(int year)
Write a test program that displays the number of days in year from 2000 to 2020.
Sections 6.10–6.11
*6.17
(Display matrix of 0s and 1s) Write a method that displays an n-by-n matrix using the following header: public static void printMatrix(int n)
Each element is 0 or 1, which is generated randomly. Write a test program that prompts the user to enter n and displays an n-by-n matrix. Here is a sample run: Enter n: 3 0 1 0 0 0 0 1 1 1
**6.18 (Check password ) Some websites impose certain rules for passwords. Write a method that checks whether a string is a valid password. Suppose the password rules are as follows: ■ ■ ■
*6.19
A password must have at least eight characters. A password consists of only letters and digits. A password must contain at least two digits.
Write a program that prompts the user to enter a password and displays Valid Password if the rules are followed or Invalid Password otherwise. (The MyTriangle class) Create a class named MyTriangle that contains the following two methods: /** Return true if the sum of any two sides is * greater than the third side. */ public static boolean isValid( double side1, double side2, double side3)
Programming Exercises 239 /** Return the area of the triangle. */ public static double area( double side1, double side2, double side3)
*6.20
Write a test program that reads three sides for a triangle and computes the area if the input is valid. Otherwise, it displays that the input is invalid. The formula for computing the area of a triangle is given in Programming Exercise 2.19. (Count the letters in a string) Write a method that counts the number of letters in a string using the following header: public static int countLetters(String s)
Write a test program that prompts the user to enter a string and displays the number of letters in the string.
*6.21
(Phone keypads) The international standard letter/number mapping for telephones is shown in Programming Exercise 4.15. Write a method that returns a number, given an uppercase letter, as follows: int getNumber(char uppercaseLetter)
Write a test program that prompts the user to enter a phone number as a string. The input number may contain letters. The program translates a letter (uppercase or lowercase) to a digit and leaves all other characters intact. Here is a sample run of the program: Enter a string: 1-800-Flowers 1-800-3569377
Enter a string: 1800flowers 18003569377
**6.22
(Math: approximate the square root) There are several techniques for implementing the sqrt method in the Math class. One such technique is known as the Babylonian method. It approximates the square root of a number, n, by repeatedly performing a calculation using the following formula: nextGuess = (lastGuess + n / lastGuess) / 2
When nextGuess and lastGuess are almost identical, nextGuess is the approximated square root. The initial guess can be any positive value (e.g., 1). This value will be the starting value for lastGuess. If the difference between nextGuess and lastGuess is less than a very small number, such as 0.0001, you can claim that nextGuess is the approximated square root of n. If not, nextGuess becomes lastGuess and the approximation process continues. Implement the following method that returns the square root of n. public static double sqrt(long n)
*6.23
(Occurrences of a specified character) Write a method that finds the number of occurrences of a specified character in a string using the following header: public static int count(String str, char a)
240 Chapter 6
Methods For example, count("Welcome", 'e') returns 2. Write a test program that prompts the user to enter a string followed by a character and displays the number of occurrences of the character in the string.
Sections 6.10–6.12
**6.24
(Display current date and time) Listing 2.7, ShowCurrentTime.java, displays the current time. Improve this example to display the current date and time. The calendar example in Listing 6.12, PrintCalendar.java, should give you some ideas on how to find the year, month, and day.
**6.25
(Convert milliseconds to hours, minutes, and seconds) Write a method that converts milliseconds to hours, minutes, and seconds using the following header: public static String convertMillis(long millis)
The method returns a string as hours:minutes:seconds. For example, convertMillis(5500) returns a string 0:0:5, convertMillis(100000) returns a string 0:1:40, and convertMillis(555550000) returns a string 154:19:10.
Comprehensive
**6.26
(Palindromic prime) A palindromic prime is a prime number and also palindromic. For example, 131 is a prime and also a palindromic prime, as are 313 and 757. Write a program that displays the first 100 palindromic prime numbers. Display 10 numbers per line, separated by exactly one space, as follows: 2 3 5 7 11 101 131 151 181 191 313 353 373 383 727 757 787 797 919 929 ...
**6.27
(Emirp) An emirp (prime spelled backward) is a nonpalindromic prime number whose reversal is also a prime. For example, 17 is a prime and 71 is a prime, so 17 and 71 are emirps. Write a program that displays the first 100 emirps. Display 10 numbers per line, separated by exactly one space, as follows: 13 17 31 37 71 73 79 97 107 113 149 157 167 179 199 311 337 347 359 389 ...
**6.28
**6.29
(Mersenne prime) A prime number is called a Mersenne prime if it can be written in the form 2p - 1 for some positive integer p. Write a program that finds all Mersenne primes with p … 31 and displays the output as follows: p
2^p –1
2 3 5 ...
3 7 31
(Twin primes) Twin primes are a pair of prime numbers that differ by 2. For example, 3 and 5 are twin primes, 5 and 7 are twin primes, and 11 and 13 are twin primes. Write a program to find all twin primes less than 1,000. Display the output as follows: (3, 5) (5, 7) ...
Programming Exercises 241 **6.30
(Game: craps) Craps is a popular dice game played in casinos. Write a program to play a variation of the game, as follows: Roll two dice. Each die has six faces representing values 1, 2, …, and 6, respectively. Check the sum of the two dice. If the sum is 2, 3, or 12 (called craps), you lose; if the sum is 7 or 11 (called natural), you win; if the sum is another value (i.e., 4, 5, 6, 8, 9, or 10), a point is established. Continue to roll the dice until either a 7 or the same point value is rolled. If 7 is rolled, you lose. Otherwise, you win. Your program acts as a single player. Here are some sample runs. You rolled 5 + 6 = 11 You win
You rolled 1 + 2 = 3 You lose
You rolled 4 + 4 = 8 point is 8 You rolled 6 + 2 = 8 You win
You rolled 3 + 2 = 5 point is 5 You rolled 2 + 5 = 7 You lose
**6.31
(Financial: credit card number validation) Credit card numbers follow certain patterns. A credit card number must have between 13 and 16 digits. It must start with: ■ ■ ■ ■
4 for Visa cards 5 for Master cards 37 for American Express cards 6 for Discover cards
In 1954, Hans Luhn of IBM proposed an algorithm for validating credit card numbers. The algorithm is useful to determine whether a card number is entered correctly or whether a credit card is scanned correctly by a scanner. Credit card numbers are generated following this validity check, commonly known as the Luhn check or the Mod 10 check, which can be described as follows (for illustration, consider the card number 4388576018402626): 1. Double every second digit from right to left. If doubling of a digit results in a two-digit number, add up the two digits to get a single-digit number. 4388576018402626 2*2=4 2*2=4 4*2=8 1*2=2 6 * 2 = 12 (1 + 2 = 3) 5 * 2 = 10 (1 + 0 = 1) 8 * 2 = 16 (1 + 6 = 7) 4*2=8
242 Chapter 6
Methods 2. Now add all single-digit numbers from Step 1. 4 + 4 + 8 + 2 + 3 + 1 + 7 + 8 = 37 3. Add all digits in the odd places from right to left in the card number. 6 + 6 + 0 + 8 + 0 + 7 + 8 + 3 = 38 4. Sum the results from Step 2 and Step 3. 37 + 38 = 75 5. If the result from Step 4 is divisible by 10, the card number is valid; otherwise, it is invalid. For example, the number 4388576018402626 is invalid, but the number 4388576018410707 is valid. Write a program that prompts the user to enter a credit card number as a long integer. Display whether the number is valid or invalid. Design your program to use the following methods: /** Return true if the card number is valid */ public static boolean isValid(long number) /** Get the result from Step 2 */ public static int sumOfDoubleEvenPlace(long number) /** Return this number if it is a single digit, otherwise, * return the sum of the two digits */ public static int getDigit(int number) /** Return sum of odd-place digits in number */ public static int sumOfOddPlace(long number) /** Return true if the digit d is a prefix for number */ public static boolean prefixMatched(long number, int d) /** Return the number of digits in d */ public static int getSize(long d) /** Return the first k number of digits from number. If the * number of digits in number is less than k, return number. */ public static long getPrefix(long number, int k)
Here are sample runs of the program: (You may also implement this program by reading the input as a string and processing the string to validate the credit card.) Enter a credit card number as a long integer: 4388576018410707 4388576018410707 is valid
Enter a credit card number as a long integer: 4388576018402626 4388576018402626 is invalid
**6.32 **6.33
(Game: chance of winning at craps) Revise Exercise 6.30 to run it 10,000 times and display the number of winning games. (Current date and time) Invoking System.currentTimeMillis() returns the elapsed time in milliseconds since midnight of January 1, 1970. Write a program that displays the date and time. Here is a sample run: Current date and time is May 16, 2012 10:34:23
Programming Exercises 243 **6.34 6.35
(Print calendar) Programming Exercise 3.21 uses Zeller’s congruence to calculate the day of the week. Simplify Listing 6.12, PrintCalendar.java, using Zeller’s algorithm to get the start day of the month. (Geometry: area of a pentagon) The area of a pentagon can be computed using the following formula: Area =
5 * s2 p 4 * tan¢ ≤ 5
Write a method that returns the area of a pentagon using the following header: public static double area(double side)
Write a main method that prompts the user to enter the side of a pentagon and displays its area. Here is a sample run:
Enter the side: 5.5 The area of the pentagon is 52.04444136781625
*6.36
(Geometry: area of a regular polygon) A regular polygon is an n-sided polygon in which all sides are of the same length and all angles have the same degree (i.e., the polygon is both equilateral and equiangular). The formula for computing the area of a regular polygon is Area =
n * s2 p 4 * tan¢ ≤ n
Write a method that returns the area of a regular polygon using the following header: public static double area(int n, double side)
Write a main method that prompts the user to enter the number of sides and the side of a regular polygon and displays its area. Here is a sample run:
Enter the number of sides: 5 Enter the side: 6.5 The area of the polygon is 72.69017017488385
6.37
(Format an integer) Write a method with the following header to format the integer with the specified width. public static String format(int number, int width)
The method returns a string for the number with one or more prefix 0s. The size of the string is the width. For example, format(34, 4) returns 0034 and format(34, 5) returns 00034. If the number is longer than the width, the method
244 Chapter 6
Methods returns the string representation for the number. For example, format(34, 1) returns 34.
*6.38 6.39
Write a test program that prompts the user to enter a number and its width and displays a string returned by invoking format(number, width). (Generate random characters) Use the methods in RandomCharacter in Listing 6.10 to print 100 uppercase letters and then 100 single digits, printing ten per line. (Geometry: point position) Programming Exercise 3.32 shows how to test whether a point is on the left side of a directed line, on the right, or on the same line. Write the methods with the following headers: /** Return true if point (x2, y2) is on the left side of the * directed line from (x0, y0) to (x1, y1) */ public static boolean leftOfTheLine(double x0, double y0, double x1, double y1, double x2, double y2) /** Return true if point (x2, y2) is on the same * line from (x0, y0) to (x1, y1) */ public static boolean onTheSameLine(double x0, double y0, double x1, double y1, double x2, double y2) /** Return true if point (x2, y2) is on the * line segment from (x0, y0) to (x1, y1) */ public static boolean onTheLineSegment(double x0, double y0, double x1, double y1, double x2, double y2)
Write a program that prompts the user to enter the three points for p0, p1, and p2 and displays whether p2 is on the left of the line from p0 to p1, right, the same line, or on the line segment. Here are some sample runs: Enter three points for p0, p1, and p2: 1 1 2 2 1.5 1.5 (1.5, 1.5) is on the line segment from (1.0, 1.0) to (2.0, 2.0)
Enter three points for p0, p1, and p2: 1 1 2 2 3 3 (3.0, 3.0) is on the same line from (1.0, 1.0) to (2.0, 2.0)
Enter three points for p0, p1, and p2: 1 1 2 2 1 1.5 (1.0, 1.5) is on the left side of the line from (1.0, 1.0) to (2.0, 2.0)
Enter three points for p0, p1, and p2: 1 1 2 2 1 -1 (1.0, -1.0) is on the right side of the line from (1.0, 1.0) to (2.0, 2.0)
CHAPTER
7 SINGLE-DIMENSIONAL ARRAYS Objectives ■
To describe why arrays are necessary in programming (§7.1).
■
To declare array reference variables and create arrays (§§7.2.1–7.2.2).
■
To obtain array size using arrayRefVar.length and know default values in an array (§7.2.3).
■
To access array elements using indexes (§7.2.4).
■
To declare, create, and initialize an array using an array initializer (§7.2.5).
■
To program common array operations (displaying arrays, summing all elements, finding the minimum and maximum elements, random shuffling, and shifting elements) (§7.2.6).
■
To simplify programming using the for each loops (§7.2.7).
■
To apply arrays in application development (AnalyzeNumbers, DeckOfCards) (§§7.3–7.4).
■
To copy contents from one array to another (§7.5).
■
To develop and invoke methods with array arguments and return values (§§7.6–7.8).
■
To define a method with a variable-length argument list (§7.9).
■
To search elements using the linear (§7.10.1) or binary (§7.10.2) search algorithm.
■
To sort an array using the selection sort approach (§7.11).
■
To use the methods in the java.util.Arrays class (§7.12).
■
To pass arguments to the main method from the command line (§7.13).
246 Chapter 7
Single-Dimensional Arrays
7.1 Introduction Key Point problem why array?
A single array variable can reference a large collection of data. Often you will have to store a large number of values during the execution of a program. Suppose, for instance, that you need to read 100 numbers, compute their average, and find out how many numbers are above the average. Your program first reads the numbers and computes their average, then compares each number with the average to determine whether it is above the average. In order to accomplish this task, the numbers must all be stored in variables. You have to declare 100 variables and repeatedly write almost identical code 100 times. Writing a program this way would be impractical. So, how do you solve this problem? An efficient, organized approach is needed. Java and most other high-level languages provide a data structure, the array, which stores a fixed-size sequential collection of elements of the same type. In the present case, you can store all 100 numbers into an array and access them through a single array variable. This chapter introduces single-dimensional arrays. The next chapter will introduce twodimensional and multidimensional arrays.
7.2 Array Basics Key Point index
Once an array is created, its size is fixed. An array reference variable is used to access the elements in an array using an index. An array is used to store a collection of data, but often we find it more useful to think of an array as a collection of variables of the same type. Instead of declaring individual variables, such as number0, number1, . . . , and number99, you declare one array variable such as numbers and use numbers[0], numbers[1], . . . , and numbers[99] to represent individual variables. This section introduces how to declare array variables, create arrays, and process arrays using indexes.
7.2.1 element type
Declaring Array Variables
To use an array in a program, you must declare a variable to reference the array and specify the array’s element type. Here is the syntax for declaring an array variable: elementType[] arrayRefVar;
The elementType can be any data type, and all elements in the array will have the same data type. For example, the following code declares a variable myList that references an array of double elements. double[] myList;
Note You can also use elementType arrayRefVar[] to declare an array variable. This style comes from the C/C++ language and was adopted in Java to accommodate C/C++ programmers. The style elementType[] arrayRefVar is preferred.
preferred syntax
7.2.2
null
Creating Arrays
Unlike declarations for primitive data type variables, the declaration of an array variable does not allocate any space in memory for the array. It creates only a storage location for the reference to an array. If a variable does not contain a reference to an array, the value of the variable is null. You cannot assign elements to an array unless it has already been created. After an
7.2 Array Basics 247 array variable is declared, you can create an array by using the new operator and assign its reference to the variable with the following syntax: arrayRefVar = new elementType[arraySize];
new operator
This statement does two things: (1) it creates an array using new elementType[arraySize]; (2) it assigns the reference of the newly created array to the variable arrayRefVar. Declaring an array variable, creating an array, and assigning the reference of the array to the variable can be combined in one statement as: elementType[] arrayRefVar = new elementType[arraySize];
or elementType arrayRefVar[] = new elementType[arraySize];
Here is an example of such a statement: double[] myList = new double[10];
This statement declares an array variable, myList, creates an array of ten elements of double type, and assigns its reference to myList. To assign values to the elements, use the syntax: arrayRefVar[index] = value;
For example, the following code initializes the array. myList[0] myList[1] myList[2] myList[3] myList[4] myList[5] myList[6] myList[7] myList[8] myList[9]
= = = = = = = = = =
5.6; 4.5; 3.3; 13.2; 4.0; 34.33; 34.0; 45.45; 99.993; 11123;
This array is illustrated in Figure 7.1.
double[] myList = new double[10]; myList reference
Array reference variable Array element at index 5
myList[0]
5.6
myList[1]
4.5
myList[2]
3.3
myList[3]
13.2
myList[4]
4.0
myList[5]
34.33
myList[6]
34.0
myList[7]
45.45
myList[8]
99.993
myList[9]
11123
Element value
FIGURE 7.1 The array myList has ten elements of double type and int indices from 0 to 9.
248 Chapter 7
Single-Dimensional Arrays Note An array variable that appears to hold an array actually contains a reference to that array. Strictly speaking, an array variable and an array are different, but most of the time the distinction can be ignored. Thus it is all right to say, for simplicity, that myList is an array, instead of stating, at greater length, that myList is a variable that contains a reference to an array of ten double elements.
array vs. array variable
7.2.3 array length
default values
When space for an array is allocated, the array size must be given, specifying the number of elements that can be stored in it. The size of an array cannot be changed after the array is created. Size can be obtained using arrayRefVar.length. For example, myList.length is 10. When an array is created, its elements are assigned the default value of 0 for the numeric primitive data types, \u0000 for char types, and false for boolean types.
7.2.4 0 based
indexed variable
Array Size and Default Values
Accessing Array Elements
The array elements are accessed through the index. Array indices are 0 based; that is, they range from 0 to arrayRefVar.length-1. In the example in Figure 7.1, myList holds ten double values, and the indices are from 0 to 9. Each element in the array is represented using the following syntax, known as an indexed variable: arrayRefVar[index];
For example, myList[9] represents the last element in the array myList.
Caution Some programming languages use parentheses to reference an array element, as in myList(9), but Java uses brackets, as in myList[9].
An indexed variable can be used in the same way as a regular variable. For example, the following code adds the values in myList[0] and myList[1] to myList[2]. myList[2] = myList[0] + myList[1];
The following loop assigns 0 to myList[0], 1 to myList[1], . . . , and 9 to myList[9]: for (int i = 0; i < myList.length; i++) { myList[i] = i; }
7.2.5 array initializer
Array Initializers
Java has a shorthand notation, known as the array initializer, which combines the declaration, creation, and initialization of an array in one statement using the following syntax: elementType[] arrayRefVar = {value0, value1, ..., valuek};
For example, the statement double[] myList = {1.9, 2.9, 3.4, 3.5};
declares, creates, and initializes the array myList with four elements, which is equivalent to the following statements: double[] myList = new double[4]; myList[0] = 1.9; myList[1] = 2.9;
7.2 Array Basics 249 myList[2] = 3.4; myList[3] = 3.5;
Caution The new operator is not used in the array-initializer syntax. Using an array initializer, you have to declare, create, and initialize the array all in one statement. Splitting it would cause a syntax error. Thus, the next statement is wrong: double[] myList; myList = {1.9, 2.9, 3.4, 3.5};
7.2.6
Processing Arrays
When processing array elements, you will often use a for loop—for two reasons: ■
All of the elements in an array are of the same type. They are evenly processed in the same fashion repeatedly using a loop.
■
Since the size of the array is known, it is natural to use a for loop.
Assume the array is created as follows: double[] myList = new double[10];
The following are some examples of processing arrays. 1. Initializing arrays with input values: The following loop initializes the array myList with user input values. java.util.Scanner input = new java.util.Scanner(System.in); System.out.print("Enter " + myList.length + " values: "); for (int i = 0; i < myList.length; i++) myList[i] = input.nextDouble();
2. Initializing arrays with random values: The following loop initializes the array myList with random values between 0.0 and 100.0, but less than 100.0. for (int i = 0; i < myList.length; i++) { myList[i] = Math.random() * 100; }
3. Displaying arrays: To print an array, you have to print each element in the array using a loop like the following: for (int i = 0; i < myList.length; i++) { System.out.print(myList[i] + " "); }
Tip For an array of the char[] type, it can be printed using one print statement. For example, the following code displays Dallas: char[] city = {'D', 'a', 'l', 'l', 'a', 's'}; System.out.println(city);
4. Summing all elements: Use a variable named total to store the sum. Initially total is 0. Add each element in the array to total using a loop like this: double total = 0; for (int i = 0; i < myList.length; i++) { total += myList[i]; }
print character array
250 Chapter 7
Single-Dimensional Arrays 5. Finding the largest element: Use a variable named max to store the largest element. Initially max is myList[0]. To find the largest element in the array myList, compare each element with max, and update max if the element is greater than max. double max = myList[0]; for (int i = 1; i < myList.length; i++) { if (myList[i] > max) max = myList[i]; }
6. Finding the smallest index of the largest element: Often you need to locate the largest element in an array. If an array has multiple elements with the same largest value, find the smallest index of such an element. Suppose the array myList is {1, 5, 3, 4, 5, 5}. The largest element is 5 and the smallest index for 5 is 1. Use a variable named max to store the largest element and a variable named indexOfMax to denote the index of the largest element. Initially max is myList[0], and indexOfMax is 0. Compare each element in myList with max, and update max and indexOfMax if the element is greater than max. double max = myList[0]; int indexOfMax = 0; for (int i = 1; i < myList.length; i++) { if (myList[i] > max) { max = myList[i]; indexOfMax = i; } } Random shuffling
7. Random shuffling: In many applications, you need to randomly reorder the elements in an array. This is called shuffling. To accomplish this, for each element myList[i], randomly generate an index j and swap myList[i] with myList[j], as follows:
VideoNote
Random shuffling
myList for (int i = myList.length – 1; i > 0; i––) { i [0] . // Generate an index j randomly with 0 <= j <= i int j = (int)(Math.random() [1] * (i + 1)); A random index [j] . // Swap myList[i] with myList[j] double temp = myList[i]; myList[i] = myList[j]; myList[j] = temp; [i] }
swap
8. Shifting elements: Sometimes you need to shift the elements left or right. Here is an example of shifting the elements one position to the left and filling the last element with the first element: double temp = myList[0]; // Retain the first element // Shift elements left for (int i = 1; i < myList.length; i++) { myList[i - 1] = myList[i]; }
myList
// Move the first element to fill in the last position myList[myList.length - 1] = temp;
9. Simplifying coding: Arrays can be used to greatly simplify coding for certain tasks. For example, suppose you wish to obtain the English name of a given month by its number. If the month names are stored in an array, the month name for a given month can be
7.2 Array Basics 251 accessed simply via the index. The following code prompts the user to enter a month number and displays its month name: String[] months = {"January", "February", ..., "December"}; System.out.print("Enter a month number (1 to 12): "); int monthNumber = input.nextInt(); System.out.println("The month is " + months[monthNumber - 1]);
If you didn’t use the months array, you would have to determine the month name using a lengthy multi-way if-else statement as follows: if (monthNumber == 1) System.out.println("The month is January"); else if (monthNumber == 2) System.out.println("The month is February"); ... else System.out.println("The month is December");
7.2.7
Foreach Loops
Java supports a convenient for loop, known as a foreach loop, which enables you to traverse the array sequentially without using an index variable. For example, the following code displays all the elements in the array myList: for (double e: myList) { System.out.println(e); }
You can read the code as “for each element e in myList, do the following.” Note that the variable, e, must be declared as the same type as the elements in myList. In general, the syntax for a foreach loop is for (elementType element: arrayRefVar) { // Process the element }
You still have to use an index variable if you wish to traverse the array in a different order or change the elements in the array.
Caution Accessing an array out of bounds is a common programming error that throws a runtime ArrayIndexOutOfBoundsException. To avoid it, make sure that you do not use an index beyond arrayRefVar.length – 1. Programmers often mistakenly reference the first element in an array with index 1, but it should be 0. This is called the off-by-one error. Another common off-by-one error in a loop is using <= where < should be used. For example, the following loop is wrong.
ArrayIndexOutOfBoundsException
off-by-one error
for (int i = 0; i <= list.length; i++) System.out.print(list[i] + " ");
The <= should be replaced by <.
7.1 7.2
How do you declare an array reference variable and how do you create an array? When is the memory allocated for an array?
✓
Check Point
252 Chapter 7
Single-Dimensional Arrays 7.3
What is the output of the following code? int x = 30; int[] numbers = new int[x]; x = 60; System.out.println("x is " + x); System.out.println("The size of numbers is " + numbers.length);
7.4
Indicate true or false for the following statements: ■ Every element in an array has the same type. ■ The array size is fixed after an array reference variable is declared. ■ ■
7.5
The array size is fixed after it is created. The elements in an array must be a primitive data type.
Which of the following statements are valid? int i = new int(30); double d[] = new double[30]; char[] r = new char(1..30); int i[] = (3, 4, 3, 2); float f[] = {2.3, 4.5, 6.6}; char[] c = new char();
7.6 7.7 7.8
How do you access elements in an array? What is the array index type? What is the lowest index? What is the representation of the third element in an array named a? Write statements to do the following: a. Create an array to hold 10 double values. b. Assign the value 5.5 to the last element in the array. c. Display the sum of the first two elements. d. Write a loop that computes the sum of all elements in the array. e. Write a loop that finds the minimum element in the array. f. Randomly generate an index and display the element of this index in the array. g. Use an array initializer to create another array with the initial values 3.5, 5.5, 4.52, and 5.6.
7.9 7.10
What happens when your program attempts to access an array element with an invalid index? Identify and fix the errors in the following code: 1 2 3 4 5 6 7 8
7.11
public class Test { public static void main(String[] args) { double[100] r; for (int i = 0; i < r.length(); i++); r(i) = Math.random * 100; } }
What is the output of the following code? 1 2 3
public class Test { public static void main(String[] args) { int list[] = {1, 2, 3, 4, 5, 6};
7.3 Case Study: Analyzing Numbers 253 4 5 6 7 8 9 10
for (int i = 1; i < list.length; i++) list[i] = list[i - 1]; for (int i = 0; i < list.length; i++) System.out.print(list[i] + " "); } }
7.3 Case Study: Analyzing Numbers The problem is to write a program that finds the number of items above the average of all items.
Key Point
Now you can write a program using arrays to solve the problem proposed at the beginning of this chapter. The problem is to read 100 numbers, get the average of these numbers, and find the number of the items greater than the average. To be flexible for handling any number of input, we will let the user enter the number of input, rather than fixing it to 100. Listing 7.1 gives a solution.
LISTING 7.1 AnalyzeNumbers.java 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
public class AnalyzeNumbers { public static void main(String[] args) { numbers[0] java.util.Scanner input = new java.util.Scanner(System.in); numbers[1]: System.out.print("Enter the number of items: "); numbers[2]: int n = input.nextInt(); double [] numbers = new double[n]; double sum = 0; numbers[i]:
System.out.print("Enter the numbers: "); for (int i = 0; i < n; i++) { numbers[i] = input.nextDouble(); sum += numbers[i]; }
numbers[n - 3]: numbers[n - 2]: numbers[n - 1]:
double average = sum / n; int count = 0; // The number of elements above average for (int i = 0; i < n; i++) if (numbers[i] > average) count++; System.out.println("Average is " + average); System.out.println("Number of elements above the average is " + count); } }
Enter the number of items: 10 Enter the numbers: 3.4 5 6 1 6.5 7.8 3.5 8.5 6.3 9.5 Average is 5.75 Number of elements above the average is 6
The program prompts the user to enter the array size (line 5) and creates an array with the specified size (line 6). The program reads the input, stores numbers into the array (line 11), adds each number to sum in line 11, and obtains the average (line 15). It then compares
. . .
create array
store number in array
get average
above average?
254 Chapter 7
Single-Dimensional Arrays each number in the array with the average to count the number of values above the average (lines 17–20).
7.4 Case Study: Deck of Cards Key Point
The problem is to create a program that will randomly select four cards from a deck of cards. Say you want to write a program that will pick four cards at random from a deck of 52 cards. All the cards can be represented using an array named deck, filled with initial values 0 to 51, as follows: int[] deck = new int[52];
VideoNote
// Initialize cards for (int i = 0; i < deck.length; i++) deck[i] = i;
Deck of cards
Card numbers 0 to 12, 13 to 25, 26 to 38, and 39 to 51 represent 13 Spades, 13 Hearts, 13 Diamonds, and 13 Clubs, respectively, as shown in Figure 7.2. cardNumber / 13 determines the suit of the card and cardNumber % 13 determines the rank of the card, as shown in Figure 7.3. After shuffling the array deck, pick the first four cards from deck. The program displays the cards from these four card numbers.
0 . . . 12 13 . . . 25 26 . . . 38 39 . . . 51
deck [0] 0 . . . . . . [12] 12 [13] 13 . . . . . . [25] 25 [26] 26 . . . . . . [38] 38 [39] 39 . . . . . . [51] 51
13 Spades ( )
13 Hearts ( )
13 Diamonds ( )
13 Clubs ( )
FIGURE 7.2
Random shuffle
deck [0] 6 [1] 48 [2] 11 [3] 24 [4] . [5] . . . . . . . [25] . [26] . . . . . . . [38] . [39] . . . . . . . [51] .
Card number 6 is the 7 (6 % 13 = 6) of Spades (7 / 13 is 0) Card number 48 is the 10 (48 % 13 = 9) of Clubs (48 / 13 is 3) Card number 11 is the Queen (11 % 13 = 11) of Spades (11 / 13 is 0) Card number 24 is the Queen (24 % 13 = 11) of Hearts (24 / 13 is 1)
52 cards are stored in an array named deck.
0
Spades
1
Hearts
2
Diamonds
3
Clubs
cardNumber / 13 =
0
Ace
1
2
. cardNumber % 13 =
FIGURE 7.3 CardNumber identifies a card’s suit and rank number.
. 10
Jack
11
Queen
12
King
7.4 Case Study: Deck of Cards 255 Listing 7.2 gives the solution to the problem.
LISTING 7.2 DeckOfCards.java 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
public class DeckOfCards { public static void main(String[] args) { int[] deck = new int[52]; String[] suits = {"Spades", "Hearts", "Diamonds", "Clubs"}; String[] ranks = {"Ace", "2", "3", "4", "5", "6", "7", "8", "9", "10", "Jack", "Queen", "King"}; // Initialize the cards for (int i = 0; i < deck.length; i++) deck[i] = i; // Shuffle the cards for (int i = 0; i < deck.length; i++) { // Generate an index randomly int index = (int)(Math.random() * deck.length); int temp = deck[i]; deck[i] = deck[index]; deck[index] = temp; } // Display the first four cards for (int i = 0; i < 4; i++) { String suit = suits[deck[i] / 13]; String rank = ranks[deck[i] % 13]; System.out.println("Card number " + deck[i] + ": " + rank + " of " + suit); } } }
Card Card Card Card
number number number number
6: 7 of Spades 48: 10 of Clubs 11: Queen of Spades 24: Queen of Hearts
The program creates an array suits for four suits (line 4) and an array ranks for 13 cards in a suit (lines 5–6). Each element in these arrays is a string. The program initializes deck with values 0 to 51 in lines 9–10. The deck value 0 represents the card Ace of Spades, 1 represents the card 2 of Spades, 13 represents the card Ace of Hearts, and 14 represents the card 2 of Hearts. Lines 13–19 randomly shuffle the deck. After a deck is shuffled, deck[i] contains an arbitrary value. deck[i] / 13 is 0, 1, 2, or 3, which determines the suit (line 23). deck[i] % 13 is a value between 0 and 12, which determines the rank (line 24). If the suits array is not defined, you would have to determine the suit using a lengthy multi-way if-else statement as follows: if (deck[i] / 13 == 0) System.out.print("suit else if (deck[i] / 13 == System.out.print("suit else if (deck[i] / 13 == System.out.print("suit else System.out.print("suit
is Spades"); 1) is Hearts"); 2) is Diamonds"); is Clubs");
create array deck array of strings array of strings
initialize deck
shuffle deck
suit of a card rank of a card
256 Chapter 7
Single-Dimensional Arrays With suits = {"Spades", "Hearts", "Diamonds", "Clubs"} created in an array, suits[deck / 13] gives the suit for the deck. Using arrays greatly simplifies the solution for this program.
✓
Check Point
7.12
Will the program pick four random cards if you replace lines 22–27 in Listing 7.2 DeckOfCards.java with the following code? for (int i = 0; i < 4; i++) { int cardNumber = (int)(Math.random() * deck.length); String suit = suits[cardNumber / 13]; String rank = ranks[cardNumber % 13]; System.out.println("Card number " + cardNumber + ": " + rank + " of " + suit); }
7.5 Copying Arrays Key Point
To copy the contents of one array into another, you have to copy the array’s individual elements into the other array. Often, in a program, you need to duplicate an array or a part of an array. In such cases you could attempt to use the assignment statement (=), as follows: list2 = list1;
copy reference
garbage collection
However, this statement does not copy the contents of the array referenced by list1 to list2, but instead merely copies the reference value from list1 to list2. After this statement, list1 and list2 reference the same array, as shown in Figure 7.4. The array previously referenced by list2 is no longer referenced; it becomes garbage, which will be automatically collected by the Java Virtual Machine (this process is called garbage collection).
Before the assignment list2 = list1; list1 Contents of list1
list2
After the assignment list2 = list1; list1
Contents of list1
list2 Contents of list2
Contents of list2
FIGURE 7.4 Before the assignment statement, list1 and list2 point to separate memory locations. After the assignment, the reference of the list1 array is passed to list2. In Java, you can use assignment statements to copy primitive data type variables, but not arrays. Assigning one array variable to another array variable actually copies one reference to another and makes both variables point to the same memory location. There are three ways to copy arrays: ■
Use a loop to copy individual elements one by one.
■
Use the static arraycopy method in the System class.
■
Use the clone method to copy arrays; this will be introduced in Chapter 13, Abstract Classes and Interfaces.
7.6 Passing Arrays to Methods 257 You can write a loop to copy every element from the source array to the corresponding element in the target array. The following code, for instance, copies sourceArray to targetArray using a for loop. int[] sourceArray = {2, 3, 1, 5, 10}; int[] targetArray = new int[sourceArray.length]; for (int i = 0; i < sourceArray.length; i++) { targetArray[i] = sourceArray[i]; }
Another approach is to use the arraycopy method in the java.lang.System class to copy arrays instead of using a loop. The syntax for arraycopy is:
arraycopy method
arraycopy(sourceArray, srcPos, targetArray, tarPos, length);
The parameters srcPos and tarPos indicate the starting positions in sourceArray and targetArray, respectively. The number of elements copied from sourceArray to targetArray is indicated by length. For example, you can rewrite the loop using the following statement: System.arraycopy(sourceArray, 0, targetArray, 0, sourceArray.length);
The arraycopy method does not allocate memory space for the target array. The target array must have already been created with its memory space allocated. After the copying takes place, targetArray and sourceArray have the same content but independent memory locations.
Note The arraycopy method violates the Java naming convention. By convention, this method should be named arrayCopy (i.e., with an uppercase C).
7.13
Use the arraycopy method to copy the following array to a target array t: int[] source = {3, 4, 5};
7.14
✓
Check Point
Once an array is created, its size cannot be changed. Does the following code resize the array? int[] myList; myList = new int[10]; // Sometime later you want to assign a new array to myList myList = new int[20];
7.6 Passing Arrays to Methods When passing an array to a method, the reference of the array is passed to the method. Just as you can pass primitive type values to methods, you can also pass arrays to methods. For example, the following method displays the elements in an int array: public static void printArray(int[] array) { for (int i = 0; i < array.length; i++) { System.out.print(array[i] + " "); } }
You can invoke it by passing an array. For example, the following statement invokes the printArray method to display 3, 1, 2, 6, 4, and 2. printArray(new int[]{3, 1, 2, 6, 4, 2});
Key Point
258 Chapter 7
Single-Dimensional Arrays Note The preceding statement creates an array using the following syntax: new elementType[]{value0, value1, ..., valuek};
There is no explicit reference variable for the array. Such array is called an anonymous array.
anonymous array
pass-by-value
Java uses pass-by-value to pass arguments to a method. There are important differences between passing the values of variables of primitive data types and passing arrays. ■
For an argument of a primitive type, the argument’s value is passed.
■
For an argument of an array type, the value of the argument is a reference to an array; this reference value is passed to the method. Semantically, it can be best described as pass-by-sharing, that is, the array in the method is the same as the array being passed. Thus, if you change the array in the method, you will see the change outside the method.
pass-by-sharing
Take the following code, for example: public class Test { public static void main(String[] args) { int x = 1; // x represents an int value int[] y = new int[10]; // y represents an array of int values m(x, y); // Invoke m with arguments x and y System.out.println("x is " + x); System.out.println("y[0] is " + y[0]); } public static void m(int number, int[] numbers) { number = 1001; // Assign a new value to number numbers[0] = 5555; // Assign a new value to numbers[0] } }
x is 1 y[0] is 5555
You may wonder why after m is invoked, x remains 1, but y[0] become 5555. This is because y and numbers, although they are independent variables, reference the same array, as illustrated in Figure 7.5. When m(x, y) is invoked, the values of x and y are passed to number and numbers. Since y contains the reference value to the array, numbers now contains the same reference value to the same array. Stack Activation record for method m reference int[] numbers: int number: 1 Activation record for the main method int[] y: reference int x: 1
Heap
An array of ten int values is stored here
Arrays are stored in a heap.
FIGURE 7.5 The primitive type value in x is passed to number, and the reference value in y is passed to numbers.
7.6 Passing Arrays to Methods 259 Note Arrays are objects in Java (objects are introduced in Chapter 9). The JVM stores the objects in an area of memory called the heap, which is used for dynamic memory allocation.
heap
Listing 7.3 gives another program that shows the difference between passing a primitive data type value and an array reference variable to a method. The program contains two methods for swapping elements in an array. The first method, named swap, fails to swap two int arguments. The second method, named swapFirstTwoInArray, successfully swaps the first two elements in the array argument.
LISTING 7.3 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
TestPassArray.java
public class TestPassArray { /** Main method */ public static void main(String[] args) { int[] a = {1, 2}; // Swap elements using the swap method System.out.println("Before invoking swap"); System.out.println("array is {" + a[0] + ", " + a[1] + "}"); swap(a[0], a[1]); System.out.println("After invoking swap"); System.out.println("array is {" + a[0] + ", " + a[1] + "}"); // Swap elements using the swapFirstTwoInArray method System.out.println("Before invoking swapFirstTwoInArray"); System.out.println("array is {" + a[0] + ", " + a[1] + "}"); swapFirstTwoInArray(a); System.out.println("After invoking swapFirstTwoInArray"); System.out.println("array is {" + a[0] + ", " + a[1] + "}"); } /** Swap two variables */ public static void swap(int n1, int n2) { int temp = n1; n1 = n2; n2 = temp; } /** Swap the first two elements in the array */ public static void swapFirstTwoInArray(int[] array) { int temp = array[0]; array[0] = array[1]; array[1] = temp; } }
Before invoking swap array is {1, 2} After invoking swap array is {1, 2} Before invoking swapFirstTwoInArray array is {1, 2} After invoking swapFirstTwoInArray array is {2, 1}
false swap
swap array elements
260 Chapter 7
Single-Dimensional Arrays As shown in Figure 7.6, the two elements are not swapped using the swap method. However, they are swapped using the swapFirstTwoInArray method. Since the parameters in the swap method are primitive type, the values of a[0] and a[1] are passed to n1 and n2 inside the method when invoking swap(a[0], a[1]). The memory locations for n1 and n2 are independent of the ones for a[0] and a[1]. The contents of the array are not affected by this call. Stack
Activation record for the swap method n2: 2 n1: 1 Activation record for the main method int[] a reference
Invoke swap(int n1, int n2). The primitive type values in a[0] and a[1] are passed to the swap method.
Heap
a[0]: 1 a[1]: 2 The arrays are stored in a heap.
Stack Activation record for the swapFirstTwoInArray method int[] array reference
Activation record for the main method int[] a reference
Invoke swapFirstTwoInArray(int[] array). The reference value in a is passed to the swapFirstTwoInArray method.
FIGURE 7.6 When passing an array to a method, the reference of the array is passed to the method. The parameter in the swapFirstTwoInArray method is an array. As shown in Figure 7.6, the reference of the array is passed to the method. Thus the variables a (outside the method) and array (inside the method) both refer to the same array in the same memory location. Therefore, swapping array[0] with array[1] inside the method swapFirstTwoInArray is the same as swapping a[0] with a[1] outside of the method.
7.7 Returning an Array from a Method Key Point
create array
return array
When a method returns an array, the reference of the array is returned. You can pass arrays when invoking a method. A method may also return an array. For example, the following method returns an array that is the reversal of another array. l public static int[] reverse(int[] list) { 2 int[] result = new int[list.length]; 3 4 for (int i = 0, j = result.length - 1; 5 i < list.length; i++, j--) { 6 result[j] = list[i]; list 7 } 8 result 9 return result; 10 }
Line 2 creates a new array result. Lines 4–7 copy elements from array list to array result. Line 9 returns the array. For example, the following statement returns a new array list2 with elements 6, 5, 4, 3, 2, 1. int[] list1 = {1, 2, 3, 4, 5, 6}; int[] list2 = reverse(list1);
7.8 Case Study: Counting the Occurrences of Each Letter 261 7.15
Suppose the following code is written to reverse the contents in an array, explain why it is wrong. How do you fix it? int[] list = {1, 2, 3, 5, 4};
✓
Check Point
for (int i = 0, j = list.length - 1; i < list.length; i++, j--) { // Swap list[i] with list[j] int temp = list[i]; list[i] = list[j]; list[j] = temp; }
7.8 Case Study: Counting the Occurrences of Each Letter This section presents a program to count the occurrences of each letter in an array of characters.
Key Point
The program given in Listing 7.4 does the following: 1. Generates 100 lowercase letters randomly and assigns them to an array of characters, as shown in Figure 7.7a. You can obtain a random letter by using the getRandomLowerCaseLetter() method in the RandomCharacter class in Listing 6.10. 2. Count the occurrences of each letter in the array. To do so, create an array, say counts, of 26 int values, each of which counts the occurrences of a letter, as shown in Figure 7.7b. That is, counts[0] counts the number of a’s, counts[1] counts the number of b’s, and so on.
chars[0]
counts[0]
chars[1]
counts[1]
…
…
…
…
…
…
…
…
chars[98]
counts[24]
chars[99]
counts[25]
(a)
(b)
FIGURE 7.7 The chars array stores 100 characters, and the counts array stores 26 counts, each of which counts the occurrences of a letter.
LISTING 7.4 CountLettersInArray.java 1 2 3 4 5 6 7 8 9 10
public class CountLettersInArray { /** Main method */ public static void main(String[] args) { // Declare and create an array char[] chars = createArray(); // Display the array System.out.println("The lowercase letters are:"); displayArray(chars);
create array
pass array
262 Chapter 7 return array
pass array
increase count
Single-Dimensional Arrays 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 63 64 65 66
// Count the occurrences of each letter int[] counts = countLetters(chars); // Display counts System.out.println(); System.out.println("The occurrences of each letter are:"); displayCounts(counts); } /** Create an array of characters */ public static char[] createArray() { // Declare an array of characters and create it char[] chars = new char[100]; // Create lowercase letters randomly and assign // them to the array for (int i = 0; i < chars.length; i++) chars[i] = RandomCharacter.getRandomLowerCaseLetter(); // Return the array return chars; } /** Display the array of characters */ public static void displayArray(char[] chars) { // Display the characters in the array 20 on each line for (int i = 0; i < chars.length; i++) { if ((i + 1) % 20 == 0) System.out.println(chars[i]); else System.out.print(chars[i] + " "); } } /** Count the occurrences of each letter */ public static int[] countLetters(char[] chars) { // Declare and create an array of 26 int int[] counts = new int[26]; // For each lowercase letter in the array, count it for (int i = 0; i < chars.length; i++) counts[chars[i] - 'a']++; return counts; } /** Display counts */ public static void displayCounts(int[] counts) { for (int i = 0; i < counts.length; i++) { if ((i + 1) % 10 == 0) System.out.println(counts[i] + " " + (char)(i + 'a')); else System.out.print(counts[i] + " " + (char)(i + 'a') + " "); } } }
7.8 Case Study: Counting the Occurrences of Each Letter 263 The e y s c a z h w q e
lowercase l s r i b c k r d w g d e g f i w n t g a m f w p
letters k j v j a m p w i n d x x w c d g u q t
The 5 a 2 k 3 u
occurrences 3 b 4 c 4 d 3 l 4 m 6 n 5 v 8 w 3 x
are: h a b v u n m z o o t x r e n
z q u h n
n a l y w
w m o v f
b p z z c
t l j y r
v o v z f
of each letter are: 4 e 4 f 4 g 3 h 3 i 3 j 4 o 3 p 3 q 4 r 2 s 4 t 3 y 6 z
The createArray method (lines 21–32) generates an array of 100 random lowercase letters. Line 5 invokes the method and assigns the array to chars. What would be wrong if you rewrote the code as follows? char[] chars = new char[100]; chars = createArray();
You would be creating two arrays. The first line would create an array by using new char[100]. The second line would create an array by invoking createArray() and assign the reference of the array to chars. The array created in the first line would be garbage because it is no longer referenced, and as mentioned earlier Java automatically collects garbage behind the scenes. Your program would compile and run correctly, but it would create an array unnecessarily. Invoking getRandomLowerCaseLetter() (line 28) returns a random lowercase letter. This method is defined in the RandomCharacter class in Listing 6.10. The countLetters method (lines 46–55) returns an array of 26 int values, each of which stores the number of occurrences of a letter. The method processes each letter in the array and increases its count by one. A brute-force approach to count the occurrences of each letter might be as follows: for (int i = 0; i < chars.length; i++) if (chars[i] == 'a') counts[0]++; else if (chars[i] == 'b') counts[1]++; ...
But a better solution is given in lines 51–52. for (int i = 0; i < chars.length; i++) counts[chars[i] - 'a']++;
If the letter (chars[i]) is a, the corresponding count is counts['a' - 'a'] (i.e., counts[0]). If the letter is b, the corresponding count is counts['b' - 'a'] (i.e., counts[1]), since the Unicode of b is one more than that of a. If the letter is z, the corresponding count is counts['z' - 'a'] (i.e., counts[25]), since the Unicode of z is 25 more than that of a. Figure 7.8 shows the call stack and heap during and after executing createArray. See Checkpoint Question 7.18 to show the call stack and heap for other methods in the program.
264 Chapter 7
Single-Dimensional Arrays Stack Activation record for the createArray method char[] chars: ref
Heap
Stack
Array of 100 characters
Heap Array of 100 characters
Activation record for the main method char[] chars: ref
Activation record for the main method char[] chars: ref (a) Executing createArray in line 5
(b) After exiting createArray in line 5
FIGURE 7.8 (a) An array of 100 characters is created when executing createArray. (b) This array is returned and assigned to the variable chars in the main method.
✓
Check Point
7.16
True or false? When an array is passed to a method, a new array is created and passed to the method.
7.17
Show the output of the following two programs:
public class Test { public static void main(String[] args) { int number = 0; int[] numbers = new int[1]; m(number, numbers);
public class Test { public static void main(String[] args) { int[] list = {1, 2, 3, 4, 5}; reverse(list); for (int i = 0; i < list.length; i++) System.out.print(list[i] + " "); }
System.out.println("number is " + number + " and numbers[0] is " + numbers[0]);
public static void reverse(int[] list) { int[] newList = new int[list.length];
} public static void m(int x, int[] y) { x = 3; y[0] = 3; }
for (int i = 0; i < list.length; i++) newList[i] = list[list.length - 1 - i]; list = newList;
}
} } (b)
(a)
7.18
Where are the arrays stored during execution? Show the contents of the stack and heap during and after executing displayArray, countLetters, displayCounts in Listing 7.4.
7.9 Variable-Length Argument Lists Key Point
A variable number of arguments of the same type can be passed to a method and treated as an array. You can pass a variable number of arguments of the same type to a method. The parameter in the method is declared as follows: typeName... parameterName
In the method declaration, you specify the type followed by an ellipsis (...). Only one variable-length parameter may be specified in a method, and this parameter must be the last parameter. Any regular parameters must precede it.
7.10 Searching Arrays 265 Java treats a variable-length parameter as an array. You can pass an array or a variable number of arguments to a variable-length parameter. When invoking a method with a variable number of arguments, Java creates an array and passes the arguments to it. Listing 7.5 contains a method that prints the maximum value in a list of an unspecified number of values.
LISTING 7.5 VarArgsDemo.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
public class VarArgsDemo { public static void main(String[] args) { printMax(34, 3, 3, 2, 56.5); printMax(new double[]{1, 2, 3}); } public static void printMax(double... numbers) { if (numbers.length == 0) { System.out.println("No argument passed"); return; }
pass variable-length arg list pass an array arg
a variable-length arg parameter
double result = numbers[0]; for (int i = 1; i < numbers.length; i++) if (numbers[i] > result) result = numbers[i]; System.out.println("The max value is " + result); } }
Line 3 invokes the printMax method with a variable-length argument list passed to the array numbers. If no arguments are passed, the length of the array is 0 (line 8). Line 4 invokes the printMax method with an array.
7.19
What is wrong in the following method header? public static void print(String... strings, double... numbers) public static void print(double... numbers, String name) public static double... print(double d1, double d2)
7.20
✓
Check Point
Can you invoke the printMax method in Listing 7.5 using the following statements? printMax(1, 2, 2, 1, 4); printMax(new double[]{1, 2, 3}); printMax(new int[]{1, 2, 3});
7.10 Searching Arrays If an array is sorted, binary search is more efficient than linear search for finding an element in the array. Searching is the process of looking for a specific element in an array—for example, discovering whether a certain score is included in a list of scores. Searching is a common task in computer programming. Many algorithms and data structures are devoted to searching. This section discusses two commonly used approaches, linear search and binary search.
7.10.1
The Linear Search Approach
The linear search approach compares the key element key sequentially with each element in the array. It continues to do so until the key matches an element in the array or the array is exhausted without a match being found. If a match is made, the linear search returns the index
Key Point
linear search binary search
266 Chapter 7
Single-Dimensional Arrays of the element in the array that matches the key. If no match is found, the search returns -1. The linearSearch method in Listing 7.6 gives the solution.
linear search animation on Companion Website
LISTING 7.6
LinearSearch.java
1 public class LinearSearch { 2 /** The method for finding a key in the list */ 3 public static int linearSearch(int[] list, int key) { 4 for (int i = 0; i < list.length; i++) { 5 if (key == list[i]) [0] [1] [2] … 6 return i; 7 } list 8 return -1; key Compare key with list[i] for i = 0, 1, … 9 } 10 }
To better understand this method, trace it with the following statements: 1 2 3 4
int[] int i int j int k
list = {1, 4, 4, 2, 5, -3, 6, 2}; = linearSearch(list, 4); // Returns 1 = linearSearch(list, -4); // Returns -1 = linearSearch(list, -3); // Returns 5
The linear search method compares the key with each element in the array. The elements can be in any order. On average, the algorithm will have to examine half of the elements in an array before finding the key, if it exists. Since the execution time of a linear search increases linearly as the number of array elements increases, linear search is inefficient for a large array.
7.10.2
The Binary Search Approach
Binary search is the other common search approach for a list of values. For binary search to work, the elements in the array must already be ordered. Assume that the array is in ascending order. The binary search first compares the key with the element in the middle of the array. Consider the following three cases:
binary search animation on Companion Website
■
If the key is less than the middle element, you need to continue to search for the key only in the first half of the array.
■
If the key is equal to the middle element, the search ends with a match.
■
If the key is greater than the middle element, you need to continue to search for the key only in the second half of the array.
Clearly, the binary search method eliminates at least half of the array after each comparison. Sometimes you eliminate half of the elements, and sometimes you eliminate half plus one. Suppose that the array has n elements. For convenience, let n be a power of 2. After the first comparison, n/2 elements are left for further search; after the second comparison, (n/2)/2 elements are left. After the kth comparison, n/2k elements are left for further search. When k = log2n, only one element is left in the array, and you need only one more comparison. Therefore, in the worst case when using the binary search approach, you need log2n+1 comparisons to find an element in the sorted array. In the worst case for a list of 1024 (210) elements, binary search requires only 11 comparisons, whereas a linear search requires 1023 comparisons in the worst case. The portion of the array being searched shrinks by half after each comparison. Let low and high denote, respectively, the first index and last index of the array that is currently being searched. Initially, low is 0 and high is list.length–1. Let mid denote the index of the middle element, so mid is (low + high)/2. Figure 7.9 shows how to find key 11 in the list {2, 4, 7, 10, 11, 45, 50, 59, 60, 66, 69, 70, 79} using binary search.
7.10 Searching Arrays 267 You now know how the binary search works. The next task is to implement it in Java. Don’t rush to give a complete implementation. Implement it incrementally, one step at a time. You may start with the first iteration of the search, as shown in Figure 7.10a. It compares the key with the middle element in the list whose low index is 0 and high index is list.length - 1. If key < list[mid], set the high index to mid - 1; if key == list[mid], a match is found and return mid; if key > list[mid], set the low index to mid + 1. Next consider implementing the method to perform the search repeatedly by adding a loop, as shown in Figure 7.10b. The search ends if the key is found, or if the key is not found when low > high. When the key is not found, low is the insertion point where a key would be inserted to maintain the order of the list. It is more useful to return the insertion point than -1. The method must return a negative value to indicate that the key is not in the list. Can it simply return –low? No. If the key is less than list[0], low would be 0. -0 is 0. This would indicate that the key matches list[0]. A good choice is to let the method return –low – 1 if the key is not in the list. Returning –low – 1 indicates not only that the key is not in the list, but also where the key would be inserted.
low
key is 11 key 50
mid
high
[0] [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] list
2
4
low
7
10 11 45 50 59 60 66 69 70 79
mid
high
[0] [1] [2] [3] [4] [5] key 7
list
2
4
7
10 11 45 low mid high [3] [4] [5]
key 11
list
10 11 45
FIGURE 7.9 Binary search eliminates half of the list from further consideration after each comparison.
public static int binarySearch( int[] list, int key) { int low = 0; int high = list.length - 1;
public static int binarySearch( int[] list, int key) { int low = 0; int high = list.length - 1;
int mid = (low + high) / 2; if (key < list[mid]) high = mid - 1; else if (key == list[mid]) return mid; else low = mid + 1;
while (high >= low) { int mid = (low + high) / 2; if (key < list[mid]) high = mid - 1; else if (key == list[mid]) return mid; else low = mid + 1; } return -1; // Not found }
} (a) Version 1
FIGURE 7.10 Binary search is implemented incrementally.
(b) Version 2
why not -1?
268 Chapter 7
Single-Dimensional Arrays The complete program is given in Listing 7.7.
LISTING 7.7
first half
second half
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
BinarySearch.java
public class BinarySearch { /** Use binary search to find the key in the list */ public static int binarySearch(int[] list, int key) { int low = 0; int high = list.length - 1; while (high >= low) { int mid = (low + high) / 2; if (key < list[mid]) high = mid - 1; else if (key == list[mid]) return mid; else low = mid + 1; } return –low - 1; // Now high < low, key not found } }
The binary search returns the index of the search key if it is contained in the list (line 12). Otherwise, it returns –low – 1 (line 17). What would happen if we replaced (high >= low) in line 7 with (high > low)? The search would miss a possible matching element. Consider a list with just one element. The search would miss the element. Does the method still work if there are duplicate elements in the list? Yes, as long as the elements are sorted in increasing order. The method returns the index of one of the matching elements if the element is in the list. To better understand this method, trace it with the following statements and identify low and high when the method returns. int[] int i int j int k int l int m
list = {2, 4, 7, 10, 11, 45, 50, 59, 60, 66, 69, 70, 79}; = BinarySearch.binarySearch(list, 2); // Returns 0 = BinarySearch.binarySearch(list, 11); // Returns 4 = BinarySearch.binarySearch(list, 12); // Returns –6 = BinarySearch.binarySearch(list, 1); // Returns –1 = BinarySearch.binarySearch(list, 3); // Returns –2
Here is the table that lists the low and high values when the method exits and the value returned from invoking the method. Low
High
Value Returned
binarySearch(list, 2)
Method
0
1
0
binarySearch(list, 11)
3
5
4
binarySearch(list, 12)
5
4
-6
binarySearch(list, 1)
0
-1
-1
binarySearch(list, 3)
1
0
-2
Note binary search benefits
Linear search is useful for finding an element in a small array or an unsorted array, but it is inefficient for large arrays. Binary search is more efficient, but it requires that the array be presorted.
7.11 Sorting Arrays 269 7.21
If high is a very large integer such as the maximum int value 2147483647, (low + high) / 2 may cause overflow. How do you fix it to avoid overflow?
7.22
Use Figure 7.9 as an example to show how to apply the binary search approach to a search for key 10 and key 12 in list {2, 4, 7, 10, 11, 45, 50, 59, 60, 66, 69, 70, 79}. If the binary search method returns -4, is the key in the list? Where should the key be inserted if you wish to insert the key into the list?
7.23
✓
Check Point
7.11 Sorting Arrays Sorting, like searching, is a common task in computer programming. Many different algorithms have been developed for sorting. This section introduces an intuitive sorting algorithm: selection sort. Suppose that you want to sort a list in ascending order. Selection sort finds the smallest number in the list and swaps it with the first element. It then finds the smallest number remaining and swaps it with the second element, and so on, until only a single number remains. Figure 7.11 shows how to sort the list {2, 9, 5, 4, 8, 1, 6} using selection sort.
Key Point
VideoNote
Selection sort selection sort
swap Select 1 (the smallest) and swap it with 2 (the first) in the list.
2
9
5
4
1
6
8
2
6
Select 2 (the smallest) and swap it with 9 (the first) in the remaining list.
8
swap The number 1 is now in the correct position and thus no longer needs to be considered.
1
9
5
4
swap The number 2 is now in the correct position and thus no longer needs to be considered.
1
2
5
4
8
9
6
Select 4 (the smallest) and swap it with 5 (the first) in the remaining list.
The number 4 is now in the correct position and thus no longer needs to be considered.
1
2
4
5
8
9
6
5 is the smallest and in the right position. No swap is necessary.
6
Select 6 (the smallest) and swap it with 8 (the first) in the remaining list.
swap The number 5 is now in the correct position and thus no longer needs to be considered.
1
2
4
5
8
9
swap The number 6 is now in the correct position and thus no longer needs to be considered.
1
2
4
5
6
9
8
Select 8 (the smallest) and swap it with 9 (the first) in the remaining list.
The number 8 is now in the correct position and thus no longer needs to be considered.
1
2
4
5
6
8
9
Since there is only one element remaining in the list, the sort is completed.
FIGURE 7.11 Selection sort repeatedly selects the smallest number and swaps it with the first number in the list. You know how the selection-sort approach works. The task now is to implement it in Java. Beginners find it difficult to develop a complete solution on the first attempt. Start by writing the code for the first iteration to find the smallest element in the list and swap it with the first element, and then observe what would be different for the second iteration, the third, and so on. The insight this gives will enable you to write a loop that generalizes all the iterations.
selection sort animation on Companion Website
270 Chapter 7
Single-Dimensional Arrays The solution can be described as follows: for (int i = 0; i < list.length - 1; i++) { select the smallest element in list[i..list.length-1]; swap the smallest with list[i], if necessary; // list[i] is in its correct position. // The next iteration applies on list[i+1..list.length-1] }
Listing 7.8 implements the solution.
LISTING 7.8 SelectionSort.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
select
swap
public class SelectionSort { /** The method for sorting the numbers */ public static void selectionSort(double[] list) { for (int i = 0; i < list.length - 1; i++) { // Find the minimum in the list[i..list.length-1] double currentMin = list[i]; int currentMinIndex = i; for (int j = i + 1; j < list.length; j++) { if (currentMin > list[j]) { currentMin = list[j]; currentMinIndex = j; } } // Swap list[i] with list[currentMinIndex] if necessary if (currentMinIndex != i) { list[currentMinIndex] = list[i]; list[i] = currentMin; } } } }
The selectionSort(double[] list) method sorts any array of double elements. The method is implemented with a nested for loop. The outer loop (with the loop control variable i) (line 4) is iterated in order to find the smallest element in the list, which ranges from list[i] to list[list.length-1], and exchange it with list[i]. The variable i is initially 0. After each iteration of the outer loop, list[i] is in the right place. Eventually, all the elements are put in the right place; therefore, the whole list is sorted. To understand this method better, trace it with the following statements: double[] list = {1, 9, 4.5, 6.6, 5.7, -4.5}; SelectionSort.selectionSort(list);
✓
Check Point
7.24 7.25
Use Figure 7.11 as an example to show how to apply the selection-sort approach to sort {3.4, 5, 3, 3.5, 2.2, 1.9, 2}. How do you modify the selectionSort method in Listing 7.8 to sort numbers in decreasing order?
7.12 The Arrays Class Key Point
The java.util.Arrays class contains useful methods for common array operations such as sorting and searching.
7.12 The Arrays Class 271 The java.util.Arrays class contains various static methods for sorting and searching arrays, comparing arrays, filling array elements, and returning a string representation of the array. These methods are overloaded for all primitive types. You can use the sort or parallelSort method to sort a whole array or a partial array. For example, the following code sorts an array of numbers and an array of characters.
sort parallelSort
double[] numbers = {6.0, 4.4, 1.9, 2.9, 3.4, 3.5}; java.util.Arrays.sort(numbers); // Sort the whole array java.util.Arrays.parallelSort(numbers); // Sort the whole array char[] chars = {'a', 'A', '4', 'F', 'D', 'P'}; java.util.Arrays.sort(chars, 1, 3); // Sort part of the array java.util.Arrays.parallelSort(chars, 1, 3); // Sort part of the array
Invoking sort(numbers) sorts the whole array numbers. Invoking sort(chars, 1, 3) sorts a partial array from chars[1] to chars[3-1]. parallelSort is more efficient if your computer has multiple processors. You can use the binarySearch method to search for a key in an array. The array must be presorted in increasing order. If the key is not in the array, the method returns –(insertionIndex + 1). For example, the following code searches the keys in an array of integers and an array of characters.
binarySearch
int[] list = {2, 4, 7, 10, 11, 45, 50, 59, 60, 66, 69, 70, 79}; System.out.println("1. Index is " + java.util.Arrays.binarySearch(list, 11)); System.out.println("2. Index is " + java.util.Arrays.binarySearch(list, 12)); char[] chars = {'a', 'c', 'g', 'x', 'y', 'z'}; System.out.println("3. Index is " + java.util.Arrays.binarySearch(chars, 'a')); System.out.println("4. Index is " + java.util.Arrays.binarySearch(chars, 't'));
The output of the preceding code is 1. Index is 4 2. Index is -6 3. Index is 0 4. Index is -4 You can use the equals method to check whether two arrays are strictly equal. Two arrays are strictly equal if their corresponding elements are the same. In the following code, list1 and list2 are equal, but list2 and list3 are not.
equals
int[] list1 = {2, 4, 7, 10}; int[] list2 = {2, 4, 7, 10}; int[] list3 = {4, 2, 7, 10}; System.out.println(java.util.Arrays.equals(list1, list2)); // true System.out.println(java.util.Arrays.equals(list2, list3)); // false
You can use the fill method to fill in all or part of the array. For example, the following code fills list1 with 5 and fills 8 into elements list2[1] through list2[5-1]. int[] list1 = {2, 4, 7, 10}; int[] list2 = {2, 4, 7, 7, 7, 10}; java.util.Arrays.fill(list1, 5); // Fill 5 to the whole array java.util.Arrays.fill(list2, 1, 5, 8); // Fill 8 to a partial array
fill
272 Chapter 7
Single-Dimensional Arrays You can also use the toString method to return a string that represents all elements in the array. This is a quick and simple way to display all elements in the array. For example, the following code
toString
int[] list = {2, 4, 7, 10}; System.out.println(Arrays.toString(list));
displays [2, 4, 7, 10].
✓
Check Point
7.26 7.27 7.28
What types of array can be sorted using the java.util.Arrays.sort method? Does this sort method create a new array? To apply java.util.Arrays.binarySearch(array, key), should the array be sorted in increasing order, in decreasing order, or neither? Show the output of the following code: int[] list1 = {2, 4, 7, 10}; java.util.Arrays.fill(list1, 7); System.out.println(java.util.Arrays.toString(list1)); int[] list2 = {2, 4, 7, 10}; System.out.println(java.util.Arrays.toString(list2)); System.out.print(java.util.Arrays.equals(list1, list2));
7.13 Command-Line Arguments Key Point
VideoNote
Command-line arguments
The main method can receive string arguments from the command line. Perhaps you have already noticed the unusual header for the main method, which has the parameter args of String[] type. It is clear that args is an array of strings. The main method is just like a regular method with a parameter. You can call a regular method by passing actual parameters. Can you pass arguments to main? Yes, of course you can. In the following examples, the main method in class TestMain is invoked by a method in A.
public class A { public static void main(String[] args) { String[] strings = {"New York", "Boston", "Atlanta"}; TestMain.main(strings); } }
public class TestMain { public static void main(String[] args) { for (int i = 0; i < args.length; i++) System.out.println(args[i]); } }
A main method is just a regular method. Furthermore, you can pass arguments from the command line.
7.13.1
Passing Strings to the main Method
You can pass strings to a main method from the command line when you run the program. The following command line, for example, starts the program TestMain with three strings: arg0, arg1, and arg2: java TestMain arg0 arg1 arg2
arg0, arg1, and arg2 are strings, but they don’t have to appear in double quotes on the command line. The strings are separated by a space. A string that contains a space must be enclosed in double quotes. Consider the following command line: java TestMain "First num" alpha 53
7.13 Command-Line Arguments 273 It starts the program with three strings: First num, alpha, and 53. Since First num is a string, it is enclosed in double quotes. Note that 53 is actually treated as a string. You can use "53" instead of 53 in the command line. When the main method is invoked, the Java interpreter creates an array to hold the command-line arguments and pass the array reference to args. For example, if you invoke a program with n arguments, the Java interpreter creates an array like this one: args = new String[n];
The Java interpreter then passes args to invoke the main method.
Note If you run the program with no strings passed, the array is created with new String[0]. In this case, the array is empty with length 0. args references to this empty array. Therefore, args is not null, but args.length is 0.
7.13.2
Case Study: Calculator
Suppose you are to develop a program that performs arithmetic operations on integers. The program receives an expression in one string argument. The expression consists of an integer followed by an operator and another integer. For example, to add two integers, use this command: java Calculator 2 + 3
The program will display the following output: 2 + 3 = 5
Figure 7.12 shows sample runs of the program. The strings passed to the main program are stored in args, which is an array of strings. The first string is stored in args[0], and args.length is the number of strings passed. Here are the steps in the program: 1. Use args.length to determine whether the expression has been provided as three arguments in the command line. If not, terminate the program using System.exit(1). 2. Perform a binary arithmetic operation on the operands args[0] and args[2] using the operator in args[1].
Add Subtract Multiply Divide
FIGURE 7.12 The program takes three arguments (operand1 operator operand2) from the command line and displays the expression and the result of the arithmetic operation.
VideoNote
Command-line argument
274 Chapter 7
Single-Dimensional Arrays The program is shown in Listing 7.9.
LISTING 7.9 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
check argument
check operator
Calculator.java
public class Calculator { /** Main method */ public static void main(String[] args) { // Check number of strings passed if (args.length != 3) { System.out.println( "Usage: java Calculator operand1 operator operand2"); System.exit(0); } // The result of the operation int result = 0; // Determine the operator switch (args[1].charAt(0)) { case '+': result = Integer.parseInt(args[0]) + Integer.parseInt(args[2]); break; case '-': result = Integer.parseInt(args[0]) Integer.parseInt(args[2]); break; case '.': result = Integer.parseInt(args[0]) * Integer.parseInt(args[2]); break; case '/': result = Integer.parseInt(args[0]) / Integer.parseInt(args[2]); } // Display result System.out.println(args[0] + ' ' + args[1] + ' ' + args[2] + " = " + result); } }
Integer.parseInt(args[0]) (line 16) converts a digital string into an integer. The string must consist of digits. If not, the program will terminate abnormally. We used the . symbol for multiplication, not the common * symbol. The reason for this is that the * symbol refers to all the files in the current directory when it is used on a command line. The following program displays all the files in the current directory when issuing the command java Test *: public class Test { public static void main(String[] args) { for (int i = 0; i < args.length; i++) System.out.println(args[i]); } }
To circumvent this problem, we will have to use a different symbol for the multiplication operator.
✓
Check Point
7.29
This book declares the main method as public static void main(String[] args)
Can it be replaced by one of the following lines? public static void main(String args[]) public static void main(String[] x)
Chapter Summary 275 public static void main(String x[]) static void main(String x[])
7.30
Show the output of the following program when invoked using 1. java Test I have a dream 2. java Test “1 2 3” 3. java Test public class Test { public static void main(String[] args) { System.out.println("Number of strings is " + args.length); for (int i = 0; i < args.length; i++) System.out.println(args[i]); } }
KEY TERMS anonymous array 258 array 246 array initializer 248 binary search 265 garbage collection 256
index 246 indexed variable 248 linear search 265 off-by-one error 251 selection sort 269
CHAPTER SUMMARY 1. A variable is declared as an array type using the syntax elementType[]
arrayRefVar
or elementType arrayRefVar[]. The style elementType[] arrayRefVar is preferred, although elementType arrayRefVar[] is legal.
2. Unlike declarations for primitive data type variables, the declaration of an array variable does not allocate any space in memory for the array. An array variable is not a primitive data type variable. An array variable contains a reference to an array.
3. You cannot assign elements to an array unless it has already been created. You can create an array by using the new operator with the following syntax: new elementType[arraySize].
4. Each element in the array is represented using the syntax arrayRefVar[index]. An index must be an integer or an integer expression.
5. After an array is created, its size becomes permanent and can be obtained using arrayRefVar.length. Since the index of an array always begins with 0, the last index is always arrayRefVar.length - 1. An out-of-bounds error will occur if you
attempt to reference elements beyond the bounds of an array.
6. Programmers often mistakenly reference the first element in an array with index 1, but it should be 0. This is called the index off-by-one error.
276 Chapter 7
Single-Dimensional Arrays 7. When an array is created, its elements are assigned the default value of 0 for the numeric primitive data types, \u0000 for char types, and false for boolean types.
8. Java has a shorthand notation, known as the array initializer, which combines declaring an array, creating an array, and initializing an array in one statement, using the syntax elementType[] arrayRefVar = {value0, value1, ..., valuek}.
9. When you pass an array argument to a method, you are actually passing the reference of the array; that is, the called method can modify the elements in the caller’s original array.
10. If an array is sorted, binary search is more efficient than linear search for finding an element in the array.
11. Selection sort finds the smallest number in the list and swaps it with the first element. It then finds the smallest number remaining and swaps it with the first element in the remaining list, and so on, until only a single number remains.
QUIZ Answer the quiz for this chapter online at www.cs.armstrong.edu/liang/intro10e/quiz.html.
PROGRAMMING EXERCISES Sections 7.2–7.5
*7.1 (Assign grades) Write a program that reads student scores, gets the best score, and then assigns grades based on the following scheme: Grade is A if score is Ú best - 10 Grade is B if score is Ú best - 20; Grade is C if score is Ú best - 30; Grade is D if score is Ú best - 40; Grade is F otherwise. The program prompts the user to enter the total number of students, then prompts the user to enter all of the scores, and concludes by displaying the grades. Here is a sample run:
Enter the number of students: 4 Enter 4 Student Student Student Student
scores: 0 score 1 score 2 score 3 score
40 is is is is
55 40 55 70 58
70 58 and grade and grade and grade and grade
is is is is
C B A B
7.2 (Reverse the numbers entered) Write a program that reads ten integers and displays them in the reverse of the order in which they were read.
Programming Exercises 277 **7.3 (Count occurrence of numbers) Write a program that reads the integers between 1 and 100 and counts the occurrences of each. Assume the input ends with 0. Here is a sample run of the program:
Enter the integers between 1 and 100: 2 5 6 5 4 3 23 43 2 0 2 occurs 2 times 3 occurs 1 time 4 occurs 1 time 5 occurs 2 times 6 occurs 1 time 23 occurs 1 time 43 occurs 1 time
Note that if a number occurs more than one time, the plural word “times” is used in the output.
7.4 (Analyze scores) Write a program that reads an unspecified number of scores and
**7.5
determines how many scores are above or equal to the average and how many scores are below the average. Enter a negative number to signify the end of the input. Assume that the maximum number of scores is 100. (Print distinct numbers) Write a program that reads in ten numbers and displays the number of distinct numbers and the distinct numbers separated by exactly one space (i.e., if a number appears multiple times, it is displayed only once). (Hint: Read a number and store it to an array if it is new. If the number is already in the array, ignore it.) After the input, the array contains the distinct numbers. Here is the sample run of the program:
Enter ten numbers: 1 2 3 2 1 6 3 4 5 2 The number of distinct number is 6 The distinct numbers are: 1 2 3 6 4 5
*7.6 (Revise Listing 5.15, PrimeNumber.java) Listing 5.15 determines whether a num-
*7.7
ber n is prime by checking whether 2, 3, 4, 5, 6, . . . , n/2 is a divisor. If a divisor is found, n is not prime. A more efficient approach is to check whether any of the prime numbers less than or equal to 2n can divide n evenly. If not, n is prime. Rewrite Listing 5.15 to display the first 50 prime numbers using this approach. You need to use an array to store the prime numbers and later use them to check whether they are possible divisors for n. (Count single digits) Write a program that generates 100 random integers between 0 and 9 and displays the count for each number. (Hint: Use an array of ten integers, say counts, to store the counts for the number of 0s, 1s, . . . , 9s.)
Sections 7.6–7.8
7.8 (Average an array) Write two overloaded methods that return the average of an array with the following headers: public static int average(int[] array) public static double average(double[] array)
Write a test program that prompts the user to enter ten double values, invokes this method, and displays the average value.
278 Chapter 7
Single-Dimensional Arrays 7.9 (Find the smallest element) Write a method that finds the smallest element in an array of double values using the following header: public static double min(double[] array)
Write a test program that prompts the user to enter ten numbers, invokes this method to return the minimum value, and displays the minimum value. Here is a sample run of the program:
Enter ten numbers: 1.9 2.5 3.7 2 1.5 6 3 4 5 2 The minimum number is: 1.5
7.10 (Find the index of the smallest element) Write a method that returns the index of the smallest element in an array of integers. If the number of such elements is greater than 1, return the smallest index. Use the following header: public static int indexOfSmallestElement(double[] array)
*7.11
Write a test program that prompts the user to enter ten numbers, invokes this method to return the index of the smallest element, and displays the index. (Statistics: compute deviation) Programming Exercise 5.45 computes the standard deviation of numbers. This exercise uses a different but equivalent formula to compute the standard deviation of n numbers. n
mean =
a xi
i=1
n
n
=
x1 + x2 + g + xn n
deviation =
2 a (xi - mean)
i=1
H
n - 1
To compute the standard deviation with this formula, you have to store the individual numbers using an array, so that they can be used after the mean is obtained. Your program should contain the following methods: /** Compute the deviation of double values */ public static double deviation(double[] x) /** Compute the mean of an array of double values */ public static double mean(double[] x)
Write a test program that prompts the user to enter ten numbers and displays the mean and standard deviation, as shown in the following sample run:
Enter ten numbers: 1.9 2.5 3.7 2 1 6 3 4 5 2 The mean is 3.11 The standard deviation is 1.55738
*7.12 (Reverse an array) The
reverse method in Section 7.7 reverses an array by copying it to a new array. Rewrite the method that reverses the array passed in the argument and returns this array. Write a test program that prompts the user to
Programming Exercises 279 enter ten numbers, invokes the method to reverse the numbers, and displays the numbers.
Section 7.9
*7.13 (Random number chooser) Write a method that returns a random number between 1 and 54, excluding the numbers passed in the argument. The method header is specified as follows: public static int getRandom(int... numbers)
7.14 (Computing gcd) Write a method that returns the gcd of an unspecified number of integers. The method header is specified as follows: public static int gcd(int... numbers)
Write a test program that prompts the user to enter five numbers, invokes the method to find the gcd of these numbers, and displays the gcd.
Sections 7.10–7.12
7.15 (Eliminate duplicates) Write a method that returns a new array by eliminating the duplicate values in the array using the following method header: public static int[] eliminateDuplicates(int[] list)
Write a test program that reads in ten integers, invokes the method, and displays the result. Here is the sample run of the program:
Enter ten numbers: 1 2 3 2 1 6 3 4 5 2 The distinct numbers are: 1 2 3 6 4 5
7.16 (Execution time) Write a program that randomly generates an array of 100,000 integers and a key. Estimate the execution time of invoking the linearSearch method in Listing 7.6. Sort the array and estimate the execution time of invoking the binarySearch method in Listing 7.7. You can use the following code template to obtain the execution time: long startTime = System.currentTimeMillis(); perform the task; long endTime = System.currentTimeMillis(); long executionTime = endTime - startTime;
**7.17 (Sort students) Write a program that prompts the user to enter the number of stu**7.18
dents, the students’ names, and their scores, and prints student names in decreasing order of their scores. (Bubble sort) Write a sort method that uses the bubble-sort algorithm. The bubblesort algorithm makes several passes through the array. On each pass, successive neighboring pairs are compared. If a pair is not in order, its values are swapped; otherwise, the values remain unchanged. The technique is called a bubble sort or sinking sort because the smaller values gradually “bubble” their way to the top and the larger values “sink” to the bottom. Write a test program that reads in ten double numbers, invokes the method, and displays the sorted numbers.
280 Chapter 7
Single-Dimensional Arrays **7.19 (Sorted?) Write the following method that returns true if the list is already sorted in increasing order. public static boolean isSorted(int[] list)
Write a test program that prompts the user to enter a list and displays whether the list is sorted or not. Here is a sample run. Note that the first number in the input indicates the number of the elements in the list. This number is not part of the list.
Enter list: 8 10 1 5 16 61 9 11 1 The list is not sorted
Enter list: 10 1 1 3 4 4 5 7 9 11 21 The list is already sorted
*7.20 (Revise selection sort) In Section 7.11, you used selection sort to sort an array.
***7.21
The selection-sort method repeatedly finds the smallest number in the current array and swaps it with the first. Rewrite this program by finding the largest number and swapping it with the last. Write a test program that reads in ten double numbers, invokes the method, and displays the sorted numbers. (Game: bean machine) The bean machine, also known as a quincunx or the Galton box, is a device for statistics experiments named after English scientist Sir Francis Galton. It consists of an upright board with evenly spaced nails (or pegs) in a triangular form, as shown in Figure 7.13.
(a)
(b)
(c)
FIGURE 7.13 Each ball takes a random path and falls into a slot. Balls are dropped from the opening of the board. Every time a ball hits a nail, it has a 50% chance of falling to the left or to the right. The piles of balls are accumulated in the slots at the bottom of the board. Write a program that simulates the bean machine. Your program should prompt the user to enter the number of the balls and the number of the slots in the machine. Simulate the falling of each ball by printing its path. For example, the path for the ball in Figure 7.13b is LLRRLLR and the path for the ball in Figure 7.13c is
Programming Exercises 281 RLRRLRR. Display the final buildup of the balls in the slots in a histogram. Here is a sample run of the program:
Enter the number of balls to drop: 5 Enter the number of slots in the bean machine: 8 LRLRLRR RRLLLRR LLRLLRR RRLLLLL LRLRRLR O O OOO
(Hint: Create an array named slots. Each element in slots stores the number of balls in a slot. Each ball falls into a slot via a path. The number of Rs in a path is the position of the slot where the ball falls. For example, for the path LRLRLRR, the ball falls into slots[4], and for the path is RRLLLLL, the ball falls into slots[2].)
***7.22 (Game: Eight Queens) The classic Eight Queens puzzle is to place eight queens on a chessboard such that no two queens can attack each other (i.e., no two queens are on the same row, same column, or same diagonal). There are many possible solutions. Write a program that displays one such solution. A sample output is shown below: |Q| | | | | | | | | | | | |Q| | | | | | | | | | | |Q| | | | | | |Q| | | | | |Q| | | | | | | | | | | | |Q| | | |Q| | | | | | | | | | |Q| | | | |
**7.23 (Game: locker puzzle) A school has 100 lockers and 100 students. All lockers are
**7.24
closed on the first day of school. As the students enter, the first student, denoted S1, opens every locker. Then the second student, S2, begins with the second locker, denoted L2, and closes every other locker. Student S3 begins with the third locker and changes every third locker (closes it if it was open, and opens it if it was closed). Student S4 begins with locker L4 and changes every fourth locker. Student S5 starts with L5 and changes every fifth locker, and so on, until student S100 changes L100. After all the students have passed through the building and changed the lockers, which lockers are open? Write a program to find your answer and display all open locker numbers separated by exactly one space. (Hint: Use an array of 100 Boolean elements, each of which indicates whether a locker is open (true) or closed (false). Initially, all lockers are closed.) (Simulation: coupon collector’s problem) Coupon collector is a classic statistics problem with many practical applications. The problem is to pick objects from a set of objects repeatedly and find out how many picks are needed for all the
VideoNote
Coupon collector’s problem
282 Chapter 7
Single-Dimensional Arrays objects to be picked at least once. A variation of the problem is to pick cards from a shuffled deck of 52 cards repeatedly and find out how many picks are needed before you see one of each suit. Assume a picked card is placed back in the deck before picking another. Write a program to simulate the number of picks needed to get four cards from each suit and display the four cards picked (it is possible a card may be picked twice). Here is a sample run of the program:
Queen of Spades 5 of Clubs Queen of Hearts 4 of Diamonds Number of picks: 12
7.25 (Algebra: solve quadratic equations) Write a method for solving a quadratic equation using the following header: public static int solveQuadratic(double[] eqn, double[] roots)
7.26
The coefficients of a quadratic equation ax2 + bx + c = 0 are passed to the array eqn and the real roots are stored in roots. The method returns the number of real roots. See Programming Exercise 3.1 on how to solve a quadratic equation. Write a program that prompts the user to enter values for a, b, and c and displays the number of real roots and all real roots. (Strictly identical arrays) The arrays list1 and list2 are strictly identical if their corresponding elements are equal. Write a method that returns true if list1 and list2 are strictly identical, using the following header: public static boolean equals(int[] list1, int[] list2)
Write a test program that prompts the user to enter two lists of integers and displays whether the two are strictly identical. Here are the sample runs. Note that the first number in the input indicates the number of the elements in the list. This number is not part of the list.
Enter list1: 5 2 5 6 1 6 Enter list2: 5 2 5 6 1 6 Two lists are strictly identical
Enter list1: 5 2 5 6 6 1 Enter list2: 5 2 5 6 1 6 Two lists are not strictly identical
7.27 (Identical arrays) The arrays
list1 and list2 are identical if they have the same contents. Write a method that returns true if list1 and list2 are identical, using the following header:
public static boolean equals(int[] list1, int[] list2)
Programming Exercises 283 Write a test program that prompts the user to enter two lists of integers and displays whether the two are identical. Here are the sample runs. Note that the first number in the input indicates the number of the elements in the list. This number is not part of the list.
Enter list1: 5 2 5 6 6 1 Enter list2: 5 5 2 6 1 6 Two lists are identical
Enter list1: 5 5 5 6 6 1 Enter list2: 5 2 5 6 1 6 Two lists are not identical
*7.28 (Math: combinations) Write a program that prompts the user to enter 10 integers and displays all combinations of picking two numbers from the 10.
*7.29 (Game: pick four cards) Write a program that picks four cards from a deck of 52
*7.30
cards and computes their sum. An Ace, King, Queen, and Jack represent 1, 13, 12, and 11, respectively. Your program should display the number of picks that yields the sum of 24. (Pattern recognition: consecutive four equal numbers) Write the following method that tests whether the array has four consecutive numbers with the same value. public static boolean isConsecutiveFour(int[] values)
Write a test program that prompts the user to enter a series of integers and displays if the series contains four consecutive numbers with the same value. Your program should first prompt the user to enter the input size—i.e., the number of values in the series. Here are sample runs:
Enter the number of values: 8 Enter the values: 3 4 5 5 5 5 4 5 The list has consecutive fours
Enter the number of values: 9 Enter the values: 3 4 5 5 6 5 5 4 5 The list has no consecutive fours
**7.31 (Merge two sorted lists) Write the following method that merges two sorted lists into a new sorted list. public static int[] merge(int[] list1, int[] list2)
VideoNote
Consecutive four
284 Chapter 7
Single-Dimensional Arrays Implement the method in a way that takes at most list1.length + list2. length comparisons. Write a test program that prompts the user to enter two sorted lists and displays the merged list. Here is a sample run. Note that the first number in the input indicates the number of the elements in the list. This number is not part of the list.
Enter list1: 5 1 5 16 61 111 Enter list2: 4 2 4 5 6 The merged list is 1 2 4 5 5 6 16 61 111
**7.32 (Partition of a list) Write the following method that partitions the list using the first element, called a pivot. public static int partition(int[] list)
After the partition, the elements in the list are rearranged so that all the elements before the pivot are less than or equal to the pivot and the elements after the pivot are greater than the pivot. The method returns the index where the pivot is located in the new list. For example, suppose the list is {5, 2, 9, 3, 6, 8}. After the partition, the list becomes {3, 2, 5, 9, 6, 8}. Implement the method in a way that takes at most list.length comparisons. Write a test program that prompts the user to enter a list and displays the list after the partition. Here is a sample run. Note that the first number in the input indicates the number of the elements in the list. This number is not part of the list.
Enter list: 8 10 1 5 16 61 9 11 1 After the partition, the list is 9 1 5 1 10 61 11 16
*7.33 (Culture: Chinese Zodiac) Simplify Listing 3.9 using an array of strings to store **7.34
the animal names. (Sort characters in a string) Write a method that returns a sorted string using the following header: public static String sort(String s)
***7.35
For example, sort("acb") returns abc. Write a test program that prompts the user to enter a string and displays the sorted string. (Game: hangman) Write a hangman game that randomly generates a word and prompts the user to guess one letter at a time, as shown in the sample run. Each letter in the word is displayed as an asterisk. When the user makes a correct guess, the actual letter is then displayed. When the user finishes a word, display
Programming Exercises 285 the number of misses and ask the user whether to continue to play with another word. Declare an array to store words, as follows: // Add any words you wish in this array String[] words = {"write", "that", ...};
(Guess) Enter a letter in word ******* > p (Guess) Enter a letter in word p****** > r (Guess) Enter a letter in word pr**r** > p p is already in the word (Guess) Enter a letter in word pr**r** > o (Guess) Enter a letter in word pro*r** > g (Guess) Enter a letter in word progr** > n n is not in the word (Guess) Enter a letter in word progr** > m (Guess) Enter a letter in word progr*m > a The word is program. You missed 1 time Do you want to guess another word? Enter y or n>
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CHAPTER
8 MULTIDIMENSIONAL ARRAYS Objectives ■
To give examples of representing data using two-dimensional arrays (§8.1).
■
To declare variables for two-dimensional arrays, create arrays, and access array elements in a two-dimensional array using row and column indexes (§8.2).
■
To program common operations for two-dimensional arrays (displaying arrays, summing all elements, finding the minimum and maximum elements, and random shuffling) (§8.3).
■
To pass two-dimensional arrays to methods (§8.4).
■
To write a program for grading multiple-choice questions using twodimensional arrays (§8.5).
■
To solve the closest-pair problem using two-dimensional arrays (§8.6).
■
To check a Sudoku solution using two-dimensional arrays (§8.7).
■
To use multidimensional arrays (§8.8).
288 Chapter 8
Multidimensional Arrays
8.1 Introduction Key Point
problem
Data in a table or a matrix can be represented using a two-dimensional array. The preceding chapter introduced how to use one-dimensional arrays to store linear collections of elements. You can use a two-dimensional array to store a matrix or a table. For example, the following table that lists the distances between cities can be stored using a twodimensional array named distances.
Distance Table (in miles) Chicago Boston New York Atlanta Miami Dallas Houston
Chicago
Boston
New York
Atlanta
Miami
Dallas
Houston
0 983 787 714 1375 967 1087
983 0 214 1102 1763 1723 1842
787 214 0 888 1549 1548 1627
714 1102 888 0 661 781 810
1375 1763 1549 661 0 1426 1187
967 1723 1548 781 1426 0 239
1087 1842 1627 810 1187 239 0
double[][] distances = { {0, 983, 787, 714, 1375, 967, 1087}, {983, 0, 214, 1102, 1763, 1723, 1842}, {787, 214, 0, 888, 1549, 1548, 1627}, {714, 1102, 888, 0, 661, 781, 810}, {1375, 1763, 1549, 661, 0, 1426, 1187}, {967, 1723, 1548, 781, 1426, 0, 239}, {1087, 1842, 1627, 810, 1187, 239, 0}, };
8.2 Two-Dimensional Array Basics Key Point
An element in a two-dimensional array is accessed through a row and column index. How do you declare a variable for two-dimensional arrays? How do you create a twodimensional array? How do you access elements in a two-dimensional array? This section addresses these issues.
8.2.1
Declaring Variables of Two-Dimensional Arrays and Creating Two-Dimensional Arrays
The syntax for declaring a two-dimensional array is: elementType[][] arrayRefVar;
or elementType arrayRefVar[][]; // Allowed, but not preferred
As an example, here is how you would declare a two-dimensional array variable matrix of int values: int[][] matrix;
8.2 Two-Dimensional Array Basics 289 or int matrix[][]; // This style is allowed, but not preferred
You can create a two-dimensional array of 5-by-5 int values and assign it to matrix using this syntax: matrix = new int[5][5];
Two subscripts are used in a two-dimensional array, one for the row and the other for the column. As in a one-dimensional array, the index for each subscript is of the int type and starts from 0, as shown in Figure 8.1a. [0][1][2][3][4]
[0][1][2][3][4]
[0][1][2]
[0] 0
0
0
0
0
[0] 0
0
0
0
0
[0] 1
2
3
[1] 0
0
0
0
0
[1] 0
0
0
0
0
[1] 4
5
6
[2] 0
0
0
0
0
[2] 0
7
0
0
0
[2] 7
8
9
[3] 0
0
0
0
0
[3] 0
0
0
0
0
[3] 10 11 12
[4] 0
0
0
0
0
[4] 0
0
0
0
0
matrix = new int[5][5];
matrix[2][1] = 7;
(a)
(b)
int[][] array = { {1, 2, 3}, {4, 5, 6}, {7, 8, 9}, {10, 11, 12} }; (c)
FIGURE 8.1 The index of each subscript of a two-dimensional array is an int value, starting from 0. To assign the value 7 to a specific element at row 2 and column 1, as shown in Figure 8.1b, you can use the following syntax: matrix[2][1] = 7;
Caution It is a common mistake to use matrix[2, 1] to access the element at row 2 and column 1. In Java, each subscript must be enclosed in a pair of square brackets.
You can also use an array initializer to declare, create, and initialize a two-dimensional array. For example, the following code in (a) creates an array with the specified initial values, as shown in Figure 8.1c. This is equivalent to the code in (b). int[][] array = { {1, 2, 3}, {4, 5, 6}, {7, 8, 9}, {10, 11, 12} }; (a)
8.2.2
Equivalent
int[][] array array[0][0] = array[1][0] = array[2][0] = array[3][0] =
= new int[4][3]; 1; array[0][1] = 2; array[0][2] = 4; array[1][1] = 5; array[1][2] = 7; array[2][1] = 8; array[2][2] = 10; array[3][1] = 11; array[3][2]
(b)
Obtaining the Lengths of Two-Dimensional Arrays
A two-dimensional array is actually an array in which each element is a one-dimensional array. The length of an array x is the number of elements in the array, which can be obtained using x.length. x[0], x[1], . . . , and x[x.length-1] are arrays. Their lengths can be obtained using x[0].length, x[1].length, . . . , and x[x.length-1].length.
3; 6; 9; = 12;
290 Chapter 8
Multidimensional Arrays For example, suppose x = new int[3][4], x[0], x[1], and x[2] are one-dimensional arrays and each contains four elements, as shown in Figure 8.2. x.length is 3, and x[0].length, x[1].length, and x[2].length are 4. x
x[0][0] x[0][1] x[0][2] x[0][3]
x[0].length is 4
x[1][0] x[1][1] x[1][2] x[1][3]
x[1].length is 4
x[2][0] x[2][1] x[2][2] x[2][3]
x[2].length is 4
x[0] x[1] x[2] x.length is 3
FIGURE 8.2 A two-dimensional array is a one-dimensional array in which each element is another one-dimensional array.
8.2.3 ragged array
Ragged Arrays
Each row in a two-dimensional array is itself an array. Thus, the rows can have different lengths. An array of this kind is known as a ragged array. Here is an example of creating a ragged array: int[][] triangleArray = { {1, 2, 3, 4, 5}, {2, 3, 4, 5}, {3, 4, 5}, {4, 5}, {5} };
1 2 3 4 5 2 3 4 5 3 4 5 4 5 5
As you can see, triangleArray[0].length is 5, triangleArray[1].length is 4, triangleArray[2].length is 3, triangleArray[3].length is 2, and triangleArray[4].length is 1. If you don’t know the values in a ragged array in advance, but do know the sizes—say, the same as before—you can create a ragged array using the following syntax: int[][] triangleArray = new int[5][]; triangleArray[0] = new int[5]; triangleArray[1] = new int[4]; triangleArray[2] = new int[3]; triangleArray[3] = new int[2]; triangleArray[4] = new int[1];
You can now assign values to the array. For example, triangleArray[0][3] = 50; triangleArray[4][0] = 45;
Note The syntax new int[5][] for creating an array requires the first index to be specified. The syntax new int[][] would be wrong.
8.3 Processing Two-Dimensional Arrays 291 8.1
Declare an array reference variable for a two-dimensional array of int values, create a 4-by-5 int matrix, and assign it to the variable.
8.2 8.3
Can the rows in a two-dimensional array have different lengths? What is the output of the following code?
✓
Check Point
int[][] array = new int[5][6]; int[] x = {1, 2}; array[0] = x; System.out.println("array[0][1] is " + array[0][1]);
8.4
Which of the following statements are valid? int[][] int[] x int[][] int[][] int[][] int[][]
r = y z m n
= new int[2]; new int[]; = new int[3][]; = {{1, 2}}; = {{1, 2}, {2, 3}}; = {{1, 2}, {2, 3}, };
8.3 Processing Two-Dimensional Arrays Nested for loops are often used to process a two-dimensional array. Suppose an array matrix is created as follows: int[][] matrix = new int[10][10];
The following are some examples of processing two-dimensional arrays. 1. Initializing arrays with input values. The following loop initializes the array with user input values: java.util.Scanner input = new Scanner(System.in); System.out.println("Enter " + matrix.length + " rows and " + matrix[0].length + " columns: "); for (int row = 0; row < matrix.length; row++) { for (int column = 0; column < matrix[row].length; column++) { matrix[row][column] = input.nextInt(); } }
2. Initializing arrays with random values. The following loop initializes the array with random values between 0 and 99: for (int row = 0; row < matrix.length; row++) { for (int column = 0; column < matrix[row].length; column++) { matrix[row][column] = (int)(Math.random() * 100); } }
3. Printing arrays. To print a two-dimensional array, you have to print each element in the array using a loop like the following: for (int row = 0; row < matrix.length; row++) { for (int column = 0; column < matrix[row].length; column++) { System.out.print(matrix[row][column] + " "); } System.out.println(); }
Key Point
292 Chapter 8
Multidimensional Arrays 4. Summing all elements. Use a variable named total to store the sum. Initially total is 0. Add each element in the array to total using a loop like this: int total = 0; for (int row = 0; row < matrix.length; row++) { for (int column = 0; column < matrix[row].length; column++) { total += matrix[row][column]; } }
5. Summing elements by column. For each column, use a variable named total to store its sum. Add each element in the column to total using a loop like this: for (int column = 0; column < matrix[0].length; column++) { int total = 0; for (int row = 0; row < matrix.length; row++) total += matrix[row][column]; System.out.println("Sum for column " + column + " is " + total); }
VideoNote
Find the row with the largest sum
6. Which row has the largest sum? Use variables maxRow and indexOfMaxRow to track the largest sum and index of the row. For each row, compute its sum and update maxRow and indexOfMaxRow if the new sum is greater. int maxRow = 0; int indexOfMaxRow = 0; // Get sum of the first row in maxRow for (int column = 0; column < matrix[0].length; column++) { maxRow += matrix[0][column]; } for (int row = 1; row < matrix.length; row++) { int totalOfThisRow = 0; for (int column = 0; column < matrix[row].length; column++) totalOfThisRow += matrix[row][column]; if (totalOfThisRow > maxRow) { maxRow = totalOfThisRow; indexOfMaxRow = row; } } System.out.println("Row " + indexOfMaxRow + " has the maximum sum of " + maxRow);
7. Random shuffling. Shuffling the elements in a one-dimensional array was introduced in Section 7.2.6. How do you shuffle all the elements in a two-dimensional array? To accomplish this, for each element matrix[i][j], randomly generate indices i1 and j1 and swap matrix[i][j] with matrix[i1][j1], as follows: for (int i for (int int i1 int j1
= j = =
0; i < matrix.length; i++) { = 0; j < matrix[i].length; j++) { (int)(Math.random() * matrix.length); (int)(Math.random() * matrix[i].length);
// Swap matrix[i][j] with matrix[i1][j1]
8.4 Passing Two-Dimensional Arrays to Methods 293 int temp = matrix[i][j]; matrix[i][j] = matrix[i1][j1]; matrix[i1][j1] = temp; } }
8.5
Show the output of the following code: int[][] array = {{1, 2}, {3, 4}, for (int i = array.length - 1; i for (int j = array[i].length System.out.print(array[i][j] System.out.println(); }
8.6
{5, 6}}; >= 0; i——) { 1; j >= 0; j——) + " ");
✓
Check Point
Show the output of the following code: int[][] array = {{1, 2}, {3, 4}, {5, 6}}; int sum = 0; for (int i = 0; i < array.length; i++) sum += array[i][0]; System.out.println(sum);
8.4 Passing Two-Dimensional Arrays to Methods When passing a two-dimensional array to a method, the reference of the array is passed to the method.
Key Point
You can pass a two-dimensional array to a method just as you pass a one-dimensional array. You can also return an array from a method. Listing 8.1 gives an example with two methods. The first method, getArray(), returns a two-dimensional array, and the second method, sum(int[][] m), returns the sum of all the elements in a matrix.
LISTING 8.1 PassTwoDimensionalArray.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
import java.util.Scanner; public class PassTwoDimensionalArray { public static void main(String[] args) { int[][] m = getArray(); // Get an array // Display sum of elements System.out.println("\nSum of all elements is " + sum(m));
get array
pass array
} public static int[][] getArray() { // Create a Scanner Scanner input = new Scanner(System.in); // Enter array values int[][] m = new int[3][4]; System.out.println("Enter " + m.length + " rows and " + m[0].length + " columns: "); for (int i = 0; i < m.length; i++) for (int j = 0; j < m[i].length; j++) m[i][j] = input.nextInt();
getArray method
294 Chapter 8
Multidimensional Arrays 23 24 25 26 27 28 29 30 31 32 33 34 35 36
return array
sum method
return m; } public static int sum(int[][] m) { int total = 0; for (int row = 0; row < m.length; row++) { for (int column = 0; column < m[row].length; column++) { total += m[row][column]; } } return total; } }
Enter 3 rows and 4 columns: 1 2 3 4 5 6 7 8 9 10 11 12 Sum of all elements is 78
The method getArray prompts the user to enter values for the array (lines 11–24) and returns the array (line 23). The method sum (lines 26–35) has a two-dimensional array argument. You can obtain the number of rows using m.length (line 28) and the number of columns in a specified row using m[row].length (line 29).
✓
Check Point
8.7
Show the output of the following code: public class Test { public static void main(String[] args) { int[][] array = {{1, 2, 3, 4}, {5, 6, 7, 8}}; System.out.println(m1(array)[0]); System.out.println(m1(array)[1]); } public static int[] m1(int[][] m) { int[] result = new int[2]; result[0] = m.length; result[1] = m[0].length; return result; } }
8.5 Case Study: Grading a Multiple-Choice Test Key Point VideoNote
Grade multiple-choice test
The problem is to write a program that will grade multiple-choice tests. Suppose you need to write a program that grades multiple-choice tests. Assume there are eight students and ten questions, and the answers are stored in a two-dimensional array. Each row records a student’s answers to the questions, as shown in the following array.
8.5 Case Study: Grading a Multiple-Choice Test 295 Students’ Answers to the Questions: 0 1 2 3 4 5 6 7 8 9 Student Student Student Student Student Student Student Student
0 1 2 3 4 5 6 7
A D E C A B B E
B B D B B B B B
A A D A D E A E
C B A E C C C C
C C C D C C C C
D A B C D D D D
E E E E E E E E
E E E E E E E E
A A A A A A A A
D D D D D D D D
The key is stored in a one-dimensional array: Key to the Questions: 0 1 2 3 4 5 6 7 8 9 Key D B D C C D A E A D
Your program grades the test and displays the result. It compares each student’s answers with the key, counts the number of correct answers, and displays it. Listing 8.2 gives the program.
LISTING 8.2 GradeExam.java 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
public class GradeExam { /** Main method */ public static void main(String[] args) { // Students' answers to the questions char[][] answers = { {'A', 'B', 'A', 'C', 'C', 'D', 'E', 'E', {'D', 'B', 'A', 'B', 'C', 'A', 'E', 'E', {'E', 'D', 'D', 'A', 'C', 'B', 'E', 'E', {'C', 'B', 'A', 'E', 'D', 'C', 'E', 'E', {'A', 'B', 'D', 'C', 'C', 'D', 'E', 'E', {'B', 'B', 'E', 'C', 'C', 'D', 'E', 'E', {'B', 'B', 'A', 'C', 'C', 'D', 'E', 'E', {'E', 'B', 'E', 'C', 'C', 'D', 'E', 'E',
2-D array 'A', 'A', 'A', 'A', 'A', 'A', 'A', 'A',
'D'}, 'D'}, 'D'}, 'D'}, 'D'}, 'D'}, 'D'}, 'D'}};
// Key to the questions char[] keys = {'D', 'B', 'D', 'C', 'C', 'D', 'A', 'E', 'A', 'D'}; // Grade all answers for (int i = 0; i < answers.length; i++) { // Grade one student int correctCount = 0; for (int j = 0; j < answers[i].length; j++) { if (answers[i][j] == keys[j]) correctCount++; } System.out.println("Student " + i + "'s correct count is " + correctCount); } } }
1-D array
compare with key
296 Chapter 8
Multidimensional Arrays Student Student Student Student Student Student Student Student
0's 1's 2's 3's 4's 5's 6's 7's
correct correct correct correct correct correct correct correct
count count count count count count count count
is is is is is is is is
7 6 5 4 8 7 7 7
The statement in lines 5–13 declares, creates, and initializes a two-dimensional array of characters and assigns the reference to answers of the char[][] type. The statement in line 16 declares, creates, and initializes an array of char values and assigns the reference to keys of the char[] type. Each row in the array answers stores a student’s answer, which is graded by comparing it with the key in the array keys. The result is displayed immediately after a student’s answer is graded.
8.6 Case Study: Finding the Closest Pair Key Point
closest-pair animation on the Companion Website
This section presents a geometric problem for finding the closest pair of points. Given a set of points, the closest-pair problem is to find the two points that are nearest to each other. In Figure 8.3, for example, points (1, 1) and (2, 0.5) are closest to each other. There are several ways to solve this problem. An intuitive approach is to compute the distances between all pairs of points and find the one with the minimum distance, as implemented in Listing 8.3.
(–1, 3)
(3, 3) (4, 2) (1, 1) (2, 0.5) (4, –0.5)
(–1, –1)
FIGURE 8.3
(2, –1)
x 0 –1 1 –1 2 1 3 2 4 2 5 3 6 4 7 4
Points can be represented in a two-dimensional array.
LISTING 8.3 FindNearestPoints.java
number of points
1 2 3 4 5 6 7 8 9
import java.util.Scanner; public class FindNearestPoints { public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.print("Enter the number of points: "); int numberOfPoints = input.nextInt(); // Create an array to store points
y 3 –1 1 0.5 –1 3 2 –0.5
8.6 Case Study: Finding the Closest Pair 297 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
double[][] points = new double[numberOfPoints][2]; System.out.print("Enter " + numberOfPoints + " points: "); for (int i = 0; i < points.length; i++) { points[i][0] = input.nextDouble(); points[i][1] = input.nextDouble(); } // p1 and p2 are the indices in the points' array int p1 = 0, p2 = 1; // Initial two points double shortestDistance = distance(points[p1][0], points[p1][1], points[p2][0], points[p2][1]); // Initialize shortestDistance // Compute distance for every two points for (int i = 0; i < points.length; i++) { for (int j = i + 1; j < points.length; j++) { double distance = distance(points[i][0], points[i][1], points[j][0], points[j][1]); // Find distance if (shortestDistance > distance) { p1 = i; // Update p1 p2 = j; // Update p2 shortestDistance = distance; // Update shortestDistance } } } // Display result System.out.println("The closest two points are " + "(" + points[p1][0] + ", " + points[p1][1] + ") and (" + points[p2][0] + ", " + points[p2][1] + ")"); } /** Compute the distance between two points (x1, y1) and (x2, y2)*/ public static double distance( double x1, double y1, double x2, double y2) { return Math.sqrt((x2 - x1) * (x2 - x1) + (y2 - y1) * (y2 - y1)); } }
Enter the number of points: 8 Enter 8 points: -1 3 -1 -1 1 1 2 0.5 2 -1 3 3 The closest two points are (1, 1) and (2, 0.5)
4 2 4 -0.5
The program prompts the user to enter the number of points (lines 6–7). The points are read from the console and stored in a two-dimensional array named points (lines 12–15). The program uses the variable shortestDistance (line 19) to store the distance between the two nearest points, and the indices of these two points in the points array are stored in p1 and p2 (line 18). For each point at index i, the program computes the distance between points[i] and points[j] for all j > i (lines 23–34). Whenever a shorter distance is found, the variable shortestDistance and p1 and p2 are updated (lines 28–32). The distance between two points (x1, y1) and (x2, y2) can be computed using the formula 2(x2 - x1)2 + (y2 - y1)2 (lines 43–46). The program assumes that the plane has at least two points. You can easily modify the program to handle the case if the plane has zero or one point.
2-D array read points
track two points track shortestDistance
for each point i for each point j distance between i and j distance between two points
update shortestDistance
298 Chapter 8
Multidimensional Arrays Note that there might be more than one closest pair of points with the same minimum distance. The program finds one such pair. You may modify the program to find all closest pairs in Programming Exercise 8.8.
multiple closest pairs
Tip It is cumbersome to enter all points from the keyboard. You may store the input in a file, say FindNearestPoints.txt, and compile and run the program using the following command:
input file
java FindNearestPoints < FindNearestPoints.txt
8.7 Case Study: Sudoku Key Point VideoNote
Sudoku
fixed cells free cells
The problem is to check whether a given Sudoku solution is correct. This section presents an interesting problem of a sort that appears in the newspaper every day. It is a number-placement puzzle, commonly known as Sudoku. This is a very challenging problem. To make it accessible to the novice, this section presents a simplified version of the Sudoku problem, which is to verify whether a Sudoku solution is correct. The complete program for finding a Sudoku solution is presented in Supplement VI.A. Sudoku is a 9 * 9 grid divided into smaller 3 * 3 boxes (also called regions or blocks), as shown in Figure 8.4a. Some cells, called fixed cells, are populated with numbers from 1 to 9. The objective is to fill the empty cells, also called free cells, with the numbers 1 to 9 so that every row, every column, and every 3 * 3 box contains the numbers 1 to 9, as shown in Figure 8.4b. 5
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Solution
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(a) Puzzle
FIGURE 8.4 representing a grid
(b) Solution
The Sudoku puzzle in (a) is solved in (b).
For convenience, we use value 0 to indicate a free cell, as shown in Figure 8.5a. The grid can be naturally represented using a two-dimensional array, as shown in Figure 8.5b. 5
3
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(a)
FIGURE 8.5
int[][] grid = {{5, 3, 0, 0, {6, 0, 0, 1, {0, 9, 8, 0, {8, 0, 0, 0, {4, 0, 0, 8, {7, 0, 0, 0, {0, 6, 0, 0, {0, 0, 0, 4, {0, 0, 0, 0, };
7, 9, 0, 6, 0, 2, 0, 1, 8,
0, 5, 0, 0, 3, 0, 0, 9, 0,
0, 0, 0, 0, 0, 0, 2, 0, 0,
(b)
A grid can be represented using a two-dimensional array.
0, 0, 6, 0, 0, 0, 8, 0, 7,
0}, 0}, 0}, 3}, 1}, 6}, 0}, 5}, 9}
8.7 Case Study: Sudoku 299 To find a solution for the puzzle, we must replace each 0 in the grid with an appropriate number from 1 to 9. For the solution to the puzzle in Figure 8.5, the grid should be as shown in Figure 8.6. Once a solution to a Sudoku puzzle is found, how do you verify that it is correct? Here are two approaches: ■
Check if every row has numbers from 1 to 9, every column has numbers from 1 to 9, and every small box has numbers from 1 to 9.
■
Check each cell. Each cell must be a number from 1 to 9 and the cell must be unique on every row, every column, and every small box. A solution grid is {{5, 3, 4, 6, 7, 8, {6, 7, 2, 1, 9, 5, {1, 9, 8, 3, 4, 2, {8, 5, 9, 7, 6, 1, {4, 2, 6, 8, 5, 3, {7, 1, 3, 9, 2, 4, {9, 6, 1, 5, 3, 7, {2, 8, 7, 4, 1, 9, {3, 4, 5, 2, 8, 6, };
9, 3, 5, 4, 7, 8, 2, 6, 1,
1, 4, 6, 2, 9, 5, 8, 3, 7,
2}, 8}, 7}, 3}, 1}, 6}, 4}, 5}, 9}
FIGURE 8.6 A solution is stored in grid. The program in Listing 8.4 prompts the user to enter a solution and reports whether it is valid. We use the second approach in the program to check whether the solution is correct.
LISTING 8.4 CheckSudokuSolution.java 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
import java.util.Scanner; public class CheckSudokuSolution { public static void main(String[] args) { // Read a Sudoku solution int[][] grid = readASolution(); System.out.println(isValid(grid) ? "Valid solution" : "Invalid solution");
read input solution valid?
} /** Read a Sudoku solution from the console */ public static int[][] readASolution() { // Create a Scanner Scanner input = new Scanner(System.in);
read solution
System.out.println("Enter a Sudoku puzzle solution:"); int[][] grid = new int[9][9]; for (int i = 0; i < 9; i++) for (int j = 0; j < 9; j++) grid[i][j] = input.nextInt(); return grid; } /** Check whether a solution is valid */ public static boolean isValid(int[][] grid) {
check solution
300 Chapter 8
Multidimensional Arrays
check rows
check columns
check small boxes
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
for (int i = 0; i < 9; i++) for (int j = 0; j < 9; j++) if (grid[i][j] < 1 || grid[i][j] > 9 || !isValid(i, j, grid)) return false; return true; // The solution is valid } /** Check whether grid[i][j] is valid in the grid */ public static boolean isValid(int i, int j, int[][] grid) { // Check whether grid[i][j] is unique in i's row for (int column = 0; column < 9; column++) if (column != j && grid[i][column] == grid[i][j]) return false; // Check whether grid[i][j] is unique in j's column for (int row = 0; row < 9; row++) if (row != i && grid[row][j] == grid[i][j]) return false; // Check whether grid[i][j] is unique in the 3-by-3 box for (int row = (i / 3) * 3; row < (i / 3) * 3 + 3; row++) for (int col = (j / 3) * 3; col < (j / 3) * 3 + 3; col++) if (row != i && col != j && grid[row][col] == grid[i][j]) return false; return true; // The current value at grid[i][j] is valid } }
Enter a Sudoku puzzle solution: 9 6 3 1 7 4 2 5 8 1 7 8 3 2 5 6 4 9 2 5 4 6 8 9 7 3 1 8 2 1 4 3 7 5 9 6 4 9 6 8 5 2 3 1 7 7 3 5 9 6 1 8 2 4 5 8 9 7 1 3 4 6 2 3 1 7 2 4 6 9 8 5 6 4 2 5 9 8 1 7 3 Valid solution
isValid method
overloaded isValid method
The program invokes the readASolution() method (line 6) to read a Sudoku solution and return a two-dimensional array representing a Sudoku grid. The isValid(grid) method checks whether the values in the grid are valid by verifying that each value is between 1 and 9 and that each value is valid in the grid (lines 27–34). The isValid(i, j, grid) method checks whether the value at grid[i][j] is valid. It checks whether grid[i][j] appears more than once in row i (lines 39–41), in column j (lines 44–46), and in the 3 * 3 box (lines 49–52). How do you locate all the cells in the same box? For any grid[i][j], the starting cell of the 3 * 3 box that contains it is grid[(i / 3) * 3][(j / 3) * 3], as illustrated in Figure 8.7.
8.8 Multidimensional Arrays 301 grid[0][6]
grid[0][0]
For any grid[i][j] in this 3 by 3 box, its starting cell is grid[3*(i/3)][3*(j/3)] (i.e., grid[0][6]). For example, for grid[2][8], i=2 and j=8, 3*(i/3)=0 and 3*(j/3)=6.
grid[6][3] For any grid[i][j] in this 3 by 3 box, its starting cell is grid[3*(i/3)][3*(j/3)] (i.e., grid[6][3]). For example, for grid[8][5], i=8 and j=5, 3*(i/3)=6 and 3*(j/3)=3.
FIGURE 8.7 The location of the first cell in a 3 * 3 box determines the locations of other cells in the box. With this observation, you can easily identify all the cells in the box. For instance, if grid[r][c] is the starting cell of a 3 * 3 box, the cells in the box can be traversed in a
nested loop as follows: // Get all cells in a 3-by-3 box starting at grid[r][c] for (int row = r; row < r + 3; row++) for (int col = c; col < c + 3; col++) // grid[row][col] is in the box
It is cumbersome to enter 81 numbers from the console. When you test the program, you may store the input in a file, say CheckSudokuSolution.txt (see www.cs.armstrong.edu/liang/ data/CheckSudokuSolution.txt), and run the program using the following command:
input file
java CheckSudokuSolution < CheckSudokuSolution.txt
8.8 Multidimensional Arrays A two-dimensional array consists of an array of one-dimensional arrays and a threedimensional array consists of an array of two-dimensional arrays. In the preceding section, you used a two-dimensional array to represent a matrix or a table. Occasionally, you will need to represent n-dimensional data structures. In Java, you can create n-dimensional arrays for any integer n. The way to declare two-dimensional array variables and create two-dimensional arrays can be generalized to declare n-dimensional array variables and create n-dimensional arrays for n 7 = 3. For example, you may use a three-dimensional array to store exam scores for a class of six students with five exams, and each exam has two parts (multiple-choice and essay). The following syntax declares a three-dimensional array variable scores, creates an array, and assigns its reference to scores. double[][][] scores = new double[6][5][2];
You can also use the short-hand notation to create and initialize the array as follows: double[][][] scores = {{7.5, 20.5}, {9.0, {{4.5, 21.5}, {9.0, {{6.5, 30.5}, {9.4, {{6.5, 23.5}, {9.4, {{8.5, 26.5}, {9.4, {{9.5, 20.5}, {9.4,
{ 22.5}, 22.5}, 10.5}, 32.5}, 52.5}, 42.5},
{15, {15, {11, {13, {13, {13,
33.5}, 34.5}, 33.5}, 34.5}, 36.5}, 31.5},
{13, {12, {11, {11, {13, {12,
21.5}, 20.5}, 23.5}, 20.5}, 24.5}, 20.5},
{15, {14, {10, {16, {16, {16,
2.5}}, 9.5}}, 2.5}}, 7.5}}, 2.5}}, 6.5}}};
Key Point
302 Chapter 8
Multidimensional Arrays scores[0][1][0] refers to the multiple-choice score for the first student’s second exam, which is 9.0. scores[0][1][1] refers to the essay score for the first student’s second exam, which is 22.5. This is depicted in the following figure:
Which student
Which exam
scores [i]
[j]
Multiple-choice or essay
[k]
A multidimensional array is actually an array in which each element is another array. A threedimensional array consists of an array of two-dimensional arrays. A two-dimensional array consists of an array of one-dimensional arrays. For example, suppose x = new int[2] [2][5], and x[0] and x[1] are two-dimensional arrays. X[0][0], x[0][1], x[1][0], and x[1][1] are one-dimensional arrays and each contains five elements. x.length is 2, x[0].length and x[1].length are 2, and X[0][0].length, x[0][1].length, x[1][0].length, and x[1][1].length are 5.
8.8.1
Case Study: Daily Temperature and Humidity
Suppose a meteorology station records the temperature and humidity every hour of every day and stores the data for the past ten days in a text file named Weather.txt (see www .cs.armstrong.edu/liang/data/Weather.txt). Each line of the file consists of four numbers that indicate the day, hour, temperature, and humidity. The contents of the file may look like the one in (a). Day
Temperature Hour
1 1 . . . 10 10
Day Humidity
1 2
76.4 77.7
0.92 0.93
23 24
97.7 98.7
0.71 0.74
Temperature Hour
10 1 . . . 10 1
(a)
Humidity
24 2
98.7 77.7
0.74 0.93
23 1
97.7 76.4
0.71 0.92
(b)
Note that the lines in the file are not necessarily in increasing order of day and hour. For example, the file may appear as shown in (b). Your task is to write a program that calculates the average daily temperature and humidity for the 10 days. You can use the input redirection to read the file and store the data in a three-dimensional array named data. The first index of data ranges from 0 to 9 and represents 10 days, the second index ranges from 0 to 23 and represents 24 hours, and the third index ranges from 0 to 1 and represents temperature and humidity, as depicted in the following figure: Which day
Which hour
data [ i ] [ j ] [ k ]
Temperature or humidity
8.8 Multidimensional Arrays 303 Note that the days are numbered from 1 to 10 and the hours from 1 to 24 in the file. Because the array index starts from 0, data[0][0][0] stores the temperature in day 1 at hour 1 and data[9][23][1] stores the humidity in day 10 at hour 24. The program is given in Listing 8.5.
LISTING 8.5 Weather.java 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
import java.util.Scanner; public class Weather { public static void main(String[] args) { final int NUMBER_OF_DAYS = 10; final int NUMBER_OF_HOURS = 24; double[][][] data = new double[NUMBER_OF_DAYS][NUMBER_OF_HOURS][2]; Scanner input = new Scanner(System.in); // Read input using input redirection from a file for (int k = 0; k < NUMBER_OF_DAYS * NUMBER_OF_HOURS; k++) { int day = input.nextInt(); int hour = input.nextInt(); double temperature = input.nextDouble(); double humidity = input.nextDouble(); data[day - 1][hour - 1][0] = temperature; data[day - 1][hour - 1][1] = humidity; } // Find the average daily temperature and humidity for (int i = 0; i < NUMBER_OF_DAYS; i++) { double dailyTemperatureTotal = 0, dailyHumidityTotal = 0; for (int j = 0; j < NUMBER_OF_HOURS; j++) { dailyTemperatureTotal += data[i][j][0]; dailyHumidityTotal += data[i][j][1]; } // Display result System.out.println("Day " + i + "'s average temperature is " + dailyTemperatureTotal / NUMBER_OF_HOURS); System.out.println("Day " + i + "'s average humidity is " + dailyHumidityTotal / NUMBER_OF_HOURS); } } }
Day Day Day Day . . Day Day
0's 0's 1's 1's . 9's 9's
average average average average
temperature humidity is temperature humidity is
is 77.7708 0.929583 is 77.3125 0.929583
average temperature is 79.3542 average humidity is 0.9125
You can use the following command to run the program: java Weather < Weather.txt
A three-dimensional array for storing temperature and humidity is created in line 8. The loop in lines 12–19 reads the input to the array. You can enter the input from the keyboard, but
three-dimensional array
304 Chapter 8
Multidimensional Arrays doing so will be awkward. For convenience, we store the data in a file and use input redirection to read the data from the file. The loop in lines 24–27 adds all temperatures for each hour in a day to dailyTemperatureTotal and all humidity for each hour to dailyHumidityTotal. The average daily temperature and humidity are displayed in lines 30–33.
8.8.2
Case Study: Guessing Birthdays
Listing 3.3, GuessBirthday.java, gives a program that guesses a birthday. The program can be simplified by storing the numbers in five sets in a three-dimensional array, and it prompts the user for the answers using a loop, as shown in Listing 8.6. The sample run of the program can be the same as shown in Listing 4.3.
LISTING 8.6 GuessBirthdayUsingArray.java
three-dimensional array
Set i
add to day
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
import java.util.Scanner; public class GuessBirthdayUsingArray { public static void main(String[] args) { int day = 0; // Day to be determined int answer; int[][][] dates {{ 1, 3, 5, { 9, 11, 13, {17, 19, 21, {25, 27, 29, {{ 2, 3, 6, {10, 11, 14, {18, 19, 22, {26, 27, 30, {{ 4, 5, 6, {12, 13, 14, {20, 21, 22, {28, 29, 30, {{ 8, 9, 10, {12, 13, 14, {24, 25, 26, {28, 29, 30, {{16, 17, 18, {20, 21, 22, {24, 25, 26, {28, 29, 30,
= { 7}, 15}, 23}, 31}}, 7}, 15}, 23}, 31}}, 7}, 15}, 23}, 31}}, 11}, 15}, 27}, 31}}, 19}, 23}, 27}, 31}}};
// Create a Scanner Scanner input = new Scanner(System.in); for (int i = 0; i < 5; i++) { System.out.println("Is your birthday in Set" + (i + 1) + "?"); for (int j = 0; j < 4; j++) { for (int k = 0; k < 4; k++) System.out.printf("%4d", dates[i][j][k]); System.out.println(); } System.out.print("\nEnter 0 for No and 1 for Yes: "); answer = input.nextInt(); if (answer == 1) day += dates[i][0][0];
Programming Exercises 305 46 47 48 49 50
} System.out.println("Your birthday is " + day); } }
A three-dimensional array dates is created in Lines 8–28. This array stores five sets of numbers. Each set is a 4-by-4 two-dimensional array. The loop starting from line 33 displays the numbers in each set and prompts the user to answer whether the birthday is in the set (lines 41–42). If the day is in the set, the first number (dates[i][0][0]) in the set is added to variable day (line 45).
8.8
Declare an array variable for a three-dimensional array, create a 4 * 6 * 5 int array, and assign its reference to the variable.
8.9
Assume int[][][] x = new char[12][5][2], how many elements are in the array? What are x.length, x[2].length, and x[0][0].length? Show the output of the following code:
8.10
int[][][] array = {{{1, 2}, {3, 4}}, {{5, 6},{7, 8}}}; System.out.println(array[0][0][0]); System.out.println(array[1][1][1]);
CHAPTER SUMMARY 1. A two-dimensional array can be used to store a table. 2. A variable for two-dimensional arrays can be declared using the syntax: elementType[][] arrayVar.
3. A two-dimensional array can be created using the syntax:
new
elementType
[ROW_SIZE][COLUMN_SIZE].
4. Each element in a two-dimensional array is represented using the syntax: arrayVar[rowIndex][columnIndex].
5. You can create and initialize a two-dimensional array using an array initializer with the syntax: elementType[][] arrayVar = {{row values}, . . . , {row values}}.
6. You can use arrays of arrays to form multidimensional arrays. For example, a variable for three-dimensional arrays can be declared as elementType[][][] arrayVar, and a three-dimensional array can be created using new elementType[size1][size2] [size3].
QUIZ Answer the quiz for this chapter online at www.cs.armstrong.edu/liang/intro10e/quiz.html.
PROGRAMMING EXERCISES *8.1
(Sum elements column by column) Write a method that returns the sum of all the elements in a specified column in a matrix using the following header: public static double sumColumn(double[][] m, int columnIndex)
✓
Check Point
306 Chapter 8
Multidimensional Arrays Write a test program that reads a 3-by-4 matrix and displays the sum of each column. Here is a sample run: Enter a 3-by-4 matrix row by row: 1.5 2 3 4 5.5 6 7 8 9.5 Sum Sum Sum Sum
*8.2
1 3 1 of the of the of the of the
elements elements elements elements
at at at at
column column column column
0 1 2 3
is is is is
16.5 9.0 13.0 13.0
(Sum the major diagonal in a matrix) Write a method that sums all the numbers in the major diagonal in an n * n matrix of double values using the following header: public static double sumMajorDiagonal(double[][] m)
Write a test program that reads a 4-by-4 matrix and displays the sum of all its elements on the major diagonal. Here is a sample run: Enter a 4-by-4 matrix row by row: 1 2 3 4.0 5 6.5 7 8 9 10 11 12 13 14 15 16 Sum of the elements in the major diagonal is 34.5
*8.3 **8.4
8.5
(Sort students on grades) Rewrite Listing 8.2, GradeExam.java, to display the students in increasing order of the number of correct answers. (Compute the weekly hours for each employee) Suppose the weekly hours for all employees are stored in a two-dimensional array. Each row records an employee’s seven-day work hours with seven columns. For example, the following array stores the work hours for eight employees. Write a program that displays employees and their total hours in decreasing order of the total hours. Su
M
T
W
Th
F
Sa
Employee 0
2
4
3
4
5
8
8
Employee 1
7
3
4
3
3
4
4
Employee 2
3
3
4
3
3
2
2
Employee 3
9
3
4
7
3
4
1
Employee 4
3
5
4
3
6
3
8
Employee 5
3
4
4
6
3
4
4
Employee 6
3
7
4
8
3
8
4
Employee 7
6
3
5
9
2
7
9
(Algebra: add two matrices) Write a method to add two matrices. The header of the method is as follows: public static double[][] addMatrix(double[][] a, double[][] b)
Programming Exercises 307 In order to be added, the two matrices must have the same dimensions and the same or compatible types of elements. Let c be the resulting matrix. Each element cij is aij + bij. For example, for two 3 * 3 matrices a and b, c is a11 £ a21 a31
a12 a22 a32
b11 a13 a23 ≥ + £ b21 a33 b31
b12 b22 b32
a11 + b11 b13 b23 ≥ = £ a21 + b21 b33 a31 + b31
a12 + b12 a22 + b22 a32 + b32
a13 + b13 a23 + b23 ≥ a33 + b33
Write a test program that prompts the user to enter two 3 * 3 matrices and displays their sum. Here is a sample run: Enter matrix1: 1 2 3 4 5 6 7 8 9 Enter matrix2: 0 2 4 1 4.5 2.2 1.1 The matrices are added as follows 1.0 2.0 3.0 0.0 2.0 4.0 4.0 5.0 6.0 + 1.0 4.5 2.2 = 7.0 8.0 9.0 1.1 4.3 5.2
**8.6
4.3 5.2 1.0 4.0 7.0 5.0 9.5 8.2 8.1 12.3 14.2
(Algebra: multiply two matrices) Write a method to multiply two matrices. The header of the method is: public static double[][] multiplyMatrix(double[][] a, double[][] b)
To multiply matrix a by matrix b, the number of columns in a must be the same as the number of rows in b, and the two matrices must have elements of the same or compatible types. Let c be the result of the multiplication. Assume the column size of matrix a is n. Each element cij is ai1 * b1j + ai2 * b2j + c + ain * bnj. For example, for two 3 * 3 matrices a and b, c is a11 £ a21 a31
a12 a22 a32
b11 a13 a23 ≥ * £ b21 a33 b31
b12 b22 b32
c11 b13 b23 ≥ = £ c21 b33 c31
c12 c22 c32
c13 c23 ≥ c33
where cij = ai1 * b1j + ai2 * b2j + ai3 * b3j. Write a test program that prompts the user to enter two 3 * 3 matrices and displays their product. Here is a sample run: Enter matrix1: 1 2 3 4 5 6 7 8 9 Enter matrix2: 0 2 4 1 4.5 2.2 1.1 4.3 5.2 The multiplication of the matrices is 1 2 3 0 2.0 4.0 5.3 23.9 24 4 5 6 * 1 4.5 2.2 = 11.6 56.3 58.2 7 8 9 1.1 4.3 5.2 17.9 88.7 92.4
*8.7
(Points nearest to each other) Listing 8.3 gives a program that finds two points in a two-dimensional space nearest to each other. Revise the program so that it finds two points in a three-dimensional space nearest to each other. Use a two-dimensional array to represent the points. Test the program using the following points: double[][] points = {{-1, 0, 3}, {-1, -1, -1}, {4, 1, 1}, {2, 0.5, 9}, {3.5, 2, -1}, {3, 1.5, 3}, {-1.5, 4, 2}, {5.5, 4, -0.5}};
VideoNote
Multiply two matrices
308 Chapter 8
Multidimensional Arrays The formula for computing the distance between two points (x1, y1, z1) and (x2, y2, z2) is 2(x2 - x1)2 + (y2 - y1)2 + (z2 - z1)2.
**8.8
(All closest pairs of points) Revise Listing 8.3, FindNearestPoints.java, to display all closest pairs of points with the same minimum distance. Here is a sample run: Enter the number of points: 8 Enter 8 points: 0 0 1 1 -1 -1 2 2 -2 -2 -3 -3 -4 -4 5 5 The closest two points are (0.0, 0.0) and (1.0, 1.0) The closest two points are (0.0, 0.0) and (-1.0, -1.0) The closest two points are (1.0, 1.0) and (2.0, 2.0) The closest two points are (-1.0, -1.0) and (-2.0, -2.0) The closest two points are (-2.0, -2.0) and (-3.0, -3.0) The closest two points are (-3.0, -3.0) and (-4.0, -4.0) Their distance is 1.4142135623730951
***8.9
(Game: play a tic-tac-toe game) In a game of tic-tac-toe, two players take turns marking an available cell in a 3 * 3 grid with their respective tokens (either X or O). When one player has placed three tokens in a horizontal, vertical, or diagonal row on the grid, the game is over and that player has won. A draw (no winner) occurs when all the cells on the grid have been filled with tokens and neither player has achieved a win. Create a program for playing tic-tac-toe. The program prompts two players to enter an X token and O token alternately. Whenever a token is entered, the program redisplays the board on the console and determines the status of the game (win, draw, or continue). Here is a sample run: ——————-—————— | | | | ——————-—————— | | | | ——————-—————— | | | | ——————-—————— Enter a row (0, 1, or 2) for player X: 1 Enter a column (0, 1, or 2) for player X: 1 ——————-—————— | | | | ——————-—————— | | X | | ——————-—————— | | | | ——————-—————— Enter a row (0, 1, or 2) for player O: 1 Enter a column (0, 1, or 2) for player O: 2 ——————-—————— | | | | ——————-—————— | | X | O | ——————-—————— | | | | ——————-——————
Programming Exercises 309 Enter a row (0, 1, or 2) for player X: . . . ——————-—————— | X | | | ——————-—————— | O | X | O | ——————-—————— | | | X | ——————-—————— X player won
*8.10
(Largest row and column) Write a program that randomly fills in 0s and 1s into a 4-by-4 matrix, prints the matrix, and finds the first row and column with the most 1s. Here is a sample run of the program: 0011 0011 1101 1010 The largest row index: 2 The largest column index: 2
**8.11
(Game: nine heads and tails) Nine coins are placed in a 3-by-3 matrix with some face up and some face down. You can represent the state of the coins using a 3-by-3 matrix with values 0 (heads) and 1 (tails). Here are some examples: 0 0 0 0 1 0 0 0 0
1 0 1 0 0 1 1 0 0
1 1 0 1 0 0 0 0 1
1 0 1 1 1 0 1 0 0
1 0 0 1 1 1 1 1 0
Each state can also be represented using a binary number. For example, the preceding matrices correspond to the numbers 000010000 101001100 110100001 101110100 100111110
There are a total of 512 possibilities, so you can use decimal numbers 0, 1, 2, 3, . . . , and 511 to represent all states of the matrix. Write a program that prompts the user to enter a number between 0 and 511 and displays the corresponding matrix with the characters H and T. Here is a sample run:
Enter a number between 0 and 511: 7 H H H H H H T T T
The user entered 7, which corresponds to 000000111. Since 0 stands for H and 1 for T, the output is correct.
**8.12
(Financial application: compute tax) Rewrite Listing 3.5, ComputeTax.java, using arrays. For each filing status, there are six tax rates. Each rate is applied to a certain amount of taxable income. For example, from the taxable income of $400,000 for a single filer, $8,350 is taxed at 10%, (33,950 - 8,350) at 15%,
310 Chapter 8
Multidimensional Arrays (82,250 - 33,950) at 25%, (171,550 - 82,550) at 28%, (372,550 - 82,250) at 33%, and (400,000 - 372,950) at 36%. The six rates are the same for all filing statuses, which can be represented in the following array: double[] rates = {0.10, 0.15, 0.25, 0.28, 0.33, 0.35};
The brackets for each rate for all the filing statuses can be represented in a twodimensional array as follows: int[][] brackets = { {8350, 33950, 82250, 171550, 372950}, // Single filer {16700, 67900, 137050, 20885, 372950}, // Married jointly // -or qualifying widow(er) {8350, 33950, 68525, 104425, 186475}, // Married separately {11950, 45500, 117450, 190200, 372950} // Head of household };
Suppose the taxable income is $400,000 for single filers. The tax can be computed as follows: tax = brackets[0][0] * rates[0] + (brackets[0][1] – brackets[0][0]) * rates[1] (brackets[0][2] – brackets[0][1]) * rates[2] (brackets[0][3] – brackets[0][2]) * rates[3] (brackets[0][4] – brackets[0][3]) * rates[4] (400000 – brackets[0][4]) * rates[5]
*8.13
+ + + +
(Locate the largest element) Write the following method that returns the location of the largest element in a two-dimensional array. public static int[] locateLargest(double[][] a)
The return value is a one-dimensional array that contains two elements. These two elements indicate the row and column indices of the largest element in the two-dimensional array. Write a test program that prompts the user to enter a twodimensional array and displays the location of the largest element in the array. Here is a sample run:
Enter the number of rows and columns of the array: 3 4 Enter the array: 23.5 35 2 10 4.5 3 45 3.5 35 44 5.5 9.6 The location of the largest element is at (1, 2)
**8.14
(Explore matrix) Write a program that prompts the user to enter the length of a square matrix, randomly fills in 0s and 1s into the matrix, prints the matrix, and finds the rows, columns, and diagonals with all 0s or 1s. Here is a sample run of the program:
Programming Exercises 311 Enter the size for 0111 0000 0100 1111 All 0s on row 1 All 1s on row 3 No same numbers on No same numbers on No same numbers on
*8.15
the matrix: 4
a column the major diagonal the sub-diagonal
(Geometry: same line?) Programming Exercise 6.39 gives a method for testing whether three points are on the same line. Write the following method to test whether all the points in the array points are on the same line. public static boolean sameLine(double[][] points)
Write a program that prompts the user to enter five points and displays whether they are on the same line. Here are sample runs:
Enter five points: 3.4 2 6.5 9.5 2.3 2.3 5.5 5 -5 4 The five points are not on the same line
Enter five points: 1 1 2 2 3 3 4 4 5 5 The five points are on the same line
*8.16
(Sort two-dimensional array) Write a method to sort a two-dimensional array using the following header: public static void sort(int m[][])
The method performs a primary sort on rows and a secondary sort on columns. For example, the following array {{4, 2},{1, 7},{4, 5},{1, 2},{1, 1},{4, 1}}
will be sorted to {{1, 1},{1, 2},{1, 7},{4, 1},{4, 2},{4, 5}}.
***8.17
(Financial tsunami) Banks lend money to each other. In tough economic times, if a bank goes bankrupt, it may not be able to pay back the loan. A bank’s total assets are its current balance plus its loans to other banks. The diagram in Figure 8.8 shows five banks. The banks’ current balances are 25, 125, 175, 75, and 181 million dollars, respectively. The directed edge from node 1 to node 2 indicates that bank 1 lends 40 million dollars to bank 2.
312 Chapter 8
Multidimensional Arrays 125 1
100.5
25
85 75
0
3
125 125
320.5
125 181
FIGURE 8.8
4
40 75 2
175
Banks lend money to each other. If a bank’s total assets are under a certain limit, the bank is unsafe. The money it borrowed cannot be returned to the lender, and the lender cannot count the loan in its total assets. Consequently, the lender may also be unsafe, if its total assets are under the limit. Write a program to find all the unsafe banks. Your program reads the input as follows. It first reads two integers n and limit, where n indicates the number of banks and limit is the minimum total assets for keeping a bank safe. It then reads n lines that describe the information for n banks with IDs from 0 to n-1. The first number in the line is the bank’s balance, the second number indicates the number of banks that borrowed money from the bank, and the rest are pairs of two numbers. Each pair describes a borrower. The first number in the pair is the borrower’s ID and the second is the amount borrowed. For example, the input for the five banks in Figure 8.8 is as follows (note that the limit is 201): 5 201 25 2 1 100.5 4 320.5 125 2 2 40 3 85 175 2 0 125 3 75 75 1 0 125 181 1 2 125
The total assets of bank 3 are (75 + 125), which is under 201, so bank 3 is unsafe. After bank 3 becomes unsafe, the total assets of bank 1 fall below (125 + 40). Thus, bank 1 is also unsafe. The output of the program should be Unsafe banks are 3 1
*8.18
(Hint: Use a two-dimensional array borrowers to represent loans. borrowers[i][j] indicates the loan that bank i loans to bank j. Once bank j becomes unsafe, borrowers[i][j] should be set to 0.) (Shuffle rows) Write a method that shuffles the rows in a two-dimensional int array using the following header: public static void shuffle(int[][] m)
Write a test program that shuffles the following matrix: int[][] m = {{1, 2}, {3, 4}, {5, 6}, {7, 8}, {9, 10}};
**8.19
(Pattern recognition: four consecutive equal numbers) Write the following method that tests whether a two-dimensional array has four consecutive numbers of the same value, either horizontally, vertically, or diagonally. public static boolean isConsecutiveFour(int[][] values)
Programming Exercises 313 Write a test program that prompts the user to enter the number of rows and columns of a two-dimensional array and then the values in the array and displays true if the array contains four consecutive numbers with the same value. Otherwise, display false. Here are some examples of the true cases: 0 1 0 3 1 6 1
0 1 0 3 1 6 1
0 1 0 3 1 6 1
0 1 0 3 1 6 1
0 1 6 8 6 0 1
0 1 6 8 6 0 1
0 1 6 8 6 0 1
0 1 6 8 6 0 1
5 6 2 1 8 2 9
5 5 2 1 8 2 9
5 6 2 1 6 2 9
9 6 2 1 8 2 9
6 5 6 1 1 9 1
6 5 6 1 1 9 1
6 5 6 6 1 9 1
6 9 6 1 1 9 1
1 3 6 1 4 0 7
1 5 6 1 4 0 7
1 3 6 1 4 0 7
1 3 9 1 4 0 7
3 3 3 3 4 0 7
3 5 3 3 4 0 7
3 6 3 3 4 0 7
3 3 3 9 4 0 7
***8.20
(Game: connect four) Connect four is a two-player board game in which the players alternately drop colored disks into a seven-column, six-row vertically suspended grid, as shown below.
The objective of the game is to connect four same-colored disks in a row, a column, or a diagonal before your opponent can do likewise. The program prompts two players to drop a red or yellow disk alternately. In the preceding figure, the red disk is shown in a dark color and the yellow in a light color. Whenever a disk is dropped, the program redisplays the board on the console and determines the status of the game (win, draw, or continue). Here is a sample run: | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | ——————————————— Drop a red disk at column (0–6): 0 | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |R| | | | | | | ———————————————
314 Chapter 8
Multidimensional Arrays Drop a yellow disk at column (0–6): 3 | | | | | | | | | | |R|
| | | | | |
| | | | | | | | | | |Y|
| | | | | |
| | | | | |
| | | | | |
. . . . . . . . . Drop a yellow disk at column (0–6): 6 | | | | | | | | | | | | | | | | | | | |R| | | | | | | |Y|R|Y| | | | |R|Y|Y|Y|Y| |R|Y|R|Y|R|R|R| ——————————————— The yellow player won
*8.21
(Central city) Given a set of cities, the central city is the city that has the shortest total distance to all other cities. Write a program that prompts the user to enter the number of the cities and the locations of the cities (coordinates), and finds the central city and its total distance to all other cities. Enter the number of cities: 5 Enter the coordinates of the cities: 2.5 5 5.1 3 1 9 5.4 54 5.5 2.1 The central city is at (2.5, 5.0) The total distance to all other cities is 60.81
*8.22
VideoNote
Even number of 1s
*8.23
(Even number of 1s) Write a program that generates a 6-by-6 two-dimensional matrix filled with 0s and 1s, displays the matrix, and checks if every row and every column have an even number of 1s. (Game: find the flipped cell) Suppose you are given a 6-by-6 matrix filled with 0s and 1s. All rows and all columns have an even number of 1s. Let the user flip one cell (i.e., flip from 1 to 0 or from 0 to 1) and write a program to find which cell was flipped. Your program should prompt the user to enter a 6-by-6 array with 0s and 1s and find the first row r and first column c where the even number of the 1s property is violated (i.e., the number of 1s is not even). The flipped cell is at (r, c). Here is a sample run: Enter a 6-by-6 matrix row by row: 1 1 1 0 1 1 1 1 1 1 0 0 0 1 0 1 1 1 1 1 1 1 1 1 0 1 1 1 1 0 1 0 0 0 0 1 The flipped cell is at (0, 1)
Programming Exercises 315 *8.24 *8.25
(Check Sudoku solution) Listing 8.4 checks whether a solution is valid by checking whether every number is valid in the board. Rewrite the program by checking whether every row, every column, and every small box has the numbers 1 to 9. (Markov matrix) An n * n matrix is called a positive Markov matrix if each element is positive and the sum of the elements in each column is 1. Write the following method to check whether a matrix is a Markov matrix. public static boolean isMarkovMatrix(double[][] m)
Write a test program that prompts the user to enter a 3 * 3 matrix of double values and tests whether it is a Markov matrix. Here are sample runs:
Enter a 3-by-3 matrix row by row: 0.15 0.875 0.375 0.55 0.005 0.225 0.30 0.12 0.4 It is a Markov matrix
Enter a 3-by-3 matrix row by row: 0.95 -0.875 0.375 0.65 0.005 0.225 0.30 0.22 -0.4 It is not a Markov matrix
*8.26
(Row sorting) Implement the following method to sort the rows in a twodimensional array. A new array is returned and the original array is intact. public static double[][] sortRows(double[][] m)
Write a test program that prompts the user to enter a 3 * 3 matrix of double values and displays a new row-sorted matrix. Here is a sample run:
Enter a 3-by-3 matrix row by row: 0.15 0.875 0.375 0.55 0.005 0.225 0.30 0.12 0.4 The row-sorted array is 0.15 0.375 0.875 0.005 0.225 0.55 0.12 0.30 0.4
*8.27
(Column sorting) Implement the following method to sort the columns in a twodimensional array. A new array is returned and the original array is intact. public static double[][] sortColumns(double[][] m)
316 Chapter 8
Multidimensional Arrays Write a test program that prompts the user to enter a 3 * 3 matrix of double values and displays a new column-sorted matrix. Here is a sample run: Enter a 3-by-3 matrix row by row: 0.15 0.875 0.375 0.55 0.005 0.225 0.30 0.12 0.4 The column-sorted array is 0.15 0.0050 0.225 0.3 0.12 0.375 0.55 0.875 0.4
8.28
(Strictly identical arrays) The two-dimensional arrays m1 and m2 are strictly identical if their corresponding elements are equal. Write a method that returns true if m1 and m2 are strictly identical, using the following header: public static boolean equals(int[][] m1, int[][] m2)
Write a test program that prompts the user to enter two 3 * 3 arrays of integers and displays whether the two are strictly identical. Here are the sample runs. Enter list1: 51 22 25 6 1 4 24 54 6 Enter list2: 51 22 25 6 1 4 24 54 6 The two arrays are strictly identical
Enter list1: 51 25 22 6 1 4 24 54 6 Enter list2: 51 22 25 6 1 4 24 54 6 The two arrays are not strictly identical
8.29
(Identical arrays) The two-dimensional arrays m1 and m2 are identical if they have the same contents. Write a method that returns true if m1 and m2 are identical, using the following header: public static boolean equals(int[][] m1, int[][] m2)
Write a test program that prompts the user to enter two 3 * 3 arrays of integers and displays whether the two are identical. Here are the sample runs. Enter list1: 51 25 22 6 1 4 24 54 6 Enter list2: 51 22 25 6 1 4 24 54 6 The two arrays are identical
Enter list1: 51 5 22 6 1 4 24 54 6 Enter list2: 51 22 25 6 1 4 24 54 6 The two arrays are not identical
Programming Exercises 317 *8.30
(Algebra: solve linear equations) Write a method that solves the following 2 * 2 system of linear equations: a00x + a01y = b0 a10x + a11y = b1
x =
b0a11 - b1a01 a00a11 - a01a10
y =
b1a00 - b0a10 a00a11 - a01a10
The method header is public static double[] linearEquation(double[][] a, double[] b)
*8.31
The method returns null if a00a11 - a01a10 is 0. Write a test program that prompts the user to enter a00, a01, a10, a11, b0, and b1, and displays the result. If a00a11 - a01a10 is 0, report that “The equation has no solution.” A sample run is similar to Programming Exercise 3.3. (Geometry: intersecting point) Write a method that returns the intersecting point of two lines. The intersecting point of the two lines can be found by using the formula shown in Programming Exercise 3.25. Assume that (x1, y1) and (x2, y2) are the two points on line 1 and (x3, y3) and (x4, y4) are on line 2. The method header is public static double[] getIntersectingPoint(double[][] points)
*8.32
The points are stored in a 4-by-2 two-dimensional array points with (points[0][0], points[0][1]) for (x1, y1). The method returns the intersecting point or null if the two lines are parallel. Write a program that prompts the user to enter four points and displays the intersecting point. See Programming Exercise 3.25 for a sample run. (Geometry: area of a triangle) Write a method that returns the area of a triangle using the following header: public static double getTriangleArea(double[][] points)
The points are stored in a 3-by-2 two-dimensional array points with points[0] [0] and points[0][1] for (x1, y1). The triangle area can be computed using the formula in Programming Exercise 2.19. The method returns 0 if the three points are on the same line. Write a program that prompts the user to enter three points of a triangle and displays the triangle's area. Here is a sample run of the program: Enter x1, y1, x2, y2, x3, y3: 2.5 2 5 -1.0 4.0 2.0 The area of the triangle is 2.25
Enter x1, y1, x2, y2, x3, y3: 2 2 4.5 4.5 6 6 The three points are on the same line
*8.33
(Geometry: polygon subareas) A convex 4-vertex polygon is divided into four triangles, as shown in Figure 8.9. Write a program that prompts the user to enter the coordinates of four vertices and displays the areas of the four triangles in increasing order. Here is a sample run: Enter x1, y1, x2, y2, x3, y3, x4, y4: -2.5 2 4 4 3 -2 -2 -3.5 The areas are 6.17 7.96 8.08 10.42
318 Chapter 8
Multidimensional Arrays v2 (x2, y2)
v1 (x1, y1)
v3 (x3, y3)
v4 (x4, y4)
FIGURE 8.9
*8.34
A 4-vertex polygon is defined by four vertices.
(Geometry: rightmost lowest point) In computational geometry, often you need to find the rightmost lowest point in a set of points. Write the following method that returns the rightmost lowest point in a set of points. public static double[] getRightmostLowestPoint(double[][] points)
Write a test program that prompts the user to enter the coordinates of six points and displays the rightmost lowest point. Here is a sample run: Enter 6 points: 1.5 2.5 -3 4.5 5.6 -7 6.5 -7 8 1 10 2.5 The rightmost lowest point is (6.5, -7.0)
**8.35
(Largest block) Given a square matrix with the elements 0 or 1, write a program to find a maximum square submatrix whose elements are all 1s. Your program should prompt the user to enter the number of rows in the matrix. The program then displays the location of the first element in the maximum square submatrix and the number of the rows in the submatrix. Here is a sample run:
Enter the number of rows in the matrix: 5 Enter the matrix row by row: 1 0 1 0 1 1 1 1 0 1 1 0 1 1 1 1 0 1 1 1 1 0 1 1 1 The maximum square submatrix is at (2, 2) with size 3
Your program should implement and use the following method to find the maximum square submatrix: public static int[] findLargestBlock(int[][] m)
The return value is an array that consists of three values. The first two values are the row and column indices for the first element in the submatrix, and the third value is the number of the rows in the submatrix.
**8.36
(Latin square) A Latin square is an n-by-n array filled with n different Latin letters, each occurring exactly once in each row and once in each column. Write a
Programming Exercises 319 program that prompts the user to enter the number n and the array of characters, as shown in the sample output, and checks if the input array is a Latin square. The characters are the first n characters starting from A. Enter number n: 4 Enter 4 rows of letters separated by spaces: A B C D B A D C C D B A D C A B The input array is a Latin square
Enter number n: 3 Enter 3 rows of letters separated by spaces: A F D Wrong input: the letters must be from A to C
**8.37
(Guess the capitals) Write a program that repeatedly prompts the user to enter a capital for a state. Upon receiving the user input, the program reports whether the answer is correct. Assume that 50 states and their capitals are stored in a twodimensional array, as shown in Figure 8.10. The program prompts the user to answer all states’ capitals and displays the total correct count. The user’s answer is not case-sensitive. Alabama Alaska Arizona ... ...
Montgomery Juneau Phoenix ... ...
FIGURE 8.10 A two-dimensional array stores states and their capitals. Here is a sample run: What is the The correct What is the Your answer What is the ... The correct
capital of Alabama? Montogomery answer should be Montgomery capital of Alaska? Juneau is correct capital of Arizona? ... count is 35
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CHAPTER
OBJECTS AND CLASSES Objectives ■
To describe objects and classes, and use classes to model objects (§9.2).
■
To use UML graphical notation to describe classes and objects (§9.2).
■
To demonstrate how to define classes and create objects (§9.3).
■
To create objects using constructors (§9.4).
■
To access objects via object reference variables (§9.5).
■
To define a reference variable using a reference type (§9.5.1).
■
To access an object’s data and methods using the object member access operator (.) (§9.5.2).
■
To define data fields of reference types and assign default values for an object’s data fields (§9.5.3).
■
To distinguish between object reference variables and primitive data type variables (§9.5.4).
■
To use the Java library classes Date, Random, and Point2D (§9.6).
■
To distinguish between instance and static variables and methods (§9.7).
■
To define private data fields with appropriate getter and setter methods (§9.8).
■
To encapsulate data fields to make classes easy to maintain (§9.9).
■
To develop methods with object arguments and differentiate between primitive-type arguments and object-type arguments (§9.10).
■
To store and process objects in arrays (§9.11).
■
To create immutable objects from immutable classes to protect the contents of objects (§9.12).
■
To determine the scope of variables in the context of a class (§9.13).
■
To use the keyword this to refer to the calling object itself (§9.14).
9
322 Chapter 9
Objects and Classes
9.1 Introduction Key Point
Object-oriented programming enables you to develop large-scale software and GUIs effectively. Having learned the material in the preceding chapters, you are able to solve many programming problems using selections, loops, methods, and arrays. However, these Java features are not sufficient for developing graphical user interfaces and large-scale software systems. Suppose you want to develop a graphical user interface (GUI, pronounced goo-ee) as shown in Figure 9.1. How would you program it?
why OOP?
Button
Label
FIGURE 9.1
Text Field Check Box
Radio Button
Combo Box
The GUI objects are created from classes.
This chapter introduces object-oriented programming, which you can use to develop GUI and large-scale software systems.
9.2 Defining Classes for Objects Key Point VideoNote
Define classes and objects object state of an object properties attributes data fields behavior actions
class contract instantiation instance
data field method constructors
A class defines the properties and behaviors for objects. Object-oriented programming (OOP) involves programming using objects. An object represents an entity in the real world that can be distinctly identified. For example, a student, a desk, a circle, a button, and even a loan can all be viewed as objects. An object has a unique identity, state, and behavior. ■
The state of an object (also known as its properties or attributes) is represented by data fields with their current values. A circle object, for example, has a data field radius, which is the property that characterizes a circle. A rectangle object has the data fields width and height, which are the properties that characterize a rectangle.
■
The behavior of an object (also known as its actions) is defined by methods. To invoke a method on an object is to ask the object to perform an action. For example, you may define methods named getArea() and getPerimeter() for circle objects. A circle object may invoke getArea() to return its area and getPerimeter() to return its perimeter. You may also define the setRadius(radius) method. A circle object can invoke this method to change its radius.
Objects of the same type are defined using a common class. A class is a template, blueprint, or contract that defines what an object’s data fields and methods will be. An object is an instance of a class. You can create many instances of a class. Creating an instance is referred to as instantiation. The terms object and instance are often interchangeable. The relationship between classes and objects is analogous to that between an apple-pie recipe and apple pies: You can make as many apple pies as you want from a single recipe. Figure 9.2 shows a class named Circle and its three objects. A Java class uses variables to define data fields and methods to define actions. Additionally, a class provides methods of a special type, known as constructors, which are invoked to create a new object. A constructor can perform any action, but constructors are designed to perform initializing actions, such as initializing the data fields of objects. Figure 9.3 shows an example of defining the class for circle objects.
9.2 Defining Classes for Objects 323 A class template
Class Name: Circle Data Fields: radius is _____ Methods: getArea getPerimeter setRadius
Circle Object 1
Circle Object 2
Circle Object 3
Data Fields: radius is 1
Data Fields: radius is 25
Data Fields: radius is 125
Three objects of the Circle class
FIGURE 9.2 A class is a template for creating objects.
class Circle { /** The radius of this circle */ double radius = 1;
Data field
/** Construct a circle object */ Circle() { } Constructors /** Construct a circle object */ Circle(double newRadius) { radius = newRadius; } /** Return the area of this circle */ double getArea() { return radius * radius * Math.PI; } /** Return the perimeter of this circle */ double getPerimeter() { return 2 * radius * Math.PI; }
Method
/** Set new radius for this circle */ double setRadius(double newRadius) { radius = newRadius; } }
FIGURE 9.3 A class is a construct that defines objects of the same type.
The Circle class is different from all of the other classes you have seen thus far. It does not have a main method and therefore cannot be run; it is merely a definition for circle objects. The class that contains the main method will be referred to in this book, for convenience, as the main class. The illustration of class templates and objects in Figure 9.2 can be standardized using Unified Modeling Language (UML) notation. This notation, as shown in Figure 9.4, is called a UML class diagram, or simply a class diagram. In the class diagram, the data field is denoted as dataFieldName: dataFieldType
The constructor is denoted as ClassName(parameterName: parameterType)
main class Unified Modeling Language (UML) class diagram
324 Chapter 9
Objects and Classes UML Class Diagram
Class name
Circle radius: double
Data fields
Circle()
Constructors and methods
Circle(newRadius: double) getArea(): double getPerimeter(): double setRadius(newRadius: double): void
FIGURE 9.4
circle1: Circle
circle2: Circle
circle3: Circle
radius = 1
radius = 25
radius = 125
UML notation for objects
Classes and objects can be represented using UML notation.
The method is denoted as methodName(parameterName: parameterType): returnType
9.3 Example: Defining Classes and Creating Objects Key Point
Classes are definitions for objects and objects are created from classes. This section gives two examples of defining classes and uses the classes to create objects. Listing 9.1 is a program that defines the Circle class and uses it to create objects. The program constructs three circle objects with radius 1, 25, and 125 and displays the radius and area of each of the three circles. It then changes the radius of the second object to 100 and displays its new radius and area.
Note To avoid a naming conflict with several enhanced versions of the Circle class introduced later in the chapter, the Circle class in this example is named SimpleCircle. For simplicity, we will still refer to the class in the text as Circle.
avoid naming conflicts
LISTING 9.1 TestSimpleCircle.java main class main method
create object
create object
create object
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
public class TestSimpleCircle { /** Main method */ public static void main(String[] args) { // Create a circle with radius 1 SimpleCircle circle1 = new SimpleCircle(); System.out.println("The area of the circle of radius " + circle1.radius + " is " + circle1.getArea()); // Create a circle with radius 25 SimpleCircle circle2 = new SimpleCircle(25); System.out.println("The area of the circle of radius " + circle2.radius + " is " + circle2.getArea()); // Create a circle with radius 125 SimpleCircle circle3 = new SimpleCircle(125); System.out.println("The area of the circle of radius " + circle3.radius + " is " + circle3.getArea()); // Modify circle radius circle2.radius = 100; // or circle2.setRadius(100) System.out.println("The area of the circle of radius " + circle2.radius + " is " + circle2.getArea());
9.3 Example: Defining Classes and Creating Objects 325 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
} } // Define the circle class with two constructors class SimpleCircle { double radius;
class SimpleCircle data field
/** Construct a circle with radius 1 */ SimpleCircle() { radius = 1; }
no-arg constructor
/** Construct a circle with a specified radius */ SimpleCircle(double newRadius) { radius = newRadius; }
second constructor
/** Return the area of this circle */ double getArea() { return radius * radius * Math.PI; }
getArea
/** Return the perimeter of this circle */ double getPerimeter() { return 2 * radius * Math.PI; }
getPerimeter
/** Set a new radius for this circle */ void setRadius(double newRadius) { radius = newRadius; }
setRadius
}
The The The The
area area area area
of of of of
the the the the
circle circle circle circle
of of of of
radius radius radius radius
1.0 is 3.141592653589793 25.0 is 1963.4954084936207 125.0 is 49087.385212340516 100.0 is 31415.926535897932
The program contains two classes. The first of these, TestSimpleCircle, is the main class. Its sole purpose is to test the second class, SimpleCircle. Such a program that uses the class is often referred to as a client of the class. When you run the program, the Java runtime system invokes the main method in the main class. You can put the two classes into one file, but only one class in the file can be a public class. Furthermore, the public class must have the same name as the file name. Therefore, the file name is TestSimpleCircle.java, since TestSimpleCircle is public. Each class in the source code is compiled into a .class file. When you compile TestSimpleCircle.java, two class files TestSimpleCircle.class and SimpleCircle.class are generated, as shown in Figure 9.5. // File TestSimpleCircle.java public class TestSimpleCircle { … } class SimpleCircle { … }
generates
TestSimpleCircle.class
generates
SimpleCircle.class
Java compiled Compiler by
FIGURE 9.5 Each class in the source code file is compiled into a .class file.
client public class
326 Chapter 9
Objects and Classes The main class contains the main method (line 3) that creates three objects. As in creating an array, the new operator is used to create an object from the constructor: new SimpleCircle() creates an object with radius 1 (line 5), new SimpleCircle(25) creates an object with radius 25 (line 10), and new SimpleCircle(125) creates an object with radius 125 (line 15). These three objects (referenced by circle1, circle2, and circle3) have different data but the same methods. Therefore, you can compute their respective areas by using the getArea() method. The data fields can be accessed via the reference of the object using circle1.radius, circle2.radius, and circle3.radius, respectively. The object can invoke its method via the reference of the object using circle1.getArea(), circle2.getArea(), and circle3.getArea(), respectively. These three objects are independent. The radius of circle2 is changed to 100 in line 20. The object’s new radius and area are displayed in lines 21–22. There are many ways to write Java programs. For instance, you can combine the two classes in the example into one, as shown in Listing 9.2.
LISTING 9.2 SimpleCircle.java main method
data field
no-arg constructor
second constructor
method
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
public class SimpleCircle { /** Main method */ public static void main(String[] args) { // Create a circle with radius 1 SimpleCircle circle1 = new SimpleCircle(); System.out.println("The area of the circle of radius " + circle1.radius + " is " + circle1.getArea()); // Create a circle with radius 25 SimpleCircle circle2 = new SimpleCircle(25); System.out.println("The area of the circle of radius " + circle2.radius + " is " + circle2.getArea()); // Create a circle with radius 125 SimpleCircle circle3 = new SimpleCircle(125); System.out.println("The area of the circle of radius " + circle3.radius + " is " + circle3.getArea()); // Modify circle radius circle2.radius = 100; System.out.println("The area of the circle of radius " + circle2.radius + " is " + circle2.getArea()); } double radius; /** Construct a circle with radius 1 */ SimpleCircle() { radius = 1; } /** Construct a circle with a specified radius */ SimpleCircle(double newRadius) { radius = newRadius; } /** Return the area of this circle */ double getArea() { return radius * radius * Math.PI; }
9.3 Example: Defining Classes and Creating Objects 327 42 43 44 45 46 47 48 49 50 51
/** Return the perimeter of this circle */ double getPerimeter() { return 2 * radius * Math.PI; } /** Set a new radius for this circle */ void setRadius(double newRadius) { radius = newRadius; } }
Since the combined class has a main method, it can be executed by the Java interpreter. The main method is the same as that in Listing 9.1. This demonstrates that you can test a class by simply adding a main method in the same class. As another example, consider television sets. Each TV is an object with states (current channel, current volume level, power on or off) and behaviors (change channels, adjust volume, turn on/off). You can use a class to model TV sets. The UML diagram for the class is shown in Figure 9.6.
TV channel: int volumeLevel: int The + sign indicates public modifier
The current channel (1 to 120) of this TV.
on: boolean
The current volume level (1 to 7) of this TV. Indicates whether this TV is on/off.
+TV() +turnOn(): void +turnOff(): void
Constructs a default TV object. Turns on this TV. Turns off this TV.
+setChannel(newChannel: int): void +setVolume(newVolumeLevel: int): void +channelUp(): void
Sets a new channel for this TV. Sets a new volume level for this TV. Increases the channel number by 1. Decreases the channel number by 1.
+channelDown(): void +volumeUp(): void +volumeDown(): void
Increases the volume level by 1. Decreases the volume level by 1.
FIGURE 9.6 The TV class models TV sets.
Listing 9.3 gives a program that defines the TV class.
LISTING 9.3 TV.java 1 2 3 4 5 6 7 8 9 10 11 12 13
public class TV { int channel = 1; // Default channel is 1 int volumeLevel = 1; // Default volume level is 1 boolean on = false; // TV is off
data fields
public TV() { }
constructor
public void turnOn() { on = true; }
turn on TV
public void turnOff() {
turn off TV
328 Chapter 9
set a new channel
set a new volume
increase channel
decrease channel
increase volume
decrease volume
Objects and Classes 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
on = false; } public void setChannel(int newChannel) { if (on && newChannel >= 1 && newChannel <= 120) channel = newChannel; } public void setVolume(int newVolumeLevel) { if (on && newVolumeLevel >= 1 && newVolumeLevel <= 7) volumeLevel = newVolumeLevel; } public void channelUp() { if (on && channel < 120) channel++; } public void channelDown() { if (on && channel > 1) channel—–; } public void volumeUp() { if (on && volumeLevel < 7) volumeLevel++; } public void volumeDown() { if (on && volumeLevel > 1) volumeLevel—–; } }
The constructor and methods in the TV class are defined public so they can be accessed from other classes. Note that the channel and volume level are not changed if the TV is not on. Before either of these is changed, its current value is checked to ensure that it is within the correct range. Listing 9.4 gives a program that uses the TV class to create two objects.
LISTING 9.4 TestTV.java main method
create a TV turn on set a new channel set a new volume create a TV turn on increase channel increase volume display state
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
public class TestTV { public static void main(String[] args) { TV tv1 = new TV(); tv1.turnOn(); tv1.setChannel(30); tv1.setVolume(3); TV tv2 = new TV(); tv2.turnOn(); tv2.channelUp(); tv2.channelUp(); tv2.volumeUp(); System.out.println("tv1's + " and volume level is System.out.println("tv2's + " and volume level is } }
channel is " + tv1.channel " + tv1.volumeLevel); channel is " + tv2.channel " + tv2.volumeLevel);
9.4 Constructing Objects Using Constructors 329 tv1's channel is 30 and volume level is 3 tv2's channel is 3 and volume level is 2
The program creates two objects in lines 3 and 8 and invokes the methods on the objects to perform actions for setting channels and volume levels and for increasing channels and volumes. The program displays the state of the objects in lines 14–17. The methods are invoked using syntax such as tv1.turnOn() (line 4). The data fields are accessed using syntax such as tv1.channel (line 14). These examples have given you a glimpse of classes and objects. You may have many questions regarding constructors, objects, reference variables, accessing data fields, and invoking object’s methods. The sections that follow discuss these issues in detail.
9.1 9.2 9.3 9.4
Describe the relationship between an object and its defining class. How do you define a class? How do you declare an object’s reference variable? How do you create an object?
✓
Check Point
9.4 Constructing Objects Using Constructors A constructor is invoked to create an object using the new operator. Constructors are a special kind of method. They have three peculiarities:
Key Point
■
A constructor must have the same name as the class itself.
constructor’s name
■
Constructors do not have a return type—not even void.
no return type
■
Constructors are invoked using the new operator when an object is created. Constructors play the role of initializing objects.
new operator
The constructor has exactly the same name as its defining class. Like regular methods, constructors can be overloaded (i.e., multiple constructors can have the same name but different signatures), making it easy to construct objects with different initial data values. It is a common mistake to put the void keyword in front of a constructor. For example, public void Circle() { }
In this case, Circle() is a method, not a constructor. Constructors are used to construct objects. To construct an object from a class, invoke a constructor of the class using the new operator, as follows:
overloaded constructors
no void
constructing objects
new ClassName(arguments);
For example, new Circle() creates an object of the Circle class using the first constructor defined in the Circle class, and new Circle(25) creates an object using the second constructor defined in the Circle class. A class normally provides a constructor without arguments (e.g., Circle()). Such a constructor is referred to as a no-arg or no-argument constructor. A class may be defined without constructors. In this case, a public no-arg constructor with an empty body is implicitly defined in the class. This constructor, called a default constructor, is provided automatically only if no constructors are explicitly defined in the class.
9.5 9.6
What are the differences between constructors and methods? When will a class have a default constructor?
no-arg constructor default constructor
✓
Check Point
330 Chapter 9
Objects and Classes
9.5 Accessing Objects via Reference Variables Key Point
An object’s data and methods can be accessed through the dot (.) operator via the object’s reference variable. Newly created objects are allocated in the memory. They can be accessed via reference variables.
9.5.1 reference variable
Reference Variables and Reference Types
Objects are accessed via the object’s reference variables, which contain references to the objects. Such variables are declared using the following syntax: ClassName objectRefVar;
reference type
A class is essentially a programmer-defined type. A class is a reference type, which means that a variable of the class type can reference an instance of the class. The following statement declares the variable myCircle to be of the Circle type: Circle myCircle;
The variable myCircle can reference a Circle object. The next statement creates an object and assigns its reference to myCircle: myCircle = new Circle();
You can write a single statement that combines the declaration of an object reference variable, the creation of an object, and the assigning of an object reference to the variable with the following syntax: ClassName objectRefVar = new ClassName();
Here is an example: Circle myCircle = new Circle();
The variable myCircle holds a reference to a Circle object.
Note An object reference variable that appears to hold an object actually contains a reference to that object. Strictly speaking, an object reference variable and an object are different, but most of the time the distinction can be ignored. Therefore, it is fine, for simplicity, to say that myCircle is a Circle object rather than use the longer-winded description that myCircle is a variable that contains a reference to a Circle object.
object vs. object reference variable
Note Arrays are treated as objects in Java. Arrays are created using the new operator. An array variable is actually a variable that contains a reference to an array.
array object
9.5.2 dot operator (.)
Accessing an Object’s Data and Methods
In OOP terminology, an object’s member refers to its data fields and methods. After an object is created, its data can be accessed and its methods can be invoked using the dot operator (.), also known as the object member access operator: ■ objectRefVar.dataField
references a data field in the object.
■ objectRefVar.method(arguments)
invokes a method on the object.
9.5 Accessing Objects via Reference Variables 331 For example, myCircle.radius references the radius in myCircle, and myCircle .getArea() invokes the getArea method on myCircle. Methods are invoked as operations on objects. The data field radius is referred to as an instance variable, because it is dependent on a specific instance. For the same reason, the method getArea is referred to as an instance method, because you can invoke it only on a specific instance. The object on which an instance method is invoked is called a calling object.
instance variable instance method calling object
Caution Recall that you use Math.methodName(arguments) (e.g., Math.pow(3, 2.5)) to invoke a method in the Math class. Can you invoke getArea() using Circle.getArea()? The answer is no. All the methods in the Math class are static methods, which are defined using the static keyword. However, getArea() is an instance method, and thus nonstatic. It must be invoked from an object using objectRefVar.methodName(arguments) (e.g., myCircle.getArea()). Further explanation is given in Section 9.7, Static Variables, Constants, and Methods.
invoking methods
Note Usually you create an object and assign it to a variable, and then later you can use the variable to reference the object. Occasionally an object does not need to be referenced later. In this case, you can create an object without explicitly assigning it to a variable using the syntax: new Circle();
or System.out.println("Area is " + new Circle(5).getArea());
The former statement creates a Circle object. The latter creates a Circle object and invokes its getArea method to return its area. An object created in this way is known as an anonymous object.
9.5.3
anonymous object
Reference Data Fields and the null Value
The data fields can be of reference types. For example, the following Student class contains a data field name of the String type. String is a predefined Java class.
reference data fields
class Student { String name; // name has the default value null int age; // age has the default value 0 boolean isScienceMajor; // isScienceMajor has default value false char gender; // gender has default value '\u0000' }
If a data field of a reference type does not reference any object, the data field holds a special Java value, null. null is a literal just like true and false. While true and false are Boolean literals, null is a literal for a reference type. The default value of a data field is null for a reference type, 0 for a numeric type, false for a boolean type, and \u0000 for a char type. However, Java assigns no default value to a local variable inside a method. The following code displays the default values of the data fields name, age, isScienceMajor, and gender for a Student object: class Test { public static void main(String[] args) { Student student = new Student(); System.out.println("name? " + student.name);
null value
default field values
332 Chapter 9
Objects and Classes System.out.println("age? " + student.age); System.out.println("isScienceMajor? " + student.isScienceMajor); System.out.println("gender? " + student.gender); } }
The following code has a compile error, because the local variables x and y are not initialized: class Test { public static void main(String[] args) { int x; // x has no default value String y; // y has no default value System.out.println("x is " + x); System.out.println("y is " + y); } }
Caution NullPointerException is a common runtime error. It occurs when you invoke a method on a reference variable with a null value. Make sure you assign an object
NullPointerException
reference to the variable before invoking the method through the reference variable (See Checkpoint Question 9.11c).
9.5.4
Differences between Variables of Primitive Types and Reference Types
Every variable represents a memory location that holds a value. When you declare a variable, you are telling the compiler what type of value the variable can hold. For a variable of a primitive type, the value is of the primitive type. For a variable of a reference type, the value is a reference to where an object is located. For example, as shown in Figure 9.7, the value of int variable i is int value 1, and the value of Circle object c holds a reference to where the contents of the Circle object are stored in memory. When you assign one variable to another, the other variable is set to the same value. For a variable of a primitive type, the real value of one variable is assigned to the other variable. For a variable of a reference type, the reference of one variable is assigned to the other variable. As shown in Figure 9.8, the assignment statement i = j copies the contents of j into i Created using new Circle() Primitive type
int i = 1
Object type
Circle c c
1
i
c: Circle
reference
radius = 1
FIGURE 9.7 A variable of a primitive type holds a value of the primitive type, and a variable of a reference type holds a reference to where an object is stored in memory. Primitive type assignment i = j Before:
FIGURE 9.8
After:
i
1
i
2
j
2
j
2
Primitive variable j is copied to variable i.
9.5 Accessing Objects via Reference Variables 333 for primitive variables. As shown in Figure 9.9, the assignment statement c1 = c2 copies the reference of c2 into c1 for reference variables. After the assignment, variables c1 and c2 refer to the same object. Object type assignment c1 = c2 Before:
After:
c1
c1
c2
c2
c2: Circle
c1: Circle
c2: Circle
c1: Circle
radius = 9
radius = 5
radius = 9
radius = 5
FIGURE 9.9 Reference variable c2 is copied to variable c1.
Note As illustrated in Figure 9.9, after the assignment statement c1 = c2, c1 points to the same object referenced by c2. The object previously referenced by c1 is no longer useful and therefore is now known as garbage. Garbage occupies memory space, so the Java runtime system detects garbage and automatically reclaims the space it occupies. This process is called garbage collection.
garbage garbage collection
Tip If you know that an object is no longer needed, you can explicitly assign null to a reference variable for the object. The JVM will automatically collect the space if the object is not referenced by any reference variable.
9.7 9.8 9.9 9.10 9.11 1 2 3 4 5
Check Point
What is an anonymous object? What is NullPointerException? Is an array an object or a primitive type value? Can an array contain elements of an object type? Describe the default value for the elements of an array. What is wrong with each of the following programs? public class ShowErrors { public static void main(String[] args) { ShowErrors t = new ShowErrors(5); } }
1 2 3 4 5 6
public class ShowErrors { public static void main(String[] args) { ShowErrors t = new ShowErrors(); t.x(); } }
(a) 1 2 3 4 5 6 7 8
✓
Which operator is used to access a data field or invoke a method from an object?
public class ShowErrors { public void method1() { Circle c; System.out.println("What is radius " + c.getRadius()); c = new Circle(); } }
(c)
(b) 1 2 3 4 5 6 7 8 9 10
public class ShowErrors { public static void main(String[] args) { C c = new C(5.0); System.out.println(c.value); } } class C { int value = 2; } (d)
334 Chapter 9
Objects and Classes 9.12
What is wrong in the following code? 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
9.13
class Test { public static void main(String[] args) { A a = new A(); a.print(); } } class A { String s; A(String newS) { s = newS; } public void print() { System.out.print(s); } }
What is the output of the following code? public class A { boolean x; public static void main(String[] args) { A a = new A(); System.out.println(a.x); } }
9.6 Using Classes from the Java Library Key Point
VideoNote
The Java API contains a rich set of classes for developing Java programs. Listing 9.1 defined the SimpleCircle class and created objects from the class. You will frequently use the classes in the Java library to develop programs. This section gives some examples of the classes in the Java library.
9.6.1
Use classes
java.util.Date class
The Date Class
In Listing 2.7, ShowCurrentTime.java, you learned how to obtain the current time using System.currentTimeMillis(). You used the division and remainder operators to extract the current second, minute, and hour. Java provides a system-independent encapsulation of date and time in the java.util.Date class, as shown in Figure 9.10. java.util.Date
+Date()
Constructs a Date object for the current time.
+Date(elapseTime: long) +toString(): String
Constructs a Date object for a given time in milliseconds elapsed since January 1, 1970, GMT. Returns a string representing the date and time.
+getTime(): long
Returns the number of milliseconds since January 1,
+setTime(elapseTime: long): void
1970, GMT. Sets a new elapse time in the object.
FIGURE 9.10 A Date object represents a specific date and time.
9.6 Using Classes from the Java Library 335 You can use the no-arg constructor in the Date class to create an instance for the current date and time, the getTime() method to return the elapsed time since January 1, 1970, GMT, and the toString() method to return the date and time as a string. For example, the following code java.util.Date date = new java.util.Date(); System.out.println("The elapsed time since Jan 1, 1970 is " + date.getTime() + " milliseconds"); System.out.println(date.toString());
create object get elapsed time invoke toString
displays the output like this: The elapsed time since Jan 1, 1970 is 1324903419651 milliseconds Mon Dec 26 07:43:39 EST 2011
The Date class has another constructor, Date(long elapseTime), which can be used to construct a Date object for a given time in milliseconds elapsed since January 1, 1970, GMT.
9.6.2
The Random Class
You have used Math.random() to obtain a random double value between 0.0 and 1.0 (excluding 1.0). Another way to generate random numbers is to use the java.util.Random class, as shown in Figure 9.11, which can generate a random int, long, double, float, and boolean value.
java.util.Random +Random()
Constructs a Random object with the current time as its seed.
+Random(seed: long)
Constructs a Random object with a specified seed.
+nextInt(): int
Returns a random int value.
+nextInt(n: int): int
Returns a random int value between 0 and n (excluding n).
+nextLong(): long
Returns a random long value.
+nextDouble(): double
Returns a random double value between 0.0 and 1.0 (excluding 1.0).
+nextFloat(): float
Returns a random float value between 0.0F and 1.0F (excluding 1.0F).
+nextBoolean(): boolean
Returns a random boolean value.
FIGURE 9.11 A Random object can be used to generate random values.
When you create a Random object, you have to specify a seed or use the default seed. A seed is a number used to initialize a random number generator. The no-arg constructor creates a Random object using the current elapsed time as its seed. If two Random objects have the same seed, they will generate identical sequences of numbers. For example, the following code creates two Random objects with the same seed, 3. Random random1 = new Random(3); System.out.print("From random1: "); for (int i = 0; i < 10; i++) System.out.print(random1.nextInt(1000) + " "); Random random2 = new Random(3); System.out.print("\nFrom random2: "); for (int i = 0; i < 10; i++) System.out.print(random2.nextInt(1000) + " ");
336 Chapter 9
Objects and Classes The code generates the same sequence of random int values: From random1: 734 660 210 581 128 202 549 564 459 961 From random2: 734 660 210 581 128 202 549 564 459 961
Note The ability to generate the same sequence of random values is useful in software testing and many other applications. In software testing, often you need to reproduce the test cases from a fixed sequence of random numbers.
same sequence
9.6.3
The Point2D Class
Java API has a conveninent Point2D class in the javafx.geometry package for representing a point in a two-dimensional plane. The UML diagram for the class is shown in Figure 9.12.
javafx.geometry.Point2D +Point2D(x: double, y: double) +distance(x: double, y: double): double +distance(p: Point2D): double +getX(): double +getY(): double +toString(): String
Constructs a Point2D object with the specified x- and y-coordinates. Returns the distance between this point and the specified point (x, y). Returns the distance between this point and the specified point p. Returns the x-coordinate from this point. Returns the y-coordinate from this point. Returns a string representation for the point.
FIGURE 9.12 A Point2D object represents a point with x- and y-coordinates. You can create a Point2D object for a point with the specified x- and y-coordinates, use the distance method to compute the distance from this point to another point, and use the toString() method to return a string representation of the point. Lisitng 9.5 gives an example of using this class.
LISTING 9.5 TestPoint2D.java
create an object invoke toString()
get distance
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
import java.util.Scanner; import javafx.geometry.Point2D; public class TestPoint2D { public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.print("Enter point1's x-, y-coordinates: "); double x1 = input.nextDouble(); double y1 = input.nextDouble(); System.out.print("Enter point2's x-, y-coordinates: "); double x2 = input.nextDouble(); double y2 = input.nextDouble(); Point2D p1 = new Point2D(x1, y1); Point2D p2 = new Point2D(x2, y2); System.out.println("p1 is " + p1.toString()); System.out.println("p2 is " + p2.toString()); System.out.println("The distance between p1 and p2 is " + p1.distance(p2)); } }
9.7 Static Variables, Constants, and Methods 337 Enter point1's x-, y-coordinates: 1.5 5.5 Enter point2's x-, y-coordinates: -5.3 -4.4 p1 is Point2D [x = 1.5, y = 5.5] p2 is Point2D [x = -5.3, y = -4.4] The distance between p1 and p2 is 12.010412149464313
This program creates two objects of the Point2D class (lines 15–16). The toString() method returns a string that describes the object (lines 17–18). Invoking p1.distance(p2) returns the distance between the two points (line 20).
9.14 9.15 9.16
How do you create a Date for the current time? How do you display the current time? How do you create a Point2D? Suppose p1 and p2 are two instances of Point2D? How do you obtain the distance between the two points? Which packages contain the classes Date, Random, Point2D, System, and Math?
✓
Check Point
9.7 Static Variables, Constants, and Methods A static variable is shared by all objects of the class. A static method cannot access instance members of the class. The data field radius in the circle class is known as an instance variable. An instance variable is tied to a specific instance of the class; it is not shared among objects of the same class. For example, suppose that you create the following objects: Circle circle1 = new Circle(); Circle circle2 = new Circle(5);
The radius in circle1 is independent of the radius in circle2 and is stored in a different memory location. Changes made to circle1’s radius do not affect circle2’s radius, and vice versa. If you want all the instances of a class to share data, use static variables, also known as class variables. Static variables store values for the variables in a common memory location. Because of this common location, if one object changes the value of a static variable, all objects of the same class are affected. Java supports static methods as well as static variables. Static methods can be called without creating an instance of the class. Let’s modify the Circle class by adding a static variable numberOfObjects to count the number of circle objects created. When the first object of this class is created, numberOfObjects is 1. When the second object is created, numberOfObjects becomes 2. The UML of the new circle class is shown in Figure 9.13. The Circle class defines the instance variable radius and the static variable numberOfObjects, the instance methods getRadius, setRadius, and getArea, and the static method getNumberOfObjects. (Note that static variables and methods are underlined in the UML class diagram.) To declare a static variable or define a static method, put the modifier static in the variable or method declaration. The static variable numberOfObjects and the static method getNumberOfObjects() can be declared as follows:
Key Point Static vs. instance instance variable
VideoNote
Static vs. instance
static variable
static method
static int numberOfObjects;
declare static variable
static int getNumberObjects() { return numberOfObjects; }
define static method
338 Chapter 9
Objects and Classes
UML Notation: underline: static variables or methods instantiate
Circle
circle1: Circle
Memory
radius = 1 numberOfObjects = 2
1
radius
2
numberOfObjects
5
radius
radius: double numberOfObjects: int getNumberOfObjects(): int getArea(): double
instantiate
After two Circle Objects were created, numberOfObjects is 2.
circle2: Circle radius = 5 numberOfObjects = 2
FIGURE 9.13 Instance variables belong to the instances and have memory storage independent of one another. Static variables are shared by all the instances of the same class.
declare constant
Constants in a class are shared by all objects of the class. Thus, constants should be declared as final static. For example, the constant PI in the Math class is defined as: final static double PI = 3.14159265358979323846;
The new circle class, named CircleWithStaticMembers, is defined in Listing 9.6:
LISTING 9.6 CircleWithStaticMembers.java
static variable
increase by 1
increase by 1
static method
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
public class CircleWithStaticMembers { /** The radius of the circle */ double radius; /** The number of objects created */ static int numberOfObjects = 0; /** Construct a circle with radius 1 */ CircleWithStaticMembers() { radius = 1; numberOfObjects++; } /** Construct a circle with a specified radius */ CircleWithStaticMembers(double newRadius) { radius = newRadius; numberOfObjects++; } /** Return numberOfObjects */ static int getNumberOfObjects() { return numberOfObjects; } /** Return the area of this circle */ double getArea() { return radius * radius * Math.PI; } }
Method getNumberOfObjects() in CircleWithStaticMembers is a static method. All the methods in the Math class are static. The main method is static, too.
9.7 Static Variables, Constants, and Methods 339 Instance methods (e.g., getArea()) and instance data (e.g., radius) belong to instances and can be used only after the instances are created. They are accessed via a reference variable. Static methods (e.g., getNumberOfObjects()) and static data (e.g., numberOfObjects) can be accessed from a reference variable or from their class name. The program in Listing 9.7 demonstrates how to use instance and static variables and methods and illustrates the effects of using them.
LISTING 9.7 TestCircleWithStaticMembers.java 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
public class TestCircleWithStaticMembers { /** Main method */ public static void main(String[] args) { System.out.println("Before creating objects"); System.out.println("The number of Circle objects is " + CircleWithStaticMembers.numberOfObjects);
static variable
// Create c1 CircleWithStaticMembers c1 = new CircleWithStaticMembers(); // Display c1 BEFORE c2 is created System.out.println("\nAfter creating c1"); System.out.println("c1: radius (" + c1.radius + ") and number of Circle objects (" + c1.numberOfObjects + ")");
instance variable static variable
// Create c2 CircleWithStaticMembers c2 = new CircleWithStaticMembers(5); // Modify c1 c1.radius = 9; // Display c1 and c2 AFTER c2 was created System.out.println("\nAfter creating c2 and modifying c1"); System.out.println("c1: radius (" + c1.radius + ") and number of Circle objects (" + c1.numberOfObjects + ")"); System.out.println("c2: radius (" + c2.radius + ") and number of Circle objects (" + c2.numberOfObjects + ")"); } }
Before creating objects The number of Circle objects is 0 After creating c1 c1: radius (1.0) and number of Circle objects (1) After creating c2 and modifying c1 c1: radius (9.0) and number of Circle objects (2) c2: radius (5.0) and number of Circle objects (2)
When you compile TestCircleWithStaticMembers.java, the Java compiler automatically compiles CircleWithStaticMembers.java if it has not been compiled since the last change. Static variables and methods can be accessed without creating objects. Line 6 displays the number of objects, which is 0, since no objects have been created.
instance variable
static variable
static variable
340 Chapter 9
Objects and Classes The main method creates two circles, c1 and c2 (lines 9, 18). The instance variable radius in c1 is modified to become 9 (line 21). This change does not affect the instance variable radius in c2, since these two instance variables are independent. The static variable numberOfObjects becomes 1 after c1 is created (line 9), and it becomes 2 after c2 is created (line 18). Note that PI is a constant defined in Math, and Math.PI references the constant. c1.numberOfObjects (line 27) and c2.numberOfObjects (line 30) are better replaced by CircleWithStaticMembers.numberOfObjects. This improves readability, because other programmers can easily recognize the static variable. You can also replace CircleWithStaticMembers.numberOfObjects with CircleWithStaticMembers. getNumberOfObjects().
Tip Use ClassName.methodName(arguments) to invoke a static method and ClassName.staticVariable to access a static variable. This improves readability, because this makes the static method and data easy to spot.
use class name
An instance method can invoke an instance or static method and access an instance or static data field. A static method can invoke a static method and access a static data field. However, a static method cannot invoke an instance method or access an instance data field, since static methods and static data fields don’t belong to a particular object. The relationship between static and instance members is summarized in the following diagram: invoke access An instance method
invoke
An instance method
access
An instance data field A static method
invoke
A static method
access
A static data field
invoke access
An instance method An instance data field A static method A static data field
For example, the following code is wrong. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
public class A { int i = 5; static int k = 2; public static void main(String[] args) { int j = i; // Wrong because i is an instance variable m1(); // Wrong because m1() is an instance method } public void m1() { // Correct since instance and static variables and methods // can be used in an instance method i = i + k + m2(i, k); } public static int m2(int i, int j) { return (int)(Math.pow(i, j)); } }
9.7 Static Variables, Constants, and Methods 341 Note that if you replace the preceding code with the following new code, the program would be fine, because the instance data field i and method m1 are now accessed from an object a (lines 7–8): 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
public class A { int i = 5; static int k = 2; public static void main(String[] args) { A a = new A(); int j = a.i; // OK, a.i accesses the object's instance variable a.m1(); // OK. a.m1() invokes the object's instance method } public void m1() { i = i + k + m2(i, k); } public static int m2(int i, int j) { return (int)(Math.pow(i, j)); } }
Design Guide How do you decide whether a variable or a method should be an instance one or a static one? A variable or a method that is dependent on a specific instance of the class should be an instance variable or method. A variable or a method that is not dependent on a specific instance of the class should be a static variable or method. For example, every circle has its own radius, so the radius is dependent on a specific circle. Therefore, radius is an instance variable of the Circle class. Since the getArea method is dependent on a specific circle, it is an instance method. None of the methods in the Math class, such as random, pow, sin, and cos, is dependent on a specific instance. Therefore, these methods are static methods. The main method is static and can be invoked directly from a class.
instance or static?
Caution It is a common design error to define an instance method that should have been defined as static. For example, the method factorial(int n) should be defined as static, as shown next, because it is independent of any specific instance. public class Test { public int factorial(int n) { int result = 1; for (int i = 1; i <= n; i ++) result *= i;
public class Test { public static int factorial(int n) { int result = 1; for (int i = 1; i <= n; i++) result *= i;
return result;
return result;
}
}
}
} (a) Wrong design
9.17
common design error
(b) Correct design
Suppose that the class F is defined in (a). Let f be an instance of F. Which of the statements in (b) are correct?
✓
Check Point
342 Chapter 9
Objects and Classes public class F { int i; static String s; void imethod() { } static void smethod() { }
System.out.println(f.i); System.out.println(f.s); f.imethod(); f.smethod(); System.out.println(F.i); System.out.println(F.s); F.imethod(); F.smethod();
} (a)
9.18
(b)
Add the static keyword in the place of ? if appropriate. public class Test { int count; public ? void main(String[] args) { ... } public ? int getCount() { return count; } public ? int factorial(int n) { int result = 1; for (int i = 1; i <= n; i++) result *= i; return result; } }
9.19
Can you invoke an instance method or reference an instance variable from a static method? Can you invoke a static method or reference a static variable from an instance method? What is wrong in the following code? 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
public class C { public static void main(String[] args) { method1(); } public void method1() { method2(); } public static void method2() { System.out.println("What is radius " + c.getRadius()); } Circle c = new Circle(); }
9.8 Visibility Modifiers Key Point
package-private (or package-access)
Visibility modifiers can be used to specify the visibility of a class and its members. You can use the public visibility modifier for classes, methods, and data fields to denote that they can be accessed from any other classes. If no visibility modifier is used, then by default the classes, methods, and data fields are accessible by any class in the same package. This is known as package-private or package-access.
9.8 Visibility Modifiers 343 Note Packages can be used to organize classes. To do so, you need to add the following line as the first noncomment and nonblank statement in the program:
using packages
package packageName;
If a class is defined without the package statement, it is said to be placed in the default package. Java recommends that you place classes into packages rather than using a default package. For simplicity, however, this book uses default packages. For more information on packages, see Supplement III.E, Packages.
In addition to the public and default visibility modifiers, Java provides the private and protected modifiers for class members. This section introduces the private modifier. The protected modifier will be introduced in Section 11.14, The protected Data and Methods. The private modifier makes methods and data fields accessible only from within its own class. Figure 9.14 illustrates how a public, default, and private data field or method in class C1 can be accessed from a class C2 in the same package and from a class C3 in a different package. package p1;
package p1;
package p2;
public class C1 { public int x; int y; private int z;
public class C2 { void aMethod() { C1 o = new C1(); can access o.x; can access o.y; cannot access o.z;
public class C3 { void aMethod() { C1 o = new C1(); can access o.x; cannot access o.y; cannot access o.z;
public void m1() { } void m2() { } private void m3() { } }
can invoke o.m1(); can invoke o.m2(); cannot invoke o.m3();
can invoke o.m1(); cannot invoke o.m2(); cannot invoke o.m3();
}
}
}
}
FIGURE 9.14 The private modifier restricts access to its defining class, the default modifier restricts access to a package, and the public modifier enables unrestricted access. If a class is not defined as public, it can be accessed only within the same package. As shown in Figure 9.15, C1 can be accessed from C2 but not from C3. package p1;
package p1;
package p2;
class C1 { ... }
public class C2 { can access C1 }
public class C3 { cannot access C1; can access C2; }
FIGURE 9.15 A nonpublic class has package-access. A visibility modifier specifies how data fields and methods in a class can be accessed from outside the class. There is no restriction on accessing data fields and methods from inside the class. As shown in Figure 9.16b, an object c of class C cannot access its private members, because c is in the Test class. As shown in Figure 9.16a, an object c of class C can access its private members, because c is defined inside its own class.
inside access
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public class C { private boolean x; public static void main(String[] args) { C c = new C(); System.out.println(c.x); System.out.println(c.convert()); }
public class Test { public static void main(String[] args) { C c = new C(); System.out.println(c.x); System.out.println(c.convert()); } }
private int convert() { return x ? 1 : -1; } } (a) This is okay because object c is used inside the class C.
(b) This is wrong because x and convert are private in class C.
FIGURE 9.16 An object can access its private members if it is defined in its own class.
Caution The private modifier applies only to the members of a class. The public modifier can apply to a class or members of a class. Using the modifiers public and private on local variables would cause a compile error.
Note In most cases, the constructor should be public. However, if you want to prohibit the user from creating an instance of a class, use a private constructor. For example, there is no reason to create an instance from the Math class, because all of its data fields and methods are static. To prevent the user from creating objects from the Math class, the constructor in java.lang.Math is defined as follows:
private constructor
private Math() { }
9.9 Data Field Encapsulation Key Point Data field encapsulation
Making data fields private protects data and makes the class easy to maintain. The data fields radius and numberOfObjects in the CircleWithStaticMembers class in Listing 9.6 can be modified directly (e.g., c1.radius = 5 or CircleWithStaticMembers .numberOfObjects = 10). This is not a good practice—for two reasons: ■
First, data may be tampered with. For example, numberOfObjects is to count the number of objects created, but it may be mistakenly set to an arbitrary value (e.g., CircleWithStaticMembers.numberOfObjects = 10).
■
Second, the class becomes difficult to maintain and vulnerable to bugs. Suppose you want to modify the CircleWithStaticMembers class to ensure that the radius is nonnegative after other programs have already used the class. You have to change not only the CircleWithStaticMembers class but also the programs that use it, because the clients may have modified the radius directly (e.g., c1.radius = -5).
VideoNote
Data field encapsulation
data field encapsulation
To prevent direct modifications of data fields, you should declare the data fields private, using the private modifier. This is known as data field encapsulation.
9.9 Data Field Encapsulation 345 A private data field cannot be accessed by an object from outside the class that defines the private field. However, a client often needs to retrieve and modify a data field. To make a private data field accessible, provide a getter method to return its value. To enable a private data field to be updated, provide a setter method to set a new value. A getter method is also referred to as an accessor and a setter to a mutator. A getter method has the following signature:
getter (or accessor) setter (or mutator)
public returnType getPropertyName()
If the returnType is boolean, the getter method should be defined as follows by convention:
boolean accessor
public boolean isPropertyName()
A setter method has the following signature: public void setPropertyName(dataType propertyValue)
Let’s create a new circle class with a private data-field radius and its associated accessor and mutator methods. The class diagram is shown in Figure 9.17. The new circle class, named CircleWithPrivateDataFields, is defined in Listing 9.8:
The - sign indicates a private modifier
Circle -radius: double -numberOfObjects: int
The radius of this circle (default: 1.0). The number of circle objects created.
+Circle()
Constructs a default circle object.
+Circle(radius: double)
Constructs a circle object with the specified radius.
+getRadius(): double
Returns the radius of this circle. Sets a new radius for this circle.
+setRadius(radius: double): void +getNumberOfObjects(): int +getArea(): double
Returns the number of circle objects created. Returns the area of this circle.
FIGURE 9.17 The Circle class encapsulates circle properties and provides getter/setter and other methods.
LISTING 9.8 CircleWithPrivateDataFields.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
public class CircleWithPrivateDataFields { /** The radius of the circle */ private double radius = 1; /** The number of objects created */ private static int numberOfObjects = 0;
encapsulate radius
encapsulate numberOfObjects
/** Construct a circle with radius 1 */ public CircleWithPrivateDataFields() { numberOfObjects++; } /** Construct a circle with a specified radius */ public CircleWithPrivateDataFields(double newRadius) { radius = newRadius; numberOfObjects++;
346 Chapter 9
accessor method
mutator method
accessor method
Objects and Classes 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38
} /** Return radius */ public double getRadius() { return radius; } /** Set a new radius */ public void setRadius(double newRadius) { radius = (newRadius >= 0) ? newRadius : 0; } /** Return numberOfObjects */ public static int getNumberOfObjects() { return numberOfObjects; } /** Return the area of this circle */ public double getArea() { return radius * radius * Math.PI; } }
The getRadius() method (lines 20–22) returns the radius, and the setRadius(newRadius) method (line 25–27) sets a new radius for the object. If the new radius is negative, 0 is set as the radius for the object. Since these methods are the only ways to read and modify the radius, you have total control over how the radius property is accessed. If you have to change the implementation of these methods, you don’t need to change the client programs. This makes the class easy to maintain. Listing 9.9 gives a client program that uses the Circle class to create a Circle object and modifies the radius using the setRadius method.
LISTING 9.9 TestCircleWithPrivateDataFields.java
invoke public method
invoke public method
invoke public method
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
public class TestCircleWithPrivateDataFields { /** Main method */ public static void main(String[] args) { // Create a circle with radius 5.0 CircleWithPrivateDataFields myCircle = new CircleWithPrivateDataFields(5.0); System.out.println("The area of the circle of radius " + myCircle.getRadius() + " is " + myCircle.getArea()); // Increase myCircle's radius by 10% myCircle.setRadius(myCircle.getRadius() * 1.1); System.out.println("The area of the circle of radius " + myCircle.getRadius() + " is " + myCircle.getArea()); System.out.println("The number of objects created is " + CircleWithPrivateDataFields.getNumberOfObjects()); } }
The data field radius is declared private. Private data can be accessed only within their defining class, so you cannot use myCircle.radius in the client program. A compile error would occur if you attempted to access private data from a client. Since numberOfObjects is private, it cannot be modified. This prevents tampering. For example, the user cannot set numberOfObjects to 100. The only way to make it 100 is to create 100 objects of the Circle class.
9.10 Passing Objects to Methods 347 Suppose you combined TestCircleWithPrivateDataFields and Circle into one class by moving the main method in TestCircleWithPrivateDataFields into Circle. Could you use myCircle.radius in the main method? See Checkpoint Question 9.22 for the answer.
Design Guide To prevent data from being tampered with and to make the class easy to maintain, declare data fields private.
9.20 9.21 9.22
What is an accessor method? What is a mutator method? What are the naming conventions for accessor methods and mutator methods? What are the benefits of data field encapsulation?
✓
Check Point
In the following code, radius is private in the Circle class, and myCircle is an object of the Circle class. Does the highlighted code cause any problems? If so, explain why. public class Circle { private double radius = 1; /** Find the area of this circle */ public double getArea() { return radius * radius * Math.PI; } public static void main(String[] args) { Circle myCircle = new Circle(); System.out.println("Radius is " + myCircle.radius); } }
9.10 Passing Objects to Methods Passing an object to a method is to pass the reference of the object. You can pass objects to methods. Like passing an array, passing an object is actually passing the reference of the object. The following code passes the myCircle object as an argument to the printCircle method: 1 2 3 4 5 6 7 8 9 10 11 12 13
public class Test { public static void main(String[] args) { // CircleWithPrivateDataFields is defined in Listing 9.8 CircleWithPrivateDataFields myCircle = new CircleWithPrivateDataFields(5.0); printCircle(myCircle); }
Key Point
pass an object
public static void printCircle(CircleWithPrivateDataFields c) { System.out.println("The area of the circle of radius " + c.getRadius() + " is " + c.getArea()); } }
Java uses exactly one mode of passing arguments: pass-by-value. In the preceding code, the value of myCircle is passed to the printCircle method. This value is a reference to a Circle object. The program in Listing 9.10 demonstrates the difference between passing a primitive type value and passing a reference value.
pass-by-value
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Objects and Classes
LISTING 9.10 TestPassObject.java
pass object
object parameter
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
public class TestPassObject { /** Main method */ public static void main(String[] args) { // Create a Circle object with radius 1 CircleWithPrivateDataFields myCircle = new CircleWithPrivateDataFields(1); // Print areas for radius 1, 2, 3, 4, and 5. int n = 5; printAreas(myCircle, n); // See myCircle.radius and times System.out.println("\n" + "Radius is " + myCircle.getRadius()); System.out.println("n is " + n); } /** Print a table of areas for radius */ public static void printAreas( CircleWithPrivateDataFields c, int times) { System.out.println("Radius \t\tArea"); while (times >= 1) { System.out.println(c.getRadius() + "\t\t" + c.getArea()); c.setRadius(c.getRadius() + 1); times——; } } }
Radius Area 1.0 3.141592653589793 2.0 12.566370614359172 3.0 29.274333882308138 4.0 50.26548245743669 5.0 79.53981633974483 Radius is 6.0 n is 5
pass-by-sharing
The CircleWithPrivateDataFields class is defined in Listing 9.8. The program passes a CircleWithPrivateDataFields object myCircle and an integer value from n to invoke printAreas(myCircle, n) (line 10), which prints a table of areas for radii 1, 2, 3, 4, 5, as shown in the sample output. Figure 9.18 shows the call stack for executing the methods in the program. Note that the objects are stored in a heap (see Section 7.6). When passing an argument of a primitive data type, the value of the argument is passed. In this case, the value of n (5) is passed to times. Inside the printAreas method, the content of times is changed; this does not affect the content of n. When passing an argument of a reference type, the reference of the object is passed. In this case, c contains a reference for the object that is also referenced via myCircle. Therefore, changing the properties of the object through c inside the printAreas method has the same effect as doing so outside the method through the variable myCircle. Pass-by-value on references can be best described semantically as pass-by-sharing; that is, the object referenced in the method is the same as the object being passed.
9.10 Passing Objects to Methods 349 Stack
Pass-by-value (here the value is 5)
Activation record for the printArea method int times: 5 Circle c: reference
Heap
Pass-by-value (here the value is the reference for the object)
Activation record for the main method int n: 5 myCircle: reference
A Circle object
FIGURE 9.18 The value of n is passed to times, and the reference to myCircle is passed to c in the printAreas method.
9.23
Describe the difference between passing a parameter of a primitive type and passing a parameter of a reference type. Show the output of the following programs:
public class Test { public static void main(String[] args) { Count myCount = new Count(); int times = 0;
public Count(int c) { count = c; } public Count() { count = 1; }
System.out.println("count is " + myCount.count); System.out.println("times is " + times); }
public static void increment(Count c, int times) { c.count++; times++; } }
9.24
Show the output of the following program: public class Test { public static void main(String[] args) { Circle circle1 = new Circle(1); Circle circle2 = new Circle(2); swap1(circle1, circle2); System.out.println("After swap1: circle1 = " + circle1.radius + " circle2 = " + circle2.radius); swap2(circle1, circle2); System.out.println("After swap2: circle1 = " + circle1.radius + " circle2 = " + circle2.radius); } public static void swap1(Circle x, Circle y) { Circle temp = x; x = y; y = temp; }
Check Point
public class Count { public int count;
for (int i = 0; i < 100; i++) increment(myCount, times);
}
✓
350 Chapter 9
Objects and Classes public static void swap2(Circle x, Circle y) { double temp = x.radius; x.radius = y.radius; y.radius = temp; } } class Circle { double radius; Circle(double newRadius) { radius = newRadius; } }
9.25
Show the output of the following code:
public class Test { public static void main(String[] args) { int[] a = {1, 2}; swap(a[0], a[1]); System.out.println("a[0] = " + a[0] + " a[1] = " + a[1]); }
public class Test { public static void main(String[] args) { int[] a = {1, 2}; swap(a); System.out.println("a[0] = " + a[0] + " a[1] = " + a[1]); }
public static void swap(int n1, int n2) { int temp = n1; n1 = n2; n2 = temp; } }
public static void swap(int[] a) { int temp = a[0]; a[0] = a[1]; a[1] = temp; } }
(a)
public class Test { public static void main(String[] args) { T t = new T(); swap(t); System.out.println("e1 = " + t.e1 + " e2 = " + t.e2); } public static void swap(T t) { int temp = t.e1; t.e1 = t.e2; t.e2 = temp; } }
(b)
public class Test { public static void main(String[] args) { T t1 = new T(); T t2 = new T(); System.out.println("t1's i = " + t1.i + " and j = " + t1.j); System.out.println("t2's i = " + t2.i + " and j = " + t2.j); } } class T { static int i = 0; int j = 0;
class T { int e1 = 1; int e2 = 2; }
T() { i++; j = 1; } } (c)
(d)
9.11 Array of Objects 351 9.26
What is the output of the following programs?
import java.util.Date;
import java.util.Date;
public class Test { public static void main(String[] args) { Date date = null; m1(date); System.out.println(date); }
public class Test { public static void main(String[] args) { Date date = new Date(1234567); m1(date); System.out.println(date.getTime()); } public static void m1(Date date) { date = new Date(7654321); }
public static void m1(Date date) { date = new Date(); } }
} (a)
(b)
import java.util.Date;
import java.util.Date;
public class Test { public static void main(String[] args) { Date date = new Date(1234567); m1(date); System.out.println(date.getTime()); }
public class Test { public static void main(String[] args) { Date date = new Date(1234567); m1(date); System.out.println(date.getTime()); }
public static void m1(Date date) { date.setTime(7654321); } }
public static void m1(Date date) { date = null; } }
(c)
(d)
9.11 Array of Objects An array can hold objects as well as primitive type values. Chapter 7, Single-Dimensional Arrays, described how to create arrays of primitive type elements. You can also create arrays of objects. For example, the following statement declares and creates an array of ten Circle objects: Circle[] circleArray = new Circle[10];
To initialize circleArray, you can use a for loop like this one: for (int i = 0; i < circleArray.length; i++) { circleArray[i] = new Circle(); }
An array of objects is actually an array of reference variables. So, invoking circleArray[1]. getArea() involves two levels of referencing, as shown in Figure 9.19. circleArray references the entire array; circleArray[1] references a Circle object.
Note When an array of objects is created using the new operator, each element in the array is a reference variable with a default value of null.
Key Point
352 Chapter 9
Objects and Classes circleArray reference
circleArray[0]
Circle object 0
circleArray[1] …
Circle object 1
circleArray[9]
Circle object 9
FIGURE 9.19 In an array of objects, an element of the array contains a reference to an object. Listing 9.11 gives an example that demonstrates how to use an array of objects. The program summarizes the areas of an array of circles. The program creates circleArray, an array composed of five Circle objects; it then initializes circle radii with random values and displays the total area of the circles in the array.
LISTING 9.11 TotalArea.java
array of objects
return array of objects
pass array of 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
public class TotalArea { /** Main method */ public static void main(String[] args) { // Declare circleArray CircleWithPrivateDataFields[] circleArray; // Create circleArray circleArray = createCircleArray(); // Print circleArray and total areas of the circles printCircleArray(circleArray); } /** Create an array of Circle objects */ public static CircleWithPrivateDataFields[] createCircleArray() { CircleWithPrivateDataFields[] circleArray = new CircleWithPrivateDataFields[5]; for (int i = 0; i < circleArray.length; i++) { circleArray[i] = new CircleWithPrivateDataFields(Math.random() * 100); } // Return Circle array return circleArray; } /** Print an array of circles and their total area */ public static void printCircleArray( CircleWithPrivateDataFields[] circleArray) { System.out.printf("%-30s%-15s\n", "Radius", "Area"); for (int i = 0; i < circleArray.length; i++) { System.out.printf("%-30f%-15f\n", circleArray[i].getRadius(), circleArray[i].getArea()); } System.out.println("—————————————————————————————————————————-"); // Compute and display the result System.out.printf("%-30s%-15f\n", "The total area of circles is", sum(circleArray) ); }
9.12 Immutable Objects and Classes 353 43 44 45 46 47 48 49 50 51 52 53 54 55
/** Add circle areas */ public static double sum(CircleWithPrivateDataFields[] circleArray) { // Initialize sum double sum = 0;
pass array of objects
// Add areas to sum for (int i = 0; i < circleArray.length; i++) sum += circleArray[i].getArea(); return sum; } }
Radius Area 70.577708 15649.941866 44.152266 6124.291736 24.867853 1942.792644 5.680718 101.380949 36.734246 4239.280350 —————————————————————————————————————————————The total area of circles is 28056.687544
The program invokes createCircleArray() (line 8) to create an array of five circle objects. Several circle classes were introduced in this chapter. This example uses the CircleWithPrivateDataFields class introduced in Section 9.9, Data Field Encapsulation. The circle radii are randomly generated using the Math.random() method (line 21). The createCircleArray method returns an array of CircleWithPrivateDataFields objects (line 25). The array is passed to the printCircleArray method, which displays the radius and area of each circle and the total area of the circles. The sum of the circle areas is computed by invoking the sum method (line 41), which takes the array of CircleWithPrivateDataFields objects as the argument and returns a double value for the total area.
9.27
What is wrong in the following code? 1 2 3 4 5 6 7
public class Test { public static void main(String[] args) { java.util.Date[] dates = new java.util.Date[10]; System.out.println(dates[0]); System.out.println(dates[0].toString()); } }
✓
Check Point
9.12 Immutable Objects and Classes You can define immutable classes to create immutable objects. The contents of immutable objects cannot be changed. Normally, you create an object and allow its contents to be changed later. However, occasionally it is desirable to create an object whose contents cannot be changed once the object has been created. We call such an object as immutable object and its class as immutable class. The String class, for example, is immutable. If you deleted the setter method in the CircleWithPrivateDataFields class in Listing 9.9, the class would be immutable, because radius is private and cannot be changed without a setter method.
Key Point
immutable object immutable class
354 Chapter 9
Objects and Classes If a class is immutable, then all its data fields must be private and it cannot contain public setter methods for any data fields. A class with all private data fields and no mutators is not necessarily immutable. For example, the following Student class has all private data fields and no setter methods, but it is not an immutable class.
Student class
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
public class Student { private int id; private String name; private java.util.Date dateCreated; public Student(int ssn, String newName) { id = ssn; name = newName; dateCreated = new java.util.Date(); } public int getId() { return id; } public String getName() { return name; } public java.util.Date getDateCreated() { return dateCreated; } }
As shown in the following code, the data field dateCreated is returned using the getDateCreated() method. This is a reference to a Date object. Through this reference, the content for dateCreated can be changed. public class Test { public static void main(String[] args) { Student student = new Student(111223333, "John"); java.util.Date dateCreated = student.getDateCreated(); dateCreated.setTime(200000); // Now dateCreated field is changed! } }
For a class to be immutable, it must meet the following requirements: ■
All data fields must be private.
■
There can’t be any mutator methods for data fields.
■
No accessor methods can return a reference to a data field that is mutable.
Interested readers may refer to Supplement III.U for an extended discussion on immutable objects.
✓
Check Point
9.28 9.29 9.30
If a class contains only private data fields and no setter methods, is the class immutable? If all the data fields in a class are private and of primitive types, and the class doesn’t contain any setter methods, is the class immutable? Is the following class immutable? public class A { private int[] values; public int[] getValues() {
9.13 The Scope of Variables 355 return values; } }
9.13 The Scope of Variables The scope of instance and static variables is the entire class, regardless of where the variables are declared. Section 6.9 discussed local variables and their scope rules. Local variables are declared and used inside a method locally. This section discusses the scope rules of all the variables in the context of a class. Instance and static variables in a class are referred to as the class’s variables or data fields. A variable defined inside a method is referred to as a local variable. The scope of a class’s variables is the entire class, regardless of where the variables are declared. A class’s variables and methods can appear in any order in the class, as shown in Figure 9.20a. The exception is when a data field is initialized based on a reference to another data field. In such cases, the other data field must be declared first, as shown in Figure 9.20b. For consistency, this book declares data fields at the beginning of the class. public class Circle { public double findArea() { return radius * radius * Math.PI; }
Key Point
class’s variables
public class F { private int i ; private int j = i + 1; }
private double radius = 1; } (a) The variable radius and method findArea() can be declared in any order.
(b) i has to be declared before j because j’s initial value is dependent on i.
FIGURE 9.20 Members of a class can be declared in any order, with one exception. You can declare a class’s variable only once, but you can declare the same variable name in a method many times in different nonnesting blocks. If a local variable has the same name as a class’s variable, the local variable takes precedence and the class’s variable with the same name is hidden. For example, in the following program, x is defined both as an instance variable and as a local variable in the method. public class F { private int x = 0; // Instance variable private int y = 0; public F() { } public void p() { int x = 1; // Local variable System.out.println("x = " + x); System.out.println("y = " + y); } }
What is the output for f.p(), where f is an instance of F? The output for f.p() is 1 for x and 0 for y. Here is why: ■ x
is declared as a data field with the initial value of 0 in the class, but it is also declared in the method p() with an initial value of 1. The latter x is referenced in the System.out.println statement.
■ y
is declared outside the method p(), but y is accessible inside the method.
hidden variables
356 Chapter 9
Objects and Classes Tip To avoid confusion and mistakes, do not use the names of instance or static variables as local variable names, except for method parameters.
✓
Check Point
9.31
What is the output of the following program? public class Test { private static int i = 0; private static int j = 0; public static void main(String[] args) { int i = 2; int k = 3; { int j = 3; System.out.println("i + j is " + i + j); } k = i + j; System.out.println("k is " + k); System.out.println("j is " + j); } }
9.14 The this Reference Key Point this keyword
The keyword this refers to the object itself. It can also be used inside a constructor to invoke another constructor of the same class. The this keyword is the name of a reference that an object can use to refer to itself. You can use the this keyword to reference the object’s instance members. For example, the following code in (a) uses this to reference the object’s radius and invokes its getArea() method explicitly. The this reference is normally omitted, as shown in (b). However, the this reference is needed to reference hidden data fields or invoke an overloaded constructor. public class Circle { private double radius;
public class Circle { private double radius;
...
... public double getArea() { return this.radius * this.radius * Math.PI; }
public double getArea() { return radius * radius * Math.PI; }
Equivalent
public String toString() { return "radius: " + radius + "area: " + getArea() ; }
public String toString() { return "radius: " + this.radius + "area: " + this.getArea() ; } }
} (a)
9.14.1 hidden data fields
(b)
Using this to Reference Hidden Data Fields
The this keyword can be used to reference a class’s hidden data fields. For example, a datafield name is often used as the parameter name in a setter method for the data field. In this case, the data field is hidden in the setter method. You need to reference the hidden data-field name in the method in order to set a new value to it. A hidden static variable can be accessed
9.14 The this Reference 357 simply by using the ClassName.staticVariable reference. A hidden instance variable can be accessed by using the keyword this, as shown in Figure 9.21a. public class F { private int i = 5; private static double k = 0; public void setI(int i) { this.i = i; } public static void setK(double k) { F.k = k; }
Suppose that f1 and f2 are two objects of F. Invoking f1.setI(10) is to execute this.i = 10, where this refers f1 Invoking f2.setI(45) is to execute this.i = 45, where this refers f2 Invoking F.setK(33) is to execute F.k = 33. setK is a static method
// Other methods omitted } (a)
(b)
FIGURE 9.21 The keyword this refers to the calling object that invokes the method. The this keyword gives us a way to reference the object that invokes an instance method. To invoke f1.setI(10), this.i = i is executed, which assigns the value of parameter i to the data field i of this calling object f1. The keyword this refers to the object that invokes the instance method setI, as shown in Figure 9.21b. The line F.k = k means that the value in parameter k is assigned to the static data field k of the class, which is shared by all the objects of the class.
9.14.2
Using this to Invoke a Constructor
The this keyword can be used to invoke another constructor of the same class. For example, you can rewrite the Circle class as follows: public class Circle { private double radius; public Circle(double radius) { this.radius = radius; } The this keyword is used to reference the hidden data field radius of the object being constructed. public Circle() { this(1.0); } The this keyword is used to invoke another constructor. ... }
The line this(1.0) in the second constructor invokes the first constructor with a double value argument.
Note Java requires that the this(arg-list) statement appear first in the constructor before any other executable statements.
Tip If a class has multiple constructors, it is better to implement them using this(arg-list) as much as possible. In general, a constructor with no or fewer arguments can invoke a constructor with more arguments using this(arg-list). This syntax often simplifies coding and makes the class easier to read and to maintain.
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✓
Check Point
9.32 9.33
Describe the role of the this keyword. What is wrong in the following code? 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
9.34
public class C { private int p; public C() { System.out.println("C's no-arg constructor invoked"); this(0); } public C(int p) { p = p; } public void setP(int p) { p = p; } }
What is wrong in the following code? public class Test { private int id; public void m1() { this.id = 45; } public void m2() { Test.id = 45; } }
KEY TERMS action 322 anonymous object 331 attribute 322 behavior 322 class 322 class’s variable 355 client 325 constructor 322 data field 322 data field encapsulation 344 default constructor 329 dot operator (.) 330 getter (or accessor) 345 instance 322 instance method 331 instance variable 331 instantiation 322 immutable class 353
immutable object 353 no-arg constructor 329 null value 331 object 322 object-oriented programming (OOP) 322 package-private (or package-access) 342 private constructor 344 property 322 public class 325 reference type 330 reference variable 330 setter (or mutator) 345 state 322 static method 337 static variable 337 this keyword 356 Unified Modeling Language (UML) 323
Quiz 359
CHAPTER SUMMARY 1. A class is a template for objects. It defines the properties of objects and provides constructors for creating objects and methods for manipulating them.
2. A class is also a data type. You can use it to declare object reference variables. An object reference variable that appears to hold an object actually contains a reference to that object. Strictly speaking, an object reference variable and an object are different, but most of the time the distinction can be ignored.
3. An object is an instance of a class. You use the new operator to create an object, and the dot operator (.) to access members of that object through its reference variable.
4. An instance variable or method belongs to an instance of a class. Its use is associated with individual instances. A static variable is a variable shared by all instances of the same class. A static method is a method that can be invoked without using instances.
5. Every instance of a class can access the class’s static variables and methods. For clarity, however, it is better to invoke static variables and methods using ClassName.variable and ClassName.method.
6. Visibility modifiers specify how the class, method, and data are accessed. A public class, method, or data is accessible to all clients. A private method or data is accessible only inside the class.
7. You can provide a getter (accessor) method or a setter (mutator) method to enable clients to see or modify the data.
8. A getter method has the signature
public returnType getPropertyName(). If the returnType is boolean, the get method should be defined as public boolean isPropertyName(). A setter method has the signature public void setPropertyName(dataType propertyValue).
9. All parameters are passed to methods using pass-by-value. For a parameter of a primitive type, the actual value is passed; for a parameter of a reference type, the reference for the object is passed.
10. A Java array is an object that can contain primitive type values or object type values. When an array of objects is created, its elements are assigned the default value of null.
11. Once it is created, an immutable object cannot be modified. To prevent users from modifying an object, you can define immutable classes.
12. The scope of instance and static variables is the entire class, regardless of where the variables are declared. Instance and static variables can be declared anywhere in the class. For consistency, they are declared at the beginning of the class in this book.
13. The keyword this can be used to refer to the calling object. It can also be used inside a constructor to invoke another constructor of the same class.
QUIZ Answer the quiz for this chapter online at www.cs.armstrong.edu/liang/intro10e/quiz.html.
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PROGRAMMING EXERCISES Pedagogical Note The exercises in Chapters 9–13 help you achieve three objectives:
three objectives
■ Design classes and draw UML class diagrams. ■ Implement classes from the UML. ■ Use classes to develop applications.
Students can download solutions for the UML diagrams for the even-numbered exercises from the Companion Website, and instructors can download all solutions from the same site.
Sections 9.2–9.5
9.1
(The Rectangle class) Following the example of the Circle class in Section 9.2, design a class named Rectangle to represent a rectangle. The class contains: ■ ■ ■ ■ ■
9.2
Two double data fields named width and height that specify the width and height of the rectangle. The default values are 1 for both width and height. A no-arg constructor that creates a default rectangle. A constructor that creates a rectangle with the specified width and height. A method named getArea() that returns the area of this rectangle. A method named getPerimeter() that returns the perimeter.
Draw the UML diagram for the class and then implement the class. Write a test program that creates two Rectangle objects—one with width 4 and height 40 and the other with width 3.5 and height 35.9. Display the width, height, area, and perimeter of each rectangle in this order. (The Stock class) Following the example of the Circle class in Section 9.2, design a class named Stock that contains: ■ ■ ■ ■ ■ ■
A string data field named symbol for the stock’s symbol. A string data field named name for the stock’s name. A double data field named previousClosingPrice that stores the stock price for the previous day. A double data field named currentPrice that stores the stock price for the current time. A constructor that creates a stock with the specified symbol and name. A method named getChangePercent() that returns the percentage changed from previousClosingPrice to currentPrice.
Draw the UML diagram for the class and then implement the class. Write a test program that creates a Stock object with the stock symbol ORCL, the name Oracle Corporation, and the previous closing price of 34.5. Set a new current price to 34.35 and display the price-change percentage.
Section 9.6
*9.3
*9.4
(Use the Date class) Write a program that creates a Date object, sets its elapsed time to 10000, 100000, 1000000, 10000000, 100000000, 1000000000, 10000000000, and 100000000000, and displays the date and time using the toString() method, respectively. (Use the Random class) Write a program that creates a Random object with seed 1000 and displays the first 50 random integers between 0 and 100 using the nextInt(100) method.
Programming Exercises 361 *9.5
(Use the GregorianCalendar class) Java API has the GregorianCalendar class in the java.util package, which you can use to obtain the year, month, and day of a date. The no-arg constructor constructs an instance for the current date, and the methods get(GregorianCalendar.YEAR), get(GregorianCalendar.MONTH), and get(GregorianCalendar.DAY_OF_MONTH) return the year, month, and day. Write a program to perform two tasks: ■ ■
Display the current year, month, and day. The GregorianCalendar class has the setTimeInMillis(long), which can be used to set a specified elapsed time since January 1, 1970. Set the value to 1234567898765L and display the year, month, and day.
Sections 9.7–9.9
*9.6
(Stopwatch) Design a class named StopWatch. The class contains: ■ ■ ■ ■ ■
9.7
Private data fields startTime and endTime with getter methods. A no-arg constructor that initializes startTime with the current time. A method named start() that resets the startTime to the current time. A method named stop() that sets the endTime to the current time. A method named getElapsedTime() that returns the elapsed time for the stopwatch in milliseconds.
Draw the UML diagram for the class and then implement the class. Write a test program that measures the execution time of sorting 100,000 numbers using selection sort. (The Account class) Design a class named Account that contains: ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■
A private int data field named id for the account (default 0). A private double data field named balance for the account (default 0). A private double data field named annualInterestRate that stores the current interest rate (default 0). Assume all accounts have the same interest rate. A private Date data field named dateCreated that stores the date when the account was created. A no-arg constructor that creates a default account. A constructor that creates an account with the specified id and initial balance. The accessor and mutator methods for id, balance, and annualInterestRate. The accessor method for dateCreated. A method named getMonthlyInterestRate() that returns the monthly interest rate. A method named getMonthlyInterest() that returns the monthly interest. A method named withdraw that withdraws a specified amount from the account. A method named deposit that deposits a specified amount to the account.
Draw the UML diagram for the class and then implement the class. (Hint: The method getMonthlyInterest() is to return monthly interest, not the interest rate. Monthly interest is balance * monthlyInterestRate. monthlyInterestRate is annualInterestRate / 12. Note that annualInterestRate is a percentage, e.g., like 4.5%. You need to divide it by 100.) Write a test program that creates an Account object with an account ID of 1122, a balance of $20,000, and an annual interest rate of 4.5%. Use the withdraw method to withdraw $2,500, use the deposit method to deposit $3,000, and print the balance, the monthly interest, and the date when this account was created.
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Objects and Classes 9.8
(The Fan class) Design a class named Fan to represent a fan. The class contains: ■
VideoNote
The Fan class ■ ■ ■ ■ ■ ■ ■
Three constants named SLOW, MEDIUM, and FAST with the values 1, 2, and 3 to denote the fan speed. A private int data field named speed that specifies the speed of the fan (the default is SLOW). A private boolean data field named on that specifies whether the fan is on (the default is false). A private double data field named radius that specifies the radius of the fan (the default is 5). A string data field named color that specifies the color of the fan (the default is blue). The accessor and mutator methods for all four data fields. A no-arg constructor that creates a default fan. A method named toString() that returns a string description for the fan. If the fan is on, the method returns the fan speed, color, and radius in one combined string. If the fan is not on, the method returns the fan color and radius along with the string “fan is off” in one combined string.
Draw the UML diagram for the class and then implement the class. Write a test program that creates two Fan objects. Assign maximum speed, radius 10, color yellow, and turn it on to the first object. Assign medium speed, radius 5, color blue, and turn it off to the second object. Display the objects by invoking their toString method.
**9.9
(Geometry: n-sided regular polygon) In an n-sided regular polygon, all sides have the same length and all angles have the same degree (i.e., the polygon is both equilateral and equiangular). Design a class named RegularPolygon that contains: ■ ■ ■ ■ ■ ■ ■ ■ ■ ■
A private int data field named n that defines the number of sides in the polygon with default value 3. A private double data field named side that stores the length of the side with default value 1. A private double data field named x that defines the x-coordinate of the polygon’s center with default value 0. A private double data field named y that defines the y-coordinate of the polygon’s center with default value 0. A no-arg constructor that creates a regular polygon with default values. A constructor that creates a regular polygon with the specified number of sides and length of side, centered at (0, 0). A constructor that creates a regular polygon with the specified number of sides, length of side, and x- and y-coordinates. The accessor and mutator methods for all data fields. The method getPerimeter() that returns the perimeter of the polygon. The method getArea() that returns the area of the polygon. The formula for computing the area of a regular polygon is Area =
n * s2 p 4 * tan¢ ≤ n
.
Draw the UML diagram for the class and then implement the class. Write a test program that creates three RegularPolygon objects, created using the no-arg constructor, using RegularPolygon(6, 4), and using RegularPolygon(10, 4, 5.6, 7.8). For each object, display its perimeter and area.
Programming Exercises 363 *9.10
(Algebra: quadratic equations) Design a class named QuadraticEquation for a quadratic equation ax2 + bx + x = 0. The class contains: ■ ■ ■ ■ ■
Private data fields a, b, and c that represent three coefficients. A constructor for the arguments for a, b, and c. Three getter methods for a, b, and c. A method named getDiscriminant() that returns the discriminant, which is b2 - 4ac. The methods named getRoot1() and getRoot2() for returning two roots of the equation r1 =
*9.11
-b + 2b2 - 4ac 2a
and r2 =
-b - 2b2 - 4ac 2a
These methods are useful only if the discriminant is nonnegative. Let these methods return 0 if the discriminant is negative. Draw the UML diagram for the class and then implement the class. Write a test program that prompts the user to enter values for a, b, and c and displays the result based on the discriminant. If the discriminant is positive, display the two roots. If the discriminant is 0, display the one root. Otherwise, display “The equation has no roots.” See Programming Exercise 3.1 for sample runs. (Algebra: 2 * 2 linear equations) Design a class named LinearEquation for a 2 * 2 system of linear equations: ed - bf ax + by = e x = ad - bc cx + dy = f
y =
af - ec ad - bc
The class contains: ■ ■ ■ ■ ■
**9.12
**9.13
Private data fields a, b, c, d, e, and f. A constructor with the arguments for a, b, c, d, e, and f. Six getter methods for a, b, c, d, e, and f. A method named isSolvable() that returns true if ad - bc is not 0. Methods getX() and getY() that return the solution for the equation.
Draw the UML diagram for the class and then implement the class. Write a test program that prompts the user to enter a, b, c, d, e, and f and displays the result. If ad - bc is 0, report that “The equation has no solution.” See Programming Exercise 3.3 for sample runs. (Geometry: intersecting point) Suppose two line segments intersect. The two endpoints for the first line segment are (x1, y1) and (x2, y2) and for the second line segment are (x3, y3) and (x4, y4). Write a program that prompts the user to enter these four endpoints and displays the intersecting point. As discussed in Programming Exercise 3.25, the intersecting point can be found by solving a linear equation. Use the LinearEquation class in Programming Exercise 9.11 to solve this equation. See Programming Exercise 3.25 for sample runs. (The Location class) Design a class named Location for locating a maximal value and its location in a two-dimensional array. The class contains public data fields row, column, and maxValue that store the maximal value and its indices in a two-dimensional array with row and column as int types and maxValue as a double type. Write the following method that returns the location of the largest element in a two-dimensional array: public static Location locateLargest(double[][] a)
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Objects and Classes The return value is an instance of Location. Write a test program that prompts the user to enter a two-dimensional array and displays the location of the largest element in the array. Here is a sample run: Enter the number of rows and columns in the array: Enter the array: 23.5 35 2 10 4.5 3 45 3.5 35 44 5.5 9.6 The location of the largest element is 45 at (1, 2)
3 4
CHAPTER
10 OBJECT-ORIENTED THINKING Objectives ■
To apply class abstraction to develop software (§10.2).
■
To explore the differences between the procedural paradigm and object-oriented paradigm (§10.3).
■
To discover the relationships between classes (§10.4).
■
To design programs using the object-oriented paradigm (§§10.5–10.6).
■
To create objects for primitive values using the wrapper classes (Byte, Short, Integer, Long, Float, Double, Character, and Boolean) (§10.7).
■
To simplify programming using automatic conversion between primitive types and wrapper class types (§10.8).
■
To use the BigInteger and BigDecimal classes for computing very large numbers with arbitrary precisions (§10.9).
■
To use the String class to process immutable strings (§10.10).
■
To use the StringBuilder and StringBuffer classes to process mutable strings (§10.11).
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10.1 Introduction Key Point
The focus of this chapter is on class design and explores the differences between procedural programming and object-oriented programming. The preceding chapter introduced objects and classes. You learned how to define classes, create objects, and use objects from several classes in the Java API (e.g., Circle, Date, Random, and Point2D). This book’s approach is to teach problem solving and fundamental programming techniques before object-oriented programming. This chapter shows how procedural and object-oriented programming differ. You will see the benefits of object-oriented programming and learn to use it effectively. Our focus here is on class design. We will use several examples to illustrate the advantages of the object-oriented approach. The examples involve designing new classes and using them in applications and introducing new classes in the Java API.
10.2 Class Abstraction and Encapsulation Key Point
class abstraction
class’s contract class encapsulation abstract data type
Class abstraction is the separation of class implementation from the use of a class. The details of implementation are encapsulated and hidden from the user. This is known as class encapsulation. In Chapter 6, you learned about method abstraction and used it in stepwise refinement. Java provides many levels of abstraction, and class abstraction separates class implementation from how the class is used. The creator of a class describes the functions of the class and lets the user know how the class can be used. The collection of methods and fields that are accessible from outside the class, together with the description of how these members are expected to behave, serves as the class’s contract. As shown in Figure 10.1, the user of the class does not need to know how the class is implemented. The details of implementation are encapsulated and hidden from the user. This is called class encapsulation. For example, you can create a Circle object and find the area of the circle without knowing how the area is computed. For this reason, a class is also known as an abstract data type (ADT). Class implementation is like a black box hidden from the clients
Class
Class Contract (signatures of public methods and public constants)
Clients use the class through the contract of the class
FIGURE 10.1 Class abstraction separates class implementation from the use of the class. Class abstraction and encapsulation are two sides of the same coin. Many real-life examples illustrate the concept of class abstraction. Consider, for instance, building a computer system. Your personal computer has many components—a CPU, memory, disk, motherboard, fan, and so on. Each component can be viewed as an object that has properties and methods. To get the components to work together, you need know only how each component is used and how it interacts with the others. You don’t need to know how the components work internally. The internal implementation is encapsulated and hidden from you. You can build a computer without knowing how a component is implemented. The computer-system analogy precisely mirrors the object-oriented approach. Each component can be viewed as an object of the class for the component. For example, you might have a class that models all kinds of fans for use in a computer, with properties such as fan size and speed and methods such as start and stop. A specific fan is an instance of this class with specific property values. As another example, consider getting a loan. A specific loan can be viewed as an object of a Loan class. The interest rate, loan amount, and loan period are its data properties, and
10.2 Class Abstraction and Encapsulation 367 computing the monthly payment and total payment are its methods. When you buy a car, a loan object is created by instantiating the class with your loan interest rate, loan amount, and loan period. You can then use the methods to find the monthly payment and total payment of your loan. As a user of the Loan class, you don’t need to know how these methods are implemented. Listing 2.9, ComputeLoan.java, presented a program for computing loan payments. That program cannot be reused in other programs because the code for computing the payments is in the main method. One way to fix this problem is to define static methods for computing the monthly payment and total payment. However, this solution has limitations. Suppose you wish to associate a date with the loan. There is no good way to tie a date with a loan without using objects. The traditional procedural programming paradigm is action-driven, and data are separated from actions. The object-oriented programming paradigm focuses on objects, and actions are defined along with the data in objects. To tie a date with a loan, you can define a loan class with a date along with the loan’s other properties as data fields. A loan object now contains data and actions for manipulating and processing data, and the loan data and actions are integrated in one object. Figure 10.2 shows the UML class diagram for the Loan class.
VideoNote
The Loan class
Loan -annualInterestRate: double
The annual interest rate of the loan (default: 2.5).
-numberOfYears: int
The number of years for the loan (default: 1).
-loanAmount: double
The loan amount (default: 1000).
-loanDate: java.util.Date
The date this loan was created.
+Loan()
Constructs a default Loan object.
+Loan(annualInterestRate: double, numberOfYears: int,loanAmount: double)
Constructs a loan with specified interest rate, years, and loan amount.
+getAnnualInterestRate(): double
Returns the annual interest rate of this loan.
+getNumberOfYears(): int
Returns the number of the years of this loan.
+getLoanAmount(): double +getLoanDate(): java.util.Date
Returns the amount of this loan.
+setAnnualInterestRate( annualInterestRate: double): void
Sets a new annual interest rate for this loan.
+setNumberOfYears( numberOfYears: int): void +setLoanAmount( loanAmount: double): void +getMonthlyPayment(): double
Sets a new number of years for this loan.
+getTotalPayment(): double
Returns the total payment for this loan.
Returns the date of the creation of this loan.
Sets a new amount for this loan. Returns the monthly payment for this loan.
FIGURE 10.2 The Loan class models the properties and behaviors of loans. The UML diagram in Figure 10.2 serves as the contract for the Loan class. Throughout this book, you will play the roles of both class user and class developer. Remember that a class user can use the class without knowing how the class is implemented. Assume that the Loan class is available. The program in Listing 10.1 uses that class.
LISTING 10.1 TestLoanClass.java 1 2 3 4 5
import java.util.Scanner; public class TestLoanClass { /** Main method */ public static void main(String[] args) {
368 Chapter 10
create Loan object
invoke instance method invoke instance method
Object-Oriented Thinking 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
// Create a Scanner Scanner input = new Scanner(System.in); // Enter annual interest rate System.out.print( "Enter annual interest rate, for example, 8.25: "); double annualInterestRate = input.nextDouble(); // Enter number of years System.out.print("Enter number of years as an integer: "); int numberOfYears = input.nextInt(); // Enter loan amount System.out.print("Enter loan amount, for example, 120000.95: "); double loanAmount = input.nextDouble(); // Create a Loan object Loan loan = new Loan(annualInterestRate, numberOfYears, loanAmount); // Display loan date, monthly payment, and total payment System.out.printf("The loan was created on %s\n" + "The monthly payment is %.2f\nThe total payment is %.2f\n", loan.getLoanDate().toString(), loan.getMonthlyPayment(), loan.getTotalPayment()); } }
Enter annual interest rate, for example, 8.25: 2.5 Enter number of years as an integer: 5 Enter loan amount, for example, 120000.95: 1000 The loan was created on Sat Jun 16 21:12:50 EDT 2012 The monthly payment is 17.75 The total payment is 1064.84
The main method reads the interest rate, the payment period (in years), and the loan amount; creates a Loan object; and then obtains the monthly payment (line 29) and the total payment (line 30) using the instance methods in the Loan class. The Loan class can be implemented as in Listing 10.2.
LISTING 10.2 Loan.java
no-arg constructor
1 2 3 4 5 6 7 8 9 10 11 12
public class Loan { private double annualInterestRate; private int numberOfYears; private double loanAmount; private java.util.Date loanDate; /** Default constructor */ public Loan() { this(2.5, 1, 1000); } /** Construct a loan with specified annual interest rate,
10.2 Class Abstraction and Encapsulation 369 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 64 65 66 67 68 69 70 71
number of years, and loan amount */ public Loan(double annualInterestRate, int numberOfYears, double loanAmount) { this.annualInterestRate = annualInterestRate; this.numberOfYears = numberOfYears; this.loanAmount = loanAmount; loanDate = new java.util.Date(); } /** Return annualInterestRate */ public double getAnnualInterestRate() { return annualInterestRate; } /** Set a new annualInterestRate */ public void setAnnualInterestRate(double annualInterestRate) { this.annualInterestRate = annualInterestRate; } /** Return numberOfYears */ public int getNumberOfYears() { return numberOfYears; } /** Set a new numberOfYears */ public void setNumberOfYears(int numberOfYears) { this.numberOfYears = numberOfYears; } /** Return loanAmount */ public double getLoanAmount() { return loanAmount; } /** Set a new loanAmount */ public void setLoanAmount(double loanAmount) { this.loanAmount = loanAmount; } /** Find monthly payment */ public double getMonthlyPayment() { double monthlyInterestRate = annualInterestRate / 1200; double monthlyPayment = loanAmount * monthlyInterestRate / (1 (1 / Math.pow(1 + monthlyInterestRate, numberOfYears * 12))); return monthlyPayment; } /** Find total payment */ public double getTotalPayment() { double totalPayment = getMonthlyPayment() * numberOfYears * 12; return totalPayment; } /** Return loan date */ public java.util.Date getLoanDate() { return loanDate; } }
constructor
370 Chapter 10
Object-Oriented Thinking From a class developer’s perspective, a class is designed for use by many different customers. In order to be useful in a wide range of applications, a class should provide a variety of ways for customization through constructors, properties, and methods. The Loan class contains two constructors, four getter methods, three setter methods, and the methods for finding the monthly payment and the total payment. You can construct a Loan object by using the no-arg constructor or the constructor with three parameters: annual interest rate, number of years, and loan amount. When a loan object is created, its date is stored in the loanDate field. The getLoanDate method returns the date. The methods—getAnnualInterest, getNumberOfYears, and getLoanAmount—return the annual interest rate, payment years, and loan amount, respectively. All the data properties and methods in this class are tied to a specific instance of the Loan class. Therefore, they are instance variables and methods.
Important Pedagogical Tip Use the UML diagram for the Loan class shown in Figure 10.2 to write a test program that uses the Loan class even though you don’t know how the Loan class is implemented. This has three benefits: ■ It demonstrates that developing a class and using a class are two separate tasks. ■ It enables you to skip the complex implementation of certain classes without inter-
rupting the sequence of this book. ■ It is easier to learn how to implement a class if you are familiar with it by using the class.
For all the class examples from now on, create an object from the class and try using its methods before turning your attention to its implementation.
✓
Check Point
10.1
If you redefine the Loan class in Listing 10.2 without setter methods, is the class immutable?
10.3 Thinking in Objects Key Point
The procedural paradigm focuses on designing methods. The object-oriented paradigm couples data and methods together into objects. Software design using the object-oriented paradigm focuses on objects and operations on objects. Chapters 1–8 introduced fundamental programming techniques for problem solving using loops, methods, and arrays. Knowing these techniques lays a solid foundation for objectoriented programming. Classes provide more flexibility and modularity for building reusable software. This section improves the solution for a problem introduced in Chapter 3 using the object-oriented approach. From these improvements, you will gain insight into the differences between procedural and object-oriented programming and see the benefits of developing reusable code using objects and classes. Listing 3.4, ComputeAndInterpretBMI.java, presented a program for computing body mass index. The code cannot be reused in other programs, because the code is in the main method. To make it reusable, define a static method to compute body mass index as follows: public static double getBMI(double weight, double height)
VideoNote
The BMI class
This method is useful for computing body mass index for a specified weight and height. However, it has limitations. Suppose you need to associate the weight and height with a person’s name and birth date. You could declare separate variables to store these values, but these values would not be tightly coupled. The ideal way to couple them is to create an object that contains them all. Since these values are tied to individual objects, they should be stored in instance data fields. You can define a class named BMI as shown in Figure 10.3.
10.3 Thinking in Objects 371 The getter methods for these data fields are provided in the class, but omitted in the UML diagram for brevity. BMI The name of the person.
-name: String -age: int -weight: double -height: double
The age of the person. The weight of the person in pounds. The height of the person in inches.
+BMI(name: String, age: int, weight: double, height: double)
Creates a BMI object with the specified name, age, weight, and height.
+BMI(name: String, weight: double, height: double)
Creates a BMI object with the specified name, weight, height, and a default age 20.
+getBMI(): double +getStatus(): String
Returns the BMI. Returns the BMI status (e.g., normal, overweight, etc.).
FIGURE 10.3 The BMI class encapsulates BMI information. Assume that the BMI class is available. Listing 10.3 gives a test program that uses this class.
LISTING 10.3 UseBMIClass.java 1 2 3 4 5 6 7 8 9 10 11
public class UseBMIClass { public static void main(String[] args) { BMI bmi1 = new BMI("Kim Yang", 18, 145, 70); System.out.println("The BMI for " + bmi1.getName() + " is " + bmi1.getBMI() + " " + bmi1.getStatus()); BMI bmi2 = new BMI("Susan King", 215, 70); System.out.println("The BMI for " + bmi2.getName() + " is " + bmi2.getBMI() + " " + bmi2.getStatus());
create an object invoke instance method
create an object invoke instance method
} }
The BMI for Kim Yang is 20.81 Normal The BMI for Susan King is 30.85 Obese
Line 3 creates the object bmi1 for Kim Yang and line 7 creates the object bmi2 for Susan King. You can use the instance methods getName(), getBMI(), and getStatus() to return the BMI information in a BMI object. The BMI class can be implemented as in Listing 10.4.
LISTING 10.4 BMI.java 1 2 3 4 5 6 7 8 9 10
public class BMI { private String name; private int age; private double weight; // in pounds private double height; // in inches public static final double KILOGRAMS_PER_POUND = 0.45359237; public static final double METERS_PER_INCH = 0.0254; public BMI(String name, int age, double weight, double height) { this.name = name;
constructor
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Object-Oriented Thinking
constructor
getBMI
getStatus
procedural vs. object-oriented paradigms
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
this.age = age; this.weight = weight; this.height = height; } public BMI(String name, double weight, double height) { this(name, 20, weight, height); } public double getBMI() { double bmi = weight * KILOGRAMS_PER_POUND / ((height * METERS_PER_INCH) * (height * METERS_PER_INCH)); return Math.round(bmi * 100) / 100.0; } public String getStatus() { double bmi = getBMI(); if (bmi < 18.5) return "Underweight"; else if (bmi < 25) return "Normal"; else if (bmi < 30) return "Overweight"; else return "Obese"; } public String getName() { return name; } public int getAge() { return age; } public double getWeight() { return weight; } public double getHeight() { return height; } }
The mathematical formula for computing the BMI using weight and height is given in Section 3.8. The instance method getBMI() returns the BMI. Since the weight and height are instance data fields in the object, the getBMI() method can use these properties to compute the BMI for the object. The instance method getStatus() returns a string that interprets the BMI. The interpretation is also given in Section 3.8. This example demonstrates the advantages of the object-oriented paradigm over the procedural paradigm. The procedural paradigm focuses on designing methods. The object-oriented paradigm couples data and methods together into objects. Software design using the objectoriented paradigm focuses on objects and operations on objects. The object-oriented approach combines the power of the procedural paradigm with an added dimension that integrates data with operations into objects. In procedural programming, data and operations on the data are separate, and this methodology requires passing data to methods. Object-oriented programming places data and
10.4 Class Relationships 373 the operations that pertain to them in an object. This approach solves many of the problems inherent in procedural programming. The object-oriented programming approach organizes programs in a way that mirrors the real world, in which all objects are associated with both attributes and activities. Using objects improves software reusability and makes programs easier to develop and easier to maintain. Programming in Java involves thinking in terms of objects; a Java program can be viewed as a collection of cooperating objects.
10.2
✓
Is the BMI class defined in Listing 10.4 immutable?
Check Point
10.4 Class Relationships To design classes, you need to explore the relationships among classes. The common relationships among classes are association, aggregation, composition, and inheritance.
Key Point
This section explores association, aggregation, and composition. The inheritance relationship will be introduced in the next chapter.
10.4.1
Association
Association is a general binary relationship that describes an activity between two classes. For example, a student taking a course is an association between the Student class and the Course class, and a faculty member teaching a course is an association between the Faculty class and the Course class. These associations can be represented in UML graphical notation, as shown in Figure 10.4.
Take Student
5..60
association
Teach Course
0..3
1 Teacher
Faculty
FIGURE 10.4 This UML diagram shows that a student may take any number of courses, a faculty member may teach at most three courses, a course may have from five to sixty students, and a course is taught by only one faculty member. An association is illustrated by a solid line between two classes with an optional label that describes the relationship. In Figure 10.4, the labels are Take and Teach. Each relationship may have an optional small black triangle that indicates the direction of the relationship. In this figure, the direction indicates that a student takes a course (as opposed to a course taking a student). Each class involved in the relationship may have a role name that describes the role it plays in the relationship. In Figure 10.4, teacher is the role name for Faculty. Each class involved in an association may specify a multiplicity, which is placed at the side of the class to specify how many of the class’s objects are involved in the relationship in UML. A multiplicity could be a number or an interval that specifies how many of the class’s objects are involved in the relationship. The character * means an unlimited number of objects, and the interval m..n indicates that the number of objects is between m and n, inclusively. In Figure 10.4, each student may take any number of courses, and each course must have at least five and at most sixty students. Each course is taught by only one faculty member, and a faculty member may teach from zero to three courses per semester. In Java code, you can implement associations by using data fields and methods. For example, the relationships in Figure 10.4 may be implemented using the classes in Figure 10.5. The
multiplicity
374 Chapter 10
Object-Oriented Thinking relation “a student takes a course” is implemented using the addCourse method in the Student class and the addStuent method in the Course class. The relation “a faculty teaches a course” is implemented using the addCourse method in the Faculty class and the setFaculty method in the Course class. The Student class may use a list to store the courses that the student is taking, the Faculty class may use a list to store the courses that the faculty is teaching, and the Course class may use a list to store students enrolled in the course and a data field to store the instructor who teaches the course.
public class Student { private Course[] courseList;
public class Course { private Student[] classList; private Faculty faculty;
public void addCourse( Course s) { ... }
public class Faculty { private Course[] courseList; public void addCourse( Course c) { ... }
public void addStudent( Student s) { ... }
}
}
public void setFaculty( Faculty faculty) { ... } }
FIGURE 10.5 The association relations are implemented using data fields and methods in classes.
Note There are many possible ways to implement relationships. For example, the student and faculty information in the Course class can be omitted, since they are already in the Student and Faculty class. Likewise, if you don’t need to know the courses a student takes or a faculty member teaches, the data field courseList and the addCourse method in Student or Faculty can be omitted.
many possible implementations
10.4.2 aggregation aggregating object aggregated object aggregated class aggregating class composition
Aggregation and Composition
Aggregation is a special form of association that represents an ownership relationship between two objects. Aggregation models has-a relationships. The owner object is called an aggregating object, and its class is called an aggregating class. The subject object is called an aggregated object, and its class is called an aggregated class. An object can be owned by several other aggregating objects. If an object is exclusively owned by an aggregating object, the relationship between the object and its aggregating object is referred to as a composition. For example, “a student has a name” is a composition relationship between the Student class and the Name class, whereas “a student has an address” is an aggregation relationship between the Student class and the Address class, since an address can be shared by several students. In UML, a filled diamond is attached to an aggregating class (in this case, Student) to denote the composition relationship with an aggregated class (Name), and an empty diamond is attached to an aggregating class (Student) to denote the aggregation relationship with an aggregated class (Address), as shown in Figure 10.6. Composition
Name
1
1
Student
Aggregation
1..3
1
Address
FIGURE 10.6 Each student has a name and an address. In Figure 10.6, each student has only one multiplicity—address—and each address can be shared by up to 3 students. Each student has one name, and a name is unique for each student.
10.4 Class Relationships 375 An aggregation relationship is usually represented as a data field in the aggregating class. For example, the relationships in Figure 10.6 may be implemented using the classes in Figure 10.7. The relation “a student has a name” and “a student has an address” are implemented in the data field name and address in the Student class. public class Name { ... }
public class Student { private Name name; private Address address; ... }
Aggregated class
Aggregating class
public class Address { ... }
Aggregated class
FIGURE 10.7 The composition relations are implemented using data fields in classes. Aggregation may exist between objects of the same class. For example, a person may have a supervisor. This is illustrated in Figure 10.8.
Person
1
1
Supervisor
FIGURE 10.8 A person may have a supervisor. In the relationship “a person has a supervisor,” a supervisor can be represented as a data field in the Person class, as follows: public class Person { // The type for the data is the class itself private Person supervisor; ... }
If a person can have several supervisors, as shown in Figure 10.9a, you may use an array to store supervisors, as shown in Figure 10.9b. 1 Person Supervisor
m
public class Person { ... private Person[] supervisors; }
(a)
(b)
FIGURE 10.9 A person can have several supervisors.
Note Since aggregation and composition relationships are represented using classes in the same way, we will not differentiate them and call both compositions for simplicity.
10.3 10.4 10.5 10.6
What are common relationships among classes? What is association? What is aggregation? What is composition? What is UML notation of aggregation and composition? Why both aggregation and composition are together referred to as composition?
aggregation or composition
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10.5 Case Study: Designing the Course Class Key Point
This section designs a class for modeling courses. This book’s philosophy is teaching by example and learning by doing. The book provides a wide variety of examples to demonstrate object-oriented programming. This section and the next offer additional examples on designing classes. Suppose you need to process course information. Each course has a name and has students enrolled. You should be able to add/drop a student to/from the course. You can use a class to model the courses, as shown in Figure 10.10. Course -courseName: String -students: String[] -numberOfStudents: int
The name of the course. An array to store the students for the course. The number of students (default: 0).
+Course(courseName: String) +getCourseName(): String
Creates a course with the specified name. Returns the course name.
+addStudent(student: String): void
Adds a new student to the course.
+dropStudent(student: String): void +getStudents(): String[] +getNumberOfStudents(): int
Drops a student from the course. Returns the students for the course. Returns the number of students for the course.
FIGURE 10.10 The Course class models the courses. A Course object can be created using the constructor Course(String name) by passing a course name. You can add students to the course using the addStudent(String student) method, drop a student from the course using the dropStudent(String student) method, and return all the students in the course using the getStudents() method. Suppose the Course class is available; Listing 10.5 gives a test class that creates two courses and adds students to them.
LISTING 10.5 TestCourse.java create a course
add a student
number of students return students
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
public class TestCourse { public static void main(String[] args) { Course course1 = new Course("Data Structures"); Course course2 = new Course("Database Systems"); course1.addStudent("Peter Jones"); course1.addStudent("Kim Smith"); course1.addStudent("Anne Kennedy"); course2.addStudent("Peter Jones"); course2.addStudent("Steve Smith"); System.out.println("Number of students in course1: " + course1.getNumberOfStudents()); String[] students = course1.getStudents(); for (int i = 0; i < course1.getNumberOfStudents(); i++) System.out.print(students[i] + ", "); System.out.println(); System.out.print("Number of students in course2: " + course2.getNumberOfStudents()); } }
10.5 Case Study: Designing the Course Class 377 Number of students in course1: 3 Peter Jones, Kim Smith, Anne Kennedy, Number of students in course2: 2
The Course class is implemented in Listing 10.6. It uses an array to store the students in the course. For simplicity, assume that the maximum course enrollment is 100. The array is created using new String[100] in line 3. The addStudent method (line 10) adds a student to the array. Whenever a new student is added to the course, numberOfStudents is increased (line 12). The getStudents method returns the array. The dropStudent method (line 27) is left as an exercise.
LISTING 10.6 Course.java 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
public class Course { private String courseName; private String[] students = new String[100]; private int numberOfStudents; public Course(String courseName) { this.courseName = courseName; }
create students
add a course
public void addStudent(String student) { students[numberOfStudents] = student; numberOfStudents++; } public String[] getStudents() { return students; }
return students
public int getNumberOfStudents() { return numberOfStudents; }
number of students
public String getCourseName() { return courseName; } public void dropStudent(String student) { // Left as an exercise in Programming Exercise 10.9 } }
The array size is fixed to be 100 (line 3), so you cannot have more than 100 students in the course. You can improve the class by automatically increasing the array size in Programming Exercise 10.9. When you create a Course object, an array object is created. A Course object contains a reference to the array. For simplicity, you can say that the Course object contains the array. The user can create a Course object and manipulate it through the public methods addStudent, dropStudent, getNumberOfStudents, and getStudents. However, the user doesn’t need to know how these methods are implemented. The Course class encapsulates the internal implementation. This example uses an array to store students, but you could use a different data structure to store students. The program that uses Course does not need to change as long as the contract of the public methods remains unchanged.
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10.6 Case Study: Designing a Class for Stacks stack
Key Point
This section designs a class for modeling stacks. Recall that a stack is a data structure that holds data in a last-in, first-out fashion, as shown in Figure 10.11. Data3
Data2
Data1
Data2 Data1
Data1 Data3
Data1
Data2 Data2 Data1
Data3 Data2 Data1
Data1
FIGURE 10.11 A stack holds data in a last-in, first-out fashion.
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The StackOfIntegers class
Stacks have many applications. For example, the compiler uses a stack to process method invocations. When a method is invoked, its parameters and local variables are pushed into a stack. When a method calls another method, the new method’s parameters and local variables are pushed into the stack. When a method finishes its work and returns to its caller, its associated space is released from the stack. You can define a class to model stacks. For simplicity, assume the stack holds the int values. So name the stack class StackOfIntegers. The UML diagram for the class is shown in Figure 10.12.
StackOfIntegers -elements: int[] -size: int +StackOfIntegers() +StackOfIntegers(capacity: int) +empty(): boolean +peek(): int
An array to store integers in the stack. The number of integers in the stack.
+push(value: int): void
Constructs an empty stack with a default capacity of 16. Constructs an empty stack with a specified capacity. Returns true if the stack is empty. Returns the integer at the top of the stack without removing it from the stack. Stores an integer into the top of the stack.
+pop(): int
Removes the integer at the top of the stack and returns it.
+getSize(): int
Returns the number of elements in the stack.
FIGURE 10.12 The StackOfIntegers class encapsulates the stack storage and provides the operations for manipulating the stack. Suppose that the class is available. The test program in Listing 10.7 uses the class to create a stack (line 3), store ten integers 0, 1, 2, . . . , and 9 (line 6), and displays them in reverse order (line 9).
LISTING 10.7 TestStackOfIntegers.java create a stack
1 2 3
public class TestStackOfIntegers { public static void main(String[] args) { StackOfIntegers stack = new StackOfIntegers();
10.6 Case Study: Designing a Class for Stacks 379 4 5 6 7 8 9 10 11
for (int i = 0; i < 10; i++) stack.push(i);
push to stack
while (!stack.empty()) System.out.print(stack.pop() + " ");
pop from stack
} }
9 8 7 6 5 4 3 2 1 0
How do you implement the StackOfIntegers class? The elements in the stack are stored in an array named elements. When you create a stack, the array is also created. The no-arg constructor creates an array with the default capacity of 16. The variable size counts the number of elements in the stack, and size – 1 is the index of the element at the top of the stack, as shown in Figure 10.13. For an empty stack, size is 0.
elements[capacity 1] . . . elements[size 1]
top capacity
. .
size
. elements[1] elements[0]
bottom
FIGURE 10.13 The StackOfIntegers class encapsulates the stack storage and provides the operations for manipulating the stack. The StackOfIntegers class is implemented in Listing 10.8. The methods empty(), peek(), pop(), and getSize() are easy to implement. To implement push(int value), assign value to elements[size] if size < capacity (line 24). If the stack is full (i.e., size >= capacity), create a new array of twice the current capacity (line 19), copy the contents of the current array to the new array (line 20), and assign the reference of the new array to the current array in the stack (line 21). Now you can add the new value to the array (line 24).
LISTING 10.8 StackOfIntegers.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
public class StackOfIntegers { private int[] elements; private int size; public static final int DEFAULT_CAPACITY = 16; /** Construct a stack with the default capacity 16 */ public StackOfIntegers() { this (DEFAULT_CAPACITY); } /** Construct a stack with the specified maximum capacity */ public StackOfIntegers(int capacity) { elements = new int[capacity]; }
max capacity 16
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double the capacity
add to stack
/** Push a new integer to the top of the stack */ public void push(int value) { if (size >= elements.length) { int[] temp = new int[elements.length * 2]; System.arraycopy(elements, 0, temp, 0, elements.length); elements = temp; } elements[size++] = value; } /** Return and remove the top element from the stack */ public int pop() { return elements[——size]; } /** Return the top element from the stack */ public int peek() { return elements[size - 1]; } /** Test whether the stack is empty */ public boolean empty() { return size == 0; } /** Return the number of elements in the stack */ public int getSize() { return size; } }
10.7 Processing Primitive Data Type Values as Objects Key Point
why wrapper class?
A primitive type value is not an object, but it can be wrapped in an object using a wrapper class in the Java API. Owing to performance considerations, primitive data type values are not objects in Java. Because of the overhead of processing objects, the language’s performance would be adversely affected if primitive data type values were treated as objects. However, many Java methods require the use of objects as arguments. Java offers a convenient way to incorporate, or wrap, a primitive data type into an object (e.g., wrapping int into the Integer class, wrapping double into the Double class, and wrapping char into the Character class,). By using a wrapper class, you can process primitive data type values as objects. Java provides Boolean, Character, Double, Float, Byte, Short, Integer, and Long wrapper classes in the java.lang package for primitive data types. The Boolean class wraps a Boolean value true or false. This section uses Integer and Double as examples to introduce the numeric wrapper classes.
Note naming convention
Most wrapper class names for a primitive type are the same as the primitive data type name with the first letter capitalized. The exceptions are Integer and Character.
Numeric wrapper classes are very similar to each other. Each contains the methods doubleValue(), floatValue(), intValue(), longValue(), shortValue(), and byteValue(). These methods “convert” objects into primitive type values. The key features of Integer and Double are shown in Figure 10.14.
10.7 Processing Primitive Data Type Values as Objects 381 java.lang.Integer
java.lang.Double
-value: int
-value: double
+MAX_VALUE: int
+MAX_VALUE: double
+MIN_VALUE: int
+MIN_VALUE: double
+Integer(value: int)
+Double(value: double)
+Integer(s: String)
+Double(s: String)
+byteValue(): byte
+byteValue(): byte
+shortValue(): short
+shortValue(): short
+intValue(): int
+intValue(): int
+longValue(): long
+longValue(): long
+floatValue(): float
+floatValue(): float
+doubleValue(): double
+doubleValue(): double
+compareTo(o: Integer): int
+compareTo(o: Double): int
+toString(): String
+toString(): String
+valueOf(s: String): Integer
+valueOf(s: String): Double
+valueOf(s: String, radix: int): Integer
+valueOf(s: String, radix: int): Double
+parseInt(s: String): int
+parseDouble(s: String): double
+parseInt(s: String, radix: int): int
+parseDouble(s: String, radix: int): double
FIGURE 10.14 The wrapper classes provide constructors, constants, and conversion methods for manipulating various data types. You can construct a wrapper object either from a primitive data type value or from a string representing the numeric value—for example, new Double(5.0), new Double("5.0"), new Integer(5), and new Integer("5"). The wrapper classes do not have no-arg constructors. The instances of all wrapper classes are immutable; this means that, once the objects are created, their internal values cannot be changed. Each numeric wrapper class has the constants MAX_VALUE and MIN_VALUE. MAX_VALUE represents the maximum value of the corresponding primitive data type. For Byte, Short, Integer, and Long, MIN_VALUE represents the minimum byte, short, int, and long values. For Float and Double, MIN_VALUE represents the minimum positive float and double values. The following statements display the maximum integer (2,147,483,647), the minimum positive float (1.4E–45), and the maximum double floating-point number (1.79769313486231570e + 308d).
constructors
no no-arg constructor immutable constants
System.out.println("The maximum integer is " + Integer.MAX_VALUE); System.out.println("The minimum positive float is " + Float.MIN_VALUE); System.out.println( "The maximum double-precision floating-point number is " + Double.MAX_VALUE);
Each numeric wrapper class contains the methods doubleValue(), floatValue(), intValue(), longValue(), and shortValue() for returning a double, float, int, long, or short value for the wrapper object. For example,
conversion methods
new Double(12.4).intValue() returns 12; new Integer(12).doubleValue() returns 12.0;
Recall that the String class contains the compareTo method for comparing two strings. The numeric wrapper classes contain the compareTo method for comparing two numbers
compareTo method
382 Chapter 10
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static valueOf methods
The numeric wrapper classes have a useful static method, valueOf (String s). This method creates a new object initialized to the value represented by the specified string. For example, Double doubleObject = Double.valueOf("12.4"); Integer integerObject = Integer.valueOf("12");
static parsing methods
You have used the parseInt method in the Integer class to parse a numeric string into an int value and the parseDouble method in the Double class to parse a numeric string into a double value. Each numeric wrapper class has two overloaded parsing methods to parse a numeric string into an appropriate numeric value based on 10 (decimal) or any specified radix (e.g., 2 for binary, 8 for octal, and 16 for hexadecimal). // These two methods are in the Byte class public static byte parseByte(String s) public static byte parseByte(String s, int radix) // These two methods are in the Short class public static short parseShort(String s) public static short parseShort(String s, int radix) // These two methods are in the Integer class public static int parseInt(String s) public static int parseInt(String s, int radix) // These two methods are in the Long class public static long parseLong(String s) public static long parseLong(String s, int radix) // These two methods are in the Float class public static float parseFloat(String s) public static float parseFloat(String s, int radix) // These two methods are in the Double class public static double parseDouble(String s) public static double parseDouble(String s, int radix)
For example, Integer.parseInt("11", Integer.parseInt("12", Integer.parseInt("13", Integer.parseInt("1A",
converting decimal to hex
2) returns 3; 8) returns 10; 10) returns 13; 16) returns 26;
Integer.parseInt("12", 2) would raise a runtime exception because 12 is not a binary number. Note that you can convert a decimal number into a hex number using the format method. For example, String.format("%x", 26) returns 1A;
10.8 Automatic Conversion between Primitive Types and Wrapper Class Types 383 10.7 Describe primitive-type wrapper classes. 10.8 Can each of the following statements be compiled?
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a. Integer i = new Integer("23"); b. Integer i = new Integer(23); c. Integer i = Integer.valueOf("23"); d. Integer i = Integer.parseInt("23", 8); e. Double d = new Double(); f. Double d = Double.valueOf("23.45"); g. int i = (Integer.valueOf("23")).intValue(); h. double d = (Double.valueOf("23.4")).doubleValue(); i. int i = (Double.valueOf("23.4")).intValue(); j. String s = (Double.valueOf("23.4")).toString();
10.9 How do you convert an integer into a string? How do you convert a numeric string into an integer? How do you convert a double number into a string? How do you convert a numeric string into a double value? 10.10 Show the output of the following code. public class Test { public static void main(String[] args) { Integer x = new Integer(3); System.out.println(x.intValue()); System.out.println(x.compareTo(new Integer(4))); } }
10.11 What is the output of the following code? public class Test { public static void main(String[] args) { System.out.println(Integer.parseInt("10")); System.out.println(Integer.parseInt("10", 10)); System.out.println(Integer.parseInt("10", 16)); System.out.println(Integer.parseInt("11")); System.out.println(Integer.parseInt("11", 10)); System.out.println(Integer.parseInt("11", 16)); } }
10.8 Automatic Conversion between Primitive Types and Wrapper Class Types A primitive type value can be automatically converted to an object using a wrapper class, and vice versa, depending on the context. Converting a primitive value to a wrapper object is called boxing. The reverse conversion is called unboxing. Java allows primitive types and wrapper classes to be converted automatically. The compiler will automatically box a primitive value that appears in a context requiring an object, and will unbox an object that appears in a context requiring a primitive value. This is called autoboxing and autounboxing.
Key Point boxing unboxing autoboxing autounboxing
384 Chapter 10
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Integer intObject = new Integer (2);
Equivalent
(a)
Integer intObject = 2;
autoboxing
(b)
Consider the following example: 1 2
Integer[] intArray = {1, 2, 3}; System.out.println(intArray[0] + intArray[1] + intArray[2]);
In line 1, the primitive values 1, 2, and 3 are automatically boxed into objects new Integer(1), new Integer(2), and new Integer(3). In line 2, the objects intArray[0], intArray[1], and intArray[2] are automatically unboxed into int values that are added together.
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10.12 What are autoboxing and autounboxing? Are the following statements correct? a. b. c. d. e. f.
Integer x = 3 + new Integer(5); Integer x = 3; Double x = 3; Double x = 3.0; int x = new Integer(3); int x = new Integer(3) + new Integer(4);
10.13 Show the output of the following code? public class Test { public static void main(String[] args) { Double x = 3.5; System.out.println(x.intValue()); System.out.println(x.compareTo(4.5)); } }
10.9 The BigInteger and BigDecimal Classes Key Point
immutable
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Process large numbers
The BigInteger and BigDecimal classes can be used to represent integers or decimal numbers of any size and precision. If you need to compute with very large integers or high-precision floating-point values, you can use the BigInteger and BigDecimal classes in the java.math package. Both are immutable. The largest integer of the long type is Long.MAX_VALUE (i.e., 9223372036854775807). An instance of BigInteger can represent an integer of any size. You can use new BigInteger(String) and new BigDecimal(String) to create an instance of BigInteger and BigDecimal, use the add, subtract, multiply, divide, and remainder methods to perform arithmetic operations, and use the compareTo method to compare two big numbers. For example, the following code creates two BigInteger objects and multiplies them. BigInteger a = new BigInteger("9223372036854775807"); BigInteger b = new BigInteger("2"); BigInteger c = a.multiply(b); // 9223372036854775807 * 2 System.out.println(c);
10.9 The BigInteger and BigDecimal Classes 385 The output is 18446744073709551614. There is no limit to the precision of a BigDecimal object. The divide method may throw an ArithmeticException if the result cannot be terminated. However, you can use the overloaded divide(BigDecimal d, int scale, int roundingMode) method to specify a scale and a rounding mode to avoid this exception, where scale is the maximum number of digits after the decimal point. For example, the following code creates two BigDecimal objects and performs division with scale 20 and rounding mode BigDecimal.ROUND_UP. BigDecimal a = new BigDecimal(1.0); BigDecimal b = new BigDecimal(3); BigDecimal c = a.divide(b, 20, BigDecimal.ROUND_UP); System.out.println(c);
The output is 0.33333333333333333334. Note that the factorial of an integer can be very large. Listing 10.9 gives a method that can return the factorial of any integer.
LISTING 10.9 LargeFactorial.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
import java.math.*; public class LargeFactorial { public static void main(String[] args) { System.out.println("50! is \n" + factorial(50)); } public static BigInteger factorial(long n) { BigInteger result = BigInteger.ONE; for (int i = 1; i <= n; i++) result = result.multiply(new BigInteger(i + ""));
constant multiply
return result; } }
50! is 30414093201713378043612608166064768844377641568960512000000000000
BigInteger.ONE (line 9) is a constant defined in the BigInteger class. BigInteger.ONE is the same as new BigInteger("1"). A new result is obtained by invoking the multiply method (line 11).
10.14 What is the output of the following code? public class Test { public static void main(String[] args) { java.math.BigInteger x = new java.math.BigInteger("3"); java.math.BigInteger y = new java.math.BigInteger("7"); java.math.BigInteger z = x.add(y); System.out.println("x is " + x); System.out.println("y is " + y); System.out.println("z is " + z); } }
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10.10 The String Class Key Point
VideoNote
The String class
A String object is immutable: Its content cannot be changed once the string is created. Strings were introduced in Section 4.4. You know strings are objects. You can invoke the charAt(index) method to obtain a character at the specified index from a string, the length() method to return the size of a string, the substring method to return a substring in a string, and the indexOf and lastIndexOf methods to return the first or last index of a matching character or a substring. We will take a closer look at strings in this section. The String class has 13 constructors and more than 40 methods for manipulating strings. Not only is it very useful in programming, but it is also a good example for learning classes and objects.
10.10.1
Constructing a String
You can create a string object from a string literal or from an array of characters. To create a string from a string literal, use the syntax: String newString = new String(stringLiteral);
The argument stringLiteral is a sequence of characters enclosed inside double quotes. The following statement creates a String object message for the string literal "Welcome to Java": String message = new String("Welcome to Java"); string literal object
Java treats a string literal as a String object. Thus, the following statement is valid: String message = "Welcome to Java";
You can also create a string from an array of characters. For example, the following statements create the string "Good Day": char[] charArray = {'G', 'o', 'o', 'd', ' ', 'D', 'a', 'y'}; String message = new String(charArray);
Note A String variable holds a reference to a String object that stores a string value. Strictly speaking, the terms String variable, String object, and string value are different, but most of the time the distinctions between them can be ignored. For simplicity, the term string will often be used to refer to String variable, String object, and string value.
String variable, String
object, string value
10.10.2 immutable
Immutable Strings and Interned Strings
A String object is immutable; its contents cannot be changed. Does the following code change the contents of the string? String s = "Java"; s = "HTML";
The answer is no. The first statement creates a String object with the content "Java" and assigns its reference to s. The second statement creates a new String object with the content "HTML" and assigns its reference to s. The first String object still exists after the assignment, but it can no longer be accessed, because variable s now points to the new object, as shown in Figure 10.15.
10.10 The String Class 387 After executing String s = "Java"; s
: String
After executing s = "HTML"; : String
s
String object for "Java"
String object for "Java"
Contents cannot be changed
This string object is now unreferenced
: String String object for "HTML"
FIGURE 10.15 Strings are immutable; once created, their contents cannot be changed. Because strings are immutable and are ubiquitous in programming, the JVM uses a unique instance for string literals with the same character sequence in order to improve efficiency and save memory. Such an instance is called an interned string. For example, the following statements:
String s1 = "Welcome to Java";
s1 s3
String s2 = new String("Welcome to Java");
interned string
: String Interned string object for "Welcome to Java"
String s3 = "Welcome to Java"; System.out.println("s1 == s2 is " + (s1 == s2)); s2 System.out.println("s1 == s3 is " + (s1 == s3));
: String A string object for "Welcome to Java"
display s1 == s2 is false s1 == s3 is true
In the preceding statements, s1 and s3 refer to the same interned string—"Welcome to Java"—so s1 == s3 is true. However, s1 == s2 is false, because s1 and s2 are two different string objects, even though they have the same contents.
10.10.3
Replacing and Splitting Strings
The String class provides the methods for replacing and splitting strings, as shown in Figure 10.16.
java.lang.String +replace(oldChar: char, newChar: char): String
Returns a new string that replaces all matching characters in this string with the new character.
+replaceFirst(oldString: String, newString: String): String
Returns a new string that replaces the first matching substring in this string with the new substring.
+replaceAll(oldString: String, newString: String): String
Returns a new string that replaces all matching substrings in this string with the new substring.
+split(delimiter: String): String[]
Returns an array of strings consisting of the substrings split by the delimiter.
FIGURE 10.16 The String class contains the methods for replacing and splitting strings.
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replace replaceFirst replace replace split
"Welcome".replace('e', 'A') returns a new string, WAlcomA. "Welcome".replaceFirst("e", "AB") returns a new string, WABlcome. "Welcome".replace("e", "AB") returns a new string, WABlcomAB. "Welcome".replace("el", "AB") returns a new string, WABcome.
The split method can be used to extract tokens from a string with the specified delimiters. For example, the following code String[] tokens = "Java#HTML#Perl".split("#"); for (int i = 0; i < tokens.length; i++) System.out.print(tokens[i] + " ");
displays Java HTML Perl
10.10.4 why regular expression? regular expression regex matches(regex)
Matching, Replacing and Splitting by Patterns
Often you will need to write code that validates user input, such as to check whether the input is a number, a string with all lowercase letters, or a Social Security number. How do you write this type of code? A simple and effective way to accomplish this task is to use the regular expression. A regular expression (abbreviated regex) is a string that describes a pattern for matching a set of strings. You can match, replace, or split a string by specifying a pattern. This is an extremely useful and powerful feature. Let us begin with the matches method in the String class. At first glance, the matches method is very similar to the equals method. For example, the following two statements both evaluate to true. "Java".matches("Java"); "Java".equals("Java");
However, the matches method is more powerful. It can match not only a fixed string, but also a set of strings that follow a pattern. For example, the following statements all evaluate to true: "Java is fun".matches("Java.*") "Java is cool".matches("Java.*") "Java is powerful".matches("Java.*")
Java.* in the preceding statements is a regular expression. It describes a string pattern that begins with Java followed by any zero or more characters. Here, the substring matches any zero or more characters. The following statement evaluates to true. "440-02-4534".matches("\\d{3}-\\d{2}-\\d{4}")
Here \\d represents a single digit, and \\d{3} represents three digits.
10.10 The String Class 389 The replaceAll, replaceFirst, and split methods can be used with a regular expression. For example, the following statement returns a new string that replaces $, +, or # in a+b$#c with the string NNN. String s = "a+b$#c".replaceAll("[$+#]", "NNN"); System.out.println(s);
replaceAll(regex)
Here the regular expression [$+#] specifies a pattern that matches $, +, or #. So, the output is aNNNbNNNNNNc. The following statement splits the string into an array of strings delimited by punctuation marks. String[] tokens = "Java,C?C#,C++".split("[.,:;?]");
split(regex)
for (int i = 0; i < tokens.length; i++) System.out.println(tokens[i]);
In this example, the regular expression [.,:;?] specifies a pattern that matches ., ,, :, ;, or ?. Each of these characters is a delimiter for splitting the string. Thus, the string is split into Java, C, C#, and C++, which are stored in array tokens. Regular expression patterns are complex for beginning students to understand. For this reason, simple patterns are introduced in this section. Please refer to Appendix H, Regular Expressions, to learn more about these patterns.
10.10.5
further studies
Conversion between Strings and Arrays
Strings are not arrays, but a string can be converted into an array, and vice versa. To convert a string into an array of characters, use the toCharArray method. For example, the following statement converts the string Java to an array.
toCharArray
char[] chars = "Java".toCharArray();
Thus, chars[0] is J, chars[1] is a, chars[2] is v, and chars[3] is a. You can also use the getChars(int srcBegin, int srcEnd, char[] dst, int dstBegin) method to copy a substring of the string from index srcBegin to index srcEnd-1 into a character array dst starting from index dstBegin. For example, the following code copies a substring "3720" in "CS3720" from index 2 to index 6-1 into the character array dst starting from index 4. char[] dst = {'J', 'A', 'V', 'A', '1', '3', '0', '1'}; "CS3720".getChars(2, 6, dst, 4);
getChars
Thus, dst becomes {'J', 'A', 'V', 'A', '3', '7', '2', '0'}. To convert an array of characters into a string, use the String(char[]) constructor or the valueOf(char[]) method. For example, the following statement constructs a string from an array using the String constructor. String str = new String(new char[]{'J', 'a', 'v', 'a'});
The next statement constructs a string from an array using the valueOf method. String str = String.valueOf(new char[]{'J', 'a', 'v', 'a'});
10.10.6
Converting Characters and Numeric Values to Strings
Recall that you can use Double.parseDouble(str) or Integer.parseInt(str) to convert a string to a double value or an int value and you can convert a character or a number into a string by using the string concatenating operator. Another way of converting a
valueOf
390 Chapter 10
Object-Oriented Thinking number into a string is to use the overloaded static valueOf method. This method can also be used to convert a character or an array of characters into a string, as shown in Figure 10.17.
overloaded valueOf
java.lang.String +valueOf(c: char): String
Returns a string consisting of the character c.
+valueOf(data: char[]): String
Returns a string consisting of the characters in the array.
+valueOf(d: double): String
Returns a string representing the double value.
+valueOf(f: float): String
Returns a string representing the float value.
+valueOf(i: int): String
Returns a string representing the int value.
+valueOf(l: long): String
Returns a string representing the long value.
+valueOf(b: boolean): String
Returns a string representing the boolean value.
FIGURE 10.17 The String class contains the static methods for creating strings from primitive type values. For example, to convert a double value 5.44 to a string, use String.valueOf(5.44). The return value is a string consisting of the characters '5', '.', '4', and '4'.
10.10.7
Formatting Strings
The String class contains the static format method to return a formatted string. The syntax to invoke this method is: String.format(format, item1, item2, ..., itemk)
This method is similar to the printf method except that the format method returns a formatted string, whereas the printf method displays a formatted string. For example, String s = String.format("%7.2f%6d%-4s", 45.556, 14, "AB"); System.out.println(s);
displays 45.56
14AB
Note that System.out.printf(format, item1, item2, ..., itemk);
is equivalent to System.out.print( String.format(format, item1, item2, ..., itemk));
where the square box ( ) denotes a blank space.
✓
Check Point
10.15 Suppose that s1, s2, s3, and s4 are four strings, given as follows: String String String String
s1 s2 s3 s4
= = = =
"Welcome to Java"; s1; new String("Welcome to Java"); "Welcome to Java";
What are the results of the following expressions? a. b.
s1 == s2 s1 == s3
10.10 The String Class 391 c. s1 == s4 d. s1.equals(s3) e. s1.equals(s4) f. "Welcome to Java".replace("Java", "HTML") g. s1.replace('o', 'T') h. s1.replaceAll("o", "T") i. s1.replaceFirst("o", "T")
j.
s1.toCharArray()
10.16 To create the string Welcome
to Java, you may use a statement like this:
String s = "Welcome to Java";
or: String s = new String("Welcome to Java");
Which one is better? Why?
10.17 What is the output of the following code? String s1 = "Welcome to Java"; String s2 = s1.replace("o", "abc"); System.out.println(s1); System.out.println(s2);
10.18 Let
s1 be "Welcome" and s2 be "welcome". Write the code for the following statements:
a. Replace all occurrences of the character e with E in s1 and assign the new string to s2. b. Split Welcome to Java and HTML into an array tokens delimited by a space and assign the first two tokens into s1 and s2.
10.19 10.20 10.21 10.22
Does any method in the String class change the contents of the string? Suppose string s is created using new String(); what is s.length()? How do you convert a char, an array of characters, or a number to a string? Why does the following code cause a NullPointerException? 1 2 3 4 5 6 7 8 9 10 11 12
public class Test { private String text; public Test(String s) { String text = s; } public static void main(String[] args) { Test test = new Test("ABC"); System.out.println(test.text.toLowerCase()); } }
10.23 What is wrong in the following program? 1 2 3
public class Test { String text;
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public void Test(String s) { text = s; } public static void main(String[] args) { Test test = new Test("ABC"); System.out.println(test); } }
10.24 Show the output of the following code. public class Test { public static void main(String[] args) { System.out.println("Hi, ABC, good".matches("ABC ")); System.out.println("Hi, ABC, good".matches(".*ABC.*")); System.out.println("A,B;C".replaceAll(",;", "#")); System.out.println("A,B;C".replaceAll("[,;]", "#")); String[] tokens = "A,B;C".split("[,;]"); for (int i = 0; i < tokens.length; i++) System.out.print(tokens[i] + " "); } }
10.25 Show the output of the following code. public class Test { public static void main(String[] args) { String s = "Hi, Good Morning"; System.out.println(m(s)); } public static int m(String s) { int count = 0; for (int i = 0; i < s.length(); i++) if (Character.isUpperCase(s.charAt(i))) count++; return count; } }
10.11 The StringBuilder and StringBuffer Classes Key Point
StringBuilder
The StringBuilder and StringBuffer classes are similar to the String class except that the String class is immutable. In general, the StringBuilder and StringBuffer classes can be used wherever a string is used. StringBuilder and StringBuffer are more flexible than String. You can add, insert, or append new contents into StringBuilder and StringBuffer objects, whereas the value of a String object is fixed once the string is created. The StringBuilder class is similar to StringBuffer except that the methods for modifying the buffer in StringBuffer are synchronized, which means that only one task is allowed to execute the methods. Use StringBuffer if the class might be accessed by multiple tasks concurrently, because synchronization is needed in this case to prevent corruptions to
10.11 The StringBuilder and StringBuffer Classes 393 StringBuffer. Concurrent programming will be introduced in Chapter 30. Using StringBuilder is more efficient if it is accessed by just a single task, because no synchronization is needed in this case. The constructors and methods in StringBuffer and StringBuilder are almost the same. This section covers StringBuilder. You can replace StringBuilder in all occurrences in this section by StringBuffer. The program can compile and run with-
out any other changes. The StringBuilder class has three constructors and more than 30 methods for managing the builder and modifying strings in the builder. You can create an empty string builder or a string builder from a string using the constructors, as shown in Figure 10.18.
StringBuilder constructors
java.lang.StringBuilder +StringBuilder()
Constructs an empty string builder with capacity 16.
+StringBuilder(capacity: int)
Constructs a string builder with the specified capacity.
+StringBuilder(s: String)
Constructs a string builder with the specified string.
FIGURE 10.18 The StringBuilder class contains the constructors for creating instances of StringBuilder.
10.11.1
Modifying Strings in the StringBuilder
You can append new contents at the end of a string builder, insert new contents at a specified position in a string builder, and delete or replace characters in a string builder, using the methods listed in Figure 10.19.
java.lang.StringBuilder +append(data: char[]): StringBuilder +append(data: char[], offset: int, len: int): StringBuilder
Appends a char array into this string builder. Appends a subarray in data into this string builder.
+append(v: aPrimitiveType): StringBuilder
Appends a primitive type value as a string to this builder.
+append(s: String): StringBuilder
Appends a string to this string builder.
+delete(startIndex: int, endIndex: int): StringBuilder +deleteCharAt(index: int): StringBuilder
Deletes characters from startIndex to endIndex-1.
+insert(index: int, data: char[], offset: int, len: int): StringBuilder +insert(offset: int, data: char[]): StringBuilder +insert(offset: int, b: aPrimitiveType): StringBuilder
Inserts a subarray of the data in the array into the builder at the specified index. Inserts data into this builder at the position offset.
+insert(offset: int, s: String): StringBuilder +replace(startIndex: int, endIndex: int, s: String): StringBuilder +reverse(): StringBuilder +setCharAt(index: int, ch: char): void
Deletes a character at the specified index.
Inserts a value converted to a string into this builder. Inserts a string into this builder at the position offset. Replaces the characters in this builder from startIndex to endIndex-1 with the specified string. Reverses the characters in the builder. Sets a new character at the specified index in this builder.
FIGURE 10.19 The StringBuilder class contains the methods for modifying string builders.
394 Chapter 10
Object-Oriented Thinking The StringBuilder class provides several overloaded methods to append boolean, char, char[], double, float, int, long, and String into a string builder. For example, the following code appends strings and characters into stringBuilder to form a new string, Welcome to Java.
append
StringBuilder stringBuilder = new StringBuilder(); stringBuilder.append("Welcome"); stringBuilder.append(' '); stringBuilder.append("to"); stringBuilder.append(' '); stringBuilder.append("Java");
The StringBuilder class also contains overloaded methods to insert boolean, char, char array, double, float, int, long, and String into a string builder. Consider the following code: insert
stringBuilder.insert(11, "HTML and ");
Suppose stringBuilder contains Welcome to Java before the insert method is applied. This code inserts "HTML and " at position 11 in stringBuilder (just before the J). The new stringBuilder is Welcome to HTML and Java. You can also delete characters from a string in the builder using the two delete methods, reverse the string using the reverse method, replace characters using the replace method, or set a new character in a string using the setCharAt method. For example, suppose stringBuilder contains Welcome to Java before each of the following methods is applied: delete deleteCharAt reverse replace setCharAt
stringBuilder.delete(8, 11) changes the builder to Welcome Java. stringBuilder.deleteCharAt(8) changes the builder to Welcome o Java. stringBuilder.reverse() changes the builder to avaJ ot emocleW. stringBuilder.replace(11, 15, "HTML") changes the builder to Welcome to HTML. stringBuilder.setCharAt(0, 'w') sets the builder to welcome to Java.
All these modification methods except setCharAt do two things:
ignore return value
■
Change the contents of the string builder
■
Return the reference of the string builder
For example, the following statement StringBuilder stringBuilder1 = stringBuilder.reverse();
reverses the string in the builder and assigns the builder’s reference to stringBuilder1. Thus, stringBuilder and stringBuilder1 both point to the same StringBuilder object. Recall that a value-returning method can be invoked as a statement, if you are not interested in the return value of the method. In this case, the return value is simply ignored. For example, in the following statement stringBuilder.reverse();
the return value is ignored.
Tip String or StringBuilder?
If a string does not require any change, use String rather than StringBuilder. Java can perform some optimizations for String, such as sharing interned strings.
10.11 The StringBuilder and StringBuffer Classes 395
10.11.2
The toString, capacity, length, setLength, and charAt Methods
The StringBuilder class provides the additional methods for manipulating a string builder and obtaining its properties, as shown in Figure 10.20. java.lang.StringBuilder +toString(): String +capacity(): int
Returns a string object from the string builder. Returns the capacity of this string builder.
+charAt(index: int): char
Returns the character at the specified index.
+length(): int +setLength(newLength: int): void
Returns the number of characters in this builder. Sets a new length in this builder.
+substring(startIndex: int): String
Returns a substring starting at startIndex.
+substring(startIndex: int, endIndex: int): String +trimToSize(): void
Returns a substring from startIndex to endIndex-1. Reduces the storage size used for the string builder.
FIGURE 10.20 The StringBuilder class contains the methods for modifying string builders. The capacity() method returns the current capacity of the string builder. The capacity is the number of characters the string builder is able to store without having to increase its size. The length() method returns the number of characters actually stored in the string builder. The setLength(newLength) method sets the length of the string builder. If the newLength argument is less than the current length of the string builder, the string builder is truncated to contain exactly the number of characters given by the newLength argument. If the newLength argument is greater than or equal to the current length, sufficient null characters (\u0000) are appended to the string builder so that length becomes the newLength argument. The newLength argument must be greater than or equal to 0. The charAt(index) method returns the character at a specific index in the string builder. The index is 0 based. The first character of a string builder is at index 0, the next at index 1, and so on. The index argument must be greater than or equal to 0, and less than the length of the string builder.
capacity() length() setLength(int)
charAt(int)
Note The length of the string is always less than or equal to the capacity of the builder. The length is the actual size of the string stored in the builder, and the capacity is the current size of the builder. The builder’s capacity is automatically increased if more characters are added to exceed its capacity. Internally, a string builder is an array of characters, so the builder’s capacity is the size of the array. If the builder’s capacity is exceeded, the array is replaced by a new array. The new array size is 2 * (the previous array size + 1).
length and capacity
Tip You can use new StringBuilder(initialCapacity) to create a StringBuilder with a specified initial capacity. By carefully choosing the initial capacity, you can make your program more efficient. If the capacity is always larger than the actual length of the builder, the JVM will never need to reallocate memory for the builder. On the other hand, if the capacity is too large, you will waste memory space. You can use the trimToSize() method to reduce the capacity to the actual size.
initial capacity
trimToSize()
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10.11.3
Case Study: Ignoring Nonalphanumeric Characters When Checking Palindromes
Listing 5.14, Palindrome.java, considered all the characters in a string to check whether it is a palindrome. Write a new program that ignores nonalphanumeric characters in checking whether a string is a palindrome. Here are the steps to solve the problem: 1. Filter the string by removing the nonalphanumeric characters. This can be done by creating an empty string builder, adding each alphanumeric character in the string to a string builder, and returning the string from the string builder. You can use the isLetterOrDigit(ch) method in the Character class to check whether character ch is a letter or a digit. 2. Obtain a new string that is the reversal of the filtered string. Compare the reversed string with the filtered string using the equals method. The complete program is shown in Listing 10.10.
LISTING 10.10 PalindromeIgnoreNonAlphanumeric.java
check palindrome
add letter or digit
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
import java.util.Scanner; public class PalindromeIgnoreNonAlphanumeric { /** Main method */ public static void main(String[] args) { // Create a Scanner Scanner input = new Scanner(System.in); // Prompt the user to enter a string System.out.print("Enter a string: "); String s = input.nextLine(); // Display result System.out.println("Ignoring nonalphanumeric characters, \nis " + s + " a palindrome? " + isPalindrome(s)); } /** Return true if a string is a palindrome */ public static boolean isPalindrome(String s) { // Create a new string by eliminating nonalphanumeric chars String s1 = filter(s); // Create a new string that is the reversal of s1 String s2 = reverse(s1); // Check if the reversal is the same as the original string return s2.equals(s1); } /** Create a new string by eliminating nonalphanumeric chars */ public static String filter(String s) { // Create a string builder StringBuilder stringBuilder = new StringBuilder(); // Examine each char in the string to skip alphanumeric char for (int i = 0; i < s.length(); i++) { if (Character.isLetterOrDigit(s.charAt(i))) { stringBuilder.append(s.charAt(i)); }
10.11 The StringBuilder and StringBuffer Classes 397 40 41 42 43 44 45 46 47 48 49 50 51 52
} // Return a new filtered string return stringBuilder.toString(); } /** Create a new string by reversing a specified string */ public static String reverse(String s) { StringBuilder stringBuilder = new StringBuilder(s); stringBuilder.reverse(); // Invoke reverse in StringBuilder return stringBuilder.toString(); } }
Enter a string: abcb?a Ignoring nonalphanumeric characters, is abcb?a a palindrome? true
Enter a string: abcc> +ArrayList()
Creates an empty list.
+add(o: E): void
Appends a new element o at the end of this list.
+add(index: int, o: E): void
Adds a new element o at the specified index in this list.
+clear(): void
Removes all the elements from this list.
+contains(o: Object): boolean
Returns true if this list contains the element o.
+get(index: int): E
Returns the element from this list at the specified index.
+indexOf(o: Object): int
Returns the index of the first matching element in this list.
+isEmpty(): boolean
Returns true if this list contains no elements.
+lastIndexOf(o: Object): int
Returns the index of the last matching element in this list.
+remove(o: Object): boolean
Removes the first element o from this list. Returns true if an element is removed.
+size(): int +remove(index: int): boolean
Returns the number of elements in this list. Removes the element at the specified index. Returns true if an element is removed.
+set(index: int, o: E): E
Sets the element at the specified index.
FIGURE 11.3 An ArrayList stores an unlimited number of objects.
ArrayList is known as a generic class with a generic type E. You can specify a concrete type to replace E when creating an ArrayList. For example, the following statement creates an ArrayList and assigns its reference to variable cities. This ArrayList object can be used to store strings. ArrayList cities = new ArrayList();
The following statement creates an ArrayList and assigns its reference to variable dates. This ArrayList object can be used to store dates. ArrayList dates = new ArrayList ();
Note Since JDK 7, the statement ArrayList list = new ArrayList();
can be simplified by ArrayList list = new ArrayList<>();
The concrete type is no longer required in the constructor thanks to a feature called type inference. The compiler is able to infer the type from the variable declaration. More discussions on generics including how to define custom generic classes and methods will be introduced in Chapter 19, Generics.
type inference
Listing 11.8 gives an example of using ArrayList to store objects.
LISTING 11.8 1 2
TestArrayList.java
import java.util.ArrayList;
import ArrayList
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create ArrayList
add element
list size contains element? element index is empty?
remove element
remove element
toString()
get element
create ArrayList
Inheritance and Polymorphism 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 54 55 56 57 58 59 60 61 62
public class TestArrayList { public static void main(String[] args) { // Create a list to store cities ArrayList cityList = new ArrayList<>(); // Add some cities in the list cityList.add("London"); // cityList now contains [London] cityList.add("Denver"); // cityList now contains [London, Denver] cityList.add("Paris"); // cityList now contains [London, Denver, Paris] cityList.add("Miami"); // cityList now contains [London, Denver, Paris, Miami] cityList.add("Seoul"); // Contains [London, Denver, Paris, Miami, Seoul] cityList.add("Tokyo"); // Contains [London, Denver, Paris, Miami, Seoul, Tokyo] System.out.println("List size? " + cityList.size()); System.out.println("Is Miami in the list? " + cityList.contains("Miami")); System.out.println("The location of Denver in the list? " + cityList.indexOf("Denver")); System.out.println("Is the list empty? " + cityList.isEmpty()); // Print false // Insert a new city at index 2 cityList.add(2, "Xian"); // Contains [London, Denver, Xian, Paris, Miami, Seoul, Tokyo] // Remove a city from the list cityList.remove("Miami"); // Contains [London, Denver, Xian, Paris, Seoul, Tokyo] // Remove a city at index 1 cityList.remove(1); // Contains [London, Xian, Paris, Seoul, Tokyo] // Display the contents in the list System.out.println(cityList.toString()); // Display the contents in the list in reverse order for (int i = cityList.size() - 1; i >= 0; i––) System.out.print(cityList.get(i) + " "); System.out.println(); // Create a list to store two circles ArrayList list = new ArrayList<>(); // Add two circles list.add(new CircleFromSimpleGeometricObject(2)); list.add(new CircleFromSimpleGeometricObject(3)); // Display the area of the first circle in the list System.out.println("The area of the circle? " + list.get(0).getArea()); } }
11.11 The ArrayList Class 435 List size? 6 Is Miami in the list? True The location of Denver in the list? 1 Is the list empty? false [London, Xian, Paris, Seoul, Tokyo] Tokyo Seoul Paris Xian London The area of the circle? 12.566370614359172
Since the ArrayList is in the java.util package, it is imported in line 1. The program creates an ArrayList of strings using its no-arg constructor and assigns the reference to cityList (line 6). The add method (lines 9–19) adds strings to the end of list. So, after cityList.add("London") (line 9), the list contains
add(Object)
[London]
After cityList.add("Denver") (line 11), the list contains [London, Denver]
After adding Paris, Miami, Seoul, and Tokyo (lines 13–19), the list contains [London, Denver, Paris, Miami, Seoul, Tokyo]
Invoking size() (line 22) returns the size of the list, which is currently 6. Invoking contains("Miami") (line 24) checks whether the object is in the list. In this case, it returns true, since Miami is in the list. Invoking indexOf("Denver") (line 26) returns the index of Denver in the list, which is 1. If Denver were not in the list, it would return -1. The isEmpty() method (line 28) checks whether the list is empty. It returns false, since the list is not empty. The statement cityList.add(2, "Xian") (line 31) inserts an object into the list at the specified index. After this statement, the list becomes
size()
add(index, Object)
[London, Denver, Xian, Paris, Miami, Seoul, Tokyo]
The statement cityList.remove("Miami") (line 35) removes the object from the list. After this statement, the list becomes
remove(Object)
[London, Denver, Xian, Paris, Seoul, Tokyo]
The statement cityList.remove(1) (line 39) removes the object at the specified index from the list. After this statement, the list becomes
remove(index)
[London, Xian, Paris, Seoul, Tokyo]
The statement in line 43 is same as System.out.println(cityList);
The toString() method returns a string representation of the list in the form of [e0.toString(), e1.toString(), ..., ek.toString()], where e0, e1, . . . , and ek are the elements in the list. The get(index) method (line 47) returns the object at the specified index. ArrayList objects can be used like arrays, but there are many differences. Table 11.1 lists their similarities and differences. Once an array is created, its size is fixed. You can access an array element using the square-bracket notation (e.g., a[index]). When an ArrayList is created, its size is 0.
toString()
get(index)
array vs. ArrayList
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TABLE 11.1 Differences and Similarities between Arrays and ArrayList Operation
Array
ArrayList
Creating an array/ArrayList
String[] a = new String[10]
ArrayList list = new ArrayList<>();
Accessing an element
a[index]
list.get(index);
Updating an element
a[index] = "London";
list.set(index, "London");
Returning size
a.length
list.size();
Adding a new element
list.add("London");
Inserting a new element
list.add(index, "London");
Removing an element
list.remove(index);
Removing an element
list.remove(Object);
Removing all elements
list.clear();
You cannot use the get(index) and set(index, element) methods if the element is not in the list. It is easy to add, insert, and remove elements in a list, but it is rather complex to add, insert, and remove elements in an array. You have to write code to manipulate the array in order to perform these operations. Note that you can sort an array using the java.util. Arrays.sort(array) method. To sort an array list, use the java.util.Collections. sort(arraylist) method. Suppose you want to create an ArrayList for storing integers. Can you use the following code to create a list? ArrayList list = new ArrayList<>();
No. This will not work because the elements stored in an ArrayList must be of an object type. You cannot use a primitive data type such as int to replace a generic type. However, you can create an ArrayList for storing Integer objects as follows: ArrayList list = new ArrayList<>();
Listing 11.9 gives a program that prompts the user to enter a sequence of numbers and displays the distinct numbers in the sequence. Assume that the input ends with 0 and 0 is not counted as a number in the sequence.
LISTING 11.9 DistinctNumbers.java
create an array list
contained in list? add to list
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
import java.util.ArrayList; import java.util.Scanner; public class DistinctNumbers { public static void main(String[] args) { ArrayList list = new ArrayList<>(); Scanner input = new Scanner(System.in); System.out.print("Enter integers (input ends with 0): "); int value; do { value = input.nextInt(); // Read a value from the input if (!list.contains(value) && value != 0) list.add(value); // Add the value if it is not in the list } while (value != 0);
11.11 The ArrayList Class 437 18 19 20 21 22 23
// Display the distinct numbers for (int i = 0; i < list.size(); i++) System.out.print(list.get(i) + " "); } }
Enter numbers (input ends with 0): 1 2 3 2 1 6 3 4 5 4 5 1 2 3 0 The distinct numbers are: 1 2 3 6 4 5
The program creates an ArrayList for Integer objects (line 6) and repeatedly reads a value in the loop (lines 12–17). For each value, if it is not in the list (line 15), add it to the list (line 16). You can rewrite this program using an array to store the elements rather than using an ArrayList. However, it is simpler to implement this program using an ArrayList for two reasons. ■
First, the size of an ArrayList is flexible so you don’t have to specify its size in advance. When creating an array, its size must be specified.
■
Second, ArrayList contains many useful methods. For example, you can test whether an element is in the list using the contains method. If you use an array, you have to write additional code to implement this method. You can traverse the elements in an array using a foreach loop. The elements in an array list can also be traversed using a foreach loop using the following syntax: for (elementType element: arrayList) { // Process the element }
For example, you can replace the code in lines 20-21 using the following code: for (int number: list) System.out.print(number + “ “);
11.30 How do you do the following? a. Create an ArrayList for storing double values? b. Append an object to a list? c. Insert an object at the beginning of a list? d. Find the number of objects in a list? e. Remove a given object from a list? f. Remove the last object from the list? g. Check whether a given object is in a list? h. Retrieve an object at a specified index from a list?
11.31 Identify the errors in the following code. ArrayList list = new ArrayList<>(); list.add("Denver"); list.add("Austin"); list.add(new java.util.Date()); String city = list.get(0); list.set(3, "Dallas"); System.out.println(list.get(3));
✓
Check Point
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Inheritance and Polymorphism 11.32 Suppose the ArrayList
list contains {"Dallas", "Dallas", "Houston", "Dallas"}. What is the list after invoking list.remove("Dallas") one time? Does the following code correctly remove all elements with value "Dallas" from
the list? If not, correct the code. for (int i = 0; i < list.size(); i++) list.remove("Dallas");
11.33 Explain why the following code displays [1,
3] rather than [2, 3].
ArrayList list = new ArrayList<>(); list.add(1); list.add(2); list.add(3); list.remove(1); System.out.println(list);
11.34 Explain why the following code is wrong. ArrayList list = new ArrayList<>(); list.add(1);
11.12 Useful Methods for Lists Key Point array to array list
Java provides the methods for creating a list from an array, for sorting a list, and finding maximum and minimum element in a list, and for shuffling a list. Often you need to create an array list from an array of objects or vice versa. You can write the code using a loop to accomplish this, but an easy way is to use the methods in the Java API. Here is an example to create an array list from an array: String[] array = {"red", "green", "blue"}; ArrayList list = new ArrayList<>(Arrays.asList(array));
array list to array
The static method asList in the Arrays class returns a list that is passed to the ArrayList constructor for creating an ArrayList. Conversely, you can use the following code to create an array of objects from an array list. String[] array1 = new String[list.size()]; list.toArray(array1);
sort a list
Invoking list.toArray(array1) copies the contents from list to array1. If the elements in a list are comparable such as integers, double, or strings, you can use the static sort method in the java.util.Collections class to sort the elements. Here are examples: Integer[] array = {3, 5, 95, 4, 15, 34, 3, 6, 5}; ArrayList list = new ArrayList<>(Arrays.asList(array)); java.util.Collections.sort(list); System.out.println(list);
max and min methods
You can use the static max and min in the java.util.Collections class to return the maximum and minimal element in a list. Here are examples: Integer[] array = {3, 5, 95, 4, 15, 34, 3, 6, 5}; ArrayList list = new ArrayList<>(Arrays.asList(array)); System.out.println(java.util.Collections.max(list)); System.out.println(java.util.Collections.min(list));
11.13 Case Study: A Custom Stack Class 439 You can use the static shuffle method in the java.util.Collections class to perform a random shuffle for the elements in a list. Here are examples:
shuffle method
Integer[] array = {3, 5, 95, 4, 15, 34, 3, 6, 5}; ArrayList list = new ArrayList<>(Arrays.asList(array)); java.util.Collections.shuffle(list); System.out.println(list);
11.35 Correct errors in the following statements: int[] array = {3, 5, 95, 4, 15, 34, 3, 6, 5}; ArrayList list = new ArrayList<>(Arrays.asList(array));
✓
Check Point
11.36 Correct errors in the following statements: int[] array = {3, 5, 95, 4, 15, 34, 3, 6, 5}; System.out.println(java.util.Collections.max(array));
11.13 Case Study: A Custom Stack Class This section designs a stack class for holding objects. Section 10.6 presented a stack class for storing int values. This section introduces a stack class to store objects. You can use an ArrayList to implement Stack, as shown in Listing 11.10. The UML diagram for the class is shown in Figure 11.4.
Key Point
VideoNote
The MyStack class MyStack -list: ArrayList
A list to store elements.
+isEmpty(): boolean
Returns true if this stack is empty.
+getSize(): int
Returns the number of elements in this stack.
+peek(): Object
Returns the top element in this stack without removing it.
+pop(): Object
Returns and removes the top element in this stack.
+push(o: Object): void
Adds a new element to the top of this stack.
FIGURE 11.4 The MyStack class encapsulates the stack storage and provides the operations for manipulating the stack.
LISTING 11.10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
MyStack.java
import java.util.ArrayList; public class MyStack { private ArrayList list = new ArrayList<>();
array list
public boolean isEmpty() { return list.isEmpty(); }
stack empty?
public int getSize() { return list.size(); }
get stack size
public Object peek() { return list.get(getSize() - 1); }
peek stack
440 Chapter 11
Inheritance and Polymorphism 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
remove
push
public Object pop() { Object o = list.get(getSize() - 1); list.remove(getSize() - 1); return o; } public void push(Object o) { list.add(o); } @Override public String toString() { return "stack: " + list.toString(); } }
An array list is created to store the elements in the stack (line 4). The isEmpty() method (lines 6–8) returns list.isEmpty(). The getSize() method (lines 10–12) returns list.size(). The peek() method (lines 14–16) retrieves the element at the top of the stack without removing it. The end of the list is the top of the stack. The pop() method (lines 18–22) removes the top element from the stack and returns it. The push(Object element) method (lines 24–26) adds the specified element to the stack. The toString() method (lines 28–31) defined in the Object class is overridden to display the contents of the stack by invoking list.toString(). The toString() method implemented in ArrayList returns a string representation of all the elements in an array list.
Design Guide In Listing 11.10, MyStack contains ArrayList. The relationship between MyStack and ArrayList is composition. While inheritance models an is-a relationship, composition models a has-a relationship. You could also implement MyStack as a subclass of ArrayList (see Programming Exercise 11.10). Using composition is better, however, because it enables you to define a completely new stack class without inheriting the unnecessary and inappropriate methods from ArrayList.
composition is-a has-a
11.14 The protected Data and Methods Key Point
why protected?
A protected member of a class can be accessed from a subclass. So far you have used the private and public keywords to specify whether data fields and methods can be accessed from outside of the class. Private members can be accessed only from inside of the class, and public members can be accessed from any other classes. Often it is desirable to allow subclasses to access data fields or methods defined in the superclass, but not to allow nonsubclasses to access these data fields and methods. To accomplish this, you can use the protected keyword. This way you can access protected data fields or methods in a superclass from its subclasses. The modifiers private, protected, and public are known as visibility or accessibility modifiers because they specify how classes and class members are accessed. The visibility of these modifiers increases in this order: Visibility increases private, default (no modifier), protected, public Table 11.2 summarizes the accessibility of the members in a class. Figure 11.5 illustrates how a public, protected, default, and private datum or method in class C1 can be accessed from a class C2 in the same package, from a subclass C3 in the same package, from a subclass C4 in a different package, and from a class C5 in a different package.
11.14 The protected Data and Methods 441 Use the private modifier to hide the members of the class completely so that they cannot be accessed directly from outside the class. Use no modifiers (the default) in order to allow the members of the class to be accessed directly from any class within the same package but not from other packages. Use the protected modifier to enable the members of the class to be accessed by the subclasses in any package or classes in the same package. Use the public modifier to enable the members of the class to be accessed by any class.
TABLE 11.2 Data and Methods Visibility Modifier on members in a class
Accessed from the same class
Accessed from the same package
Accessed from a subclass in a different package
Accessed from a different package
public
✓
✓
✓
✓
protected
✓
✓
✓
–
default (no modifier)
✓
✓
–
–
private
✓
–
–
–
package p1; public class C1 { public int x; protected int y; int z; private int u;
public class C2 { C1 o = new C1(); can access o.x; can access o.y; can access o.z; cannot access o.u;
protected void m() { } }
can invoke o.m(); }
package p2;
public class C3 extends C1 { can access x; can access y; can access z; cannot access u;
public class C4 extends C1 { can access x; can access y; cannot access z; cannot access u;
can invoke m(); }
public class C5 { C1 o = new C1(); can access o.x; cannot access o.y; cannot access o.z; cannot access o.u;
can invoke m(); }
cannot invoke o.m(); }
FIGURE 11.5 Visibility modifiers are used to control how data and methods are accessed. Your class can be used in two ways: (1) for creating instances of the class and (2) for defining subclasses by extending the class. Make the members private if they are not intended for use from outside the class. Make the members public if they are intended for the users of the class. Make the fields or methods protected if they are intended for the extenders of the class but not for the users of the class. The private and protected modifiers can be used only for members of the class. The public modifier and the default modifier (i.e., no modifier) can be used on members of the class as well as on the class. A class with no modifier (i.e., not a public class) is not accessible by classes from other packages.
442 Chapter 11
Inheritance and Polymorphism Note A subclass may override a protected method defined in its superclass and change its visibility to public. However, a subclass cannot weaken the accessibility of a method defined in the superclass. For example, if a method is defined as public in the superclass, it must be defined as public in the subclass.
change visibility
✓
Check Point
11.37 What modifier should you use on a class so that a class in the same package can access it, but a class in a different package cannot access it? 11.38 What modifier should you use so that a class in a different package cannot access the class, but its subclasses in any package can access it?
11.39 In the following code, the classes A and B are in the same package. If the question marks in (a) are replaced by blanks, can class B be compiled? If the question marks are replaced by private, can class B be compiled? If the question marks are replaced by protected, can class B be compiled? package p1;
package p1;
public class A { ? int i;
public class B extends A { public void m1(String[] args) { System.out.println(i); m(); } }
? ...
void m() {
} } (a)
(b)
11.40 In the following code, the classes A and B are in different packages. If the question marks in (a) are replaced by blanks, can class B be compiled? If the question marks are replaced by private, can class B be compiled? If the question marks are replaced by protected, can class B be compiled? package p1;
package p2;
public class A { ? int i;
public class B extends A { public void m1(String[] args) { System.out.println(i); m(); } }
? ...
void m() {
} } (a)
(b)
11.15 Preventing Extending and Overriding Key Point
Neither a final class nor a final method can be extended. A final data field is a constant. You may occasionally want to prevent classes from being extended. In such cases, use the final modifier to indicate that a class is final and cannot be a parent class. The Math class is a final class. The String, StringBuilder, and StringBuffer classes are also final classes. For example, the following class A is final and cannot be extended: public final class A { // Data fields, constructors, and methods omitted }
Chapter Summary 443 You also can define a method to be final; a final method cannot be overridden by its subclasses. For example, the following method m is final and cannot be overridden: public class Test { // Data fields, constructors, and methods omitted public final void m() { // Do something } }
Note The modifiers public, protected, private, static, abstract, and final are used on classes and class members (data and methods), except that the final modifier can also be used on local variables in a method. A final local variable is a constant inside a method.
11.41 How do you prevent a class from being extended? How do you prevent a method from being overridden? 11.42 Indicate true or false for the following statements: a. A protected datum or method can be accessed by any class in the same package. b. A protected datum or method can be accessed by any class in different packages. c. A protected datum or method can be accessed by its subclasses in any package. d. A final class can have instances. e. A final class can be extended. f. A final method can be overridden.
KEY TERMS actual type 424 casting objects 427 constructor chaining 417 declared type 424 dynamic binding 424 inheritance 410 instanceof 428 is-a relationship 440 method overriding 419 multiple inheritance 416
override 000 polymorphism
423 440 single inheritance 416 subclass 410 subtype 423 superclass 410 supertype 423 type inference 433
protected
CHAPTER SUMMARY 1. You can define a new class from an existing class. This is known as class inheritance. The new class is called a subclass, child class, or extended class. The existing class is called a superclass, parent class, or base class.
2. A constructor is used to construct an instance of a class. Unlike properties and methods, the constructors of a superclass are not inherited in the subclass. They can be invoked only from the constructors of the subclasses, using the keyword super.
✓
Check Point
444 Chapter 11
Inheritance and Polymorphism 3. A constructor may invoke an overloaded constructor or its superclass’s constructor. The call must be the first statement in the constructor. If none of them is invoked explicitly, the compiler puts super() as the first statement in the constructor, which invokes the superclass’s no-arg constructor.
4. To override a method, the method must be defined in the subclass using the same signature and the same return type as in its superclass.
5. An instance method can be overridden only if it is accessible. Thus, a private method cannot be overridden because it is not accessible outside its own class. If a method defined in a subclass is private in its superclass, the two methods are completely unrelated.
6. Like an instance method, a static method can be inherited. However, a static method cannot be overridden. If a static method defined in the superclass is redefined in a subclass, the method defined in the superclass is hidden.
7. Every class in Java is descended from the java.lang.Object class. If no superclass is specified when a class is defined, its superclass is Object.
8. If a method’s parameter type is a superclass (e.g., Object), you may pass an object to this method of any of the parameter’s subclasses (e.g., Circle or String). This is known as polymorphism.
9. It is always possible to cast an instance of a subclass to a variable of a superclass, because an instance of a subclass is always an instance of its superclass. When casting an instance of a superclass to a variable of its subclass, explicit casting must be used to confirm your intention to the compiler with the (SubclassName) cast notation.
10. A class defines a type. A type defined by a subclass is called a subtype and a type defined by its superclass is called a supertype.
11. When invoking an instance method from a reference variable, the actual type of the variable decides which implementation of the method is used at runtime. This is known as dynamic binding.
12. You can use obj
instanceof AClass to test whether an object is an instance of a
class.
13. You can use the ArrayList class to create an object to store a list of objects. 14. You can use the
protected modifier to prevent the data and methods from being accessed by nonsubclasses from a different package.
15. You can use the final modifier to indicate that a class is final and cannot be extended and to indicate that a method is final and cannot be overridden.
QUIZ Answer the quiz for this chapter online at www.cs.armstrong.edu/liang/intro10e/quiz.html.
Programming Exercises 445
PROGRAMMING EXERCISES Sections 11.2–11.4
11.1
(The Triangle class) Design a class named Triangle that extends GeometricObject. The class contains: ■ ■ ■ ■ ■ ■ ■
Three double data fields named side1, side2, and side3 with default values 1.0 to denote three sides of the triangle. A no-arg constructor that creates a default triangle. A constructor that creates a triangle with the specified side1, side2, and side3. The accessor methods for all three data fields. A method named getArea() that returns the area of this triangle. A method named getPerimeter() that returns the perimeter of this triangle. A method named toString() that returns a string description for the triangle.
For the formula to compute the area of a triangle, see Programming Exercise 2.19. The toString() method is implemented as follows: return "Triangle: side1 = " + side1 + " side2 = " + side2 + " side3 = " + side3;
Draw the UML diagrams for the classes Triangle and GeometricObject and implement the classes. Write a test program that prompts the user to enter three sides of the triangle, a color, and a Boolean value to indicate whether the triangle is filled. The program should create a Triangle object with these sides and set the color and filled properties using the input. The program should display the area, perimeter, color, and true or false to indicate whether it is filled or not.
Sections 11.5–11.14
11.2
11.3
11.4
(The Person, Student, Employee, Faculty, and Staff classes) Design a class named Person and its two subclasses named Student and Employee. Make Faculty and Staff subclasses of Employee. A person has a name, address, phone number, and email address. A student has a class status (freshman, sophomore, junior, or senior). Define the status as a constant. An employee has an office, salary, and date hired. Use the MyDate class defined in Programming Exercise 10.14 to create an object for date hired. A faculty member has office hours and a rank. A staff member has a title. Override the toString method in each class to display the class name and the person’s name. Draw the UML diagram for the classes and implement them. Write a test program that creates a Person, Student, Employee, Faculty, and Staff, and invokes their toString() methods. (Subclasses of Account) In Programming Exercise 9.7, the Account class was defined to model a bank account. An account has the properties account number, balance, annual interest rate, and date created, and methods to deposit and withdraw funds. Create two subclasses for checking and saving accounts. A checking account has an overdraft limit, but a savings account cannot be overdrawn. Draw the UML diagram for the classes and then implement them. Write a test program that creates objects of Account, SavingsAccount, and CheckingAccount and invokes their toString() methods. (Maximum element in ArrayList) Write the following method that returns the maximum value in an ArrayList of integers. The method returns null if the list is null or the list size is 0. public static Integer max(ArrayList list)
446 Chapter 11
Inheritance and Polymorphism Write a test program that prompts the user to enter a sequence of numbers ending with 0, and invokes this method to return the largest number in the input.
11.5
(The Course class) Rewrite the Course class in Listing 10.6. Use an ArrayList to replace an array to store students. Draw the new UML diagram for the class. You should not change the original contract of the Course class (i.e., the definition of the constructors and methods should not be changed, but the private members may be changed.)
11.6
(Use ArrayList) Write a program that creates an ArrayList and adds a Loan object, a Date object, a string, and a Circle object to the list, and use a loop to display all the elements in the list by invoking the object’s toString() method. (Shuffle ArrayList) Write the following method that shuffles the elements in an ArrayList of integers.
11.7
public static void shuffle(ArrayList list)
**11.8
VideoNote
New Account class
(New Account class) An Account class was specified in Programming Exercise 9.7. Design a new Account class as follows: ■ ■ ■
Add a new data field name of the String type to store the name of the customer. Add a new constructor that constructs an account with the specified name, id, and balance. Add a new data field named transactions whose type is ArrayList that stores the transaction for the accounts. Each transaction is an instance of the Transaction class. The Transaction class is defined as shown in Figure 11.6.
The getter and setter methods for these data fields are provided in the class, but omitted in the UML diagram for brevity. Transaction -date: java.util.Date
The date of this transaction.
-type: char
The type of the transaction, such as 'W' for withdrawal, 'D' for deposit.
-amount: double
The amount of the transaction.
-balance: double
The new balance after this transaction.
-description: String
The description of this transaction.
+Transaction(type: char, amount: double, balance: double, description: String)
Construct a Transaction with the specified date, type, balance, and description.
FIGURE 11.6 The Transaction class describes a transaction for a bank account. ■ ■
Modify the withdraw and deposit methods to add a transaction to the transactions array list. All other properties and methods are the same as in Programming Exercise 9.7.
Write a test program that creates an Account with annual interest rate 1.5%, balance 1000, id 1122, and name George. Deposit $30, $40, and $50 to the account and withdraw $5, $4, and $2 from the account. Print an account summary that shows account holder name, interest rate, balance, and all transactions.
Programming Exercises 447 *11.9
(Largest rows and columns) Write a program that randomly fills in 0s and 1s into an n-by-n matrix, prints the matrix, and finds the rows and columns with the most 1s. (Hint: Use two ArrayLists to store the row and column indices with the most 1s.) Here is a sample run of the program:
Enter the array size n: 4 The random array is 0011 0011 1101 1010 The largest row index: 2 The largest column index: 2, 3
11.10
11.11
(Implement MyStack using inheritance) In Listing 11.10, MyStack is implemented using composition. Define a new stack class that extends ArrayList. Draw the UML diagram for the classes and then implement MyStack. Write a test program that prompts the user to enter five strings and displays them in reverse order. (Sort ArrayList) Write the following method that sorts an ArrayList of numbers: public static void sort(ArrayList list)
11.12
Write a test program that prompts the user to enter 5 numbers, stores them in an array list, and displays them in increasing order. (Sum ArrayList) Write the following method that returns the sum of all numbers in an ArrayList: public static double sum(ArrayList list)
*11.13
Write a test program that prompts the user to enter 5 numbers, stores them in an array list, and displays their sum. (Remove duplicates) Write a method that removes the duplicate elements from an array list of integers using the following header: public static void removeDuplicate(ArrayList list)
Write a test program that prompts the user to enter 10 integers to a list and displays the distinct integers separated by exactly one space. Here is a sample run:
Enter ten integers: 34 5 3 5 6 4 33 2 2 4 The distinct integers are 34 5 3 6 4 33 2
11.14
(Combine two lists) Write a method that returns the union of two array lists of integers using the following header: public static ArrayList union( ArrayList list1, ArrayList list2)
448 Chapter 11
Inheritance and Polymorphism For example, the union of two array lists {2, 3, 1, 5} and {3, 4, 6} is {2, 3, 1, 5, 3, 4, 6}. Write a test program that prompts the user to enter two lists, each with five integers, and displays their union. The numbers are separated by exactly one space in the output. Here is a sample run:
Enter five integers for list1: 3 5 45 4 3 Enter five integers for list2: 33 51 5 4 13 The combined list is 3 5 45 4 3 33 51 5 4 13
*11.15
(Area of a convex polygon) A polygon is convex if it contains any line segments that connects two points of the polygon. Write a program that prompts the user to enter the number of points in a convex polygon, then enter the points clockwise, and display the area of the polygon. Here is a sample run of the program:
Enter the number of the points: 7 Enter the coordinates of the points: -12 0 -8.5 10 0 11.4 5.5 7.8 6 -5.5 0 -7 -3.5 -3.5 The total area is 250.075
**11.16
(Addition quiz) Rewrite Listing 5.1 RepeatAdditionQuiz.java to alert the user if an answer is entered again. Hint: use an array list to store answers. Here is a sample run:
What is 5 + 9? 12 Wrong answer. Try again. What is 5 + 9? 34 Wrong answer. Try again. What is 5 + 9? 12 You already entered 12 Wrong answer. Try again. What is 5 + 9? 14 You got it!
**11.17
(Algebra: perfect square) Write a program that prompts the user to enter an integer m and find the smallest integer n such that m * n is a perfect square. (Hint: Store all smallest factors of m into an array list. n is the product of the factors that appear an odd number of times in the array list. For example, consider m = 90, store the factors 2, 3, 3, 5 in an array list. 2 and 5 appear an odd number of times in the array list. So, n is 10.) Here are sample runs:
Enter an integer m: 1500 The smallest number n for m * n to be a perfect square is 15 m * n is 22500
Enter an integer m: 63 The smalle st number n for m * n to be a perfect square is 7 m * n is 441
CHAPTER
EXCEPTION HANDLING AND TEXT I/O Objectives ■
To get an overview of exceptions and exception handling (§12.2).
■
To explore the advantages of using exception handling (§12.2).
■
To distinguish exception types: Error (fatal) vs. Exception (nonfatal) and checked vs. unchecked (§12.3).
■
To declare exceptions in a method header (§12.4.1).
■
To throw exceptions in a method (§12.4.2).
■
To write a try-catch block to handle exceptions (§12.4.3).
■
To explain how an exception is propagated (§12.4.3).
■
To obtain information from an exception object (§12.4.4).
■
To develop applications with exception handling (§12.4.5).
■
To use the finally clause in a try-catch block (§12.5).
■
To use exceptions only for unexpected errors (§12.6).
■
To rethrow exceptions in a catch block (§12.7).
■
To create chained exceptions (§12.8).
■
To define custom exception classes (§12.9).
■
To discover file/directory properties, to delete and rename files/ directories, and to create directories using the File class (§12.10).
■
To write data to a file using the PrintWriter class (§12.11.1).
■
To use try-with-resources to ensure that the resources are closed automatically (§12.11.2).
■
To read data from a file using the Scanner class (§12.11.3).
■
To understand how data is read using a Scanner (§12.11.4).
■
To develop a program that replaces text in a file (§12.11.5).
■
To read data from the Web (§12.12).
■
To develop a Web crawler (§12.13).
12
450 Chapter 12
Exception Handling and Text I/O
12.1 Introduction Key Point
Exception handling enables a program to deal with exceptional situations and continue its normal execution. Runtime errors occur while a program is running if the JVM detects an operation that is impossible to carry out. For example, if you access an array using an index that is out of bounds, you will get a runtime error with an ArrayIndexOutOfBoundsException. If you enter a double value when your program expects an integer, you will get a runtime error with an InputMismatchException. In Java, runtime errors are thrown as exceptions. An exception is an object that represents an error or a condition that prevents execution from proceeding normally. If the exception is not handled, the program will terminate abnormally. How can you handle the exception so that the program can continue to run or else terminate gracefully? This chapter introduces this subject and text input and output.
exception
12.2 Exception-Handling Overview Key Point
VideoNote
Exception-handling advantages
read two integers
integer division
Exceptions are thrown from a method. The caller of the method can catch and handle the exception. To demonstrate exception handling, including how an exception object is created and thrown, let’s begin with the example in Listing 12.1, which reads in two integers and displays their quotient.
LISTING 12.1 Quotient.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
import java.util.Scanner; public class Quotient { public static void main(String[] args) { Scanner input = new Scanner(System.in); // Prompt the user to enter two integers System.out.print("Enter two integers: "); int number1 = input.nextInt(); int number2 = input.nextInt(); System.out.println(number1 + " / " + number2 + " is " + (number1 / number2)); } }
Enter two integers: 5 2 5 / 2 is 2
Enter two integers: 3 0 Exception in thread "main" java.lang.ArithmeticException: / by zero at Quotient.main(Quotient.java:11)
If you entered 0 for the second number, a runtime error would occur, because you cannot divide an integer by 0. (Note that a floating-point number divided by 0 does not raise an exception.) A simple way to fix this error is to add an if statement to test the second number, as shown in Listing 12.2.
12.2 Exception-Handling Overview 451
LISTING 12.2 QuotientWithIf.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
import java.util.Scanner; public class QuotientWithIf { public static void main(String[] args) { Scanner input = new Scanner(System.in); // Prompt the user to enter two integers System.out.print("Enter two integers: "); int number1 = input.nextInt(); int number2 = input.nextInt(); if (number2 != 0) System.out.println(number1 + " / " + number2 + " is " + (number1 / number2)); else System.out.println("Divisor cannot be zero ");
read two integers
test number2
} }
Enter two integers: 5 0 Divisor cannot be zero
Before introducing exception handling, let us rewrite Listing 12.2 to compute a quotient using a method, as shown in Listing 12.3.
LISTING 12.3 QuotientWithMethod.java 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
import java.util.Scanner; public class QuotientWithMethod { public static int quotient(int number1, int number2) { if (number2 == 0) { System.out.println("Divisor cannot be zero"); System.exit(1); }
quotient method
terminate the program
return number1 / number2; } public static void main(String[] args) { Scanner input = new Scanner(System.in); // Prompt the user to enter two integers System.out.print("Enter two integers: "); int number1 = input.nextInt(); int number2 = input.nextInt(); int result = quotient(number1, number2); System.out.println(number1 + " / " + number2 + " is " + result); } }
read two integers
invoke method
452 Chapter 12
Exception Handling and Text I/O Enter two integers: 5 3 5 / 3 is 1
Enter two integers: 5 0 Divisor cannot be zero
The method quotient (lines 4–11) returns the quotient of two integers. If number2 is 0, it cannot return a value, so the program is terminated in line 7. This is clearly a problem. You should not let the method terminate the program—the caller should decide whether to terminate the program. How can a method notify its caller an exception has occurred? Java enables a method to throw an exception that can be caught and handled by the caller. Listing 12.3 can be rewritten, as shown in Listing 12.4.
LISTING 12.4 QuotientWithException.java
quotient method throw exception
read two integers
try block
invoke method
catch block
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
import java.util.Scanner; public class QuotientWithException { public static int quotient(int number1, int number2) { if (number2 == 0) throw new ArithmeticException("Divisor cannot be zero"); return number1 / number2; } public static void main(String[] args) { Scanner input = new Scanner(System.in); // Prompt the user to enter two integers System.out.print("Enter two integers: "); int number1 = input.nextInt(); int number2 = input.nextInt(); try { int result = quotient(number1, number2); If an System.out.println(number1 + " / " + number2 + " is " Arithmetic Exception + result); occurs } catch (ArithmeticException ex) { System.out.println("Exception: an integer " + "cannot be divided by zero "); } System.out.println("Execution continues ..."); } }
Enter two integers: 5 3 5 / 3 is 1 Execution continues ...
12.2 Exception-Handling Overview 453 Enter two integers: 5 0 Exception: an integer cannot be divided by zero Execution continues ...
If number2 is 0, the method throws an exception (line 6) by executing throw new ArithmeticException("Divisor cannot be zero");
The value thrown, in this case new ArithmeticException("Divisor cannot be zero"), is called an exception. The execution of a throw statement is called throwing an exception. The exception is an object created from an exception class. In this case, the exception class is java.lang.ArithmeticException. The constructor ArithmeticException(str) is invoked to construct an exception object, where str is a message that describes the exception. When an exception is thrown, the normal execution flow is interrupted. As the name suggests, to “throw an exception” is to pass the exception from one place to another. The statement for invoking the method is contained in a try block and a catch block. The try block (lines 19–23) contains the code that is executed in normal circumstances. The exception is caught by the catch block. The code in the catch block is executed to handle the exception. Afterward, the statement (line 29) after the catch block is executed. The throw statement is analogous to a method call, but instead of calling a method, it calls a catch block. In this sense, a catch block is like a method definition with a parameter that matches the type of the value being thrown. Unlike a method, however, after the catch block is executed, the program control does not return to the throw statement; instead, it executes the next statement after the catch block. The identifier ex in the catch–block header
throw statement
exception throw exception
handle exception
catch (ArithmeticException ex)
acts very much like a parameter in a method. Thus, this parameter is referred to as a catch–block parameter. The type (e.g., ArithmeticException) preceding ex specifies what kind of exception the catch block can catch. Once the exception is caught, you can access the thrown value from this parameter in the body of a catch block. In summary, a template for a try-throw-catch block may look like this:
catch–block parameter
try { Code to run; A statement or a method that may throw an exception; More code to run; } catch (type ex) { Code to process the exception; }
An exception may be thrown directly by using a throw statement in a try block, or by invoking a method that may throw an exception. The main method invokes quotient (line 20). If the quotient method executes normally, it returns a value to the caller. If the quotient method encounters an exception, it throws the exception back to its caller. The caller’s catch block handles the exception. Now you can see the advantage of using exception handling: It enables a method to throw an exception to its caller, enabling the caller to handle the exception. Without this capability, the called method itself must handle the exception or terminate the program. Often the called method does not know what to do in case of error. This is typically the case for the library methods. The library method can detect the error, but only the caller knows what needs to be
advantage
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Exception Handling and Text I/O done when an error occurs. The key benefit of exception handling is separating the detection of an error (done in a called method) from the handling of an error (done in the calling method). Many library methods throw exceptions. Listing 12.5 gives an example that handles an InputMismatchException when reading an input.
LISTING 12.5 InputMismatchExceptionDemo.java 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
create a Scanner
try block
catch block
import java.util.*; public class InputMismatchExceptionDemo { public static void main(String[] args) { Scanner input = new Scanner(System.in); boolean continueInput = true; do { try { System.out.print("Enter an integer: "); int number = input.nextInt();
If an
InputMismatch Exception
occurs
// Display the result System.out.println( "The number entered is " + number);
continueInput = false; } catch (InputMismatchException ex) { System.out.println("Try again. (" + "Incorrect input: an integer is required)"); input.nextLine(); // Discard input } } while (continueInput); } }
Enter an integer: 3.5 Try again. (Incorrect input: an integer is required) Enter an integer: 4 The number entered is 4
When executing input.nextInt() (line 11), an InputMismatchException occurs if the input entered is not an integer. Suppose 3.5 is entered. An InputMismatchException occurs and the control is transferred to the catch block. The statements in the catch block are now executed. The statement input.nextLine() in line 22 discards the current input line so that the user can enter a new line of input. The variable continueInput controls the loop. Its initial value is true (line 6), and it is changed to false (line 17) when a valid input is received. Once a valid input is received, there is no need to continue the input.
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Check Point
12.1 What is the advantage of using exception handling? 12.2 Which of the following statements will throw an exception? System.out.println(1 / 0); System.out.println(1.0 / 0);
12.3 Exception Types 455 12.3 Point out the problem in the following code. Does the code throw any exceptions? long value = Long.MAX_VALUE + 1; System.out.println(value);
12.4 What does the JVM do when an exception occurs? How do you catch an exception? 12.5 What is the output of the following code? public class Test { public static void main(String[] args) { try { int value = 30; if (value < 40) throw new Exception("value is too small"); } catch (Exception ex) { System.out.println(ex.getMessage()); } System.out.println("Continue after the catch block"); } }
What would be the output if the line int value = 30;
were changed to int value = 50;
12.6 Show the output of the following code.
public class Test { public static void main(String[] args) { for (int i = 0; i < 2; i++) { System.out.print(i + " "); try { System.out.println(1 / 0); } catch (Exception ex) { } } } }
public class Test { public static void main(String[] args) { try { for (int i = 0; i < 2; i++) { System.out.print(i + " "); System.out.println(1 / 0); } } catch (Exception ex) { } } }
(a)
(b)
12.3 Exception Types Exceptions are objects, and objects are defined using classes. The root class for exceptions is java.lang.Throwable. The preceding section used the classes ArithmeticException and InputMismatchException. Are there any other types of exceptions you can use? Can you define your own exception classes? Yes. There are many predefined exception classes in the Java API. Figure 12.1 shows some of them, and in Section 12.9 you will learn how to define your own exception classes.
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Exception Handling and Text I/O ClassNotFoundException ArithmeticException IOException Exception
NullPointerException RuntimeException IndexOutOfBoundsException Many more classes
Object
Throwable
IllegalArgumentException LinkageError Many more classes Error
VirtualMachineError
Many more classes
FIGURE 12.1 Exceptions thrown are instances of the classes shown in this diagram, or of subclasses of one of these classes.
Note The class names Error, Exception, and RuntimeException are somewhat confusing. All three of these classes are exceptions, and all of the errors occur at runtime.
The Throwable class is the root of exception classes. All Java exception classes inherit directly or indirectly from Throwable. You can create your own exception classes by extending Exception or a subclass of Exception. The exception classes can be classified into three major types: system errors, exceptions, and runtime exceptions. ■
system error
System errors are thrown by the JVM and are represented in the Error class. The Error class describes internal system errors, though such errors rarely occur. If one does, there is little you can do beyond notifying the user and trying to terminate the program gracefully. Examples of subclasses of Error are listed in Table 12.1.
TABLE 12.1 Examples of Subclasses of Error
exception
Class
Reasons for Exception
LinkageError
A class has some dependency on another class, but the latter class has changed incompatibly after the compilation of the former class.
VirtualMachineError
The JVM is broken or has run out of the resources it needs in order to continue operating.
■
Exceptions are represented in the Exception class, which describes errors caused by your program and by external circumstances. These errors can be caught and handled by your program. Examples of subclasses of Exception are listed in Table 12.2.
TABLE 12.2 Examples of Subclasses of Exception Class
Reasons for Exception
ClassNotFoundException
Attempt to use a class that does not exist. This exception would occur, for example, if you tried to run a nonexistent class using the java command, or if your program were composed of, say, three class files, only two of which could be found.
IOException
Related to input/output operations, such as invalid input, reading past the end of a file, and opening a nonexistent file. Examples of subclasses of IOException are InterruptedIOException, EOFException (EOF is short for End of File), and FileNotFoundException.
12.3 Exception Types 457 ■
Runtime exceptions are represented in the RuntimeException class, which describes programming errors, such as bad casting, accessing an out-of-bounds array, and numeric errors. Runtime exceptions are generally thrown by the JVM. Examples of subclasses are listed in Table 12.3.
runtime exception
TABLE 12.3 Examples of Subclasses of RuntimeException Class
Reasons for Exception
ArithmeticException
Dividing an integer by zero. Note that floating-point arithmetic does not throw exceptions (see Appendix E, Special FloatingPoint Values).
NullPointerException
Attempt to access an object through a null reference variable.
IndexOutOfBoundsException
Index to an array is out of range.
IllegalArgumentException
A method is passed an argument that is illegal or inappropriate.
RuntimeException, Error, and their subclasses are known as unchecked exceptions. All
other exceptions are known as checked exceptions, meaning that the compiler forces the programmer to check and deal with them in a try-catch block or declare it in the method header. Declaring an exception in the method header will be covered in Section 12.4. In most cases, unchecked exceptions reflect programming logic errors that are unrecoverable. For example, a NullPointerException is thrown if you access an object through a reference variable before an object is assigned to it; an IndexOutOfBoundsException is thrown if you access an element in an array outside the bounds of the array. These are logic errors that should be corrected in the program. Unchecked exceptions can occur anywhere in a program. To avoid cumbersome overuse of try-catch blocks, Java does not mandate that you write code to catch or declare unchecked exceptions.
12.7 Describe the Java Throwable class, its subclasses, and the types of exceptions. 12.8 What RuntimeException will the following programs throw, if any?
unchecked exception checked exception
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Check Point
public class Test { public static void main(String[] args) { System.out.println(1 / 0); } }
public class Test { public static void main(String[] args) { int[] list = new int[5]; System.out.println(list[5]); } }
(a)
(b)
public class Test { public static void main(String[] args) { String s = "abc"; System.out.println(s.charAt(3)); } }
public class Test { public static void main(String[] args) { Object o = new Object(); String d = (String)o; } }
(c)
(d)
public class Test { public static void main(String[] args) { Object o = null; System.out.println(o.toString()); } }
public class Test { public static void main(String[] args) { System.out.println(1.0 / 0); } }
(e)
(f)
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12.4 More on Exception Handling Key Point
A handler for an exception is found by propagating the exception backward through a chain of method calls, starting from the current method. The preceding sections gave you an overview of exception handling and introduced several predefined exception types. This section provides an in-depth discussion of exception handling. Java’s exception-handling model is based on three operations: declaring an exception, throwing an exception, and catching an exception, as shown in Figure 12.2.
method1() {
method2() throws Exception {
try { invoke method2; } catch (Exception ex) { Process exception; }
Catch exception
Declare exception
if (an error occurs) { throw new Exception();
Throw exception
} }
}
FIGURE 12.2 Exception handling in Java consists of declaring exceptions, throwing exceptions, and catching and processing exceptions.
12.4.1 declare exception
Declaring Exceptions
In Java, the statement currently being executed belongs to a method. The Java interpreter invokes the main method to start executing a program. Every method must state the types of checked exceptions it might throw. This is known as declaring exceptions. Because system errors and runtime errors can happen to any code, Java does not require that you declare Error and RuntimeException (unchecked exceptions) explicitly in the method. However, all other exceptions thrown by the method must be explicitly declared in the method header so that the caller of the method is informed of the exception. To declare an exception in a method, use the throws keyword in the method header, as in this example: public void myMethod() throws IOException
The throws keyword indicates that myMethod might throw an IOException. If the method might throw multiple exceptions, add a list of the exceptions, separated by commas, after throws: public void myMethod() throws Exception1, Exception2, ..., ExceptionN
Note If a method does not declare exceptions in the superclass, you cannot override it to declare exceptions in the subclass.
12.4.2 throw exception
Throwing Exceptions
A program that detects an error can create an instance of an appropriate exception type and throw it. This is known as throwing an exception. Here is an example: Suppose the program detects that an argument passed to the method violates the method contract (e.g., the argument
12.4 More on Exception Handling 459 must be nonnegative, but a negative argument is passed); the program can create an instance of IllegalArgumentException and throw it, as follows: IllegalArgumentException ex = new IllegalArgumentException("Wrong Argument"); throw ex;
Or, if you prefer, you can use the following: throw new IllegalArgumentException("Wrong Argument");
Note IllegalArgumentException is an exception class in the Java API. In general,
each exception class in the Java API has at least two constructors: a no-arg constructor, and a constructor with a String argument that describes the exception. This argument is called the exception message, which can be obtained using getMessage().
exception message
Tip The keyword to declare an exception is throws, and the keyword to throw an exception is throw.
12.4.3
throws vs. throw
Catching Exceptions
You now know how to declare an exception and how to throw an exception. When an exception is thrown, it can be caught and handled in a try-catch block, as follows:
catch exception
try { statements; // Statements that may throw exceptions } catch (Exception1 exVar1) { handler for exception1; } catch (Exception2 exVar2) { handler for exception2; } ... catch (ExceptionN exVarN) { handler for exceptionN; }
If no exceptions arise during the execution of the try block, the catch blocks are skipped. If one of the statements inside the try block throws an exception, Java skips the remaining statements in the try block and starts the process of finding the code to handle the exception. The code that handles the exception is called the exception handler; it is found by propagating the exception backward through a chain of method calls, starting from the current method. Each catch block is examined in turn, from first to last, to see whether the type of the exception object is an instance of the exception class in the catch block. If so, the exception object is assigned to the variable declared, and the code in the catch block is executed. If no handler is found, Java exits this method, passes the exception to the method that invoked the method, and continues the same process to find a handler. If no handler is found in the chain of methods being invoked, the program terminates and prints an error message on the console. The process of finding a handler is called catching an exception.
exception handler exception propagation
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Exception Handling and Text I/O Suppose the main method invokes method1, method1 invokes method2, method2 invokes method3, and method3 throws an exception, as shown in Figure 12.3. Consider the following scenario:
main method { ... try { ... invoke method1; statement1; } catch (Exception1 ex1) { Process ex1; } statement2; }
■
If the exception type is Exception3, it is caught by the catch block for handling exception ex3 in method2. statement5 is skipped, and statement6 is executed.
■
If the exception type is Exception2, method2 is aborted, the control is returned to method1, and the exception is caught by the catch block for handling exception ex2 in method1. statement3 is skipped, and statement4 is executed.
■
If the exception type is Exception1, method1 is aborted, the control is returned to the main method, and the exception is caught by the catch block for handling exception ex1 in the main method. statement1 is skipped, and statement2 is executed.
■
If the exception type is not caught in method2, method1, or main, the program terminates, and statement1 and statement2 are not executed.
method1 { ... try { ... invoke method2; statement3; } catch (Exception2 ex2) { Process ex2; } statement4; }
method2 { ... try { ... invoke method3; statement5; } catch (Exception3 ex3) { Process ex3; } statement6; }
An exception is thrown in method3
Call stack method3
main method
method2
method2
method1
method1
method1
main method
main method
main method
FIGURE 12.3 If an exception is not caught in the current method, it is passed to its caller. The process is repeated until the exception is caught or passed to the main method.
Note catch block
Various exception classes can be derived from a common superclass. If a catch block catches exception objects of a superclass, it can catch all the exception objects of the subclasses of that superclass.
Note order of exception handlers
The order in which exceptions are specified in catch blocks is important. A compile error will result if a catch block for a superclass type appears before a catch block for a subclass type. For example, the ordering in (a) on the next page is erroneous, because RuntimeException is a subclass of Exception. The correct ordering should be as shown in (b).
12.4 More on Exception Handling 461 try { ... } catch (Exception ex) { ... } catch (RuntimeException ex) { ... }
try { ... } catch (RuntimeException ex) { ... } catch (Exception ex) { ... }
(a) Wrong order
(b) Correct order
Note Java forces you to deal with checked exceptions. If a method declares a checked exception (i.e., an exception other than Error or RuntimeException), you must invoke it in a try-catch block or declare to throw the exception in the calling method. For example, suppose that method p1 invokes method p2, and p2 may throw a checked exception (e.g., IOException); you have to write the code as shown in (a) or (b) below. void p1() { try { p2(); } catch (IOException ex) { ... } } (a) Catch exception
catch or declare checked exceptions
void p1() throws IOException { p2(); }
(b) Throw exception
Note You can use the new JDK 7 multi-catch feature to simplify coding for the exceptions with the same handling code. The syntax is:
JDK 7 multi-catch
catch (Exception1 | Exception2 | ... | Exceptionk ex) { // Same code for handling these exceptions }
Each exception type is separated from the next with a vertical bar (|). If one of the exceptions is caught, the handling code is executed.
12.4.4
Getting Information from Exceptions
An exception object contains valuable information about the exception. You may use the following instance methods in the java.lang.Throwable class to get information regarding the exception, as shown in Figure 12.4. The printStackTrace() method prints stack trace
methods in Throwable
java.lang.Throwable +getMessage(): String +toString(): String
Returns the message that describes this exception object. Returns the concatenation of three strings: (1) the full name of the exception class; (2) ":" (a colon and a space); (3) the getMessage() method.
+printStackTrace(): void
Prints the Throwable object and its call stack trace information on the console.
+getStackTrace(): StackTraceElement[]
Returns an array of stack trace elements representing the stack trace pertaining to this exception object.
FIGURE 12.4 Throwable is the root class for all exception objects.
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Exception Handling and Text I/O information on the console. The getStackTrace() method provides programmatic access to the stack trace information printed by printStackTrace(). Listing 12.6 gives an example that uses the methods in Throwable to display exception information. Line 4 invokes the sum method to return the sum of all the elements in the array. There is an error in line 23 that causes the ArrayIndexOutOfBoundsException, a subclass of IndexOutOfBoundsException. This exception is caught in the try-catch block. Lines 7, 8, and 9 display the stack trace, exception message, and exception object and message using the printStackTrace(), getMessage(), and toString() methods, as shown in Figure 12.5. Line 12 brings stack trace elements into an array. Each element represents a method call. You can obtain the method (line 14), class name (line 15), and exception line number (line 16) for each element.
printStackTrace()
getMessage() toString() Using getStackTrace()
FIGURE 12.5
You can use the printStackTrace(), getMessage(), toString(), and getStackTrace() methods to obtain information from exception objects.
LISTING 12.6 TestException.java
invoke sum
printStackTrace() getMessage() toString()
getStackTrace()
cause an exception
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
public class TestException { public static void main(String[] args) { try { System.out.println(sum(new int[] {1, 2, 3, 4, 5})); } catch (Exception ex) { ex.printStackTrace(); System.out.println("\n" + ex.getMessage()); System.out.println("\n" + ex.toString()); System.out.println("\nTrace Info Obtained from getStackTrace"); StackTraceElement[] traceElements = ex.getStackTrace(); for (int i = 0; i < traceElements.length; i++) { System.out.print("method " + traceElements[i].getMethodName()); System.out.print("(" + traceElements[i].getClassName() + ":"); System.out.println(traceElements[i].getLineNumber() + ")"); } } } private static int sum(int[] list) { int result = 0; for (int i = 0; i <= list.length; i++)
12.4 More on Exception Handling 463 24 25 26 27
result += list[i]; return result; } }
12.4.5
Example: Declaring, Throwing, and Catching Exceptions
This example demonstrates declaring, throwing, and catching exceptions by modifying the setRadius method in the Circle class in Listing 9.8, CircleWithPrivateDataFields.java. The new setRadius method throws an exception if the radius is negative. Listing 12.7 defines a new circle class named CircleWithException, which is the same as CircleWithPrivateDataFields except that the setRadius(double newRadius) method throws an IllegalArgumentException if the argument newRadius is negative.
LISTING 12.7 CircleWithException.java 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
public class CircleWithException { /** The radius of the circle */ private double radius; /** The number of the objects created */ private static int numberOfObjects = 0; /** Construct a circle with radius 1 */ public CircleWithException() { this(1.0); } /** Construct a circle with a specified radius */ public CircleWithException(double newRadius) { setRadius(newRadius); numberOfObjects++; } /** Return radius */ public double getRadius() { return radius; } /** Set a new radius */ public void setRadius(double newRadius) throws IllegalArgumentException { if (newRadius >= 0) radius = newRadius; else throw new IllegalArgumentException( "Radius cannot be negative"); } /** Return numberOfObjects */ public static int getNumberOfObjects() { return numberOfObjects; } /** Return the area of this circle */ public double findArea() { return radius * radius * 3.14159; } }
declare exception
throw exception
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Exception Handling and Text I/O A test program that uses the new Circle class is given in Listing 12.8.
LISTING 12.8 TestCircleWithException.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
try
catch
public class TestCircleWithException { public static void main(String[] args) { try { CircleWithException c1 = new CircleWithException(5); CircleWithException c2 = new CircleWithException(-5); CircleWithException c3 = new CircleWithException(0); } catch (IllegalArgumentException ex) { System.out.println(ex); } System.out.println("Number of objects created: " + CircleWithException.getNumberOfObjects()); } }
java.lang.IllegalArgumentException: Radius cannot be negative Number of objects created: 1
The original Circle class remains intact except that the class name is changed to CircleWithException, a new constructor CircleWithException(newRadius) is added, and the setRadius method now declares an exception and throws it if the radius is negative. The setRadius method declares to throw IllegalArgumentException in the method header (lines 25–32 in CircleWithException.java). The CircleWithException class would still compile if the throws IllegalArgumentException clause (line 26) were removed from the method declaration, since it is a subclass of RuntimeException and every method can throw RuntimeException (an unchecked exception) regardless of whether it is declared in the method header. The test program creates three CircleWithException objects—c1, c2, and c3—to test how to handle exceptions. Invoking new CircleWithException(-5) (line 5 in Listing 12.8) causes the setRadius method to be invoked, which throws an IllegalArgumentException, because the radius is negative. In the catch block, the type of the object ex is IllegalArgumentException, which matches the exception object thrown by the setRadius method, so this exception is caught by the catch block. The exception handler prints a short message, ex.toString() (line 9 in Listing 12.8), about the exception, using System.out.println(ex). Note that the execution continues in the event of the exception. If the handlers had not caught the exception, the program would have abruptly terminated. The test program would still compile if the try statement were not used, because the method throws an instance of IllegalArgumentException, a subclass of RuntimeException (an unchecked exception). If a method throws an exception other than RuntimeException or Error, the method must be invoked within a try-catch block.
✓
Check Point
12.9 What is the purpose of declaring exceptions? How do you declare an exception, and where? Can you declare multiple exceptions in a method header?
12.10 What is a checked exception, and what is an unchecked exception? 12.11 How do you throw an exception? Can you throw multiple exceptions in one throw statement?
12.12 What is the keyword throw used for? What is the keyword throws used for?
12.4 More on Exception Handling 465 12.13 Suppose that statement2 causes an exception in the following try-catch block: try { statement1; statement2; statement3; } catch (Exception1 ex1) { } catch (Exception2 ex2) { } statement4;
Answer the following questions: ■ ■
Will statement3 be executed? If the exception is not caught, will statement4 be executed?
■
If the exception is caught in the catch block, will statement4 be executed?
12.14 What is displayed when the following program is run? public class Test { public static void main(String[] args) { try { int[] list = new int[10]; System.out.println("list[10] is " + list[10]); } catch (ArithmeticException ex) { System.out.println("ArithmeticException"); } catch (RuntimeException ex) { System.out.println("RuntimeException"); } catch (Exception ex) { System.out.println("Exception"); } } }
12.15 What is displayed when the following program is run? public class Test { public static void main(String[] args) { try { method(); System.out.println("After the method call"); } catch (ArithmeticException ex) { System.out.println("ArithmeticException"); } catch (RuntimeException ex) { System.out.println("RuntimeException"); } catch (Exception e) { System.out.println("Exception"); } } static void method() throws Exception {
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Exception Handling and Text I/O System.out.println(1 / 0); } }
12.16 What is displayed when the following program is run? public class Test { public static void main(String[] args) { try { method(); System.out.println("After the method call"); } catch (RuntimeException ex) { System.out.println("RuntimeException in main"); } catch (Exception ex) { System.out.println("Exception in main"); } } static void method() throws Exception { try { String s = "abc"; System.out.println(s.charAt(3)); } catch (RuntimeException ex) { System.out.println("RuntimeException in method()"); } catch (Exception ex) { System.out.println("Exception in method()"); } } }
12.17 12.18 12.19 12.20
What does the method getMessage() do? What does the method printStackTrace() do? Does the presence of a try-catch block impose overhead when no exception occurs? Correct a compile error in the following code: public void m(int value) { if (value < 40) throw new Exception("value is too small"); }
12.5 The finally Clause Key Point
The finally clause is always executed regardless whether an exception occurred or not. Occasionally, you may want some code to be executed regardless of whether an exception occurs or is caught. Java has a finally clause that can be used to accomplish this objective. The syntax for the finally clause might look like this: try { statements; } catch (TheException ex) { handling ex; }
12.6 When to Use Exceptions 467 finally { finalStatements; }
The code in the finally block is executed under all circumstances, regardless of whether an exception occurs in the try block or is caught. Consider three possible cases: ■
If no exception arises in the try block, finalStatements is executed, and the next statement after the try statement is executed.
■
If a statement causes an exception in the try block that is caught in a catch block, the rest of the statements in the try block are skipped, the catch block is executed, and the finally clause is executed. The next statement after the try statement is executed.
■
If one of the statements causes an exception that is not caught in any catch block, the other statements in the try block are skipped, the finally clause is executed, and the exception is passed to the caller of this method.
The finally block executes even if there is a return statement prior to reaching the finally block.
Note The catch block may be omitted when the finally clause is used.
12.21 Suppose that statement2 causes an exception in the following statement: try { statement1; statement2; statement3; } catch (Exception1 ex1) { } finally { statement4; } statement5;
omit catch block
✓
Check Point
Answer the following questions: ■
If no exception occurs, will statement4 be executed, and will statement5 be executed?
■
If the exception is of type Exception1, will statement4 be executed, and will statement5 be executed?
■
If the exception is not of type Exception1, will statement4 be executed, and will statement5 be executed?
12.6 When to Use Exceptions A method should throw an exception if the error needs to be handled by its caller. The try block contains the code that is executed in normal circumstances. The catch block contains the code that is executed in exceptional circumstances. Exception handling separates error-handling code from normal programming tasks, thus making programs easier to read and to modify. Be aware, however, that exception handling usually requires more time and resources, because it requires instantiating a new exception object, rolling back the call stack, and propagating the exception through the chain of methods invoked to search for the handler.
Key Point
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Exception Handling and Text I/O An exception occurs in a method. If you want the exception to be processed by its caller, you should create an exception object and throw it. If you can handle the exception in the method where it occurs, there is no need to throw or use exceptions. In general, common exceptions that may occur in multiple classes in a project are candidates for exception classes. Simple errors that may occur in individual methods are best handled without throwing exceptions. This can be done by using if statements to check for errors. When should you use a try-catch block in the code? Use it when you have to deal with unexpected error conditions. Do not use a try-catch block to deal with simple, expected situations. For example, the following code try { System.out.println(refVar.toString()); } catch (NullPointerException ex) { System.out.println("refVar is null"); }
is better replaced by if (refVar != null) System.out.println(refVar.toString()); else System.out.println("refVar is null");
Which situations are exceptional and which are expected is sometimes difficult to decide. The point is not to abuse exception handling as a way to deal with a simple logic test.
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12.22 The following method checks whether a string is a numeric string: public static boolean isNumeric(String token) { try { Double.parseDouble(token); return true; } catch (java.lang.NumberFormatException ex) { return false; } }
Is it correct? Rewrite it without using exceptions.
12.7 Rethrowing Exceptions Key Point
Java allows an exception handler to rethrow the exception if the handler cannot process the exception or simply wants to let its caller be notified of the exception. The syntax for rethrowing an exception may look like this: try { statements; } catch (TheException ex) { perform operations before exits; throw ex; }
The statement throw ex rethrows the exception to the caller so that other handlers in the caller get a chance to process the exception ex.
12.8 Chained Exceptions 469 12.23 Suppose that statement2 causes an exception in the following statement: try { statement1; statement2; statement3; } catch (Exception1 ex1) { } catch (Exception2 ex2) { throw ex2; } finally { statement4; } statement5;
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Check Point
Answer the following questions: ■
If no exception occurs, will statement4 be executed, and will statement5 be executed?
■
If the exception is of type Exception1, will statement4 be executed, and will statement5 be executed?
■
If the exception is of type Exception2, will statement4 be executed, and will statement5 be executed?
■
If the exception is not Exception1 nor Exception2, will statement4 be executed, and will statement5 be executed?
12.8 Chained Exceptions Throwing an exception along with another exception forms a chained exception. In the preceding section, the catch block rethrows the original exception. Sometimes, you may need to throw a new exception (with additional information) along with the original exception. This is called chained exceptions. Listing 12.9 illustrates how to create and throw chained exceptions.
Key Point chained exception
LISTING 12.9 ChainedExceptionDemo.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
public class ChainedExceptionDemo { public static void main(String[] args) { try { method1(); } catch (Exception ex) { ex.printStackTrace(); } } public static void method1() throws Exception { try { method2(); } catch (Exception ex) { throw new Exception("New info from method1", ex); } }
stack trace
chained exception
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throw exception
public static void method2() throws Exception { throw new Exception("New info from method2"); } }
java.lang.Exception: New info from method1 at ChainedExceptionDemo.method1(ChainedExceptionDemo.java:16) at ChainedExceptionDemo.main(ChainedExceptionDemo.java:4) Caused by: java.lang.Exception: New info from method2 at ChainedExceptionDemo.method2(ChainedExceptionDemo.java:21) at ChainedExceptionDemo.method1(ChainedExceptionDemo.java:13) ... 1 more
The main method invokes method1 (line 4), method1 invokes method2 (line 13), and method2 throws an exception (line 21). This exception is caught in the catch block in method1 and is wrapped in a new exception in line 16. The new exception is thrown and caught in the catch block in the main method in line 6. The sample output shows the output from the printStackTrace() method in line 7. The new exception thrown from method1 is displayed first, followed by the original exception thrown from method2.
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12.24 What would be the output if line 16 is replaced by the following line? throw new Exception(“New info from method1”);
12.9 Defining Custom Exception Classes Key Point
VideoNote
Create custom exception classes
You can define a custom exception class by extending the java.lang.Exception class. Java provides quite a few exception classes. Use them whenever possible instead of defining your own exception classes. However, if you run into a problem that cannot be adequately described by the predefined exception classes, you can create your own exception class, derived from Exception or from a subclass of Exception, such as IOException. In Listing 12.7, CircleWithException.java, the setRadius method throws an exception if the radius is negative. Suppose you wish to pass the radius to the handler. In that case, you can define a custom exception class, as shown in Listing 12.10.
LISTING 12.10 InvalidRadiusException.java extends Exception
1 2 3 4 5 6 7 8 9 10 11 12 13 14
public class InvalidRadiusException extends Exception { private double radius; /** Construct an exception */ public InvalidRadiusException(double radius) { super("Invalid radius " + radius); this.radius = radius; } /** Return the radius */ public double getRadius() { return radius; } }
This custom exception class extends java.lang.Exception (line 1). The Exception class extends java.lang.Throwable. All the methods (e.g., getMessage(), toString(), and
12.9 Defining Custom Exception Classes 471 printStackTrace()) in Exception are inherited from Throwable. The Exception
class contains four constructors. Among them, the following two constructors are often used: java.lang.Exception +Exception()
Constructs an exception with no message.
+Exception(message: String)
Constructs an exception with the specified message.
Line 6 invokes the superclass’s constructor with a message. This message will be set in the exception object and can be obtained by invoking getMessage() on the object.
Tip Most exception classes in the Java API contain two constructors: a no-arg constructor and a constructor with a message parameter. To create an InvalidRadiusException, you have to pass a radius. Therefore, the setRadius method in Listing 12.7 can be modified as shown in Listing 12.11.
LISTING 12.11 TestCircleWithCustomException.java 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
public class TestCircleWithCustomException { public static void main(String[] args) { try { new CircleWithCustomException(5); new CircleWithCustomException(-5); new CircleWithCustomException(0); } catch (InvalidRadiusException ex) { System.out.println(ex); } System.out.println("Number of objects created: " + CircleWithCustomException.getNumberOfObjects()); } } class CircleWithCustomException { /** The radius of the circle */ private double radius; /** The number of objects created */ private static int numberOfObjects = 0; /** Construct a circle with radius 1 */ public CircleWithCustomException() throws InvalidRadiusException { this(1.0); } /** Construct a circle with a specified radius */ public CircleWithCustomException(double newRadius) throws InvalidRadiusException { setRadius(newRadius); numberOfObjects++; } /** Return radius */ public double getRadius() {
declare exception
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throw exception
return radius; } /** Set a new radius */ public void setRadius(double newRadius) throws InvalidRadiusException { if (newRadius >= 0) radius = newRadius; else throw new InvalidRadiusException(newRadius); } /** Return numberOfObjects */ public static int getNumberOfObjects() { return numberOfObjects; } /** Return the area of this circle */ public double findArea() { return radius * radius * 3.14159; } }
InvalidRadiusException: Invalid radius -5.0 Number of objects created: 1
The setRadius method in CircleWithCustomException throws an InvalidRadiusException when radius is negative (line 47). Since InvalidRadiusException is a checked exception, the setRadius method must declare it in the method header (line 43). Since the constructors for CircleWithCustomException invoke the setRadius method to a set a new radius and it may throw an InvalidRadiusException, the constructors are declared to throw InvalidRadiusException (lines 25, 31). Invoking new CircleWithCustomException(-5) (line 5) throws an InvalidRadiusException, which is caught by the handler. The handler displays the radius in the exception object ex.
Tip checked custom exception
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Check Point
Can you define a custom exception class by extending RuntimeException? Yes, but it is not a good way to go, because it makes your custom exception unchecked. It is better to make a custom exception checked, so that the compiler can force these exceptions to be caught in your program.
12.25 How do you define a custom exception class? 12.26 Suppose the setRadius method throws the InValidRadiusException defined in Listing 12.10. What is displayed when the following program is run? public class Test { public static void main(String[] args) { try { method(); System.out.println("After the method call"); } catch (RuntimeException ex) { System.out.println("RuntimeException in main"); }
12.10 The File Class 473 catch (Exception ex) { System.out.println("Exception in main"); } } static void method() throws Exception { try { Circle c1 = new Circle(1); c1.setRadius(-1); System.out.println(c1.getRadius()); } catch (RuntimeException ex) { System.out.println("RuntimeException in method()"); } catch (Exception ex) { System.out.println("Exception in method()"); throw ex; } } }
12.10 The File Class The File class contains the methods for obtaining the properties of a file/directory and for renaming and deleting a file/directory. Having learned exception handling, you are ready to step into file processing. Data stored in the program are temporary; they are lost when the program terminates. To permanently store the data created in a program, you need to save them in a file on a disk or other permanent storage device. The file can then be transported and read later by other programs. Since data are stored in files, this section introduces how to use the File class to obtain file/directory properties, to delete and rename files/directories, and to create directories. The next section introduces how to read/write data from/to text files. Every file is placed in a directory in the file system. An absolute file name (or full name) contains a file name with its complete path and drive letter. For example, c:\book\ Welcome.java is the absolute file name for the file Welcome.java on the Windows operating system. Here c:\book is referred to as the directory path for the file. Absolute file names are machine dependent. On the UNIX platform, the absolute file name may be /home/liang/book/Welcome.java, where /home/liang/book is the directory path for the file Welcome.java. A relative file name is in relation to the current working directory. The complete directory path for a relative file name is omitted. For example, Welcome.java is a relative file name. If the current working directory is c:\book, the absolute file name would be c:\book\Welcome.java. The File class is intended to provide an abstraction that deals with most of the machinedependent complexities of files and path names in a machine-independent fashion. The File class contains the methods for obtaining file and directory properties and for renaming and deleting files and directories, as shown in Figure 12.6. However, the File class does not contain the methods for reading and writing file contents. The file name is a string. The File class is a wrapper class for the file name and its directory path. For example, new File("c:\\book") creates a File object for the directory c:\book, and new File("c:\\book\\test.dat") creates a File object for the file c:\book\test.dat, both on Windows. You can use the File class’s isDirectory() method to check whether the object represents a directory, and the isFile() method to check whether the object represents a file.
Key Point
why file?
absolute file name
directory path
relative file name
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java.io.File +File(pathname: String) +File(parent: String, child: String)
Creates a File object for the specified path name. The path name may be a directory or a file. Creates a File object for the child under the directory parent. The child may be a file name or a subdirectory.
+File(parent: File, child: String)
Creates a File object for the child under the directory parent. The parent is a File object. In the preceding constructor, the parent is a string.
+exists(): boolean
Returns true if the file or the directory represented by the File object exists.
+canRead(): boolean
Returns true if the file represented by the File object exists and can be read.
+canWrite(): boolean
Returns true if the file represented by the File object exists and can be written.
+isDirectory(): boolean
Returns true if the File object represents a directory.
+isFile(): boolean
Returns true if the File object represents a file.
+isAbsolute(): boolean
Returns true if the File object is created using an absolute path name.
+isHidden(): boolean
Returns true if the file represented in the File object is hidden. The exact definition of hidden is system-dependent. On Windows, you can mark a file hidden in the File Properties dialog box. On Unix systems, a file is hidden if its name begins with a period(.) character.
+getAbsolutePath(): String
Returns the complete absolute file or directory name represented by the File object.
+getCanonicalPath(): String
Returns the same as getAbsolutePath() except that it removes redundant names, such as "." and "..", from the path name, resolves symbolic links (on Unix), and converts drive letters to standard uppercase (on Windows).
+getName(): String
Returns the last name of the complete directory and file name represented by the File object. For example, new File("c:\\book\\test.dat").getName() returns test.dat. Returns the complete directory and file name represented by the File object. For example, new File("c:\\book\\test.dat").getPath() returns c:\book\test.dat.
+getPath(): String +getParent(): String
Returns the complete parent directory of the current directory or the file represented by the File object. For example, new File("c:\\book\\test.dat").getParent() returns c:\book.
+lastModified(): long
Returns the time that the file was last modified. Returns the size of the file, or 0 if it does not exist or if it is a directory. Returns the files under the directory for a directory File object.
+length(): long +listFile(): File[] +delete(): boolean
Deletes the file or directory represented by this File object.The method returns true if the deletion succeeds.
+renameTo(dest: File): boolean
Renames the file or directory represented by this File object to the specified name represented in dest. The method returns true if the operation succeeds.
+mkdir(): boolean
Creates a directory represented in this File object. Returns true if the the directory is created successfully.
+mkdirs(): boolean
Same as mkdir() except that it creates directory along with its parent directories if the parent directories do not exist.
FIGURE 12.6 The File class can be used to obtain file and directory properties, to delete and rename files and directories, and to create directories.
Caution \ in file names
The directory separator for Windows is a backslash (\). The backslash is a special character in Java and should be written as \\ in a string literal (see Table 4.5).
Note Constructing a File instance does not create a file on the machine. You can create a File instance for any file name regardless whether it exists or not. You can invoke the exists() method on a File instance to check whether the file exists.
relative file name Java directory separator (/)
Do not use absolute file names in your program. If you use a file name such as c:\\book\\ Welcome.java, it will work on Windows but not on other platforms. You should use a file name relative to the current directory. For example, you may create a File object using new File("Welcome.java") for the file Welcome.java in the current directory. You may create a File object using new File("image/us.gif") for the file us.gif under the image directory in the current directory. The forward slash (/) is the Java directory separator, which
12.10 The File Class 475 is the same as on UNIX. The statement new File("image/us.gif") works on Windows, UNIX, and any other platform. Listing 12.12 demonstrates how to create a File object and use the methods in the File class to obtain its properties. The program creates a File object for the file us.gif. This file is stored under the image directory in the current directory.
LISTING 12.12 TestFileClass.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
public class TestFileClass { public static void main(String[] args) { java.io.File file = new java.io.File("image/us.gif"); System.out.println("Does it exist? " + file.exists()); System.out.println("The file has " + file.length() + " bytes"); System.out.println("Can it be read? " + file.canRead()); System.out.println("Can it be written? " + file.canWrite()); System.out.println("Is it a directory? " + file.isDirectory()); System.out.println("Is it a file? " + file.isFile()); System.out.println("Is it absolute? " + file.isAbsolute()); System.out.println("Is it hidden? " + file.isHidden()); System.out.println("Absolute path is " + file.getAbsolutePath()); System.out.println("Last modified on " + new java.util.Date(file.lastModified())); } }
create a File exists() length() canRead() canWrite() isDirectory() isFile() isAbsolute() isHidden() getAbsolutePath() lastModified()
The lastModified() method returns the date and time when the file was last modified, measured in milliseconds since the beginning of UNIX time (00:00:00 GMT, January 1, 1970). The Date class is used to display it in a readable format in lines 14–15. Figure 12.7a shows a sample run of the program on Windows, and Figure 12.7b, a sample run on UNIX. As shown in the figures, the path-naming conventions on Windows are different from those on UNIX.
(a) On Windows
FIGURE 12.7
(b) On UNIX
The program creates a File object and displays file properties.
12.27 What is wrong about creating a File object using the following statement? new File("c:\book\test.dat");
12.28 How do you check whether a file already exists? How do you delete a file? How do you rename a file? Can you find the file size (the number of bytes) using the File class? How do you create a directory? 12.29 Can you use the File class for I/O? Does creating a File object create a file on the disk?
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12.11 File Input and Output Key Point
VideoNote
Write and read data
Use the Scanner class for reading text data from a file and the PrintWriter class for writing text data to a file. A File object encapsulates the properties of a file or a path, but it does not contain the methods for creating a file or for writing/reading data to/from a file (referred to as data input and output, or I/O for short). In order to perform I/O, you need to create objects using appropriate Java I/O classes. The objects contain the methods for reading/writing data from/to a file. There are two types of files: text and binary. Text files are essentially characters on disk. This section introduces how to read/write strings and numeric values from/to a text file using the Scanner and PrintWriter classes. Binary files will be introduced in Chapter 17.
12.11.1
Writing Data Using PrintWriter
The java.io.PrintWriter class can be used to create a file and write data to a text file. First, you have to create a PrintWriter object for a text file as follows: PrintWriter output = new PrintWriter(filename);
Then, you can invoke the print, println, and printf methods on the PrintWriter object to write data to a file. Figure 12.8 summarizes frequently used methods in PrintWriter.
java.io.PrintWriter +PrintWriter(file: File) +PrintWriter(filename: String) +print(s: String): void +print(c: char): void +print(cArray: char[]): void +print(i: int): void +print(l: long): void +print(f: float): void +print(d: double): void +print(b: boolean): void Also contains the overloaded println methods.
Creates a PrintWriter object for the specified file object. Creates a PrintWriter object for the specified file-name string. Writes a string to the file. Writes a character to the file. Writes an array of characters to the file. Writes an int value to the file. Writes a long value to the file. Writes a float value to the file. Writes a double value to the file. Writes a boolean value to the file. A println method acts like a print method; additionally, it prints a line separator. The line-separator string is defined by the system. It is \r\n on Windows and \n on Unix.
Also contains the overloaded printf methods.
The printf method was introduced in §4.6, “Formatting Console Output.”
FIGURE 12.8 The PrintWriter class contains the methods for writing data to a text file. Listing 12.13 gives an example that creates an instance of PrintWriter and writes two lines to the file scores.txt. Each line consists of a first name (a string), a middle-name initial (a character), a last name (a string), and a score (an integer).
LISTING 12.13 WriteData.java throws an exception create File object file exist?
1 public class WriteData { 2 public static void main(String[] args) throws IOException { 3 java.io.File file = new java.io.File("scores.txt"); 4 if (file.exists()) { 5 System.out.println("File already exists"); 6 System.exit(1); 7 } 8
12.11 File Input and Output 477 9 10 11 12 13 14 15 16 17 18 19 20 } 21 }
// Create a file java.io.PrintWriter output = new java.io.PrintWriter(file); // Write formatted output to the file output.print("John T Smith "); output.println(90); output.print("Eric K Jones "); output.println(85);
print data John T Smith 90 scores.txt Eric K Jones 85
// Close the file output.close();
Lines 4–7 check whether the file scores.txt exists. If so, exit the program (line 6). Invoking the constructor of PrintWriter will create a new file if the file does not exist. If the file already exists, the current content in the file will be discarded without verifying with the user. Invoking the constructor of PrintWriter may throw an I/O exception. Java forces you to write the code to deal with this type of exception. For simplicity, we declare throws IOException in the main method header (line 2). You have used the System.out.print, System.out.println, and System.out. printf methods to write text to the console. System.out is a standard Java object for the console output. You can create PrintWriter objects for writing text to any file using print, println, and printf (lines 13–16). The close() method must be used to close the file (line 19). If this method is not invoked, the data may not be saved properly in the file.
12.11.2
create PrintWriter
close file
create a file
throws IOException print method
close file
Closing Resources Automatically Using try-with-resources
Programmers often forget to close the file. JDK 7 provides the followings new try-withresources syntax that automatically closes the files. try (declare and create resources) { Use the resource to process the file; }
Using the try-with-resources syntax, we rewrite the code in Listing 12.13 in Listing 12.14.
LISTING 12.14 WriteDataWithAutoClose.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
public class WriteDataWithAutoClose { public static void main(String[] args) throws Exception { java.io.File file = new java.io.File("scores.txt"); if (file.exists()) { System.out.println("File already exists"); System.exit(0); } try ( // Create a file java.io.PrintWriter output = new java.io.PrintWriter(file); ) { // Write formatted output to the file output.print("John T Smith "); output.println(90); output.print("Eric K Jones "); output.println(85); } } }
declare/create resource
use the resouce
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Exception Handling and Text I/O A resource is declared and created followed by the keyword try. Note that the resources are enclosed in the parentheses (lines 9–12). The resources must be a subtype of AutoCloseable such as a PrinterWriter that has the close() method. A resource must be declared and created in the same statement and multiple resources can be declared and created inside the parentheses. The statements in the block (lines 12–18) immediately following the resource declaration use the resource. After the block is finished, the resource’s close() method is automatically invoked to close the resource. Using try-with-resources can not only avoid errors but also make the code simpler.
12.11.3
Reading Data Using Scanner
The java.util.Scanner class was used to read strings and primitive values from the console in Section 2.3, Reading Input from the Console. A Scanner breaks its input into tokens delimited by whitespace characters. To read from the keyboard, you create a Scanner for System.in, as follows: Scanner input = new Scanner(System.in);
To read from a file, create a Scanner for a file, as follows: Scanner input = new Scanner(new File(filename));
Figure 12.9 summarizes frequently used methods in Scanner. java.util.Scanner +Scanner(source: File)
Creates a Scanner that scans tokens from the specified file.
+Scanner(source: String)
Creates a Scanner that scans tokens from the specified string.
+close()
Closes this scanner.
+hasNext(): boolean
Returns true if this scanner has more data to be read.
+next(): String
Returns next token as a string from this scanner.
+nextLine(): String
Returns a line ending with the line separator from this scanner.
+nextByte(): byte +nextShort(): short
Returns next token as a byte from this scanner. Returns next token as a short from this scanner.
+nextInt(): int
Returns next token as an int from this scanner.
+nextLong(): long
Returns next token as a long from this scanner.
+nextFloat(): float
Returns next token as a float from this scanner.
+nextDouble(): double
Returns next token as a double from this scanner.
+useDelimiter(pattern: String): Scanner
Sets this scanner’s delimiting pattern and returns this scanner.
FIGURE 12.9 The Scanner class contains the methods for scanning data. Listing 12.15 gives an example that creates an instance of Scanner and reads data from the file scores.txt.
LISTING 12.15 ReadData.java
create a File
create a Scanner
1 import java.util.Scanner; 2 3 public class ReadData { 4 public static void main(String[] args) throws Exception { 5 // Create a File instance 6 java.io.File file = new java.io.File("scores.txt"); 7 8 // Create a Scanner for the file 9 Scanner input = new Scanner(file);
12.11 File Input and Output 479 10 11 12 13 14 15 16 17 18 19 20 21 22 23 } 24 }
// Read data from a file scores.txt while (input.hasNext()) { John T Smith 90 String firstName = input.next(); Eric K Jones 85 String mi = input.next(); String lastName = input.next(); int score = input.nextInt(); System.out.println( firstName + " " + mi + " " + lastName + " " + score); } // Close the file input.close();
Note that new Scanner(String) creates a Scanner for a given string. To create a Scanner to read data from a file, you have to use the java.io.File class to create an instance of the File using the constructor new File(filename) (line 6), and use new Scanner(File) to create a Scanner for the file (line 9). Invoking the constructor new Scanner(File) may throw an I/O exception, so the main method declares throws Exception in line 4. Each iteration in the while loop reads the first name, middle initial, last name, and score from the text file (lines 12–19). The file is closed in line 22. It is not necessary to close the input file (line 22), but it is a good practice to do so to release the resources occupied by the file. You can rewrite this program using the try-with-resources syntax. See www.cs.armstrong.edu/liang/intro10e/html/ReadDataWithAutoClose.html.
12.11.4
has next? read items
close file
File class
throws Exception
close file
How Does Scanner Work?
The nextByte(), nextShort(), nextInt(), nextLong(), nextFloat(), nextDouble(), and next() methods are known as token-reading methods, because they read tokens separated by delimiters. By default, the delimiters are whitespace characters. You can use the useDelimiter(String regex) method to set a new pattern for delimiters. How does an input method work? A token-reading method first skips any delimiters (whitespace characters by default), then reads a token ending at a delimiter. The token is then automatically converted into a value of the byte, short, int, long, float, or double type for nextByte(), nextShort(), nextInt(), nextLong(), nextFloat(), and nextDouble(), respectively. For the next() method, no conversion is performed. If the token does not match the expected type, a runtime exception java.util.InputMismatchException will be thrown. Both methods next() and nextLine() read a string. The next() method reads a string delimited by delimiters, and nextLine() reads a line ending with a line separator.
token-reading method change delimiter
InputMismatchException next() vs. nextLine()
Note The line-separator string is defined by the system. It is \r\n on Windows and \n on UNIX. To get the line separator on a particular platform, use
line separator
String lineSeparator = System.getProperty("line.separator");
If you enter input from a keyboard, a line ends with the Enter key, which corresponds to the \n character.
The token-reading method does not read the delimiter after the token. If the nextLine() method is invoked after a token-reading method, this method reads characters that start from this delimiter and end with the line separator. The line separator is read, but it is not part of the string returned by nextLine().
behavior of nextLine()
480 Chapter 12
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input from file
34 567
After the following code is executed, Scanner input = new Scanner(new File("test.txt")); int intValue = input.nextInt(); String line = input.nextLine();
intValue contains 34 and line contains the characters ' ', 5, 6, and 7. input from keyboard
What happens if the input is entered from the keyboard? Suppose you enter 34, press the Enter key, then enter 567 and press the Enter key for the following code: Scanner input = new Scanner(System.in); int intValue = input.nextInt(); String line = input.nextLine();
scan a string
You will get 34 in intValue and an empty string in line. Why? Here is the reason. The token-reading method nextInt() reads in 34 and stops at the delimiter, which in this case is a line separator (the Enter key). The nextLine() method ends after reading the line separator and returns the string read before the line separator. Since there are no characters before the line separator, line is empty. You can read data from a file or from the keyboard using the Scanner class. You can also scan data from a string using the Scanner class. For example, the following code Scanner input = new Scanner("13 14"); int sum = input.nextInt() + input.nextInt(); System.out.println("Sum is " + sum);
displays The sum is 27
12.11.5
Case Study: Replacing Text
Suppose you are to write a program named ReplaceText that replaces all occurrences of a string in a text file with a new string. The file name and strings are passed as command-line arguments as follows: java ReplaceText sourceFile targetFile oldString newString
For example, invoking java ReplaceText FormatString.java t.txt StringBuilder StringBuffer
replaces all the occurrences of StringBuilder by StringBuffer in the file FormatString .java and saves the new file in t.txt. Listing 12.16 gives the program. The program checks the number of arguments passed to the main method (lines 7–11), checks whether the source and target files exist (lines 14–25), creates a Scanner for the source file (line 29), creates a PrintWriter for the target file (line 30), and repeatedly reads a line from the source file (line 33), replaces the text (line 34), and writes a new line to the target file (line 35).
LISTING 12.16 ReplaceText.java 1 2 3
import java.io.*; import java.util.*;
12.11 File Input and Output 481 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
public class ReplaceText { public static void main(String[] args) throws Exception { // Check command line parameter usage if (args.length != 4) { System.out.println( "Usage: java ReplaceText sourceFile targetFile oldStr newStr"); System.exit(1); } // Check if source file exists File sourceFile = new File(args[0]); if (!sourceFile.exists()) { System.out.println("Source file " + args[0] + " does not exist"); System.exit(2); } // Check if target file exists File targetFile = new File(args[1]); if (targetFile.exists()) { System.out.println("Target file " + args[1] + " already exists"); System.exit(3); } try ( // Create input and output files Scanner input = new Scanner(sourceFile); PrintWriter output = new PrintWriter(targetFile); ) { while (input.hasNext()) { String s1 = input.nextLine(); String s2 = s1.replaceAll(args[2], args[3]); output.println(s2); } }
check command usage
source file exists?
target file exists?
try-with-resources create a Scanner create a PrintWriter has next? read a line
} }
In a normal situation, the program is terminated after a file is copied. The program is terminated abnormally if the command-line arguments are not used properly (lines 7–11), if the source file does not exist (lines 14–18), or if the target file already exists (lines 22–25). The exit status code 1, 2, and 3 are used to indicate these abnormal terminations (lines 10, 17, 24).
12.30 How do you create a PrintWriter to write data to a file? What is the reason to declare throws Exception in the main method in Listing 12.13, WriteData.java? What would happen if the close() method were not invoked in Listing 12.13?
12.31 Show the contents of the file temp.txt after the following program is executed. public class Test { public static void main(String[] args) throws Exception { java.io.PrintWriter output = new java.io.PrintWriter("temp.txt"); output.printf("amount is %f %e\r\n", 32.32, 32.32); output.printf("amount is %5.4f %5.4e\r\n", 32.32, 32.32); output.printf("%6b\r\n", (1 > 2)); output.printf("%6s\r\n", "Java"); output.close(); } }
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482 Chapter 12
Exception Handling and Text I/O 12.32 Rewrite the code in the preceding question using a try-with-resources syntax. 12.33 How do you create a Scanner to read data from a file? What is the reason to define throws Exception in the main method in Listing 12.15, ReadData.java? What would happen if the close() method were not invoked in Listing 12.15?
12.34 What will happen if you attempt to create a Scanner for a nonexistent file? What will happen if you attempt to create a PrintWriter for an existing file?
12.35 Is the line separator the same on all platforms? What is the line separator on Windows? 12.36 Suppose you enter 45 57.8 789, then press the Enter key. Show the contents of the variables after the following code is executed. Scanner input = new Scanner(System.in); int intValue = input.nextInt(); double doubleValue = input.nextDouble(); String line = input.nextLine();
12.37 Suppose you enter 45, press the Enter key, 57.8, press the Enter key, 789, and press the Enter key. Show the contents of the variables after the following code is executed. Scanner input = new Scanner(System.in); int intValue = input.nextInt(); double doubleValue = input.nextDouble(); String line = input.nextLine();
12.12 Reading Data from the Web Key Point
Just like you can read data from a file on your computer, you can read data from a file on the Web. In addition to reading data from a local file on a computer or file server, you can also access data from a file that is on the Web if you know the file’s URL (Uniform Resource Locator— the unique address for a file on the Web). For example, www.google.com/index.html is the URL for the file index.html located on the Google Web server. When you enter the URL in a Web browser, the Web server sends the data to your browser, which renders the data graphically. Figure 12.10 illustrates how this process works. Client Web Browser
Server
Internet
Web Server
Application Program
Local files
FIGURE 12.10 The client retrieves files from a Web server. For an application program to read data from a URL, you first need to create a URL object using the java.net.URL class with this constructor: public URL(String spec) throws MalformedURLException
For example, the following statement creates a URL object for http://www.google.com/index.html. 1 2 3
try { URL url = new URL("http://www.google.com/index.html"); }
12.12 Reading Data from the Web 483 4 5 6
catch (MalformedURLException ex) { ex.printStackTrace(); }
A MalformedURLException is thrown if the URL string has a syntax error. For example, the URL string “http:www.google.com/index.html” would cause a MalformedURLException runtime error because two slashes (//) are required after the colon (:). Note that the http:// prefix is required for the URL class to recognize a valid URL. It would be wrong if you replace line 2 with the following code: URL url = new URL("www.google.com/index.html");
After a URL object is created, you can use the openStream() method defined in the URL class to open an input stream and use this stream to create a Scanner object as follows: Scanner input = new Scanner(url.openStream());
Now you can read the data from the input stream just like from a local file. The example in Listing 12.17 prompts the user to enter a URL and displays the size of the file.
LISTING 12.17 ReadFileFromURL.java 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
import java.util.Scanner; public class ReadFileFromURL { public static void main(String[] args) { System.out.print("Enter a URL: "); String URLString = new Scanner(System.in).next(); try { java.net.URL url = new java.net.URL(URLString); int count = 0; Scanner input = new Scanner(url.openStream()); while (input.hasNext()) { String line = input.nextLine(); count += line.length(); } System.out.println("The file size is " + count + " characters"); } catch (java.net.MalformedURLException ex) { System.out.println("Invalid URL"); } catch (java.io.IOException ex) { System.out.println("I/O Errors: no such file"); } } }
Enter a URL: http://cs.armstrong.edu/liang/data/Lincoln.txt The file size is 1469 characters
Enter a URL: http://www.yahoo.com The file size is 190006 characters
enter a URL
create a URL object create a Scanner object more to read? read a line
MalformedURLException
IOException
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MalformedURLException
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Check Point
The program prompts the user to enter a URL string (line 6) and creates a URL object (line 9). The constructor will throw a java.net.MalformedURLException (line 19) if the URL isn’t formed correctly. The program creates a Scanner object from the input stream for the URL (line 11). If the URL is formed correctly but does not exist, an IOException will be thrown (line 22). For example, http://google.com/index1.html uses the appropriate form, but the URL itself does not exist. An IOException would be thrown if this URL was used for this program.
12.38 How do you create a Scanner object for reading text from a URL?
12.13 Case Study: Web Crawler Key Point
Web crawler
This case study develops a program that travels the Web by following hyperlinks. The World Wide Web, abbreviated as WWW, W3, or Web, is a system of interlinked hypertext documents on the Internet. With a Web browser, you can view a document and follow the hyperlinks to view other documents. In this case study, we will develop a program that automatically traverses the documents on the Web by following the hyperlinks. This type of program is commonly known as a Web crawler. For simplicity, our program follows for the hyperlink that starts with http://. Figure 12.11 shows an example of traversing the Web. We start from a Web page that contains three URLs named URL1, URL2, and URL3. Following URL1 leads to the page that contains three URLs named URL11, URL12, and URL13. Following URL2 leads to the page that contains two URLs named URL21 and URL22. Following URL3 leads to the page that contains four URLs named URL31, URL32, and URL33, and URL34. Continue to traverse the Web following the new hyperlinks. As you see, this process may continue forever, but we will exit the program once we have traversed 100 pages. Starting URL URL1 URL2 URL3
URL1
URL2 URL11
URL3 URL31
URL21
URL12
URL22
URL32
URL13 …
…
URL33 …
…
…
…
…
URL4 …
…
FIGURE 12.11 The client retrieves files from a Web server. The program follows the URLs to traverse the Web. To ensure that each URL is traversed only once, the program maintains two lists of URLs. One list stores the URLs pending for traversing and the other stores the URLs that have already been traversed. The algorithm for this program can be described as follows: Add the starting URL to a list named listOfPendingURLs; while listOfPendingURLs is not empty and size of listOfTraversedURLs <= 100 {
12.13 Case Study: Web Crawler 485 Remove a URL from listOfPendingURLs; if this URL is not in listOfTraversedURLs { Add it to listOfTraversedURLs; Display this URL; Read the page from this URL and for each URL contained in the page { Add it to listOfPendingURLs if it is not in listOfTraversedURLs; } } }
Listing 12.18 gives the program that implements this algorithm.
LISTING 12.18 WebCrawler.java 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
import java.util.Scanner; import java.util.ArrayList; public class WebCrawler { public static void main(String[] args) { java.util.Scanner input = new java.util.Scanner(System.in); System.out.print("Enter a URL: "); String url = input.nextLine(); crawler(url); // Traverse the Web from the a starting url } public static void crawler(String startingURL) { ArrayList listOfPendingURLs = new ArrayList<>(); ArrayList listOfTraversedURLs = new ArrayList<>(); listOfPendingURLs.add(startingURL); while (!listOfPendingURLs.isEmpty() && listOfTraversedURLs.size() <= 100) { String urlString = listOfPendingURLs.remove(0); if (!listOfTraversedURLs.contains(urlString)) { listOfTraversedURLs.add(urlString); System.out.println("Craw " + urlString); for (String s: getSubURLs(urlString)) { if (!listOfTraversedURLs.contains(s)) listOfPendingURLs.add(s); }
enter a URL craw from this URL
list of pending URLs list of traversed URLs add starting URL
get the first URL URL traversed
add a new URL
} } } public static ArrayList getSubURLs(String urlString) { ArrayList list = new ArrayList<>(); try { java.net.URL url = new java.net.URL(urlString); Scanner input = new Scanner(url.openStream()); int current = 0; while (input.hasNext()) { String line = input.nextLine(); current = line.indexOf("http:", current); while (current > 0) { int endIndex = line.indexOf("\"", current); if (endIndex > 0) { // Ensure that a correct URL is found list.add(line.substring(current, endIndex)); current = line.indexOf("http:", endIndex); }
read a line search for a URL end of a URL URL ends with " extract a URL search for next URL
486 Chapter 12
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return URLs
else current = -1; } } } catch (Exception ex) { System.out.println("Error: " + ex.getMessage()); } return list; } }
Enter a URL: http://cs.armstrong.edu/liang Enter a URL: http://www.cs.armstrong.edu/liang Craw http://www.cs.armstrong.edu/liang Craw http://www.cs.armstrong.edu Craw http://www.armstrong.edu Craw http://www.pearsonhighered.com/liang ...
The program prompts the user to enter a starting URL (lines 7–8) and invokes the crawler(url) method to traverse the web (line 9). The crawler(url) method adds the starting url to listOfPendingURLs (line 16) and repeatedly processes each URL in listOfPendingURLs in a while loop (lines 17–29). It removes the first URL in the list (line 19) and processes the URL if it has not been processed (lines 20–28). To process each URL, the program first adds the URL to listOfTraversedURLs (line 21). This list stores all the URLs that have been processed. The getSubURLs(url) method returns a list of URLs in the Web page for the specified URL (line 24). The program uses a foreach loop to add each URL in the page into listOfPendingURLs if it is not in listOfTraversedURLs (lines 24–26). The getSubURLs(url) method reads each line from the Web page (line 40) and searches for the URLs in the line (line 41). Note that a correct URL cannot contain line break characters. So it is sufficient to limit the search for a URL in one line of the text in a Web page. For simplicity, we assume that a URL ends with a quotation mark " (line 43). The method obtains a URL and adds it to a list (line 45). A line may contain multiple URLs. The method continues to search for the next URL (line 46). If no URL is found in the line, current is set to -1 (line 49). The URLs contained in the page are returned in the form of a list (line 57). The program terminates when the number of traversed URLs reaches to 100 (line 18). This is a simple program to traverse the Web. Later you will learn the techniques to make the program more efficient and robust.
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Check Point
12.39 Before a URL is added to listOfPendingURLs, line 25 checks whether it has been traversed. Is it possible that listOfPendingURLs contains duplicate URLs? If so, give an example.
KEY TERMS absolute file name 473 chained exception 469 checked exception 457 declare exception 458 directory path 473
exception 450 exception propagation 459 relative file name 473 throw exception 452 unchecked exception 457
Chapter Summary 487
CHAPTER SUMMARY 1. Exception handling enables a method to throw an exception to its caller. 2. A Java exception is an instance of a class derived from
java.lang.Throwable. Java provides a number of predefined exception classes, such as Error, Exception, RuntimeException, ClassNotFoundException, NullPointerException, and ArithmeticException. You can also define your own exception class by extending Exception.
3. Exceptions occur during the execution of a method. RuntimeException and Error are unchecked exceptions; all other exceptions are checked.
4. When declaring a method, you have to declare a checked exception if the method might throw it, thus telling the compiler what can go wrong.
5. The keyword for declaring an exception is throws, and the keyword for throwing an exception is throw.
6. To invoke the method that declares checked exceptions, enclose it in a try statement. When an exception occurs during the execution of the method, the catch block catches and handles the exception.
7. If an exception is not caught in the current method, it is passed to its caller. The process is repeated until the exception is caught or passed to the main method.
8. Various exception classes can be derived from a common superclass. If a catch block catches the exception objects of a superclass, it can also catch all the exception objects of the subclasses of that superclass.
9. The order in which exceptions are specified in a catch block is important. A compile error will result if you specify an exception object of a class after an exception object of the superclass of that class.
10. When an exception occurs in a method, the method exits immediately if it does not catch the exception. If the method is required to perform some task before exiting, you can catch the exception in the method and then rethrow it to its caller.
11. The code in the finally block is executed under all circumstances, regardless of whether an exception occurs in the try block or whether an exception is caught if it occurs.
12. Exception handling separates error-handling code from normal programming tasks, thus making programs easier to read and to modify.
13. Exception handling should not be used to replace simple tests. You should perform simple test using if statements whenever possible, and reserve exception handling for dealing with situations that cannot be handled with if statements.
14. The File class is used to obtain file properties and manipulate files. It does not contain the methods for creating a file or for reading/writing data from/to a file.
15. You can use Scanner to read string and primitive data values from a text file and use PrintWriter to create a file and write data to a text file.
16. You can read from a file on the Web using the URL class.
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QUIZ Answer the quiz for this chapter online at www.cs.armstrong.edu/liang/intro10e/quiz.html.
PROGRAMMING EXERCISES Sections 12.2–12.9
*12.1 (NumberFormatException) Listing 7.9, Calculator.java, is a simple commandline calculator. Note that the program terminates if any operand is nonnumeric. Write a program with an exception handler that deals with nonnumeric operands; then write another program without using an exception handler to achieve the same objective. Your program should display a message that informs the user of the wrong operand type before exiting (see Figure 12.12).
FIGURE 12.12
The program performs arithmetic operations and detects input errors.
*12.2 (InputMismatchException) Write a program that prompts the user to read *12.3
two integers and displays their sum. Your program should prompt the user to read the number again if the input is incorrect. (ArrayIndexOutOfBoundsException) Write a program that meets the following requirements: ■ ■
Creates an array with 100 randomly chosen integers. Prompts the user to enter the index of the array, then displays the corresponding element value. If the specified index is out of bounds, display the message Out of Bounds.
*12.4 (IllegalArgumentException) Modify the
Loan class in Listing 10.2 to throw IllegalArgumentException if the loan amount, interest rate, or number of years is less than or equal to zero.
*12.5 (IllegalTriangleException) Programming Exercise 11.1 defined the Triangle class with three sides. In a triangle, the sum of any two sides is greater than the other side. The Triangle class must adhere to this rule. Create the IllegalTriangleException class, and modify the constructor of the Triangle class to throw an IllegalTriangleException object if a
triangle is created with sides that violate the rule, as follows: /** Construct a triangle with the specified sides */ public Triangle(double side1, double side2, double side3) throws IllegalTriangleException { // Implement it }
Programming Exercises 489 *12.6 (NumberFormatException) Listing 6.8 implements the
hex2Dec(String hexString) method, which converts a hex string into a decimal number. Implement the hex2Dec method to throw a NumberFormatException if the
string is not a hex string.
*12.7 (NumberFormatException) Write the bin2Dec(String
*12.8
binaryString)
method to convert a binary string into a decimal number. Implement the bin2Dec method to throw a NumberFormatException if the string is not a binary string. (HexFormatException) Exercise 12.6 implements the hex2Dec method to throw a NumberFormatException if the string is not a hex string. Define a custom exception called HexFormatException. Implement the hex2Dec method to throw a HexFormatException if the string is not a hex string.
VideoNote
HexFormatException
*12.9 (BinaryFormatException) Exercise 12.7 implements the bin2Dec method to throw a BinaryFormatException if the string is not a binary string. Define a custom exception called BinaryFormatException. Implement the bin2Dec method to throw a BinaryFormatException if the string is not a binary string.
*12.10 (OutOfMemoryError) Write a program that causes the JVM to throw an OutOfMemoryError and catches and handles this error.
Sections 12.10–12.12
**12.11 (Remove text) Write a program that removes all the occurrences of a specified string from a text file. For example, invoking java Exercise12_11 John filename
**12.12
removes the string John from the specified file. Your program should get the arguments from the command line. (Reformat Java source code) Write a program that converts the Java source code from the next-line brace style to the end-of-line brace style. For example, the following Java source in (a) uses the next-line brace style. Your program converts it to the end-of-line brace style in (b).
public class Test { public static void main(String[] args) { // Some statements } } (a) Next-line brace style
public class Test { public static void main(String[] args) { // Some statements } }
(b) End-of-line brace style
Your program can be invoked from the command line with the Java sourcecode file as the argument. It converts the Java source code to a new format. For example, the following command converts the Java source-code file Test.java to the end-of-line brace style. java Exercise12_12 Test.java
*12.13 (Count characters, words, and lines in a file) Write a program that will count the number of characters, words, and lines in a file. Words are separated by whitespace characters. The file name should be passed as a command-line argument, as shown in Figure 12.13.
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FIGURE 12.13 file.
The program displays the number of characters, words, and lines in the given
*12.14 (Process scores in a text file) Suppose that a text file contains an unspecified
*12.15
**12.16
number of scores separated by blanks. Write a program that prompts the user to enter the file, reads the scores from the file, and displays their total and average. (Write/read data) Write a program to create a file named Exercise12_15.txt if it does not exist. Write 100 integers created randomly into the file using text I/O. Integers are separated by spaces in the file. Read the data back from the file and display the data in increasing order. (Replace text) Listing 12.16, ReplaceText.java, gives a program that replaces text in a source file and saves the change into a new file. Revise the program to save the change into the original file. For example, invoking java Exercise12_16 file oldString newString
***12.17 **12.18
replaces oldString in the source file with newString. (Game: hangman) Rewrite Programming Exercise 7.35. The program reads the words stored in a text file named hangman.txt. Words are delimited by spaces. (Add package statement) Suppose you have Java source files under the directories chapter1, chapter2, . . . , chapter34. Write a program to insert the statement package chapteri; as the first line for each Java source file under the directory chapteri. Suppose chapter1, chapter2, . . . , chapter34 are under the root directory srcRootDirectory. The root directory and chapteri directory may contain other folders and files. Use the following command to run the program: java Exercise12_18 srcRootDirectory
*12.19 (Count words) Write a program that counts the number of words in President Abraham Lincoln’s Gettysburg address from http://cs.armstrong.edu/liang/data/ Lincoln.txt.
**12.20
(Remove package statement) Suppose you have Java source files under the directories chapter1, chapter2, . . . , chapter34. Write a program to remove the statement package chapteri; in the first line for each Java source file under the directory chapteri. Suppose chapter1, chapter2, . . . , chapter34 are under the root directory srcRootDirectory. The root directory and chapteri directory may contain other folders and files. Use the following command to run the program: java Exercise12_20 srcRootDirectory
*12.21 (Data sorted?) Write a program that reads the strings from file SortedStrings. txt and reports whether the strings in the files are stored in increasing order.
Programming Exercises 491
**12.22
If the strings are not sorted in the file, displays the first two strings that are out of the order. (Replace text) Revise Programming Exercise 12.16 to replace a string in a file with a new string for all files in the specified directory using the command: java Exercise12_22 dir oldString newString
**12.23 (Process scores in a text file on the Web) Suppose that the text file on the
*12.24
Web http://cs.armstrong.edu/liang/data/Scores.txt contains an unspecified number of scores. Write a program that reads the scores from the file and displays their total and average. Scores are separated by blanks. (Create large dataset) Create a data file with 1,000 lines. Each line in the file consists of a faculty member’s first name, last name, rank, and salary. The faculty member’s first name and last name for the ith line are FirstNamei and LastNamei. The rank is randomly generated as assistant, associate, and full. The salary is randomly generated as a number with two digits after the decimal point. The salary for an assistant professor should be in the range from 50,000 to 80,000, for associate professor from 60,000 to 110,000, and for full professor from 75,000 to 130,000. Save the file in Salary.txt. Here are some sample data: FirstName1 LastName1 assistant 60055.95 FirstName2 LastName2 associate 81112.45 ... FirstName1000 LastName1000 full 92255.21
*12.25 (Process large dataset) A university posts its employees’ salaries at
http:// cs.armstrong.edu/liang/data/Salary.txt. Each line in the file consists of a faculty
**12.26
**12.27
member’s first name, last name, rank, and salary (see Programming Exercise 12.24). Write a program to display the total salary for assistant professors, associate professors, full professors, and all faculty, respectively, and display the average salary for assistant professors, associate professors, full professors, and all faculty, respectively. (Create a directory) Write a program that prompts the user to enter a directory name and creates a directory using the File’s mkdirs method. The program displays the message “Directory created successfully” if a directory is created or “Directory already exists” if the directory already exists. (Replace words) Suppose you have a lot of files in a directory that contain words Exercisei_j, where i and j are digits. Write a program that pads a 0 before i if i is a single digit and 0 before j if j is a single digit. For example, the word Exercise2_1 in a file will be replaced by Exercise02_01. In Java, when you pass the symbol * from the command line, it refers to all files in the directory (see Supplement III.V). Use the following command to run your program. java Exercise12_27 *
**12.28 (Rename files) Suppose you have a lot of files in a directory named Exercisei_j, where i and j are digits. Write a program that pads a 0 before i if i is a single digit. For example, a file named Exercise2_1 in a directory will be renamed to Exercise02_1. In Java, when you pass the symbol * from the command line, it refers to all files in the directory (see Supplement III.V). Use the following command to run your program. java Exercise12_28 *
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Exception Handling and Text I/O **12.29 (Rename files) Suppose you have a lot of files in a directory named Exercisei_j, where i and j are digits. Write a program that pads a 0 before j if j is a single digit. For example, a file named Exercise2_1 in a directory will be renamed to Exercise2_01. In Java, when you pass the symbol * from the command line, it refers to all files in the directory (see Supplement III.V). Use the following command to run your program. java Exercise12_29 *
**12.30 (Occurrences of each letter) Write a program that prompts the user to enter a file name and displays the occurrences of each letter in the file. Letters are case-insensitive. Here is a sample run:
Enter a filename: Lincoln.txt Number of A's: 56 Number of B's: 134 ... Number of Z's: 9
*12.31 (Baby name popularity ranking) The popularity ranking of baby names from years 2001 to 2010 is downloaded from www.ssa.gov/oact/babynames and stored in files named babynameranking2001.txt, babynameranking2002.txt, . . . , babynameranking2010.txt. Each file contains one thousand lines. Each line contains a ranking, a boy’s name, number for the boy’s name, a girl’s name, and number for the girl’s name. For example, the first two lines in the file babynameranking2010.txt are as follows: 1
Jacob
21,875
Isabella
22,731
2
Ethan
17,866
Sophia
20,477
So, the boy’s name Jacob and girl’s name Isabella are ranked #1 and the boy’s name Ethan and girl’s name Sophia are ranked #2. 21,875 boys are named Jacob and 22,731 girls are named Isabella. Write a program that prompts the user to enter the year, gender, and followed by a name, and displays the ranking of the name for the year. Here is a sample run:
Enter the year: 2010 Enter the gender: M Enter the name: Javier Javier is ranked #190 in year 2010
Enter the year: 2010 Enter the gender: F Enter the name: ABC The name ABC is not ranked in year 2010
Programming Exercises 493 *12.32 (Ranking summary) Write a program that uses the files described in Programming Exercise 12.31 and displays a ranking summary table for the first five girl’s and boy’s names as follows: Year
Rank 1
Rank 2
Rank 3 Rank 4 Rank 5 Rank 1 Rank 2
Rank 3
2010
Isabella
Sophia
Emma
Olivia Ava
Jacob
Ethan
Michael Jayden
Rank 4
Rank 5
2009
Isabella
Emma
Olivia Sophia Ava
Jacob
Ethan
Michael Alexander William
Emily
Madison Hannah Ashley Alexis Jacob
William
... 2001
Michael Matthew Joshua
**12.33 (Search Web) Modify Listing 12.18 WebCrawler.java to search for the word Computer Programming starting from the URL http://cs.armstrong.edu/liang. Your program terminates once the word is found. Display the URL for the page that contains the word.
Christopher
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CHAPTER
13 ABSTRACT CLASSES AND INTERFACES Objectives ■
To design and use abstract classes (§13.2).
■
To generalize numeric wrapper classes, BigInteger, and BigDecimal using the abstract Number class (§13.3).
■
To process a calendar using the Calendar and GregorianCalendar classes (§13.4).
■
To specify common behavior for objects using interfaces (§13.5).
■
To define interfaces and define classes that implement interfaces (§13.5).
■
To define a natural order using the Comparable interface (§13.6).
■
To make objects cloneable using the Cloneable interface (§13.7).
■
To explore the similarities and differences among concrete classes, abstract classes, and interfaces (§13.8).
■
To design the Rational class for processing rational numbers (§13.9).
■
To design classes that follow the class-design guidelines (§13.10).
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13.1 Introduction Key Point
A superclass defines common behavior for related subclasses. An interface can be used to define common behavior for classes (including unrelated classes). You can use the java.util.Arrays.sort method to sort an array of numbers or strings. Can you apply the same sort method to sort an array of geometric objects? In order to write such code, you have to know about interfaces. An interface is for defining common behavior for classes (including unrelated classes). Before discussing interfaces, we introduce a closely related subject: abstract classes.
problem interface
13.2 Abstract Classes Key Point
VideoNote
Abstract GeometricObject class abstract class
abstract method abstract modifier
An abstract class cannot be used to create objects. An abstract class can contain abstract methods, which are implemented in concrete subclasses. In the inheritance hierarchy, classes become more specific and concrete with each new subclass. If you move from a subclass back up to a superclass, the classes become more general and less specific. Class design should ensure that a superclass contains common features of its subclasses. Sometimes a superclass is so abstract that it cannot be used to create any specific instances. Such a class is referred to as an abstract class. In Chapter 11, GeometricObject was defined as the superclass for Circle and Rectangle. GeometricObject models common features of geometric objects. Both Circle and Rectangle contain the getArea() and getPerimeter() methods for computing the area and perimeter of a circle and a rectangle. Since you can compute areas and perimeters for all geometric objects, it is better to define the getArea() and getPerimeter() methods in the GeometricObject class. However, these methods cannot be implemented in the GeometricObject class, because their implementation depends on the specific type of geometric object. Such methods are referred to as abstract methods and are denoted using the abstract modifier in the method header. After you define the methods in GeometricObject, it becomes an abstract class. Abstract classes are denoted using the abstract modifier in the class header. In UML graphic notation, the names of abstract classes and their abstract methods are italicized, as shown in Figure 13.1. Listing 13.1 gives the source code for the new GeometricObject class.
LISTING 13.1 GeometricObject.java abstract class
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
public abstract class GeometricObject { private String color = "white"; private boolean filled; private java.util.Date dateCreated; /** Construct a default geometric object */ protected GeometricObject() { dateCreated = new java.util.Date(); } /** Construct a geometric object with color and filled value */ protected GeometricObject(String color, boolean filled) { dateCreated = new java.util.Date(); this.color = color; this.filled = filled; } /** Return color */ public String getColor() { return color;
13.2 Abstract Classes 497 GeometricObject
Abstract class name is italicized
-color: String -filled: boolean -dateCreated: java.util.Date The # sign indicates protected modifier
#GeometricObject() #GeometricObject(color: string, filled: boolean) +getColor(): String +setColor(color: String): void +isFilled(): boolean +setFilled(filled: boolean): void +getDateCreated(): java.util.Date +toString(): String
Abstract methods are italicized
+getArea(): double +getPerimeter(): double
Circle
-radius: double +Circle() +Circle(radius: double) +Circle(radius: double, color: string, filled: boolean) +getRadius(): double
Methods getArea and getPerimeter are overridden in Circle and Rectangle. Superclass methods are generally omitted in the UML diagram for subclasses. Rectangle
-width: double -height: double +Rectangle() +Rectangle(width: double, height: double)
+setRadius(radius: double): void
+Rectangle(width: double, height: double, color: string, filled: boolean) +getWidth(): double
+getDiameter(): double
+setWidth(width: double): void +getHeight(): double +setHeight(height: double): void
FIGURE 13.1 The new GeometricObject class contains abstract methods. 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42
} /** Set a new color */ public void setColor(String color) { this.color = color; } /** Return filled. Since filled is boolean, * the get method is named isFilled */ public boolean isFilled() { return filled; } /** Set a new filled */ public void setFilled(boolean filled) { this.filled = filled; } /** Get dateCreated */ public java.util.Date getDateCreated() { return dateCreated; }
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abstract method
abstract method
why protected constructor?
implement Circle implement Rectangle
Abstract Classes and Interfaces 43 44 45 46 47 48 49 50 51 52 53 54 55
@Override public String toString() { return "created on " + dateCreated + "\ncolor: " + color + " and filled: " + filled; } /** Abstract method getArea */ public abstract double getArea(); /** Abstract method getPerimeter */ public abstract double getPerimeter(); }
Abstract classes are like regular classes, but you cannot create instances of abstract classes using the new operator. An abstract method is defined without implementation. Its implementation is provided by the subclasses. A class that contains abstract methods must be defined as abstract. The constructor in the abstract class is defined as protected, because it is used only by subclasses. When you create an instance of a concrete subclass, its superclass’s constructor is invoked to initialize data fields defined in the superclass. The GeometricObject abstract class defines the common features (data and methods) for geometric objects and provides appropriate constructors. Because you don’t know how to compute areas and perimeters of geometric objects, getArea() and getPerimeter() are defined as abstract methods. These methods are implemented in the subclasses. The implementation of Circle and Rectangle is the same as in Listings 13.2 and 13.3, except that they extend the GeometricObject class defined in this chapter. You can see the complete code for these two programs from www.cs.armstrong.edu/liang/intro10e/html/Circle.html and www.cs.armstrong.edu/liang/intro10e/html/Rectangle.html, respectively.
LISTING 13.2 Circle.java extends abstract GeometricObject
1 2 3
public class Circle extends GeometricObject { // Same as lines 3-48 in Listing 11.2, so omitted }
LISTING 13.3 Rectangle.java extends abstract GeometricObject
1 2 3
public class Rectangle extends GeometricObject { // Same as lines 3-51 in Listing 11.3, so omitted }
13.2.1
Why Abstract Methods?
You may be wondering what advantage is gained by defining the methods getArea() and getPerimeter() as abstract in the GeometricObject class. The example in Listing 13.4 shows the benefits of defining them in the GeometricObject class. The program creates two geometric objects, a circle and a rectangle, invokes the equalArea method to check whether they have equal areas, and invokes the displayGeometricObject method to display them.
LISTING 13.4 TestGeometricObject.java
create a circle create a rectangle
1 2 3 4 5 6
public class TestGeometricObject { /** Main method */ public static void main(String[] args) { // Create two geometric objects GeometricObject geoObject1 = new Circle(5); GeometricObject geoObject2 = new Rectangle(5, 3);
13.2 Abstract Classes 499 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.println("The two objects have the same area? " + equalArea(geoObject1, geoObject2)); // Display circle displayGeometricObject(geoObject1); // Display rectangle displayGeometricObject(geoObject2); } /** A method for comparing the areas of two geometric objects */ public static boolean equalArea(GeometricObject object1, GeometricObject object2) { return object1.getArea() == object2.getArea(); } /** A method for displaying a geometric object */ public static void displayGeometricObject(GeometricObject object) { System.out.println(); System.out.println("The area is " + object.getArea()); System.out.println("The perimeter is " + object.getPerimeter()); }
equalArea
displayGeometricObject
}
The two objects have the same area? false The area is 78.53981633974483 The perimeter is 31.41592653589793 The area is 13.0 The perimeter is 16.0
The methods getArea() and getPerimeter() defined in the GeometricObject class are overridden in the Circle class and the Rectangle class. The statements (lines 5–6) GeometricObject geoObject1 = new Circle(5); GeometricObject geoObject2 = new Rectangle(5, 3);
create a new circle and rectangle and assign them to the variables geoObject1 and geoObject2. These two variables are of the GeometricObject type. When invoking equalArea(geoObject1, geoObject2) (line 9), the getArea() method defined in the Circle class is used for object1.getArea(), since geoObject1 is a circle, and the getArea() method defined in the Rectangle class is used for object2.getArea(), since geoObject2 is a rectangle. Similarly, when invoking displayGeometricObject(geoObject1) (line 12), the methods getArea() and getPerimeter() defined in the Circle class are used, and when invoking displayGeometricObject(geoObject2) (line 15), the methods getArea and getPerimeter defined in the Rectangle class are used. The JVM dynamically determines which of these methods to invoke at runtime, depending on the actual object that invokes the method. Note that you could not define the equalArea method for comparing whether two geometric objects have the same area if the getArea method were not defined in GeometricObject. Now you have seen the benefits of defining the abstract methods in GeometricObject.
why abstract methods?
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13.2.2
Interesting Points about Abstract Classes
The following points about abstract classes are worth noting: abstract method in abstract class
■
An abstract method cannot be contained in a nonabstract class. If a subclass of an abstract superclass does not implement all the abstract methods, the subclass must be defined as abstract. In other words, in a nonabstract subclass extended from an abstract class, all the abstract methods must be implemented. Also note that abstract methods are nonstatic.
object cannot be created from abstract class
■
An abstract class cannot be instantiated using the new operator, but you can still define its constructors, which are invoked in the constructors of its subclasses. For instance, the constructors of GeometricObject are invoked in the Circle class and the Rectangle class.
abstract class without abstract method
■
A class that contains abstract methods must be abstract. However, it is possible to define an abstract class that doesn’t contain any abstract methods. In this case, you cannot create instances of the class using the new operator. This class is used as a base class for defining subclasses.
concrete method overridden to be abstract
■
A subclass can override a method from its superclass to define it as abstract. This is very unusual, but it is useful when the implementation of the method in the superclass becomes invalid in the subclass. In this case, the subclass must be defined as abstract.
superclass of abstract class may be concrete
■
A subclass can be abstract even if its superclass is concrete. For example, the Object class is concrete, but its subclasses, such as GeometricObject, may be abstract.
abstract class as type
■
You cannot create an instance from an abstract class using the new operator, but an abstract class can be used as a data type. Therefore, the following statement, which creates an array whose elements are of the GeometricObject type, is correct. GeometricObject[] objects = new GeometricObject[10];
You can then create an instance of GeometricObject and assign its reference to the array like this: objects[0] = new Circle();
✓
Check Point
13.1
Which of the following classes defines a legal abstract class?
class A { abstract void unfinished() { } } (a) class A { abstract void unfinished(); } (c) abstract class A { abstract void unfinished(); } (e)
public class abstract A { abstract void unfinished(); }
(b) abstract class A { protected void unfinished(); } (d) abstract class A { abstract int unfinished(); } (f)
13.3 Case Study: the Abstract Number Class 501 13.2 13.3
The getArea() and getPerimeter() methods may be removed from the GeometricObject class. What are the benefits of defining getArea() and getPerimeter() as abstract methods in the GeometricObject class? True or false? a. An abstract class can be used just like a nonabstract class except that you cannot use the new operator to create an instance from the abstract class. b. An abstract class can be extended. c. A subclass of a nonabstract superclass cannot be abstract. d. A subclass cannot override a concrete method in a superclass to define it as abstract. e. An abstract method must be nonstatic.
13.3 Case Study: the Abstract Number Class Number is an abstract superclass for numeric wrapper classes, BigInteger, and BigDecimal.
Key Point
Section 10.7 introduced numeric wrapper classes and Section 10.9 introduced the BigInteger and BigDecimal classes. These classes have common methods byteValue(), shortValue(), intValue(), longValue(), floatValue(), and doubleValue() for returning a byte, short, int, long, float, and double value from an object of these classes. These common methods are actually defined in the Number class, which is a superclass for the numeric wrapper classes, BigInteger, and BigDecimal, as shown in Figure 13.2. java.lang.Number +byteValue(): byte +shortValue(): short +intValue(): int +longVlaue(): long +floatValue(): float +doubleValue(): double
Double
Float
Long
Integer
Short
Byte
BigInteger
BigDecimal
FIGURE 13.2 The Number class is an abstract superclass for Double, Float, Long, Integer, Short, Byte, BigInteger and BigDecimal. Since the intValue(), longValue(), floatValue(), and doubleValue() methods cannot be implemented in the Number class, they are defined as abstract methods in the Number class. The Number class is therefore an abstract class. The byteValue() and shortValue() method are implemented from the intValue() method as follows: public byte byteValue() { return (byte)intValue(); } public short shortValue() { return (short)intValue(); }
502 Chapter 13
Abstract Classes and Interfaces With Number defined as the superclass for the numeric classes, we can define methods to perform common operations for numbers. Listing 13.5 gives a program that finds the largest number in a list of Number objects.
LISTING 13.5 LargestNumbers.java
create an array list add number to list
invoke getLargestNumber
doubleValue
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
import java.util.ArrayList; import java.math.*; public class LargestNumbers { public static void main(String[] args) { ArrayList list = new ArrayList<>(); list.add(45); // Add an integer list.add(3445.53); // Add a double // Add a BigInteger list.add(new BigInteger("3432323234344343101")); // Add a BigDecimal list.add(new BigDecimal("2.0909090989091343433344343")); System.out.println("The largest number is " + getLargestNumber(list)); } public static Number getLargestNumber(ArrayList list) { if (list == null || list.size() == 0) return null; Number number = list.get(0); for (int i = 1; i < list.size(); i++) if (number.doubleValue() < list.get(i).doubleValue()) number = list.get(i); return number; } }
The largest number is 3432323234344343101
The program creates an ArrayList of Number objects (line 6). It adds an Integer object, a Double object, a BigInteger object, and a BigDecimal object to the list (lines 7–12). Note that 45 is automatically converted into an Integer object and added to the list in line 7 and that 3445.53 is automatically converted into a Double object and added to the list in line 8 using autoboxing. Invoking the getLargestNumber method returns the largest number in the list (line 15). The getLargestNumber method returns null if the list is null or the list size is 0 (lines 19–20). To find the largest number in the list, the numbers are compared by invoking their doubleValue() method (line 24). The doubleValue() method is defined in the Number class and implemented in the concrete subclass of Number. If a number is an Integer object, the Integer’s doubleValue() is invoked. If a number is a BigDecimal object, the BigDecimal’s doubleValue() is invoked. If the doubleValue() method were not defined in the Number class, you will not be able to find the largest number among different types of numbers using the Number class.
✓
Check Point
13.4
Why do the following two lines of code compile but cause a runtime error? Number numberRef = new Integer(0); Double doubleRef = (Double)numberRef;
13.4 Case Study: Calendar and GregorianCalendar 503 13.5
Why do the following two lines of code compile but cause a runtime error? Number[] numberArray = new Integer[2]; numberArray[0] = new Double(1.5);
13.6
Show the output of the following code. public class Test { public static void main(String[] args) { Number x = 3; System.out.println(x.intValue()); System.out.println(x.doubleValue()); } }
13.7
What is wrong in the following code? (Note that the compareTo method for the Integer and Double classes was introduced in Section 10.7.) public class Test { public static void main(String[] args) { Number x = new Integer(3); System.out.println(x.intValue()); System.out.println(x.compareTo(new Integer(4))); } }
13.8
What is wrong in the following code? public class Test { public static void main(String[] args) { Number x = new Integer(3); System.out.println(x.intValue()); System.out.println((Integer)x.compareTo(new Integer(4))); } }
13.4 Case Study: Calendar and GregorianCalendar GregorianCalendar is a concrete subclass of the abstract class Calendar.
An instance of java.util.Date represents a specific instant in time with millisecond precision. java.util.Calendar is an abstract base class for extracting detailed calendar information, such as the year, month, date, hour, minute, and second. Subclasses of Calendar can implement specific calendar systems, such as the Gregorian calendar, the lunar calendar, and the Jewish calendar. Currently, java.util.GregorianCalendar for the Gregorian calendar is supported in Java, as shown in Figure 13.3. The add method is abstract in the Calendar class, because its implementation is dependent on a concrete calendar system. You can use new GregorianCalendar() to construct a default GregorianCalendar with the current time and new GregorianCalendar(year, month, date) to construct a GregorianCalendar with the specified year, month, and date. The month parameter is 0 based—that is, 0 is for January. The get(int field) method defined in the Calendar class is useful for extracting the date and time information from a Calendar object. The fields are defined as constants, as shown in Table 13.1. Listing 13.6 gives an example that displays the date and time information for the current time.
Key Point
VideoNote
Calendar and GregorianCalendar classes
abstract add method construct calendar
get(field)
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java.util.Calendar #Calendar()
Constructs a default calendar.
+get(field: int): int
Returns the value of the given calendar field.
+set(field: int, value: int): void
Sets the given calendar to the specified value.
+set(year: int, month: int, dayOfMonth: int): void
Sets the calendar with the specified year, month, and date. The month parameter is 0-based; that is, 0 is for January.
+getActualMaximum(field: int): int
Returns the maximum value that the specified calendar field could have.
+add(field: int, amount: int): void
Adds or subtracts the specified amount of time to the given calendar field.
+getTime(): java.util.Date
Returns a Date object representing this calendar’s time value (million second offset from the UNIX epoch). Sets this calendar’s time with the given Date object.
+setTime(date: java.util.Date): void
java.util.GregorianCalendar +GregorianCalendar()
Constructs a GregorianCalendar for the current time.
+GregorianCalendar(year: int, month: int, dayOfMonth: int) +GregorianCalendar(year: int, month: int, dayOfMonth: int, hour:int, minute: int, second: int)
Constructs a GregorianCalendar for the specified year, month, and date. Constructs a GregorianCalendar for the specified year, month, date, hour, minute, and second. The month parameter is 0-based, that is, 0 is for January.
FIGURE 13.3 The abstract Calendar class defines common features of various calendars.
TABLE 13.1 Field Constants in the Calendar Class Constant
Description
YEAR
The year of the calendar.
MONTH
The month of the calendar, with 0 for January.
DATE
The day of the calendar.
HOUR
The hour of the calendar (12-hour notation).
HOUR_OF_DAY
The hour of the calendar (24-hour notation).
MINUTE
The minute of the calendar.
SECOND
The second of the calendar.
DAY_OF_WEEK
The day number within the week, with 1 for Sunday.
DAY_OF_MONTH
Same as DATE.
DAY_OF_YEAR
The day number in the year, with 1 for the first day of the year.
WEEK_OF_MONTH
The week number within the month, with 1 for the first week.
WEEK_OF_YEAR
The week number within the year, with 1 for the first week.
AM_PM
Indicator for AM or PM (0 for AM and 1 for PM).
LISTING 13.6
calendar for current time extract fields in calendar
1 2 3 4 5 6 7 8
TestCalendar.java
import java.util.*; public class TestCalendar { public static void main(String[] args) { // Construct a Gregorian calendar for the current date and time Calendar calendar = new GregorianCalendar(); System.out.println("Current time is " + new Date()); System.out.println("YEAR: " + calendar.get(Calendar.YEAR));
13.4 Case Study: Calendar and GregorianCalendar 505 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
System.out.println("MONTH: " + calendar.get(Calendar.MONTH)); System.out.println("DATE: " + calendar.get(Calendar.DATE)); System.out.println("HOUR: " + calendar.get(Calendar.HOUR)); System.out.println("HOUR_OF_DAY: " + calendar.get(Calendar.HOUR_OF_DAY)); System.out.println("MINUTE: " + calendar.get(Calendar.MINUTE)); System.out.println("SECOND: " + calendar.get(Calendar.SECOND)); System.out.println("DAY_OF_WEEK: " + calendar.get(Calendar.DAY_OF_WEEK)); System.out.println("DAY_OF_MONTH: " + calendar.get(Calendar.DAY_OF_MONTH)); System.out.println("DAY_OF_YEAR: " + calendar.get(Calendar.DAY_OF_YEAR)); System.out.println("WEEK_OF_MONTH: " + calendar.get(Calendar.WEEK_OF_MONTH)); System.out.println("WEEK_OF_YEAR: " + calendar.get(Calendar.WEEK_OF_YEAR)); System.out.println("AM_PM: " + calendar.get(Calendar.AM_PM)); // Construct a calendar for September 11, 2001 Calendar calendar1 = new GregorianCalendar(2001, 8, 11); String[] dayNameOfWeek = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"}; System.out.println("September 11, 2001 is a " + dayNameOfWeek[calendar1.get(Calendar.DAY_OF_WEEK) - 1]);
create a calendar
} }
Current time is Sun Nov 27 17:48:15 EST 2011 YEAR: 2011 MONTH: 10 DATE: 27 HOUR: 5 HOUR_OF_DAY: 17 MINUTE: 48 SECOND: 15 DAY_OF_WEEK: 1 DAY_OF_MONTH: 27 DAY_OF_YEAR: 331 WEEK_OF_MONTH: 5 WEEK_OF_YEAR: 49 AM_PM: 1 September 11, 2001 is a Tuesday
The set(int field, value) method defined in the Calendar class can be used to set a field. For example, you can use calendar.set(Calendar.DAY_OF_MONTH, 1) to set the calendar to the first day of the month. The add(field, value) method adds the specified amount to a given field. For example, add(Calendar.DAY_OF_MONTH, 5) adds five days to the current time of the calendar. add(Calendar.DAY_OF_MONTH, -5) subtracts five days from the current time of the calendar. To obtain the number of days in a month, use calendar.getActualMaximum(Calendar .DAY_OF_MONTH). For example, if the calendar were for March, this method would return 31.
set(field, value)
add(field, amount)
getActualMaximum(field)
506 Chapter 13
Abstract Classes and Interfaces You can set a time represented in a Date object for the calendar by invoking calendar.setTime(date) and retrieve the time by invoking calendar.getTime().
setTime(date) getTime()
✓
Check Point
13.9 13.10 13.11 13.12
Can you create a Calendar object using the Calendar class? Which method in the Calendar class is abstract? How do you create a Calendar object for the current time? For a Calendar object c, how do you get its year, month, date, hour, minute, and second?
13.5 Interfaces Key Point VideoNote
The concept of interface
An interface is a class-like construct that contains only constants and abstract methods. In many ways an interface is similar to an abstract class, but its intent is to specify common behavior for objects of related classes or unrelated classes. For example, using appropriate interfaces, you can specify that the objects are comparable, edible, and/or cloneable. To distinguish an interface from a class, Java uses the following syntax to define an interface: modifier interface InterfaceName { /** Constant declarations */ /** Abstract method signatures */ }
Here is an example of an interface: public interface Edible { /** Describe how to eat */ public abstract String howToEat(); }
interface inheritance
An interface is treated like a special class in Java. Each interface is compiled into a separate bytecode file, just like a regular class. You can use an interface more or less the same way you use an abstract class. For example, you can use an interface as a data type for a reference variable, as the result of casting, and so on. As with an abstract class, you cannot create an instance from an interface using the new operator. You can use the Edible interface to specify whether an object is edible. This is accomplished by letting the class for the object implement this interface using the implements keyword. For example, the classes Chicken and Fruit in Listing 13.7 (lines 20, 39) implement the Edible interface. The relationship between the class and the interface is known as interface inheritance. Since interface inheritance and class inheritance are essentially the same, we will simply refer to both as inheritance.
LISTING 13.7 TestEdible.java 1 2 3 4 5 6 7 8 9 10 11 12 13
public class TestEdible { public static void main(String[] args) { Object[] objects = {new Tiger(), new Chicken(), new Apple()}; for (int i = 0; i < objects.length; i++) { if (objects[i] instanceof Edible) System.out.println(((Edible)objects[i]).howToEat()); if (objects[i] instanceof Animal) { System.out.println(((Animal)objects[i]).sound()); } } } }
13.5 Interfaces 507 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
abstract class Animal { /** Return animal sound */ public abstract String sound(); }
Animal class
class Chicken extends Animal implements Edible { @Override public String howToEat() { return "Chicken: Fry it"; }
implements Edible howToEat()
@Override public String sound() { return "Chicken: cock-a-doodle-doo"; } } class Tiger extends Animal { @Override public String sound() { return "Tiger: RROOAARR"; } }
Tiger class
abstract class Fruit implements Edible { // Data fields, constructors, and methods omitted here }
implements Edible
class Apple extends Fruit { @Override public String howToEat() { return "Apple: Make apple cider"; } }
Apple class
class Orange extends Fruit { @Override public String howToEat() { return "Orange: Make orange juice"; } }
Orange class
Tiger: RROOAARR Chicken: Fry it Chicken: cock-a-doodle-doo Apple: Make apple cider
This example uses several classes and interfaces. Their inheritance relationship is shown in Figure 13.4. The Animal class defines the sound method (line 17). It is an abstract method and will be implemented by a concrete animal class. The Chicken class implements Edible to specify that chickens are edible. When a class implements an interface, it implements all the methods defined in the interface with the exact signature and return type. The Chicken class implements the howToEat method (lines 22–24). Chicken also extends Animal to implement the sound method (lines 27–29).
508 Chapter 13
Abstract Classes and Interfaces Notation: The interface name and the method names are italicized. The dashed lines and hollow triangles are used to point to the interface.
«interface» Edible +howToEat(): String
+sound(): String
Tiger
Chicken
Fruit
Orange
Animal
Apple
FIGURE 13.4 Edible is a supertype for Chicken and Fruit. Animal is a supertype for Chicken and Tiger. Fruit is a supertype for Orange and Apple. The Fruit class implements Edible. Since it does not implement the howToEat method, Fruit must be denoted as abstract (line 39). The concrete subclasses of Fruit must implement the howToEat method. The Apple and Orange classes implement the howToEat method (lines 45, 52). The main method creates an array with three objects for Tiger, Chicken, and Apple (line 3), and invokes the howToEat method if the element is edible (line 6) and the sound method if the element is an animal (line 9). In essence, the Edible interface defines common behavior for edible objects. All edible objects have the howToEat method.
common behavior
Note Since all data fields are public static final and all methods are public abstract in an interface, Java allows these modifiers to be omitted. Therefore the following interface definitions are equivalent:
omit modifiers
public interface T { public static final int K = 1;
Equivalent
public interface T { int K = 1;
public abstract void p();
void p();
}
✓
Check Point
}
13.13 Suppose A is an interface. Can you create an instance using new A()? 13.14 Suppose A is an interface. Can you declare a reference variable x with type A like this? A x;
13.15 Which of the following is a correct interface? interface A { void print() { }; }
abstract interface A extends I1, I2 { abstract void print() { }; } (b)
(a) abstract interface A { print(); } (c)
interface A { void print(); } (d)
13.6 The Comparable Interface 509 13.16 Show the error in the following code: interface A { void m1(); } class B implements A { void m1() { System.out.println("m1"); } }
13.6 The Comparable Interface The Comparable interface defines the compareTo method for comparing objects. Suppose you want to design a generic method to find the larger of two objects of the same type, such as two students, two dates, two circles, two rectangles, or two squares. In order to accomplish this, the two objects must be comparable, so the common behavior for the objects must be comparable. Java provides the Comparable interface for this purpose. The interface is defined as follows: // Interface for comparing objects, defined in java.lang package java.lang;
Key Point
java.lang.Comparable
public interface Comparable { public int compareTo(E o); }
The compareTo method determines the order of this object with the specified object o and returns a negative integer, zero, or a positive integer if this object is less than, equal to, or greater than o. The Comparable interface is a generic interface. The generic type E is replaced by a concrete type when implementing this interface. Many classes in the Java library implement Comparable to define a natural order for objects. The classes Byte, Short, Integer, Long, Float, Double, Character, BigInteger, BigDecimal, Calendar, String, and Date all implement the Comparable interface. For example, the Integer, BigInteger, String, and Date classes are defined as follows in the Java API: public class Integer extends Number implements Comparable { // class body omitted
public class BigInteger extends Number implements Comparable { // class body omitted
@Override public int compareTo(Integer o) { // Implementation omitted }
@Override public int compareTo(BigInteger o) { // Implementation omitted }
}
}
public class String extends Object implements Comparable { // class body omitted
public class Date extends Object implements Comparable { // class body omitted
@Override public int compareTo(String o) { // Implementation omitted } }
@Override public int compareTo(Date o) { // Implementation omitted } }
510 Chapter 13
Abstract Classes and Interfaces Thus, numbers are comparable, strings are comparable, and so are dates. You can use the compareTo method to compare two numbers, two strings, and two dates. For example, the
following code 1 2 3 4 5
System.out.println(new Integer(3).compareTo(new Integer(5))); System.out.println("ABC".compareTo("ABE")); java.util.Date date1 = new java.util.Date(2013, 1, 1); java.util.Date date2 = new java.util.Date(2012, 1, 1); System.out.println(date1.compareTo(date2));
displays -1 -2 1
Line 1 displays a negative value since 3 is less than 5. Line 2 displays a negative value since ABC is less than ABE. Line 5 displays a positive value since date1 is greater than date2. Let n be an Integer object, s be a String object, and d be a Date object. All the following expressions are true.
n instanceof Integer n instanceof Object n instanceof Comparable
s instanceof String s instanceof Object s instanceof Comparable
d instanceof java.util.Date d instanceof Object d instanceof Comparable
Since all Comparable objects have the compareTo method, the java.util.Arrays .sort(Object[]) method in the Java API uses the compareTo method to compare and sorts the objects in an array, provided that the objects are instances of the Comparable interface. Listing 13.8 gives an example of sorting an array of strings and an array of BigInteger objects.
LISTING 13.8 SortComparableObjects.java
create an array sort the array
create an array
sort the array
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
import java.math.*; public class SortComparableObjects { public static void main(String[] args) { String[] cities = {"Savannah", "Boston", "Atlanta", "Tampa"}; java.util.Arrays.sort(cities); for (String city: cities) System.out.print(city + " "); System.out.println(); BigInteger[] hugeNumbers = {new BigInteger("2323231092923992"), new BigInteger("432232323239292"), new BigInteger("54623239292")}; java.util.Arrays.sort(hugeNumbers); for (BigInteger number: hugeNumbers) System.out.print(number + " "); } }
13.6 The Comparable Interface 511 Atlanta Boston Savannah Tampa 54623239292 432232323239292 2323231092923992
The program creates an array of strings (line 5) and invokes the sort method to sort the strings (line 6). The program creates an array of BigInteger objects (lines 11–13) and invokes the sort method to sort the BigInteger objects (line 14). You cannot use the sort method to sort an array of Rectangle objects, because Rectangle does not implement Comparable. However, you can define a new rectangle class that implements Comparable. The instances of this new class are comparable. Let this new class be named ComparableRectangle, as shown in Listing 13.9.
LISTING 13.9 ComparableRectangle.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
public class ComparableRectangle extends Rectangle implements Comparable { /** Construct a ComparableRectangle with specified properties */ public ComparableRectangle(double width, double height) { super(width, height); } @Override // Implement the compareTo method defined in Comparable public int compareTo(ComparableRectangle o) { if (getArea() > o.getArea()) return 1; else if (getArea() < o.getArea()) return -1; else return 0; } @Override // Implement the toString method in GeometricObject public String toString() { return super.toString() + " Area: " + getArea(); } }
ComparableRectangle extends Rectangle and implements Comparable, as shown in Figure 13.5. The keyword implements indicates that ComparableRectangle inherits all the constants from the Comparable interface and implements the methods in the interface. The compareTo method compares the areas of two rectangles. An instance of ComparableRectangle is also an instance of Rectangle, GeometricObject, Object, and Comparable. GeometricObject
«interface» java.lang.Comparable +compareTo(o: ComparableRectangle): int
Rectangle
ComparableRectangle
FIGURE 13.5 ComparableRectangle extends Rectangle and implements Comparable.
implements Comparable
implement compareTo
implement toString
512 Chapter 13
Abstract Classes and Interfaces You can now use the sort method to sort an array of ComparableRectangle objects, as in Listing 13.10.
LISTING 13.10 SortRectangles.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14
create an array
sort the array
public class SortRectangles { public static void main(String[] args) { ComparableRectangle[] rectangles = { new ComparableRectangle(3.4, 5.4), new ComparableRectangle(13.24, 55.4), new ComparableRectangle(7.4, 35.4), new ComparableRectangle(1.4, 25.4)}; java.util.Arrays.sort(rectangles); for (Rectangle rectangle: rectangles) { System.out.print(rectangle + " "); System.out.println(); } } }
An interface provides another form of generic programming. It would be difficult to use a generic sort method to sort the objects without using an interface in this example, because
Width: Width: Width: Width:
3.4 Height: 5.4 Area: 18.36 1.4 Height: 25.4 Area: 35.559999999999995 7.4 Height: 35.4 Area: 261.96 13.24 Height: 55.4 Area: 733.496
benefits of interface
multiple inheritance would be necessary to inherit Comparable and another class, such as Rectangle, at the same time. The Object class contains the equals method, which is intended for the subclasses of the Object class to override in order to compare whether the contents of the objects are the same. Suppose that the Object class contains the compareTo method, as defined in the Comparable interface; the sort method can be used to compare a list of any objects. Whether a compareTo method should be included in the Object class is debatable. Since the compareTo method is not defined in the Object class, the Comparable interface is defined in Java to enable objects to be compared if they are instances of the Comparable interface. It is strongly recommended (though not required) that compareTo should be consistent with equals. That is, for two objects o1 and o2, o1.compareTo(o2) == 0 if and only if o1.equals(o2) is true.
✓
Check Point
13.17 True or false? If a class implements Comparable, the object of the class can invoke the compareTo method. 13.18 Which of the following is the correct method header for the compareTo method in the String class? public int compareTo(String o) public int compareTo(Object o)
13.19 Can the following code be compiled? Why? Integer n1 = new Integer(3); Object n2 = new Integer(4); System.out.println(n1.compareTo(n2));
13.7 The Cloneable Interface 513 13.20 You can define the
compareTo method in a class without implementing the Comparable interface. What are the benefits of implementing the Comparable
interface? 13.21 What is wrong in the following code? public class Test { public static void main(String[] args) { Person[] persons = {new Person(3), new Person(4), new Person(1)}; java.util.Arrays.sort(persons); } } class Person { private int id; Person(int id) { this.id = id; } }
13.7 The Cloneable Interface The Cloneable interface specifies that an object can be cloned. Often it is desirable to create a copy of an object. To do this, you need to use the clone method and understand the Cloneable interface. An interface contains constants and abstract methods, but the Cloneable interface is a special case. The Cloneable interface in the java.lang package is defined as follows: package java.lang;
Key Point
java.lang.Cloneable
public interface Cloneable { }
This interface is empty. An interface with an empty body is referred to as a marker interface. A marker interface does not contain constants or methods. It is used to denote that a class possesses certain desirable properties. A class that implements the Cloneable interface is marked cloneable, and its objects can be cloned using the clone() method defined in the Object class. Many classes in the Java library (e.g., Date, Calendar, and ArrayList) implement Cloneable. Thus, the instances of these classes can be cloned. For example, the following code 1 2 3 4 5 6 7 8 9
Calendar calendar = new GregorianCalendar(2013, 2, 1); Calendar calendar1 = calendar; Calendar calendar2 = (Calendar)calendar.clone(); System.out.println("calendar == calendar1 is " + (calendar == calendar1)); System.out.println("calendar == calendar2 is " + (calendar == calendar2)); System.out.println("calendar.equals(calendar2) is " + calendar.equals(calendar2));
displays calendar == calendar1 is true calendar == calendar2 is false calendar.equals(calendar2) is true
marker interface
514 Chapter 13
Abstract Classes and Interfaces In the preceding code, line 2 copies the reference of calendar to calendar1, so calendar and calendar1 point to the same Calendar object. Line 3 creates a new object that is the clone of calendar and assigns the new object’s reference to calendar2. calendar2 and calendar are different objects with the same contents. The following code 1 2 3 4 5 6 7 8 9 10 11
ArrayList list1 = list1.add(1.5); list1.add(2.5); list1.add(3.5); ArrayList list2 = ArrayList list3 = list2.add(4.5); list3.remove(1.5); System.out.println("list1 System.out.println("list2 System.out.println("list3
new ArrayList<>();
(ArrayList)list1.clone(); list1;
is " + list1); is " + list2); is " + list3);
displays list1 is [2.5, 3.5] list2 is [1.5, 2.5, 3.5, 4.5] list3 is [2.5, 3.5]
clone arrays
In the preceding code, line 5 creates a new object that is the clone of list1 and assigns the new object’s reference to list2. list2 and list1 are different objects with the same contents. Line 6 copies the reference of list1 to list3, so list1 and list3 point to the same ArrayList object. Line 7 adds 4.5 into list2. Line 8 removes 1.5 from list3. Since list1 and list3 point to the same ArrayList, line 9 and 11 display the same content. You can clone an array using the clone method. For example, the following code 1 2 3 4 5 6
int[] list1 = {1, 2}; int[] list2 = list1.clone(); list1[0] = 7; list2[1] = 8; System.out.println("list1 is " + list1[0] + ", " + list1[1]); System.out.println("list2 is " + list2[0] + ", " + list2[1]);
displays list1 is 7, 2 list2 is 1, 8 how to implement Cloneable
To define a custom class that implements the Cloneable interface, the class must override the clone() method in the Object class. Listing 13.11 defines a class named House that implements Cloneable and Comparable.
LISTING 13.11 House.java 1 2 3 4 5 6 7 8 9 10
public class House implements Cloneable, Comparable { private int id; private double area; private java.util.Date whenBuilt; public House(int id, double area) { this.id = id; this.area = area; whenBuilt = new java.util.Date(); }
13.7 The Cloneable Interface 515 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
public int getId() { return id; } public double getArea() { return area; } public java.util.Date getWhenBuilt() { return whenBuilt; } @Override /** Override the protected clone method defined in the Object class, and strengthen its accessibility */ public Object clone() throws CloneNotSupportedException { return super.clone(); }
This exception is thrown if House does not implement Cloneable
@Override // Implement the compareTo method defined in Comparable public int compareTo(House o) { if (area > o.area) return 1; else if (area < o.area) return -1; else return 0; } }
The House class implements the clone method (lines 26–28) defined in the Object class. The header is: protected native Object clone() throws CloneNotSupportedException;
The keyword native indicates that this method is not written in Java but is implemented in the JVM for the native platform. The keyword protected restricts the method to be accessed in the same package or in a subclass. For this reason, the House class must override the method and change the visibility modifier to public so that the method can be used in any package. Since the clone method implemented for the native platform in the Object class performs the task of cloning objects, the clone method in the House class simply invokes super.clone(). The clone method defined in the Object class may throw CloneNotSupportedException. The House class implements the compareTo method (lines 31–38) defined in the Comparable interface. The method compares the areas of two houses. You can now create an object of the House class and create an identical copy from it, as follows:
CloneNotSupportedException
House house1 = new House(1, 1750.50); House house2 = (House)house1.clone();
house1 and house2 are two different objects with identical contents. The clone method in the Object class copies each field from the original object to the target object. If the field is of a primitive type, its value is copied. For example, the value of area (double type) is copied from house1 to house2. If the field is of an object, the reference of the field is copied. For example, the field whenBuilt is of the Date class, so its reference is copied into house2, as shown in Figure 13.6. Therefore, house1.whenBuilt == house2.whenBuilt is true, although house1 == house2 is false. This is referred to as a shallow copy rather than a
shallow copy
516 Chapter 13 deep copy
house1: House id = 1 area = 1750.50 whenBuilt
Abstract Classes and Interfaces deep copy, meaning that if the field is of an object type, the object’s reference is copied rather than its contents.
id = 1
1
area = 1750.50
1750.50 reference
id = 1 area = 1750.50 whenBuilt
whenBuilt: Date date object contents
house2 = house1.clone() house2: House
house1: House
Memory
Memory
whenBuilt
house2: House
area = 1750.50
1750.50 reference
1 1750.50 reference
whenBuilt
(a)
whenBuilt: Date date object contents
house2 = house1.clone()
id = 1
1
Memory
Memory 1 1750.50 reference
whenBuilt: Date date object contents
(b)
FIGURE 13.6 (a) The default clone method performs a shallow copy. (b) The custom clone method performs a deep copy. deep copy
To perform a deep copy for a House object, replace the clone() method in lines 26–28 with the following code: public Object clone() throws CloneNotSupportedException { // Perform a shallow copy House houseClone = (House)super.clone(); // Deep copy on whenBuilt houseClone.whenBuilt = (java.util.Date)(whenBuilt.clone()); return houseClone; }
or public Object clone() { try { // Perform a shallow copy House houseClone = (House)super.clone(); // Deep copy on whenBuilt houseClone.whenBuilt = (java.util.Date)(whenBuilt.clone()); return houseClone; } catch (CloneNotSupportedException ex) { return null; } }
Now if you clone a House object in the following code: House house1 = new House(1, 1750.50); House house2 = (House)house1.clone();
house1.whenBuilt == house2.whenBuilt will be false. house1 and house2 contain two different Date objects, as shown in Figure 13.6b.
13.8 Interfaces vs. Abstract Classes 517 13.22 Can you invoke the clone() method to clone an object if the class for the object does not implement the java.lang.Cloneable? Does the Date class implement Cloneable? 13.23 What would happen if the House class (defined in Listing 13.11) did not override the clone() method or if House did not implement java.lang.Cloneable? 13.24 Show the output of the following code:
✓
Check Point
java.util.Date date = new java.util.Date(); java.util.Date date1 = date; java.util.Date date2 = (java.util.Date)(date.clone()); System.out.println(date == date1); System.out.println(date == date2); System.out.println(date.equals(date2));
13.25 Show the output of the following code: ArrayList list = new ArrayList<>(); list.add("New York"); ArrayList list1 = list; ArrayList list2 = (ArrayList)(list.clone()); list.add("Atlanta"); System.out.println(list == list1); System.out.println(list == list2); System.out.println("list is " + list); System.out.println("list1 is " + list1); System.out.println("list2.get(0) is " + list2.get(0)); System.out.println("list2.size() is " + list2.size());
13.26 What is wrong in the following code? public class Test { public static void main(String[] args) { GeometricObject x = new Circle(3); GeometricObject y = x.clone(); System.out.println(x == y); } }
13.8 Interfaces vs. Abstract Classes A class can implement multiple interfaces, but it can only extend one superclass. An interface can be used more or less the same way as an abstract class, but defining an interface is different from defining an abstract class. Table 13.2 summarizes the differences.
Key Point
TABLE 13.2 Interfaces vs. Abstract Classes Variables
Constructors
Methods
Abstract class
No restrictions.
Constructors are invoked by subclasses through constructor chaining. An abstract class cannot be instantiated using the new operator.
No restrictions.
Interface
All variables must be
No constructors. An interface cannot be instantiated using the new operator.
All methods must be public abstract instance methods
public static final.
518 Chapter 13 single inheritance multiple inheritance
Abstract Classes and Interfaces Java allows only single inheritance for class extension but allows multiple extensions for interfaces. For example, public class NewClass extends BaseClass implements Interface1, ..., InterfaceN { ... }
subinterface
An interface can inherit other interfaces using the extends keyword. Such an interface is called a subinterface. For example, NewInterface in the following code is a subinterface of Interface1, . . . , and InterfaceN. public interface NewInterface extends Interface1, ... , InterfaceN { // constants and abstract methods }
A class implementing NewInterface must implement the abstract methods defined in NewInterface, Interface1, . . . , and InterfaceN. An interface can extend other interfaces but not classes. A class can extend its superclass and implement multiple interfaces. All classes share a single root, the Object class, but there is no single root for interfaces. Like a class, an interface also defines a type. A variable of an interface type can reference any instance of the class that implements the interface. If a class implements an interface, the interface is like a superclass for the class. You can use an interface as a data type and cast a variable of an interface type to its subclass, and vice versa. For example, suppose that c is an instance of Class2 in Figure 13.7. c is also an instance of Object, Class1, Interface1, Interface1_1, Interface1_2, Interface2_1, and Interface2_2. Interface2_2
Interface1_2
Interface1_1
Object
Interface1
Interface2_1
Class1
Class2
FIGURE 13.7 Class1 implements Interface1; Interface1 extends Interface1_1 and Interface1_2. Class2 extends Class1 and implements Interface2_1 and Interface2_2.
Note naming convention
Class names are nouns. Interface names may be adjectives or nouns.
Design Guide
is-a relationship is-kind-of relationship
Abstract classes and interfaces can both be used to specify common behavior of objects. How do you decide whether to use an interface or a class? In general, a strong is-a relationship that clearly describes a parent-child relationship should be modeled using classes. For example, Gregorian calendar is a calendar, so the relationship between the class java.util.GregorianCalendar and java.util.Calendar is modeled using class inheritance. A weak is-a relationship, also known as an is-kind-of relationship, indicates that an object possesses a certain property. A weak is-a relationship can be modeled using interfaces. For example, all strings are comparable, so the String class implements the Comparable interface.
13.8 Interfaces vs. Abstract Classes 519 In general, interfaces are preferred over abstract classes because an interface can define a common supertype for unrelated classes. Interfaces are more flexible than classes. Consider the Animal class. Suppose the howToEat method is defined in the Animal class, as follows: abstract class Animal { public abstract String howToEat(); }
interface preferred
Animal class
Two subclasses of Animal are defined as follows: class Chicken extends Animal { @Override public String howToEat() { return "Fry it"; } } class Duck extends Animal { @Override public String howToEat() { return "Roast it"; } }
Given this inheritance hierarchy, polymorphism enables you to hold a reference to a Chicken object or a Duck object in a variable of type Animal, as in the following code: public static void main(String[] args) { Animal animal = new Chicken(); eat(animal); animal = new Duck(); eat(animal); } public static void eat(Animal animal) { animal.howToEat(); }
The JVM dynamically decides which howToEat method to invoke based on the actual object that invokes the method. You can define a subclass of Animal. However, there is a restriction: The subclass must be for another animal (e.g., Turkey). Interfaces don’t have this restriction. Interfaces give you more flexibility than classes, because you don’t have to make everything fit into one type of class. You may define the howToEat() method in an interface and let it serve as a common supertype for other classes. For example, public static void main(String[] args) { Edible stuff = new Chicken(); eat(stuff); stuff = new Duck(); eat(stuff); stuff = new Broccoli(); eat(stuff); }
Chicken class
Duck class
520 Chapter 13
Abstract Classes and Interfaces public static void eat(Edible stuff) { stuff.howToEat(); }
Edible interface
interface Edible { public String howToEat(); }
Chicken class
class Chicken implements Edible { @Override public String howToEat() { return "Fry it"; } }
Duck class
class Duck implements Edible { @Override public String howToEat() { return "Roast it"; } }
Broccoli class
class Broccoli implements Edible { @Override public String howToEat() { return "Stir-fry it"; } }
To define a class that represents edible objects, simply let the class implement the Edible interface. The class is now a subtype of the Edible type, and any Edible object can be passed to invoke the howToEat method.
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Check Point
13.27 Give an example to show why interfaces are preferred over abstract classes. 13.28 Define the terms abstract classes and interfaces. What are the similarities and differences between abstract classes and interfaces? 13.29 True or false? a. An interface is compiled into a separate bytecode file. b. An interface can have static methods. c. An interface can extend one or more interfaces. d. An interface can extend an abstract class. e. An abstract class can extend an interface.
13.9 Case Study: The Rational Class Key Point
This section shows how to design the Rational class for representing and processing rational numbers. A rational number has a numerator and a denominator in the form a/b, where a is the numerator and b the denominator. For example, 1/3, 3/4, and 10/4 are rational numbers. A rational number cannot have a denominator of 0, but a numerator of 0 is fine. Every integer i is equivalent to a rational number i/1. Rational numbers are used in exact computations involving fractions—for example, 1/3 = 0.33333. . . . This number cannot be precisely represented in floating-point format using either the data type double or float. To obtain the exact result, we must use rational numbers.
13.9 Case Study: The Rational Class 521 Java provides data types for integers and floating-point numbers, but not for rational numbers. This section shows how to design a class to represent rational numbers. Since rational numbers share many common features with integers and floating-point numbers, and Number is the root class for numeric wrapper classes, it is appropriate to define Rational as a subclass of Number. Since rational numbers are comparable, the Rational class should also implement the Comparable interface. Figure 13.8 illustrates the Rational class and its relationship to the Number class and the Comparable interface. java.lang.Number
1 Rational
java.lang.Comparable
1 Add, Subtract, Multiply, Divide
Rational -numerator: long -denominator: long
The numerator of this rational number. The denominator of this rational number.
+Rational()
Creates a rational number with numerator 0 and denominator 1.
+Rational(numerator: long, denominator: long)
Creates a rational number with a specified numerator and denominator.
+getNumerator(): long +getDenominator(): long +add(secondRational: Rational): Rational
Returns the numerator of this rational number. Returns the denominator of this rational number. Returns the addition of this rational number with another.
+subtract(secondRational: Rational): Rational
Returns the subtraction of this rational number with another.
+multiply(secondRational: Rational): Rational
Returns the multiplication of this rational number with another.
+divide(secondRational: Rational): Rational +toString(): String
Returns the division of this rational number with another.
-gcd(n: long, d: long): long
Returns a string in the form “numerator/denominator.” Returns the numerator if denominator is 1. Returns the greatest common divisor of n and d.
FIGURE 13.8 The properties, constructors, and methods of the Rational class are illustrated in UML. A rational number consists of a numerator and a denominator. There are many equivalent rational numbers—for example, 1/3 = 2/6 = 3/9 = 4/12. The numerator and the denominator of 1/3 have no common divisor except 1, so 1/3 is said to be in lowest terms. To reduce a rational number to its lowest terms, you need to find the greatest common divisor (GCD) of the absolute values of its numerator and denominator, then divide both the numerator and denominator by this value. You can use the method for computing the GCD of two integers n and d, as suggested in Listing 5.9, GreatestCommonDivisor.java. The numerator and denominator in a Rational object are reduced to their lowest terms. As usual, let us first write a test program to create two Rational objects and test its methods. Listing 13.12 is a test program.
LISTING 13.12 TestRationalClass.java 1 2 3 4 5 6 7 8
public class TestRationalClass { /** Main method */ public static void main(String[] args) { // Create and initialize two rational numbers r1 and r2 Rational r1 = new Rational(4, 2); Rational r2 = new Rational(2, 3); // Display results
create a Rational create a Rational
522 Chapter 13 add
Abstract Classes and Interfaces 9 10 11 12 13 14 15
System.out.println(r1 System.out.println(r1 System.out.println(r1 System.out.println(r1 System.out.println(r2
+ + + + +
" " " " "
+ " + r2 + " = " + r1.add(r2)); - " + r2 + " = " + r1.subtract(r2)); * " + r2 + " = " + r1.multiply(r2)); / " + r2 + " = " + r1.divide(r2)); is " + r2.doubleValue());
} }
2 + 2 2 * 2 / 2/3
2/3 = 8/3 2/3 = 4/3 2/3 = 4/3 2/3 = 3 is 0.6666666666666666
The main method creates two rational numbers, r1 and r2 (lines 5–6), and displays the results of r1 + r2, r1 - r2, r1 x r2, and r1 / r2 (lines 9–12). To perform r1 + r2, invoke r1.add(r2) to return a new Rational object. Similarly, invoke r1.subtract(r2) for r1 - r2, r1.multiply(r2) for r1 x r2 , and r1.divide(r2) for r1 / r2. The doubleValue() method displays the double value of r2 (line 13). The doubleValue() method is defined in java.lang.Number and overridden in Rational. Note that when a string is concatenated with an object using the plus sign (+), the object’s string representation from the toString() method is used to concatenate with the string. So r1 + " + " + r2 + " = " + r1.add(r2) is equivalent to r1.toString() + " + " + r2.toString() + " = " + r1.add(r2).toString(). The Rational class is implemented in Listing 13.13.
LISTING 13.13 Rational.java 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
public class Rational extends Number implements Comparable { // Data fields for numerator and denominator private long numerator = 0; private long denominator = 1; /** Construct a rational with default properties */ public Rational() { this(0, 1); } /** Construct a rational with specified numerator and denominator */ public Rational(long numerator, long denominator) { long gcd = gcd(numerator, denominator); this.numerator = ((denominator > 0) ? 1 : -1) * numerator / gcd; this.denominator = Math.abs(denominator) / gcd; } /** Find GCD of two numbers */ private static long gcd(long n, long d) { long n1 = Math.abs(n); long n2 = Math.abs(d); int gcd = 1; for (int k = 1; k <= n1 && k <= n2; k++) { if (n1 % k == 0 && n2 % k == 0) gcd = k; } return gcd;
13.9 Case Study: The Rational Class 523 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 83 84 85 86 87 88 89
} /** Return numerator */ public long getNumerator() { return numerator; } /** Return denominator */ public long getDenominator() { return denominator; } /** Add a rational number to this rational */ public Rational add(Rational secondRational) { long n = numerator * secondRational.getDenominator() + denominator * secondRational.getNumerator(); long d = denominator * secondRational.getDenominator(); return new Rational(n, d); } /** Subtract a rational number from this rational */ public Rational subtract(Rational secondRational) { long n = numerator * secondRational.getDenominator() - denominator * secondRational.getNumerator(); long d = denominator * secondRational.getDenominator(); return new Rational(n, d); } /** Multiply a rational number by this rational */ public Rational multiply(Rational secondRational) { long n = numerator * secondRational.getNumerator(); long d = denominator * secondRational.getDenominator(); return new Rational(n, d); } /** Divide a rational number by this rational */ public Rational divide(Rational secondRational) { long n = numerator * secondRational.getDenominator(); long d = denominator * secondRational.numerator; return new Rational(n, d); } @Override public String toString() { if (denominator == 1) return numerator + ""; else return numerator + "/" + denominator; } @Override // Override the equals method in the Object class public boolean equals(Object other) { if ((this.subtract((Rational)(other))).getNumerator() == 0) return true; else return false; } @Override // Implement the abstract intValue method in Number public int intValue() {
a b
+
c d
=
ad + bc bd
a b
-
c d
=
ad - bc bd
a b
*
c d
=
ac bd
a b
,
c d
=
ad bc
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Abstract Classes and Interfaces 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117
return (int)doubleValue(); } @Override // Implement the abstract floatValue method in Number public float floatValue() { return (float)doubleValue(); } @Override // Implement the doubleValue method in Number public double doubleValue() { return numerator * 1.0 / denominator; } @Override // Implement the abstract longValue method in Number public long longValue() { return (long)doubleValue(); } @Override // Implement the compareTo method in Comparable public int compareTo(Rational o) { if (this.subtract(o).getNumerator() > 0) return 1; else if (this.subtract(o).getNumerator() < 0) return -1; else return 0; } }
The rational number is encapsulated in a Rational object. Internally, a rational number is represented in its lowest terms (line 13), and the numerator determines its sign (line 14). The denominator is always positive (line 15). The gcd method (lines 19–30 in the Rational class) is private; it is not intended for use by clients. The gcd method is only for internal use by the Rational class. The gcd method is also static, since it is not dependent on any particular Rational object. The abs(x) method (lines 20–21 in the Rational class) is defined in the Math class and returns the absolute value of x. Two Rational objects can interact with each other to perform add, subtract, multiply, and divide operations. These methods return a new Rational object (lines 43–70). The methods toString and equals in the Object class are overridden in the Rational class (lines 72–86). The toString() method returns a string representation of a Rational object in the form numerator/denominator, or simply numerator if denominator is 1. The equals(Object other) method returns true if this rational number is equal to the other rational number. The abstract methods intValue, longValue, floatValue, and doubleValue in the Number class are implemented in the Rational class (lines 88–106). These methods return the int, long, float, and double value for this rational number. The compareTo(Rational other) method in the Comparable interface is implemented in the Rational class (lines 108–116) to compare this rational number to the other rational number.
Tip immutable
The getter methods for the properties numerator and denominator are provided in the Rational class, but the setter methods are not provided, so, once a Rational object is created, its contents cannot be changed. The Rational class is immutable. The String class and the wrapper classes for primitive type values are also immutable.
13.10 Class Design Guidelines 525 Tip The numerator and denominator are represented using two variables. It is possible to use an array of two integers to represent the numerator and denominator (see Programming Exercise 13.14). The signatures of the public methods in the Rational class are not changed, although the internal representation of a rational number is changed. This is a good example to illustrate the idea that the data fields of a class should be kept private so as to encapsulate the implementation of the class from the use of the class.
The Rational class has serious limitations and can easily overflow. For example, the following code will display an incorrect result, because the denominator is too large.
encapsulation overflow
public class Test { public static void main(String[] args) { Rational r1 = new Rational(1, 123456789); Rational r2 = new Rational(1, 123456789); Rational r3 = new Rational(1, 123456789); System.out.println("r1 * r2 * r3 is " + r1.multiply(r2.multiply(r3))); } } r1 * r2 * r3 is -1/2204193661661244627
To fix it, you can implement the Rational class using the BigInteger for numerator and denominator (see Programming Exercise 13.15).
13.30 Show the output of the following code? Rational r1 = new Rational(-2, 6); System.out.println(r1.getNumerator()); System.out.println(r1.getDenominator()); System.out.println(r1.intValue()); System.out.println(r1.doubleValue());
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Check Point
13.31 Why is the following code wrong? Rational r1 = new Rational(-2, 6); Object r2 = new Rational(1, 45); System.out.println(r2.compareTo(r1));
13.32 Why is the following code wrong? Object r1 = new Rational(-2, 6); Rational r2 = new Rational(1, 45); System.out.println(r2.compareTo(r1));
13.33 How do you simplify the code in lines 82–85 in Listing 13.13 Rational.java using one line of code without using the if statement?
13.34 Trace the program carefully and show the output of the following code. Rational r1 = new Rational(1, 2); Rational r2 = new Rational(1, -2); System.out.println(r1.add(r2));
13.10 Class Design Guidelines Class design guidelines are helpful for designing sound classes. You have learned how to design classes from the preceding two examples and from many other examples in the preceding chapters. This section summarizes some of the guidelines.
Key Point
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13.10.1 coherent purpose
separate responsibilities
A class should describe a single entity, and all the class operations should logically fit together to support a coherent purpose. You can use a class for students, for example, but you should not combine students and staff in the same class, because students and staff are different entities. A single entity with many responsibilities can be broken into several classes to separate the responsibilities. The classes String, StringBuilder, and StringBuffer all deal with strings, for example, but have different responsibilities. The String class deals with immutable strings, the StringBuilder class is for creating mutable strings, and the StringBuffer class is similar to StringBuilder except that StringBuffer contains synchronized methods for updating strings.
13.10.2 naming conventions
naming consistency
no-arg constructor
independent methods intuitive meaning
independent properties
Encapsulation
A class should use the private modifier to hide its data from direct access by clients. This makes the class easy to maintain. Provide a getter method only if you want the data field to be readable, and provide a setter method only if you want the data field to be updateable. For example, the Rational class provides a getter method for numerator and denominator, but no setter method, because a Rational object is immutable.
13.10.4 easy to explain
Consistency
Follow standard Java programming style and naming conventions. Choose informative names for classes, data fields, and methods. A popular style is to place the data declaration before the constructor and place constructors before methods. Make the names consistent. It is not a good practice to choose different names for similar operations. For example, the length() method returns the size of a String, a StringBuilder, and a StringBuffer. It would be inconsistent if different names were used for this method in these classes. In general, you should consistently provide a public no-arg constructor for constructing a default instance. If a class does not support a no-arg constructor, document the reason. If no constructors are defined explicitly, a public default no-arg constructor with an empty body is assumed. If you want to prevent users from creating an object for a class, you can declare a private constructor in the class, as is the case for the Math class.
13.10.3 encapsulate data fields
Cohesion
Clarity
Cohesion, consistency, and encapsulation are good guidelines for achieving design clarity. Additionally, a class should have a clear contract that is easy to explain and easy to understand. Users can incorporate classes in many different combinations, orders, and environments. Therefore, you should design a class that imposes no restrictions on how or when the user can use it, design the properties in a way that lets the user set them in any order and with any combination of values, and design methods that function independently of their order of occurrence. For example, the Loan class contains the properties loanAmount, numberOfYears, and annualInterestRate. The values of these properties can be set in any order. Methods should be defined intuitively without causing confusion. For example, the substring(int beginIndex, int endIndex) method in the String class is somewhat confusing. The method returns a substring from beginIndex to endIndex – 1, rather than to endIndex. It would be more intuitive to return a substring from beginIndex to endIndex. You should not declare a data field that can be derived from other data fields. For example, the following Person class has two data fields: birthDate and age. Since age can be derived from birthDate, age should not be declared as a data field. public class Person { private java.util.Date birthDate;
13.10 Class Design Guidelines 527 private int age; ... }
13.10.5
Completeness
Classes are designed for use by many different customers. In order to be useful in a wide range of applications, a class should provide a variety of ways for customization through properties and methods. For example, the String class contains more than 40 methods that are useful for a variety of applications.
13.10.6
Instance vs. Static
A variable or method that is dependent on a specific instance of the class must be an instance variable or method. A variable that is shared by all the instances of a class should be declared static. For example, the variable numberOfObjects in CircleWithPrivateDataFields in Listing 9.8 is shared by all the objects of the CircleWithPrivateDataFields class and therefore is declared static. A method that is not dependent on a specific instance should be defined as a static method. For instance, the getNumberOfObjects() method in CircleWithPrivateDataFields is not tied to any specific instance and therefore is defined as a static method. Always reference static variables and methods from a class name (rather than a reference variable) to improve readability and avoid errors. Do not pass a parameter from a constructor to initialize a static data field. It is better to use a setter method to change the static data field. Thus, the following class in (a) is better replaced by (b). public class SomeThing { private int tl; private static int t2;
public class SomeThing { private int tl; private static int t2;
public SomeThing(int tl, int t2) { ... }
public SomeThing(int tl) { ... }
} public static void setT2(int t2) { SomeThing.t2 = t2; } } (a)
(b)
Instance and static are integral parts of object-oriented programming. A data field or method is either instance or static. Do not mistakenly overlook static data fields or methods. It is a common design error to define an instance method that should have been static. For example, the factorial(int n) method for computing the factorial of n should be defined static, because it is independent of any specific instance. A constructor is always instance, because it is used to create a specific instance. A static variable or method can be invoked from an instance method, but an instance variable or method cannot be invoked from a static method.
13.10.7
Inheritance vs. Aggregation
The difference between inheritance and aggregation is the difference between an is-a and a has-a relationship. For example, an apple is a fruit; thus, you would use inheritance to model the relationship between the classes Apple and Fruit. A person has a name; thus, you would use aggregation to model the relationship between the classes Person and Name.
common design error
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13.10.8
Interfaces vs. Abstract Classes
Both interfaces and abstract classes can be used to specify common behavior for objects. How do you decide whether to use an interface or a class? In general, a strong is-a relationship that clearly describes a parent–child relationship should be modeled using classes. For example, since an orange is a fruit, their relationship should be modeled using class inheritance. A weak is-a relationship, also known as an is-kind-of relationship, indicates that an object possesses a certain property. A weak is-a relationship can be modeled using interfaces. For example, all strings are comparable, so the String class implements the Comparable interface. A circle or a rectangle is a geometric object, so Circle can be designed as a subclass of GeometricObject. Circles are different and comparable based on their radii, so Circle can implement the Comparable interface. Interfaces are more flexible than abstract classes, because a subclass can extend only one superclass but can implement any number of interfaces. However, interfaces cannot contain concrete methods. The virtues of interfaces and abstract classes can be combined by creating an interface with an abstract class that implements it. Then you can use the interface or the abstract class, whichever is convenient. We will give examples of this type of design in Chapter 20, Lists, Stacks, Queues, and Priority Queues.
✓
Check Point
13.35 Describe class design guidelines.
KEY TERMS abstract class 496 abstract method 496 deep copy 516 interface 496
marker interface 513 shallow copy 515 subinterface 518
CHAPTER SUMMARY 1. Abstract classes are like regular classes with data and methods, but you cannot create instances of abstract classes using the new operator.
2. An abstract method cannot be contained in a nonabstract class. If a subclass of an abstract superclass does not implement all the inherited abstract methods of the superclass, the subclass must be defined as abstract.
3. A class that contains abstract methods must be abstract. However, it is possible to define an abstract class that doesn’t contain any abstract methods.
4. A subclass can be abstract even if its superclass is concrete. 5. An interface is a class-like construct that contains only constants and abstract methods. In many ways, an interface is similar to an abstract class, but an abstract class can contain constants and abstract methods as well as variables and concrete methods.
6. An interface is treated like a special class in Java. Each interface is compiled into a separate bytecode file, just like a regular class.
7. The java.lang.Comparable interface defines the compareTo method. Many classes in the Java library implement Comparable.
Programming Exercises 529 8. The java.lang.Cloneable interface is a marker interface. An object of the class that implements the Cloneable interface is cloneable.
9. A class can extend only one superclass but can implement one or more interfaces. 10. An interface can extend one or more interfaces.
QUIZ Answer the quiz for this chapter online at www.cs.armstrong.edu/liang/intro10e/quiz.html.
PROGRAMMING EXERCISES Sections 13.2–13.3
**13.1 (Triangle class) Design a new
Triangle class that extends the abstract GeometricObject class. Draw the UML diagram for the classes Triangle and GeometricObject and then implement the Triangle class. Write a test
*13.2
program that prompts the user to enter three sides of the triangle, a color, and a Boolean value to indicate whether the triangle is filled. The program should create a Triangle object with these sides and set the color and filled properties using the input. The program should display the area, perimeter, color, and true or false to indicate whether it is filled or not. (Shuffle ArrayList) Write the following method that shuffles an ArrayList of numbers: public static void shuffle(ArrayList list)
*13.3 (Sort ArrayList) Write the following method that sorts an ArrayList of numbers. public static void sort(ArrayList list)
**13.4 (Display calendars) Rewrite the PrintCalendar class in Listing 6.12 to display a calendar for a specified month using the Calendar and GregorianCalendar classes. Your program receives the month and year from the command line. For example: java Exercise13_04 5 2016
This displays the calendar shown in Figure 13.9.
FIGURE 13.9
The program displays a calendar for May 2016.
530 Chapter 13
Abstract Classes and Interfaces You also can run the program without the year. In this case, the year is the current year. If you run the program without specifying a month and a year, the month is the current month.
Sections 13.4–13.8
*13.5 (Enable GeometricObject comparable) Modify the GeometricObject class to implement the Comparable interface, and define a static max method in the GeometricObject class for finding the larger of two GeometricObject objects. Draw the UML diagram and implement the new GeometricObject class. Write a test program that uses the max method to find the larger of two circles and the
*13.6
larger of two rectangles. (The ComparableCircle class) Define a class named ComparableCircle that extends Circle and implements Comparable. Draw the UML diagram and implement the compareTo method to compare the circles on the basis of area. Write a test class to find the larger of two instances of ComparableCircle objects.
*13.7 (The Colorable interface) Design an interface named Colorable with a void method named howToColor(). Every class of a colorable object must implement the Colorable interface. Design a class named Square that extends GeometricObject and implements Colorable. Implement howToColor to display the message Color all four sides.
*13.8
Draw a UML diagram that involves Colorable, Square, and GeometricObject. Write a test program that creates an array of five GeometricObjects. For each object in the array, display its area and invoke its howToColor method if it is colorable. (Revise the MyStack class) Rewrite the MyStack class in Listing 11.10 to perform a deep copy of the list field.
*13.9 (Enable Circle comparable) Rewrite the Circle class in Listing 13.2 to extend GeometricObject and implement the Comparable interface. Override the equals method in the Object class. Two Circle objects are equal if their radii are the same. Draw the UML diagram that involves Circle, GeometricObject, and Comparable.
*13.10 (Enable Rectangle comparable) Rewrite the Rectangle class in Listing 13.3 to VideoNote
Redesign the Rectangle class
*13.11
extend GeometricObject and implement the Comparable interface. Override the equals method in the Object class. Two Rectangle objects are equal if their areas are the same. Draw the UML diagram that involves Rectangle, GeometricObject, and Comparable. (The Octagon class) Write a class named Octagon that extends GeometricObject and implements the Comparable and Cloneable interfaces. Assume that all eight sides of the octagon are of equal length. The area can be computed using the following formula: area = (2 + 4/22)* side * side
*13.12
Draw the UML diagram that involves Octagon, GeometricObject, Comparable, and Cloneable. Write a test program that creates an Octagon object with side value 5 and displays its area and perimeter. Create a new object using the clone method and compare the two objects using the compareTo method. (Sum the areas of geometric objects) Write a method that sums the areas of all the geometric objects in an array. The method signature is: public static double sumArea(GeometricObject[] a)
Programming Exercises 531
*13.13
Write a test program that creates an array of four objects (two circles and two rectangles) and computes their total area using the sumArea method. (Enable the Course class cloneable) Rewrite the Course class in Listing 10.6 to add a clone method to perform a deep copy on the students field.
Section 13.9
*13.14 (Demonstrate the benefits of encapsulation) Rewrite the
Rational class in Listing 13.13 using a new internal representation for the numerator and denominator. Create an array of two integers as follows: private long[] r = new long[2];
Use r[0] to represent the numerator and r[1] to represent the denominator. The signatures of the methods in the Rational class are not changed, so a client application that uses the previous Rational class can continue to use this new Rational class without being recompiled.
*13.15 (Use
BigInteger for the Rational class) Redesign and implement the Rational class in Listing 13.13 using BigInteger for the numerator and
*13.16
denominator. (Create a rational-number calculator) Write a program similar to Listing 7.9, Calculator.java. Instead of using integers, use rationals, as shown in Figure 13.10a. You will need to use the split method in the String class, introduced in Section 10.10.3, Replacing and Splitting Strings, to retrieve the numerator string and denominator string, and convert strings into integers using the Integer.parseInt method. y-axis 2 + 3i
x-axis 3 - 2i
(a)
(b)
FIGURE 13.10 (a) The program takes three arguments (operand1, operator, and operand2) from the command line and displays the expression and the result of the arithmetic operation. (b) A complex number can be interpreted as a point in a plane.
*13.17 (Math: The Complex class) A complex number is a number in the form a + bi,
where a and b are real numbers and i is 2 -1. The numbers a and b are known as the real part and imaginary part of the complex number, respectively. You can perform addition, subtraction, multiplication, and division for complex numbers using the following formulas: a + bi + c + di = (a + c) + (b + d)i a + bi - (c + di) = (a - c) + (b - d)i (a + bi)*(c + di) = (ac - bd) + (bc + ad)i (a + bi)/(c + di) = (ac + bd)/(c2 + d 2) + (bc - ad)i/(c2 + d 2)
532 Chapter 13
Abstract Classes and Interfaces You can also obtain the absolute value for a complex number using the following formula: a + bi = 2a2 + b2 (A complex number can be interpreted as a point on a plane by identifying the (a,b) values as the coordinates of the point. The absolute value of the complex number corresponds to the distance of the point to the origin, as shown in Figure 13.10b.) Design a class named Complex for representing complex numbers and the methods add, subtract, multiply, divide, and abs for performing complexnumber operations, and override toString method for returning a string representation for a complex number. The toString method returns (a + bi) as a string. If b is 0, it simply returns a. Your Complex class should also implement the Cloneable interface. Provide three constructors Complex(a, b), Complex(a), and Complex(). Complex() creates a Complex object for number 0 and Complex(a) creates a Complex object with 0 for b. Also provide the getRealPart() and getImaginaryPart() methods for returning the real and imaginary part of the complex number, respectively. Write a test program that prompts the user to enter two complex numbers and displays the result of their addition, subtraction, multiplication, division, and absolute value. Here is a sample run:
Enter the first complex number: 3.5 5.5 Enter the second complex number: -3.5 1 (3.5 + 5.5i) + (-3.5 + 1.0i) = 0.0 + 6.5i (3.5 + 5.5i) - (-3.5 + 1.0i) = 7.0 + 4.5i (3.5 + 5.5i) * (-3.5 + 1.0i) = -17.75 + -13.75i (3.5 + 5.5i) / (-3.5 + 1.0i) = -0.5094 + -1.7i |(3.5 + 5.5i)| = 6.519202405202649
13.18 (Use the Rational class) Write a program that computes the following summation series using the Rational class: 2 3 98 99 1 + + + c + + 2 3 4 99 100
13.19
You will discover that the output is incorrect because of integer overflow (too large). To fix this problem, see Programming Exercise 13.15. (Convert decimals to fractions) Write a program that prompts the user to enter a decimal number and displays the number in a fraction. Hint: read the decimal number as a string, extract the integer part and fractional part from the string, and use the BigInteger implementation of the Rational class in Programming Exercise 13.15 to obtain a rational number for the decimal number. Here are some sample runs:
Enter a decimal number: 3.25 The fraction number is 13/4
Programming Exercises 533 Enter a decimal number: -0.45452 The fraction number is -11363/25000
13.20 (Algebra: solve quadratic equations) Rewrite Programming Exercise 3.1 to obtain imaginary roots if the determinant is less than 0 using the Complex class in Programming Exercise 13.17. Here are some sample runs. Enter a, b, c: 1 3 1 The roots are -0.381966 and -2.61803
Enter a, b, c: 1 2 1 The root is -1
Enter a, b, c: 1 2 3 The roots are -1.0 + 1.4142i and -1.0 + -1.4142i
13.21 (Algebra: vertex form equations) The equation of a parabola can be expressed in either standard form (y = ax2 + bx + c) or vertex form (y = a(x - h)2 + k). Write a program that prompts the user to enter a, b, and c as integers in standard form and displays h and k in the vertex form. Here are some sample runs. Enter a, b, c: 1 3 1 h is -3/2 k is -5/4
Enter a, b, c: 2 3 4 h is -3/4 k is 23/8
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CHAPTER
14 JAVAFX BASICS Objectives ■
To distinguish between JavaFX, Swing, and AWT (§14.2).
■
To write a simple JavaFX program and understand the relationship among stages, scenes, and nodes (§14.3).
■
To create user interfaces using panes, UI controls, and shapes (§14.4).
■
To update property values automatically through property binding (§14.5).
■
To use the common properties style and rotate for nodes (§14.6).
■
To create colors using the Color class (§14.7).
■
To create fonts using the Font class (§14.8).
■
To create images using the Image class and to create image views using the ImageView class (§14.9).
■
To layout nodes using Pane, StackPane, FlowPane, GridPane, BorderPane, HBox, and VBox (§14.10).
■
To display text using the Text class and create shapes using Line, Circle, Rectangle, Ellipse, Arc, Polygon, and Polyline (§14.11).
■
To develop the reusable GUI component ClockPane for displaying an analog clock (§14.12).
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14.1 Introduction Key Point
JavaFX is an excellent pedagogical tool for learning object-oriented programming. JavaFX is a new framework for developing Java GUI programs. The JavaFX API is an excellent example of how the object-oriented principles are applied. This chapter serves two purposes. First, it presents the basics of JavaFX programming. Second, it uses JavaFX to demonstrate object-oriented design and programming. Specifically, this chapter introduces the framework of JavaFX and discusses JavaFX GUI components and their relationships. You will learn how to develop simple GUI programs using layout panes, buttons, labels, text fields, colors, fonts, images, image views, and shapes.
14.2 JavaFX vs Swing and AWT Key Point
Swing and AWT are replaced by the JavaFX platform for developing rich Internet applications. When Java was introduced, the GUI classes were bundled in a library known as the Abstract Windows Toolkit (AWT). AWT is fine for developing simple graphical user interfaces, but not for developing comprehensive GUI projects. In addition, AWT is prone to platform-specific bugs. The AWT user-interface components were replaced by a more robust, versatile, and flexible library known as Swing components. Swing components are painted directly on canvases using Java code. Swing components depend less on the target platform and use less of the native GUI resources. Swing is designed for developing desktop GUI applications. It is now replaced by a completely new GUI platform known as JavaFX. JavaFX incorporates modern GUI technologies to enable you to develop rich Internet applications. A rich Internet application (RIA) is a Web application designed to deliver the same features and functions normally associated with deskop applications. A JavaFX application can run seemlessly on a desktop and from a Web browser. Additionally, JavaFX provides a multi-touch support for touchenabled devices such as tablets and smart phones. JavaFX has a built-in 2D, 3D, animation support, video and audio playback, and runs as a stand-alone application or from a browser. This book teaches Java GUI programming using JavaFX for two reasons. First, JavaFX is much simpler to learn and use for new Java programmers. Second, Swing is essentially dead, because it will not receive any further enhancement. JavaFX is the new GUI tool for developing cross-platform-rich Internet applications on desktop computers, on hand-held devices, and on the Web.
AWT
Swing
JavaFX
why teaching JavaFX?
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Check Point
14.1 Explain the evolution of Java GUI technologies. 14.2 Explain why this book teaches Java GUI using JavaFX.
14.3 The Basic Structure of a JavaFX Program Key Point
The abstract javafx.application.Application class defines the essential framework for writing JavaFX programs. We begin by writing a simple JavaFX program that illustrates the basic structure of a JavaFX program. Every JavaFX program is defined in a class that extends javafx.application .Application, as shown in Listing 14.1:
LISTING 14.1 MyJavaFX.java 1 2 3 4
import import import import
javafx.application.Application; javafx.scene.Scene; javafx.scene.control.Button; javafx.stage.Stage;
14.3 The Basic Structure of a JavaFX Program 537 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
public class MyJavaFX extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { // Create a scene and place a button in the scene Button btOK = new Button("OK"); Scene scene = new Scene(btOK, 200, 250); primaryStage.setTitle("MyJavaFX"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } /** * The main method is only needed for the IDE with limited * JavaFX support. Not needed for running from the command line. */ public static void main(String[] args) { Application.launch(args); }
extend Application override start create a button create a scene set stage title set a scene display stage
main method launch application
}
You can test and run your program from a command window or from an IDE such as NetBeans or Eclipse. A sample run of the program is shown in Figure 14.1. Supplements II.F–H give the tips for running JavaFX programs from a command window, NetBeans, and Eclipse. A JavaFX program can run stand-alone or from a Web browser. For running a JavaFX program from a Web browser, see Supplement II.I.
FIGURE 14.1
JavaFX on NetBenas and Eclipse
A simple JavaFX displays a button in the window.
The launch method (line 22) is a static method defined in the Application class for launching a stand-alone JavaFX application. The main method (lines 21–23) is not needed if you run the program from the command line. It may be needed to launch a JavaFX program from an IDE with a limited JavaFX support. When you run a JavaFX application without a main method, JVM automatically invokes the launch method to run the application. The main class overrides the start method defined in javafx.application.Application (line 8). After a JavaFX application is launched, the JVM constructs an instance of the class using its no-arg constructor and invokes its start method. The start method normally places UI controls in a scene and displays the scene in a stage, as shown in Figure 14.2a. Line 10 creates a Button object and places it in a Scene object (line 11). A Scene object can be created using the constructor Scene(node, width, height). This constructor specifies the width and height of the scene and places the node in the scene. A Stage object is a window. A Stage object called primary stage is automatically created by the JVM when the application is launched. Line 13 sets the scene to the primary stage and line 14 displays the primary stage. JavaFX names the Stage and Scene classes using the analogy from the theater. You may think stage as the platform to support scenes and nodes as actors to perform in the scenes. You can create additional stages if needed. The JavaFX program in Listing 14.2 displays two stages, as shown in Figure 14.2b.
launch
construct application start application scene
primary stage
538 Chapter 14
JavaFX Basics Stage Scene Button
(b)
(a)
FIGURE 14.2 (a) Stage is a window for displaying a scene that contains nodes. (b) Multiple stages can be displayed in a JavaFX program.
LISTING 14.2 MultipleStageDemo.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
primary stage in start
display primary stage create second stage
display second stage main method omitted
import import import import
javafx.application.Application; javafx.scene.Scene; javafx.scene.control.Button; javafx.stage.Stage;
public class MultipleStageDemo extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { // Create a scene and place a button in the scene Scene scene = new Scene(new Button("OK"), 200, 250); primaryStage.setTitle("MyJavaFX"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage Stage stage = new Stage(); // Create a new stage stage.setTitle("Second Stage"); // Set the stage title // Set a scene with a button in the stage stage.setScene(new Scene(new Button("New Stage"), 100, 100)); stage.show(); // Display the stage } }
Note that the main method is omitted in the listing since it is identical for every JavaFX application. From now on, we will not list the main method in our JavaFX source code for brevity. By default, the user can resize the stage. To prevent the user from resizing the stage, invoke stage.setResizable(false).
main method omitted
prevent stage resizing
✓
Check Point
14.3 How do you define a JavaFX main class? What is the signature of the start method? What is a stage? What is a primary stage? Is a primary stage automatically created? How do you display a stage? Can you prevent the user from resizing the stage? Can you replace Application.launch(args) by launch(args) in line 22 in Listing 14.1? 14.4 Show the output of the following JavaFX program. import javafx.application.Application; import javafx.stage.Stage; public class Test extends Application { public Test() { System.out.println("Test constructor is invoked"); }
14.4 Panes, UI Controls, and Shapes 539 @Override // Override the start method in the Application class public void start(Stage primaryStage) { System.out.println("start method is invoked"); } public static void main(String[] args) { System.out.println("launch application"); Application.launch(args); } }
14.4 Panes, UI Controls, and Shapes Panes, UI controls, and shapes are subtypes of Node. When you run MyJavaFX in Listing 14.1, the window is displayed as shown in Figure 14.1. The button is always centered in the scene and occupies the entire window no matter how you resize it. You can fix the problem by setting the position and size properties of a button. However, a better approach is to use container classes, called panes, for automatically laying out the nodes in a desired location and size. You place nodes inside a pane and then place the pane into a scene. A node is a visual component such as a shape, an image view, a UI control, or a pane. A shape refers to a text, line, circle, ellipse, rectangle, arc, polygon, polyline, etc. A UI control refers to a label, button, check box, radio button, text field, text area, etc. A scene can be displayed in a stage, as shown in Figure 14.3a. The relationship among Stage, Scene, Node, Control, and Pane is illustrated in the UML diagram, as shown in Figure 14.3b. Note that a Scene can contain a Control or a Pane, but not a Shape or an ImageView. A Pane can contain any subtype of Node. You can create a Scene using the constructor Scene(Parent, width, height) or Scene(Parent). The dimension of the scene is automatically decided in the latter constructor. Every subclass of Node has a no-arg constructor for creating a default node. Listing 14.3 gives a program that places a button in a pane, as shown in Figure 14.4
Shape Stage 1 ImageView
1 Scene
Control *
Node
Key Point
pane node shape UI control
Shapes such as Line, Circle, Ellipse, Rectangle, Path, Polygon, Polyline, and Text are subclasses of Shape. For displaying an image. UI controls such as Label, TextField, Button, CheckBox, RadioButton, and TextArea are subclasses of Control. FlowPane
Stage Scene Parent (Pane, Control)
GridPane
Parent
BorderPane Pane
HBox
Nodes VBox StackPane (a)
(b)
FIGURE 14.3 (a) Panes are used to hold nodes. (b) Nodes can be shapes, image views, UI controls, and panes.
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LISTING 14.3 ButtonInPane.java
create a pane add a button add pane to scene
display stage main method omitted
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
import import import import import
public class ButtonInPane extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { // Create a scene and place a button in the scene StackPane pane = new StackPane(); pane.getChildren().add(new Button("OK")); Scene scene = new Scene(pane, 200, 50); primaryStage.setTitle("Button in a pane"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } }
FIGURE 14.4
ObservableList
javafx.application.Application; javafx.scene.Scene; javafx.scene.control.Button; javafx.stage.Stage; javafx.scene.layout.StackPane;
A button is placed in the center of the pane.
The program creates a StackPane (line 11) and adds a button as a child of the pane (line 12). The getChildren() method returns an instance of javafx.collections.ObservableList. ObservableList behaves very much like an ArrayList for storing a collection of elements. Invoking add(e) adds an element to the list. The StackPane places the nodes in the center of the pane on top of each other. Here, there is only one node in the pane. The StackPane respects a node’s preferred size. So you see the button displayed in its preferred size. Listing 14.4 gives an example that displays a circle in the center of the pane, as shown in Figure 14.5a.
LISTING 14.4 ShowCircle.java
create a circle set circle properties
create a pane
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
import import import import import import
javafx.application.Application; javafx.scene.Scene; javafx.scene.layout.Pane; javafx.scene.paint.Color; javafx.scene.shape.Circle; javafx.stage.Stage;
public class ShowCircle extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { // Create a circle and set its properties Circle circle = new Circle(); circle.setCenterX(100); circle.setCenterY(100); circle.setRadius(50); circle.setStroke(Color.BLACK); circle.setFill(Color.WHITE); // Create a pane to hold the circle Pane pane = new Pane();
14.4 Panes, UI Controls, and Shapes 541 21 22 23 24 25 26 27 28 29
pane.getChildren().add(circle);
add circle to pane
// Create a scene and place it in the stage Scene scene = new Scene(pane, 200, 200); primaryStage.setTitle("ShowCircle"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage
add pane to scene
display stage
} }
main method omitted
(0, 0)
(0, 0)
(100, 100)
(100, 100)
(a)
(b)
FIGURE 14.5 (a) A circle is displayed in the center of the scene. (b) The circle is not centered after the window is resized. The program creates a Circle (line 12) and sets its center at (100, 100) (lines 13–14), which is also the center for the scene, since the scene is created with the width and height of 200 (line 24). The radius of the circle is set to 50 (line 15). Note that the measurement units for graphics in Java are all in pixels. The stroke color (i.e., the color to draw the circle) is set to black (line 16). The fill color (i.e., the color to fill the circle) is set to white (line 17). You may set the color to null to specify that no color is set. The program creates a Pane (line 20) and places the circle in the pane (line 21). Note that the coordinates of the upper left corner of the pane is (0, 0) in the Java coordinate system, as shown in Figure 14.6a, as opposed to the conventional coordinate system where (0, 0) is at the center of the window, as shown in Figure 14.6b. The x-coordinate increases from left to right and the y-coordinate increases downward in the Java coordinate system. The pane is placed in the scene (line 24) and the scene is set in the stage (line 26). The circle is displayed in the center of the stage, as shown in Figure 14.5a. However, if you resize the window, the circle is not centered, as shown in Figure 14.5b. In order to display the circle centered as the window resizes, the x- and y-coordinates of the circle center need to be reset to the center of the pane. This can be done by using property binding, introduced in the next section. x
Y axis
(0, 0)
X axis
y (x, y) (0, 0) Java Coordinate System
X axis
Conventional Coordinate System
Y axis (a)
(b)
FIGURE 14.6 The Java coordinate system is measured in pixels, with (0, 0) at its upper-left corner.
pixels set color
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14.5 How do you create a Scene object? How do you set a scene in a stage? How do you place a circle into a scene? 14.6 What is a pane? What is a node? How do you place a node in a pane? Can you directly place a Shape or an ImageView into a Scene? Can you directly place a Control or a Pane into a Scene? 14.7 How do you create a Circle? How do you set its center location and radius? How do you set its stroke color and fill color?
14.5 Property Binding Key Point target object source object binding object binding property bindable object observable object
You can bind a target object to a source object. A change in the source object will be automatically reflected in the target object. JavaFX introduces a new concept called property binding that enables a target object to be bound to a source object. If the value in the source object changes, the target object is also changed automatically. The target object is called a binding object or a binding property and the source object is called a bindable object or observable object. As discussed in the preceding listing, the circle is not centered after the window is resized. In order to display the circle centered as the window resizes, the x- and y-coordinates of the circle center need to be reset to the center of the pane. This can be done by binding the centerX with pane’s width/2 and centerY with pane’s height/2, as shown in Listing 14.5.
LISTING 14.5 ShowCircleCentered.java VideoNote
Understand property binding
create a pane
create a circle bind properties
add circle to pane
add pane to scene
display stage
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
import import import import import import
javafx.application.Application; javafx.scene.Scene; javafx.scene.layout.Pane; javafx.scene.paint.Color; javafx.scene.shape.Circle; javafx.stage.Stage;
public class ShowCircleCentered extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { // Create a pane to hold the circle Pane pane = new Pane(); // Create a circle and set its properties Circle circle = new Circle(); circle.centerXProperty().bind(pane.widthProperty().divide(2)); circle.centerYProperty().bind(pane.heightProperty().divide(2)); circle.setRadius(50); circle.setStroke(Color.BLACK); circle.setFill(Color.WHITE); pane.getChildren().add(circle); // Add circle to the pane // Create a scene and place it in the stage Scene scene = new Scene(pane, 200, 200); primaryStage.setTitle("ShowCircleCentered"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } }
The Circle class has the centerX property for representing the x-coordinate of the circle center. This property like many properties in JavaFX classes can be used both as target and source in a property binding. A target listens to the changes in the source and automatically
14.5 Property Binding 543 updates itself once a change is made in the source. A target binds with a source using the bind method as follows: target.bind(source);
The bind method is defined in the javafx.beans.property.Property interface. A binding property is an instance of javafx.beans.property.Property. A source object is an instance of the javafx.beans.value.ObservableValue interface. An ObservableValue is an entity that wraps a value and allows to observe the value for changes. JavaFX defines binding properties for primitive types and strings. For a double/float/ long/int/boolean value, its binding property type is DoubleProperty/FloatProperty/ LongProperty/IntegerProperty/BooleanProperty. For a string, its binding property type is StringProperty. These properties are also subtypes of ObservableValue. So they can also be used as source objects for binding properties. By convention, each binding property (e.g., centerX) in a JavaFX class (e.g., Circle) has a getter (e.g., getCenterX()) and setter (e.g., setCenterX(double)) method for returning and setting the property’s value. It also has a getter method for returning the property itself. The naming convention for this method is the property name followed by the word Property. For example, the property getter method for centerX is centerXProperty(). We call the getCenterX() method as the value getter method, the setCenterX(double) method as the value setter method, and centerXProperty() as the property getter method. Note that getCenterX() returns a double value and centerXProperty() returns an object of the DoubleProperty type. Figure 14.7a shows the convention for defining a binding property in a class and Figure 14.7b shows a concrete example in which centerX is a binding property of the type DoubleProperty. public class SomeClassName {
the Property interface the ObservableValue interface common binding properties common ObservableValue objects
value getter method value setter method property getter method
public class Circle {
private PropertyType x;
private DoubleProperty centerX;
/** Value getter method */ public propertyValueType getX() { ... }
/** Value getter method */ public double getCenterX() { ... }
/** Value setter method */ public void setX(propertyValueType value) { ... }
/** Value setter method */ public void setCenterX(double value) { ... }
/** Property getter method */ public PropertyType xProperty() { ... }
/** Property getter method */ public DoubleProperty centerXProperty() { ... } }
} (a) x is a binding property
(b) centerX is binding property
FIGURE 14.7 A binding property has a value getter method, setter method, and property getter method. The program in Listing 14.5 is the same as in Listing 14.4 except that it binds circle’s centerX and centerY properties to half of pane’s width and height (lines 16–17). Note that circle.centerXProperty() returns centerX and pane.widthProperty() returns width. Both centerX and width are binding properties of the DoubleProperty type. The numeric binding property classes such as DoubleProperty and IntegerProperty contain the add, subtract, multiply, and divide methods for adding, subtracting, multiplying, and dividing a value in a binding property and returning a new observable property. So, pane.widthProperty().divide(2) returns a new observable property that represents half of the pane’s width. The statement circle.centerXProperty().bind(pane.widthProperty().divide(2));
is same as centerX.bind(width.divide(2));
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LISTING 14.6 BindingDemo.java
create a DoubleProperty create a DoubleProperty bind property
set a new source value
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
import javafx.beans.property.DoubleProperty; import javafx.beans.property.SimpleDoubleProperty; public class BindingDemo { public static void main(String[] args) { DoubleProperty d1 = new SimpleDoubleProperty(1); DoubleProperty d2 = new SimpleDoubleProperty(2); d1.bind(d2); System.out.println("d1 is " + d1.getValue() + " and d2 is " + d2.getValue()); d2.setValue(70.2); System.out.println("d1 is " + d1.getValue() + " and d2 is " + d2.getValue()); } }
d1 is 2.0 and d2 is 2.0 d1 is 70.2 and d2 is 70.2
The program creates an instance of DoubleProperty using SimpleDoubleProperty(1) (line 6). Note that DoubleProperty, FloatProperty, LongProperty, IntegerProperty, and BooleanProperty are abstract classes. Their concrete subclasses SimpleDoubleProperty, SimpleFloatProperty, SimpleLongProperty, SimpleIntegerProperty, and SimpleBooleanProperty are used to create instances of these properties. These classes are very much like wrapper classes Double, Float, Long, Integer, and Boolean with additional features for binding to a source object. The program binds d1 with d2 (line 8). Now the values in d1 and d2 are the same. After setting d2 to 70.2 (line 11), d1 also becomes 70.2 (line 13). The binding demonstrated in this example is known as unidirectional binding. Occasionally, it is useful to synchronize two properties so that a change in one property is reflected in another object, and vice versa. This is called a bidirectional binding. If the target and source are both binding properties and observable properties, they can be bound bidirectionally using the bindBidirectional method.
unidirectional binding bidirectional binding
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14.8 What is a binding property? What interface defines a binding property? What interface defines a source object? What are the binding object types for int, long, float, double, and boolean? Are Integer and Double binding properties? Can Integer and Double be used as source objects in a binding? 14.9 Following the JavaFX binding property naming convention, for a binding property named age of the IntegerProperty type, what is its value getter method, value setter method, and property getter method? 14.10 Can you create an object of IntegerProperty using new IntegerProperty(3)? If not, what is the correct way to create it? What will the output if line 8 is replaced by d1.bind(d2.multiply(2)) in Listing 14.6? What will the output if line 8 is replaced by d1.bind(d2.add(2)) in Listing 14.6? 14.11 What is a unidirectional binding and what is bidirectional binding? Are all binding properties capable of bidirectional binding? Write a statement to bind property d1 with property d2 bidirectionally.
14.6 Common Properties and Methods for Nodes 545
14.6 Common Properties and Methods for Nodes The abstract Node class defines many properties and methods that are common to all nodes. Nodes share many common properties. This section introduces two such properties style and rotate. JavaFX style properties are similar to cascading style sheets (CSS) used to specify the styles for HTML elements in a Web page. So, the style properties in JavaFX are called JavaFX CSS. In JavaFX, a style property is defined with a prefix –fx-. Each node has its own style properties. You can find these properties from http://docs.oracle.com/javafx/2/api/javafx/scene/doc-files/ cssref.html. For information on HTML and CSS, see Supplements V.A and V.B. If you are not familiar with HTML and CSS, you can still use JavaFX CSS. The syntax for setting a style is styleName:value. Multiple style properties for a node can be set together separated by semicolon (;). For example, the following statement circle.setStyle("-fx-stroke: black; -fx-fill: red;");
Key Point
JavaFX CSS
setStyle
sets two JavaFX CSS properties for a circle. This statement is equivalent to the following two statements. circle.setStroke(Color.BLACK); circle.setFill(Color.RED);
If an incorrect JavaFX CSS is used, your program will still compile and run, but the style is ignored. The rotate property enables you to specify an angle in degrees for rotating the node from its center. If the degree is positive, the rotation is performed clockwise; otherwise, it is performed counterclockwise. For example, the following code rotates a button 80 degrees. button.setRotate(80);
Listing 14.7 gives an example that creates a button, sets its style, and adds it to a pane. It then rotates the pane 45 degrees and set its style with border color red and background color light gray, as shown in Figure 14.8.
LISTING 14.7 NodeStyleRotateDemo.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
import import import import import
javafx.application.Application; javafx.scene.Scene; javafx.scene.control.Button; javafx.stage.Stage; javafx.scene.layout.StackPane;
public class NodeStyleRotateDemo extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { // Create a scene and place a button in the scene StackPane pane = new StackPane(); Button btOK = new Button("OK"); btOK.setStyle("-fx-border-color: blue;"); pane.getChildren().add(btOK); pane.setRotate(45); pane.setStyle( "-fx-border-color: red; -fx-background-color: lightgray;"); Scene scene = new Scene(pane, 200, 250); primaryStage.setTitle("NodeStyleRotateDemo"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage
rotate the pane set style for pane
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primaryStage.show(); // Display the stage } }
FIGURE 14.8
A pane’s style is set and it is rotated 45 degrees.
As seen in Figure 14.8, the rotate on a pane causes all its containing nodes rotated too. The Node class contains many useful methods that can be applied to all nodes. For example, you can use the contains(double x, double y) method to test where a point (x, y) is inside the boundary of a node.
contains method
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14.12 How do you set a style of a node with border color red? Modify the code to set the text color for the button to red. 14.13 Can you rotate a pane, a text, or a button? Modify the code to rotate the button 15 degrees counterclockwise?
14.7 The Color Class Key Point
The Color class can be used to create colors. JavaFX defines the abstract Paint class for painting a node. The javafx.scene.paint.Color is a concrete subclass of Paint, which is used to encapsulate colors, as shown in Figure 14.9.
The getter methods for property values are provided in the class, but omitted in the UML diagram for brevity.
javafx.scene.paint.Color
The red value of this Color (between 0.0 and 1.0).
-red: double -green: double -blue: double -opacity: double +Color(r: double, g: double, double, opacity: double) +brighter(): Color +darker(): Color +color(r: double, g: double, double): Color +color(r: double, g: double, double, opacity: double):
The green value of this Color (between 0.0 and 1.0). The blue value of this Color (between 0.0 and 1.0). The opacity of this Color (between 0.0 and 1.0). b:
Creates a Color with the specified red, green, blue, and opacity values.
b:
Creates a Color that is a brighter version of this Color. Creates a Color that is a darker version of this Color. Creates an opaque Color with the specified red, green, and blue values.
+rgb(r: int, g: int, b: int): Color +rgb(r: int, g: int, b: int, opacity: double): Color
b: Color
Creates a Color with the specified red, green, blue, and opacity values. Creates a Color with the specified red, green, and blue values in the range from 0 to 255. Creates a Color with the specified red, green, and blue values in the range from 0 to 255 and a given opacity.
FIGURE 14.9 Color encapsulates information about colors.
14.8 The Font Class 547 A color instance can be constructed using the following constructor: public Color(double r, double g, double b, double opacity);
in which r, g, and b specify a color by its red, green, and blue components with values in the range from 0.0 (darkest shade) to 1.0 (lightest shade). The opacity value defines the transparency of a color within the range from 0.0 (completely transparent) to 1.0 (completely opaque). This is known as the RGBA model, where RGBA stands for red, green, blue, and alpha. The alpha value indicates the opacity. For example,
RBGA model
Color color = new Color(0.25, 0.14, 0.333, 0.51);
The Color class is immutable. Once a Color object is created, its properties cannot be changed. The brighter() method returns a new Color with a larger red, green, and blue values and the darker() method returns a new Color with a smaller red, green, and blue values. The opacity value is the same as in the original Color object. You can also create a Color object using the static methods color(r, g, b), color(r, g, b, opacity), rgb(r, g, b), and rgb(r, g, b, opacity). Alternatively, you can use one of the many standard colors such as BEIGE, BLACK, BLUE, BROWN, CYAN, DARKGRAY, GOLD, GRAY, GREEN, LIGHTGRAY, MAGENTA, NAVY, ORANGE, PINK, RED, SILVER, WHITE, and YELLOW defined as constants in the Color class. The following code, for instance, sets the fill color of a circle to red: circle.setFill(Color.RED);
14.14 How do you create a color? What is wrong about creating a
Color using new Color(1.2, 2.3, 3.5, 4)? Which of two colors is darker, new Color(0, 0, 0, 1) or new Color(1, 1, 1, 1)? Does invoking c.darker() change the color value in c?
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14.15 How do you create a Color object with a random color? 14.16 How do you set a circle object c with blue fill color using the setFill method and using the setStyle method?
14.8 The Font Class A Font describes font name, weight, and size. You can set fonts for rendering the text. The javafx.scene.text.Font class is used to create fonts, as shown in Figure 14.10. A Font instance can be constructed using its constructors or using its static methods. A Font is defined by its name, weight, posture, and size. Times, Courier, and Arial are the examples of the font names.You can obtain a list of available font family names by invoking the static getFamilies() method. List is an interface that defines common methods for a list. ArrayList is a concrete implmentation of List. The font postures are two constants: FontPosture.ITALIC and FontPosture.REGULAR. For example, the following statements create two fonts. Font font1 = new Font("SansSerif", 16); Font font2 = Font.font("Times New Roman", FontWeight.BOLD, FontPosture.ITALIC, 12);
Listing 14.8 gives a program that displays a label using the font (Times New Roman, bold, italic, and size 20), as shown in Figure 14.11.
Key Point
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javafx.scene.text.Font -size: double -name: String
The size of this font. The name of this font. The family of this font.
-family: String +Font(size: double) +Font(name: String, size: double) +font(name: String, size: double) +font(name: String, w: FontWeight, size: double) +font(name: String, w: FontWeight, p: FontPosture, size: double)
Creates a Font with the specified size. Creates a Font with the specified full font name and size.
+getFamilies(): List +getFontNames(): List
Returns a list of font family names. Returns a list of full font names including family and weight.
Creates a Font with the specified name and size. Creates a Font with the specified name, weight, and size. Creates a Font with the specified name, weight, posture, and size.
FIGURE 14.10 Font encapsulates information about fonts.
LISTING 14.8 FontDemo.java
create a StackPane
create a Circle
create a Color add circle to the pane
create a label create a font add label to the pane
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
import import import import import import import import
javafx.application.Application; javafx.scene.Scene; javafx.scene.layout.*; javafx.scene.paint.Color; javafx.scene.shape.Circle; javafx.scene.text.*; javafx.scene.control.*; javafx.stage.Stage;
public class FontDemo extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { // Create a pane to hold the circle Pane pane = new StackPane(); // Create a circle and set its properties Circle circle = new Circle(); circle.setRadius(50); circle.setStroke(Color.BLACK); circle.setFill(new Color(0.5, 0.5, 0.5, 0.1)); pane.getChildren().add(circle); // Add circle to the pane // Create a label and set its properties Label label = new Label("JavaFX"); label.setFont(Font.font("Times New Roman", FontWeight.BOLD, FontPosture.ITALIC, 20)); pane.getChildren().add(label); // Create a scene and place it in the stage Scene scene = new Scene(pane); primaryStage.setTitle("FontDemo"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage
14.9 The Image and ImageView Classes 549 33 34 35
primaryStage.show(); // Display the stage } }
FIGURE 14.11
A label is on top of a circle displayed in the center of the scene.
The program creates a StackPane (line 14) and adds a circle and a label to it (lines 21, 27). These two statements can be combined using the following one statement: pane.getChildren().addAll(circle, label);
A StackPane places the nodes in the center and nodes are placed on top of each other. A custom color is created and set as a fill color for the circle (line 20). The program creates a label and sets a font (line 25) so the text in the label is displayed in Times New Roman, bold, italic, and 20 pixels. As you resize the window, the circle and label are displayed in the center of the window, because the circle and label are placed in the stack pane. Stack pane automatically places nodes in the center of the pane. A Font object is immutable. Once a Font object is created, its properties cannot be changed.
14.17 How do you create a
Font object with font name Courier, size 20, and weight
bold?
14.18 How do you find all available fonts on your system?
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Check Point
14.9 The Image and ImageView Classes The Image class represents a graphical image and the ImageView class can be used to display an image. The javafx.scene.image.Image class represents a graphical image and is used for loading an image from a specified filename or a URL. For example, new Image("image/us.gif") creates an Image object for the image file us.gif under the directory image in the Java class directory and new Image("http://www.cs.armstrong.edu/liang/image/us.gif") creates an Image object for the image file in the URL on the Web. The javafx.scene.image.ImageView is a node for displaying an image. An ImageView can be created from an Image object. For example, the following code creates an ImageView from an image file: Image image = new Image("image/us.gif"); ImageView imageView = new ImageView(image);
Alternatively, you can create an ImageView directly from a file or a URL as follows: ImageView imageView = new ImageView("image/us.gif");
The UML diagrams for the Image and ImageView classes are illustrated in Figures 14.12 and 14.13.
Key Point
VideoNote
Use Image and ImageView
550 Chapter 14
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javafx.scene.image.Image -error: ReadOnlyBooleanProperty -height: ReadOnlyBooleanProperty -width: ReadOnlyBooleanProperty
Indicates whether the image is loaded correctly? The height of the image. The width of the image.
-progress: ReadOnlyBooleanProperty
The approximate percentage of image’s loading that is completed.
+Image(filenameOrURL: String)
Creates an Image with contents loaded from a file or a URL.
FIGURE 14.12 Image encapsulates information about images.
The getter and setter methods for property values and a getter for property itself are provided in the class, but omitted in the UML diagram for brevity.
javafx.scene.image.ImageView -fitHeight: DoubleProperty -fitWidth: DoubleProperty
The height of the bounding box within which the image is resized to fit. The width of the bounding box within which the image is resized to fit. The x-coordinate of the ImageView origin. The y-coordinate of the ImageView origin.
-x: DoubleProperty -y: DoubleProperty -image: ObjectProperty
The image to be displayed in the image view.
+ImageView() +ImageView(image: Image) +ImageView(filenameOrURL: String)
Creates an ImageView. Creates an ImageView with the specified image. Creates an ImageView with image loaded from the specified file or URL.
FIGURE 14.13 ImageView is a node for displaying an image. Listing 14.9 displays an image in three image views, as shown in Figure 14.14.
LISTING 14.9 ShowImage.java
create an HBox create an image add an image view to pane create an image view set image view properties
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
import import import import import import import import
javafx.application.Application; javafx.scene.Scene; javafx.scene.layout.HBox; javafx.scene.layout.Pane; javafx.geometry.Insets; javafx.stage.Stage; javafx.scene.image.Image; javafx.scene.image.ImageView;
public class ShowImage extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { // Create a pane to hold the image views Pane pane = new HBox(10); pane.setPadding(new Insets(5, 5, 5, 5)); Image image = new Image("image/us.gif"); pane.getChildren().add(new ImageView(image)); ImageView imageView2 = new ImageView(image); imageView2.setFitHeight(100); imageView2.setFitWidth(100);
14.9 The Image and ImageView Classes 551 22 23 24 25 26 27 28 29 30 31 32 33 34
pane.getChildren().add(imageView2);
add an image to pane
ImageView imageView3 = new ImageView(image); imageView3.setRotate(90); pane.getChildren().add(imageView3);
create an image view rotate an image view add an image to pane
// Create a scene and place it in the stage Scene scene = new Scene(pane); primaryStage.setTitle("ShowImage"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } }
FIGURE 14.14
An image is displayed in three image views placed in a pane.
The program creates an HBox (line 14). An HBox is a pane that places all nodes horizontallly in one row. The program creates an Image, and then an ImageView for displaying the iamge, and places the ImageView in the HBox (line 17). The program creates the second ImageView (line 19), sets its fitHeight and fitWidth properties (lines 20–21) and places the ImageView into the HBox (line 22). The program creates the third ImageView (line 24), rotates it 90 degrees (line 25), and places it into the HBox (line 26). The setRotate method is defined in the Node class and can be used for any node. Note that an Image object can be shared by multiple nodes. In this case, it is shared by three ImageView. However, a node such as ImageView cannot be shared. You cannot place an ImageView multiple times into a pane or scene. Note that you must place the image file in the same directory as the class file, as shown in the following figure. Directory
ShowImage.class
image
us.gif
If you use the URL to locate the image file, the URL protocal http:// must be present. So the following code is wrong. new Image("www.cs.armstrong.edu/liang/image/us.gif");
It must be replaced by new Image("http://www.cs.armstrong.edu/liang/image/us.gif");
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Check Point
14.19 How do you create an Image from a URL or a filename? 14.20 How do you create an ImageView from an Image, or directly from a file or a URL? 14.21 Can you set an Image to multiple ImageView? Can you display the same ImageView multiple times?
14.10 Layout Panes Key Point
VideoNote
Use layout panes
JavaFX provides many types of panes for automatically laying out nodes in a desired location and size. JavaFX provides many types of panes for organizing nodes in a container, as shown in Table 14.1. You have used the layout panes Pane, StackPane, and HBox in the preceding sections for containing nodes. This section introduces the panes in more details.
TABLE 14.1 Panes for Containing and Organizing Nodes
ObservableList getChildren()
Class
Description
Pane
Base class for layout panes. It contains the getChildren() method for returning a list of nodes in the pane.
StackPane
Places the nodes on top of each other in the center of the pane.
FlowPane
Places the nodes row-by-row horizontally or column-by-column vertically.
GridPane
Places the nodes in the cells in a two-dimensional grid.
BorderPane
Places the nodes in the top, right, bottom, left, and center regions.
HBox
Places the nodes in a single row.
VBox
Places the nodes in a single column.
You have used the Pane in Listing 14.4, ShowCircle.java. A Pane is usually used as a canvas for displaying shapes. Pane is the base class for all specialized panes. You have used a specialized pane StackPane in Listing 14.3, ButtonInPane.java. Nodes are placed in the center of a StackPane. Each pane contains a list for holding nodes in the pane. This list is an instance of ObservableList, which can be obtained using pane’s getChildren() method. You can use the add(node) method to add an element to the list, use addAll(node1, node2, ...) to add a variable number of nodes to the pane.
14.10.1 FlowPane FlowPane arranges the nodes in the pane horizontally from left to right or vertically from top to bottom in the order in which they were added. When one row or one column is filled, a new row or column is started. You can specify the way the nodes are placed horizontally or vertically using one of two constants: Orientation.HORIZONTAL or Orientation.VERTICAL. You can also specify the gap between the nodes in pixels. The class diagram for FlowPane is shown in Figure 14.15. Data fields alignment, orientation, hgap, and vgap are binding properties. Each binding property in JavaFX has a getter method (e.g., getHgap()) that returns its value, a setter method (e.g., sethGap(double)) for setting a value, and a getter method that returns the property itself (e.g., hGapProperty()). For a data field of ObjectProperty type, the value getter method returns a value of type T and the property getter method returns a property value of type ObjectProperty.
14.10 Layout Panes 553 The getter and setter methods for property values and a getter for property itself are provided in the class, but omitted in the UML diagram for brevity.
javafx.scene.layout.FlowPane -alignment: ObjectProperty
The overall alignment of the content in this pane (default: Pos.LEFT).
-orientation: ObjectProperty
The orientation in this pane (default: Orientation.HORIZONTAL).
-hgap: DoubleProperty -vgap: DoubleProperty
The horizontal gap between the nodes (default: 0). The vertical gap between the nodes (default: 0).
+FlowPane() +FlowPane(hgap: double, vgap: double) +FlowPane(orientation: ObjectProperty) +FlowPane(orientation: ObjectProperty, hgap: double, vgap: double
Creates a default FlowPane. Creates a FlowPane with a specified horizontal and vertical gap. Creates a FlowPane with a specified orientation. Creates a FlowPane with a specified orientation, horizontal gap and vertical gap.
FIGURE 14.15 FlowPane lays out nodes row by row horizontally or column by column vertically. Listing 14.10 gives a program that demonstrates FlowPane. The program adds labels and text fields to a FlowPane, as shown in Figure 14.16.
LISTING 14.10 ShowFlowPane.java 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
import import import import import import import
javafx.application.Application; javafx.geometry.Insets; javafx.scene.Scene; javafx.scene.control.Label; javafx.scene.control.TextField; javafx.scene.layout.FlowPane; javafx.stage.Stage;
public class ShowFlowPane extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { // Create a pane and set its properties FlowPane pane = new FlowPane(); pane.setPadding(new Insets(11, 12, 13, 14)); pane.setHgap(5); pane.setVgap(5); // Place nodes in the pane pane.getChildren().addAll(new Label("First Name:"), new TextField(), new Label("MI:")); TextField tfMi = new TextField(); tfMi.setPrefColumnCount(1); pane.getChildren().addAll(tfMi, new Label("Last Name:"), new TextField()); // Create a scene and place it in the stage Scene scene = new Scene(pane, 200, 250); primaryStage.setTitle("ShowFlowPane"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } }
extend Application
create FlowPane
add UI controls to pane
add pane to scene place scene to stage display stage
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(a)
FIGURE 14.16
(b)
The nodes fill in the rows in the FlowPane one after another.
The program creates a FlowPane (line 13) and sets its padding property with an Insets object (line 14). An Insets object specifies the size of the border of a pane. The constructor Insets(11, 12, 13, 14) creates an Insets with the border sizes for top (11), right (12), bottom (13), and left (14) in pixels, as shown in Figure 14.17. You can also use the constructor Insets(value) to create an Insets with the same value for all four sides. The hGap and vGap properties are in lines 15–16 to specify the horizontal gap and vertical gap between two nodes in the pane, as shown in Figure 14.17. hGap
Top side Pane
vGap
Right side
Left side
Bottom side
FIGURE 14.17
You can specify hGap and vGap between the nodes in a FlowLPane.
Each FlowPane contains an object of ObservableList for holding the nodes. This list can be obtained using the getChildren() method (line 19). To add a node into a FlowPane is to add it to this list using the add(node) or addAll(node1, node2, ...) method. You can also remove a node from the list using the remove(node) method or use the removeAll() method to remove all nodes from the pane. The program adds the labels and text fields into the pane (lines 19–24). Invoking tfMi.setPrefColumnCount(1) sets the preferred column count to 1 for the MI text field (line 22). The program declares an explicit reference tfMi for a TextField object for MI. The explicit reference is necessary, because we need to reference the object directly to set its prefColumnCount property. The program adds the pane to the scene (line 27), sets the scene in the stage (line 29), and displays the stage (line 30). Note that if you resize the window, the nodes are automatically rearranged to fit in the pane. In Figure 14.16a, the first row has three nodes, but in Figure 14.16b, the first row has four nodes, because the width has been increased. Suppose you wish to add the object tfMi to a pane ten times; will ten text fields appear in the pane? No, a node such as a text field can be added to only one pane and once. Adding a node to a pane multiple times or to different panes will cause a runtime error.
Note A node can be placed only in one pane. Therefore, the relationship between a pane and a node is the composition denoted by a filled diamond, as shown in Figure 14.3b.
14.10 Layout Panes 555
14.10.2 GridPane A GridPane arranges nodes in a grid (matrix) formation. The nodes are placed in the specified column and row indices. The class diagram for GridPane is shown in Figure 14.18.
The getter and setter methods for property values and a getter for property itself are provided in the class, but omitted in the UML diagram for brevity.
javafx.scene.layout.GridPane -alignment: ObjectProperty -gridLinesVisible: BooleanProperty -hgap: DoubleProperty -vgap: DoubleProperty
The overall alignment of the content in this pane (default: Pos.LEFT). Is the grid line visible? (default: false) The horizontal gap between the nodes (default: 0). The vertical gap between the nodes (default: 0).
+GridPane() +add(child: Node, columnIndex: int, rowIndex: int): void +addColumn(columnIndex: int, children: Node...): void
Creates a GridPane.
+addRow(rowIndex: int, children: Node...): void +getColumnIndex(child: Node): int
Adds multiple nodes to the specified row.
+setColumnIndex(child: Node, columnIndex: int): void +getRowIndex(child:Node): int
Sets a node to a new column. This method repositions the node.
+setRowIndex(child: Node, rowIndex: int): void +setHalighnment(child: Node, value: HPos): void +setValighnment(child: Node, value: VPos): void
Adds a node to the specified column and row. Adds multiple nodes to the specified column.
Returns the column index for the specified node.
Returns the row index for the specified node. Sets a node to a new row. This method repositions the node. Sets the horizontal alignment for the child in the cell. Sets the vertical alignment for the child in the cell.
FIGURE 14.18 GridPane lays out nodes in the specified cell in a grid. Listing 14.11 gives a program that demonstrates GridPane. The program is similar to the one in Listing 14.10, except that it adds three labels and three text fields, and a button to the specified location in a grid, as shown in Figure 14.19.
FIGURE 14.19 indices.
The GridPane places the nodes in a grid with a specified column and row
LISTING 14.11 ShowGridPane.java 1 2 3
import javafx.application.Application; import javafx.geometry.HPos; import javafx.geometry.Insets;
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create a grid pane set properties
add label add text field
add button align button right
create a scene
display stage
remove nodes
JavaFX Basics 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
import import import import import import import
javafx.geometry.Pos; javafx.scene.Scene; javafx.scene.control.Button; javafx.scene.control.Label; javafx.scene.control.TextField; javafx.scene.layout.GridPane; javafx.stage.Stage;
public class ShowGridPane extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { // Create a pane and set its properties GridPane pane = new GridPane(); pane.setAlignment(Pos.CENTER); pane.setPadding(new Insets(11.5, 12.5, 13.5, 14.5)); pane.setHgap(5.5); pane.setVgap(5.5); // Place nodes in the pane pane.add(new Label("First Name:"), 0, 0); pane.add(new TextField(), 1, 0); pane.add(new Label("MI:"), 0, 1); pane.add(new TextField(), 1, 1); pane.add(new Label("Last Name:"), 0, 2); pane.add(new TextField(), 1, 2); Button btAdd = new Button("Add Name"); pane.add(btAdd, 1, 3); GridPane.setHalignment(btAdd, HPos.RIGHT); // Create a scene and place it in the stage Scene scene = new Scene(pane); primaryStage.setTitle("ShowGridPane"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } }
The program creates a GridPane (line 16) and sets its properties (line 17–20). The alignment is set to the center position (line 17), which causes the nodes to be placed in the center of the grid pane. If you resize the window, you will see the nodes remains in the center of the grid pane. The program adds the label in column 0 and row 0 (line 23). The column and row index starts from 0. The add method places a node in the specified column and row. Not every cell in the grid needs to be filled. A button is placed in column 1 and row 3 (line 30), but there are no nodes placed in column 0 and row 3. To remove a node from a GridPane, use pane. getChildren().remove(node). To remove all nodes, use pane.getChildren(). removeAll(). The program invokes the static setHalignment method to align the button right in the cell (line 31). Note that the scene size is not set (line 34). In this case, the scene size is automatically computed according to the sizes of the nodes placed inside the scene.
14.10.3 BorderPane A BorderPane can place nodes in five regions: top, bottom, left, right, and center, using the setTop(node), setBottom(node), setLeft(node), setRight(node), and setCenter(node) methods. The class diagram for GridPane is shown in Figure 14.20.
14.10 Layout Panes 557 The getter and setter methods for property values and a getter for property itself are provided in the class, but omitted in the UML diagram for brevity.
javafx.scene.layout.BorderPane -top: ObjectProperty -right: ObjectProperty -bottom: ObjectProperty
The node placed in the top region (default: null). The node placed in the right region (default: null).
-left: ObjectProperty
The node placed in the bottom region (default: null). The node placed in the left region (default: null).
-center: ObjectProperty
The node placed in the center region (default: null).
+BorderPane() +setAlignment(child: Node, pos: Pos)
Creates a BorderPane. Sets the alignment of the node in the BorderPane.
FIGURE 14.20 BorderPane places the nodes in top, bottom, left, right, and center regions. Listing 14.12 gives a program that demonstrates BorderPane. The program places five buttons in the five regions of the pane, as shown in Figure 14.21.
LISTING 14.12 ShowBorderPane.java 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
import import import import import import import
javafx.application.Application; javafx.geometry.Insets; javafx.scene.Scene; javafx.scene.control.Label; javafx.scene.layout.BorderPane; javafx.scene.layout.StackPane; javafx.stage.Stage;
public class ShowBorderPane extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { // Create a border pane BorderPane pane = new BorderPane(); // Place nodes in the pane pane.setTop(new CustomPane("Top")); pane.setRight(new CustomPane("Right")); pane.setBottom(new CustomPane("Bottom")); pane.setLeft(new CustomPane("Left")); pane.setCenter(new CustomPane("Center"));
create a border pane
add to top add to right add to bottom add to left add to center
// Create a scene and place it in the stage Scene scene = new Scene(pane); primaryStage.setTitle("ShowBorderPane"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } } // Define a custom pane to hold a label in the center of the pane class CustomPane extends StackPane { public CustomPane(String title) { getChildren().add(new Label(title)); setStyle("-fx-border-color: red"); setPadding(new Insets(11.5, 12.5, 13.5, 14.5)); } }
define a custom pane add a label to pane set style set padding
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FIGURE 14.21
The BorderPane places the nodes in five regions of the pane.
The program defines CustomPane that extends StackPane (line 31). The constructor of CustomPane adds a label with the specified title (line 33), sets a style for the border color, and sets a padding using insets (line 35). The program creates a BorderPane (line 13) and places five instances of CustomPane into five regions of the border pane (lines 16–20). Note that a pane is a node. So a pane can be added into another pane. To remove a node from the top region, invoke setTop(null). If a region is not occupied, no space will be allocated for this region.
14.10.4 HBox and VBox An HBox lays out its children in a single horizontal row. A VBox lays out its children in a single vertical column. Recall that a FlowPane can lay out its children in multiple rows or multiple columns, but an HBox or a VBox can lay out children only in one row or one column. The class diagrams for HBox and VBox are shown in Figures 14.22 and 14.23.
The getter and setter methods for property values and a getter for property itself are provided in the class, but omitted in the UML diagram for brevity.
javafx.scene.layout.HBox -alignment: ObjectProperty -fillHeight: BooleanProperty -spacing: DoubleProperty
The overall alignment of the children in the box (default: Pos.TOP_LEFT).
+HBox() +HBox(spacing: double) +setMargin(node: Node, value: Insets): void
Creates a default HBox. Creates an HBox with the specified horizontal gap between nodes. Sets the margin for the node in the pane.
Is resizable children fill the full height of the box (default: true). The horizontal gap between two nodes (default: 0).
FIGURE 14.22 HBox places the nodes in one row.
javafx.scene.layout.VBox
The getter and setter methods for property values and a getter for property itself are provided in the class, but omitted in the UML diagram for brevity.
-alignment: ObjectProperty -fillWidth: BooleanProperty -spacing: DoubleProperty
The overall alignment of the children in the box (default: Pos.TOP_LEFT). Is resizable children fill the full width of the box (default: true). The vertical gap between two nodes (default: 0).
+VBox() +VBox(spacing: double) +setMargin(node: Node, value: Insets): void
Creates a default VBox. Creates a VBox with the specified horizontal gap between nodes. Sets the margin for the node in the pane.
FIGURE 14.23 VBox places the nodes in one column.
14.10 Layout Panes 559 Listing 14.12 gives a program that demonstrates HBox and VBox. The program places two buttons in an HBox and five labels in a VBox, as shown in Figure 14.24.
LISTING 14.13 ShowHBoxVBox.java 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 54 55 56
import import import import import import import import import import import
javafx.application.Application; javafx.geometry.Insets; javafx.scene.Scene; javafx.scene.control.Button; javafx.scene.control.Label; javafx.scene.layout.BorderPane; javafx.scene.layout.HBox; javafx.scene.layout.VBox; javafx.stage.Stage; javafx.scene.image.Image; javafx.scene.image.ImageView;
public class ShowHBoxVBox extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { // Create a border pane BorderPane pane = new BorderPane(); // Place nodes in the pane pane.setTop(getHBox()); pane.setLeft(getVBox()); // Create a scene and place it in the stage Scene scene = new Scene(pane); primaryStage.setTitle("ShowHBoxVBox"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage
create a border pane
add an HBox to top add a VBox to left
create a scene
display stage
} private HBox getHBox() { HBox hBox = new HBox(15); hBox.setPadding(new Insets(15, 15, 15, 15)); hBox.setStyle("-fx-background-color: gold"); hBox.getChildren().add(new Button("Computer Science")); hBox.getChildren().add(new Button("Chemistry")); ImageView imageView = new ImageView(new Image("image/us.gif")); hBox.getChildren().add(imageView); return hBox; }
getHBox
private VBox getVBox() { VBox vBox = new VBox(15); vBox.setPadding(new Insets(15, 5, 5, 5)); vBox.getChildren().add(new Label("Courses"));
getVBox
add buttons to HBox
return an HBox
add a label
Label[] courses = {new Label("CSCI 1301"), new Label("CSCI 1302"), new Label("CSCI 2410"), new Label("CSCI 3720")};
} }
for (Label course: courses) { VBox.setMargin(course, new Insets(0, 0, 0, 15)); vBox.getChildren().add(course); }
set margin add a label
return vBox;
return vBox
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FIGURE 14.24 column.
The HBox places the nodes in one row and the VBox places the nodes in one
The program defines the getHBox() method. This method returns an HBox that contains two buttons and an image view (lines 30–39). The background color of the HBox is set to gold using Java CSS (line 33). The program defines the getVBox() method. This method returns a VBox that contains five labels (lines 41–55). The first label is added to the VBox in line 44 and the other four are added in line 51. The setMargin method is used to set a node’s margin when placed inside the VBox (line 50).
✓
Check Point
14.22 How do you add a node to a Pane, StackPane, FlowPane, GridPane, BorderPane, HBox, and VBox? How do you remove a node from these panes?
14.23 How do you set the alignment to right for nodes in a FlowPane, GridPane, HBox, and VBox?
14.24 How do you set the horizontal gap and vertical hap between nodes in 8 pixels in a FlowPane and GridPane and set spacing in 8 pixels in an HBox and VBox?
14.25 How do you get the column and row index of a node in a GridPane? How do you reposition a node in a GridPane?
14.26 What are the differences between a FlowPane and an HBox or a VBox?
14.11 Shapes Key Point
VideoNote
Use shapes
JavaFX provides many shape classes for drawing texts, lines, circles, rectangles, ellipses, arcs, polygons, and polylines. The Shape class is the abstract base class that defines the common properties for all shapes. Among them are the fill, stroke, and strokeWidth properties. The fill property specifies a color that fills the interior of a shape. The stroke property specifies a color that is used to draw the outline of a shape. The strokeWidth property specifies the width of the outline of a shape. This section introduces the classes Text, Line, Rectangle, Circle, Ellipse, Arc, Polygon, and Polyline for drawing texts and simple shapes. All these are subclasses of Shape, as shown in Figure 14.25.
14.11.1 Text The Text class defines a node that displays a string at a starting point (x, y), as shown in Figure 14.27a. A Text object is usually placed in a pane. The pane’s upper-left corner point is (0, 0) and the bottom-right point is (pane.getWidth(), pane.getHeight()). A string may be displayed in multiple lines separated by \n. The UML diagram for the Text class is shown in Figure 14.26. Listing 14.13 gives an example that demonstrates text, as shown in Figure 14.27b.
14.11 Shapes 561 Node
Text
Shape
Line Rectangle Circle Ellipse Arc Polygon Polyline
FIGURE 14.25
A shape is a node. The Shape class is the root of all shape classes.
The getter and setter methods for property values and a getter for property itself are provided in the class, but omitted in the UML diagram for brevity.
javafx.scene.text.Text -text: StringProperty -x: DoubleProperty -y: DoubleProperty -underline: BooleanProperty -strikethrough: BooleanProperty -font: ObjectProperty +Text() +Text(text: String) +Text(x: double, y: double, text: String)
Defines the text to be displayed. Defines the x-coordinate of text (default 0). Defines the y-coordinate of text (default 0). Defines if each line has an underline below it (default false). Defines if each line has a line through it (default false). Defines the font for the text. Creates an empty Text. Creates a Text with the specified text. Creates a Text with the specified x-, y-coordinates and text.
FIGURE 14.26 Text defines a node for displaying a text.
(getWidth(), 0)
(0, 0)
(x, y)
text is displayed
(0, getHeight())
(getWidth(), getHeight())
(a) Text(x, y, text)
FIGURE 14.27
A Text object is created to display a text.
LISTING 14.14 ShowText.java 1 2 3 4 5
import import import import import
javafx.application.Application; javafx.scene.Scene; javafx.scene.layout.Pane; javafx.scene.paint.Color; javafx.geometry.Insets;
(b) Three Text objects are displayed
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create a pane create a text set text font add text to pane create a two-line text add text to pane create a text set text color set underline set strike line add text to pane
JavaFX Basics 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
import import import import import
javafx.stage.Stage; javafx.scene.text.Text; javafx.scene.text.Font; javafx.scene.text.FontWeight; javafx.scene.text.FontPosture;
public class ShowText extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { // Create a pane to hold the texts Pane pane = new Pane(); pane.setPadding(new Insets(5, 5, 5, 5)); Text text1 = new Text(20, 20, "Programming is fun"); text1.setFont(Font.font("Courier", FontWeight.BOLD, FontPosture.ITALIC, 15)); pane.getChildren().add(text1); Text text2 = new Text(60, 60, "Programming is fun\nDisplay text"); pane.getChildren().add(text2); Text text3 = new Text(10, 100, "Programming is fun\nDisplay text"); text3.setFill(Color.RED); text3.setUnderline(true); text3.setStrikethrough(true); pane.getChildren().add(text3); // Create a scene and place it in the stage Scene scene = new Scene(pane); primaryStage.setTitle("ShowText"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } }
The program creates a Text (line 18), sets its font (line 19), and places it to the pane (line 21). The program creates another Text with multiple lines (line 23) and places it to the pane (line 24). The program creates the third Text (line 26), sets its color (line 27), sets an underline and a strike through line (lines 28–29), and places it to the pane (line 30).
14.11.2 Line A line connects two points with four parameters startX, startY, endX, and endY, as shown in Figure 14.29a. The Line class defines a line. The UML diagram for the Line class is shown in Figure 14.28. Listing 14.15 gives an example that demonstrates text, as shown in Figure 14.29b.
LISTING 14.15 ShowLine.java 1 2 3 4 5 6 7 8 9 10 11
import import import import import import
javafx.application.Application; javafx.scene.Scene; javafx.scene.layout.Pane; javafx.scene.paint.Color; javafx.stage.Stage; javafx.scene.shape.Line;
public class ShowLine extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { // Create a scene and place it in the stage
14.11 Shapes 563 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35
Scene scene = new Scene(new LinePane(), 200, 200); primaryStage.setTitle("ShowLine"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage
create a pane in scene
} } class LinePane extends Pane { public LinePane() { Line line1 = new Line(10, 10, 10, 10); line1.endXProperty().bind(widthProperty().subtract(10)); line1.endYProperty().bind(heightProperty().subtract(10)); line1.setStrokeWidth(5); line1.setStroke(Color.GREEN); getChildren().add(line1); Line line2 = new Line(10, 10, 10, 10); line2.startXProperty().bind(widthProperty().subtract(10)); line2.endYProperty().bind(heightProperty().subtract(10)); line2.setStrokeWidth(5); line2.setStroke(Color.GREEN); getChildren().add(line2);
define a custom pane create a line
set stroke width set stroke add line to pane create a line
add line to pane
} }
The getter and setter methods for property values and a getter for property itself are provided in the class, but omitted in the UML diagram for brevity.
javafx.scene.shape.Line -startX: DoubleProperty -startY: DoubleProperty -endX: DoubleProperty -endY: DoubleProperty +Line() +Line(startX: double, startY: double, endX: double, endY: double)
FIGURE 14.28
The x-coordinate of the start point. The y-coordinate of the start point. The x-coordinate of the end point. The y-coordinate of the end point. Creates an empty Line. Creates a Line with the specified starting and ending points.
The Line class defines a line.
(0, 0)
(getWidth(), 0)
(startX, startY)
(endX, endY) (0, getHeight())
(getWidth(), getHeight())
(a) Line(startX, startY, endX, endY)
FIGURE 14.29
(b) Two lines are displayed across the pane.
A Line object is created to display a line.
The program defines a custom pane class named LinePane (line 19). The custom pane class creates two lines and binds the starting and ending points of the line with the width and height of the pane (lines 22–23, 29–30) so that the two points of the lines are changed as the pane is resized.
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14.11.3 Rectangle A rectangle is defined by the parameters x, y, width, height, arcWidth, and arcHeight, as shown in Figure 14.31a. The rectangle’s upper-left corner point is at (x, y) and parameter aw (arcWidth) is the horizontal diameter of the arcs at the corner, and ah (arcHeight) is the vertical diameter of the arcs at the corner. The Rectangle class defines a rectangle. The UML diagram for the Rectangle class is shown in Figure 14.30. Listing 14.15 gives an example that demonstrates rectangles, as shown in Figure 14.31b.
The getter and setter methods for property values and a getter for property itself are provided in the class, but omitted in the UML diagram for brevity.
javafx.scene.shape.Rectangle -x: DoubleProperty
The x-coordinate of the upper-left corner of the rectangle (default 0). The y-coordinate of the upper-left corner of the rectangle (default 0). The width of the rectangle (default: 0). The height of the rectangle (default: 0).
-y:DoubleProperty -width: DoubleProperty -height: DoubleProperty -arcWidth: DoubleProperty
The arcWidth of the rectangle (default: 0). arcWidth is the horizontal diameter of the arcs at the corner (see Figure 14.31a). The arcHeight of the rectangle (default: 0). arcHeight is the vertical diameter of the arcs at the corner (see Figure 14.31a).
-arcHeight: DoubleProperty +Rectangle() +Rectanlge(x: double, y: double, width: double, height: double)
FIGURE 14.30
Creates an empty Rectangle. Creates a Rectangle with the specified upper-left corner point, width, and height.
The Rectangle class defines a rectangle.
height
aw/2 (x, y) ah/2
width (a) Rectangle(x, y, w, h)
FIGURE 14.31
(b) Multiple rectangles are displayed
(c) Transparent rectangles are displayed
A Rectangle object is created to display a rectangle.
LISTING 14.16 ShowRectangle.java 1 2 3 4 5 6 7 8 9 10 11 12
import import import import import import import
javafx.application.Application; javafx.scene.Scene; javafx.scene.layout.Pane; javafx.scene.paint.Color; javafx.stage.Stage; javafx.scene.text.Text; javafx.scene.shape.Rectangle;
public class ShowRectangle extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { // Create a pane
14.11 Shapes 565 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
Pane pane = new Pane(); // Create rectangles and add to pane Rectangle r1 = new Rectangle(25, 10, 60, 30); r1.setStroke(Color.BLACK); r1.setFill(Color.WHITE); pane.getChildren().add(new Text(10, 27, "r1")); pane.getChildren().add(r1); Rectangle r2 = new Rectangle(25, 50, 60, 30); pane.getChildren().add(new Text(10, 67, "r2")); pane.getChildren().add(r2); Rectangle r3 = new Rectangle(25, 90, 60, 30); r3.setArcWidth(15); r3.setArcHeight(25); pane.getChildren().add(new Text(10, 107, "r3")); pane.getChildren().add(r3); for (int i = 0; i < 4; i++) { Rectangle r = new Rectangle(100, 50, 100, 30); r.setRotate(i * 360 / 8); r.setStroke(Color.color(Math.random(), Math.random(), Math.random())); r.setFill(Color.WHITE); pane.getChildren().add(r); } // Create a scene and place it in the stage Scene scene = new Scene(pane, 250, 150); primaryStage.setTitle("ShowRectangle"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } }
The program creates multiple rectangles. By default, the fill color is black. So a rectangle is filled with black color. The stroke color is white by default. Line 17 sets stroke color of rectangle r1 to black. The program creates rectangle r3 (line 26) and sets its arc width and arc height (lines 27–28). So r3 is displayed as a rounded rectangle. The program repeatedly creates a rectangle (line 33), rotates it (line 34), sets a random stroke color (lines 35–36), its fill color to white (line 37), and adds the rectangle to the pane (line 38). If line 37 is replaced by the following line r.setFill(null);
the rectangle is not filled with a color. So they are displayed as shown in Figure 14.31c.
14.11.4 Circle and Ellipse You have used circles in several examples early in this chapter. A circle is defined by its parameters centerX, centerY, and radius. The Circle class defines a circle. The UML diagram for the Circle class is shown in Figure 14.32. An ellipse is defined by its parameters centerX, centerY, radiusX, and radiusY, as shown in Figure 14.34a. The Ellipse class defines an ellipse. The UML diagram for the Ellipse class is shown in Figure 14.33. Listing 14.17 gives an example that demonstrates ellipses, as shown in Figure 14.34b.
create a pane
create a rectangle r1 set r1’s properties
add r1 to pane create rectangle r2 add r2 to pane create rectangle r3 set r3’s arc width set r3’s arc height
create a rectangle rotate a rectangle
add rectangle to pane
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JavaFX Basics The getter and setter methods for property values and a getter for property itself are provided in the class, but omitted in the UML diagram for brevity.
javafx.scene.shape.Circle -centerX: DoubleProperty
The x-coordinate of the center of the circle (default 0).
-centerY: DoubleProperty -radius: DoubleProperty
The y-coordinate of the center of the circle (default 0).
+Circle()
Creates an empty Circle.
+Circle(x: double, y: double) +Circle(x: double, y: double, radius: double)
Creates a Circle with the specified center. Creates a Circle with the specified center and radius.
FIGURE 14.32
The radius of the circle (default: 0).
The Circle class defines circles. The getter and setter methods for property values and a getter for property itself are provided in the class, but omitted in the UML diagram for brevity.
javafx.scene.shape.Ellipse -centerX: DoubleProperty -centerY: DoubleProperty
The x-coordinate of the center of the ellipse (default 0).
-radiusX: DoubleProperty -radiusY: DoubleProperty
The y-coordinate of the center of the ellipse (default 0). The horizontal radius of the ellipse (default: 0). The vertical radius of the ellipse (default: 0).
+Ellipse() +Ellipse(x: double, y: double)
Creates an empty Ellipse. Creates an Ellipse with the specified center.
+Ellipse(x: double, y: double, radiusX: double, radiusY: double)
Creates an Ellipse with the specified center and radiuses.
FIGURE 14.33
The Ellipse class defines ellipses.
radiusX
radiusY
(centerX, centerY)
(a) Ellipse(centerX, centerY, radiusX, radiusY)
FIGURE 14.34
(b) Multiple ellipses are displayed.
An Ellipse object is created to display an ellipse.
LISTING 14.17 ShowEllipse.java 1 2 3 4 5 6 7 8 9
import import import import import import
javafx.application.Application; javafx.scene.Scene; javafx.scene.layout.Pane; javafx.scene.paint.Color; javafx.stage.Stage; javafx.scene.shape.Ellipse;
public class ShowEllipse extends Application { @Override // Override the start method in the Application class
14.11 Shapes 567 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
public void start(Stage primaryStage) { // Create a pane Pane pane = new Pane(); for (int i = 0; i < 16; i++) { // Create an ellipse and add it to pane Ellipse e1 = new Ellipse(150, 100, 100, 50); e1.setStroke(Color.color(Math.random(), Math.random(), Math.random())); e1.setFill(Color.WHITE); e1.setRotate(i * 180 / 16); pane.getChildren().add(e1); }
create a pane
create an ellipse set random color for stroke set fill color rotate ellipse add ellipse to pane
// Create a scene and place it in the stage Scene scene = new Scene(pane, 300, 200); primaryStage.setTitle("ShowEllipse"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } }
The program repeatedly creates an ellipse (line 16), sets a random stroke color (lines 17–18), sets its fill color to white (line 19), rotates it (line 20), and adds the rectangle to the pane (line 21).
14.11.5 Arc An arc is conceived as part of an ellipse, defined by the parameters centerX, centerY, radiusX, radiusY, startAngle, length, and an arc type (ArcType.OPEN, ArcType .CHORD, or ArcType.ROUND). The parameter startAngle is the starting angle; and length is the spanning angle (i.e., the angle covered by the arc). Angles are measured in degrees and follow the usual mathematical conventions (i.e., 0 degrees is in the easterly direction, and positive angles indicate counterclockwise rotation from the easterly direction), as shown in Figure 14.36a. The Arc class defines an arc. The UML diagram for the Arc class is shown in Figure 14.35. Listing 14.18 gives an example that demonstrates ellipses, as shown in Figure 14.36b.
javafx.scene.shape.Arc
The getter and setter methods for property values and a getter for property itself are provided in the class, but omitted in the UML diagram for brevity.
-centerX: DoubleProperty -centerY: DoubleProperty -radiusX: DoubleProperty -radiusY: DoubleProperty -startAngle: DoubleProperty -length: DoubleProperty -type: ObjectProperty
The x-coordinate of the center of the ellipse (default 0). The y-coordinate of the center of the ellipse (default 0). The horizontal radius of the ellipse (default: 0). The vertical radius of the ellipse (default: 0). The start angle of the arc in degrees. The angular extent of the arc in degrees. The closure type of the arc (ArcType.OPEN, ArcType.CHORD, ArcType.ROUND).
+Arc() +Arc(x: double, y: double, radiusX: double, radiusY: double, startAngle: double, length: double)
Creates an empty Arc. Creates an Arc with the specified arguments.
FIGURE 14.35 The Arc class defines an arc.
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radiusX
radiusY
length startAngle 0 degree
(centerX, centerY) (a) Arc(centerX, centerY, radiusX,
(b) Multiple ellipses are displayed
radiusY, startAngle, length)
FIGURE 14.36
An Arc object is created to display an arc.
LISTING 14.18 ShowArc.java
create a pane create arc1 set fill color for arc1 set arc1 as round arc add arc1 to pane create arc2 set fill color for arc2 set arc2 as round arc
add arc2 to pane create arc3 set fill color for arc3 set arc3 as chord arc
add arc3 to pane create arc4
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
import import import import import import import import
javafx.application.Application; javafx.scene.Scene; javafx.scene.layout.Pane; javafx.scene.paint.Color; javafx.stage.Stage; javafx.scene.shape.Arc; javafx.scene.shape.ArcType; javafx.scene.text.Text;
public class ShowArc extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { // Create a pane Pane pane = new Pane(); Arc arc1 = new Arc(150, 100, 80, 80, 30, 35); // Create an arc arc1.setFill(Color.RED); // Set fill color arc1.setType(ArcType.ROUND); // Set arc type pane.getChildren().add(new Text(210, 40, "arc1: round")); pane.getChildren().add(arc1); // Add arc to pane Arc arc2 = new Arc(150, 100, 80, 80, 30 + 90, 35); arc2.setFill(Color.WHITE); arc2.setType(ArcType.OPEN); arc2.setStroke(Color.BLACK); pane.getChildren().add(new Text(20, 40, "arc2: open")); pane.getChildren().add(arc2); Arc arc3 = new Arc(150, 100, 80, 80, 30 + 180, 35); arc3.setFill(Color.WHITE); arc3.setType(ArcType.CHORD); arc3.setStroke(Color.BLACK); pane.getChildren().add(new Text(20, 170, "arc3: chord")); pane.getChildren().add(arc3); Arc arc4 = new Arc(150, 100, 80, 80, 30 + 270, 35); arc4.setFill(Color.GREEN); arc4.setType(ArcType.CHORD);
14.11 Shapes 569 39 40 41 42 43 44 45 46 47 48 49
arc4.setStroke(Color.BLACK); pane.getChildren().add(new Text(210, 170, "arc4: chord")); pane.getChildren().add(arc4);
add arc4 to pane
// Create a scene and place it in the stage Scene scene = new Scene(pane, 300, 200); primaryStage.setTitle("ShowArc"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } }
The program creates an arc arc1 centered at (150, 100) with radiusX 80 and radiusY 80. The starting angle is 30 with length 35 (line 15). arc1’s arc type is set to ArcType.ROUND (line 18). Since arc1’s fill color is red, arc1 is displayed filled with red round. The program creates an arc arc3 centered at (150, 100) with radiusX 80 and radiusY 80. The starting angle is 30+180 with length 35 (line 29). Arc3’s arc type is set to ArcType. CHORD (line 31). Since arc3’s fill color is white and stroke color is black, arc3 is displayed with black outline as a chord. Angles may be negative. A negative starting angle sweeps clockwise from the easterly direction, as shown in Figure 14.37. A negative spanning angle sweeps clockwise from the starting angle. The following two statements define the same arc: new Arc(x, y, radiusX, radiusY, -30, -20); new Arc(x, y, radiusX, radiusY, -50, 20);
The first statement uses negative starting angle -30 and negative spanning angle -20, as shown in Figure 14.37a. The second statement uses negative starting angle -50 and positive spanning angle 20, as shown in Figure 14.37b.
–30
–50
–20
20
(a) Negative starting angle –30 and negative spanning angle –20
(b) Negative starting angle –50 and positive spanning angle 20
FIGURE 14.37 Angles may be negative.
Note that the trigonometric methods in the Math class use the angles in radians, but the angles in the Arc class are in degrees.
14.11.6 Polygon and Polyline The Polygon class defines a polygon that connects a sequence of points, as shown in Figure 14.38a. The Polyline class is similar to the Polygon class except that the Polyline class is not automatically closed, as shown in Figure 14.38b.
negative degrees
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(40, 20)
(40, 20) (70, 40)
(70, 40) (45, 45)
(45, 45)
(20, 60)
(20, 60)
(60, 80)
(60, 80) (a) Polygon
(b) Polyline
FIGURE 14.38 Polygon is closed and Polyline is not closed. The UML diagram for the Polygon class is shown in Figure 14.39. Listing 14.19 gives an example that creates a hexagon, as shown in Figure 14.40.
The getter and setter methods for property values and a getter for property itself are provided in the class, but omitted in the UML diagram for brevity.
javafx.scene.shape.Arc +Polygon() +Polygon(double... points) +getPoints(): ObservableList
Creates an empty Polygon. Creates a Polygon with the given points. Returns a list of double values as x-and y-coordinates of the points.
FIGURE 14.39 Polygon defines a polygon.
(x, y) x is centerX radius cos(2 /6) y is centerY radius sin(2 /6) 2 6 radius
(centerX, centerY) (a)
FIGURE 14.40
(b)
(a) A Polygon is displayed. (b) A Polyline is displayed.
LISTING 14.19 ShowPolygon.java 1 2 3 4 5 6 7
import import import import import import import
javafx.application.Application; javafx.collections.ObservableList; javafx.scene.Scene; javafx.scene.layout.Pane; javafx.scene.paint.Color; javafx.stage.Stage; javafx.scene.shape.Polygon;
14.11 Shapes 571 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
public class ShowPolygon extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { // Create a pane, a polygon, and place polygon to pane Pane pane = new Pane(); Polygon polygon = new Polygon(); pane.getChildren().add(polygon); polygon.setFill(Color.WHITE); polygon.setStroke(Color.BLACK); ObservableList list = polygon.getPoints();
create a pane create a polygon add polygon to pane
get a list of points
final double WIDTH = 200, HEIGHT = 200; double centerX = WIDTH / 2, centerY = HEIGHT / 2; double radius = Math.min(WIDTH, HEIGHT) * 0.4; // Add points to the polygon list for (int i = 0; i < 6; i++) { list.add(centerX + radius * Math.cos(2 * i * Math.PI / 6)); list.add(centerY - radius * Math.sin(2 * i * Math.PI / 6)); } // Create a scene and place it in the stage Scene scene = new Scene(pane, WIDTH, HEIGHT); primaryStage.setTitle("ShowPolygon"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage
add x-coordinate of a point add y-coordinate of a point
add pane to scene
} }
The program creates a polygon (line 14) and adds it to a pane (line 15). The polygon .getPoints() method returns an ObservableList (line 18), which contains the add method for adding an element to the list (lines 26–27). Note that the value passed to add(value) must be a double value. If an int value is passed, the int value would be automatically boxed into an Integer. This would cause an error because the ObservableList consists of Double elements. The loop adds six points to the polygon (lines 25–28). Each point is represented by its x- and y-coordinates. For each point, its x-coordinate is added to the polygon’s list (line 26) and then its y-coordinate is added to the list (line 27). The formula for computing the x- and y-coordinates for a point in the hexagon is illustrated in Figure 14.40a. If you replace Polygon by Polyline, the program displays a polyline as shown in Figure 14.40b. The Polyline class is used in the same way as Polygon except that the starting and ending point are not connected in Polyline.
14.27 14.28 14.29 14.30
How do you display a text, line, rectangle, circle, ellipse, arc, polygon, and polyline? Write code fragments to display a string rotated 45 degrees in the center of the pane. Write code fragments to display a thick line of 10 pixels from (10, 10) to (70, 30). Write code fragments to fill red color in a rectangle of width 100 and height 50 with the upper-left corner at (10, 10). 14.31 Write code fragments to display a round-cornered rectangle with width 100, height 200 with the upper-left corner at (10, 10), corner horizontal diameter 40, and corner vertical diameter 20. 14.32 Write code fragments to display an ellipse with horizontal radius 50 and vertical radius 100. 14.33 Write code fragments to display the outline of the upper half of a circle with radius 50.
✓
Check Point
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14.35 Write code fragments to display a polygon connecting the following points: (20, 40), (30, 50), (40, 90), (90, 10), (10, 30), and fill the polygon with green color. 14.36 Write code fragments to display a polyline connecting the following points: (20, 40), (30, 50), (40, 90), (90, 10), (10, 30).
14.12 Case Study: The ClockPane Class Key Point
This case study develops a class that displays a clock on a pane. The contract of the ClockPane class is shown in Figure 14.41.
javafx.scene.layout.Pane
ClockPane
The getter and setter methods for these data fields are provided in the class, but omitted in the UML diagram for brevity.
-hour: int -minute: int
The hour in the clock. The minute in the clock.
-second: int -w: double
The second in the clock. The width of the pane that contains the clock. The height of the pane that contains the clock.
-h: double +ClockPane() +ClockPane(hour: int, minute: int, second: int)
Constructs a default clock for the current time. Constructs a clock with the specified time.
+setCurrentTime(): void
Sets hour, minute, and second to current time.
FIGURE 14.41
ClockPane displays an analog clock.
Assume ClockPane is available; we write a test program in Listing 14.20 to display an analog clock and use a label to display the hour, minute, and second, as shown in Figure 14.42. (0, 0) (xEnd, yEnd) 12
handLength
9
3 (centerX, centerY)
6
(a)
(b)
FIGURE 14.42 (a) The DisplayClock program displays a clock that shows the current time. (b) The endpoint of a clock hand can be determined, given the spanning angle, the hand length, and the center point.
14.12 Case Study: The ClockPane Class 573
LISTING 14.20 DisplayClock.java 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
import import import import import import
javafx.application.Application; javafx.geometry.Pos; javafx.stage.Stage; javafx.scene.Scene; javafx.scene.control.Label; javafx.scene.layout.BorderPane;
public class DisplayClock extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { // Create a clock and a label ClockPane clock = new ClockPane(); String timeString = clock.getHour() + ":" + clock.getMinute() + ":" + clock.getSecond(); Label lblCurrentTime = new Label(timeString); // Place clock and label in border pane BorderPane pane = new BorderPane(); pane.setCenter(clock); pane.setBottom(lblCurrentTime); BorderPane.setAlignment(lblCurrentTime, Pos.TOP_CENTER);
create a clock
create a label
add a clock add a label
// Create a scene and place it in the stage Scene scene = new Scene(pane, 250, 250); primaryStage.setTitle("DisplayClock"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } }
The rest of this section explains how to implement the ClockPane class. Since you can use the class without knowing how it is implemented, you may skip the implementation if you wish. To draw a clock, you need to draw a circle and three hands for the second, minute, and hour. To draw a hand, you need to specify the two ends of the line. As shown in Figure 14.42b, one end is the center of the clock at (centerX, centerY); the other end, at (endX, endY), is determined by the following formula: endX = centerX + handLength × sin(θ) endY = centerY - handLength × cos(θ)
Since there are 60 seconds in one minute, the angle for the second hand is second × (2π/60)
The position of the minute hand is determined by the minute and second. The exact minute value combined with seconds is minute + second/60. For example, if the time is 3 minutes and 30 seconds, the total minutes are 3.5. Since there are 60 minutes in one hour, the angle for the minute hand is (minute + second/60) × (2π/60)
Since one circle is divided into 12 hours, the angle for the hour hand is (hour + minute/60 + second/(60 × 60)) × (2π/12)
skip implementation? implementation
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JavaFX Basics For simplicity in computing the angles of the minute hand and hour hand, you can omit the seconds, because they are negligibly small. Therefore, the endpoints for the second hand, minute hand, and hour hand can be computed as: secondX secondY minuteX minuteY hourX = hourY =
= centerX = centerY = centerX = centerY centerX + centerY -
+ secondHandLength × sin(second × (2π/60)) - secondHandLength × cos(second × (2π/60)) + minuteHandLength × sin(minute × (2π/60)) - minuteHandLength × cos(minute × (2π/60)) hourHandLength × sin((hour + minute/60) × (2π/12)) hourHandLength × cos((hour + minute/60) × (2π/12))
The ClockPane class is implemented in Listing 14.21.
LISTING 14.21 ClockPane.java
clock properties
no-arg constructor
constructor
set a new hour paint clock
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import import import import import import import
java.util.Calendar; java.util.GregorianCalendar; javafx.scene.layout.Pane; javafx.scene.paint.Color; javafx.scene.shape.Circle; javafx.scene.shape.Line; javafx.scene.text.Text;
public class ClockPane extends Pane { private int hour; private int minute; private int second; // Clock pane's width and height private double w = 250, h = 250; /** Construct a default clock with the current time*/ public ClockPane() { setCurrentTime(); } /** Construct a clock with specified hour, minute, and second */ public ClockPane(int hour, int minute, int second) { this.hour = hour; this.minute = minute; this.second = second; paintClock(); } /** Return hour */ public int getHour() { return hour; } /** Set a new hour */ public void setHour(int hour) { this.hour = hour; paintClock(); } /** Return minute */ public int getMinute() { return minute; }
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/** Set a new minute */ public void setMinute(int minute) { this.minute = minute; paintClock(); }
set a new minute paint clock
/** Return second */ public int getSecond() { return second; } /** Set a new second */ public void setSecond(int second) { this.second = second; paintClock(); }
set a new second paint clock
/** Return clock pane's width */ public double getW() { return w; } /** Set clock pane's width */ public void setW(double w) { this.w = w; paintClock(); }
set a new width paint clock
/** Return clock pane's height */ public double getH() { return h; } /** Set clock pane's height */ public void setH(double h) { this.h = h; paintClock(); } /* Set the current time for the clock */ public void setCurrentTime() { // Construct a calendar for the current date and time Calendar calendar = new GregorianCalendar();
set a new height paint clock
set current time
// Set current hour, minute and second this.hour = calendar.get(Calendar.HOUR_OF_DAY); this.minute = calendar.get(Calendar.MINUTE); this.second = calendar.get(Calendar.SECOND); paintClock(); // Repaint the clock
paint clock
} /** Paint the clock */ protected void paintClock() { // Initialize clock parameters double clockRadius = Math.min(w, h) * 0.8 * 0.5; double centerX = w / 2; double centerY = h / 2; // Draw circle
paint clock get radius set center
576 Chapter 14 create a circle
create texts
create second hand
create minute hand
create hour hand
clear pane add to pane
JavaFX Basics 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
Circle circle = new Circle(centerX, centerY, clockRadius); circle.setFill(Color.WHITE); circle.setStroke(Color.BLACK); Text t1 = new Text(centerX - 5, centerY - clockRadius + 12, "12"); Text t2 = new Text(centerX - clockRadius + 3, centerY + 5, "9"); Text t3 = new Text(centerX + clockRadius - 10, centerY + 3, "3"); Text t4 = new Text(centerX - 3, centerY + clockRadius - 3, "6"); // Draw second hand double sLength = clockRadius * 0.8; double secondX = centerX + sLength * Math.sin(second * (2 * Math.PI / 60)); double secondY = centerY - sLength * Math.cos(second * (2 * Math.PI / 60)); Line sLine = new Line(centerX, centerY, secondX, secondY); sLine.setStroke(Color.RED); // Draw minute hand double mLength = clockRadius * 0.65; double xMinute = centerX + mLength * Math.sin(minute * (2 * Math.PI / 60)); double minuteY = centerY - mLength * Math.cos(minute * (2 * Math.PI / 60)); Line mLine = new Line(centerX, centerY, xMinute, minuteY); mLine.setStroke(Color.BLUE); // Draw hour hand double hLength = clockRadius * 0.5; double hourX = centerX + hLength * Math.sin((hour % 12 + minute / 60.0) * (2 * Math.PI / 12)); double hourY = centerY - hLength * Math.cos((hour % 12 + minute / 60.0) * (2 * Math.PI / 12)); Line hLine = new Line(centerX, centerY, hourX, hourY); hLine.setStroke(Color.GREEN); getChildren().clear(); getChildren().addAll(circle, t1, t2, t3, t4, sLine, mLine, hLine); } }
The program displays a clock for the current time using the no-arg constructor (lines 18–20) and displays a clock for the specified hour, minute, and second using the other constructor (lines 23–28). The current hour, minute, and second is obtained by using the GregorianCalendar class (lines 86–96). The GregorianCalendar class in the Java API enables you to create a Calendar instance for the current time using its noarg constructor. You can then use its methods get(Calendar.HOUR), get(Calendar .MINUTE), and get(Calendar.SECOND) to return the hour, minute, and second from a Calendar object. The class defines the properties hour, minute, and second to store the time represented in the clock (lines 10–12) and uses the w and h properties to represent the width and height of the clock pane (line 15). The initial values of w and h are set to 250. The w and h values can be reset using the setW and setH methods (lines 69, 80). These values are used to draw a clock in the pane in the paintClock() method. The paintClock() method paints the clock (lines 99–143). The clock radius is proportional to the width and height of the pane (line 101). A circle for the clock is created at the center of the pane (line 106). The text for showing the hours 12, 3, 6, 9 are created in lines 109–112.
Chapter Summary 577 The second hand, minute hand, and hour hand are the lines created in lines 114–139. The paintClock() method places all these shapes in the pane using the addAll method in a list (line 142). Because the paintClock() method is invoked whenever a new property (hour, minute, second, w, and h) is set (lines 27, 38, 49, 60, 71, 82, 95), before adding new contents into the pane, the old contents are cleared from the pane (line 141).
KEY TERMS AWT 536 bidirectional binding 544 bindable object 542 binding object 542 binding property 542 JavaFX 536 node 539 observable object 542 pane 539
property getter method 543 primary stage 537 shape 539 Swing 536 value getter method 543 value setter method 543 UI control 539 unidirectional binding 544
CHAPTER SUMMARY 1. JavaFX is the new framework for developing rich Internet applications. JavaFX completely replaces Swing and AWT.
2. A main JavaFX class must extend javafx.application.Application and implement the start method. The primary stage is automatically created by the JVM and passed to the start method.
3. A stage is a window for displaying a scene. You can add nodes to a scene. Panes, controls, and shapes are nodes. Panes can be used as the containers for nodes.
4. A binding property can be bound to an observable source object. A change in the source object will be automatically reflected in the binding property. A binding property has a value getter method, value setter method, and property getter method.
5. The Node class defines many properties that are common to all nodes. You can apply these properties to panes, controls, and shapes.
6. You can create a
Color object with the specified red, green, blue components, and
opacity value.
7. You can create a Font object and set its name, size, weight, and posture. 8. The javafx.scene.image.Image class can be used to load an image and this image can be displayed in an ImageView object.
9. JavaFX provides many types of panes for automatically laying out nodes in a desired location and size. The Pane is the base class for all panes. It contains the getChildren() method to return an ObservableList. You can use ObservableList’s add(node) and addAll(node1, node2, ...) methods for adding nodes into a pane.
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JavaFX Basics 10. A FlowPane arranges the nodes in the pane horizontally from left to right or vertically from top to bottom in the order in which they were added. A GridPane arranges nodes in a grid (matrix) formation. The nodes are placed in the specified column and row indices. A BorderPane can place nodes in five regions: top, bottom, left, right, and center. An HBox lays out its children in a single horizontal row. A VBox lays out its children in a single vertical column.
11. JavaFX provides many shape classes for drawing texts, lines, circles, rectangles, ellipses, arcs, polygons, and polylines.
QUIZ Answer the quiz for this chapter online at www.cs.armstrong.edu/liang/intro10e/quiz.html.
PROGRAMMING EXERCISES Note The image files used in the exercises can be obtained from www.cs.armstrong.edu/ liang/intro10e/book.zip under the image folder.
download image files
Sections 14.2–14.9
14.1
(a)
(Display images) Write a program that displays four images in a grid pane, as shown in Figure 14.43a.
(b)
(c)
FIGURE 14.43 (a) Exercise 14.1 displays four images. (b) Exercise 14.2 displays a tic-tac-toe board with images. (c) Three cards are randomly selected.
*14.2 VideoNote
Display a tictactoe board
*14.3
14.4
(Tic-tac-toe board) Write a program that displays a tic-tac-toe board, as shown in Figure 14.43b. A cell may be X, O, or empty. What to display at each cell is randomly decided. The X and O are images in the files x.gif and o.gif. (Display three cards) Write a program that displays three cards randomly selected from a deck of 52, as shown in Figure 14.43c. The card image files are named 1.png, 2.png, …, 52.png and stored in the image/card directory. All three cards are distinct and selected randomly. Hint: You can select random cards by storing the numbers 1–52 to an array list, perform a random shuffle introduced in Section 11.12, and use the first three numbers in the array list as the file names for the image. (Color and font) Write a program that displays five texts vertically, as shown in Figure 14.44a. Set a random color and opacity for each text and set the font of each text to Times Roman, bold, italic, and 22 pixels.
Programming Exercises 579
(a)
(b)
(c)
FIGURE 14.44 (a) Five texts are displayed with a random color and a specified font. (b) A string is displayed around the circle. (c) A checkerboard is displayed using rectangles.
14.5 *14.6
(Characters around circle) Write a program that displays a string Welcome to Java around the circle, as shown in Figure 14.44b. Hint: You need to display each character in the right location with appropriate rotation using a loop. (Game: display a checkerboard) Write a program that displays a checkerboard in which each white and black cell is a Rectangle with a fill color black or white, as shown in Figure 14.44c.
Sections 14.10–14.11
*14.7
(Display random 0 or 1) Write a program that displays a 10-by-10 square matrix, as shown in Figure 14.45a. Each element in the matrix is 0 or 1, randomly generated. Display each number centered in a text field. Use TextField’s setText method to set value 0 or 1 as a string.
(a)
(b)
(c)
FIGURE 14.45 (a) The program randomly generates 0s and 1s. (b) Exercise 14.9 draws four fans. (c) Exercise 14.10 draws a cylinder.
14.8 *14.9 *14.10
(Display 54 cards) Expand Exercise 14.3 to display all 54 cards (including two jokers), nine per row. The image files are jokers and are named 53.jpg and 54.jpg. (Create four fans) Write a program that places four fans in a GridPane with two rows and two columns, as shown in Figure 14.45b. (Display a cylinder) Write a program that draws a cylinder, as shown in Figure 14.45b. You can use the following method to set the dashed stroke for an arc: arc.getStrokeDashArray().addAll(6.0, 21.0);
Display a random matrix
580 Chapter 14
JavaFX Basics *14.11
(Paint a smiley face) Write a program that paints a smiley face, as shown in Figure 14.46a.
(a)
FIGURE 14.46 pie chart.
(b)
(c)
(a) Exercise 14.11 paints a smiley face. (b) Exercise 14.12 paints a bar chart. (c) Exercise 14.13 paints a
**14.12
(Display a bar chart) Write a program that uses a bar chart to display the percentages of the overall grade represented by projects, quizzes, midterm exams, and the final exam, as shown in Figure 14.46b. Suppose that projects take 20 percent and are displayed in red, quizzes take 10 percent and are displayed in blue, midterm exams take 30 percent and are displayed in green, and the final exam takes 40 percent and is displayed in orange. Use the Rectangle class to display the bars. Interested readers may explore the JavaFX BarChart class for further study.
**14.13
(Display a pie chart) Write a program that uses a pie chart to display the percentages of the overall grade represented by projects, quizzes, midterm exams, and the final exam, as shown in Figure 14.46c. Suppose that projects take 20 percent and are displayed in red, quizzes take 10 percent and are displayed in blue, midterm exams take 30 percent and are displayed in green, and the final exam takes 40 percent and is displayed in orange. Use the Arc class to display the pies. Interested readers may explore the JavaFX PieChart class for further study.
14.14
(Display a rectanguloid) Write a program that displays a rectanguloid, as shown in Figure 14.47a. The cube should grow and shrink as the window grows or shrinks.
VideoNote
Display a bar chart
(a)
(b)
(c)
FIGURE 14.47 (a) Exercise 14.14 paints a rectanguloid. (b) Exercise 14.15 paints a STOP sign. (c) Exercise 14.13 paints a grid.
*14.15
(Display a STOP sign) Write a program that displays a STOP sign, as shown in Figure 14.47b. The octagon is in red and the sign is in white. (Hint: Place an octagon and a text in a stack pane.)
Programming Exercises 581 *14.16 14.17
(Display a 3 * 3 grid) Write a program that displays a 3 * 3 grid, as shown in Figure 14.47c. Use red color for vertical lines and blue for horizontals. The lines are automatically resized when the window is resized. (Game: hangman) Write a program that displays a drawing for the popular hangman game, as shown in Figure 14.48a.
(a)
(b)
(c)
FIGURE 14.48 (a) Exercise 14.17 draws a sketch for the hangman game. (c) Exercise 14.18 plots the quadratic function. (c) Exercise 14.19 plots the sine/cosine functions.
*14.18
(Plot the square function) Write a program that draws a diagram for the function f(x) = x2 (see Figure 14.48b). Hint: Add points to a polyline using the following code: Polyline polyline = new Polyline(); ObservableList list = polyline.getPoints(); double scaleFactor = 0.0125; for (int x = -100; x <= 100; x++) { list.add(x + 200.0); list.add(scaleFactor * x * x); }
**14.19
(Plot the sine and cosine functions) Write a program that plots the sine function in red and cosine in blue, as shown in Figure 14.48c. Hint: The Unicode for p is \u03c0. To display -2p, use Text(x, y, "-2\u03c0"). For a trigonometric function like sin(x), x is in radians. Use the following loop to add the points to a polyline: Polyline polyline = new Polyline(); ObservableList list = polyline.getPoints(); double scaleFactor = 50; for (int x = -170; x <= 170; x++) { list.add(x + 200.0); list.add(100 – 50 * Math.sin((x / 100.0) * 2 * Math.PI)); }
**14.20
(Draw an arrow line) Write a static method that draws an arrow line from a starting point to an ending point in a pane using the following method header: public static void drawArrowLine(double startX, double startY, double endX, double endY, Pane pane)
Write a test program that randomly draws an arrow line, as shown in Figure 14.49a.
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JavaFX Basics
(a)
(b)
(c)
FIGURE 14.49 (a) The program displays an arrow line. (b) Exercise14.21 connects the centers of two filled circles. (c) Exercise14.22 connects two circles from their perimeter.
*14.21
*14.22 *14.23
(Two circles and their distance) Write a program that draws two filled circles with radius 15 pixels, centered at random locations, with a line connecting the two circles. The distance between the two centers is displayed on the line, as shown in Figure 14.49b. (Connect two circles) Write a program that draws two circles with radius 15 pixels, centered at random locations, with a line connecting the two circles. The line should not cross inside the circles, as shown in Figure 14.49c. (Geometry: two rectangles) Write a program that prompts the user to enter the center coordinates, width, and height of two rectangles from the command line. The program displays the rectangles and a text indicating whether the two are overlapping, whether one is contained in the other, or whether they don’t overlap, as shown in Figure 14.50. See Programming Exercise 10.13 for checking the relationship between two rectangles.
(a)
FIGURE 14.50
(b)
(c)
Two rectangles are displayed.
*14.24
(Geometry: Inside a polygon?) Write a program that prompts the user to enter the coordinates of five points from the command line. The first four points form a polygon, and the program displays the polygon and a text that indicates whether the fifth point is inside the polygon, as shown in Figure 14.51a. Hint: Use the Node’s contains method to test whether a point is inside a node.
Programming Exercises 583
(a)
(b)
(c)
FIGURE 14.51 (a) The polygon and a point are displayed. (b) Exercise14.25 connects five random points on a circle. (c) Exercise 14.26 displays two clocks.
*14.25
(Random points on a circle) Modify Programming Exercise 4.6 to create five random points on a circle, form a polygon by connecting the points clockwise, and display the circle and the polygon, as shown in Figure 14.51b.
Section 14.12
14.26
(Use the ClockPane class) Write a program that displays two clocks. The hour, minute, and second values are 4, 20, 45 for the first clock and 22, 46, 15 for the second clock, as shown in Figure 14.51c.
*14.27
(Draw a detailed clock) Modify the ClockPane class in Section 14.12 to draw the clock with more details on the hours and minutes, as shown in Figure 14.52a.
(a)
(b)
(c)
FIGURE 14.52 (a) Exercise 14.27 displays a detailed clock. (b) Exercise 14.28 displays a clock with random hour and minute values. (c) Exercise 14.29 displays a bean machine.
*14.28
**14.29
(Random time) Modify the ClockPane class with three new Boolean properties— hourHandVisible, minuteHandVisible, and secondHandVisible—and their associated accessor and mutator methods. You can use the set methods to make a hand visible or invisible. Write a test program that displays only the hour and minute hands. The hour and minute values are randomly generated. The hour is between 0 and 11, and the minute is either 0 or 30, as shown in Figure 14.52b. (Game: bean machine) Write a program that displays a bean machine introduced in Programming Exercise 7.21, as shown in Figure 14.52c.
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CHAPTER
15 EVENT-DRIVEN PROGRAMMING AND ANIMATIONS Objectives ■
To get a taste of event-driven programming (§15.1).
■
To describe events, event sources, and event classes (§15.2).
■
To define handler classes, register handler objects with the source object, and write the code to handle events (§15.3).
■
To define handler classes using inner classes (§15.4).
■
To define handler classes using anonymous inner classes (§15.5).
■
To simplify event handling using lambda expressions (§15.6).
■
To develop a GUI application for a loan calculator (§15.7).
■
To write programs to deal with MouseEvents (§15.8).
■
To write programs to deal with KeyEvents (§15.9).
■
To create listeners for processing a value change in an observable object (§15.10).
■
To use the Animation, PathTransition, FadeTransition, and Timeline classes to develop animations (§15.11).
■
To develop an animation for simulating a bouncing ball (§15.12).
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Event-Driven Programming and Animations
15.1 Introduction Key Point problem
You can write code to process events such as a button click, mouse movement, and keystrokes. Suppose you wish to write a GUI program that lets the user enter a loan amount, annual interest rate, and number of years and click the Calculate button to obtain the monthly payment and total payment, as shown in Figure 15.1. How do you accomplish the task? You have to use event-driven programming to write the code to respond to the button-clicking event.
FIGURE 15.1 problem
The program computes loan payments.
Before delving into event-driven programming, it is helpful to get a taste using a simple example. The example displays two buttons in a pane, as shown in Figure 15.2.
(a)
(b)
FIGURE 15.2 (a) The program displays two buttons. (b) A message is displayed in the console when a button is clicked. To respond to a button click, you need to write the code to process the button-clicking action. The button is an event source object—where the action originates. You need to create an object capable of handling the action event on a button. This object is called an event handler, as shown in Figure 15.3.
FIGURE 15.3
button
event
handler
Clicking a button fires an action event
An event is an object
The event handler processes the event
(Event source object)
(Event object)
(Event handler object)
An event handler processes the event fired from the source object.
Not all objects can be handlers for an action event. To be a handler of an action event, two requirements must be met: EventHandler interface
setOnAction(handler)
1. The object must be an instance of the EventHandler interface. This interface defines the common behavior for all handlers. denotes that T is a generic type that is a subtype of Event. 2. The EventHandler object handler must be registered with the event source object using the method source.setOnAction(handler).
15.1 Introduction 587 The EventHandler interface contains the handle(ActionEvent) method for processing the action event. Your handler class must override this method to respond to the event. Listing 15.1 gives the code that processes the ActionEvent on the two buttons. When you click the OK button, the message “OK button clicked” is displayed. When you click the Cancel button, the message “Cancel button clicked” is displayed, as shown in Figure 15.2.
LISTING 15.1 HandleEvent.java 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
import import import import import import import import
javafx.application.Application; javafx.geometry.Pos; javafx.scene.Scene; javafx.scene.control.Button; javafx.scene.layout.HBox; javafx.stage.Stage; javafx.event.ActionEvent; javafx.event.EventHandler;
public class HandleEvent extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { // Create a pane and set its properties HBox pane = new HBox(10); pane.setAlignment(Pos.CENTER); Button btOK = new Button("OK"); Button btCancel = new Button("Cancel"); OKHandlerClass handler1 = new OKHandlerClass(); btOK.setOnAction(handler1); CancelHandlerClass handler2 = new CancelHandlerClass(); btCancel.setOnAction(handler2); pane.getChildren().addAll(btOK, btCancel);
create handler register handler create handler register handler
// Create a scene and place it in the stage Scene scene = new Scene(pane); primaryStage.setTitle("HandleEvent"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } } class OKHandlerClass implements EventHandler { @Override public void handle(ActionEvent e) { System.out.println("OK button clicked"); } }
handler class
class CancelHandlerClass implements EventHandler { @Override public void handle(ActionEvent e) { System.out.println("Cancel button clicked"); } }
handler class
Two handler classes are defined in lines 32–44. Each handler class implements EventHandler to process ActionEvent. The object handler1 is an instance of OKHandlerClass (line 18), which is registered with the button btOK (line 19). When the OK button is clicked, the handle(ActionEvent) method (line 34) in
handle event
handle event
588 Chapter 15
Event-Driven Programming and Animations OKHandlerClass is invoked to process the event. The object handler2 is an instance of CancelHandlerClass (line 20), which is registered with the button btCancel in line 21. When the Cancel button is clicked, the handle(ActionEvent) method (line 41) in CancelHandlerClass is invoked to process the event. You now have seen a glimpse of event-driven programming in JavaFX. You probably have many questions, such as why a handler class is defined to implement the EventHandler. The following sections will give you all the answers.
15.2 Events and Event Sources Key Point event-driven programming event
fire event event source object source object
An event is an object created from an event source. Firing an event means to create an event and delegate the handler to handle the event. When you run a Java GUI program, the program interacts with the user, and the events drive its execution. This is called event-driven programming. An event can be defined as a signal to the program that something has happened. Events are triggered by external user actions, such as mouse movements, mouse clicks, and keystrokes. The program can choose to respond to or ignore an event. The example in the preceding section gave you a taste of event-driven programming. The component that creates an event and fires it is called the event source object, or simply source object or source component. For example, a button is the source object for a buttonclicking action event. An event is an instance of an event class. The root class of the Java event classes is java.util.EventObject. The root class of the JavaFX event classes is javafx.event.Event. The hierarchical relationships of some event classes are shown in Figure 15.4.
ActionEvent MouseEvent EventObject
Event
InputEvent KeyEvent WindowEvent
JavaFX event classes are in the javafx.event package
FIGURE 15.4 An event in JavaFX is an object of the javafx.event.Event class. event object getSource()
An event object contains whatever properties are pertinent to the event. You can identify the source object of an event using the getSource() instance method in the EventObject class. The subclasses of EventObject deal with specific types of events, such as action events, window events, mouse events, and key events. The first three columns in Table 15.1 list some external user actions, source objects, and event types fired. For example, when clicking a button, the button creates and fires an ActionEvent, as indicated in the first line of this table. Here, the button is an event source object, and an ActionEvent is the event object fired by the source object, as shown in Figure 15.3.
Note If a component can fire an event, any subclass of the component can fire the same type of event. For example, every JavaFX shape, layout pane, and control can fire MouseEvent and KeyEvent since Node is the superclass for shapes, layout panes, and controls.
15.3 Registering Handlers and Handling Events 589 TABLE 15.1 User Action, Source Object, Event Type, Handler Interface, and Handler User Action
Source Object
Event Type Fired
Event Registration Method
Click a button
Button
ActionEvent
setOnAction(EventHandler)
Press Enter in a text field
TextField
ActionEvent
setOnAction(EventHandler)
Check or uncheck
RadioButton
ActionEvent
setOnAction(EventHandler)
Check or uncheck
CheckBox
ActionEvent
setOnAction(EventHandler)
Select a new item
ComboBox
ActionEvent
setOnAction(EventHandler)
Mouse pressed
Node, Scene
MouseEvent
setOnMousePressed(EventHandler)
Mouse released
setOnMouseReleased(EventHandler)
Mouse clicked
setOnMouseClicked(EventHandler)
Mouse entered
setOnMouseEntered(EventHandler)
Mouse exited
setOnMouseExited(EventHandler)
Mouse moved
setOnMouseMoved(EventHandler)
Mouse dragged Key pressed
setOnMouseDragged(EventHandler) Node, Scene
KeyEvent
setOnKeyPressed(EventHandler)
Key released
setOnKeyReleased(EventHandler)
Key typed
setOnKeyTyped(EventHandler)
15.1 What is an event source object? What is an event object? Describe the relationship between an event source object and an event object. 15.2 Can a button fire a MouseEvent? Can a button fire a KeyEvent? Can a button fire an ActionEvent?
✓
Check Point
15.3 Registering Handlers and Handling Events A handler is an object that must be registered with an event source object, and it must be an instance of an appropriate event-handling interface. Java uses a delegation-based model for event handling: a source object fires an event, and an object interested in the event handles it. The latter object is called an event handler or an event listener. For an object to be a handler for an event on a source object, two things are needed, as shown in Figure 15.5. 1. The handler object must be an instance of the corresponding event-handler interface to ensure that the handler has the correct method for processing the event. JavaFX defines a unified handler interface EventHandler for an event T. The handler interface contains the handle(T e) method for processing the event. For example, the handler interface for ActionEvent is EventHandler; each handler for ActionEvent should implement the handle(ActionEvent e) method for processing an ActionEvent. 2. The handler object must be registered by the source object. Registration methods depend on the event type. For ActionEvent, the method is setOnAction. For a mouse pressed event, the method is setOnMousePressed. For a key pressed event, the method is setOnKeyPressed. Let’s revisit Listing 15.1, HandleEvent.java. Since a Button object fires ActionEvent, a handler object for ActionEvent must be an instance of EventHandler, so
Key Point event delegation event handler
event-handler interface EventHandler
event handler
register handler
590 Chapter 15 User Action
Event-Driven Programming and Animations
Trigger an event
«interface» EventHandler
source: SourceClass +setOnXEventType(listener)
+handle(event: T)
(2) Register by invoking source.setOnXEventType(listener): (1) A listener object is an instance of a listener interface
listener: ListenerClass
(a) A generic source object with a generic event T «interface» EventHandler
source: javafx.scene.control.Button +setOnAction(listener)
+handle(event: ActionEvent)
(2) Register by invoking source.setOnAction(listener); (1) An action event listener is an instance of EventHandler
listener: CustomListenerClass
(b) A Button source object with an ActionEvent
FIGURE 15.5
A listener must be an instance of a listener interface and must be registered with a source object. the handler class implements EventHandler in line 34. The source object invokes setOnAction(handler) to register a handler, as follows: Button btOK = new Button("OK"); // Line 16 in Listing 15.1 OKHandlerClass handler1 = new OKHandlerClass(); // Line 18 in Listing 15.1 btOK.setOnAction(handler1); // Line 19 in Listing 15.1
create source object create handler object register handler
first version
When you click the button, the Button object fires an ActionEvent and passes it to invoke the handler’s handle(ActionEvent) method to handle the event. The event object contains information pertinent to the event, which can be obtained using the methods. For example, you can use e.getSource() to obtain the source object that fired the event. We now write a program that uses two buttons to control the size of a circle, as shown in Figure 15.6. We will develop this program incrementally. First, we write the program in Listing 15.2 that displays the user interface with a circle in the center (lines 15-19) and two buttons on the bottom (lines 21-27).
FIGURE 15.6 The user clicks the Enlarge and Shrink buttons to enlarge and shrink the size of the circle.
LISTING 15.2 ControlCircleWithoutEventHandling.java 1 2 3 4
import import import import
javafx.application.Application; javafx.geometry.Pos; javafx.scene.Scene; javafx.scene.control.Button;
15.3 Registering Handlers and Handling Events 591 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 49
import import import import import import
javafx.scene.layout.StackPane; javafx.scene.layout.HBox; javafx.scene.layout.BorderPane; javafx.scene.paint.Color; javafx.scene.shape.Circle; javafx.stage.Stage;
public class ControlCircleWithoutEventHandling extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { StackPane pane = new StackPane(); Circle circle = new Circle(50); circle.setStroke(Color.BLACK); circle.setFill(Color.WHITE); pane.getChildren().add(circle); HBox hBox = new HBox(); hBox.setSpacing(10); hBox.setAlignment(Pos.CENTER); Button btEnlarge = new Button("Enlarge"); Button btShrink = new Button("Shrink"); hBox.getChildren().add(btEnlarge); hBox.getChildren().add(btShrink);
circle
buttons
BorderPane borderPane = new BorderPane(); borderPane.setCenter(pane); borderPane.setBottom(hBox); BorderPane.setAlignment(hBox, Pos.CENTER); // Create a scene and place it in the stage Scene scene = new Scene(borderPane, 200, 150); primaryStage.setTitle("ControlCircle"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } }
How do you use the buttons to enlarge or shrink the circle? When the Enlarge button is clicked, you want the circle to be repainted with a larger radius. How can you accomplish this? You can expand and modify the program in Listing 15.2 into Listing 15.3 with the following features:
second version
1. Define a new class named CirclePane for displaying the circle in a pane (lines 51–68). This new class displays a circle and provides the enlarge and shrink methods for increasing and decreasing the radius of the circle (lines 60–62, 64–67). It is a good strategy to design a class to model a circle pane with supporting methods so that these related methods along with the circle are coupled in one object. 2. Create a CirclePane object and declare circlePane as a data field to reference this object (line 15) in the ControlCircle class. The methods in the ControlCircle class can now access the CirclePane object through this data field. a handler class named EnlargeHandler that implements EventHandler (lines 43–48). To make the reference variable circlePane accessible from the handle method, define EnlargeHandler as an inner class of the ControlCircle class. (Inner classes are defined inside another
3. Define
class. We use an inner class here and will introduce it fully in the next section.) 4. Register the handler for the Enlarge button (line 29) and implement the handle method in EnlargeHandler to invoke circlePane.enlarge() (line 46).
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LISTING 15.3 ControlCircle.java VideoNote
Handler and its registration
create/register handler
handler class
CirclePane class
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 54 55 56 57 58
import import import import import import import import import import import import
javafx.application.Application; javafx.event.ActionEvent; javafx.event.EventHandler; javafx.geometry.Pos; javafx.scene.Scene; javafx.scene.control.Button; javafx.scene.layout.StackPane; javafx.scene.layout.HBox; javafx.scene.layout.BorderPane; javafx.scene.paint.Color; javafx.scene.shape.Circle; javafx.stage.Stage;
public class ControlCircle extends Application { private CirclePane circlePane = new CirclePane(); @Override // Override the start method in the Application class public void start(Stage primaryStage) { // Hold two buttons in an HBox HBox hBox = new HBox(); hBox.setSpacing(10); hBox.setAlignment(Pos.CENTER); Button btEnlarge = new Button("Enlarge"); Button btShrink = new Button("Shrink"); hBox.getChildren().add(btEnlarge); hBox.getChildren().add(btShrink); // Create and register the handler btEnlarge.setOnAction(new EnlargeHandler()); BorderPane borderPane = new BorderPane(); borderPane.setCenter(circlePane); borderPane.setBottom(hBox); BorderPane.setAlignment(hBox, Pos.CENTER); // Create a scene and place it in the stage Scene scene = new Scene(borderPane, 200, 150); primaryStage.setTitle("ControlCircle"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } class EnlargeHandler implements EventHandler { @Override // Override the handle method public void handle(ActionEvent e) { circlePane.enlarge(); } } } class CirclePane extends StackPane { private Circle circle = new Circle(50); public CirclePane() { getChildren().add(circle); circle.setStroke(Color.BLACK); circle.setFill(Color.WHITE); }
15.4 Inner Classes 593 59 60 61 62 63 64 65 66 67 68
public void enlarge() { circle.setRadius(circle.getRadius() + 2); }
enlarge method
public void shrink() { circle.setRadius(circle.getRadius() > 2 ? circle.getRadius() - 2 : circle.getRadius()); } }
As an exercise, add the code for handling the Shrink button to display a smaller circle when the Shrink button is clicked.
15.3 15.4 15.5 15.6
Why must a handler be an instance of an appropriate handler interface? Explain how to register a handler object and how to implement a handler interface. What is the handler method for the EventHandler interface? What is the registration method for a button to register an ActionEvent handler?
the Shrink button
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Check Point
15.4 Inner Classes An inner class, or nested class, is a class defined within the scope of another class. Inner classes are useful for defining handler classes. Inner classes are used in the preceding section. This section introduces inner classes in detail. First, let us see the code in Figure 15.7. The code in Figure 15.7a defines two separate classes, Test and A. The code in Figure 15.7b defines A as an inner class in Test. public class Test { ... }
// OuterClass.java: inner class demo public class OuterClass { private int data;
public class A { ... }
/** A method in the outer class */ public void m() { // Do something }
(a)
// An inner class class InnerClass { /** A method in the inner class */ public void mi() { // Directly reference data and method // defined in its outer class data++; m(); } }
public class Test { ... // Inner class public class A { ... } } } (b)
(c)
FIGURE 15.7 Inner classes combine dependent classes into the primary class. The class InnerClass defined inside OuterClass in Figure 15.7c is another example of an inner class. An inner class may be used just like a regular class. Normally, you define
Key Point
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Event-Driven Programming and Animations a class as an inner class if it is used only by its outer class. An inner class has the following features: ■
An inner class is compiled into a class named OuterClassName$InnerClassName. class. For example, the inner class A in Test is compiled into Test$A.class in Figure 15.7b.
■
An inner class can reference the data and the methods defined in the outer class in which it nests, so you need not pass the reference of an object of the outer class to the constructor of the inner class. For this reason, inner classes can make programs simple and concise. For example, circlePane is defined in ControlCircle in Listing 15.3 (line 15). It can be referenced in the inner class EnlargeHandler in line 46.
■
An inner class can be defined with a visibility modifier subject to the same visibility rules applied to a member of the class.
■
An inner class can be defined as static. A static inner class can be accessed using the outer class name. A static inner class cannot access nonstatic members of the outer class.
■
Objects of an inner class are often created in the outer class. But you can also create an object of an inner class from another class. If the inner class is nonstatic, you must first create an instance of the outer class, then use the following syntax to create an object for the inner class: OuterClass.InnerClass innerObject = outerObject.new InnerClass();
■
If the inner class is static, use the following syntax to create an object for it: OuterClass.InnerClass innerObject = new OuterClass.InnerClass();
A simple use of inner classes is to combine dependent classes into a primary class. This reduces the number of source files. It also makes class files easy to organize since they are all named with the primary class as the prefix. For example, rather than creating the two source files Test.java and A.java as shown in Figure 15.7a, you can merge class A into class Test and create just one source file, Test.java as shown in Figure 15.7b. The resulting class files are Test.class and Test$A.class. Another practical use of inner classes is to avoid class-naming conflicts. Two versions of CirclePane are defined in Listings 15.2 and 15.3. You can define them as inner classes to avoid a conflict. A handler class is designed specifically to create a handler object for a GUI component (e.g., a button). The handler class will not be shared by other applications and therefore is appropriate to be defined inside the main class as an inner class.
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Check Point
15.7 Can an inner class be used in a class other than the class in which it nests? 15.8 Can the modifiers public, protected, private, and static be used for inner classes?
15.5 Anonymous Inner Class Handlers Key Point anonymous inner class
An anonymous inner class is an inner class without a name. It combines defining an inner class and creating an instance of the class into one step. Inner-class handlers can be shortened using anonymous inner classes. The inner class in Listing 15.3 can be replaced by an anonymous inner class as shown below.
15.5 Anonymous Inner Class Handlers 595 public void start(Stage primaryStage) { // Omitted
public void start(Stage primaryStage) { // Omitted
btEnlarge.setOnAction( new EnlargeHandler());
btEnlarge.setOnAction( new class EnlargeHandlner implements EventHandler() { public void handle(ActionEvent e) { circlePane.enlarge(); } });
} class EnlargeHandler implements EventHandler { public void handle(ActionEvent e) { circlePane.enlarge(); } }
}
(a) Inner class EnlargeListener
(b) Anonymous inner class
The syntax for an anonymous inner class is shown below new SuperClassName/InterfaceName() { // Implement or override methods in superclass or interface // Other methods if necessary }
Since an anonymous inner class is a special kind of inner class, it is treated like an inner class with the following features: ■
An anonymous inner class must always extend a superclass or implement an interface, but it cannot have an explicit extends or implements clause.
■
An anonymous inner class must implement all the abstract methods in the superclass or in the interface.
■
An anonymous inner class always uses the no-arg constructor from its superclass to create an instance. If an anonymous inner class implements an interface, the constructor is Object().
■
An anonymous inner class is compiled into a class named OuterClassName$n. class. For example, if the outer class Test has two anonymous inner classes, they are compiled into Test$1.class and Test$2.class.
Listing 15.4 gives an example that handles the events from four buttons, as shown in Figure 15.8.
FIGURE 15.8
The program handles the events from four buttons.
LISTING 15.4 AnonymousHandlerDemo.java 1 2 3 4 5
import import import import import
javafx.application.Application; javafx.event.ActionEvent; javafx.event.EventHandler; javafx.geometry.Pos; javafx.scene.Scene;
VideoNote
Anonymous handler
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anonymous handler handle event
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import javafx.scene.control.Button; import javafx.scene.layout.HBox; import javafx.stage.Stage; public class AnonymousHandlerDemo extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { // Hold two buttons in an HBox HBox hBox = new HBox(); hBox.setSpacing(10); hBox.setAlignment(Pos.CENTER); Button btNew = new Button("New"); Button btOpen = new Button("Open"); Button btSave = new Button("Save"); Button btPrint = new Button("Print"); hBox.getChildren().addAll(btNew, btOpen, btSave, btPrint); // Create and register the handler btNew.setOnAction(new EventHandler() { @Override // Override the handle method public void handle(ActionEvent e) { System.out.println("Process New"); } }); btOpen.setOnAction(new EventHandler() { @Override // Override the handle method public void handle(ActionEvent e) { System.out.println("Process Open"); } }); btSave.setOnAction(new EventHandler() { @Override // Override the handle method public void handle(ActionEvent e) { System.out.println("Process Save"); } }); btPrint.setOnAction(new EventHandler() { @Override // Override the handle method public void handle(ActionEvent e) { System.out.println("Process Print"); } }); // Create a scene and place it in the stage Scene scene = new Scene(hBox, 300, 50); primaryStage.setTitle("AnonymousHandlerDemo"); // Set title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } }
The program creates four handlers using anonymous inner classes (lines 24–50). Without using anonymous inner classes, you would have to create four separate classes. An anonymous handler works the same way as that of an inner class handler. The program is condensed using an anonymous inner class.
15.6 Simplifying Event Handling Using Lambda Expressions 597 The
anonymous
inner
classes
in
this
example
are
compiled
into
AnonymousHandlerDemo$1.class, AnonymousHandlerDemo$2.class, AnonymousHandlerDemo$3.class, and AnonymousHandlerDemo$4.class.
15.9 If class A is an inner class in class B, what is the .class file for A? If class B contains two anonymous inner classes, what are the .class file names for these two classes? 15.10 What is wrong in the following code?
public class Test extends Application { public void start(Stage stage) { Button btOK = new Button("OK"); }
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Check Point
public class Test extends Application { public void start(Stage stage) { Button btOK = new Button("OK"); btOK.setOnAction( new EventHandler { public void handle (ActionEvent e) { System.out.println (e.getSource()); } } // Something missing here
private class Handler implements EventHandler { public void handle(Action e) { System.out.println(e.getSource()); } } } } } (a)
(b)
15.6 Simplifying Event Handling Using Lambda Expressions Lambda expressions can be used to greatly simplify coding for event handling. Lambda expression is a new feature in Java 8. Lambda expressions can be viewed as an anonymous class with a concise syntax. For example, the following code in (a) can be greatly simplified using a lambda expression in (b) in three lines.
btEnlarge.setOnAction( new EventHandler() { @Override public void handle(ActionEvent e) { // Code for processing event e } } });
btEnlarge.setOnAction(e -> { // Code for processing event e });
(a) Anonymous inner class event handler
(b) Lambda expression event handler
The basic syntax for a lambda expression is either (type1 param1, type2 param2, ...) -> expression
or (type1 param1, type2 param2, ...) -> { statements; }
Key Point lambda expression
598 Chapter 15
Event-Driven Programming and Animations The data type for a parameter may be explicitly declared or implicitly inferred by the compiler. The parentheses can be omitted if there is only one parameter without an explicit data type. In the preceding example, the lambda expression is as follows e -> { // Code for processing event e }
functional interface SAM interface
The compiler treats a lambda expression as if it is an object created from an anonymous inner class. In this case, the compiler understands that the object must be an instance of EventHandler. Since the EventHandler interface defines the handle method with a parameter of the ActionEvent type, the compiler automatically recognizes that e is a parameter of the ActionEvent type, and the statements are for the body of the handle method. The EventHandler interface contains just one method. The statements in the lambda expression are all for that method. If it contains multiple methods, the compiler will not be able to compile the lambda expression. So, for the compiler to understand lambda expressions, the interface must contain exactly one abstract method. Such an interface is known as a functional interface or a Single Abstract Method (SAM) interface. Listing 15.4 can be simplified using lambda expressions as shown in Listing 15.5.
LISTING 15.5 LambdaHandlerDemo.java
lambda handler
lambda handler
lambda handler
lambda handler
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
import import import import import import import
javafx.application.Application; javafx.event.ActionEvent; javafx.geometry.Pos; javafx.scene.Scene; javafx.scene.control.Button; javafx.scene.layout.HBox; javafx.stage.Stage;
public class LambdaHandlerDemo extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { // Hold two buttons in an HBox HBox hBox = new HBox(); hBox.setSpacing(10); hBox.setAlignment(Pos.CENTER); Button btNew = new Button("New"); Button btOpen = new Button("Open"); Button btSave = new Button("Save"); Button btPrint = new Button("Print"); hBox.getChildren().addAll(btNew, btOpen, btSave, btPrint); // Create and register the handler btNew.setOnAction((ActionEvent e) -> { System.out.println("Process New"); }); btOpen.setOnAction((e) -> { System.out.println("Process Open"); }); btSave.setOnAction(e -> { System.out.println("Process Save"); }); btPrint.setOnAction(e -> System.out.println("Process Print"));
15.6 Simplifying Event Handling Using Lambda Expressions 599 36 37 38 39 40 41 42 43
// Create a scene and place it in the stage Scene scene = new Scene(hBox, 300, 50); primaryStage.setTitle("LambdaHandlerDemo"); // Set title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } }
The program creates four handlers using lambda expressions (lines 23–35). Using lambda expressions, the code is shorter and cleaner. As seen in this example, lambda expressions may have many variations. Line 23 uses a declared type. Line 27 uses an inferred type since the type can be determined by the compiler. Line 31 omits the parentheses for a single inferred type. Line 35 omits the braces for a single statement in the body. You can handle events by defining handler classes using inner classes, anonymous inner classes, or lambda expressions. We recommend that you use lambda expressions because it produces a shorter, clearer, and cleaner code.
15.11 What is a lambda expression? What is the benefit of using lambda expressions for event handling? What is the syntax of a lambda expression?
15.12 What is a functional interface? Why is a functional interface required for a lambda expression? 15.13 Show the output of the following code: public class Test { public static void main(String[] args) { Test test = new Test(); test.setAction1(() -> System.out.print("Action 1! ")); test.setAction2(e -> System.out.print(e + " ")); System.out.println(test.setAction3(e -> e * 2)); } public void setAction1(T1 t) { t.m(); } public void setAction2(T2 t) { t.m(4.5); } public double setAction3(T3 t) { return t.m(5.5); } } interface T1 { public void m(); } interface T2 { public void m(Double d); } interface T3 { public double m(Double d); }
inner class, anonymous class, or Lambda?
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15.7 Case Study: Loan Calculator Key Point
This case study develops a loan calculator using event-driven programming with GUI controls. Now, we will write the program for the loan-calculator problem presented at the beginning of this chapter. Here are the major steps in the program: 1. Create the user interface, as shown in Figure 15.9. a. Create a GridPane. Add labels, text fields, and button to the pane. b. Set the alignment of the button to the right. 2. Process the event. Create and register the handler for processing the button-clicking action event. The handler obtains the user input on the loan amount, interest rate, and number of years, computes the monthly and total payments, and displays the values in the text fields.
GridPane
Text field is right aligned
Button is right aligned
FIGURE 15.9
The program computes loan payments.
The complete program is given in Listing 15.6.
LISTING 15.6 LoanCalculator.java
text fields
button
create a grid pane
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
import import import import import import import import import
javafx.application.Application; javafx.geometry.Pos; javafx.geometry.HPos; javafx.scene.Scene; javafx.scene.control.Button; javafx.scene.control.Label; javafx.scene.control.TextField; javafx.scene.layout.GridPane; javafx.stage.Stage;
public class LoanCalculator extends Application { private TextField tfAnnualInterestRate = new TextField(); private TextField tfNumberOfYears = new TextField(); private TextField tfLoanAmount = new TextField(); private TextField tfMonthlyPayment = new TextField(); private TextField tfTotalPayment = new TextField(); private Button btCalculate = new Button("Calculate"); @Override // Override the start method in the Application class public void start(Stage primaryStage) { // Create UI GridPane gridPane = new GridPane();
15.7 Case Study: Loan Calculator 601 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 75
gridPane.setHgap(5); gridPane.setVgap(5); gridPane.add(new Label("Annual Interest Rate:"), 0, 0); gridPane.add(tfAnnualInterestRate, 1, 0); gridPane.add(new Label("Number of Years:"), 0, 1); gridPane.add(tfNumberOfYears, 1, 1); gridPane.add(new Label("Loan Amount:"), 0, 2); gridPane.add(tfLoanAmount, 1, 2); gridPane.add(new Label("Monthly Payment:"), 0, 3); gridPane.add(tfMonthlyPayment, 1, 3); gridPane.add(new Label("Total Payment:"), 0, 4); gridPane.add(tfTotalPayment, 1, 4); gridPane.add(btCalculate, 1, 5);
add to grid pane
// Set properties for UI gridPane.setAlignment(Pos.CENTER); tfAnnualInterestRate.setAlignment(Pos.BOTTOM_RIGHT); tfNumberOfYears.setAlignment(Pos.BOTTOM_RIGHT); tfLoanAmount.setAlignment(Pos.BOTTOM_RIGHT); tfMonthlyPayment.setAlignment(Pos.BOTTOM_RIGHT); tfTotalPayment.setAlignment(Pos.BOTTOM_RIGHT); tfMonthlyPayment.setEditable(false); tfTotalPayment.setEditable(false); GridPane.setHalignment(btCalculate, HPos.RIGHT); // Process events btCalculate.setOnAction(e -> calculateLoanPayment());
register handler
// Create a scene and place it in the stage Scene scene = new Scene(gridPane, 400, 250); primaryStage.setTitle("LoanCalculator"); // Set title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } private void calculateLoanPayment() { // Get values from text fields double interest = Double.parseDouble(tfAnnualInterestRate.getText()); int year = Integer.parseInt(tfNumberOfYears.getText()); double loanAmount = Double.parseDouble(tfLoanAmount.getText()); // Create a loan object. Loan defined in Listing 10.2 Loan loan = new Loan(interest, year, loanAmount); // Display monthly payment and total payment tfMonthlyPayment.setText(String.format("$%.2f", loan.getMonthlyPayment())); tfTotalPayment.setText(String.format("$%.2f", loan.getTotalPayment())); } }
The user interface is created in the start method (lines 22–46). The button is the source of the event. A handler is created and registered with the button (line 49). The button handler invokes the calculateLoanPayment() method to get the interest rate (line 60), number of years (line 62), and loan amount (line 64). Invoking tfAnnualInterestRate.getText() returns the string text in the tfAnnualInterestRate text field. The Loan class is used for
get input
create loan
set result
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Event-Driven Programming and Animations computing the loan payments. This class was introduced in Listing 10.2, Loan.java. Invoking loan.getMonthlyPayment() returns the monthly payment for the loan (line 71). The String.format method, introduced in Section 10.10.7, is used to format a number into a desirable format and returns it as a string (lines 70, 72). Invoking the setText method on a text field sets a string value in the text field.
15.8 Mouse Events Key Point
A MouseEvent is fired whenever a mouse button is pressed, released, clicked, moved, or dragged on a node or a scene. The MouseEvent object captures the event, such as the number of clicks associated with it, the location (the x- and y-coordinates) of the mouse, or which mouse button was pressed, as shown in Figure 15.10.
javafx.scene.input.MouseEvent +getButton(): MouseButton
Indicates which mouse button has been clicked.
+getClickCount(): int +getX(): double +getY(): double +getSceneX(): double
Returns the number of mouse clicks associated with this event. Returns the x-coordinate of the mouse point in the event source node. Returns the y-coordinate of the mouse point in the event source node.
+getSceneY(): double +getScreenX(): double +getScreenY(): double +isAltDown(): boolean +isControlDown(): boolean +isMetaDown(): boolean +isShiftDown(): boolean
FIGURE 15.10
Returns the x-coordinate of the mouse point in the scene. Returns the y-coordinate of the mouse point in the scene. Returns the x-coordinate of the mouse point in the screen. Returns the y-coordinate of the mouse point in the screen. Returns true if the Alt key is pressed on this event. Returns true if the Control key is pressed on this event. Returns true if the mouse Meta button is pressed on this event. Returns true if the Shift key is pressed on this event.
The MouseEvent class encapsulates information for mouse events.
detect mouse buttons
Four constants—PRIMARY, SECONDARY, MIDDLE, and NONE—are defined in MouseButton to indicate the left, right, middle, and none mouse buttons. You can use the getButton() method to detect which button is pressed. For example, getButton() == MouseButton.SECONDARY indicates that the right button was pressed. The mouse events are listed in Table 15.1. To demonstrate using mouse events, we give an example that displays a message in a pane and enables the message to be moved using a mouse. The message moves as the mouse is dragged, and it is always displayed at the mouse point. Listing 15.7 gives the program. A sample run of the program is shown in Figure 15.11.
FIGURE 15.11
You can move the message by dragging the mouse.
LISTING 15.7 MouseEventDemo.java VideoNote
Move message using the mouse
1 2 3
import javafx.application.Application; import javafx.scene.Scene; import javafx.scene.layout.Pane;
15.9 Key Events 603 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
import javafx.scene.text.Text; import javafx.stage.Stage; public class MouseEventDemo extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { // Create a pane and set its properties Pane pane = new Pane(); Text text = new Text(20, 20, "Programming is fun"); pane.getChildren().addAll(text); text.setOnMouseDragged(e -> { text.setX(e.getX()); text.setY(e.getY()); });
create a pane create a text add text to a pane lambda handler reset text position
// Create a scene and place it in the stage Scene scene = new Scene(pane, 300, 100); primaryStage.setTitle("MouseEventDemo"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } }
Each node or scene can fire mouse events. The program creates a Text (line 12) and registers a handler to handle move dragged event (line 14). Whenever a mouse is dragged, the text’s x- and y-coordinates are set to the mouse position (lines 15 and 16).
15.14 What method do you use to get the mouse-point position for a mouse event? 15.15 What methods do you use to register a handler for a mouse pressed, released, clicked, entered, exited, moved and dragged event?
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Check Point
15.9 Key Events A KeyEvent is fired whenever a key is pressed, released, or typed on a node or a scene. Key events enable the use of the keys to control and perform actions or get input from the keyboard. The KeyEvent object describes the nature of the event (namely, that a key has been pressed, released, or typed) and the value of the key, as shown in Figure 15.12.
javafx.scene.input.KeyEvent +getCharacter(): String +getCode(): KeyCode +getText(): String +isAltDown(): boolean +isControlDown(): boolean +isMetaDown(): boolean +isShiftDown(): boolean
Returns the character associated with the key in this event. Returns the key code associated with the key in this event. Returns a string describing the key code. Returns true if the Alt key is pressed on this event. Returns true if the Control key is pressed on this event. Returns true if the mouse Meta button is pressed on this event. Returns true if the Shift key is pressed on this event.
FIGURE 15.12 The KeyEvent class encapsulates information about key events. Every key event has an associated code that is returned by the getCode() method in KeyEvent. The key codes are constants defined in KeyCode. Table 15.2 lists some constants. KeyCode is an enum type. For use of enum types, see Appendix I. For the key-pressed and
Key Point
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Event-Driven Programming and Animations key-released events, getCode() returns the value as defined in the table, getText() returns a string that describes the key code, and getCharacter() returns an empty string. For the key-typed event, getCode() returns UNDEFINED and getCharacter() returns the Unicode character or a sequence of characters associated with the key-typed event.
TABLE 15.2 KeyCode Constants Constant
Description
Constant
Description
HOME
The Home key
CONTROL
The Control key
END
The End key
SHIFT
The Shift key
PAGE_UP
The Page Up key
BACK_SPACE
The Backspace key
PAGE_DOWN
The Page Down key
CAPS
The Caps Lock key
UP
The up-arrow key
NUM_LOCK
The Num Lock key
DOWN
The down-arrow key
ENTER
The Enter key
LEFT
The left-arrow key
UNDEFINED
The keyCode unknown
RIGHT
The right-arrow key
F1 to F12
The function keys from F1 to F12
ESCAPE
The Esc key
0 to 9
The number keys from 0 to 9
TAB
The Tab key
A to Z
The letter keys from A to Z
The program in Listing 15.8 displays a user-input character. The user can move the character up, down, left, and right, using the up, down, left, and right arrow keys. Figure 15.13 contains a sample run of the program.
FIGURE 15.13 The program responds to key events by displaying a character and moving it up, down, left, or right.
LISTING 15.8 KeyEventDemo.java
create a pane
register handler get the key pressed move a character
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
import import import import import
javafx.application.Application; javafx.scene.Scene; javafx.scene.layout.Pane; javafx.scene.text.Text; javafx.stage.Stage;
public class KeyEventDemo extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { // Create a pane and set its properties Pane pane = new Pane(); Text text = new Text(20, 20, "A"); pane.getChildren().add(text); text.setOnKeyPressed(e -> { switch (e.getCode()) { case DOWN: text.setY(text.getY() + 10); break; case UP: text.setY(text.getY() - 10); break; case LEFT: text.setX(text.getX() - 10); break; case RIGHT: text.setX(text.getX() + 10); break;
15.9 Key Events 605 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35
default: if (Character.isLetterOrDigit(e.getText().charAt(0))) text.setText(e.getText());
set a new character
} }); // Create a scene and place it in the stage Scene scene = new Scene(pane); primaryStage.setTitle("KeyEventDemo"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage text.requestFocus(); // text is focused to receive key input
request focus on text
} }
The program creates a pane (line 11), creates a text (line 12), and places the text into the pane (line 14). The text registers the handler for the key-pressed event in lines 15–25. When a key is pressed, the handler is invoked. The program uses e.getCode() (line 16) to obtain the key code and e.getText() (line 23) to get the character for the key. When a nonarrow key is pressed, the character is displayed (lines 22 and 23). When an arrow key is pressed, the character moves in the direction indicated by the arrow key (lines 17–20). Note that in a switch statement for an enum type value, the cases are for the enum constants (lines 16–24). The constants are unqualified. For example, using KeyCode.DOWN in the case clause would be wrong (see Appendix I). Only a focused node can receive KeyEvent. Invoking requestFocus() on text enables text to receive key input (line 33). This method must be invoked after the stage is displayed. We can now add more control for our ControlCircle example in Listing 15.3 to increase/ decrease the circle radius by clicking the left/right mouse button or by pressing the U and D keys. The new program is given in Listing 15.9.
requestFocus()
LISTING 15.9 ControlCircleWithMouseAndKey.java 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
import import import import import import import import import
javafx.application.Application; javafx.geometry.Pos; javafx.scene.Scene; javafx.scene.control.Button; javafx.scene.input.KeyCode; javafx.scene.input.MouseButton; javafx.scene.layout.HBox; javafx.scene.layout.BorderPane; javafx.stage.Stage;
public class ControlCircleWithMouseAndKey extends Application { private CirclePane circlePane = new CirclePane(); @Override // Override the start method in the Application class public void start(Stage primaryStage) { // Hold two buttons in an HBox HBox hBox = new HBox(); hBox.setSpacing(10); hBox.setAlignment(Pos.CENTER); Button btEnlarge = new Button("Enlarge"); Button btShrink = new Button("Shrink"); hBox.getChildren().add(btEnlarge); hBox.getChildren().add(btShrink); // Create and register the handler btEnlarge.setOnAction(e -> circlePane.enlarge()); btShrink.setOnAction(e -> circlePane.shrink());
button handler
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mouse-click handler
key-pressed handler U key pressed
D key pressed
request focus
circlePane.setOnMouseClicked(e -> { if (e.getButton() == MouseButton.PRIMARY) { circlePane.enlarge(); } else if (e.getButton() == MouseButton.SECONDARY) { circlePane.shrink(); } }); circlePane.setOnKeyPressed(e -> { if (e.getCode() == KeyCode.U) { circlePane.enlarge(); } else if (e.getCode() == KeyCode.D) { circlePane.shrink(); } }); BorderPane borderPane = new BorderPane(); borderPane.setCenter(circlePane); borderPane.setBottom(hBox); BorderPane.setAlignment(hBox, Pos.CENTER); // Create a scene and place it in the stage Scene scene = new Scene(borderPane, 200, 150); primaryStage.setTitle("ControlCircle"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage circlePane.requestFocus(); // Request focus on circlePane } }
The CirclePane class (line 12) is already defined in Listing 15.3 and can be reused in this program. A handler for mouse clicked events is created in lines 29–36. If the left mouse button is clicked, the circle is enlarged (lines 30–32); if the right mouse button is clicked, the circle is shrunk (lines 33–35). A handler for key pressed events is created in lines 38–45. If the U key is pressed, the circle is enlarged (lines 39–41); if the D key is pressed, the circle is shrunk (lines 42–44). Invoking requestFocus() on circlePane (line 58) makes circlePane to receive key events. Note that after you click a button, circlePane is no longer focused. To fix the problem, invoke reuquestFocus() on circlePane again after each button is clicked.
mouse clicked event key pressed event requestFocus()
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Check Point
15.16 What methods do you use to register handlers for key pressed, key released, and key typed events? In which classes are these methods defined? (See Table 15.1) 15.17 What method do you use to get the key character for a key-typed event? What method do you use to get the key code for a key-pressed or key-released event? 15.18 How do you set focus on a node so it can listen for key events?
15.10 Listeners for Observable Objects Key Point
You can add a listener to process a value change in an observable object. An instance of Observable is known as an observable object, which contains the addListener(InvalidationListener listener) method for adding a listener. The listener class must implement the InvalidationListener interface to override the invalidated(Observable o) method for handling the value change. Once
15.10 Listeners for Observable Objects 607 the value is changed in the Observable object, the listener is notified by invoking its invalidated(Observable o) method. Every binding property is an instance of Observable. Listing 15.10 gives an example of observing and handling a change in a DoubleProperty object balance.
observable object
LISTING 15.10 ObservablePropertyDemo.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
import import import import
javafx.beans.InvalidationListener; javafx.beans.Observable; javafx.beans.property.DoubleProperty; javafx.beans.property.SimpleDoubleProperty;
public class ObservablePropertyDemo { public static void main(String[] args) { DoubleProperty balance = new SimpleDoubleProperty(); balance.addListener(new InvalidationListener() { public void invalidated(Observable ov) { System.out.println("The new value is " + balance.doubleValue()); } }); balance.set(4.5); } }
The new value is 4.5
When line 16 is executed, it causes a change in balance, which notifies the listener by invoking the listener’s invalidated method. Note that the anonymous inner class in lines 9–14 can be simplified using a lambda expression as follows: balance.addListener(ov -> { System.out.println("The new value is " + balance.doubleValue()); });
Recall that in Listing 14.20 DisplayClock.java, the clock pane size is not changed when you resize the window. The problem can be fixed by adding a listener to change the clock pane size and register the listener to the window’s width and height properties, as shown in Listing 15.11.
LISTING 15.11 DisplayResizableClock.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14
import import import import import import
javafx.application.Application; javafx.geometry.Pos; javafx.stage.Stage; javafx.scene.Scene; javafx.scene.control.Label; javafx.scene.layout.BorderPane;
public class DisplayResizableClock extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { // Create a clock and a label ClockPane clock = new ClockPane(); String timeString = clock.getHour() + ":" + clock.getMinute() + ":" + clock.getSecond();
observable property add listener handle change
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create a listener set a new width for clock
create a listener set a new height for clock
15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37
Label lblCurrentTime = new Label(timeString); // Place clock and label in border pane BorderPane pane = new BorderPane(); pane.setCenter(clock); pane.setBottom(lblCurrentTime); BorderPane.setAlignment(lblCurrentTime, Pos.TOP_CENTER); // Create a scene and place it in the stage Scene scene = new Scene(pane, 250, 250); primaryStage.setTitle("DisplayClock"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage pane.widthProperty().addListener(ov -> clock.setW(pane.getWidth()) ); pane.heightProperty().addListener(ov -> clock.setH(pane.getHeight()) ); } }
The program is identical to Listing 14.19 except that you added the code in lines 29–35 to register listeners for resizing the clock pane upon a change of the width or height of the scene. The code ensures that the clock pane size is synchronized with the scene size.
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15.19 What would happen if you replace pane with scene or primaryStage in lines 29 and 33?
15.11 Animation Key Point
JavaFX provides the Animation class with the core functionality for all animations. Suppose you want to write a program that animates a rising flag, as shown in Figure 15.14. How do you accomplish the task? There are several ways to program this. An effective one is to use the subclasses of the JavaFX Animation class, which is the subject of this section.
VideoNote
Animate a rising flag
FIGURE 15.14
The animation simulates a flag rising.
The abstract Animation class provides the core functionalities for animations in JavaFX, as shown in Figure 15.15. Many concrete subclasses of Animation are provided in JavaFX. This section introduces PathTransition, FadeTransition and Timeline.
15.11 Animation 609
javafx.animation.Animation -autoReverse: BooleanProperty -cycleCount: IntegerProperty -rate: DoubleProperty
The getter and setter methods for property values and a getter for property itself are provided in the class, but omitted in the UML diagram for brevity.
Defines whether the animation reverses direction on alternating cycles. Defines the number of cycles in this animation. Defines the speed and direction for this animation.
-status: ReadOnlyObjectProperty
Read-only property to indicate the status of the animation.
+pause(): void +play(): void
Pauses the animation.
+stop(): void
Stops the animation and resets the animation.
Plays the animation from the current position.
FIGURE 15.15 The abstract Animation class is the root class for JavaFX animations. The autoReverse is a Boolean property that indicates whether an animation will reverse its direction on the next cycle. The cycleCount indicates the number of the cycles for the animation. You can use the constant Timeline.INDEFINTE to indicate an indefinite number of cycles. The rate defines the speed of the animation. A negative rate value indicates the opposite direction for the animation. The status is a read-only property that indicates the status of the animation (Animation.Status.PAUSED, Animation.Status.RUNNING, and Animation.Status.STOPPED). The methods pause(), play(), and stop() pauses, plays, and stops an animation.
15.11.1 PathTransition The PathTransition class animates the the moves of a node along a path from one end to the other over a given time. PathTransition is a subtype of Animation. The UML class diagram for the class is shown in Figure 15.16.
javafx.animation.PathTransition
The getter and setter methods for property values and a getter for property itself are provided in the class, but omitted in the UML diagram for brevity.
-duration: ObjectProperty -node: ObjectProperty -orientation: ObjectProperty -path: ObjectType
The duration of this transition. The target node of this transition. The orientation of the node along the path.
+PathTransition() +PathTransition(duration: Duration, path: Shape) +PathTransition(duration: Duration, path: Shape, node: Node)
Creates an empty PathTransition. Creates a PathTransition with the specified duration and path.
The shape whose outline is used as a path to animate the node move.
Creates a PathTransition with the specified duration, path, and node.
FIGURE 15.16 The PathTransition class defines an animation for a node along a path. The Duration class defines a duration of time. It is an immutable class. The class defines constants INDEFINTE, ONE, UNKNOWN, and ZERO to represent an indefinte duration, 1 milliseconds, unknow, and 0 duration. You can use new Duration(double millis) to create
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LISTING 15.12 PathTransitionDemo.java
create a pane
create a rectangle
create a circle
add circle to pane add rectangle to pane
create a PathTransition set transition duration set path in transition set node in transition set orientation set cycle count indefinite set auto reverse true play animation pause animation resume animation
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
import import import import import import import import import import
javafx.animation.PathTransition; javafx.animation.Timeline; javafx.application.Application; javafx.scene.Scene; javafx.scene.layout.Pane; javafx.scene.paint.Color; javafx.scene.shape.Rectangle; javafx.scene.shape.Circle; javafx.stage.Stage; javafx.util.Duration;
public class PathTransitionDemo extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { // Create a pane Pane pane = new Pane(); // Create a rectangle Rectangle rectangle = new Rectangle (0, 0, 25, 50); rectangle.setFill(Color.ORANGE); // Create a circle Circle circle = new Circle(125, 100, 50); circle.setFill(Color.WHITE); circle.setStroke(Color.BLACK); // Add circle and rectangle to the pane pane.getChildren().add(circle); pane.getChildren().add(rectangle); // Create a path transition PathTransition pt = new PathTransition(); pt.setDuration(Duration.millis(4000)); pt.setPath(circle); pt.setNode(rectangle); pt.setOrientation( PathTransition.OrientationType.ORTHOGONAL_TO_TANGENT); pt.setCycleCount(Timeline.INDEFINITE); pt.setAutoReverse(true); pt.play(); // Start animation circle.setOnMousePressed(e -> pt.pause()); circle.setOnMouseReleased(e -> pt.play()); // Create a scene and place it in the stage Scene scene = new Scene(pane, 250, 200); primaryStage.setTitle("PathTransitionDemo"); // Set the stage title
15.11 Animation 611 48 49 50 51
primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } }
(a)
FIGURE 15.17
(b)
The PathTransition animates a rectangle moving along the circle.
The program creates a pane (line 16), a rectangle (line 19), and a circle (line 23). The circle and rectangle are placed in the pane (lines 28 and 29). If the circle was not placed in the pane, you will see the screen shot as shown in Figure 15.17b. The program creates a path transition (line 32), sets its duration to 4 seconds for one cycle of animation (line 33), sets circle as the path (line 34), sets rectangle as the node (line 35), and sets the orientation to orthogonal to tangent (line 36). The cycle count is set to indefinite (line 38) so the animation continues forever. The auto reverse is set to true (line 39) so that the direction of the move is reversed in the alternating cycle. The program starts animation by invoking the play() method (line 40). If the pause() method is replaced by the stop() method in line 42, the animation will start over from the beginning when it restarts. Listing 15.13 gives the program that animates a flag rising, as shown in Figure 15.14.
LISTING 15.13 FlagRisingAnimation.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
import import import import import import import import
javafx.animation.PathTransition; javafx.application.Application; javafx.scene.Scene; javafx.scene.image.ImageView; javafx.scene.layout.Pane; javafx.scene.shape.Line; javafx.stage.Stage; javafx.util.Duration;
public class FlagRisingAnimation extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { // Create a pane Pane pane = new Pane();
create a pane
// Add an image view and add it to pane ImageView imageView = new ImageView("image/us.gif"); pane.getChildren().add(imageView);
create an image view add image view to pane
// Create a path transition PathTransition pt = new PathTransition(Duration.millis(10000),
create a path transition
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set cycle count play animation
new Line(100, 200, 100, 0), imageView); pt.setCycleCount(5); pt.play(); // Start animation // Create a scene and place it in the stage Scene scene = new Scene(pane, 250, 200);
28
29 30 31 32
primaryStage.setTitle("FlagRisingAnimation"); // Set the stage title
primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } }
The program creates a pane (line 14), an image view from an image file (line 17), and places the image view to the page (line 18). A path transition is created with duration of 10 seconds using a line as a path and the image view as the node (lines 21 and 22). The image view will move along the line. Since the line is not placed in the scene, you will not see the line in the window. The cycle count is set to 5 (line 23) so that the animation is repeated five times.
15.11.2 FadeTransition The FadeTransition class animates the change of the opacity in a node over a given time. FadeTransition is a subtype of Animation. The UML class diagram for the class is shown in Figure 15.18.
javafx.animation.FadeTransition -duration: ObjectProperty -node: ObjectProperty -fromValue: DoubleProperty -toValue: DoubleProperty -byValue: DoubleProperty
The getter and setter methods for property values and a getter for property itself are provided in the class, but omitted in the UML diagram for brevity.
The duration of this transition. The target node of this transition. The start opacity for this animation. The stop opacity for this animation. The incremental value on the opacity for this animation.
+FadeTransition() +FadeTransition(duration: Duration) +FadeTransition(duration: Duration, node: Node)
Creates an empty FadeTransition. Creates a FadeTransition with the specified duration. Creates a FadeTransition with the specified duration and node.
FIGURE 15.18 The FadeTransition class defines an animation for the change of opacity in a node. Listing 15.14 gives an example that applies a fade transition to the filled color in an ellipse, as shown in Figure 15.19.
LISTING 15.14 FadeTransitionDemo.java 1 2 3 4 5 6 7 8
import import import import import import import import
javafx.animation.FadeTransition; javafx.animation.Timeline; javafx.application.Application; javafx.scene.Scene; javafx.scene.layout.Pane; javafx.scene.paint.Color; javafx.scene.shape.Ellipse; javafx.stage.Stage;
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import javafx.util.Duration; public class FadeTransitionDemo extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { // Place an ellipse to the pane Pane pane = new Pane(); Ellipse ellipse = new Ellipse(10, 10, 100, 50); ellipse.setFill(Color.RED); ellipse.setStroke(Color.BLACK); ellipse.centerXProperty().bind(pane.widthProperty().divide(2)); ellipse.centerYProperty().bind(pane.heightProperty().divide(2)); ellipse.radiusXProperty().bind( pane.widthProperty().multiply(0.4)); ellipse.radiusYProperty().bind( pane.heightProperty().multiply(0.4)); pane.getChildren().add(ellipse); // Apply a fade transition to ellipse FadeTransition ft = new FadeTransition(Duration.millis(3000), ellipse); ft.setFromValue(1.0); ft.setToValue(0.1); ft.setCycleCount(Timeline.INDEFINITE); ft.setAutoReverse(true); ft.play(); // Start animation // Control animation ellipse.setOnMousePressed(e -> ft.pause()); ellipse.setOnMouseReleased(e -> ft.play()); // Create a scene and place it in the stage Scene scene = new Scene(pane, 200, 150); primaryStage.setTitle("FadeTransitionDemo"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } }
FIGURE 15.19
The FadeTransition animates the change of opacity in the ellipse.
The program creates a pane (line 15) and an ellipse (line 16) and places the ellipse into the pane (line 25). The ellipse’s centerX, centerY, radiusX, and radiusY properties are bound to the pane’s size (lines 19–24). A fade transition is created with a duration of 3 seconds for the ellipse (line 29). It sets the start opaque to 1.0 (line 30) and the stop opaque 0.1 (line 31). The cycle count is set to infinite so the animation is repeated indefinitely (line 32). When the mouse is pressed, the animation is paused (line 37). When the mouse is released, the animation resumes from where it was paused (line 38).
create a pane create an ellipse set ellipse fill color set ellipse stroke color bind ellipse properties
add ellipse to pane
create a FadeTransition set start opaque value set end opaque value set cycle count set auto reverse true play animation
pause animation resume animation
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15.11.3 Timeline PathTransition and FadeTransition define specialized animations. The Timeline class can be used to program any animation using one or more KeyFrames. Each KeyFrame is executed sequentially at a specified time interval. Timeline inherits from Animation. You can construct a Timeline using the constructor new Timeline(KeyFrame... keyframes). A KeyFrame can be constructed using new KeyFrame(Duration duration, EventHandler onFinished)
The handler onFinished is called when the duration for the key frame is elapsed. Listing 15.15 gives an example that displays a flashing text, as shown in Figure 15.20. The text is on and off alternating to animate flashing.
LISTING 15.15 TimelineDemo.java VideoNote
Flashing text
create a stack pane create a text add text to pane
handler for changing text set text empty
set text
create a Timeline create a KeyFrame for handler set cycle count indefinite play animation
resume animation
pause animation
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
import import import import import import import import import import import import
javafx.animation.Animation; javafx.application.Application; javafx.stage.Stage; javafx.animation.KeyFrame; javafx.animation.Timeline; javafx.event.ActionEvent; javafx.event.EventHandler; javafx.scene.Scene; javafx.scene.layout.StackPane; javafx.scene.paint.Color; javafx.scene.text.Text; javafx.util.Duration;
public class TimelineDemo extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { StackPane pane = new StackPane(); Text text = new Text(20, 50, "Programming is fun"); text.setFill(Color.RED); pane.getChildren().add(text); // Place text into the stack pane // Create a handler for changing text EventHandler eventHandler = e -> { if (text.getText().length() != 0) { text.setText(""); } else { text.setText("Programming is fun"); } }; // Create an animation for alternating text Timeline animation = new Timeline( new KeyFrame(Duration.millis(500), eventHandler)); animation.setCycleCount(Timeline.INDEFINITE); animation.play(); // Start animation // Pause and resume animation text.setOnMouseClicked(e -> { if (animation.getStatus() == Animation.Status.PAUSED) { animation.play(); } else { animation.pause();
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} }); // Create a scene and place it in the stage Scene scene = new Scene(pane, 250, 250); primaryStage.setTitle("TimelineDemo"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } }
FIGURE 15.20
The handler is called to set the text to Programming is fun or empty in turn.
The program creates a stack pane (line 17) and a text (line 18) and places the text into the pane (line 20). A handler is created to change the text to empty (lines 24–26) if it is not empty or to Progrmming is fun if it is empty (lines 27–29). A KeyFrame is created to run an action event in every half second (line 34). A Timeline animation is created to contain a key frame (lines 33 and 34). The animation is set to run indefinitely (line 35). The mouse clicked event is set for the text (lines 39–46). A mouse click on the text resumes the animation if the animation is paused (lines 40–42), and a mouse click on the text pauses the animation if the animation is running (lines 43–45). In Section 14.12, Case Study: The ClockPane Class, you drew a clock to show the current time. The clock does not tick after it is displayed. What can you do to make the clock display a new current time every second? The key to making the clock tick is to repaint it every second with a new current time. You can use a Timeline to control the repainting of the clock with the code in Listing 15.16. The sample run of the program is shown in Figure 15.21.
LISTING 15.16 ClockAnimation.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
import import import import import import import import
javafx.application.Application; javafx.stage.Stage; javafx.animation.KeyFrame; javafx.animation.Timeline; javafx.event.ActionEvent; javafx.event.EventHandler; javafx.scene.Scene; javafx.util.Duration;
public class ClockAnimation extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { ClockPane clock = new ClockPane(); // Create a clock // Create a handler for animation EventHandler eventHandler = e -> { clock.setCurrentTime(); // Set a new clock time }; // Create an animation for a running clock Timeline animation = new Timeline(
create a clock
create a handler
create a time line
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create a key frame set cycle count indefinite play animation
22 23 24 25 26 27 28 29 30 31 32
new KeyFrame(Duration.millis(1000), eventHandler)); animation.setCycleCount(Timeline.INDEFINITE); animation.play(); // Start animation // Create a scene and place it in the stage Scene scene = new Scene(clock, 250, 50); primaryStage.setTitle("ClockAnimation"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } }
FIGURE 15.21
A live clock is displayed in the window.
The program creates an instance clock of ClockPane for displaying a clock (line 13). The ClockPane class is defined in Listing 14.21. The clock is placed in the scene in line 27. An event handler is created for setting the current time in the clock (lines 16–18). This handler is called every second in the key frame in the time line animation (lines 21–24). So the clock time is updated every second in the animation.
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Check Point
15.20 How do you set the cycle count of an animation to infinite? How do you auto reverse an animation? How do you start, pause, and stop an animation? 15.21 Are PathTransition, FadeTransition, and Timeline a subtype of Animation? 15.22 How do you create a PathTransition? How do you create a FadeTransition? How do you create a Timeline? 15.23 How do you create a KeyFrame?
15.12 Case Study: Bouncing Ball Key Point
This section presents an animation that displays a ball bouncing in a pane. The program uses Timeline to animation ball bouncing, as shown in Figure 15.22.
FIGURE 15.22
A ball is bouncing in a pane.
15.12 Case Study: Bouncing Ball 617 Here are the major steps to write this program: 1. Define a subclass of Pane named BallPane to display a ball bouncing, as shown in Listing 15.17. 2. Define a subclass of Application named BounceBallControl to control the bouncing ball with mouse actions, as shown in Listing 15.18. The animation pauses when the mouse is pressed and resumes when the mouse is released. Pressing the UP and DOWN arrow keys increases/decreases animation speed. The relationship among these classes is shown in Figure 15.23.
javafx.scene.layout.Pane
BallPane
javafx.application.Application
1
1
BounceBallControl
-x: double -y: double -dx: double -dy: double -radius: double -circle: Circle -animation: Timeline +BallPane() +play(): void +pause(): void +increaseSpeed(): void +decreaseSpeed(): void +rateProperty(): DoubleProperty +moveBall(): void
FIGURE 15.23 BounceBallControl contains BallPane.
LISTING 15.17 BallPane.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
import import import import import import import
javafx.animation.KeyFrame; javafx.animation.Timeline; javafx.beans.property.DoubleProperty; javafx.scene.layout.Pane; javafx.scene.paint.Color; javafx.scene.shape.Circle; javafx.util.Duration;
public class BallPane extends Pane { public final double radius = 20; private double x = radius, y = radius; private double dx = 1, dy = 1; private Circle circle = new Circle(x, y, radius); private Timeline animation; public BallPane() { circle.setFill(Color.GREEN); // Set ball color getChildren().add(circle); // Place a ball into this pane
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create animation keep animation running start animation
play animation
pause animation
increase animation rate
decrease animation rate
change horizontal direction
change verticaal direction
set new ball position
Event-Driven Programming and Animations 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
// Create an animation for moving the ball animation = new Timeline( new KeyFrame(Duration.millis(50), e -> moveBall())); animation.setCycleCount(Timeline.INDEFINITE); animation.play(); // Start animation } public void play() { animation.play(); } public void pause() { animation.pause(); } public void increaseSpeed() { animation.setRate(animation.getRate() + 0.1); } public void decreaseSpeed() { animation.setRate( animation.getRate() > 0 ? animation.getRate() - 0.1 : 0); } public DoubleProperty rateProperty() { return animation.rateProperty(); } protected void moveBall() { // Check boundaries if (x < radius || x > getWidth() - radius) { dx *= -1; // Change ball move direction } if (y < radius || y > getHeight() - radius) { dy *= -1; // Change ball move direction } // Adjust ball position x += dx; y += dy; circle.setCenterX(x); circle.setCenterY(y); } }
BallPane extends Pane to display a moving ball (line 9). An instance of Timeline is created to control animation (lines 21 and 22). This instance contains a KeyFrame object that invokes the moveBall() method at a fixed rate. The moveBall() method moves the ball to simulate animation. The center of the ball is at (x, y), which changes to (x + dx, y + dy) on the next move (lines 58–61). When the ball is out of the horizontal boundary, the sign of dx is changed (from positive to negative or vice versa) (lines 50–52). This causes the ball to change its horizontal movement direction. When the ball is out of the vertical boundary, the sign of dy is changed (from positive to negative or vice versa) (lines 53–55). This causes the ball to change its vertical movement direction. The pause and play methods (lines 27–33) can be used to pause and resume the animation. The increaseSpeed() and decreaseSpeed() methods (lines 35–42) can be used to increase and decrease animation speed. The rateProperty()
15.12 Case Study: Bouncing Ball 619 method (lines 44–46) returns a binding property value for rate. This binding property is useful for binding the rate in future applications in the next chapter.
LISTING 15.18 BounceBallControl.java 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
import import import import
javafx.application.Application; javafx.stage.Stage; javafx.scene.Scene; javafx.scene.input.KeyCode;
public class BounceBallControl extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { BallPane ballPane = new BallPane(); // Create a ball pane // Pause and resume animation ballPane.setOnMousePressed(e -> ballPane.pause()); ballPane.setOnMouseReleased(e -> ballPane.play()); // Increase and decrease animation ballPane.setOnKeyPressed(e -> { if (e.getCode() == KeyCode.UP) { ballPane.increaseSpeed(); } else if (e.getCode() == KeyCode.DOWN) { ballPane.decreaseSpeed(); } });
create a ball pane
pause animation resume animation
increase speed
decrease speed
// Create a scene and place it in the stage Scene scene = new Scene(ballPane, 250, 150); primaryStage.setTitle("BounceBallControl"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage // Must request focus after the primary stage is displayed ballPane.requestFocus();
request focus on pane
} }
The BounceBallControl class is the main JavaFX class that extends Applicaiton to display the ball pane with control functions. The mouse-pressed and mouse-released handlers are implemented for the ball pane to pause the animation and resume the animation (lines 12 and 13). When the UP arrow key is pressed, the ball pane’s increaseSpeed() method is invoked to increase the ball’s movement (line 18). When the DOWN arror key is pressed, the ball pane’s decreaseSpeed() method is invoked to reduce the ball’s movement (line 21). Invoking ballPane.requestFocus() in line 32 sets the input focus to ballPane.
15.24 How does the program make the ball moving? 15.25 How does the code in Listing 15.17 BallPane.java change the direction of the ball movement?
15.26 What does the program do when the mouse is pressed on the ball pane? What does the program do when the mouse is released on the ball pane? 15.27 If line 32 in Listing 15.18 BounceBallControl.java is not in the program, what would happen when you press the UP or the DOWN arrow key?
15.28 If line 23 is not in Listing 15.17, what would happen?
✓
Check Point
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KEY TERMS anonymous inner class 594 event 588 event-driven programming 588 event handler 589 event-handler interface 589 event object 588 event source object 588
functional interface 598 lambda expression 597 inner class 591 key code 604 observable object 607 single abstract method interface
598
CHAPTER SUMMARY 1. The root class of the JavaFX event classes is javafx.event.Event, which is a subclass of java.util.EventObject. The subclasses of Event deal with special types of events, such as action events, window events, mouse events, and key events. If a node can fire an event, any subclass of the node can fire the same type of event.
2. The handler object’s class must implement the corresponding event-handler interface. JavaFX provides a handler interface EventHandler for every event class T. The handler interface contains the handle(T e) method for handling event e.
3. The handler object must be registered by the source object. Registration methods depend on the event type. For an action event, the method is setOnAction. For a mouse-pressed event, the method is setOnMousePressed. For a key-pressed event, the method is setOnKeyPressed.
4. An inner class, or nested class, is defined within the scope of another class. An inner class can reference the data and methods defined in the outer class in which it nests, so you need not pass the reference of the outer class to the constructor of the inner class.
5. An anonymous inner class can be used to shorten the code for event handling. Furthermore, a lambda expression can be used to greatly simplify the event-handling code for functional interface handlers.
6. A functional interface is an interface with exactly one abstract method. This is also known as a single abstract method (SAM) interface.
7. A MouseEvent is fired whenever a mouse button is pressed, released, clicked, moved, or dragged on a node or a scene. The getButton() method can be used to detect which mouse button is pressed for the event.
8. A KeyEvent is fired whenever a key is pressed, released, or typed on a node or a scene. The getCode() method can be used to return the code value for the key.
9. An instance of
Observable is known as an observable object, which contains the addListener(InvalidationListener listener) method for adding a listener.
Once the value is changed in the property, a listener is notified. The listener class should implement the InvalidationListener interface, which uses the invalidated method to handle the property value change.
10. The abstract
Animation class provides the core functionalities for animations in JavaFX. PathTransition, FadeTransition, and Timeline are specialized classes for implementing animations.
Programming Exercises 621
QUIZ Answer the quiz for this chapter online at www.cs.armstrong.edu/liang/intro10e/quiz.html.
PROGRAMMING EXERCISES Sections 15.2–15.7
*15.1 (Pick four cards) Write a program that lets the user click the Refresh button to display four cards from a deck of 52 cards, as shown in Figure 15.24a. (See the hint in Programming Exercise 14.3 on how to obtain four random cards.)
(a)
(b)
(c)
FIGURE 15.24 (a) Exercise 15.1 displays four cards randomly. (b) Exercise 15.2 rotates the rectangle. (c) Exercise 15.3 uses the buttons to move the ball.
15.2 (Rotate a rectangle) Write a program that rotates a rectangle 15 degrees right *15.3
when the Rotate button is clicked, as shown in Figure 15.24b. (Move the ball) Write a program that moves the ball in a pane. You should define a pane class for displaying the ball and provide the methods for moving the ball left, right, up, and down, as shown in Figure 15.24c. Check the boundary to prevent the ball from moving out of sight completely.
*15.4 (Create a simple calculator) Write a program to perform addition, subtraction, multiplication, and division, as shown in Figure 15.25a.
VideoNote
Simple calculator
(a)
(b)
FIGURE 15.25 (a) Exercise 15.4 performs addition, subtraction, multiplication, and division on double numbers. (b) The user enters the investment amount, years, and interest rate to compute future value.
*15.5 (Create an investment-value calculator) Write a program that calculates the future value of an investment at a given interest rate for a specified number of years. The formula for the calculation is: futureValue = investmentAmount * (1 + monthlyInterestRate)years*12
622 Chapter 15
Event-Driven Programming and Animations Use text fields for the investment amount, number of years, and annual interest rate. Display the future amount in a text field when the user clicks the Calculate button, as shown in Figure 15.25b.
Sections 15.8 and 15.9
**15.6 (Alternate two messages) Write a program to display the text Java
is fun
and Java is powerful alternately with a mouse click.
*15.7 (Change color using a mouse) Write a program that displays the color of a *15.8
circle as black when the mouse button is pressed and as white when the mouse button is released. (Display the mouse position) Write two programs, such that one displays the mouse position when the mouse button is clicked (see Figure 15.26a) and the other displays the mouse position when the mouse button is pressed and ceases to display it when the mouse button is released.
(a)
(b)
FIGURE 15.26 (a) Exercise 15.8 displays the mouse position. (b) Exercise 15.9 uses the arrow keys to draw the lines.
*15.9 (Draw lines using the arrow keys) Write a program that draws line segments
**15.10 *15.11 **15.12 VideoNote
Check mouse point location
(a)
FIGURE 15.27
using the arrow keys. The line starts from the center of the pane and draws toward east, north, west, or south when the right-arrow key, up-arrow key, leftarrow key, or down-arrow key is pressed, as shown in Figure 15.26b. (Enter and display a string) Write a program that receives a string from the keyboard and displays it on a pane. The Enter key signals the end of a string. Whenever a new string is entered, it is displayed on the pane. (Move a circle using keys) Write a program that moves a circle up, down, left, or right using the arrow keys. (Geometry: inside a circle?) Write a program that draws a fixed circle centered at (100, 60) with radius 50. Whenever the mouse is moved, display a message indicating whether the mouse point is inside the circle at the mouse point or outside of it, as shown in Figure 15.27a.
(b)
Detect whether a point is inside a circle, a rectangle, or a polygon.
(c)
Programming Exercises 623 **15.13 (Geometry: inside a rectangle?) Write a program that draws a fixed rectangle
**15.14
centered at (100, 60) with width 100 and height 40. Whenever the mouse is moved, display a message indicating whether the mouse point is inside the rectangle at the mouse point or outside of it, as shown in Figure 15.27b. To detect whether a point is inside a polygon, use the contains method defined in the Node class. (Geometry: inside a polygon?) Write a program that draws a fixed polygon with points at (40, 20), (70, 40), (60, 80), (45, 45), and (20, 60). Whenever the mouse is moved, display a message indicating whether the mouse point is inside the polygon at the mouse point or outside of it, as shown in Figure 15.27c. To detect whether a point is inside a polygon, use the contains method defined in the Node class.
**15.15 (Geometry: add and remove points) Write a program that lets the user click on a pane to dynamically create and remove points (see Figure 15.28a). When the user left-clicks the mouse (primary button), a point is created and displayed at the mouse point. The user can remove a point by pointing to it and rightclicking the mouse (secondary button).
FIGURE 15.28 (a) Exercise 15.15 allows the user to create/remove points dynamically. (b) Exercise 15.16 displays two vertices and a connecting edge.
*15.16 (Two movable vertices and their distances) Write a program that displays two circles with radius 10 at location (40, 40) and (120, 150) with a line connecting the two circles, as shown in Figure 15.28b. The distance between the circles is displayed along the line. The user can drag a circle. When that happens, the circle and its line are moved and the distance between the circles is updated.
**15.17 (Geometry: find the bounding rectangle) Write a program that enables the user to add and remove points in a two-dimensional plane dynamically, as shown in Figure 15.29a. A minimum bounding rectangle is updated as the points are added and removed. Assume that the radius of each point is 10 pixels.
(a)
(b)
(c)
FIGURE 15.29 (a) Exercise 15.17 enables the user to add/remove points dynamically and displays the bounding rectangle. (b) When you click a circle, a new circle is displayed at a random location. (c) After 20 circles are clicked, the time spent is displayed in the pane.
624 Chapter 15
Event-Driven Programming and Animations **15.18 (Move a rectangle using mouse) Write a program that displays a rectangle. You can point the mouse inside the rectangle and drag (i.e., move with mouse pressed) the rectangle wherever the mouse goes. The mouse point becomes the center of the rectangle.
**15.19 (Game: eye-hand coordination) Write a program that displays a circle of radius
**15.20
10 pixels filled with a random color at a random location on a pane, as shown in Figure 15.29b. When you click the circle, it disappears and a new randomcolor circle is displayed at another random location. After twenty circles are clicked, display the time spent in the pane, as shown in Figure 15.29c. (Geometry: display angles) Write a program that enables the user to drag the vertices of a triangle and displays the angles dynamically as the triangle shape changes, as shown in Figure 15.30a. The formula to compute angles is given in Listing 4.1.
(a)
(b)
FIGURE 15.30 (a) Exercise 15.20 enables the user to drag vertices and display the angles dynamically. (b) Exercise 15.21 enables the user to drag vertices along the circle and display the angles in the triangle dynamically.
*15.21 (Drag points) Draw a circle with three random points on the circle. Connect the points to form a triangle. Display the angles in the triangle. Use the mouse to drag a point along the perimeter of the circle. As you drag it, the triangle and angles are redisplayed dynamically, as shown in Figure 15.30b. For computing angles in a triangle, see Listing 4.1.
Section 15.10
*15.22 (Auto resize cylinder) Rewrite Programming Exercise 14.10 so that the cylin*15.23
der’s width and height are automatically resized when the window is resized. (Auto resize stop sign) Rewrite Programming Exercise 14.15 so that the stop sign’s width and height are automatically resized when the window is resized.
Section 15.11
**15.24 (Animation: palindrome) Write a program that animates a palindrome swing as shown in Figure 15.31. Press/release the mouse to pause/resume the animation.
FIGURE 15.31
The program animates a palindrome swing.
Programming Exercises 625 **15.25 (Animation: ball on curve) Write a program that animates a ball moving along a sine curve, as shown in Figure 15.32. When the ball gets to the right border, it starts over from the left. Enable the user to resume/pause the animation with a click on the left/right mouse button.
FIGURE 15.32
The program animates a ball traveling along a sine curve.
*15.26 (Change opacity) Rewrite Programming Exercise 15.24 so that the ball’s *15.27
opacity is changed as it swings. (Control a moving text) Write a program that displays a moving text, as shown in Figure 15.33a and b. The text moves from left to right circularly. When it disappears in the right, it reappears from the left. The text freezes when the mouse is pressed and moves again when the button is released.
(a)
FIGURE 15.33
(b)
(c)
(a and b) A text is moving from left to right circularly. (c) The program simulates a fan running.
**15.28 (Display a running fan) Write a program that displays a running fan, as shown **15.29
**15.30
in Figure 15.33c. Use the Pause, Resume, Reverse buttons to pause, resume, and reverse fan running. (Racing car) Write a program that simulates car racing, as shown in Figure 15.34a. The car moves from left to right. When it hits the right end, it restarts from the left and continues the same process. You can use a timer to control animation. Redraw the car with a new base coordinates (x, y), as shown in Figure 15.34b. Also let the user pause/resume the animation with a button press/release and increase/decrease the car speed by pressing the UP and DOWN arrow keys. (Slide show) Twenty-five slides are stored as image files (slide0.jpg, slide1 .jpg, . . . , slide24.jpg) in the image directory downloadable along with the source code in the book. The size of each image is 800 * 600. Write a program that automatically displays the slides repeatedly. Each slide is shown for
VideoNote
Display a running fan
626 Chapter 15
Event-Driven Programming and Animations x
x 20
x 40
y30 y20 y10 y (x, y) (a)
FIGURE 15.34
(a) The program displays a moving car. (b) You can redraw a car with a new base point.
**15.31
FIGURE 15.35
(b)
two seconds. The slides are displayed in order. When the last slide finishes, the first slide is redisplayed, and so on. Click to pause if the animation is currently playing. Click to resume if the animation is currently paused. (Geometry: pendulum) Write a program that animates a pendulum swinging, as shown in Figure 15.35. Press the UP arrow key to increase the speed and the DOWN key to decrease it. Press the S key to stop animation and the R key to resume it.
Exercise 15.31 animates a pendulum swinging.
*15.32 (Control a clock) Modify Listing 14.21, ClockPane.java, to add the animation into this class and add two methods start() and stop() to start and stop the clock. Write a program that lets the user control the clock with the Start and Stop buttons, as shown in Figure 15.36a.
(a)
(b)
(c)
FIGURE 15.36 (a) Exercise 15.32 allows the user to start and stop a clock. (b and c) The balls are dropped into the bean machine.
Programming Exercises 627 ***15.33 (Game: bean-machine animation) Write a program that animates the bean ***15.34
machine introduced in Programming Exercise 7.21. The animation terminates after ten balls are dropped, as shown in Figure 15.36b and c. (Simulation: self-avoiding random walk) A self-avoiding walk in a lattice is a path from one point to another that does not visit the same point twice. Selfavoiding walks have applications in physics, chemistry, and mathematics. They can be used to model chain-like entities such as solvents and polymers. Write a program that displays a random path that starts from the center and ends at a point on the boundary, as shown in Figure 15.37a or ends at a dead-end point (i.e., surrounded by four points that have already been visited), as shown in Figure 15.37b. Assume the size of the lattice is 16 by 16.
(a)
(b)
(c)
(d)
FIGURE 15.37 (a) A path ends at a boundary point. (b) A path ends at dead-end point. (c and d) Animation shows the progress of a path step by step.
***15.35 (Animation: self-avoiding random walk) Revise the preceding exercise to display the walk step by step in an animation, as shown in Figure 15.37c and d.
**15.36 (Simulation: self-avoiding random walk) Write a simulation program to show that the chance of getting dead-end paths increases as the grid size increases. Your program simulates lattices with size from 10 to 80. For each lattice size, simulate a self-avoiding random walk 10,000 times and display the probability of the dead-end paths, as shown in the following sample output: For a lattice of size 10, the probability of dead-end paths is 10.6% For a lattice of size 11, the probability of dead-end paths is 14.0% ... For a lattice of size 80, the probability of dead-end paths is 99.5%
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CHAPTER
16 JAVAFX UI CONTROLS AND MULTIMEDIA Objectives ■
To create graphical user interfaces with various user-interface controls (§§16.2–16.11).
■
To create a label with text and graphic using the Label class and explore properties in the abstract Labeled class (§16.2).
■
To create a button with text and graphic using the Button class and set a handler using the setOnAction method in the abstract ButtonBase class (§16.3).
■
To create a check box using the CheckBox class (§16.4).
■
To create a radio button using the RadioButton class and group radio buttons using a ToggleGroup (§16.5).
■
To enter data using the TextField class and password using the PasswordField class (§16.6).
■
To enter data in multiple lines using the TextArea class (§16.7).
■
To select a single item using ComboBox (§16.8).
■
To select a single or multiple items using ListView (§16.9).
■
To select a range of values using ScrollBar (§16.10).
■
To select a range of values using Slider and explore differences between ScrollBar and Slider (§16.11).
■
To develop a tic-tac-toe game (§16.12).
■
To view and play video and audio using the Media, MediaPlayer, and MediaView (§16.13).
■
To develop a case study for showing the national flag and playing anthem (§16.14).
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16.1 Introduction Key Point GUI
JavaFX provides many UI controls for developing a comprehensive user interface. A graphical user interface (GUI) makes a system user-friendly and easy to use. Creating a GUI requires creativity and knowledge of how UI controls work. Since the UI controls in JavaFX are very flexible and versatile, you can create a wide assortment of useful user interfaces for rich Internet applications. Oracle provides tools for visually designing and developing GUIs. This enables the programmer to rapidly assemble the elements of a GUI with minimum coding. Tools, however, cannot do everything. You have to modify the programs they produce. Consequently, before you begin to use the visual tools, you must understand the basic concepts of JavaFX GUI programming. Previous chapters used UI controls such as Button, Label, and TextField. This chapter introduces the frequently used UI controls in detail (see Figure 16.1). Button
Node
Parent ImageView
Control Covered in Chapter 14
Labeled
ButtonBase
ScrollBar
Label
CheckBox ToggleButton
Slider
MediaView
RadioButton
TextArea TextInputControl TextField
PasswordField
ListView ComboBoxBase
ComboBox
FIGURE 16.1 These UI controls are frequently used to create user interfaces.
Note Throughout this book, the prefixes lbl, bt, chk, rb, tf, pf, ta, cbo, lv, scb, sld, and mp are used to name reference variables for Label, Button, CheckBox, RadioButton, TextField, PasswordField, TextArea, ComboBox, ListView, ScrollBar, Slider, and MediaPlayer.
naming convention for controls
16.2 Labeled and Label A label is a display area for a short text, a node, or both. It is often used to label other controls (usually text fields). Labels and buttons share many common properties. These common properties are defined in the Labeled class, as shown in Figure 16.2. A Label can be constructed using one of the three constructors as shown in Figure 16.3. The graphic property can be any node such as a shape, an image, or a control. Listing 16.1 gives an example that displays several labels with text and images in the label, as shown in Figure 16.4.
LISTING 16.1 LabelWithGraphic.java 1 2 3 4 5
import import import import import
javafx.application.Application; javafx.stage.Stage; javafx.scene.Scene; javafx.scene.control.ContentDisplay; javafx.scene.control.Label;
16.2 Labeled and Label 631 The getter and setter methods for property values and a getter for property itself are provided in the class, but omitted in the UML diagram for brevity.
javafx.scene.control.Labeled -alignment: ObjectProperty
Specifies the alignment of the text and node in the labeled.
-contentDisplay: ObjectProperty
Specifies the position of the node relative to the text using the constants TOP,BOTTOM,LEFT, and RIGHT defined in ContentDisplay.
-graphic: ObjectProperty -graphicTextGap: DoubleProperty
A graphic for the labeled. The gap between the graphic and the text.
-textFill: ObjectProperty
The paint used to fill the text. A text for the labeled.
-text: StringProperty -underline: BooleanProperty -wrapText: BooleanProperty
Whether text should be underlined. Whether text should be wrapped if the text exceeds the width.
FIGURE 16.2 Labeled defines common properties for Label, Button, CheckBox, and RadioButton. javafx.scene.control.Labeled
javafx.scene.control.Label +Label() +Label(text: String)
Creates an empty label. Creates a label with the specified text.
+Label(text: String, graphic: Node)
Creates a label with the specified text and graphic.
FIGURE 16.3 Label is created to display a text or a node, or both. 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
import import import import import import import import
javafx.scene.image.Image; javafx.scene.image.ImageView; javafx.scene.layout.HBox; javafx.scene.layout.StackPane; javafx.scene.paint.Color; javafx.scene.shape.Circle; javafx.scene.shape.Rectangle; javafx.scene.shape.Ellipse;
public class LabelWithGraphic extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { ImageView us = new ImageView(new Image("image/us.gif")); Label lb1 = new Label("US\n50 States", us); lb1.setStyle("-fx-border-color: green; -fx-border-width: 2"); lb1.setContentDisplay(ContentDisplay.BOTTOM); lb1.setTextFill(Color.RED);
create a label set node position
Label lb2 = new Label("Circle", new Circle(50, 50, 25)); lb2.setContentDisplay(ContentDisplay.TOP); lb2.setTextFill(Color.ORANGE);
create a label
Label lb3 = new Label("Retangle", new Rectangle(10, 10, 50, 25)); lb3.setContentDisplay(ContentDisplay.RIGHT);
create a label
Label lb4 = new Label("Ellipse", new Ellipse(50, 50, 50, 25)); lb4.setContentDisplay(ContentDisplay.LEFT);
create a label
set node position
632 Chapter 16
JavaFX UI Controls and Multimedia 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 60
create a label
add labels to pane
Ellipse ellipse = new Ellipse(50, 50, 50, 25); ellipse.setStroke(Color.GREEN); ellipse.setFill(Color.WHITE); StackPane stackPane = new StackPane(); stackPane.getChildren().addAll(ellipse, new Label("JavaFX")); Label lb5 = new Label("A pane inside a label", stackPane); lb5.setContentDisplay(ContentDisplay.BOTTOM); HBox pane = new HBox(20); pane.getChildren().addAll(lb1, lb2, lb3, lb4, lb5); // Create a scene and place it in the stage Scene scene = new Scene(pane, 450, 150); primaryStage.setTitle("LabelWithGraphic"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } }
FIGURE 16.4
The program displays labels with texts and nodes.
The program creates a label with a text and an image (line 19). The text is US\n50 States so it is displayed in two lines. Line 21 specifies that the image is placed at the bottom of the text. The program creates a label with a text and a circle (line 24). The circle is placed on top of the text (line 25). The program creates a label with a text and a rectangle (line 28). The rectangle is placed on the right of the text (line 29). The program creates a label with a text and an ellipse (line 31). The ellipse is placed on the left of the text (line 32). The program creates an ellipse (line 34), places it along with a label to a stack pane (line 38), and creates a label with a text and the stack pane as the node (line 39). As seen from this example, you can place any node in a label. The program creates an HBox (line 42) and places all five labels into the HBox (line 43).
✓
Check Point
16.1 16.2 16.3 16.4
How do you create a label with a node without a text? How do you place a text on the right of the node in a label? Can you display multiple lines of text in a label? Can the text in a label be underlined?
16.3 Button A button is a control that triggers an action event when clicked. JavaFX provides regular buttons, toggle buttons, check box buttons, and radio buttons. The common features of these buttons are defined in ButtonBase and Labeled classes as shown in Figure 16.5. The Labeled class defines the common properties for labels and buttons. A button is just like a label except that the button has the onAction property defined in the ButtonBase class, which sets a handler for handling a button’s action.
16.3 Button 633 javafx.scene.control.Labeled The getter and setter methods for property values and a getter for property itself are provided in the class, but omitted in the UML diagram for brevity. javafx.scene.control.ButtonBase -onAction: ObjectProperty>
Defines a handler for handling a button’s action.
javafx.scene.control.Button +Button() +Button(text: String) +Button(text: String, graphic: Node)
Creates an empty button. Creates a button with the specified text. Creates a button with the specified text and graphic.
FIGURE 16.5 ButtonBase extends Labeled and defines common features for all buttons. Listing 16.2 gives a program that uses the buttons to control the movement of a text, as shown in Figure 16.6.
LISTING 16.2 ButtonDemo.java 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
import import import import import import import import import import
javafx.application.Application; javafx.stage.Stage; javafx.geometry.Pos; javafx.scene.Scene; javafx.scene.control.Button; javafx.scene.image.ImageView; javafx.scene.layout.BorderPane; javafx.scene.layout.HBox; javafx.scene.layout.Pane; javafx.scene.text.Text;
public class ButtonDemo extends Application { protected Text text = new Text(50, 50, "JavaFX Programming"); protected BorderPane getPane() { HBox paneForButtons = new HBox(20); Button btLeft = new Button("Left", new ImageView("image/left.gif")); Button btRight = new Button("Right", new ImageView("image/right.gif")); paneForButtons.getChildren().addAll(btLeft, btRight); paneForButtons.setAlignment(Pos.CENTER); paneForButtons.setStyle("-fx-border-color: green"); BorderPane pane = new BorderPane(); pane.setBottom(paneForButtons);
create a button
add buttons to pane
create a border pane add buttons to the bottom
Pane paneForText = new Pane(); paneForText.getChildren().add(text); pane.setCenter(paneForText); btLeft.setOnAction(e -> text.setX(text.getX() - 10)); btRight.setOnAction(e -> text.setX(text.getX() + 10));
add an action handler
return pane;
return a pane
634 Chapter 16
JavaFX UI Controls and Multimedia 36 37 38 39 40 41 42 43 44 45 46
set pane to scene
} @Override // Override the start method in the Application class public void start(Stage primaryStage) { // Create a scene and place it in the stage Scene scene = new Scene(getPane(), 450, 200); primaryStage.setTitle("ButtonDemo"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } }
FIGURE 16.6
The program demonstrates using buttons.
The program creates two buttons btLeft and btRight with each button containing a text and an image (lines 17–20). The buttons are placed in an HBox (line 21) and the HBox is placed in the bottom of a border pane (line 26). A text is created in line 13 and is placed in the center of the border pane (line 30). The action handler for btLeft moves the text to the left (line 32). The action handler for btRight moves the text to the right (line 33). The program purposely defines a protected getPane() method to return a pane (line 15). This method will be overridden by subclasses in the upcoming examples to add more nodes in the pane. The text is declared protected so that it can be accessed by subclasses (line 13).
getPane() protected
✓
Check Point
16.5 How do you create a button with a text and a node? Can you apply all the methods for Labeled to Button?
16.6 Why is the getPane() method protected in Listing 16.2? Why is the data field text protected?
16.7 How do you set a handler for processing a button-clicked action?
16.4 CheckBox A CheckBox is used for the user to make a selection. Like Button, CheckBox inherits all the properties such as onAction, text, graphic, alignment, graphicTextGap, textFill, contentDisplay from ButtonBase and Labeled, as shown in Figure 16.7. Additionally, it provides the selection property to indicate whether a check box is selected. Here is an example of a check box with text US, a graphic image, green text color, and black border, and initially selected. CheckBox chkUS = new CheckBox("US"); chkUS.setGraphic(new ImageView("image/usIcon.gif")); chkUS.setTextFill(Color.GREEN); chkUS.setContentDisplay(ContentDisplay.LEFT); chkUS.setStyle("-fx-border-color: black"); chkUS.setSelected(true); chkUS.setPadding(new Insets(5, 5, 5, 5));
16.4 CheckBox 635 javafx.scene.control.Labeled The getter and setter methods for property values and a getter for property itself are provided in the class, but omitted in the UML diagram for brevity. javafx.scene.control.ButtonBase -onAction: ObjectProperty>
Defines a handler for handling a button’s action.
javafx.scene.control.CheckBox -selected: BooleanProperty
Indicates whether this check box is checked.
+CheckBox()
Creates an empty check box. Creates a check box with the specified text.
+CheckBox(text: String)
FIGURE 16.7 CheckBox contains the properties inherited from ButtonBase and Labeled. When a check box is clicked (checked or unchecked), it fires an ActionEvent. To see if a check box is selected, use the isSelected() method. We now write a program that adds two check boxes named Bold and Italic to the preceding example to let the user specify whether the message is in bold or italic, as shown in Figure 16.8.
VBox containing two check boxes
FIGURE 16.8
The program demonstrates check boxes.
There are at least two approaches to writing this program. The first is to revise the preceding ButtonDemo class to insert the code for adding the check boxes and processing their events. The second is to define a subclass that extends ButtonDemo. Please implement the first approach as an exercise. Listing 16.3 gives the code to implement the second approach.
LISTING 16.3 CheckBoxDemo.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14
import import import import import import import import import
javafx.event.ActionEvent; javafx.event.EventHandler; javafx.geometry.Insets; javafx.scene.control.CheckBox; javafx.scene.layout.BorderPane; javafx.scene.layout.VBox; javafx.scene.text.Font; javafx.scene.text.FontPosture; javafx.scene.text.FontWeight;
public class CheckBoxDemo extends ButtonDemo { @Override // Override the getPane() method in the super class protected BorderPane getPane() { BorderPane pane = super.getPane();
Application
ButtonDemo
CheckBoxDemo
override getPane() invoke super.getPane()
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create fonts
pane for check boxes
create check boxes
create a handler
set handler for action
return a pane main method omitted
Font fontBoldItalic = Font.font("Times New Roman", FontWeight.BOLD, FontPosture.ITALIC, 20); Font fontBold = Font.font("Times New Roman", FontWeight.BOLD, FontPosture.REGULAR, 20); Font fontItalic = Font.font("Times New Roman", FontWeight.NORMAL, FontPosture.ITALIC, 20); Font fontNormal = Font.font("Times New Roman", FontWeight.NORMAL, FontPosture.REGULAR, 20); text.setFont(fontNormal); VBox paneForCheckBoxes = new VBox(20); paneForCheckBoxes.setPadding(new Insets(5, 5, 5, 5)); paneForCheckBoxes.setStyle("-fx-border-color: green"); CheckBox chkBold = new CheckBox("Bold"); CheckBox chkItalic = new CheckBox("Italic"); paneForCheckBoxes.getChildren().addAll(chkBold, chkItalic); pane.setRight(paneForCheckBoxes); EventHandler handler = e -> { if (chkBold.isSelected() && chkItalic.isSelected()) { text.setFont(fontBoldItalic); // Both check boxes checked } else if (chkBold.isSelected()) { text.setFont(fontBold); // The Bold check box checked } else if (chkItalic.isSelected()) { text.setFont(fontItalic); // The Italic check box checked } else { text.setFont(fontNormal); // Both check boxes unchecked } }; chkBold.setOnAction(handler); chkItalic.setOnAction(handler); return pane; // Return a new pane } }
CheckBoxDemo extends ButtonDemo and overrides the getPane() method (line 13). The new getPane() method invokes the super.getPane() method from the ButtonDemo class to obtain a border pane that contains the buttons and a text (line 14). The check boxes are created and added to paneForCheckBoxes (lines 30–32). paneForCheckBoxes is added to the border pane (lines 33). The handler for processing the action event on check boxes is created in lines 35–48. It sets the appropriate font based on the status of the check boxes. The start method for this JavaFX program is defined in ButtonDemo and inherited in CheckBoxDemo. So when you run CheckBoxDemo, the start method in ButtonDemo is invoked. Since the getPane() method is overridden in CheckBoxDemo, the method in CheckBoxDemo is invoked from line 41 in Listing 16.2, ButtonDemo.java.
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16.8 How do you test if a check box is selected? 16.9 Can you apply all the methods for Labeled to CheckBox? 16.10 Can you set a node for the graphic property in a check box?
16.5 RadioButton 637
16.5 RadioButton Radio buttons, also known as option buttons, enable the user to choose a single item from a group of choices. In appearance radio buttons resemble check boxes, but check boxes display a square that is either checked or blank, whereas radio buttons display a circle that is either filled (if selected) or blank (if not selected). RadioButton is a subclass of ToggleButton. The difference between a radio button and a toggle button is that a radio button displays a circle, but a toggle button is rendered similar to a button. The UML diagrams for ToggleButton and RadioButton are shown in Figure 16.9.
option buttons
The getter and setter methods for property values and a getter for property itself are provided in the class, but omitted in the UML diagram for brevity. javafx.scene.control.ToggleButton -selected: BooleanProperty -toggleGroup: ObjectProperty +ToggleButton() +ToggleButton(text: String) +ToggleButton(text: String, graphic: Node)
Indicates whether the button is selected. Specifies the button group to which the button belongs.
Creates an empty toggle button. Creates a toggle button with the specified text. Creates a toggle button with the specified text and graphic.
javafx.scene.control.RadioButton +RadioButton() +RadioButton(text: String)
Creates an empty radio button. Creates a radio button with the specified text.
FIGURE 16.9 ToggleButton and RadioButton are specialized buttons for making selections. Here is an example of a radio button with text US, a graphic image, green text color, and black border, and initially selected. RadioButton rbUS = new RadioButton("US"); rbUS.setGraphic(new ImageView("image/usIcon.gif")); rbUS.setTextFill(Color.GREEN); rbUS.setContentDisplay(ContentDisplay.LEFT); rbUS.setStyle("-fx-border-color: black"); rbUS.setSelected(true); rbUS.setPadding(new Insets(5, 5, 5,));
To group radio buttons, you need to create an instance of ToggleGroup and set a radio button’s toggleGroup property to join the group, as follows: ToggleGroup group = new ToggleGroup(); rbRed.setToggleGroup(group); rbGreen.setToggleGroup(group); rbBlue.setToggleGroup(group);
This code creates a button group for radio buttons rbRed, rbGreen, and rbBlue so that buttons rbRed, rbGreen, and rbBlue are selected mutually exclusively. Without grouping, these buttons would be independent. When a radio button is changed (selected or deselected), it fires an ActionEvent. To see if a radio button is selected, use the isSelected() method.
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JavaFX UI Controls and Multimedia We now give a program that adds three radio buttons named Red, Green, and Blue to the preceding example to let the user choose the color of the message, as shown in Figure 16.10.
VBox containing three radio buttons
FIGURE 16.10
Application
ButtonDemo
CheckBoxDemo
RadioButtonDemo
override getPane() invoke super.getPane() pane for radio buttons
create radio buttons
add to border pane group radio buttons
handle radio button
The program demonstrates using radio buttons.
Again there are at least two approaches to writing this program. The first is to revise the preceding CheckBoxDemo class to insert the code for adding the radio buttons and processing their events. The second is to define a subclass that extends CheckBoxDemo. Listing 16.4 gives the code to implement the second approach.
LISTING 16.4 RadioButtonDemo.java 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
import import import import import import
javafx.geometry.Insets; javafx.scene.control.RadioButton; javafx.scene.control.ToggleGroup; javafx.scene.layout.BorderPane; javafx.scene.layout.VBox; javafx.scene.paint.Color;
public class RadioButtonDemo extends CheckBoxDemo { @Override // Override the getPane() method in the super class protected BorderPane getPane() { BorderPane pane = super.getPane(); VBox paneForRadioButtons = new VBox(20); paneForRadioButtons.setPadding(new Insets(5, 5, 5, 5)); paneForRadioButtons.setStyle("-fx-border-color: green"); paneForRadioButtons.setStyle ("-fx-border-width: 2px; -fx-border-color: green"); RadioButton rbRed = new RadioButton("Red"); RadioButton rbGreen = new RadioButton("Green"); RadioButton rbBlue = new RadioButton("Blue"); paneForRadioButtons.getChildren().addAll(rbRed, rbGreen, rbBlue); pane.setLeft(paneForRadioButtons); ToggleGroup group = new ToggleGroup(); rbRed.setToggleGroup(group); rbGreen.setToggleGroup(group); rbBlue.setToggleGroup(group); rbRed.setOnAction(e -> { if (rbRed.isSelected()) { text.setFill(Color.RED); } }); rbGreen.setOnAction(e -> { if (rbGreen.isSelected()) { text.setFill(Color.GREEN);
16.6 TextField 639 38 39 40 41 42 43 44 45 46 47 48 49
} }); rbBlue.setOnAction(e -> { if (rbBlue.isSelected()) { text.setFill(Color.BLUE); } }); return pane;
return border pane
} }
main method omitted
RadioButtonDemo extends CheckBoxDemo and overrides the getPane() method (line 10). The new getPane() method invokes the getPane() method from the CheckBoxDemo class to create a border pane that contains the check boxes, buttons, and a text (line 11). This border pane is returned from invoking super.getPane(). The radio buttons are created and added to paneForRadioButtons (lines 18–21). paneForRadioButtons is added to the border pane (lines 22). The radio buttons are grouped together in lines 24–27. The handlers for processing the action event on radio buttons are created in lines 29–45. It sets the appropriate color based on the status of the radio buttons. The start method for this JavaFX program is defined in ButtonDemo and inherited in CheckBoxDemo and then in RadioButtonDemo. So when you run RadioButtonDemo, the start method in ButtonDemo is invoked. Since the getPane() method is overridden in RadioButtonDemo, the method in RadioButtonDemo is invoked from line 41 in Listing 16.2, ButtonDemo.java.
16.11 16.12 16.13 16.14
How do you test if a radio button is selected? Can you apply all the methods for Labeled to RadioButton? Can you set any node in the graphic property in a radio button? How do you group radio buttons?
16.6 TextField A text field can be used to enter or display a string. TextField is a subclass of TextInputControl. Figure 16.11 lists the properties and constructors in TextField. Here is an example of creating a noneditable text field with red text color, a specified font, and right horizontal alignment: TextField tfMessage = new TextField("T-Strom"); tfMessage.setEditable(false); tfMessage.setStyle("-fx-text-fill: red"); tfMessage.setFont(Font.font("Times", 20)); tfMessage.setAlignment(Pos.BASELINE_RIGHT);
When you move the cursor in the text field and press the Enter key, it fires an ActionEvent. Listing 16.5 gives a program that adds a text field to the preceding example to let the user set a new message, as shown in Figure 16.12.
LISTING 16.5 TextFieldDemo.java 1 2
import javafx.geometry.Insets; import javafx.geometry.Pos;
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JavaFX UI Controls and Multimedia The getter and setter methods for property values and a getter for property itself are provided in the class, but omitted in the UML diagram for brevity.
javafx.scene.control.TextInputControl -text: StringProperty
The text content of this control.
-editable: BooleanProperty
Indicates whether the text can be edited by the user.
javafx.scene.control.TextField -alignment: ObjectProperty -prefColumnCount: IntegerProperty -onAction: ObjectProperty> +TextField() +TextField(text: String)
Specifies how the text should be aligned in the text field. Specifies the preferred number of columns in the text field. Specifies the handler for processing the action event on the text field. Creates an empty text field. Creates a text field with the specified text.
FIGURE 16.11 TextField enables the user to enter or display a string.
Application
ButtonDemo
CheckBoxDemo
FIGURE 16.12 RadioButtonDemo
TextFieldDemo
override getPane() invoke super.getPane() pane for label and text field
create text field
add to border pane handle text field action return border pane main method omitted
3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
The program demonstrates using text fields.
import javafx.scene.control.Label; import javafx.scene.control.TextField; import javafx.scene.layout.BorderPane; public class TextFieldDemo extends RadioButtonDemo { @Override // Override the getPane() method in the super class protected BorderPane getPane() { BorderPane pane = super.getPane(); BorderPane paneForTextField = new BorderPane(); paneForTextField.setPadding(new Insets(5, 5, 5, 5)); paneForTextField.setStyle("-fx-border-color: green"); paneForTextField.setLeft(new Label("Enter a new message: ")); TextField tf = new TextField(); tf.setAlignment(Pos.BOTTOM_RIGHT); paneForTextField.setCenter(tf); pane.setTop(paneForTextField); tf.setOnAction(e -> text.setText(tf.getText())); return pane; } }
16.6 TextArea 641 TextFieldDemo extends RadioButtonDemo (line 7) and adds a label and a text field to let the user enter a new text (lines 12–19). After you set a new text in the text field and press the Enter key, a new message is displayed (line 22). Pressing the Enter key on the text field triggers an action event.
Note If a text field is used for entering a password, use PasswordField to replace TextField. PasswordField extends TextField and hides the input text with echo characters ******.
16.15 16.16 16.17 16.18
Can you disable editing of a text field? Can you apply all the methods for TextInputControl to TextField? Can you set a node as the graphic property in a text field?
PasswordField
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How do you align the text in a text field to the right?
16.7 TextArea A TextArea enables the user to enter multiple lines of text. If you want to let the user enter multiple lines of text, you may create several instances of TextField. A better alternative, however, is to use TextArea, which enables the user to enter multiple lines of text. Figure 16.13 lists the properties and constructors in TextArea.
Key Point
The getter and setter methods for property values and a getter for property itself are provided in the class, but omitted in the UML diagram for brevity. javafx.scene.control.TextInputControl -text: StringProperty -editable: BooleanProperty
The text content of this control. Indicates whether the text can be edited by the user.
javafx.scene.control.TextArea -prefColumnCount: IntegerProperty -prefRowCount: IntegerProperty -wrapText: BooleanProperty
Specifies the preferred number of text columns.
+TextArea() +TextArea(text: String)
Creates an empty text area. Creates a text area with the specified text.
Specifies the preferred number of text rows. Specifies whether the text is wrapped to the next line.
FIGURE 16.13 TextArea enables the user to enter or display multiple lines of characters. Here is an example of creating a text area with 5 rows and 20 columns, wrapped to the next line, red text color, and Courier font 20 pixels. TextArea taNote = new TextArea("This is a text area"); taNote.setPrefColumnCount(20); taNote.setPrefRowCount(5); taNote.setWrapText(true); taNote.setStyle("-fx-text-fill: red"); taNote.setFont(Font.font("Times", 20));
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JavaFX UI Controls and Multimedia TextArea provides scrolling, but often it is useful to create a ScrollPane object to hold an instance of TextArea and let ScrollPane handle scrolling for TextArea, as follows: // Create a scroll pane to hold text area ScrollPane scrollPane = new ScrollPane(taNote);
Tip You can place any node in a ScrollPane. ScrollPane provides vertical and horizontal scrolling automatically if the control is too large to fit in the viewing area.
ScrollPane
We now give a program that displays an image and a short text in a label, and a long text in a text area, as shown in Figure 16.14.
DescriptionPane A label showing an image and a text
FIGURE 16.14
A text area inside a scroll pane
The program displays an image in a label, a title in a label, and text in the text area. Here are the major steps in the program: 1. Define a class named DescriptionPane that extends BorderPane, as shown in Listing 16.6. This class contains a text area inside a scroll pane, and a label for displaying an image icon and a title. The class DescriptionPane will be reused in later examples. 2. Define a class named TextAreaDemo that extends Application, as shown in Listing 16.7. Create an instance of DescriptionPane and add it to the scene. The relationship between DescriptionPane and TextAreaDemo is shown in Figure 16.15.
javafx.scene.layout.BorderPane
DescriptionPane
javafx.application.Application
1
1
TextAreaDemo
-lblImageTitle: Label -taDescription: TextArea +setImageView(im: ImageView) +setDescription(text: String)
FIGURE 16.15 TextAreaDemo uses DescriptionPane to display an image, title, and text description of a national flag.
LISTING 16.6 DescriptionPane.java 1 2 3
import javafx.geometry.Insets; import javafx.scene.control.Label; import javafx.scene.control.ContentDisplay;
16.6 TextArea 643 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
import import import import import
javafx.scene.control.ScrollPane; javafx.scene.control.TextArea; javafx.scene.image.ImageView; javafx.scene.layout.BorderPane; javafx.scene.text.Font;
public class DescriptionPane extends BorderPane { /** Label for displaying an image and a title */ private Label lblImageTitle = new Label(); /** Text area for displaying text */ private TextArea taDescription = new TextArea(); public DescriptionPane() { // Center the icon and text and place the text under the icon lblImageTitle.setContentDisplay(ContentDisplay.TOP); lblImageTitle.setPrefSize(200, 100);
label
text area
label properties
// Set the font in the label and the text field lblImageTitle.setFont(new Font("SansSerif", 16)); taDescription.setFont(new Font("Serif", 14)); taDescription.setWrapText(true); taDescription.setEditable(false);
wrap text read only
// Create a scroll pane to hold the text area ScrollPane scrollPane = new ScrollPane(taDescription);
scroll pane
// Place label and scroll pane in the border pane setLeft(lblImageTitle); setCenter(scrollPane); setPadding(new Insets(5, 5, 5, 5)); } /** Set the title */ public void setTitle(String title) { lblImageTitle.setText(title); } /** Set the image view */ public void setImageView(ImageView icon) { lblImageTitle.setGraphic(icon); } /** Set the text description */ public void setDescription(String text) { taDescription.setText(text); } }
The text area is inside a ScrollPane (line 30), which provides scrolling functions for the text area. The wrapText property is set to true (line 26) so that the line is automatically wrapped when the text cannot fit in one line. The text area is set as noneditable (line 27), so you cannot edit the description in the text area. It is not necessary to define a separate class for DescriptionPane in this example. However, this class was defined for reuse in the next section, where you will use it to display a description pane for various images.
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LISTING 16.7 TextAreaDemo.java
create descriptionPane
set title set image
add descriptionPane to scene
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
import import import import
javafx.application.Application; javafx.stage.Stage; javafx.scene.Scene; javafx.scene.image.ImageView;
public class TextAreaDemo extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { // Declare and create a description pane DescriptionPane descriptionPane = new DescriptionPane(); // Set title, text, and image in the description pane descriptionPane.setTitle("Canada"); String description = "The Canadian national flag ..."; descriptionPane.setImageView(new ImageView("image/ca.gif")); descriptionPane.setDescription(description); // Create a scene and place it in the stage Scene scene = new Scene(descriptionPane, 450, 200); primaryStage.setTitle("TextAreaDemo"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } }
The program creates an instance of DescriptionPane (line 10), and sets the title (line 13), image (line 15), and text in the description pane (line 16). DescriptionPane is a subclass of Pane. DescriptionPane contains a label for displaying an image and a title, and a text area for displaying a description of the image.
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16.19 16.20 16.21 16.22
How do you create a text area with 10 rows and 20 columns? How do you obtain the text from a text area? Can you disable editing of a text area? What method do you use to wrap text to the next line in a text area?
16.8 ComboBox Key Point
A combo box, also known as a choice list or drop-down list, contains a list of items from which the user can choose. A combo box is useful for limiting a user’s range of choices and avoids the cumbersome validation of data input. Figure 16.16 lists several frequently used properties and constructors in ComboBox. ComboBox is defined as a generic class. The generic type T specifies the element type for the elements stored in a combo box. The following statements create a combo box with four items, red color, and value set to the first item. ComboBox cbo = new ComboBox<>(); cbo.getItems().addAll("Item 1", "Item 2", "Item 3", "Item 4"); cbo.setStyle("-fx-color: red"); cbo.setValue("Item 1");
16.8 ComboBox 645 The getter and setter methods for property values and a getter for property itself are provided in the class, but omitted in the UML diagram for brevity. javafx.scene.control.ComboBoxBase -value: ObjectProperty
The value selected in the combo box.
-editable: BooleanProperty -onAction: ObjectProperty>
Specifies whether the combo box allows user input. Specifies the handler for processing the action event.
javafx.scene.control.ComboBox -items: ObjectProperty>
The items in the combo box popup.
-visibleRowCount: IntegerProperty
The maximum number of visible rows of the items in the combo box popup.
+ComboBox() +ComboBox(items: ObservableList)
Creates an empty combo box.
FIGURE 16.16
Creates a combo box with the specified items.
ComboBox enables the user to select an item from a list of items.
ComboBox inherits from ComboBoxBase. ComboBox can fire an ActionEvent. Whenever an item is selected, an ActionEvent is fired. ObservableList is a subinterface of java.util.List. So you can apply all the methods defined in List for an ObservableList. For convenience, JavaFX provides the static method FXCollections.observableArrayList(arrayOfElements) for creating an ObservableList from an array of elements. Listing 16.8 gives a program that lets the user view an image and a description of a country’s flag by selecting the country from a combo box, as shown in Figure 16.17.
ComboBox
DescriptionPane
FIGURE 16.17 Information about a country, including an image and a description of its flag, is displayed when the country is selected in the combo box. Here are the major steps in the program: 1. Create the user interface. Create a combo box with country names as its selection values. Create a DescriptionPane object (the DescriptionPane class was introduced in the preceding section). Place the combo box at the top of the border pane and the description pane in the center of the border pane. 2. Process the event. Create a handler for handling action event from the combo box to set the flag title, image, and text in the description pane for the selected country name.
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LISTING 16.8 ComboBoxDemo.java
countries
image views
description
combo box
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 54 55 56 57 58
import import import import import import import import import
javafx.application.Application; javafx.stage.Stage; javafx.collections.FXCollections; javafx.collections.ObservableList; javafx.scene.Scene; javafx.scene.control.ComboBox; javafx.scene.control.Label; javafx.scene.image.ImageView; javafx.scene.layout.BorderPane;
public class ComboBoxDemo extends Application { // Declare an array of Strings for flag titles private String[] flagTitles = {"Canada", "China", "Denmark", "France", "Germany", "India", "Norway", "United Kingdom", "United States of America"}; // Declare an ImageView array for the national flags of 9 countries private ImageView[] flagImage = {new ImageView("image/ca.gif"), new ImageView("image/china.gif"), new ImageView("image/denmark.gif"), new ImageView("image/fr.gif"), new ImageView("image/germany.gif"), new ImageView("image/india.gif"), new ImageView("image/norway.gif"), new ImageView("image/uk.gif"), new ImageView("image/us.gif")}; // Declare an array of strings for flag descriptions private String[] flagDescription = new String[9]; // Declare and create a description pane private DescriptionPane descriptionPane = new DescriptionPane(); // Create a combo box for selecting countries private ComboBox cbo = new ComboBox<>(); // flagTitles; @Override // Override the start method in the Application class public void start(Stage primaryStage) { // Set text description flagDescription[0] = "The Canadian national flag ..."; flagDescription[1] = "Description for China ... "; flagDescription[2] = "Description for Denmark ... "; flagDescription[3] = "Description for France ... "; flagDescription[4] = "Description for Germany ... "; flagDescription[5] = "Description for India ... "; flagDescription[6] = "Description for Norway ... "; flagDescription[7] = "Description for UK ... "; flagDescription[8] = "Description for US ... "; // Set the first country (Canada) for display setDisplay(0); // Add combo box and description pane to the border pane BorderPane pane = new BorderPane(); BorderPane paneForComboBox = new BorderPane(); paneForComboBox.setLeft(new Label("Select a country: ")); paneForComboBox.setCenter(cbo); pane.setTop(paneForComboBox);
16.9 ListView 647 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
cbo.setPrefWidth(400); cbo.setValue("Canada"); ObservableList items = FXCollections.observableArrayList(flagTitles); cbo.getItems().addAll(items); pane.setCenter(descriptionPane);
set combo box value observable list add to combo box
// Display the selected country cbo.setOnAction(e -> setDisplay(items.indexOf(cbo.getValue()))); // Create a scene and place it in the stage Scene scene = new Scene(pane, 450, 170); primaryStage.setTitle("ComboBoxDemo"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } /** Set display information on the description pane */ public void setDisplay(int index) { descriptionPane.setTitle(flagTitles[index]); descriptionPane.setImageView(flagImage[index]); descriptionPane.setDescription(flagDescription[index]); } }
The program stores the flag information in three arrays: flagTitles, flagImage, and flagDescription (lines 13–28). The array flagTitles contains the names of nine countries, the array flagImage contains image views of the nine countries’ flags, and the array flagDescription contains descriptions of the flags. The program creates an instance of DescriptionPane (line 31), which was presented in Listing 16.6, DescriptionPane.java. The program creates a combo box with values from flagTitles (lines 62–63). The getItems() method returns a list from the combo box (line 64) and the addAll method adds multiple items into the list. When the user selects an item in the combo box, the action event triggers the execution of the handler. The handler finds the selected index (line 68) and invokes the setDisplay(int index) method to set its corresponding flag title, flag image, and flag description on the pane (lines 78–82).
16.23 How do you create a combo box and add three items to it? 16.24 How do you retrieve an item from a combo box? How do you retrieve a selected item from a combo box?
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16.25 How do you get the number of items in a combo box? How do you retrieve an item at a specified index in a combo box? 16.26 What events would a ComboBox fire upon selecting a new item?
16.9 ListView A list view is a control that basically performs the same function as a combo box, but it enables the user to choose a single value or multiple values. Figure 16.18 lists several frequently used properties and constructors in ListView. ListView is defined as a generic class. The generic type T specifies the element type for the elements stored in a list view.
Key Point
VideoNote
Use ListView
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JavaFX UI Controls and Multimedia The getter and setter methods for property values and a getter for property itself are provided in the class, but omitted in the UML diagram for brevity.
javafx.scene.control.ListView -items: ObjectProperty>
The items in the list view.
-orientation: BooleanProperty
Indicates whether the items are displayed horizontally or vertically in the list view.
-selectionModel: ObjectProperty>
Specifies how items are selected. The SelectionModel is also used to obtain the selected items.
+ListView() +ListView(items: ObservableList)
Creates an empty list view.
FIGURE 16.18
Creates a list view with the specified items.
ListView enables the user to select one or multiple items from a list of items.
The getSelectionModel() method returns an instance of SelectionModel, which contains the methods for setting a selection mode and obtaining selected indices and items. The selection mode is defined in one of the two constants SelectionMode.MULTIPLE and SelectionMode.SINGLE, which indicates whether a single item or multiple items can be selected. The default value is SelectionMode.SINGLE. Figure 16.19a shows a single selection and Figure 16.19b–c show multiple selections.
(a) Single selection
(b) Multiple selection
(c) Multiple selection
FIGURE 16.19 SelecitonMode has two selection modes: single selection and multipleinterval selection. The following statements create a list view of six items with multiple selections allowed. ObservableList items = FXCollections.observableArrayList("Item 1", "Item 2", "Item 3", "Item 4", "Item 5", "Item 6"); ListView lv = new ListView<>(items); lv.getSelectionModel().setSelectionMode(SelectionMode.MULTIPLE);
The selection model in a list view has the selectedItemProperty property, which is an instance of Observable. As discussed in Section 15.10, you can add a listener to this property for handling the property change as follows: lv.getSelectionModel().selectedItemProperty().addListener( new InvalidationListener() { public void invalidated(Observable ov) { System.out.println("Selected indices: " + lv.getSelectionModel().getSelectedIndices());
16.9 ListView 649 System.out.println("Selected items: " + lv.getSelectionModel().getSelectedItems()); } });
This anonymous inner class can be simplified using a lambda expression as follows: lv.getSelectionModel().selectedItemProperty().addListener(ov -> { System.out.println("Selected indices: " + lv.getSelectionModel().getSelectedIndices()); System.out.println("Selected items: " + lv.getSelectionModel().getSelectedItems()); });
Listing 16.9 gives a program that lets users select the countries in a list view and displays the flags of the selected countries in the image views. Figure 16.20 shows a sample run of the program.
FlowPane
ListView inside a scroll pane
An ImageView is displayed
FIGURE 16.20 When the countries in the list view are selected, corresponding images of their flags are displayed in the image views. Here are the major steps in the program: 1. Create the user interface. Create a list view with nine country names as selection values, and place the list view inside a scroll pane. Place the scroll pane on the left of a border pane. Create nine image views to be used to display the countries’ flag images. Create a flow pane to hold the image views and place the pane in the center of the border pane. 2. Process the event. Create a listener
to implement the invalidated method in the InvalidationListener interface to place the selected countries’ flag image views
in the pane.
LISTING 16.9 ListViewDemo.java 1 2 3 4 5 6 7 8 9 10 11 12
import import import import import import import import import import
javafx.application.Application; javafx.stage.Stage; javafx.collections.FXCollections; javafx.scene.Scene; javafx.scene.control.ListView; javafx.scene.control.ScrollPane; javafx.scene.control.SelectionMode; javafx.scene.image.ImageView; javafx.scene.layout.BorderPane; javafx.scene.layout.FlowPane;
public class ListViewDemo extends Application {
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create a list view set list view properties
place list view in pane
listen to item selected
add image views of selected items
JavaFX UI Controls and Multimedia 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
// Declare an array of Strings for flag titles private String[] flagTitles = {"Canada", "China", "Denmark", "France", "Germany", "India", "Norway", "United Kingdom", "United States of America"}; // Declare an ImageView array for the national flags of 9 countries private ImageView[] ImageViews = { new ImageView("image/ca.gif"), new ImageView("image/china.gif"), new ImageView("image/denmark.gif"), new ImageView("image/fr.gif"), new ImageView("image/germany.gif"), new ImageView("image/india.gif"), new ImageView("image/norway.gif"), new ImageView("image/uk.gif"), new ImageView("image/us.gif") }; @Override // Override the start method in the Application class public void start(Stage primaryStage) { ListView lv = new ListView<> (FXCollections.observableArrayList(flagTitles)); lv.setPrefSize(400, 400); lv.getSelectionModel().setSelectionMode(SelectionMode.MULTIPLE); // Create a pane to hold image views FlowPane imagePane = new FlowPane(10, 10); BorderPane pane = new BorderPane(); pane.setLeft(new ScrollPane(lv)); pane.setCenter(imagePane); lv.getSelectionModel().selectedItemProperty().addListener( ov -> { imagePane.getChildren().clear(); for (Integer i: lv.getSelectionModel().getSelectedIndices()) { imagePane.getChildren().add(ImageViews[i]); } }); // Create a scene and place it in the stage Scene scene = new Scene(pane, 450, 170); primaryStage.setTitle("ListViewDemo"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } }
The program creates an array of strings for countries (lines 14–16) and an array of nine image views for displaying flag images for nine countries (lines 19–29) in the same order as in the array of countries. The items in the list view are from the array of countries (line 34). Thus, the index 0 of the image view array corresponds to the first country in the list view. The list view is placed in a scroll pane (line 41) so that it can be scrolled when the number of items in the list extends beyond the viewing area. By default, the selection mode of the list view is single. The selection mode for the list view is set to multiple (line 36), which allows the user to select multiple items in the list view. When the user selects countries in the list view, the listener’s handler (lines 44–50) is executed, which gets the indices of the selected items and adds their corresponding image views to the flow pane.
16.10 ScrollBar 651 16.27 How do you create an observable list with an array of strings? 16.28 How do you set the orientation in a list view? 16.29 What selection modes are available for a list view? What is the default selection
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mode? How do you set a selection mode?
16.30 How do you obtain the selected items and selected indices?
16.10 ScrollBar ScrollBar is a control that enables the user to select from a range of values.
Figure 16.21 shows a scroll bar. Normally, the user changes the value of a scroll bar by making a gesture with the mouse. For example, the user can drag the scroll bar’s thumb, click on the scroll bar track, or the scroll bar’s left or right buttons. Minimal value
Key Point
Maximal value
Track
Thumb Left button
Right button
FIGURE 16.21 A scroll bar represents a range of values graphically. ScrollBar has the following properties, as shown in Figure 16.22.
The getter and setter methods for property values and a getter for property itself are provided in the class, but omitted in the UML diagram for brevity. javafx.scene.control.ScrollBar -blockIncrement: DoubleProperty -max: DoubleProperty
The amount to adjust the scroll bar if the track of the bar is clicked (default: 10). The maximum value represented by this scroll bar (default: 100).
-min: DoubleProperty -unitIncrement: DoubleProperty
The minimum value represented by this scroll bar (default: 0). The amount to adjust the scroll bar when the increment() and decrement() methods are called (default: 1).
-value: DoubleProperty -visibleAmount: DoubleProperty -orientation: ObjectProperty
Current value of the scroll bar (default: 0). The width of the scroll bar (default: 15). Specifies the orientation of the scroll bar (default: HORIZONTAL).
+ScrollBar() +increment() +decrement()
Creates a default horizontal scroll bar. Increments the value of the scroll bar by unitIncrement. Decrements the value of the scroll bar by unitIncrement.
FIGURE 16.22 ScrollBar enables the user to select from a range of values.
Note The width of the scroll bar’s track corresponds to max + visibleAmount. When a scroll bar is set to its maximum value, the left side of the bubble is at max, and the right side is at max + visibleAmount.
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JavaFX UI Controls and Multimedia When the user changes the value of the scroll bar, it notifies the listener of the change. You can register a listener on the scroll bar’s valueProperty for responding to this change as follows: ScrollBar sb = new ScrollBar(); sb.valueProperty().addListener(ov -> { System.out.println("old value: " + oldVal); System.out.println("new value: " + newVal); });
Listing 16.10 gives a program that uses horizontal and vertical scroll bars to move a text displayed on a pane. The horizontal scroll bar is used to move the text to the left and the right, and the vertical scroll bar to move it up and down. A sample run of the program is shown in Figure 16.23.
Text
Vertical scroll bar Horizontal scroll bar
FIGURE 16.23
The scroll bars move the message on a pane horizontally and vertically.
Here are the major steps in the program: 1. Create the user interface. Create a Text object and place it in the center of the border pane. Create a vertical scroll bar and place it on the right of the border pane. Create a horizontal scroll bar and place it at the bottom of the border pane. 2. Process the event. Create listeners to move the text according to the bar movement in the scroll bars upon the change of the value property.
LISTING 16.10 ScrollBarDemo.java
horizontal scroll bar vertical scroll bar
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
import import import import import import import import
javafx.application.Application; javafx.stage.Stage; javafx.geometry.Orientation; javafx.scene.Scene; javafx.scene.control.ScrollBar; javafx.scene.layout.BorderPane; javafx.scene.layout.Pane; javafx.scene.text.Text;
public class ScrollBarDemo extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { Text text = new Text(20, 20, "JavaFX Programming"); ScrollBar sbHorizontal = new ScrollBar(); ScrollBar sbVertical = new ScrollBar(); sbVertical.setOrientation(Orientation.VERTICAL); // Create a text in a pane
16.10 ScrollBar 653 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
Pane paneForText = new Pane(); paneForText.getChildren().add(text); // Create a border pane to hold text and scroll bars BorderPane pane = new BorderPane(); pane.setCenter(paneForText); pane.setBottom(sbHorizontal); pane.setRight(sbVertical); // Listener for horizontal scroll bar value change sbHorizontal.valueProperty().addListener(ov -> text.setX(sbHorizontal.getValue() * paneForText.getWidth() / sbHorizontal.getMax())); // Listener for vertical scroll bar value change sbVertical.valueProperty().addListener(ov -> text.setY(sbVertical.getValue() * paneForText.getHeight() / sbVertical.getMax()));
add text to a pane
border pane
set new location for text
set new location for text
// Create a scene and place it in the stage Scene scene = new Scene(pane, 450, 170); primaryStage.setTitle("ScrollBarDemo"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } }
The program creates a text (line 13) and two scroll bars (sbHorizontal and sbVertical) (lines 15–16). The text is placed in a pane (line 21) that is then placed in the center of the border pane (line 25). If the text were directly placed in the center of the border pane, the position of the text cannot be changed by resetting its x and y properties. The sbHorizontal and sbVertical are placed on the right and at the bottom of the border pane (lines 26–27), respectively. You can specify the properties of the scroll bar. By default, the property value is 100 for max, 0 for min, 10 for blockIncrement, and 15 for visibleAmount. A listener is registered to listen for the sbHorizontal value property change (lines 30–32). When the value of the scroll bar changes, the listener is notified by invoking the handler to set a new x value for the text that corresponds to the current value of sbHorizontal (lines 31–32). A listener is registered to listen for the sbVertical value property change (lines 35–37). When the value of the scroll bar changes, the listener is notified by invoking the handler to set a new y value for the text that corresponds to the current value of sbVertical (lines 36–37). Alternatively, the code in lines 30–37 can be replaced by using binding properties as follows: text.xProperty().bind(sbHorizontal.valueProperty(). multiply(paneForText.widthProperty()). divide(sbHorizontal.maxProperty())); text.yProperty().bind(sbVertical.valueProperty().multiply( paneForText.heightProperty().divide( sbVertical.maxProperty())));
16.31 How do you create a horizontal scroll bar? How do you create a vertical scroll bar? 16.32 How do you write the code to respond to the value property change of a scroll bar? 16.33 How do you get the value from a scroll bar? How do you get the maximum value from a scroll bar?
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16.11 Slider Key Point
Slider is similar to ScrollBar, but Slider has more properties and can appear in many forms.
Figure 16.24 shows two sliders. Slider lets the user graphically select a value by sliding a knob within a bounded interval. The slider can show both major tick marks and minor tick marks between them. The number of pixels between the tick marks is specified by the majorTickUnit and minorTickUnit properties. Sliders can be displayed horizontally or vertically, with or without ticks, and with or without labels.
VideoNote
Use Slider
Vertical slider
Text
Horizontal slider
FIGURE 16.24
The sliders move the message on a pane horizontally and vertically.
The frequently used constructors and properties in Slider are shown in Figure 16.25.
The getter and setter methods for property values and a getter for property itself are provided in the class, but omitted in the UML diagram for brevity. javafx.scene.control.Slider -blockIncrement: DoubleProperty -max: DoubleProperty -min: DoubleProperty -value: DoubleProperty -orientation: ObjectProperty -majorTickUnit: DoubleProperty -minorTickCount: IntegerProperty -showTickLabels: BooleanProperty -showTickMarks: BooleanProperty +Slider() +Slider(min: double, max: double, value: double)
The amount to adjust the slider if the track of the bar is clicked (default: 10). The maximum value represented by this slider (default: 100). The minimum value represented by this slider (default: 0). Current value of the slider (default: 0). Specifies the orientation of the slider (default: HORIZONTAL). The unit distance between major tick marks. The number of minor ticks to place between two major ticks. Specifies whether the labels for tick marks are shown. Specifies whether the tick marks are shown. Creates a default horizontal slider. Creates a slider with the specified min, max, and value.
FIGURE 16.25 Slider enables the user to select from a range of values.
Note The values of a vertical scroll bar increase from top to bottom, but the values of a vertical slider decrease from top to bottom.
You can add a listener to listen for the value property change in a slider in the same way as in a scroll bar. We now rewrite the program in the preceding section using the sliders to move a text displayed on a pane in Listing 16.11. A sample run of the program is shown in Figure 16.24.
16.11 Slider 655
LISTING 16.11 SliderDemo.java 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
import import import import import import import import
javafx.application.Application; javafx.stage.Stage; javafx.geometry.Orientation; javafx.scene.Scene; javafx.scene.control.Slider; javafx.scene.layout.BorderPane; javafx.scene.layout.Pane; javafx.scene.text.Text;
public class SliderDemo extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { Text text = new Text(20, 20, "JavaFX Programming"); Slider slHorizontal = new Slider(); slHorizontal.setShowTickLabels(true); slHorizontal.setShowTickMarks(true);
horizontal slider set slider properties
Slider slVertical = new Slider(); slVertical.setOrientation(Orientation.VERTICAL); slVertical.setShowTickLabels(true); slVertical.setShowTickMarks(true); slVertical.setValue(100);
vertical slider set slider properties
// Create a text in a pane Pane paneForText = new Pane(); paneForText.getChildren().add(text); // Create a border pane to hold text and scroll bars BorderPane pane = new BorderPane(); pane.setCenter(paneForText); pane.setBottom(slHorizontal); pane.setRight(slVertical);
add text to a pane
border pane
slHorizontal.valueProperty().addListener(ov -> text.setX(slHorizontal.getValue() * paneForText.getWidth() / slHorizontal.getMax()));
set new location for text
slVertical.valueProperty().addListener(ov -> text.setY((slVertical.getMax() - slVertical.getValue()) * paneForText.getHeight() / slVertical.getMax()));
set new location for text
// Create a scene and place it in the stage Scene scene = new Scene(pane, 450, 170); primaryStage.setTitle("SliderDemo"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } }
Slider is similar to ScrollBar but has more features. As shown in this example, you can specify labels, major ticks, and minor ticks on a Slider (lines 16–17). A listener is registered to listen for the slHorizontal value property change (lines 35–37) and another one is for the sbVertical value property change (lines 39–41). When
the value of the slider changes, the listener is notified by invoking the handler to set a new position for the text (lines 36–37, 40–41). Note that since the value of a vertical slider decreases from top to bottom, the corresponding y value for the text is adjusted accordingly.
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JavaFX UI Controls and Multimedia The code in lines 35–41 can be replaced by using binding properties as follows: text.xProperty().bind(slHorizontal.valueProperty(). multiply(paneForText.widthProperty()). divide(slHorizontal.maxProperty())); text.yProperty().bind((slVertical.maxProperty().subtract( slVertical.valueProperty()).multiply( paneForText.heightProperty().divide( slVertical.maxProperty()))));
Listing 15.17 gives a program that displays a bouncing ball. You can add a slider to control the speed of the ball movement as shown in Figure 16.26. The new program is given in Listing 16.12.
FIGURE 16.26
You can increase or decrease the speed of the ball using a slider.
LISTING 16.12 BounceBallSlider.java
create a ball pane create a slider set max value for slider bind rate with slider value create a border pane add ball pane to center add slider to the bottom
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
import import import import import
javafx.application.Application; javafx.stage.Stage; javafx.scene.Scene; javafx.scene.control.Slider; javafx.scene.layout.BorderPane;
public class BounceBallSlider extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { BallPane ballPane = new BallPane(); Slider slSpeed = new Slider(); slSpeed.setMax(20); ballPane.rateProperty().bind(slSpeed.valueProperty()); BorderPane pane = new BorderPane(); pane.setCenter(ballPane); pane.setBottom(slSpeed); // Create a scene and place it in the stage Scene scene = new Scene(pane, 250, 250); primaryStage.setTitle("BounceBallSlider"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } }
The BallPane class defined in Listing 15.17 animates a ball bouncing in a pane. The rateProperty() method in BallPane returns a property value for animation rate.
16.12 Case Study: Developing a Tic-Tac-Toe Game 657 The animation stops if the rate is 0. If the rate is greater than 20, the animation will be too fast. So, we purposely set the rate to a value between 0 and 20. This value is bound to the slider value (line 13). So the slider max value is set to 20 (line 12).
16.34 How do you create a horizontal slider? How do you create a vertical slider? 16.35 How do you add a listener to handle the property value change of a slider? 16.36 How do you get the value from a slider? How do you get the maximum value from a
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16.12 Case Study: Developing a Tic-Tac-Toe Game This section develops a program for playing tic-tac-toe. From the many examples in this and earlier chapters you have learned about objects, classes, arrays, class inheritance, GUI, and event-driven programming. Now it is time to put what you have learned to work in developing comprehensive projects. In this section, we will develop a JavaFX program with which to play the popular game of tic-tac-toe. Two players take turns marking an available cell in a 3 * 3 grid with their respective tokens (either X or O). When one player has placed three tokens in a horizontal, vertical, or diagonal row on the grid, the game is over and that player has won. A draw (no winner) occurs when all the cells on the grid have been filled with tokens and neither player has achieved a win. Figure 16.27 shows the representative sample runs of the game.
(a) The X player won the game
FIGURE 16.27
(b) Draw — no winners
Key Point
VideoNote
TicTacToe
(c) The O player won the game
Two players play a tic-tac-toe game.
All the examples you have seen so far show simple behaviors that are easy to model with classes. The behavior of the tic-tac-toe game is somewhat more complex. To define classes that model the behavior, you need to study and understand the game. Assume that all the cells are initially empty, and that the first player takes the X token and the second player the O token. To mark a cell, the player points the mouse to the cell and clicks it. If the cell is empty, the token (X or O) is displayed. If the cell is already filled, the player’s action is ignored. From the preceding description, it is obvious that a cell is a GUI object that handles the mouse-click event and displays tokens. There are many choices for this object. We will use a pane to model a cell and to display a token (X or O). How do you know the state of the cell (empty, X, or O)? You use a property named token of the char type in the Cell class. The Cell class is responsible for drawing the token when an empty cell is clicked, so you need to write the code for listening to the mouse-clicked action and for painting the shapes for tokens X and O. The Cell class can be defined as shown in Figure 16.28.
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Cell -token: char
Token used in the cell (default: ' ').
+getToken(): char +setToken(token: char): void
Returns the token in the cell. Sets a new token in the cell.
-handleMouseClick(): void
Handles a mouse click event.
FIGURE 16.28
The Cell class displays the token in a cell.
The tic-tac-toe board consists of nine cells, created using new Cell[3][3]. To determine which player’s turn it is, you can introduce a variable named whoseTurn of the char type. whoseTurn is initially 'X', then changes to 'O', and subsequently changes between 'X' and 'O' whenever a new cell is occupied. When the game is over, set whoseTurn to ' '. How do you know whether the game is over, whether there is a winner, and who the winner, if any? You can define a method named isWon(char token) to check whether a specified token has won and a method named isFull() to check whether all the cells are occupied. Clearly, two classes emerge from the foregoing analysis. One is the Cell class, which handles operations for a single cell; the other is the TicTacToe class, which plays the whole game and deals with all the cells. The relationship between these two classes is shown in Figure 16.29.
javafx.application.Application
Cell 9 1
TicTacToe -whoseTurn: char -cell: Cell[][]
Indicates which player has the turn, initially X. A 3 3, two-dimensional array for cells.
-lblStatus: Label
A label to display game status.
+TicTacToe() +isFull(): boolean +isWon(token: char): boolean
Constructs the TicTacToe user interface. Returns true if all cells are filled. Returns true if a player with the specified token has won.
FIGURE 16.29 The TicTacToe class contains nine cells. Since the Cell class is only to support the TicTacToe class, it can be defined as an inner class in TicTacToe. The complete program is given in Listing 16.13.
LISTING 16.13 TicTacToe.java 1 2 3 4 5
import import import import import
javafx.application.Application; javafx.stage.Stage; javafx.scene.Scene; javafx.scene.control.Label; javafx.scene.layout.BorderPane;
16.12 Case Study: Developing a Tic-Tac-Toe Game 659 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 58 59 60 61 62 63 64 65
import import import import import
javafx.scene.layout.GridPane; javafx.scene.layout.Pane; javafx.scene.paint.Color; javafx.scene.shape.Line; javafx.scene.shape.Ellipse;
public class TicTacToe extends Application { // Indicate which player has a turn, initially it is the X player private char whoseTurn = 'X';
main class TicTacToe
// Create and initialize cell private Cell[][] cell = new Cell[3][3]; // Create and initialize a status label private Label lblStatus = new Label("X's turn to play"); @Override // Override the start method in the Application class public void start(Stage primaryStage) { // Pane to hold cell GridPane pane = new GridPane(); for (int i = 0; i < 3; i++) for (int j = 0; j < 3; j++) pane.add(cell[i][j] = new Cell(), j, i); BorderPane borderPane = new BorderPane(); borderPane.setCenter(pane); borderPane.setBottom(lblStatus);
hold nine cells
create a cell
tic-tac-toe cells in center label at bottom
// Create a scene and place it in the stage Scene scene = new Scene(borderPane, 450, 170); primaryStage.setTitle("TicTacToe"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } /** Determine if the cell are all occupied */ public boolean isFull() { for (int i = 0; i < 3; i++) for (int j = 0; j < 3; j++) if (cell[i][j].getToken() == ' ') return false;
check isFull
return true; } /** Determine if the player with public boolean isWon(char token) for (int i = 0; i < 3; i++) if (cell[i][0].getToken() == && cell[i][1].getToken() && cell[i][2].getToken() return true; }
the specified token wins */ { check rows token == token == token) {
for (int j = 0; j < 3; j++) if (cell[0][j].getToken() == token && cell[1][j].getToken() == token && cell[2][j].getToken() == token) { return true; }
check columns
660 Chapter 16 check major diagonal
check subdiagonal
inner class Cell
register listener
display X
display O
JavaFX UI Controls and Multimedia 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 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125
if (cell[0][0].getToken() == token && cell[1][1].getToken() == token && cell[2][2].getToken() == token) { return true; } if (cell[0][2].getToken() == token && cell[1][1].getToken() == token && cell[2][0].getToken() == token) { return true; } return false; } // An inner class for a cell public class Cell extends Pane { // Token used for this cell private char token = ' '; public Cell() { setStyle("-fx-border-color: black"); this.setPrefSize(2000, 2000); this.setOnMouseClicked(e -> handleMouseClick()); } /** Return token */ public char getToken() { return token; } /** Set a new token */ public void setToken(char c) { token = c; if (token == 'X') { Line line1 = new Line(10, 10, this.getWidth() - 10, this.getHeight() - 10); line1.endXProperty().bind(this.widthProperty().subtract(10)); line1.endYProperty().bind(this.heightProperty().subtract(10)); Line line2 = new Line(10, this.getHeight() - 10, this.getWidth() - 10, 10); line2.startYProperty().bind( this.heightProperty().subtract(10)); line2.endXProperty().bind(this.widthProperty().subtract(10)); // Add the lines to the pane this.getChildren().addAll(line1, line2); } else if (token == 'O') { Ellipse ellipse = new Ellipse(this.getWidth() / 2, this.getHeight() / 2, this.getWidth() / 2 - 10, this.getHeight() / 2 - 10); ellipse.centerXProperty().bind( this.widthProperty().divide(2)); ellipse.centerYProperty().bind( this.heightProperty().divide(2)); ellipse.radiusXProperty().bind( this.widthProperty().divide(2).subtract(10));
16.12 Case Study: Developing a Tic-Tac-Toe Game 661 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159
ellipse.radiusYProperty().bind( this.heightProperty().divide(2).subtract(10)); ellipse.setStroke(Color.BLACK); ellipse.setFill(Color.WHITE); getChildren().add(ellipse); // Add the ellipse to the pane } } /* Handle a mouse click event */ private void handleMouseClick() { // If cell is empty and game is not over if (token == ' ' && whoseTurn != ' ') { setToken(whoseTurn); // Set token in the cell
handle mouse click
// Check game status if (isWon(whoseTurn)) { lblStatus.setText(whoseTurn + " won! The game is over"); whoseTurn = ' '; // Game is over } else if (isFull()) { lblStatus.setText("Draw! The game is over"); whoseTurn = ' '; // Game is over } else { // Change the turn whoseTurn = (whoseTurn == 'X') ? 'O' : 'X'; // Display whose turn lblStatus.setText(whoseTurn + "'s turn"); } } } } }
The TicTacToe class initializes the user interface with nine cells placed in a grid pane (lines 25–28). A label named lblStatus is used to show the status of the game (line 20). The variable whoseTurn (line 14) is used to track the next type of token to be placed in a cell. The methods isFull (lines 42–49) and isWon (lines 52–80) are for checking the status of the game. Since Cell is an inner class in TicTacToe, the variable (whoseTurn) and methods (isFull and isWon) defined in TicTacToe can be referenced from the Cell class. The inner class makes programs simple and concise. If Cell were not defined as an inner class of TicTacToe, you would have to pass an object of TicTacToe to Cell in order for the variables and methods in TicTacToe to be used in Cell. The listener for the mouse-click action is registered for the cell (line 90). If an empty cell is clicked and the game is not over, a token is set in the cell (line 138). If the game is over, whoseTurn is set to ' ' (lines 144, 148). Otherwise, whoseTurn is alternated to a new turn (line 152).
Tip Use an incremental approach in developing and testing a Java project of this kind. For example, this program can be divided into five steps: 1.
Lay out the user interface and display a fixed token X on a cell.
2.
Enable the cell to display a fixed token X upon a mouse click.
3.
Coordinate between the two players so as to display tokens X and O alternately.
4.
Check whether a player wins, or whether all the cells are occupied without a winner.
5.
Implement displaying a message on the label upon each move by a player.
incremental development and testing
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16.37 When the game starts, what value is in whoseTurn? When the game is over, what value is in whoseTurn?
16.38 What happens when the user clicks on an empty cell if the game is not over? What happens when the user clicks on an empty cell if the game is over? 16.39 How does the program check whether a player wins? How does the program check whether all cells are filled?
16.13 Video and Audio Key Point
VideoNote
Use Media, MediaPlayer, and MediaView
You can use the Media class to obtain the source of the media, the MediaPlayer class to play and control the media, and the MediaView class to display the video. Media (video and audio) is essential in developing rich Internet applications. JavaFX provides the Media, MediaPlayer, and MediaView classes for working with media. Currently, JavaFX supports MP3, AIFF, WAV, and MPEG-4 audio formats and FLV and MPEG-4 video formats. The Media class represents a media source with properties duration, width, and height, as shown in Figure 16.30. You can construct a Media object from an Internet URL string. The getter and setter methods for property values and a getter for property itself are provided in the class, but omitted in the UML diagram for brevity.
javafx.scene.media.Media -duration: ReadOnlyObjectProperty -width: ReadOnlyIntegerProperty -height: ReadOnlyIntegerProperty
The durations in seconds of the source media.
+Media(source: String)
Creates a Media from a URL source.
The width in pixels of the source video. The height in pixels of the source video.
FIGURE 16.30 Media represents a media source such as a video or an audio. The MediaPlayer class plays and controls the media with properties such as autoPlay, currentCount, cycleCount, mute, volume, and totalDuration, as shown in Figure 16.31. You can construct a MediaPlayer object from a media and use the pause() and play() method to pause and resume playing. The getter and setter methods for property values and a getter for property itself are provided in the class, but omitted in the UML diagram for brevity. javafx.scene.media.MediaPlayer -autoPlay: BooleanProperty -currentCount: ReadOnlyIntegerProperty -cycleCount: IntegerProperty -mute: BooleanProperty -volume: DoubleProperty -totalDuration: ReadOnlyObjectProperty
Specifies whether the playing should start automatically. The number of completed playback cycles. Specifies the number of time the media will be played. Specifies whether the audio is muted. The volume for the audio. The amount of time to play the media from start to finish.
+MediaPlayer(media: Media) +play(): void +pause(): void +seek(): void
Creates a player for a specified media. Plays the media. Pauses the media. Seeks the player to a new playback time.
FIGURE 16.31 MediaPlayer plays and controls a media.
16.13 Video and Audio 663 The MediaView class is a subclass of Node that provides a view of the Media being played by a MediaPlayer. The MediaView class provides the properties for viewing the media, as shown in Figure 16.32.
The getter and setter methods for property values and a getter for property itself are provided in the class, but omitted in the UML diagram for brevity. javafx.scene.media.MediaView -x: DoubleProperty
Specifies the current x-coordinate of the media view.
-y: DoubleProperty -mediaPlayer: ObjectProperty -fitWidth: DoubleProperty
Specifies the current y-coordinate of the media view. Specifies a media player for the media view.
-fitHeight: DoubleProperty
Specifies the height of the view for the media to fit.
+MediaView() +MediaView(mediaPlayer: MediaPlayer)
Creates an empty media view. Creates a media view with the specified media player.
Specifies the width of the view for the media to fit.
FIGURE 16.32 MediaView provides the properties for viewing the media. Listing 16.14 gives an example that displays a video in a view, as shown in Figure 16.33. You can use the play/pause button to play or pause the video and use the rewind button to restart the video, and use the slider to control the volume of the audio.
FIGURE 16.33 The program controls and plays a video.
LISTING 16.14 MediaDemo.java 1 2 3 4 5 6 7 8 9 10 11
import import import import import import import import import import import
javafx.application.Application; javafx.stage.Stage; javafx.geometry.Pos; javafx.scene.Scene; javafx.scene.control.Button; javafx.scene.control.Label; javafx.scene.control.Slider; javafx.scene.layout.BorderPane; javafx.scene.layout.HBox; javafx.scene.layout.Region; javafx.scene.media.Media;
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create a media create a media player create a media view create a play/pause button add handler for button action play media
pause media
create a rewind button create a handler for rewinding create a slider for volume
set current volume bind volume with slider
add buttons, slider to hBox
place media view in a pane
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
import javafx.scene.media.MediaPlayer; import javafx.scene.media.MediaView; import javafx.util.Duration; public class MediaDemo extends Application { private static final String MEDIA_URL = "http://cs.armstrong.edu/liang/common/sample.mp4"; @Override // Override the start method in the Application class public void start(Stage primaryStage) { Media media = new Media(MEDIA_URL); MediaPlayer mediaPlayer = new MediaPlayer(media); MediaView mediaView = new MediaView(mediaPlayer); Button playButton = new Button(">"); playButton.setOnAction(e -> { if (playButton.getText().equals(">")) { mediaPlayer.play(); playButton.setText("||"); } else { mediaPlayer.pause(); playButton.setText(">"); } }); Button rewindButton = new Button("<<"); rewindButton.setOnAction(e -> mediaPlayer.seek(Duration.ZERO)); Slider slVolume = new Slider(); slVolume.setPrefWidth(150); slVolume.setMaxWidth(Region.USE_PREF_SIZE); slVolume.setMinWidth(30); slVolume.setValue(50); mediaPlayer.volumeProperty().bind( slVolume.valueProperty().divide(100)); HBox hBox = new HBox(10); hBox.setAlignment(Pos.CENTER); hBox.getChildren().addAll(playButton, rewindButton, new Label("Volume"), slVolume); BorderPane pane = new BorderPane(); pane.setCenter(mediaView); pane.setBottom(hBox); // Create a scene and place it in the stage Scene scene = new Scene(pane, 650, 500); primaryStage.setTitle("MediaDemo"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } }
The source of the media is a URL string defined in lines 17 and 18. The program creates a Media object from this URL (line 22), a MediaPlayer from the Media object (line 23), and a MediaView from the MediaPlayer object (line 24). The relationship among these three objects is shown in Figure 16.34.
16.14 Case Study: National Flags and Anthems 665 media: Media
mediaPlayer: MediaPlayer
mediaView: MediaView
FIGURE 16.34 The media represents the source, the media player controls the playing, and the media view displays the video. A Media object supports live streaming. You can now download a large media file and play it in the same time. A Media object can be shared by multiple media players and different views can use the same MediaPlayer object. A play button is created (line 26) to play/pause the media (line 29). The button’s text is changed to || (line 30) if the button’s current text is > (line 28). If the button’s current text is ||, it is changed to > (line 33) and the player is paused (line 32). A rewind button is created (line 37) to reset the playback time to the beginning of the media stream by invoking seek(Duration.ZERO) (line 38). A slider is created (line 40) to set the volume. The media player’s volume property is bound to the slider (lines 45 and 46). The buttons and slider are placed in an HBox (lines 48–51) and the media view is placed in the center of the border pane (line 54) and the HBox is placed at the bottom of the border pane (line 55).
16.40 How do you create a Media from a URL? How do you create a MediaPlayer? How do you create a MediaView? 16.41 If the URL is typed as cs.armstrong.edu/liang/common/sample.mp4 without http:// in front of it, will it work?
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Check Point
16.42 Can you place a Media in multiple MediaPlayers? Can you place a MediaPlayer in multiple MediaViews? Can you place a MediaView in multiple Panes?
16.14 Case Study: National Flags and Anthems This case study presents a program that displays a nation’s flag and plays its anthem. The images for seven national flags, named flag0.gif, flag1.gif, . . . , flag6.gif for Denmark, Germany, China, India, Norway, United Kingdom, and United States are stored under www.cs.armstrong.edu/liang/common/image. The audio consists of national anthems for these seven nations, named anthem0.mp3, anthem1.mp3, . . . , and anthem6.mp3. They are stored under www.cs.armstrong.edu/liang/common/audio. The program enables the user to select a nation from a combo box and then displays its flag and plays its anthem. The user can suspend the audio by clicking the || button and resume it by clicking the < button, as shown in Figure 16.35.
FIGURE 16.35
The program displays a national flag and plays its anthem.
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JavaFX UI Controls and Multimedia The program is given in Listing 16.15.
LISTING 16.15 FlagAnthem.java VideoNote
Audio and image
URLBase for image and audio track current image/audio
image array media player array
load image load audio
create play button handle button action
pause audio
play audio
create image view create combo box create observable list
process combo selection choose a new nation play audio
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 54 55 56 57
import import import import import import import import import import import import import import import
javafx.application.Application; javafx.collections.FXCollections; javafx.collections.ObservableList; javafx.stage.Stage; javafx.geometry.Pos; javafx.scene.Scene; javafx.scene.control.Button; javafx.scene.control.ComboBox; javafx.scene.control.Label; javafx.scene.image.Image; javafx.scene.image.ImageView; javafx.scene.layout.BorderPane; javafx.scene.layout.HBox; javafx.scene.media.Media; javafx.scene.media.MediaPlayer;
public class FlagAnthem extends Application { private final static int NUMBER_OF_NATIONS = 7; private final static String URLBase = "http://cs.armstrong.edu/liang/common"; private int currentIndex = 0; @Override // Override the start method in the Application class public void start(Stage primaryStage) { Image[] images = new Image[NUMBER_OF_NATIONS]; MediaPlayer[] mp = new MediaPlayer[NUMBER_OF_NATIONS]; // Load images and audio for (int i = 0; i < NUMBER_OF_NATIONS; i++) { images[i] = new Image(URLBase + "/image/flag" + i + ".gif"); mp[i] = new MediaPlayer(new Media( URLBase + "/audio/anthem/anthem" + i + ".mp3")); } Button btPlayPause = new Button(">"); btPlayPause.setOnAction(e -> { if (btPlayPause.getText().equals(">")) { btPlayPause.setText("||"); mp[currentIndex].pause(); } else { btPlayPause.setText(">"); mp[currentIndex].play(); } }); ImageView imageView = new ImageView(images[currentIndex]); ComboBox cboNation = new ComboBox<>(); ObservableList items = FXCollections.observableArrayList ("Denmark", "Germany", "China", "India", "Norway", "UK", "US"); cboNation.getItems().addAll(items); cboNation.setValue(items.get(0)); cboNation.setOnAction(e -> { mp[currentIndex].stop(); currentIndex = items.indexOf(cboNation.getValue()); imageView.setImage(images[currentIndex]); mp[currentIndex].play(); });
Chapter Summary 667 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75
HBox hBox = new HBox(10); hBox.getChildren().addAll(btPlayPause, new Label("Select a nation: "), cboNation); hBox.setAlignment(Pos.CENTER); // Create a pane to hold nodes BorderPane pane = new BorderPane(); pane.setCenter(imageView); pane.setBottom(hBox); // Create a scene and place it in the stage Scene scene = new Scene(pane, 350, 270); primaryStage.setTitle("FlagAnthem"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage } }
The program loads the image and audio from the Internet (lines 29–33). A play/pause button is created to control the playing of the audio (line 35). When the button is clicked, if the button’s current text is > (line 37), its text is changed to || (line 38) and the player is paused (line 39); If the button’s current text is ||, it is changed to > (line 41) and the player is paused (line 42). An image view is created to display a flag image (line 46). A combo box is created for selecting a nation (line 47–49). When a new country name in the combo box is selected, the current audio is stopped (line 53) and the newly selected nation’s image is displayed (line 55) and the new anthem is played (line 56). JavaFX also provides the AudioClip class for creating auto clips. An AudioClip object can be created using new AudioClip(URL). An audio clip stores the audio in memory. AudioClip is more efficient for playing a small audio clip in the program than using MediaPlayer. AudioClip has the similar methods as in the MediaPlayer class.
16.43 In Listing 16.15, which code sets the initial image icon and which code plays the audio? 16.44 In Listing 16.15, what does the program do when a new nation is selected in the combo box?
CHAPTER SUMMARY 1. The abstract
Labeled class is the base class for Label, Button, CheckBox, and RadioButton. It defines properties alignment, contentDisplay, text, graphic, graphicTextGap, textFill, underline, and wrapText.
2. The abstract
ButtonBase class is the base class for Button, CheckBox, and RadioButton. It defines the onAction property for specifying a handler for action events.
3. The abstract TextInputContorl class is the base class for TextField and TextArea. It defines the properties text and editable.
4. A
TextField fires an action event when clicking the Enter key with the text field focused. A TextArea is often used for editing a multiline text.
5.
ComboBox and ListView are generic classes for storing elements of type T. The elements in a combo box or a list view are stored in an observable list.
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Check Point
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JavaFX UI Controls and Multimedia 6. A ComboBox fires an action event when a new item is selected. 7. You can set a single item or multiple item selection for a ListView and add a listener for processing selected items.
8. You can use a ScrollBar or Slider to select a range of values and add a listener to the value property to respond to the change of the value.
9. JavaFX provides the Media class for loading a media, the MediaPlayer class for controlling a media, and the MediaView for displaying a media.
QUIZ Answer the quiz for this chapter online at www.cs.armstrong.edu/liang/intro10e/quiz.html.
PROGRAMMING EXERCISES Sections 16.2–16.5
*16.1 (Use radio buttons) Write a GUI program as shown in Figure 16.36a. You can use buttons to move the message to the left and right and use the radio buttons to change the color for the message displayed.
StackPane
HBox (a)
(b)
FIGURE 16.36 (a) The 6= and = 7 buttons move the message, and the radio buttons change the color for the message. (b) The program displays a circle, rectangle, and ellipse when you select a shape type.
*16.2 (Select geometric figures) Write a program that draws various figures, as shown in Figure 16.36b. The user selects a figure from a radio button and uses a check box to specify whether it is filled. **16.3 (Traffic lights) Write a program that simulates a traffic light. The program lets the user select one of three lights: red, yellow, or green. When a radio button is selected, the light is turned on. Only one light can be on at a time (see Figure 16.37a). No light is on when the program starts.
Programming Exercises 669
(a)
(b)
(c)
FIGURE 16.37 (a) The radio buttons are grouped to let you turn only one light on at a time. (b) The program converts miles to kilometers, and vice versa. (c) The program converts between decimal, hex, and binary numbers.
*16.4 (Create a miles/kilometers converter) Write a program that converts miles and kilometers, as shown in Figure 16.37b. If you enter a value in the Mile text field and press the Enter key, the corresponding kilometer measurement is displayed in the Kilometer text field. Likewise, if you enter a value in the Kilometer text field and press the Enter key, the corresponding miles is displayed in the Mile text field. *16.5 (Convert numbers) Write a program that converts between decimal, hex, and binary numbers, as shown in Figure 16.37c. When you enter a decimal value in the decimalvalue text field and press the Enter key, its corresponding hex and binary numbers are displayed in the other two text fields. Likewise, you can enter values in the other fields and convert them accordingly. (Hint: Use the Integer.parseInt(s, radix) method to parse a string to a decimal and use Integer.toHexString(decimal) and Integer.toBinaryString(decimal) to obtain a hex number or a binary number from a decimal.) *16.6 (Demonstrate TextField properties) Write a program that sets the horizontalalignment and column-size properties of a text field dynamically, as shown in Figure 16.38a.
(a)
(b)
FIGURE 16.38 (a) You can set a text field’s properties for the horizontal alignment and column size dynamically. (b) The program displays the time specified in the text fields.
VideoNote
Use radio buttons and text fields
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JavaFX UI Controls and Multimedia *16.7 (Set clock time) Write a program that displays a clock and sets the time with the input from three text fields, as shown in Figure 16.38b. Use the ClockPane in Listing 14.21. Resize the clock to the center of the pane. **16.8 (Geometry: two circles intersect?) Write a program that enables the user to specify the location and size of the circles and displays whether the two circles intersect, as shown in Figure 16.39a. Enable the user to point the mouse inside a circle and drag it. As the circle is being dragged, the circle’s center coordinates in the text fields are updated.
(a)
FIGURE 16.39
(b)
Check whether two circles and two rectangles are overlapping.
**16.9 (Geometry: two rectangles intersect?) Write a program that enables the user to specify the location and size of the rectangles and displays whether the two rectangles intersect, as shown in Figure 16.39b. Enable the user to point the mouse inside a rectangle and drag it. As the rectangle is being dragged, the rectangle’s center coordinates in the text fields are updated.
Sections 16.6–16.8
**16.10 (Text viewer) Write a program that displays a text file in a text area, as shown in Figure 16.40a. The user enters a file name in a text field and clicks the View button; the file is then displayed in a text area.
(a)
(b)
FIGURE 16.40 (a) The program displays the text from a file in a text area. (b) The program displays a histogram that shows the occurrences of each letter in the file.
Programming Exercises 671 **16.11 (Create a histogram for occurrences of letters) Write a program that reads a file and displays a histogram to show the occurrences of each letter in the file, as shown in Figure 16.40b. The file name is entered from a text field. Pressing the Enter key on the text field causes the program to start to read and process the file and displays the histogram. The histogram is displayed in the center of the window. Define a class named Histogram that extends Pane. The class contains the property counts that is an array of 26 elements. counts[0] stores the number of A, counts[1] the number of B, and so on. The class also contains a setter method for setting a new counts and displaying the histogram for the new counts. *16.12 (Demonstrate TextArea properties) Write a program that demonstrates the properties of a text area. The program uses a check box to indicate whether the text is wrapped onto next line, as shown in Figure 16.41a.
(a)
(b)
FIGURE 16.41 (a) You can set the options to enable text editing and text wrapping. (b) The program displays a table for monthly payments and total payments on a given loan based on various interest rates.
*16.13 (Compare loans with various interest rates) Rewrite Programming Exercise 5.21 to create a GUI, as shown in Figure 16.41b. Your program should let the user enter the loan amount and loan period in the number of years from text fields, and it should display the monthly and total payments for each interest rate starting from 5 percent to 8 percent, with increments of one-eighth, in a text area. **16.14 (Select a font) Write a program that can dynamically change the font of a text in a label displayed on a stack pane. The text can be displayed in bold and italic at the same time. You can select the font name or font size from combo boxes, as shown in Figure 16.42a. The available font names can be obtained using Font.getFamilies(). The combo box for the font size is initialized with numbers from 1 to 100.
(a)
VideoNote
Set fonts
(b)
FIGURE 16.42 You can dynamically set the font for the message. (b) You can set the alignment and text-position properties of a label dynamically.
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JavaFX UI Controls and Multimedia **16.15 (Demonstrate Label properties) Write a program to let the user dynamically set the properties contentDisplay and graphicTextGap, as shown in Figure 16.42b. *16.16 (Use ComboBox and ListView) Write a program that demonstrates selecting items in a list. The program uses a combo box to specify a selection mode, as shown in Figure 16.43a. When you select items, they are displayed in a label below the list.
(b)
(a)
(c)
FIGURE 16.43 (a) You can choose single or multiple selection mode in a list. (b) The color changes in the text as you adjust the scroll bars. (c) The program simulates a running fan.
Sections 16.6–16.8
**16.17 (Use ScrollBar and
Slider) Write a program that uses scroll bars or sliders to select the color for a text, as shown in Figure 16.43b. Four horizontal scroll bars are used for selecting the colors: red, green, blue, and opacity percentages.
**16.18 (Simulation: a running fan) Rewrite Programming Exercise 15.28 to add a slider to control the speed of the fan, as shown in Figure 16.43c.
**16.19 (Control a group of fans) Write a program that displays three fans in a group, with control buttons to start and stop all of them, as shown in Figure 16.44.
FIGURE 16.44
The program runs and controls a group of fans.
*16.20 (Count-up stopwatch) Write a program that simulates a stopwatch, as shown in Figure 16.45a. When the user clicks the Start button, the button’s label is changed to Pause, as shown in Figure 16.45b. When the user clicks the Pause
Programming Exercises 673 button, the button’s label is changed to Resume, as shown in Figure 16.45c. The Clear button resets the count to 0 and resets the button’s label to Start.
(a)
FIGURE 16.45
(b)
(c)
(d)
(a–c) The program counts up the time. (d) The program counts down the time.
*16.21 (Count-down stopwatch) Write a program that allows the user to enter time in seconds in the text field and press the Enter key to count down the seconds, as shown in Figure 16.45d. The remaining seconds are redisplayed every one second. When the seconds are expired, the program starts to play music continuously. 16.22 (Play, loop, and stop a sound clip) Write a program that meets the following requirements: ■ ■ ■
Get an audio file from the class directory using AudioClip. Place three buttons labeled Play, Loop, and Stop, as shown in Figure 16.46a. If you click the Play button, the audio file is played once. If you click the Loop button, the audio file keeps playing repeatedly. If you click the Stop button, the playing stops.
(a)
(b)
FIGURE 16.46 (a) Click Play to play an audio clip once, click Loop to play an audio repeatedly, and click Stop to terminate playing. (b) The program lets the user specify image files, an audio file, and the animation speed.
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JavaFX UI Controls and Multimedia **16.23 (Create an image animator with audio) Create animation in Figure 16.46b to meet the following requirements: ■ ■
■
Allow the user to specify the animation speed in a text field. Get the number of iamges and image’s file-name prefix from the user. For example, if the user enters n for the number of images and L for the image prefix, then the files are L1.gif, L2.gif, and so on, to Ln.gif. Assume that the images are stored in the image directory, a subdirectory of the program’s class directory. The animation displays the images one after the other. Allow the user to specify an audio file URL. The audio is played while the animation runs.
**16.24 (Revise Listing 16.14 MediaDemo.java) Add a slider to enable the user to set the current time for the video and a label to display the current time and the total time for the video. As shown in Figure 16.47a, the total time is 5 minutes and 3 seconds and the current time is 3 minutes and 58 seconds. As the video plays, the slider value and current time are continuously updated.
(a)
(b)
FIGURE 16.47 (a) A slider for current video time and a label to show the current time and total time are added. (b) You can set the speed for each car.
**16.25 (Racing cars) Write a program that simulates four cars racing, as shown in Figure 16.47b. You can set the speed for each car, with maximum 100. **16.26 (Simulation: raise flag and play anthem) Write a program that displays a flag rising up, as shown in Figure 15.14. As the national flag rises, play the national anthem. (You may use a flag image and anthem audio file from Listing 16.15.)
Comprehensive
**16.27 (Display country flag and flag description) Listing 16.4, ComboBoxDemo.java, gives a program that lets the user view a country’s flag image and description by selecting the country from a combo box. The description is a string coded in the program. Rewrite the program to read the text description from a file. Suppose that the descriptions are stored in the files description0.txt, . . . , and
Programming Exercises 675 description8.txt under the text directory for the nine countries Canada, China, Denmark, France, Germany, India, Norway, United Kingdom, and United States, in this order. **16.28 (Slide show) Programming Exercise 15.30 developed a slide show using images. Rewrite that program to develop a slide show using text files. Suppose ten text files named slide0.txt, slide1.txt, . . . , and slide9.txt are stored in the text directory. Each slide displays the text from one file. Each slide is shown for one second, and the slides are displayed in order. When the last slide finishes, the first slide is redisplayed, and so on. Use a text area to display the slide. ***16.29 (Display a calendar) Write a program that displays the calendar for the current month. You can use the Prior and Next buttons to show the calendar of the previous or next month. Display the dates in the current month in black and display the dates in the previous month and next month in gray, as shown in Figure 16.48.
FIGURE 16.48
The program displays the calendar for the current month.
**16.30 (Pattern recognition: consecutive four equal numbers) Write a GUI program for Programming Exercise 8.19, as shown in Figure 16.49a–b. Let the user enter the numbers in the text fields in a grid of 6 rows and 7 columns. The user can click the Solve button to highlight a sequence of four equal numbers, if it exists. Initially, the values in the text fields are filled with numbers from 0 to 9 randomly.
(a)
(b)
(c)
FIGURE 16.49 (a–b) Clicking the Solve button highlights the four consecutive numbers in a row, a column, or a diagonal. (c) The program enables two players to play the connect-four game.
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JavaFX UI Controls and Multimedia ***16.31 (Game: connect four) Programming Exercise 8.20 enables two players to play the connect-four game on the console. Rewrite a GUI version for the program, as shown in Figure 16.49c. The program enables two players to place red and yellow discs in turn. To place a disk, the player needs to click an available cell. An available cell is unoccupied and its downward neighbor is occupied. The program flashes the four winning cells if a player wins and reports no winners if all cells are occupied with no winners.
CHAPTER
17 BINARY I/O Objectives ■
To discover how I/O is processed in Java (§17.2).
■
To distinguish between text I/O and binary I/O (§17.3).
■
To read and write bytes using FileInputStream and FileOutputStream (§17.4.1).
■
To filter data using the base classes FilterInputStream and FilterOutputStream (§17.4.2).
■
To read and write primitive values and strings using DataInputStream and DataOutputStream (§17.4.3).
■
To improve I/O performance by using BufferedInputStream and BufferedOutputStream (§17.4.4).
■
To write a program that copies a file (§17.5).
■
To store and restore objects using ObjectOutputStream and ObjectInputStream (§17.6).
■
To implement the Serializable interface to make objects serializable (§17.6.1).
■
To serialize arrays (§17.6.2).
■
To read and write files using the RandomAccessFile class (§17.7).
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Binary I/O
17.1 Introduction Key Point text file binary file
why binary I/O?
text I/O binary I/O
Java provides many classes for performing text I/O and binary I/O. Files can be classified as either text or binary. A file that can be processed (read, created, or modified) using a text editor such as Notepad on Windows or vi on UNIX is called a text file. All the other files are called binary files. You cannot read binary files using a text editor—they are designed to be read by programs. For example, Java source programs are text files and can be read by a text editor, but Java class files are binary files and are read by the JVM. Although it is not technically precise and correct, you can envision a text file as consisting of a sequence of characters and a binary file as consisting of a sequence of bits. Characters in a text file are encoded using a character encoding scheme such as ASCII or Unicode. For example, the decimal integer 199 is stored as a sequence of three characters 1, 9, 9 in a text file, and the same integer is stored as a byte-type value C7 in a binary file, because decimal 199 equals hex C7 (199 = 12 * 161 + 7). The advantage of binary files is that they are more efficient to process than text files. Java offers many classes for performing file input and output. These can be categorized as text I/O classes and binary I/O classes. In Section 12.11, File Input and Output, you learned how to read and write strings and numeric values from/to a text file using Scanner and PrintWriter. This chapter introduces the classes for performing binary I/O.
17.2 How Is Text I/O Handled in Java? Key Point
Text data are read using the Scanner class and written using the PrintWriter class. Recall that a File object encapsulates the properties of a file or a path but does not contain the methods for reading/writing data from/to a file. In order to perform I/O, you need to create objects using appropriate Java I/O classes. The objects contain the methods for reading/ writing data from/to a file. For example, to write text to a file named temp.txt, you can create an object using the PrintWriter class as follows: PrintWriter output = new PrintWriter("temp.txt");
You can now invoke the print method on the object to write a string to the file. For example, the following statement writes Java 101 to the file. output.print("Java 101");
The next statement closes the file. output.close();
There are many I/O classes for various purposes. In general, these can be classified as input classes and output classes. An input class contains the methods to read data, and an output class contains the methods to write data. PrintWriter is an example of an output class, and Scanner is an example of an input class. The following code creates an input object for the file temp.txt and reads data from the file. Scanner input = new Scanner(new File("temp.txt")); System.out.println(input.nextLine());
stream input stream output stream
If temp.txt contains the text Java 101, input.nextLine() returns the string "Java 101". Figure 17.1 illustrates Java I/O programming. An input object reads a stream of data from a file, and an output object writes a stream of data to a file. An input object is also called an input stream and an output object an output stream.
17.3 Text I/O vs. Binary I/O 679 Program Input stream Input object created from an input class
01011...1001
Output object created from an output class
File
11001...1011
File
Output stream
FIGURE 17.1 The program receives data through an input object and sends data through an output object.
17.1 What is a text file and what is a binary file? Can you view a text file or a binary file using a text editor?
17.2 How do you read or write text data in Java? What is a stream?
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17.3 Text I/O vs. Binary I/O Binary I/O does not involve encoding or decoding and thus is more efficient than text I/O. Computers do not differentiate between binary files and text files. All files are stored in binary format, and thus all files are essentially binary files. Text I/O is built upon binary I/O to provide a level of abstraction for character encoding and decoding, as shown in Figure 17.2a. Encoding and decoding are automatically performed for text I/O. The JVM converts Unicode to a file-specific encoding when writing a character, and it converts a file-specific encoding to Unicode when reading a character. For example, suppose you write the string "199" using text I/O to a file, each character is written to the file. Since the Unicode for character 1 is 0x0031, the Unicode 0x0031 is converted to a code that depends on the encoding scheme for the file. (Note that the prefix 0x denotes a hex number.) In the United States, the default encoding for text files on Windows is ASCII. The ASCII code for character 1 is 49 (0x31 in
Text I/O program The Unicode of the character
Encoding/ Decoding
The encoding of the character is stored in the file 00110001 00111001 00111001
e.g., " 199"
0x31
0x39
0x39
(a)
Binary I/O program A byte is read/written
The same byte in the file
e.g., 199
11000111 0xC7 (b)
FIGURE 17.2 Text I/O requires encoding and decoding, whereas binary I/O does not.
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Binary I/O hex) and for character 9 is 57 (0x39 in hex). Thus, to write the characters 199, three bytes— 0x31, 0x39, and 0x39—are sent to the output, as shown in Figure 17.2a. Binary I/O does not require conversions. If you write a numeric value to a file using binary I/O, the exact value in the memory is copied into the file. For example, a byte-type value 199 is represented as 0xC7 (199 = 12 * 161 + 7) in the memory and appears exactly as 0xC7 in the file, as shown in Figure 17.2b. When you read a byte using binary I/O, one byte value is read from the input. In general, you should use text input to read a file created by a text editor or a text output program, and use binary input to read a file created by a Java binary output program. Binary I/O is more efficient than text I/O, because binary I/O does not require encoding and decoding. Binary files are independent of the encoding scheme on the host machine and thus are portable. Java programs on any machine can read a binary file created by a Java program. This is why Java class files are binary files. Java class files can run on a JVM on any machine.
Note For consistency, this book uses the extension .txt to name text files and .dat to name binary files.
.txt and .dat
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17.3 What are the differences between text I/O and binary I/O? 17.4 How is a Java character represented in the memory, and how is a character represented in a text file?
17.5 If you write the string "ABC" to an ASCII text file, what values are stored in the file? 17.6 If you write the string "100" to an ASCII text file, what values are stored in the file? If you write a numeric byte-type value 100 using binary I/O, what values are stored in the file? 17.7 What is the encoding scheme for representing a character in a Java program? By default, what is the encoding scheme for a text file on Windows?
17.4 Binary I/O Classes Key Point
The abstract InputStream is the root class for reading binary data, and the abstract OutputStream is the root class for writing binary data. The design of the Java I/O classes is a good example of applying inheritance, where common operations are generalized in superclasses, and subclasses provide specialized operations. Figure 17.3 lists some of the classes for performing binary I/O. InputStream is the root for FileInputStream DataInputStream InputStream
FilterInputStream BufferedInputStream ObjectInputStream
Object FileOutputStream DataOutputStream OutputStream
FilterOutputStream BufferedOutputStream ObjectOutputStream
FIGURE 17.3 InputStream, OutputStream, and their subclasses are for performing binary I/O.
17.4 Binary I/O Classes 681 java.io.InputStream +read(): int
Reads the next byte of data from the input stream. The value byte is returned as an int value in the range 0 to 255. If no byte is available because the end of the stream has been reached, the value –1 is returned.
+read(b: byte[]): int
Reads up to b.length bytes into array b from the input stream and returns the actual number of bytes read. Returns –1 at the end of the stream. Reads bytes from the input stream and stores them in b[off], b[off+1], . . ., b[off+len-1]. The actual number of bytes read is returned. Returns –1 at the end of the stream.
+read(b: byte[], off: int, len: int): int +available(): int
Returns an estimate of the number of bytes that can be read from the input stream.
+close(): void +skip(n: long): long
Closes this input stream and releases any system resources occupied by it. Skips over and discards n bytes of data from this input stream. The actual number of bytes skipped is returned.
+markSupported(): boolean
Tests whether this input stream supports the mark and reset methods.
+mark(readlimit: int): void +reset(): void
Marks the current position in this input stream. Repositions this stream to the position at the time the mark method was last called on this input stream.
FIGURE 17.4 The abstract InputStream class defines the methods for the input stream of bytes.
binary input classes, and OutputStream is the root for binary output classes. Figures 17.4 and 17.5 list all the methods in the classes InputStream and OutputStream.
Note All the methods in the binary I/O classes are declared to throw java.io.IOException or a subclass of java.io.IOException.
throws IOException
java.io.OutputStream +write(int b): void
Writes the specified byte to this output stream. The parameter b is an int value. (byte)b is written to the output stream.
+write(b: byte[]): void
Writes all the bytes in array b to the output stream.
+write(b: byte[], off: int, len: int): void
Writes b[off], b[off+1],. . ., b[off+len-1] into the output stream.
+close(): void
Closes this output stream and releases any system resources occupied by it.
+flush(): void
Flushes this output stream and forces any buffered output bytes to be written out.
FIGURE 17.5 The abstract OutputStream class defines the methods for the output stream of bytes.
17.4.1 FileInputStream/FileOutputStream FileInputStream/FileOutputStream is for reading/writing bytes from/to files. All the methods in these classes are inherited from InputStream and OutputStream. FileInputStream/FileOutputStream does not introduce new methods. To construct a FileInputStream, use the constructors shown in Figure 17.6. A java.io.FileNotFoundException will occur if you attempt to create a FileInputStream with a nonexistent file. To construct a FileOutputStream, use the constructors shown in Figure 17.7. If the file does not exist, a new file will be created. If the file already exists, the first two constructors will delete the current content of the file. To retain the current content and append new data into the file, use the last two constructors and pass true to the append parameter.
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Binary I/O java.io.InputStream
javo.io.FileInputStream +FileInputStream(file: File)
Creates a FileInputStream from a File object.
+FileInputStream(filename: String)
Creates a FileInputStream from a file name.
FIGURE 17.6 FileInputStream inputs a stream of bytes from a file. java.io.OutputStream
java.io.FileOutputStream +FileOutputStream(file: File) +FileOutputStream(filename: String) +FileOutputStream(file: File, append: boolean) +FileOutputStream(filename: String, append: boolean)
Creates a FileOutputStream from a File object. Creates a FileOutputStream from a file name. If append is true, data are appended to the existing file. If append is true, data are appended to the existing file.
FIGURE 17.7 FileOutputStream outputs a stream of bytes to a file. IOException
Almost all the methods in the I/O classes throw java.io.IOException. Therefore, you have to declare to throw java.io.IOException in the method or place the code in a trycatch block, as shown below: Declaring exception in the method public static void main(String[] args) throws IOException { // Perform I/O operations }
Using try-catch block public static void main(String[] args) { try { // Perform I/O operations } catch (IOException ex) { ex.printStackTrace(); } }
Listing 17.1 uses binary I/O to write ten byte values from 1 to 10 to a file named temp.dat and reads them back from the file.
LISTING 17.1 TestFileStream.java import
output stream
output
1 2 3 4 5 6 7 8 9 10 11 12 13 14
import java.io.*; public class TestFileStream { public static void main(String[] args) throws IOException { try ( // Create an output stream to the file FileOutputStream output = new FileOutputStream("temp.dat"); ) { // Output values to the file for (int i = 1; i <= 10; i++) output.write(i); } try (
17.4 Binary I/O Classes 683 15 16 17 18 19 20 21 22 23 24
// Create an input stream for the file FileInputStream input = new FileInputStream("temp.dat"); ) { // Read values from the file int value; while ((value = input.read()) != -1) System.out.print(value + " "); }
input stream
input
} }
1 2 3 4 5 6 7 8 9 10
The program uses the try-with-resources to declare and create input and output streams so that they will be automatically closed after they are used. The java.io.InputStream and java.io.OutputStream classes implement the AutoClosable interface. The AutoClosable interface defines the close() method that closes a resource. Any object of the AutoClosable type can be used with the try-with-resources syntax for automatic closing. A FileOutputStream is created for the file temp.dat in line 7. The for loop writes ten byte values into the file (lines 10–11). Invoking write(i) is the same as invoking write((byte)i). Line 16 creates a FileInputStream for the file temp.dat. Values are read from the file and displayed on the console in lines 19–21. The expression ((value = input.read()) != -1) (line 20) reads a byte from input.read(), assigns it to value, and checks whether it is –1. The input value of –1 signifies the end of a file. The file temp.dat created in this example is a binary file. It can be read from a Java program but not from a text editor, as shown in Figure 17.8.
AutoClosable
end of a file
Binary data
FIGURE 17.8
A binary file cannot be displayed in text mode.
Tip When a stream is no longer needed, always close it using the close() method or automatically close it using a try-with-resource statement. Not closing streams may cause data corruption in the output file, or other programming errors.
close stream
Note The root directory for the file is the classpath directory. For the example in this book, the root directory is c:\book, so the file temp.dat is located at c:\book. If you wish to place temp.dat in a specific directory, replace line 6 with
where is the file?
FileOutputStream output = new FileOutputStream ("directory/temp.dat");
Note An instance of FileInputStream can be used as an argument to construct a Scanner, and an instance of FileOutputStream can be used as an argument to construct a PrintWriter. You can create a PrintWriter to append text into a file using
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Binary I/O new PrintWriter(new FileOutputStream("temp.txt", true));
If temp.txt does not exist, it is created. If temp.txt already exists, new data are appended to the file.
17.4.2 FilterInputStream/FilterOutputStream Filter streams are streams that filter bytes for some purpose. The basic byte input stream provides a read method that can be used only for reading bytes. If you want to read integers, doubles, or strings, you need a filter class to wrap the byte input stream. Using a filter class enables you to read integers, doubles, and strings instead of bytes and characters. FilterInputStream and FilterOutputStream are the base classes for filtering data. When you need to process primitive numeric types, use DataInputStream and DataOutputStream to filter bytes.
17.4.3 DataInputStream/DataOutputStream DataInputStream reads bytes from the stream and converts them into appropriate primitive-type values or strings. DataOutputStream converts primitive-type values or strings into bytes and outputs the bytes to the stream. DataInputStream extends FilterInputStream and implements the DataInput interface, as shown in Figure 17.9. DataOutputStream extends FilterOutputStream and implements the DataOutput interface, as shown in Figure 17.10.
«interface» java.io.DataInput
InputStream
FilterInputStream
DataInputStream +DataInputStream( in: InputStream)
+readBoolean(): boolean
Reads a Boolean from the input stream.
+readByte(): byte
Reads a byte from the input stream.
+readChar(): char
Reads a character from the input stream.
+readFloat(): float
Reads a float from the input stream.
+readDouble(): double
Reads a double from the input stream.
+readInt(): int
Reads an int from the input stream.
+readLong(): long
Reads a long from the input stream.
+readShort(): short
Reads a short from the input stream.
+readLine(): String
Reads a line of characters from input.
+readUTF(): String
Reads a string in UTF format.
FIGURE 17.9 DataInputStream filters an input stream of bytes into primitive data-type values and strings.
DataInputStream implements the methods defined in the DataInput interface to read primitive data-type values and strings. DataOutputStream implements the methods defined in the DataOutput interface to write primitive data-type values and strings. Primitive values are copied from memory to the output without any conversions. Characters in a string may be written in several ways, as discussed in the next section.
Characters and Strings in Binary I/O A Unicode character consists of two bytes. The writeChar(char c) method writes the Unicode of character c to the output. The writeChars(String s) method writes the Unicode for each character in the string s to the output. The writeBytes(String s) method writes the lower byte of the Unicode for each character in the string s to the output. The high byte of the Unicode is discarded. The writeBytes method is suitable for strings that consist
17.4 Binary I/O Classes 685 OutputStream
«interface» java.io.DataOutput +writeBoolean(b: boolean): void +writeByte(v: int): void
Writes a Boolean to the output stream. Writes the eight low-order bits of the argument v to the output stream.
+writeBytes(s: String): void
Writes the lower byte of the characters in a string to the output stream.
+writeChar(c: char): void
Writes a character (composed of 2 bytes) to the output stream.
+writeChars(s: String): void
Writes every character in the string s to the output stream, in order, 2 bytes per character.
FilterOutputStream
DataOutputStream +DataOutputStream (out: OutputStream)
+writeFloat(v: float): void
Writes a float value to the output stream.
+writeDouble(v: double): void
Writes a double value to the output stream.
+writeInt(v: int): void
Writes an int value to the output stream.
+writeLong(v: long): void
Writes a long value to the output stream.
+writeShort(v: short): void
Writes a short value to the output stream.
+writeUTF(s: String): void
Writes s string in UTF format.
FIGURE 17.10 DataOutputStream enables you to write primitive data-type values and strings into an output stream. of ASCII characters, since an ASCII code is stored only in the lower byte of a Unicode. If a string consists of non-ASCII characters, you have to use the writeChars method to write the string. The writeUTF(String s) method writes two bytes of length information to the output stream, followed by the modified UTF-8 representation of every character in the string s. UTF-8 is a coding scheme that allows systems to operate with both ASCII and Unicode. Most operating systems use ASCII. Java uses Unicode. The ASCII character set is a subset of the Unicode character set. Since most applications need only the ASCII character set, it is a waste to represent an 8-bit ASCII character as a 16-bit Unicode character. The modified UTF-8 scheme stores a character using one, two, or three bytes. Characters are coded in one byte if their code is less than or equal to 0x7F, in two bytes if their code is greater than 0x7F and less than or equal to 0x7FF, or in three bytes if their code is greater than 0x7FF. The initial bits of a UTF-8 character indicate whether a character is stored in one byte, two bytes, or three bytes. If the first bit is 0, it is a one-byte character. If the first bits are 110, it is the first byte of a two-byte sequence. If the first bits are 1110, it is the first byte of a threebyte sequence. The information that indicates the number of characters in a string is stored in the first two bytes preceding the UTF-8 characters. For example, writeUTF("ABCDEF") actually writes eight bytes (i.e., 00 06 41 42 43 44 45 46) to the file, because the first two bytes store the number of characters in the string. The writeUTF(String s) method converts a string into a series of bytes in the UTF-8 format and writes them into an output stream. The readUTF() method reads a string that has been written using the writeUTF method. The UTF-8 format has the advantage of saving a byte for each ASCII character, because a Unicode character takes up two bytes and an ASCII character in UTF-8 only one byte. If most of the characters in a long string are regular ASCII characters, using UTF-8 is more efficient.
Creating DataInputStream/DataOutputStream DataInputStream/DataOutputStream are created using the following constructors (see Figures 17.9 and 17.10): public DataInputStream(InputStream instream) public DataOutputStream(OutputStream outstream)
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Binary I/O The following statements create data streams. The first statement creates an input stream for the file in.dat; the second statement creates an output stream for the file out.dat. DataInputStream input = new DataInputStream(new FileInputStream("in.dat")); DataOutputStream output = new DataOutputStream(new FileOutputStream("out.dat"));
Listing 17.2 writes student names and scores to a file named temp.dat and reads the data back from the file.
LISTING 17.2 TestDataStream.java
output stream
output
input stream
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
import java.io.*; public class TestDataStream { public static void main(String[] args) throws IOException { try ( // Create an output stream for file temp.dat DataOutputStream output = new DataOutputStream(new FileOutputStream("temp.dat")); ) { // Write student test scores to the file output.writeUTF("John"); output.writeDouble(85.5); output.writeUTF("Jim"); output.writeDouble(185.5); output.writeUTF("George"); output.writeDouble(105.25); } try ( // Create an input stream for file temp.dat DataInputStream input = new DataInputStream(new FileInputStream("temp.dat")); ) { // Read student test scores from the file System.out.println(input.readUTF() + " " + input.readDouble()); System.out.println(input.readUTF() + " " + input.readDouble()); System.out.println(input.readUTF() + " " + input.readDouble()); } } }
John 85.5 Susan 185.5 Kim 105.25
A DataOutputStream is created for file temp.dat in lines 6 and 7. Student names and scores are written to the file in lines 10–15. A DataInputStream is created for the same file in lines 19–20. Student names and scores are read back from the file and displayed on the console in lines 23–25. DataInputStream and DataOutputStream read and write Java primitive-type values and strings in a machine-independent fashion, thereby enabling you to write a data file on one machine and read it on another machine that has a different operating system or file structure. An application uses a data output stream to write data that can later be read by a program using a data input stream. DataInputStream filters data from an input stream into appropriate primitive-type values or strings. DataOutputStream converts primitive-type values or strings into bytes and
17.4 Binary I/O Classes 687 outputs the bytes to an output stream. You can view DataInputStream/FileInputStream and DataOutputStream/FileOutputStream working in a pipe line as shown in Figure 17.11.
DataInputStream int, double, string …
FileInputStream 01000110011 …
DataOutputStream int, double, string …
External File
FileOutputStream
External File
01000110011 …
FIGURE 17.11 DataInputStream filters an input stream of byte to data and DataOutputStream converts data into a stream of bytes.
Caution You have to read data in the same order and format in which they are stored. For example, since names are written in UTF-8 using writeUTF, you must read names using readUTF.
Detecting the End of a File If you keep reading data at the end of an InputStream, an EOFException will occur. This exception can be used to detect the end of a file, as shown in Listing 17.3.
EOFException
LISTING 17.3 DetectEndOfFile.java 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
import java.io.*; public class DetectEndOfFile { public static void main(String[] args) { try { try (DataOutputStream output = new DataOutputStream(new FileOutputStream("test.dat"))) { output.writeDouble(4.5); output.writeDouble(43.25); output.writeDouble(3.2); } try (DataInputStream input = new DataInputStream(new FileInputStream("test.dat"))) { while (true) System.out.println(input.readDouble()); } } catch (EOFException ex) { System.out.println("All data were read"); } catch (IOException ex) { ex.printStackTrace(); } } }
output stream output
input stream
input
EOFException
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Binary I/O 4.5 43.25 3.2 All data were read
The program writes three double values to the file using DataOutputStream (lines 6–11) and reads the data using DataInputStream (lines 13–17). When reading past the end of the file, an EOFException is thrown. The exception is caught in line 19.
17.4.4 BufferedInputStream/BufferedOutputStream BufferedInputStream/BufferedOutputStream can be used to speed up input and output by reducing the number of disk reads and writes. Using BufferedInputStream, the whole block of data on the disk is read into the buffer in the memory once. The individual data are then delivered to your program from the buffer, as shown in Figure 17.12a. Using BufferedOutputStream, the individual data are first written to the buffer in the memory. When the buffer is full, all data in the buffer are written to the disk once, as shown in Figure 17.12b.
BufferedInputStream A block of data
Buffer
BufferedOutputStream
Program Read individual data
A block of data
(a)
Buffer
Program Write individual data
(b)
FIGURE 17.12 Buffer I/O places data in a buffer for fast processing. BufferedInputStream/BufferedOutputStream does not contain new methods. All the methods in BufferedInputStream/BufferedOutputStream are inherited from the InputStream/OutputStream classes. BufferedInputStream/BufferedOutputStream manages a buffer behind the scene and automatically reads/writes data from/to disk on demand. You can wrap a BufferedInputStream/BufferedOutputStream on any InputStream/OutputStream using the constructors shown in Figures 17.13 and 17.14.
java.io.InputStream
java.io.FilterInputStream
java.io.BufferedInputStream +BufferedInputStream(in: InputStream)
Creates a BufferedInputStream from an InputStream object.
+BufferedInputStream(in: InputStream, bufferSize: int)
Creates a BufferedInputStream from an InputStream object with specified buffer size.
FIGURE 17.13 BufferedInputStream buffers an input stream.
17.4 Binary I/O Classes 689 java.io.OutputStream
java.io.FilterOutputStream
java.io.BufferedOutputStream +BufferedOutputStream(out: OutputStream)
Creates a BufferedOutputStream from an OutputStream object.
+BufferedOutputStream(out: OutputStream, bufferSize: int)
Creates a BufferedOutputStream from an OutputStream object with specified size.
FIGURE 17.14 BufferedOutputStream buffers an output stream. If no buffer size is specified, the default size is 512 bytes. You can improve the performance of the TestDataStream program in Listing 17.2 by adding buffers in the stream in lines 6–7 and lines 19–20, as follows: DataOutputStream output = new DataOutputStream( new BufferedOutputStream(new FileOutputStream("temp.dat"))); DataInputStream input = new DataInputStream( new BufferedInputStream(new FileInputStream("temp.dat")));
Tip You should always use buffered I/O to speed up input and output. For small files, you may not notice performance improvements. However, for large files—over 100 MB— you will see substantial improvements using buffered I/O.
17.8 Why do you have to declare to throw IOException in the method or use a try-catch block to handle IOException for Java I/O programs?
17.9 Why should you always close streams? How do you close streams? 17.10 The read() method in InputStream reads a byte. Why does it return an 17.11 17.12 17.13 17.14 17.15 17.16 17.17
int
instead of a byte? Find the abstract methods in InputStream and OutputStream. Does FileInputStream/FileOutputStream introduce any new methods beyond the methods inherited from InputStream/OutputStream? How do you create a FileInputStream/FileOutputStream? What will happen if you attempt to create an input stream on a nonexistent file? What will happen if you attempt to create an output stream on an existing file? Can you append data to an existing file? How do you append data to an existing text file using java.io.PrintWriter? Suppose a file contains an unspecified number of double values that were written to the file using the writeDouble method using a DataOutputStream, how do you write a program to read all these values? How do you detect the end of a file? What is written to a file using writeByte(91) on a FileOutputStream? How do you check the end of a file in an input stream (FileInputStream, DataInputStream)? What is wrong in the following code? import java.io.*; public class Test {
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Binary I/O public static void main(String[] args) { try ( FileInputStream fis = new FileInputStream("test.dat"); ) { } catch (IOException ex) { ex.printStackTrace(); } catch (FileNotFoundException ex) { ex.printStackTrace(); } } }
17.18 Suppose you run the following program on Windows using the default ASCII encoding after the program is finished, how many bytes are there in the file t.txt? Show the contents of each byte. public class Test { public static void main(String[] args) throws java.io.IOException { try (java.io.PrintWriter output = new java.io.PrintWriter("t.txt"); ) { output.printf("%s", "1234"); output.printf("%s", "5678"); output.close(); } } }
17.19 After the following program is finished, how many bytes are there in the file t.dat? Show the contents of each byte. import java.io.*; public class Test { public static void main(String[] args) throws IOException { try (DataOutputStream output = new DataOutputStream( new FileOutputStream("t.dat")); ) { output.writeInt(1234); output.writeInt(5678); output.close(); } } }
17.20 For each of the following statements on a DataOutputStream output, how many bytes are sent to the output? output.writeChar('A'); output.writeChars("BC"); output.writeUTF("DEF");
17.21 What are the advantages of using buffered streams? Are the following statements correct? BufferedInputStream input1 = new BufferedInputStream(new FileInputStream("t.dat")); DataInputStream input2 = new DataInputStream( new BufferedInputStream(new FileInputStream("t.dat"))); DataOutputStream output = new DataOutputStream( new BufferedOutputStream(new FileOutputStream("t.dat")));
17.5 Case Study: Copying Files 691
17.5 Case Study: Copying Files This section develops a useful utility for copying files. In this section, you will learn how to write a program that lets users copy files. The user needs to provide a source file and a target file as command-line arguments using the command: java Copy source target
Key Point
VideoNote
Copy file
The program copies the source file to the target file and displays the number of bytes in the file. The program should alert the user if the source file does not exist or if the target file already exists. A sample run of the program is shown in Figure 17.15.
File exists Delete file Copy Source does not exist
FIGURE 17.15
The program copies a file.
To copy the contents from a source file to a target file, it is appropriate to use an input stream to read bytes from the source file and an output stream to send bytes to the target file, regardless of the file’s contents. The source file and the target file are specified from the command line. Create an InputFileStream for the source file and an OutputFileStream for the target file. Use the read() method to read a byte from the input stream, and then use the write(b) method to write the byte to the output stream. Use BufferedInputStream and BufferedOutputStream to improve the performance. Listing 17.4 gives the solution to the problem.
LISTING 17.4 Copy.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
import java.io.*; public class Copy { /** Main method @param args[0] for sourcefile @param args[1] for target file */ public static void main(String[] args) throws IOException { // Check command-line parameter usage if (args.length != 2) { System.out.println( "Usage: java Copy sourceFile targetfile"); System.exit(1); } // Check if source file exists File sourceFile = new File(args[0]); if (!sourceFile.exists()) { System.out.println("Source file " + args[0] + " does not exist");
check usage
source file
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Binary I/O 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
target file
input stream
output stream
read write
System.exit(2); } // Check if target file exists File targetFile = new File(args[1]); if (targetFile.exists()) { System.out.println("Target file " + args[1] + " already exists"); System.exit(3); } try ( // Create an input stream BufferedInputStream input = new BufferedInputStream(new FileInputStream(sourceFile)); // Create an output stream BufferedOutputStream output = new BufferedOutputStream(new FileOutputStream(targetFile)); ) { // Continuously read a byte from input and write it to output int r, numberOfBytesCopied = 0; while ((r = input.read()) != -1) { output.write((byte)r); numberOfBytesCopied++; } // Display the file size System.out.println(numberOfBytesCopied + " bytes copied"); } } }
The program first checks whether the user has passed the two required arguments from the command line in lines 10–14. The program uses the File class to check whether the source file and target file exist. If the source file does not exist (lines 18–22) or if the target file already exists (lines 25–30), the program ends. An input stream is created using BufferedInputStream wrapped on FileInputStream in lines 34 and 35, and an output stream is created using BufferedOutputStream wrapped on FileOutputStream in lines 38 and 39. The expression ((r = input.read()) != -1) (line 43) reads a byte from input.read(), assigns it to r, and checks whether it is -1. The input value of -1 signifies the end of a file. The program continuously reads bytes from the input stream and sends them to the output stream until all of the bytes have been read.
✓
Check Point
17.22 How does the program check if a file already exists? 17.23 How does the program detect the end of the file while reading data? 17.24 How does the program count the number of bytes read from the file?
17.6 Object I/O Key Point
ObjectInputStream/ObjectOutputStream classes can be used to read/write
serializable objects. DataInputStream/DataOutputStream enables you to perform I/O for primitive-type values and strings. ObjectInputStream/ObjectOutputStream enables you to perform I/O
17.6 Object I/O 693 for objects in addition to primitive-type values and strings. Since ObjectInputStream/ ObjectOutputStream contains all the functions of DataInputStream/ DataOutputStream, you can replace DataInputStream/DataOutputStream completely with ObjectInputStream/ObjectOutputStream. ObjectInputStream extends InputStream and implements ObjectInput and ObjectStreamConstants, as shown in Figure 17.16. ObjectInput is a subinterface of DataInput (DataInput is shown in Figure 17.9). ObjectStreamConstants contains the constants to support ObjectInputStream/ObjectOutputStream.
VideoNote
Object I/O
«interface» ObjectStreamConstants java.io.InputStream «interface» java.io.DataInput
java.io.ObjectInputStream
«interface» java.io.ObjectInput
+ObjectInputStream(in: InputStream)
+readObject(): Object
Reads an object.
FIGURE 17.16 ObjectInputStream can read objects, primitive-type values, and strings. ObjectOutputStream extends OutputStream and implements ObjectOutput and ObjectStreamConstants, as shown in Figure 17.17. ObjectOutput is a subinterface of DataOutput (DataOutput is shown in Figure 17.10). «interface» ObjectStreamConstants java.io.OutputStream «interface» java.io.DataOutput
java.io.ObjectOutputStream +ObjectOutputStream(out: OutputStream)
«interface» java.io.ObjectOutput +writeObject(o: Object): void
FIGURE 17.17 ObjectOutputStream can write objects, primitive-type values, and strings. You can wrap an ObjectInputStream/ObjectOutputStream on any InputStream/ OutputStream using the following constructors: // Create an ObjectInputStream public ObjectInputStream(InputStream in) // Create an ObjectOutputStream public ObjectOutputStream(OutputStream out)
Listing 17.5 writes student names, scores, and the current date to a file named object.dat.
LISTING 17.5 TestObjectOutputStream.java 1 2 3
import java.io.*; public class TestObjectOutputStream {
Writes an object.
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output stream
output string output object
4 5 6 7 8 9 10 11 12 13 14 15
public static void main(String[] args) throws IOException { try ( // Create an output stream for file object.dat ObjectOutputStream output = new ObjectOutputStream(new FileOutputStream("object.dat")); ) { // Write a string, double value, and object to the file output.writeUTF("John"); output.writeDouble(85.5); output.writeObject(new java.util.Date()); } } }
An ObjectOutputStream is created to write data into the file object.dat in lines 6 and 7. A string, a double value, and an object are written to the file in lines 10–12. To improve performance, you may add a buffer in the stream using the following statement to replace lines 6 and 7: ObjectOutputStream output = new ObjectOutputStream( new BufferedOutputStream(new FileOutputStream("object.dat")));
Multiple objects or primitives can be written to the stream. The objects must be read back from the corresponding ObjectInputStream with the same types and in the same order as they were written. Java’s safe casting should be used to get the desired type. Listing 17.6 reads data from object.dat.
LISTING 17.6 TestObjectInputStream.java
input stream
input string input object
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
import java.io.*; public class TestObjectInputStream { public static void main(String[] args) throws ClassNotFoundException, IOException { try ( // Create an input stream for file object.dat ObjectInputStream input = new ObjectInputStream(new FileInputStream("object.dat")); ) { // Read a string, double value, and object from the file String name = input.readUTF(); double score = input.readDouble(); java.util.Date date = (java.util.Date)(input.readObject()); System.out.println(name + " " + score + " " + date); } } }
John 85.5 Sun Dec 04 10:35:31 EST 2011
ClassNotFoundException
The readObject() method may throw java.lang.ClassNotFoundException, because when the JVM restores an object, it first loads the class for the object if the class has not been loaded. Since ClassNotFoundException is a checked exception, the main method declares to throw it in line 5. An ObjectInputStream is created to read input from object.dat in lines 7 and 8. You have to read the data from the file in the same order and format as they were written to the file. A string, a double value, and an object are read in lines 11–13. Since readObject() returns an Object, it is cast into Date and assigned to a Date variable in line 13.
17.6 Object I/O 695
17.6.1
The Serializable Interface
Not every object can be written to an output stream. Objects that can be so written are said to be serializable. A serializable object is an instance of the java.io.Serializable interface, so the object’s class must implement Serializable. The Serializable interface is a marker interface. Since it has no methods, you don’t need to add additional code in your class that implements Serializable. Implementing this interface enables the Java serialization mechanism to automate the process of storing objects and arrays. To appreciate this automation feature, consider what you otherwise need to do in order to store an object. Suppose you wish to store an ArrayList object. To do this you need to store all the elements in the list. Each element is an object that may contain other objects. As you can see, this would be a very tedious process. Fortunately, you don’t have to go through it manually. Java provides a built-in mechanism to automate the process of writing objects. This process is referred as object serialization, which is implemented in ObjectOutputStream. In contrast, the process of reading objects is referred as object deserialization, which is implemented in ObjectInputStream. Many classes in the Java API implement Serializable. All the wrapper classes for primitive type values, java.math.BigInteger, java.math.BigDecimal, java.lang.String, java.lang.StringBuilder, java.lang.StringBuffer, java.util.Date, and java.util.ArrayList implement java.io.Serializable. Attempting to store an object that does not support the Serializable interface would cause a NotSerializableException. When a serializable object is stored, the class of the object is encoded; this includes the class name and the signature of the class, the values of the object’s instance variables, and the closure of any other objects referenced by the object. The values of the object’s static variables are not stored.
serializable
serialization deserialization
NotSerializableException
Note Nonserializable fields If an object is an instance of Serializable but contains nonserializable instance data fields, can it be serialized? The answer is no. To enable the object to be serialized, mark these data fields with the transient keyword to tell the JVM to ignore them when writing the object to an object stream. Consider the following class: public class C implements java.io.Serializable { private int v1; private static double v2; private transient A v3 = new A(); } class A { } // A is not serializable
When an object of the C class is serialized, only variable v1 is serialized. Variable v2 is not serialized because it is a static variable, and variable v3 is not serialized because it is marked transient. If v3 were not marked transient, a java.io.NotSerializableException would occur.
Note Duplicate objects If an object is written to an object stream more than once, will it be stored in multiple copies? No, it will not. When an object is written for the first time, a serial number is created for it. The JVM writes the complete contents of the object along with the serial number into the object stream. After the first time, only the serial number is stored if the
transient
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Binary I/O same object is written again. When the objects are read back, their references are the same since only one object is actually created in the memory.
17.6.2
Serializing Arrays
An array is serializable if all its elements are serializable. An entire array can be saved into a file using writeObject and later can be restored using readObject. Listing 17.7 stores an array of five int values and an array of three strings and reads them back to display on the console.
LISTING 17.7 TestObjectStreamForArray.java 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
output stream
store array
input stream
restore array
import java.io.*; public class TestObjectStreamForArray { public static void main(String[] args) throws ClassNotFoundException, IOException { int[] numbers = {1, 2, 3, 4, 5}; String[] strings = {"John", "Susan", "Kim"}; try ( // Create an output stream for file array.dat ObjectOutputStream output = new ObjectOutputStream(new FileOutputStream("array.dat", true)); ) { // Write arrays to the object output stream output.writeObject(numbers); output.writeObject(strings); } try ( // Create an input stream for file array.dat ObjectInputStream input = new ObjectInputStream(new FileInputStream("array.dat")); ) { int[] newNumbers = (int[])(input.readObject()); String[] newStrings = (String[])(input.readObject()); // Display arrays for (int i = 0; i < newNumbers.length; i++) System.out.print(newNumbers[i] + " "); System.out.println(); for (int i = 0; i < newStrings.length; i++) System.out.print(newStrings[i] + " "); } } }
1 2 3 4 5 John Susan Kim
Lines 14 and 15 write two arrays into file array.dat. Lines 22 and 23 read two arrays back in the same order they were written. Since readObject() returns Object, casting is used to cast the objects into int[] and String[].
✓
Check Point
17.25 What types of objects can be stored using the ObjectOutputStream? What is the method for writing an object? What is the method for reading an object? What is the return type of the method that reads an object from ObjectInputStream?
17.7 Random-Access Files 697 17.26 If you serialize two objects of the same type, will they take the same amount of space? If not, give an example.
17.27 Is it true that any instance of java.io.Serializable can be successfully serialized? Are the static variables in an object serialized? How do you mark an instance variable not to be serialized?
17.28 Can you write an array to an ObjectOutputStream? 17.29 Is it true that DataInputStream/DataOutputStream can always be replaced by ObjectInputStream/ObjectOutputStream?
17.30 What will happen when you attempt to run the following code? import java.io.*; public class Test { public static void main(String[] args) throws IOException { try ( ObjectOutputStream output = new ObjectOutputStream(new FileOutputStream("object.dat")); ) { output.writeObject(new A()); } } } class A implements Serializable { B b = new B(); } class B { }
17.7 Random-Access Files Java provides the RandomAccessFile class to allow data to be read from and written to at any locations in the file. All of the streams you have used so far are known as read-only or write-only streams. These streams are called sequential streams. A file that is opened using a sequential stream is called a sequential-access file. The contents of a sequential-access file cannot be updated. However, it is often necessary to modify files. Java provides the RandomAccessFile class to allow data to be read from and written to at any locations in a file. A file that is opened using the RandomAccessFile class is known as a random-access file. The RandomAccessFile class implements the DataInput and DataOutput interfaces, as shown in Figure 17.18. The DataInput interface (see Figure 17.9) defines the methods for reading primitive-type values and strings (e.g., readInt, readDouble, readChar, readBoolean, readUTF) and the DataOutput interface (see Figure 17.10) defines the methods for writing primitive-type values and strings (e.g., writeInt, writeDouble, writeChar, writeBoolean, writeUTF). When creating a RandomAccessFile, you can specify one of two modes: r or rw. Mode r means that the stream is read-only, and mode rw indicates that the stream allows both read and write. For example, the following statement creates a new stream, raf, that allows the program to read from and write to the file test.dat: RandomAccessFile raf = new RandomAccessFile("test.dat", "rw");
If test.dat already exists, raf is created to access it; if test.dat does not exist, a new file named test.dat is created, and raf is created to access the new file. The method raf.length() returns the number of bytes in test.dat at any given time. If you append new data into the file, raf.length() increases.
Key Point read-only write-only sequential-access file
random-access file
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«interface» java.io.DataInput
«interface» java.io.DataOutput
java.io.RandomAccessFile +RandomAccessFile(file: File, mode: String)
Creates a RandomAccessFile stream with the specified File object and mode.
+RandomAccessFile(name: String, mode: String)
Creates a RandomAccessFile stream with the specified file name string and mode.
+close(): void
Closes the stream and releases the resource associated with it.
+getFilePointer(): long
Returns the offset, in bytes, from the beginning of the file to where the next read or write occurs. Returns the number of bytes in this file.
+length(): long +read(): int
Reads a byte of data from this file and returns –1 at the end of stream.
+read(b: byte[]): int
Reads up to b.length bytes of data from this file into an array of bytes.
+read(b: byte[], off: int, len: int): int
Reads up to len bytes of data from this file into an array of bytes.
+seek(pos: long): void
Sets the offset (in bytes specified in pos) from the beginning of the stream to where the next read or write occurs.
+setLength(newLength: long): void
Sets a new length for this file.
+skipBytes(int n): int
Skips over n bytes of input.
+write(b: byte[]): void
Writes b.length bytes from the specified byte array to this file, starting at the current file pointer.
+write(b: byte[], off: int, len: int): void
Writes len bytes from the specified byte array, starting at offset off, to this file.
FIGURE 17.18 RandomAccessFile implements the DataInput and DataOutput interfaces with additional methods to support random access.
Tip If the file is not intended to be modified, open it with the r mode. This prevents unintentional modification of the file.
A random-access file consists of a sequence of bytes. A special marker called a file pointer is positioned at one of these bytes. A read or write operation takes place at the location of the file pointer. When a file is opened, the file pointer is set at the beginning of the file. When you read or write data to the file, the file pointer moves forward to the next data item. For example, if you read an int value using readInt(), the JVM reads 4 bytes from the file pointer, and now the file pointer is 4 bytes ahead of the previous location, as shown in Figure 17.19. For a RandomAccessFile raf, you can use the raf.seek(position) method to move the file pointer to a specified position. raf.seek(0) moves it to the beginning of the file, and raf.seek(raf.length()) moves it to the end of the file. Listing 17.8 demonstrates
file pointer
File pointer
File
byte byte …
byte byte byte byte byte
…
byte byte byte byte byte
(a) Before readInt()
…
byte byte byte byte byte
(b) After readInt()
File pointer
File
byte byte
…
byte byte byte byte byte
FIGURE 17.19 After an int value is read, the file pointer is moved 4 bytes ahead.
17.7 Random-Access Files 699 RandomAccessFile. A large case study of using RandomAccessFile to organize an
address book is given in Supplement VI.D.
LISTING 17.8 TestRandomAccessFile.jav 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
import java.io.*; public class TestRandomAccessFile { public static void main(String[] args) throws IOException { try ( // Create a random access file RandomAccessFile inout = new RandomAccessFile("inout.dat", "rw"); ) { // Clear the file to destroy the old contents if exists inout.setLength(0); // Write new integers to the file for (int i = 0; i < 200; i++) inout.writeInt(i);
RandomAccessFile
empty file
write
// Display the current length of the file System.out.println("Current file length is " + inout.length()); // Retrieve the first number inout.seek(0); // Move the file pointer to the beginning System.out.println("The first number is " + inout.readInt()); // Retrieve the second number inout.seek(1 * 4); // Move the file pointer to the second number System.out.println("The second number is " + inout.readInt()); // Retrieve the tenth number inout.seek(9 * 4); // Move the file pointer to the tenth number System.out.println("The tenth number is " + inout.readInt()); // Modify the eleventh number inout.writeInt(555); // Append a new number inout.seek(inout.length()); // Move the file pointer to the end inout.writeInt(999); // Display the new length System.out.println("The new length is " + inout.length()); // Retrieve the new eleventh number inout.seek(10 * 4); // Move the file pointer to the eleventh number System.out.println("The eleventh number is " + inout.readInt()); } } }
Current file length is 800 The first number is 0 The second number is 1 The tenth number is 9 The new length is 804 The eleventh number is 555
move pointer read
700 Chapter 17
Binary I/O A RandomAccessFile is created for the file named inout.dat with mode rw to allow both read and write operations in line 6. inout.setLength(0) sets the length to 0 in line 9. This, in effect, destroys the old contents of the file. The for loop writes 200 int values from 0 to 199 into the file in lines 12 and 13. Since each int value takes 4 bytes, the total length of the file returned from inout.length() is now 800 (line 16), as shown in the sample output. Invoking inout.seek(0) in line 19 sets the file pointer to the beginning of the file. inout.readInt() reads the first value in line 20 and moves the file pointer to the next number. The second number is read in line 24. inout.seek(9 * 4) (line 27) moves the file pointer to the tenth number. inout.readInt() reads the tenth number and moves the file pointer to the eleventh number in line 28. inout.write(555) writes a new eleventh number at the current position (line 31). The previous eleventh number is destroyed. inout.seek(inout.length()) moves the file pointer to the end of the file (line 34). inout.writeInt(999) writes a 999 to the file (line 35). Now the length of the file is increased by 4, so inout.length() returns 804 (line 38). inout.seek(10 * 4) moves the file pointer to the eleventh number in line 41. The new eleventh number, 555, is displayed in line 42.
✓
Check Point
17.31 Can
RandomAccessFile streams read and write a data file created by DataOutputStream? Can RandomAccessFile streams read and write objects?
17.32 Create a RandomAccessFile stream for the file address.dat to allow the updating of student information in the file. Create a DataOutputStream for the file address.dat. Explain the differences between these two statements.
17.33 What happens if the file test.dat does not exist when you attempt to compile and run the following code? import java.io.*; public class Test { public static void main(String[] args) { try ( RandomAccessFile raf = new RandomAccessFile("test.dat", "r"); ) { int i = raf.readInt(); } catch (IOException ex) { System.out.println("IO exception"); } } }
KEY TERMS binary I/O 678 deserialization 695 file pointer 698 random-access file 697
sequential-access file serialization 695 stream 678 text I/O 678
697
Programming Exercises 701
CHAPTER SUMMARY 1. I/O can be classified into text I/O and binary I/O. Text I/O interprets data in sequences of characters. Binary I/O interprets data as raw binary values. How text is stored in a file depends on the encoding scheme for the file. Java automatically performs encoding and decoding for text I/O.
2. The
InputStream and OutputStream classes are the roots of all binary I/O classes. FileInputStream/FileOutputStream associates a file for input/output. BufferedInputStream/BufferedOutputStream can be used to wrap any binary I/O stream to improve performance. DataInputStream/DataOutputStream can be used to read/write primitive values and strings.
3.
ObjectInputStream/ObjectOutputStream can be used to read/write objects in addition to primitive values and strings. To enable object serialization, the object’s defining class must implement the java.io.Serializable marker interface.
4. The RandomAccessFile class enables you to read and write data to a file. You can open a file with the r mode to indicate that it is read-only or with the rw mode to indicate that it is updateable. Since the RandomAccessFile class implements DataInput and DataOutput interfaces, many methods in RandomAccessFile are the same as those in DataInputStream and DataOutputStream.
QUIZ Answer the quiz for this chapter online at www.cs.armstrong.edu/liang/intro10e/quiz.html.
PROGRAMMING EXERCISES Section 17.3
*17.1 (Create a text file) Write a program to create a file named Exercise17_01.txt if it does not exist. Append new data to it if it already exists. Write 100 integers created randomly into the file using text I/O. Integers are separated by a space.
Section 17.4
*17.2 (Create a binary data file) Write a program to create a file named Exercise17_02.dat if it does not exist. Append new data to it if it already exists. Write 100 integers created randomly into the file using binary I/O. *17.3 (Sum all the integers in a binary data file) Suppose a binary data file named Exercise17_03.dat has been created and its data are created using writeInt(int) in DataOutputStream. The file contains an unspecified number of integers. Write a program to find the sum of the integers. *17.4 (Convert a text file into UTF) Write a program that reads lines of characters from a text file and writes each line as a UTF-8 string into a binary file. Display the sizes of the text file and the binary file. Use the following command to run the program: java Exercise17_04 Welcome.java Welcome.utf
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Binary I/O Section 17.6
*17.5 (Store objects and arrays in a file) Write a program that stores an array of the five int values 1, 2, 3, 4, and 5, a Date object for the current time, and the double value 5.5 into the file named Exercise17_05.dat.
*17.6 (Store
Loan objects) The Loan class in Listing 10.2 does not implement Serializable. Rewrite the Loan class to implement Serializable. Write a program that creates five Loan objects and stores them in a file named
Exercise17_06.dat.
*17.7 (Restore objects from a file) Suppose a file named Exercise17_07.dat has been created using the ObjectOutputStream. The file contains Loan objects. The Loan class in Listing 10.2 does not implement Serializable. Rewrite the Loan class to implement Serializable. Write a program that reads the Loan objects from the file and displays the total loan amount. Suppose you don’t know how many Loan objects are there in the file, use EOFException to end the loop.
Section 17.7
*17.8 (Update count) Suppose you wish to track how many times a program has been executed. You can store an int to count the file. Increase the count by 1 each time this program is executed. Let the program be Exercise17_08 and store the count in Exercise17_08.dat. ***17.9 (Address book) Write a program that stores, retrieves, adds, and updates addresses as shown in Figure 17.20. Use a fixed-length string for storing each attribute in the address. Use random access file for reading and writing an address. Assume that the size of name, street, city, state, and zip is 32, 32, 20, 2, 5 bytes, respectively.
FIGURE 17.20
The application can store, retrieve, and update addresses from a file.
Comprehensive
*17.10 (Split files) Suppose you want to back up a huge file (e.g., a 10-GB AVI file) to a VideoNote
Split a large file
CD-R. You can achieve it by splitting the file into smaller pieces and backing up these pieces separately. Write a utility program that splits a large file into smaller ones using the following command: java Exercise17_10 SourceFile numberOfPieces
The command creates the files SourceFile.1, SourceFile.2, . . . , SourceFile.n, where n is numberOfPieces and the output files are about the same size. **17.11 (Split files GUI) Rewrite Exercise 17.10 with a GUI, as shown in Figure 17.21a. *17.12 (Combine files) Write a utility program that combines the files together into a new file using the following command: java Exercise17_12 SourceFile1 . . . SourceFilen TargetFile
The command combines SourceFile1, . . . , and SourceFilen into TargetFile.
Programming Exercises 703
(b)
(a)
FIGURE 17.21
(a) The program splits a file. (b) The program combines files into a new file.
*17.13 (Combine files GUI) Rewrite Exercise 17.12 with a GUI, as shown in Figure 17.21b.
17.14 (Encrypt files) Encode the file by adding 5 to every byte in the file. Write a program that prompts the user to enter an input file name and an output file name and saves the encrypted version of the input file to the output file. 17.15 (Decrypt files) Suppose a file is encrypted using the scheme in Programming Exercise 17.14. Write a program to decode an encrypted file. Your program should prompt the user to enter an input file name for the encrypted file and an output file name for the unencrypted version of the input file. 17.16 (Frequency of characters) Write a program that prompts the user to enter the name of an ASCII text file and displays the frequency of the characters in the file. **17.17 (BitOutputStream) Implement a class named BitOutputStream, as shown in Figure 17.22, for writing bits to an output stream. The writeBit(char bit) method stores the bit in a byte variable. When you create a BitOutputStream, the byte is empty. After invoking writeBit('1'), the byte becomes 00000001. After invoking writeBit("0101"), the byte becomes 00010101. The first three bits are not filled yet. When a byte is full, it is sent to the output stream. Now the byte is reset to empty. You must close the stream by invoking the close() method. If the byte is neither empty nor full, the close() method first fills the zeros to make a full 8 bits in the byte, and then outputs the byte and closes the stream. For a hint, see Programming Exercise 5.44. Write a test program that sends the bits 010000100100001001101 to the file named Exercise17_17.dat.
BitOutputStream +BitOutputStream(file: File)
Creates a BitOutputStream to writes bits to the file.
+writeBit(char bit): void
Writes a bit '0' or '1' to the output stream. Writes a string of bits to the output stream. This method must be invoked to close the stream.
+writeBit(String bit): void +close(): void
FIGURE 17.22 BitOutputStream outputs a stream of bits to a file.
*17.18 (View bits) Write the following method that displays the bit representation for the last byte in an integer: public static String getBits(int value)
For a hint, see Programming Exercise 5.44. Write a program that prompts the user to enter a file name, reads bytes from the file, and displays each byte’s binary representation.
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Binary I/O *17.19 (View hex) Write a program that prompts the user to enter a file name, reads bytes from the file, and displays each byte’s hex representation. (Hint: You can first convert the byte value into an 8-bit string, then convert the bit string into a twodigit hex string.)
**17.20 (Binary editor) Write a GUI application that lets the user enter a file name in the text field and press the Enter key to display its binary representation in a text area. The user can also modify the binary code and save it back to the file, as shown in Figure 17.23a.
(a)
FIGURE 17.23
(b)
The programs enable the user to manipulate the contents of the file in (a) binary and (b) hex.
**17.21 (Hex editor) Write a GUI application that lets the user enter a file name in the text field and press the Enter key to display its hex representation in a text area. The user can also modify the hex code and save it back to the file, as shown in Figure 17.23b.
CHAPTER
18 RECURSION Objectives ■
To describe what a recursive method is and the benefits of using recursion (§18.1).
■
To develop recursive methods for recursive mathematical functions (§§18.2–18.3).
■
To explain how recursive method calls are handled in a call stack (§§18.2–18.3).
■
To solve problems using recursion (§18.4).
■
To use an overloaded helper method to design a recursive method (§18.5).
■
To implement a selection sort using recursion (§18.5.1).
■
To implement a binary search using recursion (§18.5.2).
■
To get the directory size using recursion (§18.6).
■
To solve the Tower of Hanoi problem using recursion (§18.7).
■
To draw fractals using recursion (§18.8).
■
To discover the relationship and difference between recursion and iteration (§18.9).
■
To know tail-recursive methods and why they are desirable (§18.10).
706 Chapter 18
Recursion
18.1 Introduction Key Point
Recursion is a technique that leads to elegant solutions to problems that are difficult to program using simple loops. Suppose you want to find all the files under a directory that contain a particular word. How do you solve this problem? There are several ways to do so. An intuitive and effective solution is to use recursion by searching the files in the subdirectories recursively. H-trees, depicted in Figure 18.1, are used in a very large-scale integration (VLSI) design as a clock distribution network for routing timing signals to all parts of a chip with equal propagation delays. How do you write a program to display H-trees? A good approach is to use recursion.
search word problem
H-tree problem
(a)
FIGURE 18.1
(b)
(c)
(d)
An H-tree can be displayed using recursion. To use recursion is to program using recursive methods—that is, to use methods that invoke themselves. Recursion is a useful programming technique. In some cases, it enables you to develop a natural, straightforward, simple solution to an otherwise difficult problem. This chapter introduces the concepts and techniques of recursive programming and illustrates with examples of how to “think recursively.”
recursive method
18.2 Case Study: Computing Factorials Key Point
A recursive method is one that invokes itself. Many mathematical functions are defined using recursion. Let’s begin with a simple example. The factorial of a number n can be recursively defined as follows: 0! = 1; n! = n × (n - 1)!; n > 0
base case or stopping condition
recursive call
How do you find n! for a given n? To find 1! is easy, because you know that 0! is 1, and 1! is 1 × 0!. Assuming that you know (n - 1)!, you can obtain n! immediately by using n × (n - 1)!. Thus, the problem of computing n! is reduced to computing (n - 1)!. When computing (n - 1)!, you can apply the same idea recursively until n is reduced to 0. Let factorial(n) be the method for computing n!. If you call the method with n = 0, it immediately returns the result. The method knows how to solve the simplest case, which is referred to as the base case or the stopping condition. If you call the method with n > 0, it reduces the problem into a subproblem for computing the factorial of n - 1. The subproblem is essentially the same as the original problem, but it is simpler or smaller. Because the subproblem has the same property as the original problem, you can call the method with a different argument, which is referred to as a recursive call. The recursive algorithm for computing factorial(n) can be simply described as follows: if (n == 0) return 1;
18.2 Case Study: Computing Factorials 707 else return n * factorial(n - 1);
A recursive call can result in many more recursive calls, because the method keeps on dividing a subproblem into new subproblems. For a recursive method to terminate, the problem must eventually be reduced to a stopping case, at which point the method returns a result to its caller. The caller then performs a computation and returns the result to its own caller. This process continues until the result is passed back to the original caller. The original problem can now be solved by multiplying n by the result of factorial(n - 1). Listing 18.1 gives a complete program that prompts the user to enter a nonnegative integer and displays the factorial for the number.
LISTING 18.1 ComputeFactorial.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
import java.util.Scanner; public class ComputeFactorial { /** Main method */ public static void main(String[] args) { // Create a Scanner Scanner input = new Scanner(System.in); System.out.print("Enter a nonnegative integer: "); int n = input.nextInt(); // Display factorial System.out.println("Factorial of " + n + " is " + factorial(n)); } /** Return the factorial for the specified number */ public static long factorial(int n) { if (n == 0) // Base case return 1; else return n * factorial(n - 1); // Recursive call }
base case
recursion
}
Enter a nonnegative integer: 4 Factorial of 4 is 24
Enter a nonnegative integer: 10 Factorial of 10 is 3628800
The factorial method (lines 16–21) is essentially a direct translation of the recursive mathematical definition for the factorial into Java code. The call to factorial is recursive because it calls itself. The parameter passed to factorial is decremented until it reaches the base case of 0. You see how to write a recursive method. How does recursion work behind the scenes? Figure 18.2 illustrates the execution of the recursive calls, starting with n = 4. The use of stack space for recursive calls is shown in Figure 18.3.
how does it work?
708 Chapter 18
Recursion factorial(4) Step 0: executes factorial(4) Step 9: return 24 return 4 * factorial(3) Step 1: executes factorial(3) Step 8: return 6 return 3 * factorial(2) Step 2: executes factorial(2) Step 7: return 2 return 2 * factorial(1) Step 3: executes factorial(1) Step 6: return 1 return 1 * factorial(0) Step 5: return 1
Step 4: executes factorial(0) return 1
FIGURE 18.2 Invoking factorial(4) spawns recursive calls to factorial.
5
Activation record for factorial(1) n: 1
Activation record for factorial(1) n: 1
Activation record for factorial(2) n: 2
Activation record for factorial(2) n: 2
Activation record for factorial(2) n: 2
Activation record for factorial(3) n: 3
Activation record for factorial(3) n: 3
Activation record for factorial(3) n: 3
Activation record for factorial(3) n: 3
Activation record for factorial(4) n: 4
Activation record for factorial(4) n: 4
Activation record for factorial(4) n: 4
Activation record for factorial(4) n: 4
4
3
2
1
Activation record for factorial(4) n: 4
6
Activation record for factorial(1) n: 1 Activation record for factorial(2) n: 2
7
Activation record for factorial(0) n: 0
Activation record for factorial(2) n: 2
Activation record for factorial(3) n: 3
Activation record for factorial(3) n: 3
Activation record for factorial(4) n: 4
Activation record for factorial(4) n: 4
8
Activation record for factorial(3) n: 3 Activation record for factorial(4) n: 4
9
Activation record for factorial(4) n: 4
FIGURE 18.3 When factorial(4) is being executed, the factorial method is called recursively, causing stack space to dynamically change.
18.3 Case Study: Computing Fibonacci Numbers 709 Pedagogical Note It is simpler and more efficient to implement the factorial method using a loop. However, we use the recursive factorial method here to demonstrate the concept of recursion. Later in this chapter, we will present some problems that are inherently recursive and are difficult to solve without using recursion. If recursion does not reduce the problem in a manner that allows it to eventually converge into the base case or a base case is not specified, infinite recursion can occur. For example, suppose you mistakenly write the factorial method as follows:
infinite recursion
public static long factorial(int n) { return n * factorial(n - 1); }
The method runs infinitely and causes a StackOverflowError.
The example discussed in this section shows a recursive method that invokes itself. This is known as direct recursion. It is also possible to create indirect recursion. This occurs when method A invokes method B, which in turn invokes method A. There can even be several more methods involved in the recursion. For example, method A invokes method B, which invokes method C, which invokes method A.
18.1 What is a recursive method? What is an infinite recursion? 18.2 How many times is the factorial method in Listing 18.1 invoked for factorial(6)? 18.3 Show the output of the following programs and identify base cases and recursive calls. public class Test { public static void main(String[] args) { System.out.println( "Sum is " + xMethod(5)); } public static int xMethod(int n) { if (n == 1) return 1; else return n + xMethod(n - 1); }
direct recursion indirect recursion
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Check Point
public class Test { public static void main(String[] args) { xMethod(1234567); } public static void xMethod(int n) { if (n > 0) { System.out.print(n % 10); xMethod(n / 10); } } }
}
18.4 Write a recursive mathematical definition for computing 2n for a positive integer n. 18.5 Write a recursive mathematical definition for computing xn for a positive integer n and a real number x.
18.6 Write a recursive mathematical definition for computing 1 + 2 + 3 + c + n for a positive integer n.
18.3 Case Study: Computing Fibonacci Numbers In some cases, recursion enables you to create an intuitive, straightforward, simple solution to a problem. The factorial method in the preceding section could easily be rewritten without using recursion. In this section, we show an example for creating an intuitive solution to a problem using recursion. Consider the well-known Fibonacci-series problem:
Key Point
710 Chapter 18
Recursion The series: 0 1 indexes: 0 1
1 2
2 3
3 4
5 5
8 6
13 7
21 8
34 9
55 10
89 … 11
The Fibonacci series begins with 0 and 1, and each subsequent number is the sum of the preceding two. The series can be recursively defined as: fib(0) = 0; fib(1) = 1; fib(index) = fib(index - 2) + fib(index - 1); index >= 2
The Fibonacci series was named for Leonardo Fibonacci, a medieval mathematician, who originated it to model the growth of the rabbit population. It can be applied in numeric optimization and in various other areas. How do you find fib(index) for a given index? It is easy to find fib(2), because you know fib(0) and fib(1). Assuming that you know fib(index - 2) and fib(index - 1), you can obtain fib(index) immediately. Thus, the problem of computing fib(index) is reduced to computing fib(index - 2) and fib(index - 1). When doing so, you apply the idea recursively until index is reduced to 0 or 1. The base case is index = 0 or index = 1. If you call the method with index = 0 or index = 1, it immediately returns the result. If you call the method with index >= 2, it divides the problem into two subproblems for computing fib(index - 1) and fib(index - 2) using recursive calls. The recursive algorithm for computing fib(index) can be simply described as follows: if (index == 0) return 0; else if (index == 1) return 1; else return fib(index - 1) + fib(index - 2);
Listing 18.2 gives a complete program that prompts the user to enter an index and computes the Fibonacci number for that index.
LISTING 18.2 ComputeFibonacci.java
base case
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
import java.util.Scanner; public class ComputeFibonacci { /** Main method */ public static void main(String[] args) { // Create a Scanner Scanner input = new Scanner(System.in); System.out.print("Enter an index for a Fibonacci number: "); int index = input.nextInt(); // Find and display the Fibonacci number System.out.println("The Fibonacci number at index " + index + " is " + fib(index)); } /** The method for finding the Fibonacci number */ public static long fib(long index) { if (index == 0) // Base case return 0;
18.3 Case Study: Computing Fibonacci Numbers 711 20 21 22 23 24 25
else if (index == 1) // Base case return 1; else // Reduction and recursive calls return fib(index - 1) + fib(index - 2);
base case
recursion
} }
Enter an index for a Fibonacci number: 1 The Fibonacci number at index 1 is 1
Enter an index for a Fibonacci number: 6 The Fibonacci number at index 6 is 8
Enter an index for a Fibonacci number: 7 The Fibonacci number at index 7 is 13
The program does not show the considerable amount of work done behind the scenes by the computer. Figure 18.4, however, shows the successive recursive calls for evaluating fib(4). The original method, fib(4), makes two recursive calls, fib(3) and fib(2), and then returns fib(3) + fib(2). But in what order are these methods called? In Java, operands are evaluated from left to right, so fib(2) is called after fib(3) is completely evaluated. The labels in Figure 18.4 show the order in which the methods are called. fib(4) 0: call fib(4)
17: return fib(4)
return fib(3) + fib(2)
10: return fib(3)
1: call fib(3)
16: return fib(2)
return fib(2) + fib(1) 7: return fib(2) 2: call fib(2)
4: return fib(1)
return fib(1) + fib(0) 8: call fib(1)
9: return fib(1)
return fib(1) + fib(0)
11: call fib(2)
13: return fib(1)
14: return fib(0) 12: call fib(1) 15: return fib(0)
return 1
return 1
5: call fib(0) 3: call fib(1) 6: return fib(0)
return 1
return 0
FIGURE 18.4 Invoking fib(4) spawns recursive calls to fib. As shown in Figure 18.4, there are many duplicated recursive calls. For instance, fib(2) is called twice, fib(1) three times, and fib(0) twice. In general, computing fib(index) requires roughly twice as many recursive calls as does computing fib(index - 1). As you try larger index values, the number of calls substantially increases, as shown in Table 18.1.
TABLE 18.1 Number of Recursive Calls in fib(index) 2
3
4
10
20
30
40
50
# of calls 3
5
9
177
21891
2,692,537
331,160,281
2,075,316,483
index
return 0
712 Chapter 18
Recursion Pedagogical Note The recursive implementation of the fib method is very simple and straightforward, but it isn’t efficient, since it requires more time and memory to run recursive methods. See Programming Exercise 18.2 for an efficient solution using loops. Though it is not practical, the recursive fib method is a good example of how to write recursive methods.
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Check Point
18.7 Show the output of the following two programs:
public class Test { public static void main(String[] args) { xMethod(5); }
public class Test { public static void main(String[] args) { xMethod(5); }
public static void xMethod(int n) { if (n > 0) { System.out.print(n + " "); xMethod(n - 1); } } }
public static void xMethod(int n) { if (n > 0) { xMethod(n - 1); System.out.print(n + " "); } } }
18.8 What is wrong in the following method? public class Test { public static void main(String[] args) { xMethod(1234567); } public static void xMethod(double n) { if (n != 0) { System.out.print(n); xMethod(n / 10); } }
public class Test { public static void main(String[] args) { Test test = new Test(); System.out.println(test.toString()); } public Test() { Test test = new Test(); } }
}
18.9 How many times is the fib method in Listing 18.2 invoked for fib(6)?
18.4 Problem Solving Using Recursion Key Point recursion characteristics
If you think recursively, you can solve many problems using recursion. The preceding sections presented two classic recursion examples. All recursive methods have the following characteristics:
if-else
■
The method is implemented using an if-else or a switch statement that leads to different cases.
base cases
■
One or more base cases (the simplest case) are used to stop recursion.
reduction
■
Every recursive call reduces the original problem, bringing it increasingly closer to a base case until it becomes that case.
In general, to solve a problem using recursion, you break it into subproblems. Each subproblem is the same as the original problem but smaller in size. You can apply the same approach to each subproblem to solve it recursively.
18.4 Problem Solving Using Recursion 713 Recursion is everywhere. It is fun to think recursively. Consider drinking coffee. You may describe the procedure recursively as follows:
think recursively
public static void drinkCoffee(Cup cup) { if (!cup.isEmpty()) { cup.takeOneSip(); // Take one sip drinkCoffee(cup); } }
Assume cup is an object for a cup of coffee with the instance methods isEmpty() and takeOneSip(). You can break the problem into two subproblems: one is to drink one sip of coffee and the other is to drink the rest of the coffee in the cup. The second problem is the same as the original problem but smaller in size. The base case for the problem is when the cup is empty. Consider the problem of printing a message n times. You can break the problem into two subproblems: one is to print the message one time and the other is to print it n - 1 times. The second problem is the same as the original problem but it is smaller in size. The base case for the problem is n == 0. You can solve this problem using recursion as follows: public static void nPrintln(String message, int times) { if (times >= 1) { System.out.println(message); nPrintln(message, times - 1); } // The base case is times == 0 }
Note that the fib method in the preceding section returns a value to its caller, but the drinkCoffee and nPrintln methods are void and they do not return a value. If you think recursively, you can use recursion to solve many of the problems presented in earlier chapters of this book. Consider the palindrome problem in Listing 5.14. Recall that a string is a palindrome if it reads the same from the left and from the right. For example, “mom” and “dad” are palindromes, but “uncle” and “aunt” are not. The problem of checking whether a string is a palindrome can be divided into two subproblems: ■
Check whether the first character and the last character of the string are equal.
■
Ignore the two end characters and check whether the rest of the substring is a palindrome.
recursive call
think recursively
The second subproblem is the same as the original problem but smaller in size. There are two base cases: (1) the two end characters are not the same, and (2) the string size is 0 or 1. In case 1, the string is not a palindrome; in case 2, the string is a palindrome. The recursive method for this problem can be implemented as shown in Listing 18.3.
LISTING 18.3 RecursivePalindromeUsingSubstring.java 1 2 3 4 5 6 7 8 9 10 11 12
public class RecursivePalindromeUsingSubstring { public static boolean isPalindrome(String s) { if (s.length() <= 1) // Base case return true; else if (s.charAt(0) != s.charAt(s.length() - 1)) // Base case return false; else return isPalindrome(s.substring(1, s.length() - 1)); } public static void main(String[] args) { System.out.println("Is moon a palindrome? "
method header base case base case
recursive call
714 Chapter 18
Recursion 13 14 15 16 17 18 19 20 21 Is Is Is Is Is
+ isPalindrome("moon")); System.out.println("Is noon a palindrome? " + isPalindrome("noon")); System.out.println("Is a a palindrome? " + isPalindrome("a")); System.out.println("Is aba a palindrome? " + isPalindrome("aba")); System.out.println("Is ab a palindrome? " + isPalindrome("ab")); } } moon a palindrome? false noon a palindrome? true a a palindrome? true aba a palindrome? true ab a palindrome? false
The substring method in line 8 creates a new string that is the same as the original string except without the first and last characters. Checking whether a string is a palindrome is equivalent to checking whether the substring is a palindrome if the two end characters in the original string are the same.
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Check Point
18.10 Describe the characteristics of recursive methods. 18.11 For the isPalindrome method in Listing 18.3, what are the base cases? How many times is this method called when invoking isPalindrome("abdxcxdba")? 18.12 Show the call stack for isPalindrome("abcba") using the method defined in Listing 18.3.
18.5 Recursive Helper Methods Key Point
Sometimes you can find a solution to the original problem by defining a recursive function to a problem similar to the original problem. This new method is called a recursive helper method. The original problem can be solved by invoking the recursive helper method. The recursive isPalindrome method in Listing 18.3 is not efficient, because it creates a new string for every recursive call. To avoid creating new strings, you can use the low and high indices to indicate the range of the substring. These two indices must be passed to the recursive method. Since the original method is isPalindrome(String s), you have to create the new method isPalindrome(String s, int low, int high) to accept additional information on the string, as shown in Listing 18.4.
LISTING 18.4 RecursivePalindrome.java
helper method base case base case
1 2 3 4 5 6 7 8 9 10 11 12 13
public class RecursivePalindrome { public static boolean isPalindrome(String s) { return isPalindrome(s, 0, s.length() - 1); } private static boolean isPalindrome(String s, int low, int high) { if (high <= low) // Base case return true; else if (s.charAt(low) != s.charAt(high)) // Base case return false; else return isPalindrome(s, low + 1, high - 1); }
18.5 Recursive Helper Methods 715 14 15 16 17 18 19 20 21 22 23 24
public static void main(String[] args) { System.out.println("Is moon a palindrome? " + isPalindrome("moon")); System.out.println("Is noon a palindrome? " + isPalindrome("noon")); System.out.println("Is a a palindrome? " + isPalindrome("a")); System.out.println("Is aba a palindrome? " + isPalindrome("aba")); System.out.println("Is ab a palindrome? " + isPalindrome("ab")); } }
Two overloaded isPalindrome methods are defined. The first, isPalindrome(String s), checks whether a string is a palindrome, and the second, isPalindrome(String s, int low, int high), checks whether a substring s(low..high) is a palindrome. The first method passes the string s with low = 0 and high = s.length() – 1 to the second method. The second method can be invoked recursively to check a palindrome in an ever-shrinking substring. It is a common design technique in recursive programming to define a second method that receives additional parameters. Such a method is known as a recursive helper method. Helper methods are very useful in designing recursive solutions for problems involving strings and arrays. The sections that follow give two more examples.
18.5.1
recursive helper method
Recursive Selection Sort
Selection sort was introduced in Section 7.11. Recall that it finds the smallest element in the list and swaps it with the first element. It then finds the smallest element remaining and swaps it with the first element in the remaining list, and so on until the remaining list contains only a single element. The problem can be divided into two subproblems: ■
Find the smallest element in the list and swap it with the first element.
■
Ignore the first element and sort the remaining smaller list recursively.
The base case is that the list contains only one element. Listing 18.5 gives the recursive sort method.
LISTING 18.5 RecursiveSelectionSort.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
public class RecursiveSelectionSort { public static void sort(double[] list) { sort(list, 0, list.length - 1); // Sort the entire list } private static void sort(double[] list, int low, int high) { if (low < high) { // Find the smallest number and its index in list[low .. high] int indexOfMin = low; double min = list[low]; for (int i = low + 1; i <= high; i++) { if (list[i] < min) { min = list[i]; indexOfMin = i; } } // Swap the smallest in list[low .. high] with list[low] list[indexOfMin] = list[low]; list[low] = min;
helper method base case
716 Chapter 18 recursive call
Recursion 22 23 24 25 26
// Sort the remaining list[low+1 .. high] sort(list, low + 1, high); } } }
Two overloaded sort methods are defined. The first method, sort(double[] list), sorts an array in list[0..list.length - 1] and the second method, sort(double[] list, int low, int high), sorts an array in list[low..high]. The second method can be invoked recursively to sort an ever-shrinking subarray.
18.5.2 VideoNote
Binary search
Recursive Binary Search
Binary search was introduced in Section 7.10.2. For binary search to work, the elements in the array must be in increasing order. The binary search first compares the key with the element in the middle of the array. Consider the following three cases: ■
Case 1: If the key is less than the middle element, recursively search for the key in the first half of the array.
■
Case 2: If the key is equal to the middle element, the search ends with a match.
■
Case 3: If the key is greater than the middle element, recursively search for the key in the second half of the array.
Case 1 and Case 3 reduce the search to a smaller list. Case 2 is a base case when there is a match. Another base case is that the search is exhausted without a match. Listing 18.6 gives a clear, simple solution for the binary search problem using recursion.
LISTING 18.6 Recursive Binary Search Method
helper method base case
recursive call base case recursive call
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
public class RecursiveBinarySearch { public static int recursiveBinarySearch(int[] list, int key) { int low = 0; int high = list.length - 1; return recursiveBinarySearch(list, key, low, high); } private static int recursiveBinarySearch(int[] list, int key, int low, int high) { if (low > high) // The list has been exhausted without a match return -low - 1; int mid = (low + high) / 2; if (key < list[mid]) return recursiveBinarySearch(list, key, low, mid - 1); else if (key == list[mid]) return mid; else return recursiveBinarySearch(list, key, mid + 1, high); } }
The first method finds a key in the whole list. The second method finds a key in the list with index from low to high. The first binarySearch method passes the initial array with low = 0 and high = list.length - 1 to the second binarySearch method. The second method is invoked recursively to find the key in an ever-shrinking subarray.
18.6 Case Study: Finding the Directory Size 717 18.13 Show the call stack for
isPalindrome("abcba") using the method defined in
Listing 18.4.
18.14 Show the call stack for selectionSort(new
double[]{2, 3, 5, 1}) using the
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Check Point
method defined in Listing 18.5. 18.15 What is a recursive helper method?
18.6 Case Study: Finding the Directory Size Recursive methods are efficient for solving problems with recursive structures. The preceding examples can easily be solved without using recursion. This section presents a problem that is difficult to solve without using recursion. The problem is to find the size of a directory. The size of a directory is the sum of the sizes of all files in the directory. A directory d may contain subdirectories. Suppose a directory contains files f1, f2, c , fm and subdirectories d1, d2, c , dn, as shown in Figure 18.5.
Key Point
VideoNote
Directory size
directory
f1
f2
...
fm
d1
d2
...
dn
FIGURE 18.5 A directory contains files and subdirectories. The size of the directory can be defined recursively as follows: size(d) = size(f1) + size(f2) + c + size(fm) + size(d1) + size(d2) + c + size(dn) The File class, introduced in Section 12.10, can be used to represent a file or a directory and obtain the properties for files and directories. Two methods in the File class are useful for this problem: ■
The length() method returns the size of a file.
■
The listFiles() method returns an array of File objects under a directory.
Listing 18.7 gives a program that prompts the user to enter a directory or a file and displays its size.
LISTING 18.7 DirectorySize.java 1 2 3 4 5 6 7 8 9 10 11 12 13
import java.io.File; import java.util.Scanner; public class DirectorySize { public static void main(String[] args) { // Prompt the user to enter a directory or a file System.out.print("Enter a directory or a file: "); Scanner input = new Scanner(System.in); String directory = input.nextLine(); // Display the size System.out.println(getSize(new File(directory)) + " bytes"); }
invoke method
718 Chapter 18
Recursion 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
getSize method
is directory? all subitems recursive call
base case
public static long getSize(File file) { long size = 0; // Store the total size of all files if (file.isDirectory()) { File[] files = file.listFiles(); // All files and subdirectories for (int i = 0; files != null && i < files.length; i++) { size += getSize(files[i]); // Recursive call } } else { // Base case size += file.length(); } return size; } }
Enter a directory or a file: c:\book 48619631 bytes
Enter a directory or a file: c:\book\Welcome.java 172 bytes
Enter a directory or a file: c:\book\NonExistentFile 0 bytes
If the file object represents a directory (line 18), each subitem (file or subdirectory) in the directory is recursively invoked to obtain its size (line 21). If the file object represents a file (line 24), the file size is obtained and added to the total size (line 25). What happens if an incorrect or a nonexistent directory is entered? The program will detect that it is not a directory and invoke file.length() (line 25), which returns 0. Thus, in this case, the getSize method will return 0.
Tip To avoid mistakes, it is a good practice to test all cases. For example, you should test the program for an input of file, an empty directory, a nonexistent directory, and a nonexistent file.
testing all cases
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Check Point
18.16 What is the base case for the getSize method? 18.17 How does the program get all files and directories under a given directory? 18.18 How many times will the getSize method be invoked for a directory if the directory has three subdirectories and each subdirectory has four files? 18.19 Will the program work if the directory is empty (i.e., it does not contain any files)?
18.20 Will the program work if line 20 is replaced by the following code? for (int i = 0; i < files.length; i++)
18.7 Case Study: Tower of Hanoi 719 18.21 Will the program work if lines 20–21 is replaced by the following code? for (File file: files) size += getSize(file); // Recursive call
18.7 Case Study: Tower of Hanoi The Tower of Hanoi problem is a classic problem that can be solved easily using recursion, but it is difficult to solve otherwise.
Key Point
The problem involves moving a specified number of disks of distinct sizes from one tower to another while observing the following rules: ■
There are n disks labeled 1, 2, 3, . . . , n and three towers labeled A, B, and C.
■
No disk can be on top of a smaller disk at any time.
■
All the disks are initially placed on tower A.
■
Only one disk can be moved at a time, and it must be the smallest disk on a tower.
The objective of the problem is to move all the disks from A to B with the assistance of C. For example, if you have three disks, the steps to move all of the disks from A to B are shown in Figure 18.6.
0
4 1 2 3
A
B
A
C
Original position
3
1 2
B
C
Step 4: Move disk 3 from A to B
1
5
2 3
1
A
B
C
Step 1: Move disk 1 from A to B 2
1
3
2
A
B
C
Step 5: Move disk 1 from C to A 6
3
1
2
1
2 3
A
B
C
A
B
Step 2: Move disk 2 from A to C 3
C
Step 6: Move disk 2 from C to B 7
A
1 2 3
1 2
3
B
C
Step 3: Move disk 1 from B to C
A
B
C
Step 7: Move disk 1 from A to B
FIGURE 18.6 The goal of the Tower of Hanoi problem is to move disks from tower A to tower B without breaking the rules.
720 Chapter 18
Recursion Note The Tower of Hanoi is a classic computer-science problem, to which many websites are devoted. One of them worth looking at is www.cut-the-knot.com/recurrence/hanoi.shtml.
In the case of three disks, you can find the solution manually. For a larger number of disks, however—even for four—the problem is quite complex. Fortunately, the problem has an inherently recursive nature, which leads to a straightforward recursive solution. The base case for the problem is n = 1. If n == 1, you could simply move the disk from A to B. When n > 1, you could split the original problem into the following three subproblems and solve them sequentially. 1. Move the first n - 1 disks from A to C recursively with the assistance of tower B, as shown in Step 1 in Figure 18.7. 2. Move disk n from A to B, as shown in Step 2 in Figure 18.7. 3. Move n - 1 disks from C to B recursively with the assistance of tower A, as shown in Step 3 in Figure 18.7.
0 2 n – 1 disks
n – 1 disks
. . .
A
. . . B Original position
A B C Step 2: Move disk n from A to B
C
1
3 n – 1 disks
n – 1 disks
. . .
. . . A B C Step 1: Move the first n – 1 disks from A to C recursively
A
B
C
Step 3: Move n – 1 disks from C to B recursively
FIGURE 18.7 The Tower of Hanoi problem can be decomposed into three subproblems. The following method moves n disks from the fromTower to the toTower with the assistance of the auxTower: void moveDisks(int n, char fromTower, char toTower, char auxTower)
The algorithm for the method can be described as: if (n == 1) // Stopping condition Move disk 1 from the fromTower to the toTower; else { moveDisks(n - 1, fromTower, auxTower, toTower); Move disk n from the fromTower to the toTower; moveDisks(n - 1, auxTower, toTower, fromTower); }
18.7 Case Study: Tower of Hanoi 721 Listing 18.8 gives a program that prompts the user to enter the number of disks and invokes the recursive method moveDisks to display the solution for moving the disks.
LISTING 18.8 TowerOfHanoi.java 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
import java.util.Scanner; public class TowerOfHanoi { /** Main method */ public static void main(String[] args) { // Create a Scanner Scanner input = new Scanner(System.in); System.out.print("Enter number of disks: "); int n = input.nextInt(); // Find the solution recursively System.out.println("The moves are:"); moveDisks(n, 'A', 'B', 'C'); } /** The method for finding the solution to move n disks from fromTower to toTower with auxTower */ public static void moveDisks(int n, char fromTower, char toTower, char auxTower) { if (n == 1) // Stopping condition System.out.println("Move disk " + n + " from " + fromTower + " to " + toTower); else { moveDisks(n - 1, fromTower, auxTower, toTower); System.out.println("Move disk " + n + " from " + fromTower + " to " + toTower); moveDisks(n - 1, auxTower, toTower, fromTower); } } }
Enter number of disks: 4 The moves are: Move disk 1 from A to C Move disk 2 from A to B Move disk 1 from C to B Move disk 3 from A to C Move disk 1 from B to A Move disk 2 from B to C Move disk 1 from A to C Move disk 4 from A to B Move disk 1 from C to B Move disk 2 from C to A Move disk 1 from B to A Move disk 3 from C to B Move disk 1 from A to C Move disk 2 from A to B Move disk 1 from C to B
base case
recursion
recursion
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Recursion This problem is inherently recursive. Using recursion makes it possible to find a natural, simple solution. It would be difficult to solve the problem without using recursion. Consider tracing the program for n = 3. The successive recursive calls are shown in Figure 18.8. As you can see, writing the program is easier than tracing the recursive calls. The system uses stacks to manage the calls behind the scenes. To some extent, recursion provides a level of abstraction that hides iterations and other details from the user.
moveDisks(3,'A','B','C') moveDisks(2,'A','C','B') move disk 3 from A to B moveDisks(2,'C','B','A')
moveDisks(2,'A','C','B') moveDisks(1,'A','B','C') move disk 2 from A to C moveDisks(1,'B','C','A')
moveDisks(2,'C','B','A') moveDisks(1,'C','A','B') move disk 2 from C to B moveDisks(1,'A','B','C')
moveDisks(1,'A','B','C')
moveDisks(1,'B','C','A')
moveDisks(1,'C','A','B')
moveDisks(1,'A','B','C')
move disk 1 from A to B
move disk 1 from B to C
move disk 1 from C to A
move disk 1 from A to B
FIGURE 18.8 Invoking moveDisks(3, 'A', 'B', 'C') spawns calls to moveDisks recursively.
✓
Check Point
18.22 How many times is the moveDisks method in Listing 18.8 invoked for moveDisks(5, 'A', 'B', 'C')?
18.8 Case Study: Fractals Key Point
VideoNote
Fractal (Sierpinski triangle)
Using recursion is ideal for displaying fractals, because fractals are inherently recursive. A fractal is a geometrical figure, but unlike triangles, circles, and rectangles, fractals can be divided into parts, each of which is a reduced-size copy of the whole. There are many interesting examples of fractals. This section introduces a simple fractal, the Sierpinski triangle, named after a famous Polish mathematician. A Sierpinski triangle is created as follows: 1. Begin with an equilateral triangle, which is considered to be a Sierpinski fractal of order (or level) 0, as shown in Figure 18.9a. 2. Connect the midpoints of the sides of the triangle of order 0 to create a Sierpinski triangle of order 1 (Figure 18.9b). 3. Leave the center triangle intact. Connect the midpoints of the sides of the three other triangles to create a Sierpinski triangle of order 2 (Figure 18.9c). 4. You can repeat the same process recursively to create a Sierpinski triangle of order 3, 4, . . . , and so on (Figure 18.9d). The problem is inherently recursive. How do you develop a recursive solution for it? Consider the base case when the order is 0. It is easy to draw a Sierpinski triangle of order 0. How do you draw a Sierpinski triangle of order 1? The problem can be reduced to drawing three Sierpinski triangles of order 0. How do you draw a Sierpinski triangle of order 2? The problem can be reduced to drawing three Sierpinski triangles of order 1, so the problem of
18.8 Case Study: Fractals 723
FIGURE 18.9
(a) Order 0
(b) Order 1
(c) Order 2
(d) Order 3
A Sierpinski triangle is a pattern of recursive triangles.
drawing a Sierpinski triangle of order n can be reduced to drawing three Sierpinski triangles of order n - 1. Listing 18.9 gives a program that displays a Sierpinski triangle of any order, as shown in Figure 18.9. You can enter an order in a text field to display a Sierpinski triangle of the specified order.
LISTING 18.9 SierpinskiTriangle.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
import import import import import import import import import import import import
javafx.application.Application; javafx.geometry.Point2D; javafx.geometry.Pos; javafx.scene.Scene; javafx.scene.control.Label; javafx.scene.control.TextField; javafx.scene.layout.BorderPane; javafx.scene.layout.HBox; javafx.scene.layout.Pane; javafx.scene.paint.Color; javafx.scene.shape.Polygon; javafx.stage.Stage;
public class SierpinskiTriangle extends Application { @Override // Override the start method in the Application class public void start(Stage primaryStage) { SierpinskiTrianglePane trianglePane = new SierpinskiTrianglePane(); TextField tfOrder = new TextField(); tfOrder.setOnAction(
recursive triangle pane
724 Chapter 18 listener for text field
hold label and text field
listener for resizing
three initial points
clear the pane draw a triangle
create a triangle
Recursion 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 68 69 70 71 72 73 74 75 76 77 78 79
e -> trianglePane.setOrder(Integer.parseInt(tfOrder.getText()))); tfOrder.setPrefColumnCount(4); tfOrder.setAlignment(Pos.BOTTOM_RIGHT); // Pane to hold label, text field, and a button HBox hBox = new HBox(10); hBox.getChildren().addAll(new Label("Enter an order: "), tfOrder); hBox.setAlignment(Pos.CENTER); BorderPane borderPane = new BorderPane(); borderPane.setCenter(trianglePane); borderPane.setBottom(hBox); // Create a scene and place it in the stage Scene scene = new Scene(borderPane, 200, 210); primaryStage.setTitle("SierpinskiTriangle"); // Set the stage title primaryStage.setScene(scene); // Place the scene in the stage primaryStage.show(); // Display the stage scene.widthProperty().addListener(ov -> trianglePane.paint()); scene.heightProperty().addListener(ov -> trianglePane.paint()); } /** Pane for displaying triangles */ static class SierpinskiTrianglePane extends Pane { private int order = 0; /** Set a new order */ public void setOrder(int order) { this.order = order; paint(); } SierpinskiTrianglePane() { } protected void paint() { // Select three points in proportion to the pane size Point2D p1 = new Point2D(getWidth() / 2, 10); Point2D p2 = new Point2D(10, getHeight() - 10); Point2D p3 = new Point2D(getWidth() - 10, getHeight() - 10); this.getChildren().clear(); // Clear the pane before redisplay displayTriangles(order, p1, p2, p3); } private void displayTriangles(int order, Point2D p1, Point2D p2, Point2D p3) { if (order == 0) { // Draw a triangle to connect three points Polygon triangle = new Polygon(); triangle.getPoints().addAll(p1.getX(), p1.getY(), p2.getX(), p2.getY(), p3.getX(), p3.getY()); triangle.setStroke(Color.BLACK); triangle.setFill(Color.WHITE); this.getChildren().add(triangle); } else {
18.8 Case Study: Fractals 725 80 81 82 83 84 85 86 87 88 89 90 91 92
// Get the midpoint on each edge in the triangle Point2D p12 = p1.midpoint(p2); Point2D p23 = p2.midpoint(p3); Point2D p31 = p3.midpoint(p1); // Recursively display displayTriangles(order displayTriangles(order displayTriangles(order
three triangles - 1, p1, p12, p31); - 1, p12, p2, p23); - 1, p31, p23, p3);
top subtriangle left subtriangle right subtriangle
} } } }
The initial triangle has three points set in proportion to the pane size (lines 58–60). If order == 0, the displayTriangles(order, p1, p2, p3) method displays a triangle that connects the three points p1, p2, and p3 in lines 71–77, as shown in Figure 18.10a. Otherwise, it performs the following tasks:
displayTriangle method
1. Obtain the midpoint between p1 and p2 (line 81), the midpoint between p2 and p3 (line 82), and the midpoint between p3 and p1 (line 83), as shown in Figure 18.10b. 2. Recursively invoke displayTriangles with a reduced order to display three smaller Sierpinski triangles (lines 86–88). Note that each small Sierpinski triangle is structurally identical to the original big Sierpinski triangle except that the order of a small triangle is one less, as shown in Figure 18.10b. p1
Draw the Sierpinski triangle displayTriangles(order, p1, p2, p3)
p2
p3 (a) p1
Recursively draw the small Sierpinski triangle displayTriangles( order - 1, p12, p2, p23) p2
p12
Recursively draw the small Sierpinski triangle displayTriangles( order - 1, p1, p12, p31)
p31 Recursively draw the small Sierpinski triangle displayTriangles( order - 1, p31, p23, p3) p23
p3
(b)
FIGURE 18.10 Drawing a Sierpinski triangle spawns calls to draw three small Sierpinski triangles recursively. A Sierpinski triangle is displayed in a SierpinskiTrianglePane. The order property in the inner class SierpinskiTrianglePane specifies the order for the Sierpinski triangle. The Point2D class, introduced in Section 9.8, The Point2D Class, represents a point with
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Recursion x- and y-coordinates. Invoking p1.midpoint(p2) returns a new Point2D object that is the midpoint between p1 and p2 (lines 81–83).
✓
Check Point
18.23 How do you obtain the midpoint between two points? 18.24 What is the base case for the displayTriangles method? 18.25 How many times is the displayTriangles method invoked for a Sierpinski triangle of order 0, order 1, order 2, and order n?
18.26 What happens if you enter a negative order? How do you fix this problem in the code? 18.27 Instead of drawing a triangle using a polygon, rewrite the code to draw a triangle by drawing three lines to connect the points in lines 71–77.
18.9 Recursion vs. Iteration Key Point
Recursion is an alternative form of program control. It is essentially repetition without a loop. When you use loops, you specify a loop body. The repetition of the loop body is controlled by the loop control structure. In recursion, the method itself is called repeatedly. A selection statement must be used to control whether to call the method recursively or not. Recursion bears substantial overhead. Each time the program calls a method, the system must allocate memory for all of the method’s local variables and parameters. This can consume considerable memory and requires extra time to manage the memory. Any problem that can be solved recursively can be solved nonrecursively with iterations. Recursion has some negative aspects: it uses up too much time and too much memory. Why, then, should you use it? In some cases, using recursion enables you to specify a clear, simple solution for an inherently recursive problem that would otherwise be difficult to obtain. Examples are the directory-size problem, the Tower of Hanoi problem, and the fractal problem, which are rather difficult to solve without using recursion. The decision whether to use recursion or iteration should be based on the nature of, and your understanding of, the problem you are trying to solve. The rule of thumb is to use whichever approach can best develop an intuitive solution that naturally mirrors the problem. If an iterative solution is obvious, use it. It will generally be more efficient than the recursive option.
recursion overhead
recursion advantages
recursion or iteration?
Note StackOverflowError
Recursive programs can run out of memory, causing a StackOverflowError.
Tip If you are concerned about your program’s performance, avoid using recursion, because it takes more time and consumes more memory than iteration. In general, recursion can be used to solve the inherent recursive problems such as Tower of Hanoi, recursive directories, and Sierpinski triangles.
performance concern
✓
Check Point
18.28 Which of the following statements are true? a. Any recursive method can be converted into a nonrecursive method. b. Recursive methods take more time and memory to execute than nonrecursive methods. c. Recursive methods are always simpler than nonrecursive methods. d. There is always a selection statement in a recursive method to check whether a base case is reached.
18.29 What is a cause for a stack-overflow exception?
18.10 Tail Recursion 727
18.10 Tail Recursion A tail recursive method is efficient for reducing stack size. A recursive method is said to be tail recursive if there are no pending operations to be performed on return from a recursive call, as illustrated in Figure 18.11a. However, method B in Figure 18.11b is not tail recursive because there are pending operations after a method call is returned.
Recursive method A ... ... ... Invoke method A recursively (a) Tail recursion
Key Point tail recursion
Recursive method B ... ... Invoke method B recursively ... ... (b) Nontail recursion
FIGURE 18.11 A tail-recursive method has no pending operations after a recursive call. For example, the recursive isPalindrome method (lines 6–13) in Listing 18.4 is tail recursive because there are no pending operations after recursively invoking isPalindrome in line 12. However, the recursive factorial method (lines 16–21) in Listing 18.1 is not tail recursive, because there is a pending operation, namely multiplication, to be performed on return from each recursive call. Tail recursion is desirable: because the method ends when the last recursive call ends, there is no need to store the intermediate calls in the stack. Compilers can optimize tail recursion to reduce stack size. A nontail-recursive method can often be converted to a tail-recursive method by using auxiliary parameters. These parameters are used to contain the result. The idea is to incorporate the pending operations into the auxiliary parameters in such a way that the recursive call no longer has a pending operation. You can define a new auxiliary recursive method with the auxiliary parameters. This method may overload the original method with the same name but a different signature. For example, the factorial method in Listing 18.1 is written in a tailrecursive way in Listing 18.10.
LISTING 18.10 ComputeFactorialTailRecursion.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14
public class ComputeFactorialTailRecursion { /** Return the factorial for a specified number */ public static long factorial(int n) { return factorial(n, 1); // Call auxiliary method } /** Auxiliary tail-recursive method for factorial */ private static long factorial(int n, int result) { if (n == 0) return result; else return factorial(n - 1, n * result); // Recursive call } }
The first factorial method (line 3) simply invokes the second auxiliary method (line 4). The second method contains an auxiliary parameter result that stores the result for the factorial of n. This method is invoked recursively in line 12. There is no pending operation after
original method invoke auxiliary method
auxiliary method
recursive call
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Recursion a call is returned. The final result is returned in line 10, which is also the return value from invoking factorial(n, 1) in line 4.
✓
Check Point
18.30 Identify tail-recursive methods in this chapter. 18.31 Rewrite the fib method in Listing 18.2 using tail recursion.
KEY TERMS base case 706 direct recursion 709 indirect recursion 709 infinite recursion 709
recursive helper method 715 recursive method 706 stopping condition 706 tail recursion 727
CHAPTER SUMMARY 1. A recursive method is one that directly or indirectly invokes itself. For a recursive method to terminate, there must be one or more base cases.
2. Recursion is an alternative form of program control. It is essentially repetition without a loop control. It can be used to write simple, clear solutions for inherently recursive problems that would otherwise be difficult to solve.
3. Sometimes the original method needs to be modified to receive additional parameters in order to be invoked recursively. A recursive helper method can be defined for this purpose.
4. Recursion bears substantial overhead. Each time the program calls a method, the system must allocate memory for all of the method’s local variables and parameters. This can consume considerable memory and requires extra time to manage the memory.
5. A recursive method is said to be tail recursive if there are no pending operations to be performed on return from a recursive call. Some compilers can optimize tail recursion to reduce stack size.
QUIZ Answer the quiz for this chapter online at www.cs.armstrong.edu/liang/intro10e/quiz.html.
PROGRAMMING EXERCISES Sections 18.2–18.3
*18.1 (Factorial) Using the BigInteger class introduced in Section 10.9, you can find the factorial for a large number (e.g., 100!). Implement the factorial method using recursion. Write a program that prompts the user to enter an integer and displays its factorial.
*18.2 (Fibonacci numbers) Rewrite the fib method in Listing 18.2 using iterations. Hint: To compute fib(n) without recursion, you need to obtain fib(n - 2) and fib(n - 1) first. Let f0 and f1 denote the two previous Fibonacci
Programming Exercises 729 numbers. The current Fibonacci number would then be f0 + f1. The algorithm can be described as follows: f0 = 0; // For fib(0) f1 = 1; // For fib(1) for (int i = 1; i <= n; i++) { currentFib = f0 + f1; f0 = f1; f1 = currentFib; } // After the loop, currentFib is fib(n)
Write a test program that prompts the user to enter an index and displays its Fibonacci number.
*18.3 (Compute greatest common divisor using recursion) The gcd(m,
n) can also
be defined recursively as follows: ■ ■
If m % n is 0, gcd(m, n) is n. Otherwise, gcd(m, n) is gcd(n, m % n).
Write a recursive method to find the GCD. Write a test program that prompts the user to enter two integers and displays their GCD. 18.4 (Sum series) Write a recursive method to compute the following series: m(i) = 1 +
1 1 1 + + c + 2 3 i
Write a test program that displays m(i) for i = 1, 2, . . ., 10.
18.5 (Sum series) Write a recursive method to compute the following series: m(i) =
2 3 4 5 6 i 1 + + + + + + c + 3 5 7 9 11 13 2i + 1
Write a test program that displays m(i) for i = 1, 2, . . ., 10.
*18.6 (Sum series) Write a recursive method to compute the following series: m(i) =
2 i 1 + + c + 2 3 i + 1
Write a test program that displays m(i) for i = 1, 2, . . ., 10.
*18.7 (Fibonacci series) Modify Listing 18.2, ComputeFibonacci.java, so that the program finds the number of times the fib method is called. (Hint: Use a static variable and increment it every time the method is called.)
Section 18.4
*18.8 (Print the digits in an integer reversely) Write a recursive method that displays an int value reversely on the console using the following header: public static void reverseDisplay(int value)
For example, reverseDisplay(12345) displays 54321. Write a test program that prompts the user to enter an integer and displays its reversal. *18.9 (Print the characters in a string reversely) Write a recursive method that displays a string reversely on the console using the following header: public static void reverseDisplay(String value)
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Recursion For example, reverseDisplay("abcd") displays dcba. Write a test program that prompts the user to enter a string and displays its reversal. *18.10 (Occurrences of a specified character in a string) Write a recursive method that finds the number of occurrences of a specified letter in a string using the following method header: public static int count(String str, char a)
For example, count("Welcome", 'e') returns 2. Write a test program that prompts the user to enter a string and a character, and displays the number of occurrences for the character in the string. *18.11 (Sum the digits in an integer using recursion) Write a recursive method that computes the sum of the digits in an integer. Use the following method header: public static int sumDigits(long n)
For example, sumDigits(234) returns 2 + 3 + 4 = 9. Write a test program that prompts the user to enter an integer and displays its sum.
Section 18.5
**18.12 (Print the characters in a string reversely) Rewrite Programming Exercise 18.9 using a helper method to pass the substring high index to the method. The helper method header is: public static void reverseDisplay(String value, int high)
*18.13 (Find the largest number in an array) Write a recursive method that returns the largest integer in an array. Write a test program that prompts the user to enter a list of eight integers and displays the largest element. *18.14 (Find the number of uppercase letters in a string) Write a recursive method to return the number of uppercase letters in a string. Write a test program that prompts the user to enter a string and displays the number of uppercase letters in the string.
*18.15 (Occurrences of a specified character in a string) Rewrite Programming Exercise 18.10 using a helper method to pass the substring high index to the method. The helper method header is: public static int count(String str, char a, int high)
*18.16 (Find the number of uppercase letters in an array) Write a recursive method to return the number of uppercase letters in an array of characters. You need to define the following two methods. The second one is a recursive helper method. public static int count(char[] chars) public static int count(char[] chars, int high)
Write a test program that prompts the user to enter a list of characters in one line and displays the number of uppercase letters in the list.
*18.17 (Occurrences of a specified character in an array) Write a recursive method that finds the number of occurrences of a specified character in an array. You need to define the following two methods. The second one is a recursive helper method. public static int count(char[] chars, char ch) public static int count(char[] chars, char ch, int high)
Programming Exercises 731 Write a test program that prompts the user to enter a list of characters in one line, and a character, and displays the number of occurrences of the character in the list.
Sections 18.6–18.10
*18.18 (Tower of Hanoi) Modify Listing 18.8, TowerOfHanoi.java, so that the program finds the number of moves needed to move n disks from tower A to tower B. (Hint: Use a static variable and increment it every time the method is called.) *18.19 (Sierpinski triangle) Revise Listing 18.9 to develop a program that lets the user use the + and – buttons to increase or decrease the current order by 1, as shown in Figure 18.12a. The initial order is 0. If the current order is 0, the Decrease button is ignored.
(a)
(b)
FIGURE 18.12 (a) Programming Exercise 18.19 uses the + and – buttons to increase or decrease the current order by 1. (b) Programming Exercise 18.20 draws ovals using a recursive method.
*18.20 (Display circles) Write a Java program that displays ovals, as shown in Figure 18.12b. The circles are centered in the pane. The gap between two adjacent circles is 10 pixels, and the gap between the border of the pane and the largest circle is also 10. *18.21 (Decimal to binary) Write a recursive method that converts a decimal number into a binary number as a string. The method header is: public static String dec2Bin(int value)
Write a test program that prompts the user to enter a decimal number and displays its binary equivalent.
*18.22 (Decimal to hex) Write a recursive method that converts a decimal number into a hex number as a string. The method header is: public static String dec2Hex(int value)
Write a test program that prompts the user to enter a decimal number and displays its hex equivalent. *18.23 (Binary to decimal) Write a recursive method that parses a binary number as a string into a decimal integer. The method header is: public static int bin2Dec(String binaryString)
Write a test program that prompts the user to enter a binary string and displays its decimal equivalent.
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Recursion *18.24 (Hex to decimal) Write a recursive method that parses a hex number as a string into a decimal integer. The method header is: public static int hex2Dec(String hexString)
Write a test program that prompts the user to enter a hex string and displays its decimal equivalent.
**18.25 (String permutation) Write a recursive method to print all the permutations of a string. For example, for the string abc, the permuation is abc acb bac bca cab cba
(Hint: Define the following two methods. The second is a helper method.) public static void displayPermutation(String s) public static void displayPermutation(String s1, String s2)
The first method simply invokes displayPermutation(" ", s). The second method uses a loop to move a character from s2 to s1 and recursively invokes it with a new s1 and s2. The base case is that s2 is empty and prints s1 to the console. Write a test program that prompts the user to enter a string and displays all its permutations. **18.26 (Create a maze) Write a program that will find a path in a maze, as shown in Figure 18.13a. The maze is represented by an 8 * 8 board. The path must meet the following conditions: ■
The path is between the upper-left corner cell and the lower-right corner cell in the maze.
(a) Correct path
(b) Illegal path
FIGURE 18.13 The program finds a path from the upper-left corner to the bottom-right corner.
Programming Exercises 733 ■
■
The program enables the user to place or remove a mark on a cell. A path consists of adjacent unmarked cells. Two cells are said to be adjacent if they are horizontal or vertical neighbors, but not if they are diagonal neighbors. The path does not contain cells that form a square. The path in Figure 18.13b, for example, does not meet this condition. (The condition makes a path easy to identify on the board.)
**18.27 (Koch snowflake fractal) The text presented the Sierpinski triangle fractal. In this exercise, you will write a program to display another fractal, called the Koch snowflake, named after a famous Swedish mathematician. A Koch snowflake is created as follows: 1. Begin with an equilateral triangle, which is considered to be the Koch fractal of order (or level) 0, as shown in Figure 18.14a. 2. Divide each line in the shape into three equal line segments and draw an outward equilateral triangle with the middle line segment as the base to create a Koch fractal of order 1, as shown in Figure 18.14b. 3. Repeat Step 2 to create a Koch fractal of order 2, 3, . . . , and so on, as shown in Figure 18.14c–d.
(a)
FIGURE 18.14
(b)
(c)
(d)
A Koch snowflake is a fractal starting with a triangle.
**18.28 (Nonrecursive directory size) Rewrite Listing 18.7, DirectorySize.java, without using recursion. *18.29 (Number of files in a directory) Write a program that prompts the user to enter a directory and displays the number of the files in the directory.
**18.30 (Find words) Write a program that finds all occurrences of a word in all the files under a directory, recursively. Pass the parameters from the command line as follows: java Exercise18_30 dirName word
**18.31 (Replace words) Write a program that replaces all occurrences of a word with a new word in all the files under a directory, recursively. Pass the parameters from the command line as follows: java Exercise18_31 dirName oldWord newWord
***18.32 (Game: Knight’s Tour) The Knight’s Tour is an ancient puzzle. The objective is to move a knight, starting from any square on a chessboard, to every other square once, as shown in Figure 18.15a. Note that the knight makes only L-shaped moves (two spaces in one direction and one space in a perpendicular direction). As shown in Figure 18.15b, the knight can move to eight squares. Write
VideoNote
Search a string in a directory
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Recursion a program that displays the moves for the knight, as shown in Figure 18.15c. When you click a cell, the knight is placed at the cell. This cell will be starting point for the knight. Clicking the Solve button to display the path for a solution.
0
1
2
3
4
5
6
7
0 1 2 3 4 5 6 7 (a)
(b)
(c)
FIGURE 18.15 (a) A knight traverses all squares once. (b) A knight makes an L-shaped move. (c) A program displays a Knight’s Tour path. (Hint: A brute-force approach for this problem is to move the knight from one square to another available square arbitrarily. Using such an approach, your program will take a long time to finish. A better approach is to employ some heuristics. A knight has two, three, four, six, or eight possible moves, depending on its location. Intuitively, you should attempt to move the knight to the least accessible squares first and leave those more accessible squares open, so there will be a better chance of success at the end of the search.) ***18.33 (Game: Knight’s Tour animation) Write a program for the Knight’s Tour problem. Your program should let the user move a knight to any starting square and click the Solve button to animate a knight moving along the path, as shown in Figure 18.16.
FIGURE 18.16
A knight traverses along the path.
**18.34 (Game: Eight Queens) The Eight Queens problem is to find a solution to place a queen in each row on a chessboard such that no two queens can attack each other. Write a program to solve the Eight Queens problem using recursion and display the result as shown in Figure 18.17.
Programming Exercises 735
FIGURE 18.17
The program displays a solution to the Eight Queens problem.
**18.35 (H-tree fractal) An H-tree (introduced at the beginning of this chapter in Figure 18.1) is a fractal defined as follows: 1. Begin with a letter H. The three lines of the H are of the same length, as shown in Figure 18.1a. 2. The letter H (in its sans-serif form, H) has four endpoints. Draw an H centered at each of the four endpoints to an H-tree of order 1, as shown in Figure 18.1b. These Hs are half the size of the H that contains the four endpoints. 3. Repeat Step 2 to create an H-tree of order 2, 3, . . . , and so on, as shown in Figure 18.1c–d. Write a program that draws an H-tree, as shown in Figure 18.1. 18.36 (Sierpinski triangle) Write a program that lets the user to enter the order and display the filled Sierpinski triangles as shown in Figure 18.18.
FIGURE 18.18
A filled Sierpinski triangle is displayed.
**18.37 (Hilbert curve) The Hilbert curve, first described by German mathematician David Hilbert in 1891, is a space-filling curve that visits every point in a square grid with a size of 2 * 2, 4 * 4, 8 * 8, 16 * 16, or any other power of 2. Write a program that displays a Hilbert curve for the specified order, as shown in Figure 18.19.
736 Chapter 18
(a)
FIGURE 18.19
Recursion
(b)
(c)
(d)
A Hilbert curve with the specified order is drawn.
**18.38 (Recursive tree) Write a program to display a recursive tree as shown in Figure 18.20.
VideoNote
Recursive tree
(a)
FIGURE 18.20
(b)
(c)
(d)
A recursive tree with the specified depth is drawn.
**18.39 (Dragging the tree) Revise Programming Exercise 18.38 to move the tree to where the mouse is dragged.
CHAPTER
19 GENERICS Objectives ■
To descriibe the benefits of generics (§19.2).
■
To use generic classes and interfaces (§19.2).
■
To define generic classes and interfaces (§19.3).
■
To explain why generic types can improve reliability and readability (§19.3).
■
To define and use generic methods and bounded generic types (§19.4).
■
To develop a generic sort method to sort an array of Comparable objects (§19.5).
■
To use raw types for backward compatibility (§19.6).
■
To explain why wildcard generic types are necessary (§19.7).
■
To describe generic type erasure and list certain restrictions and limitations on generic types caused by type erasure (§19.8).
■
To design and implement generic matrix classes (§19.9).
738 Chapter 19
Generics
19.1 Introduction Key Point what is generics?
why generics?
Generics enable you to detect errors at compile time rather than at runtime. You have used a generic class ArrayList in Chapter 11 and generic interface Comparable in Chapter 13. Generics let you parameterize types. With this capability, you can define a class or a method with generic types that the compiler can replace with concrete types. For example, Java defines a generic ArrayList class for storing the elements of a generic type. From this generic class, you can create an ArrayList object for holding strings and an ArrayList object for holding numbers. Here, strings and numbers are concrete types that replace the generic type. The key benefit of generics is to enable errors to be detected at compile time rather than at runtime. A generic class or method permits you to specify allowable types of objects that the class or method can work with. If you attempt to use an incompatible object, the compiler will detect that error. This chapter explains how to define and use generic classes, interfaces, and methods and demonstrates how generics can be used to improve software reliability and readability. It can be intertwined with Chapter 13, Abstract Classes and Interfaces.
19.2 Motivations and Benefits Key Point
The motivation for using Java generics is to detect errors at compile time. Java has allowed you to define generic classes, interfaces, and methods since JDK 1.5. Several interfaces and classes in the Java API were modified using generics. For example, prior to JDK 1.5 the java.lang.Comparable interface was defined as shown in Figure 19.1a, but since JDK 1.5 it is modified as shown in Figure 19.1b. package java.lang;
package java.lang;
public interface Comparable { public int compareTo(Object o) }
public interface Comparable { public int compareTo(T o) }
(a) Prior to JDK 1.5
(b) JDK 1.5
FIGURE 19.1 The java.lang.Comparable interface was modified in JDK 1.5 with a generic type formal generic type actual concrete type generic instantiation
Here, represents a formal generic type, which can be replaced later with an actual concrete type. Replacing a generic type is called a generic instantiation. By convention, a single capital letter such as E or T is used to denote a formal generic type. To see the benefits of using generics, let us examine the code in Figure 19.2. The statement in Figure 19.2a declares that c is a reference variable whose type is Comparable and invokes the compareTo method to compare a Date object with a string. The code compiles fine, but it has a runtime error because a string cannot be compared with a date.
Comparable c = new Date(); System.out.println(c.compareTo("red"));
Comparable c = new Date(); System.out.println(c.compareTo("red"));
(a) Prior to JDK 1.5
(b) JDK 1.5
FIGURE 19.2 The new generic type detects possible errors at compile time. The statement in Figure 19.2b declares that c is a reference variable whose type is Comparable and invokes the compareTo method to compare a Date object with a string. This code generates a compile error, because the argument passed to the compareTo
19.2 Motivations and Benefits 739 method must be of the Date type. Since the errors can be detected at compile time rather than at runtime, the generic type makes the program more reliable. ArrayList was introduced in Section 11.11, The ArrayList Class. This class has been a generic class since JDK 1.5. Figure 19.3 shows the class diagram for ArrayList before and since JDK 1.5, respectively.
java.util.ArrayList
reliable
java.util.ArrayList
+ArrayList() +add(o: Object): void +add(index: int, o: Object): void +clear(): void +contains(o: Object): boolean
+ArrayList() +add(o: E): void +add(index: int, o: E): void +clear(): void +contains(o: Object): boolean
+get(index:int): Object +indexOf(o: Object): int +isEmpty(): boolean +lastIndexOf(o: Object): int +remove(o: Object): boolean +size(): int +remove(index: int): boolean +set(index: int, o: Object): Object
+get(index:int): E +indexOf(o: Object): int +isEmpty(): boolean +lastIndexOf(o: Object): int +remove(o: Object): boolean +size(): int +remove(index: int): boolean +set(index: int, o: E): E
(a) ArrayList before JDK 1.5
(b) ArrayList since JDK 1.5
FIGURE 19.3 ArrayList is a generic class since JDK 1.5. For example, the following statement creates a list for strings: ArrayList list = new ArrayList<>();
You can now add only strings into the list. For instance,
only strings allowed
list.add("Red");
If you attempt to add a nonstring, a compile error will occur. For example, the following statement is now illegal, because list can contain only strings. list.add(new Integer(1));
Generic types must be reference types. You cannot replace a generic type with a primitive type such as int, double, or char. For example, the following statement is wrong:
generic reference type
ArrayList intList = new ArrayList<>();
To create an ArrayList object for int values, you have to use: ArrayList intList = new ArrayList<>();
You can add an int value to intList. For example, intList.add(5);
Java automatically wraps 5 into new Integer(5). This is called autoboxing, as introduced in Section 10.8, Automatic Conversion between Primitive Types and Wrapper Class Types.
autoboxing
740 Chapter 19
Generics Casting is not needed to retrieve a value from a list with a specified element type, because the compiler already knows the element type. For example, the following statements create a list that contains strings, add strings to the list, and retrieve strings from the list.
no casting needed
1 2 3 4
ArrayList list = new ArrayList<>(); list.add("Red"); list.add("White"); String s = list.get(0); // No casting is needed
Prior to JDK 1.5, without using generics, you would have had to cast the return value to String as: String s = (String)(list.get(0)); // Casting needed prior to JDK 1.5
If the elements are of wrapper types, such as Integer, Double, and Character, you can directly assign an element to a primitive type variable. This is called autounboxing, as introduced in Section 10.8. For example, see the following code:
autounboxing
1 2 3 4 5
ArrayList list = new ArrayList<>(); list.add(5.5); // 5.5 is automatically converted to new Double(5.5) list.add(3.0); // 3.0 is automatically converted to new Double(3.0) Double doubleObject = list.get(0); // No casting is needed double d = list.get(1); // Automatically converted to double
In lines 2 and 3, 5.5 and 3.0 are automatically converted into Double objects and added to list. In line 4, the first element in list is assigned to a Double variable. No casting is necessary, because list is declared for Double objects. In line 5, the second element in list is assigned to a double variable. The object in list.get(1) is automatically converted into a primitive type value.
✓
Check Point
19.1 Are there any compile errors in (a) and (b)? ArrayList dates = new ArrayList(); dates.add(new Date()); dates.add(new String());
ArrayList dates = new ArrayList<>(); dates.add(new Date()); dates.add(new String()); (b) Since JDK 1.5
(a) Prior to JDK 1.5
19.2 What is wrong in (a)? Is the code in (b) correct? ArrayList dates = new ArrayList(); dates.add(new Date()); Date date = dates.get(0);
ArrayList dates = new ArrayList<>(); dates.add(new Date()); Date date = dates.get(0);
(a) Prior to JDK 1.5
(b) Since JDK 1.5
19.3 What are the benefits of using generic types?
19.3 Defining Generic Classes and Interfaces Key Point
A generic type can be defined for a class or interface. A concrete type must be specified when using the class to create an object or using the class or interface to declare a reference variable.
19.3 Defining Generic Classes and Interfaces 741 Let us revise the stack class in Section 11.13, Case Study: A Custom Stack Class, to generalize the element type with a generic type. The new stack class, named GenericStack, is shown in Figure 19.4 and is implemented in Listing 19.1.
GenericStack -list: java.util.ArrayList
An array list to store elements.
+GenericStack()
Creates an empty stack.
+getSize(): int
Returns the number of elements in this stack.
+peek(): E
Returns the top element in this stack.
+pop(): E
Returns and removes the top element in this stack.
+push(o: E): void
Adds a new element to the top of this stack.
+isEmpty(): boolean
Returns true if the stack is empty.
FIGURE 19.4 The GenericStack class encapsulates the stack storage and provides the operations for manipulating the stack.
LISTING 19.1 GenericStack.java 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
public class GenericStack { private java.util.ArrayList list = new java.util.ArrayList<>();
generic type E declared generic array list
public int getSize() { return list.size(); }
getSize
public E peek() { return list.get(getSize() - 1); }
peek
public void push(E o) { list.add(o); }
push
public E pop() { E o = list.get(getSize() - 1); list.remove(getSize() - 1); return o; }
pop
public boolean isEmpty() { return list.isEmpty(); }
isEmpty
@Override public String toString() { return "stack: " + list.toString(); } }
The following example creates a stack to hold strings and adds three strings to the stack: GenericStack stack1 = new GenericStack<>(); stack1.push("London"); stack1.push("Paris"); stack1.push("Berlin");
742 Chapter 19
Generics This example creates a stack to hold integers and adds three integers to the stack: GenericStack stack2 = new GenericStack<>(); stack2.push(1); // autoboxing 1 to new Integer(1) stack2.push(2); stack2.push(3);
benefits of using generic types
Instead of using a generic type, you could simply make the type element Object, which can accommodate any object type. However, using generic types can improve software reliability and readability, because certain errors can be detected at compile time rather than at runtime. For example, because stack1 is declared GenericStack, only strings can be added to the stack. It would be a compile error if you attempted to add an integer to stack1.
Caution generic class constructor
To create a stack of strings, you use new GenericStack() or new GenericStack<>(). This could mislead you into thinking that the constructor of GenericStack should be defined as public GenericStack()
This is wrong. It should be defined as public GenericStack()
Note multiple generic parameters
Occasionally, a generic class may have more than one parameter. In this case, place the parameters together inside the brackets, separated by commas—for example, .
Note inheritance with generics
You can define a class or an interface as a subtype of a generic class or interface. For example, the java.lang.String class is defined to implement the Comparable interface in the Java API as follows: public class String implements Comparable
✓
Check Point
19.4 What is the generic definition for java.lang.Comparable in the Java API? 19.5 Since you create an instance of ArrayList of strings using
new ArrayList(), should the constructor in the ArrayList class be
defined as public ArrayList()
19.6 Can a generic class have multiple generic parameters? 19.7 How do you declare a generic type in a class?
19.4 Generic Methods Key Point generic method
A generic type can be defined for a static method. You can define generic interfaces (e.g., the Comparable interface in Figure 19.1b) and classes (e.g., the GenericStack class in Listing 19.1). You can also use generic types to define generic methods. For example, Listing 19.2 defines a generic method print (lines 10–14) to print an array of objects. Line 6 passes an array of integer objects to invoke the generic print method. Line 7 invokes print with an array of strings.
19.4 Generic Methods 743
LISTING 19.2 GenericMethodDemo.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
public class GenericMethodDemo { public static void main(String[] args ) { Integer[] integers = {1, 2, 3, 4, 5}; String[] strings = {"London", "Paris", "New York", "Austin"}; GenericMethodDemo.print(integers); GenericMethodDemo.print(strings); } public static void print(E[] list) { for (int i = 0; i < list.length; i++) System.out.print(list[i] + " "); System.out.println(); }
generic method
}
To declare a generic method, you place the generic type immediately after the keyword static in the method header. For example,
declare a generic method
public static void print(E[] list)
To invoke a generic method, prefix the method name with the actual type in angle brackets. For example,
invoke generic method
GenericMethodDemo.print(integers); GenericMethodDemo.print(strings);
or simply invoke it as follows: print(integers); print(strings);
In the latter case, the actual type is not explicitly specified. The compiler automatically discovers the actual type. A generic type can be specified as a subtype of another type. Such a generic type is called bounded. For example, Listing 19.3 revises the equalArea method in Listing 13.4, TestGeometricObject.java, to test whether two geometric objects have the same area. The bounded generic type (line 7) specifies that E is a generic subtype of GeometricObject. You must invoke equalArea by passing two instances of GeometricObject.
bounded generic type
LISTING 19.3 BoundedTypeDemo.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14
public class BoundedTypeDemo { public static void main(String[] args ) { Rectangle rectangle = new Rectangle(2, 2); Circle circle = new Circle(2);
Rectangle in Listing 13.3 Circle in Listing 13.2
System.out.println("Same area? " + equalArea(rectangle, circle)); } public static boolean equalArea( E object1, E object2) { return object1.getArea() == object2.getArea(); } }
bounded generic type
744 Chapter 19
Generics Note An unbounded generic type is the same as .
Note To define a generic type for a class, place it after the class name, such as GenericStack. To define a generic type for a method, place the generic type before the method return type, such as void max(E o1, E o2).
generic class parameter vs. generic method parameter
✓
Check Point
19.8 How do you declare a generic method? How do you invoke a generic method? 19.9 What is a bounded generic type?
19.5 Case Study: Sorting an Array of Objects Key Point
You can develop a generic method for sorting an array of Comparable objects. This section presents a generic method for sorting an array of Comparable objects. The objects are instances of the Comparable interface, and they are compared using the compareTo method. To test the method, the program sorts an array of integers, an array of double numbers, an array of characters, and an array of strings. The program is shown in Listing 19.4.
LISTING 19.4 GenericSort.java
sort Integer objects sort Double objects sort Character objects sort 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
public class GenericSort { public static void main(String[] args) { // Create an Integer array Integer[] intArray = {new Integer(2), new Integer(4), new Integer(3)}; // Create a Double array Double[] doubleArray = {new Double(3.4), new Double(1.3), new Double(-22.1)}; // Create a Character array Character[] charArray = {new Character('a'), new Character('J'), new Character('r')}; // Create a String array String[] stringArray = {"Tom", "Susan", "Kim"}; // Sort the arrays sort(intArray); sort(doubleArray); sort(charArray); sort(stringArray); // Display the sorted arrays System.out.print("Sorted Integer objects: "); printList(intArray); System.out.print("Sorted Double objects: "); printList(doubleArray); System.out.print("Sorted Character objects: "); printList(charArray); System.out.print("Sorted String objects: "); printList(stringArray); }
19.5 Case Study: Sorting an Array of Objects 745 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
/** Sort an array of comparable objects */ public static > void sort(E[] list) { E currentMin; int currentMinIndex;
generic sort method
for (int i = 0; i < list.length - 1; i++) { // Find the minimum in the list[i+1..list.length-2] currentMin = list[i]; currentMinIndex = i; for (int j = i + 1; j < list.length; j++) { if (currentMin.compareTo(list[j]) > 0) { currentMin = list[j]; currentMinIndex = j; } }
compareTo
// Swap list[i] with list[currentMinIndex] if necessary; if (currentMinIndex != i) { list[currentMinIndex] = list[i]; list[i] = currentMin; } } } /** Print an array of objects */ public static void printList(Object[] list) { for (int i = 0; i < list.length; i++) System.out.print(list[i] + " "); System.out.println(); } }
Sorted Sorted Sorted Sorted
Integer objects: 2 3 4 Double objects: -22.1 1.3 3.4 Character objects: J a r String objects: Kim Susan Tom
The algorithm for the sort method is the same as in Listing 7.8, SelectionSort.java. The sort method in that program sorts an array of double values. The sort method in this example can sort an array of any object type, provided that the objects are also instances of the Comparable interface. The generic type is defined as > (line 36). This has two meanings. First, it specifies that E is a subtype of Comparable. Second, it specifies that the elements to be compared are of the E type as well. The sort method uses the compareTo method to determine the order of the objects in the array (line 46). Integer, Double, Character, and String implement Comparable, so the objects of these classes can be compared using the compareTo method. The program creates arrays of Integer objects, Double objects, Character objects, and String objects (lines 4–16) and invoke the sort method to sort these arrays (lines 19–22).
19.10 Given int[]
list = {1, 2, -1}, can you invoke sort(list) using the sort method in Listing 19.4? 19.11 Given int[] list = {new Integer(1), new Integer(2), new Integer(-1)}, can you invoke sort(list) using the sort method in Listing 19.4?
✓
Check Point
746 Chapter 19
Generics
19.6 Raw Types and Backward Compatibility Key Point
A generic class or interface used without specifying a concrete type, called a raw type, enables backward compatibility with earlier versions of Java. You can use a generic class without specifying a concrete type like this: GenericStack stack = new GenericStack(); // raw type
This is roughly equivalent to GenericStack stack = new GenericStack();
raw type backward compatibility
A generic class such as GenericStack and ArrayList used without a type parameter is called a raw type. Using raw types allows for backward compatibility with earlier versions of Java. For example, a generic type has been used in java.lang.Comparable since JDK 1.5, but a lot of code still uses the raw type Comparable, as shown in Listing 19.5:
LISTING 19.5 Max.java raw type
1 2 3 4 5 6 7 8 9
public class Max { /** Return the maximum of two objects */ public static Comparable max(Comparable o1, Comparable o2) { if (o1.compareTo(o2) > 0) return o1; else return o2; } }
Comparable o1 and Comparable o2 are raw type declarations. Be careful: raw types are unsafe. For example, you might invoke the max method using Max.max("Welcome", 23); // 23 is autoboxed into new Integer(23)
Xlint:unchecked
This would cause a runtime error, because you cannot compare a string with an integer object. The Java compiler displays a warning on line 3 when compiled with the option –Xlint:unchecked, as shown in Figure 19.5.
FIGURE 19.5
The unchecked warnings are displayed using the compiler option
–Xlint:unchecked.
A better way to write the max method is to use a generic type, as shown in Listing 19.6.
LISTING 19.6 MaxUsingGenericType.java bounded type
1 2 3 4 5 6
public class MaxUsingGenericType { /** Return the maximum of two objects */ public static > E max(E o1, E o2) { if (o1.compareTo(o2) > 0) return o1; else
19.7 Wildcard Generic Types 747 7 8 9
return o2; } }
If you invoke the max method using // 23 is autoboxed into new Integer(23) MaxUsingGenericType.max("Welcome", 23);
a compile error will be displayed, because the two arguments of the max method in MaxUsingGenericType must have the same type (e.g., two strings or two integer objects). Furthermore, the type E must be a subtype of Comparable. As another example, in the following code you can declare a raw type stack in line 1, assign new GenericStack to it in line 2, and push a string and an integer object to the stack in lines 3 and 4. 1 2 3 4
GenericStack stack; stack = new GenericStack(); stack.push("Welcome to Java"); stack.push(new Integer(2));
However, line 4 is unsafe because the stack is intended to store strings, but an Integer object is added into the stack. Line 3 should be okay, but the compiler will show warnings for both line 3 and line 4, because it cannot follow the semantic meaning of the program. All the compiler knows is that stack is a raw type, and performing certain operations is unsafe. Therefore, warnings are displayed to alert potential problems.
Tip Since raw types are unsafe, this book will not use them from here on.
19.12 What is a raw type? Why is a raw type unsafe? Why is the raw type allowed in Java? 19.13 What is the syntax to declare an ArrayList reference variable using the raw type and assign a raw type ArrayList object to it?
✓
Check Point
19.7 Wildcard Generic Types You can use unbounded wildcards, bounded wildcards, or lower-bound wildcards to specify a range for a generic type.
Key Point
What are wildcard generic types and why are they needed? Listing 19.7 gives an example to demonstrate the needs. The example defines a generic max method for finding the maximum in a stack of numbers (lines 12–22). The main method creates a stack of integer objects, adds three integers to the stack, and invokes the max method to find the maximum number in the stack.
LISTING 19.7 WildCardNeedDemo.java 1 2 3 4 5 6 7 8 9 10
public class WildCardNeedDemo { public static void main(String[] args ) { GenericStack intStack = new GenericStack<>(); intStack.push(1); // 1 is autoboxed into new Integer(1) intStack.push(2); intStack.push(-2); System.out.print("The max number is " + max(intStack)); }
GenericStack
type
748 Chapter 19 GenericStack
type
unbounded wildcard bounded wildcard lower-bound wildcard
Generics 11 12 13 14 15 16 17 18 19 20 21 22 23
/** Find the maximum in a stack of numbers */ public static double max(GenericStack stack) { double max = stack.pop().doubleValue(); // Initialize max while (!stack.isEmpty()) { double value = stack.pop().doubleValue(); if (value > max) max = value; } return max; } }
The program in Listing 19.7 has a compile error in line 8 because intStack is not an instance of GenericStack. Thus, you cannot invoke max(intStack). The fact is that Integer is a subtype of Number, but GenericStack is not a subtype of GenericStack. To circumvent this problem, use wildcard generic types. A wildcard generic type has three forms: ? and ? extends T, as well as ? super T, where T is a generic type. The first form, ?, called an unbounded wildcard, is the same as ? extends Object. The second form, ? extends T, called a bounded wildcard, represents T or a subtype of T. The third form, ? super T, called a lower-bound wildcard, denotes T or a supertype of T. You can fix the error by replacing line 12 in Listing 19.7 as follows: public static double max(GenericStack stack) {
is a wildcard type that represents Number or a subtype of Number, so it is legal to invoke max(new GenericStack()) or max(new GenericStack()). Listing 19.8 shows an example of using the ? wildcard in the print method that prints objects in a stack and empties the stack. is a wildcard that represents any object type. It is equivalent to . What happens if you replace GenericStack with GenericStack? It would be wrong to invoke print(intStack), because intStack is not an instance of GenericStack. Please note that GenericStack is not a subtype of GenericStack, even though Integer is a subtype of Object.
LISTING 19.8 AnyWildCardDemo.java GenericStack
type
wildcard type
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
public class AnyWildCardDemo { public static void main(String[] args ) { GenericStack intStack = new GenericStack<>(); intStack.push(1); // 1 is autoboxed into new Integer(1) intStack.push(2); intStack.push(-2); print(intStack); } /** Prints objects and empties the stack */ public static void print(GenericStack stack) { while (!stack.isEmpty()) { System.out.print(stack.pop() + " "); } } }
19.7 Wildcard Generic Types 749 When is the wildcard needed? Consider the example in Listing 19.9. The example creates a stack of strings in stack1 (line 3) and a stack of objects in stack2 (line 4), and invokes add(stack1, stack2) (line 8) to add the strings in stack1 into stack2. GenericStack is used to declare stack2 in line 13. If is replaced by , a compile error will occur on add(stack1, stack2) in line 8, because stack1’s type is GenericStack and stack2’s type is GenericStack. represents type T or a supertype of T. Object is a supertype of String.
why
LISTING 19.9 SuperWildCardDemo.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
public class SuperWildCardDemo { public static void main(String[] args) { GenericStack stack1 = new GenericStack<>(); GenericStack stack2 = new GenericStack<>(); stack2.push("Java"); stack2.push(2); stack1.push("Sun"); add(stack1, stack2); AnyWildCardDemo.print(stack2); }
GenericStack
type
public static void add(GenericStack stack1, GenericStack stack2) { while (!stack1.isEmpty()) stack2.push(stack1.pop()); }
type
}
This program will also work if the method header in lines 12–13 is modified as follows: public static void add(GenericStack stack1, GenericStack stack2)
The inheritance relationship involving generic types and wildcard types is summarized in Figure 19.6. In this figure, A and B represent classes or interfaces, and E is a generic type parameter. Object
?
Object
E’s superclass
? super E
E
E’s subclass
A
A
? extends E
A
A
A
A
FIGURE 19.6 The relationship between generic types and wildcard types.
19.14 Is GenericStack the same as GenericStack? 19.15 What are an unbounded wildcard, a bounded wildcard, and a lower-bound wildcard? 19.16 What happens if lines 12–13 in Listing 19.9 are changed to public static 
