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Objects First with Java

8553 Prelims (i-xxviii) 27/8/02 11:01 am Page i

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8553 Prelims (i-xxviii) 27/8/02 11:01 am Page ii

Objects First with Java

A Practical Introduction using BlueJ

8553 Prelims (i-xxviii) 27/8/02 11:01 am Page iii

Pearson Education Limited

Edinburgh Gate

Harlow

Essex CM20 2JE

and Associated Companies throughout the world

Visit us on the World Wide Web at:

www.pearsoneduc.com

First published 2003

© Pearson Education Limited 2003

been asserted by them in accordance with the Copyright, Designs and Patents Act 1988. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without either the prior written permission of the publisher or a licence permitting restricted copying in the united Kingdom issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London W1P 0LP. The programs in this book have been included for their instructional value. They have been tested with care but are not guaranteed for any particular purpose. The publisher does not offer any warranties or representations nor does it accept any liabilities with respect to the programs. Many of the designations used by manufacturers and sellers to distinguish their products are claimed as trademarks. Pearson Education has made every attempt to supply trademark information about manufacturers and their products mentioned in this book. Java™ and Sun™ are trademarks or registered trademarks of Sun Microsystems Inc. Star Wars is a registered trademark of Lucasfilm Ltd.

ISBN 0130 44929 6

British Library Cataloguing-in-Publication Data

A catalogue record for this book is available from the British Library

10 9 8 7 6 5 4 3 2 1

07 06 05 04 03

Typeset in 10/12pt TimesNewRomanPS by 30

Printed by Ashford Colour Press Ltd., Gosport

8553 Prelims (i-xxviii) 27/8/02 11:01 am Page iv

To my family

Helen, John, Hannah, Ben and Sarah

djb

To my family

Leah, Sophie and Feena

mk

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8553 Prelims (i-xxviii) 27/8/02 11:01 am Page vi

Foreword by James Gosling, Sun Microsystems xvii

Preface to the instructor xviii

List of projects discussed in detail in this book xxv

Acknowledgements xxvii

Part 1 Foundations of object orientation 1

Chapter 1 Objects and classes 3

1.1 Objects and classes 3

1.2 Creating objects 4

1.3 Calling methods 5

1.4 Parameters 6

1.5 Data types 7

1.6 Multiple instances 8

1.7 State8

1.8 What is in an object? 9

1.9 Object interaction 10

1.10 Source code 11

1.11 Another example 13

1.12 Return values 13

1.13 Objects as parameters 13

1.14 Summary 15

Chapter 2 Understanding class definitions 17

2.1 Ticket machines 17

2.1.1 Exploring the behavior of a naïve ticket machine 18

2.2 Examining a class definition 19

2.3 Fields, constructors and methods 21

2.3.1 Fields 22

Contents

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2.3.2 Constructors 24

2.4 Passing data via parameters 25

2.5 Assignment 27

2.6 Accessor methods 27

2.7 Mutator methods 29

2.7 Printing from methods 31

2.9 Summary of the naïve ticket machine 33

2.10 Reflecting on the design of the ticket machine 34

2.11 Making choices: the conditional statement 35

2.12 A further conditional-statement example 38

2.13 Local variables 39

2.14 Fields, parameters and local variables 40

2.15 Summary of the better ticket machine 41

2.16 Reviewing a familiar example 42

2.17 Summary 45

Chapter 3 Object interaction 49

3.1 The clock example 49

3.2 Abstraction and modularization 50

3.3 Abstraction in software 51

3.4 Modularization in the clock example 51

3.5 Implementing the clock display 52

3.6 Class diagrams versus object diagrams 53

3.7 Primitive types and object types 54

3.8 The

ClockDisplaysource code 56

3.8.1 Class

NumberDisplay56

3.8.2 String concatenation 58

3.8.3 The modulo operator 59

3.8.4 Class

ClockDisplay59

3.9 Objects creating objects 62

3.10 Multiple constructors 64

3.11 Method calls 64

3.11.1 Internal method calls 64

3.11.2 External method calls 65

3.11.3 Summary of the clock display 66

3.12 Another example of object interaction 67

3.12.1 The mail system example 67

3.12.2 The

thiskey word 68

3.13 Using a debugger 70

3.13.1 Setting breakpoints 71viii

Contents

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3.13.2 Single stepping 72

3.13.3 Stepping into methods 74

3.14 Method calling revisited 74

3.15 Summary 75

Chapter 4 Grouping objects 77

4.1 Grouping objects in flexible-size collections 77

4.2 A personal notebook 78

4.3 A first look at library classes 78

4.3.1 An example of using a library 79

4.4 Object structures with collections 80

4.5 Numbering within collections 82

4.6 Removing an item from a collection 83

4.7 Processing a whole collection 84

4.7.1 The while loop 85

4.7.2 Iterating over a collection 87

4.7.3 Index access versus iterators 88

4.8 Summary of the notebook example 88

4.9 Another example: an auction system 89

4.9.1 The

Lotclass 89

4.9.2 The

Auctionclass 90

4.9.3 Casting 92

4.10 Fixed-size collections 94

4.10.1 A log-file analyzer 94

4.10.2 Declaring array variables 96

4.10.3 Creating array objects 97

4.10.4 Using array objects 98

4.10.5 Analyzing the log file 99

4.10.6 The for loop 100

4.11 Summary 103

Chapter 5 More sophisticated behavior 105

5.1 Documentation for library classes 106

5.2 The TechSupport system 106

5.2.1 Exploring the TechSupport system 107

5.2.2 Reading the code 108

5.3 Reading class documentation 112

5.3.1 Interfaces versus implementation 113

5.3.2 Using library-class methods 114

Contentsix

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5.3.3 Checking string equality 115

5.4 Adding random behavior 116

5.4.1 The

Randomclass 117

5.4.2 Random numbers with limited range 118

5.4.3 Generating random responses 119

5.5 Packages and import 121

5.6 Using maps for associations 122

5.6.1 The concept of a map 123

5.6.2 Using a

HashMap123

5.6.3 Using a map for the TechSupport system 124

5.7 Using sets 126

5.8 Tokenizing strings 127

5.9 Finishing the TechSupport system 129

5.10 Writing class documentation 130

5.10.1 Using

javadocin BlueJ 131

5.10.2 Elements of class documentation 131

5.11 Public versus private 133

5.11.1 Information hiding 133

5.11.2 Private methods and public fields 134

5.12 Learning about classes from their interfaces 135

5.13 Class variables and constants 138

5.13.1 The

statickey word 138

5.13.2 Constants 139

5.14 Summary 140

Chapter 6 Well-behaved objects 143

6.1 Testing and debugging 144

6.2 Unit testing within BlueJ 144

6.2.1 Using inspectors 148

6.2.2 Positive versus negative testing 150

6.3 Test automation 150

6.3.1 Regression testing 150

6.3.2 Automated checking of test results 153

6.4 Modularization and interfaces 154

6.5 A debugging scenario 156

6.6 Commenting and style 157

6.7 Manual walkthroughs 158

6.7.1 A high-level walkthrough 158

6.7.2 Checking state with a walkthrough 160

6.7.3 Verbal walkthroughs 162

6.8 Print statements 163x

Contents

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6.8.1 Turning debugging information on or off 165

6.9 Debuggers 166

6.10 Putting the techniques into practice 167

6.11 Summary 167

Chapter 7 Designing classes 169

7.1 Introduction 170

7.2 The world-of-zuul game example 171

7.3 Introduction to coupling and cohesion 173

7.4 Code duplication 174

7.5 Making extensions 177

7.5.1 The task 177

7.5.2 Finding the relevant source code 177

7.6 Coupling 179

7.6.1 Using encapsulation to reduce coupling 179

7.7 Responsibility-driven design 183

7.7.1 Responsibilities and coupling 184

7.8 Localizing change 186

7.9 Implicit coupling 186

7.10 Thinking ahead 189

7.11 Cohesion 190

7.11.1 Cohesion of methods 190

7.11.2 Cohesion of classes 191

7.11.3 Cohesion for readability 192

7.11.4 Cohesion for reuse 193

7.12 Refactoring 194

7.12.1 Refactoring and testing 194

7.12.2 An example of refactoring 195

7.13 Design guidelines 197

7.14 Executing without BlueJ 199

7.14.1 Class methods 199

7.14.2 The main method 199

7.14.3 Limitations of class methods 200

7.15 Summary 200

Part 2 Application Structures 203

Chapter 8 Improving structure with inheritance 205

8.1 The DoME example 205

Contentsxi

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8.1.1 DoME classes and objects 206

8.1.2 DoME source code 208

8.1.3 Discussion of the DoME application 214

8.2 Using Inheritance 215

8.3 Inheritance hierarchies 216

8.4 Inheritance in Java 217

8.4.1 Inheritance and access rights 218

8.4.2 Inheritance and initialization 218

8.5 DoME: adding other item types 220

8.6 Advantages of inheritance (so far) 222

8.7 Subtyping 223

8.7.1 Subclasses and subtypes 224

8.7.2 Subtyping and assignment 225

8.7.3 Subtyping and parameter passing 226

8.7.4 Polymorphic variables 227

8.8 The

Objectclass 227

8.9 Polymorphic collections 228

8.9.1 Element types 229

8.9.2 Casting revisited 229

8.9.3 Wrapper classes 230

8.10 The collection hierarchy 231

8.11 Summary 232

Chapter 9 More about inheritance 235

9.1 The problem: DoME's print method 235

9.2 Static type and dynamic type 237

9.2.1 Calling

printfrom Database238

9.3 Overriding 239

9.4 Dynamic method lookup 241

9.5 Super call in methods 244

9.6 Method polymorphism 245

9.7 Object methods:

toString245

9.8 Protected access 248

9.9 Another example of inheritance with overriding 250

9.10 Summary 253

Chapter 10 Further abstraction techniques 255

10.1 Simulations 255

10.2 The foxes-and-rabbits simulation 256xii

Contents

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10.2.1 The foxes-and-rabbits project 257

10.2.2 The

Rabbitclass 259

10.2.3 The

Foxclass 262

10.2.4 The

Simulatorclass: setup 265

10.2.5 The

Simulatorclass: a simulation step 268

10.2.6 Taking steps to improve the simulation 270

10.3 Abstract classes 270

10.3.1 The

Animalsuperclass 271

10.3.2 Abstract methods 271

10.3.3 Abstract classes 274

10.4 More abstract methods 275

10.5 Multiple inheritance 277

10.5.1 An

Actorclass 277

10.5.2 Flexibility through abstraction 278

10.5.3 Selective drawing 279

10.5.4 Drawable actors: multiple inheritance 280

10.6 Interfaces 280

10.6.1 An

Actorinterface 280

10.6.2 Multiple inheritance of interfaces 281

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