"Kent is a master at creating code that communicates well, is easy to understand, and is a pleasure to read. Every chapter of this book contains excellent explanations and insights into the smaller but important decisions we continuously have to make when creating quality code and classes."

-Erich Gamma, IBM Distinguished Engineer

"Many teams have a master developer who makes a rapid stream of good decisions all day long. Their code is easy to understand, quick to modify, and feels safe and comfortable to work with. If you ask how they thought to write something the way they did, they always have a good reason. This book will help you become the master developer on your team. The breadth and depth of topics will engage veteran programmers, who will pick up new tricks and improve on old habits, while the clarity makes it accessible to even novice developers."

-Russ Rufer, Silicon Valley Patterns Group

"Many people don't realize how readable code can be and how valuable that readability is. Kent has taught me so much, I'm glad this book gives everyone the chance to learn from him."

-Martin Fowler, chief scientist, ThoughtWorks

"Code should be worth reading, not just by the compiler, but by humans. Kent Beck distilled his experience into a cohesive collection of implementation patterns. These nuggets of advice will make your code truly worth reading."

-Gregor Hohpe, author of Enterprise Integration Patterns

"In this book Kent Beck shows how writing clear and readable code follows from the application of simple principles. Implementation Patterns will help developers write intention revealing code that is both easy to understand and flexible towards future extensions. A must read for developers who are serious about their code."

-Sven Gorts

"Implementation Patterns bridges the gap between design and coding. Beck introduces a new way of thinking about programming by basing his discussion on values and principles."

-Diomidis Spinellis, author of Code Reading and Code Quality

Software Expert Kent Beck Presents a Catalog of Patterns Infinitely Useful for Everyday Programming

Great code doesn't just function: it clearly and consistently communicates your intentions, allowing other programmers to understand your code, rely on it, and modify it with confidence. But great code doesn't just happen. It is the outcome of hundreds of small but critical decisions programmers make every single day. Now, legendary software innovator Kent Beck-known worldwide for creating Extreme Programming and pioneering software patterns and test-driven development-focuses on these critical decisions, unearthing powerful "implementation patterns" for writing programs that are simpler, clearer, better organized, and more cost effective.

Beck collects 77 patterns for handling everyday programming tasks and writing more readable code. This new collection of patterns addresses many aspects of development, including class, state, behavior, method, collections, frameworks, and more. He uses diagrams, stories, examples, and essays to engage the reader as he illuminates the patterns. You'll find proven solutions for handling everything from naming variables to checking exceptions.

This book covers

• The value of communicating through code and the philosophy behind patterns
• How and when to create classes, and how classes encode logic
• Best practices for storing and retrieving state
• Behavior: patterns for representing logic, including alternative paths
• Writing, naming, and decomposing methods
• Choosing and using collections
• Implementation pattern variations for use in building frameworks

Implementation Patterns will help programmers at all experience levels, especially those who have benefited from software patterns or agile methods. It will also be an indispensable resource for development teams seeking to work together more efficiently and build more maintainable software. No other programming book will touch your day-to-day work more often.

Kent Beck, one of the software industry's most creative and acclaimed leaders, consistently challenges software engineering dogma and promotes ideas like patterns, test-driven development, and Extreme Programming. Currently affiliated with Three Rivers Institute and Agitar Software, he is the author of many Addison-Wesley titles, including Test-Driven Development (2003) and, with Cynthia Andres, Extreme Programming Explained, Second Edition (2005).

Preface xv

Chapter 1: Introduction 1

Tour Guide 3

And Now... 4

Chapter 2: Patterns 5

Chapter 3: A Theory of Programming 9

Values 10

Principles 13

Conclusion 18

Chapter 4: Motivation 19

Chapter 5: Class 21

Class 22

Simple Superclass Name 23

Qualified Subclass Name 24

Abstract Interface 24

Interface 26

Abstract Class 26

Versioned Interface 27

Value Object 28

Specialization 31

Subclass 32

Implementor 34

Inner Class 34

Instance-Specific Behavior 36

Conditional 36

Delegation 38

Pluggable Selector 40

Anonymous Inner Class 41

Library Class 41

Conclusion 42

Chapter 6: State 43

State 44

Access 45

Direct Access 46

Indirect Access 47

Common State 47

Variable State 48

Extrinsic State 50

Variable 50

Local Variable 51

Field 52

Parameter 53

Collecting Parameter 55

Optional Parameter 56

Var Args 56

Parameter Object 57

Constant 58

Role-Suggesting Name 58

Declared Type 60

Initialization 61

Eager Initialization 61

Lazy Initialization 62

Conclusion 62

Chapter 7: Behavior 63

Control Flow 64

Main Flow 64

Message 65

Choosing Message 65

Double Dispatch 66

Decomposing (Sequencing) Message 67

Reversing Message 67

Inviting Message 68

Explaining Message 69

Exceptional Flow 70

Guard Clause 70

Exception 72

Checked Exceptions 72

Exception Propagation 73

Conclusion 73

Chapter 8: Methods 75

Composed Method 77

Intention-Revealing Name 79

Method Visibility 80

Method Object 82

Overridden Method 83

Overloaded Method 83

Method Return Type 84

Method Comment 85

Helper Method 85

Debug Print Method 86

Conversion 87

Conversion Method 87

Conversion Constructor 88

Creation 88

Complete Constructor 89

Factory Method 90

Internal Factory 91

Collection Accessor Method 91

Boolean Setting Method 93

Query Method 93

Equality Method 94

Getting Method 95

Setting Method 96

Safe Copy 97

Conclusion 98

Chapter 9: Collections 99

Metaphors 100

Issues 101

Interfaces 103

Implementations 107

Collections 110

Extending Collections 114

Conclusion 115

Chapter 10: Evolving Frameworks 117

Changing Frameworks without Changing Applications 117

Incompatible Upgrades 118

Encouraging Compatible Change 120

Conclusion 129

Appendix A: Performance Measurement 131

Example 131

API 132

Implementation 133

MethodTimer 134

Canceling Overhead 136

Tests 136

Conclusion 142

Bibliography 145

General Programming 145

Philosophy 147

Java 148

Index 149