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The Definitive Refactoring Guide, Fully Revamped for Ruby

With refactoring, programmers can transform even the most chaotic software into well-designed systems that are far easier to evolve and maintain. What’s more, they can do it one step at a time, through a series of simple, proven steps. Now, there’s an authoritative and extensively updated version of Martin Fowler’s classic refactoring book that utilizes Ruby examples and idioms throughout–not code adapted from Java or any other environment.

The authors introduce a detailed catalog of more than 70 proven Ruby refactorings, with specific guidance on when to apply each of them, step-by-step instructions for using them, and example code illustrating how they work. Many of the authors’ refactorings use powerful Ruby-specific features, and all code samples are available for download.

Leveraging Fowler’s original concepts, the authors show how to perform refactoring in a controlled, efficient, incremental manner, so you methodically improve your code’s structure without introducing new bugs. Whatever your role in writing or maintaining Ruby code, this book will be an indispensable resource.

This book will help you

•    Understand the core principles of refactoring and the reasons for doing it

•    Recognize “bad smells” in your Ruby code

•    Rework bad designs into well-designed code, one step at a time

•    Build tests to make sure your refactorings work properly

•    Understand the challenges of refactoring and how they can be overcome

•    Compose methods to package code properly

•    Move features between objects to place responsibilities where they fit best

•    Organize data to make it easier to work with

•    Simplify conditional expressions and make more effective use of polymorphism

•    Create interfaces that are easier to understand and use

•    Generalize more effectively

•    Perform larger refactorings that transform entire software systems and may take months or years

•    Successfully refactor Ruby on Rails code

Refactoring: Ruby Edition: Ruby Edition

Foreword . . . xiii

Preface . . . xv

Acknowledgments . . . xx

About the Authors . . . xxii

Chapter 1: Refactoring, a First Example . . . 1

The Starting Point . . . 2

The First Step in Refactoring . . . 6

Decomposing and Redistributing the Statement Method . . . 7

Replacing the Conditional Logic on Price Code with Polymorphism . . . 32

Final Thoughts . . . 50

Chapter 2: Principles in Refactoring . . . 51

Where Did Refactoring Come From? . . . 51

Defining Refactoring . . . 52

Why Should You Refactor? . . . 54

When Should You Refactor? . . . 57

Why Refactoring Works . . . 60

What Do I Tell My Manager? . . . 61

Indirection and Refactoring . . . 61

Problems with Refactoring . . . 63

Refactoring and Design. . . . 67

It Takes A While to Create Nothing . . . 69

Refactoring and Performance . . . 70

Optimizing a Payroll System . . . 71

Chapter 3: Bad Smells in Code . . . 73

Duplicated Code . . . 74

Long Method . . . 74

Large Class . . . 76

Long Parameter List . . . 76

Divergent Change . . . 77

Shotgun Surgery . . . 78

Feature Envy . . . 78

Data Clumps . . . 79

Primitive Obsession . . . 79

Case Statements . . . 80

Parallel Inheritance Hierarchies . . . 81

Lazy Class . . . 81

Speculative Generality . . . 81

Temporary Field . . . 82

Message Chains . . . 82

Middle Man . . . 83

Inappropriate Intimacy . . . 83

Alternative Classes with Different Interfaces . . . 83

Incomplete Library Class . . . 84

Data Class . . . 84

Refused Bequest . . . 84

Comments . . . 85

Metaprogramming Madness . . . 86

Disjointed API . . . 86

Repetitive Boilerplate . . . 86

Chapter 4: Building Tests . . . 87

The Value of Self-Testing Code . . . 87

The Test::Unit Testing Framework . . . 88

Developer and Quality Assurance Tests . . . 91

Adding More Tests . . . 92

Chapter 5: Toward a Catalog of Refactorings . . . 97

Format of the Refactorings . . . 97

Finding References . . . 99

Chapter 6: Composing Methods . . . 101

Extract Method . . . 102

Inline Method . . . 108

Inline Temp . . . 110

Replace Temp with Query. . . 111

Replace Temp with Chain . . . 114

Introduce Explaining Variable . . . 117

Split Temporary Variable . . . 121

Remove Assignments to Parameters . . . 124

Replace Method with Method Object . . . 127

Substitute Algorithm . . . 131

Replace Loop with Collection Closure Method . . . 133

Extract Surrounding Method . . . 135

Introduce Class Annotation . . . 139

Introduce Named Parameter . . . 142

Remove Named Parameter . . . 147

Remove Unused Default Parameter . . . 150

Dynamic Method Definition . . . 152

Replace Dynamic Receptor with Dynamic Method Definition . . . 158

Isolate Dynamic Receptor . . . 160

Move Eval from Runtime to Parse Time . . . 165

Chapter 7: Moving Features Between Objects . . . 167

Move Method . . . 167

Move Field . . . 172

Extract Class . . . 175

Inline Class . . . 179

Hide Delegate . . . 181

Remove Middle Man . . . 185

Chapter 8: Organizing Data . . . 187

Self Encapsulate Field . . . 188

Replace Data Value with Object . . . 191

Change Value to Reference . . . 194

Change Reference to Value . . . 198

Replace Array with Object . . . 201

Replace Hash with Object . . . 206

Change Unidirectional Association to Bidirectional . . . 210

Change Bidirectional Association to Unidirectional . . . 213

Replace Magic Number with Symbolic Constant . . . 217

Encapsulate Collection . . . 219

Replace Record with Data Class . . . 224

Replace Type Code with Polymorphism . . . 225

Replace Type Code with Module Extension . . . 232

Replace Type Code with State/Strategy . . . 239

Replace Subclass with Fields . . . 251

Lazily Initialized Attribute . . . 255

Eagerly Initialized Attribute . . . 257

Chapter 9: Simplifying Conditional Expressions . . . 261

Decompose Conditional . . . 261

Recompose Conditional . . . 264

Consolidate Conditional Expression . . . 265

Consolidate Duplicate Conditional Fragments . . . 268

Remove Control Flag . . . 269

Replace Nested Conditional with Guard Clauses . . . 274

Replace Conditional with Polymorphism . . . 279

Introduce Null Object . . . 284

Introduce Assertion . . . 292

Chapter 10: Making Method Calls Simpler . . . 297

Rename Method . . . 298

Add Parameter . . . 300

Remove Parameter . . . 302

Separate Query from Modifier . . . 303

Parameterize Method . . . 307

Replace Parameter with Explicit Methods . . . 310

Preserve Whole Object . . . 313

Replace Parameter with Method . . . 317

Introduce Parameter Object . . . 320

Remove Setting Method . . . 324

Hide Method . . . 327

Replace Constructor with Factory Method . . . 328

Replace Error Code with Exception . . . 332

Replace Exception with Test . . . 337

Introduce Gateway . . . 341

Introduce Expression Builder . . . 346

Chapter 11: Dealing with Generalization . . . 353

Pull Up Method . . . 353

Push Down Method . . . 356

Extract Module . . . 357

Inline Module . . . 362

Extract Subclass . . . 363

Introduce Inheritance . . . 368

Collapse Heirarchy . . . 371

Form Template Method . . . 372

Replace Inheritance with Delegation . . . 386

Replace Delegation with Hierarchy . . . 389

Replace Abstract Superclass with Module . . . 392

Chapter 12: Big Refactorings . . . 397

The Nature of the Game . . . 397

Why Big Refactorings Are Important . . . 398

Four Big Refactorings . . . 398

Tease Apart Inheritance . . . 399

Convert Procedural Design to Objects . . . 405

Separate Domain from Presentation . . . 406

Extract Hierarchy . . . 412

Chapter 13: Putting It All Together . . . 417

References . . . 421

Index . . . 423