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