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Growing Object-Oriented Software, Guided by Tests

By Steve Freeman, Nat Pryce

Published by Addison-Wesley Professional

Published Date: Oct 12, 2009


Foreword by Kent Beck


"The authors of this book have led a revolution in the craft of programming by controlling the environment in which software grows.” --Ward Cunningham


“At last, a book suffused with code that exposes the deep symbiosis between TDD and OOD. This one's a keeper.” --Robert C. Martin


“If you want to be an expert in the state of the art in TDD, you need to understand the ideas in this book.”--Michael Feathers


Test-Driven Development (TDD) is now an established technique for delivering better software faster. TDD is based on a simple idea: Write tests for your code before you write the code itself. However, this "simple" idea takes skill and judgment to do well. Now there's a practical guide to TDD that takes you beyond the basic concepts. Drawing on a decade of experience building real-world systems, two TDD pioneers show how to let tests guide your development and “grow” software that is coherent, reliable, and maintainable.


Steve Freeman and Nat Pryce describe the processes they use, the design principles they strive to achieve, and some of the tools that help them get the job done. Through an extended worked example, you’ll learn how TDD works at multiple levels, using tests to drive the features and the object-oriented structure of the code, and using Mock Objects to discover and then describe relationships between objects. Along the way, the book systematically addresses challenges that development teams encounter with TDD--from integrating TDD into your processes to testing your most difficult features. Coverage includes


•   Implementing TDD effectively: getting started, and maintaining your momentum

    throughout the project

•   Creating cleaner, more expressive, more sustainable code

•   Using tests to stay relentlessly focused on sustaining quality

•   Understanding how TDD, Mock Objects, and Object-Oriented Design come together

    in the context of a real software development project

•   Using Mock Objects to guide object-oriented designs

•   Succeeding where TDD is difficult: managing complex test data, and testing persistence

    and concurrency


Table of Contents

Foreword     xv

Preface     xvii

Acknowledgments     xxi

About the Authors     xxiii




Chapter 1: What Is the Point of Test-Driven Development?     3

Software Development as a Learning Process     3

Feedback Is the Fundamental Tool     4

Practices That Support Change     5

Test-Driven Development in a Nutshell     6

The Bigger Picture     7

Testing End-to-End     8

Levels of Testing     9

External and Internal Quality     10


Chapter 2: Test-Driven Development with Objects     13

A Web of Objects     13

Values and Objects     13

Follow the Messages     14

Tell, Don’t Ask     17

But Sometimes Ask     17

Unit-Testing the Collaborating Objects     18

Support for TDD with Mock     19


Chapter 3: An Introduction to the Tools     21

Stop Me If You’ve Heard This One Before     21

A Minimal Introduction to JUnit 4     21

Hamcrest Matchers and assertThat()     24

jMock2: Mock Objects     25




Chapter 4: Kick-Starting the Test-Driven Cycle     31

Introduction     31

First, Test a Walking Skeleton     32

Deciding the Shape of the Walking Skeleton     33

Build Sources of Feedback     35

Expose Uncertainty Early     36


Chapter 5: Maintaining the Test-Driven Cycle     39

Introduction     39

Start Each Feature with an Acceptance Test     39

Separate Tests That Measure Progress from Those That Catch Regressions     40

Start Testing with the Simplest Success Case     41

Write the Test That You’d Want to Read     42

Watch the Test Fail     42

Develop from the Inputs to the Outputs     43

Unit-Test Behavior, Not Methods     43

Listen to the Tests     44

Tuning the Cycle     45


Chapter 6: Object-Oriented Style     47

Introduction     47

Designing for Maintainability     47

Internals vs. Peers     50

No And’s, Or’s, or But’s     51

Object Peer Stereotypes     52

Composite Simpler Than the Sum of Its Parts     53

Context Independence     54

Hiding the Right Information     55

An Opinionated View     56


Chapter 7: Achieving Object-Oriented Design     57

How Writing a Test First Helps the Design     57

Communication over Classification     58

Value Types     59

Where Do Objects Come From?     60

Identify Relationships with Interfaces     63

Refactor Interfaces Too     63

Compose Objects to Describe System Behavior     64

Building Up to Higher-Level Programming     65

And What about Classes?     67


Chapter 8: Building on Third-Party Code     69

Introduction     69

Only Mock Types That You Own     69

Mock Application Objects in Integration Tests     71




Chapter 9: Commissioning an Auction Sniper     75

To Begin at the Beginning     75

Communicating with an Auction     78

Getting There Safely     79

This Isn’t Real     81


Chapter 10: The Walking Skeleton     83

Get the Skeleton out of the Closet     83

Our Very First Test     84

Some Initial Choices     86


Chapter 11: Passing the First Test     89

Building the Test Rig     89

Failing and Passing the Test     95

The Necessary Minimum     102


Chapter 12: Getting Ready to Bid     105

An Introduction to the Market     105

A Test for Bidding     106

The AuctionMessageTranslator     112

Unpacking a Price Message     118

Finish the Job     121


Chapter 13: The Sniper Makes a Bid     123

Introducing AuctionSniper     123

Sending a Bid     126

Tidying Up the Implementation     131

Defer Decisions     136

Emergent Design     137


Chapter 14: The Sniper Wins the Auction     139

First, a Failing Test     139

Who Knows about Bidders?     140

The Sniper Has More to Say     143

The Sniper Acquires Some State     144

The Sniper Wins     146

Making Steady Progress     148


Chapter 15: Towards a Real User Interface     149

A More Realistic Implementation     149

Displaying Price Details     152

Simplifying Sniper Events     159

Follow Through     164

Final Polish     168

Observations     171


Chapter 16: Sniping for Multiple Items     175

Testing for Multiple Items     175

Adding Items through the User Interface     183

Observations     189


Chapter 17: Teasing Apart Main     191

Finding a Role     191

Extracting the Chat     192

Extracting the Connection     195

Extracting the SnipersTableModel     197

Observations     201


Chapter 18: Filling In the Details     205

A More Useful Application     205

Stop When We’ve Had Enough     205

Observations     212


Chapter 19: Handling Failure     215

What If It Doesn’t Work?     215

Detecting the Failure     217

Displaying the Failure     218

Disconnecting the Sniper     219

Recording the Failure     221

Observations     225




Chapter 20: Listening to the Tests     229

Introduction     229

I Need to Mock an Object I Can’t Replace (without Magic)     230

Logging Is a Feature     233

Mocking Concrete Classes     235

Don’t Mock Values     237

Bloated Constructor     238

Confused Object     240

Too Many Dependencies     241

Too Many Expectations     242

What the Tests Will Tell Us (If We’re Listening)     244


Chapter 21: Test Readability     247

Introduction     247

Test Names Describe Features     248

Canonical Test Structure     251

Streamline the Test Code     252

Assertions and Expectations     254

Literals and Variables     255


Chapter 22: Constructing Complex Test Data     257

Introduction     257

Test Data Builders     258

Creating Similar Objects     259

Combining Builders     261

Emphasizing the Domain Model with Factory Methods     261

Removing Duplication at the Point of Use     262

Communication First     264


Chapter 23: Test Diagnostics     267

Design to Fail     267

Small, Focused, Well-Named Tests     268

Explanatory Assertion Messages     268

Highlight Detail with Matchers     268

Self-Describing Value     269

Obviously Canned Value     270

Tracer Object     270

Explicitly Assert That Expectations Were Satisfied     271

Diagnostics Are a First-Class Feature     271


Chapter 24: Test Flexibility     273

Introduction     273

Test for Information, Not Representation     274

Precise Assertions     275

Precise Expectations     277

“Guinea Pig” Objects     284




Chapter 25: Testing Persistence     289

Introduction     289

Isolate Tests That Affect Persistent State     290

Make Tests Transaction Boundaries Explicit     292

Testing an Object That Performs Persistence Operations     294

Testing That Objects Can Be Persisted     297

But Database Tests Are S-l-o-w!     300


Chapter 26: Unit Testing and Threads     301

Introduction     301

Separating Functionality and Concurrency Policy     302

Unit-Testing Synchronization     306

Stress-Testing Passive Objects     311

Synchronizing the Test Thread with Background Threads     312

The Limitations of Unit Stress Tests     313


Chapter 27: Testing Asynchronous Code     315

Introduction     315

Sampling or Listening     316

Two Implementations     318

Runaway Tests     322

Lost Updates     323

Testing That an Action Has No Effect     325

Distinguish Synchronizations and Assertions     326

Externalize Event Sources     326


Afterword: A Brief History of Mock Objects     329

Appendix A: jMock2 Cheat Sheet     335

Appendix B: Writing a Hamcrest Matcher     343

Bibliography     347

Index     349



Growing Object-Oriented Software, Guided by Tests

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