It turns out that everything you need to know about Object-Oriented Design (OOD) can be
learned from the "99 Bottles of Beer" song.
Well, perhaps not everything, but quite certainly a great many things.
The song is simultaneously easy to understand and full of hidden complexity, which makes it the
perfect skeleton upon which to hang lessons in OOD. The lessons embedded within the song are
so useful, and so broad, that over the last three years it has become a core part of the curriculum
of Sandi Metz’s Practical Object-Oriented Design course.
The thoughts in this book reflect countless hours of discussion and collaboration between Sandi,
Katrina Owen, and TJ Stankus. These ideas have been battle-tested by hundreds of students, and
refined by a series of deeply thoughtful co-instructors, beginning with Katrina. While none of the
authors have the hubris to claim perfect understanding, all have learned a great deal about
Object-Oriented Design from teaching this song, and feel compelled to write it all down.
Therefore, this book, now in its second edition. We hope that you find it both useful and
enjoyable.
Author(s): Sandi Metz, Katrina Owen, TJ Stankus
Edition: 2
Publisher: Potato Canyon Software, LLC
Year: 2020
Language: English
Pages: 499
Colophon
Your Rights As A Reader
Dedication
Preface
What This Book Is About
Who Should Read This Book
Before You Read This Book
How To Read This Book
Code Examples
Errata
About the Authors
About the Translators
Introduction
1. Rediscovering Simplicity
1.1. Simplifying Code
1.1.1. Incomprehensibly Concise
1.1.2. Speculatively General
1.1.3. Concretely Abstract
1.1.4. Shameless Green
1.2. Judging Code
1.2.1. Evaluating Code Based on Opinion
1.2.2. Evaluating Code Based on Facts
1.2.3. Comparing Solutions
1.3. Summary
2. Test Driving Shameless Green
2.1. Understanding Testing
2.2. Writing the First Test
2.3. Removing Duplication
2.4. Tolerating Duplication
2.5. Hewing to the Plan
2.6. Exposing Responsibilities
2.7. Choosing Names
2.8. Revealing Intentions
2.9. Writing Cost-Effective Tests
2.10. Avoiding the Echo-Chamber
2.11. Considering Options
2.12. Summary
3. Unearthing Concepts
3.1. Listening to Change
3.2. Starting With the Open/Closed Principle
3.3. Recognizing Code Smells
3.4. Identifying the Best Point of Attack
3.5. Refactoring Systematically
3.6. Following the Flocking Rules
3.7. Converging on Abstractions
3.7.1. Focusing on Difference
3.7.2. Simplifying Hard Problems
3.7.3. Naming Concepts
3.7.4. Making Methodical Transformations
3.7.5. Refactoring Gradually
3.8. Summary
4. Practicing Horizontal Refactoring
4.1. Replacing Difference With Sameness
4.2. Equivocating About Names
4.3. Deriving Names From Responsibilities
4.4. Choosing Meaningful Defaults
4.5. Seeking Stable Landing Points
4.6. Obeying the Liskov Substitution Principle
4.7. Taking Bigger Steps
4.8. Discovering Deeper Abstractions
4.9. Depending on Abstractions
4.10. Summary
5. Separating Responsibilities
5.1. Selecting the Target Code Smell
5.1.1. Identifying Patterns in Code
5.1.2. Spotting Common Qualities
5.1.3. Enumerating Flocked Method Commonalities
5.1.4. Insisting Upon Messages
5.2. Extracting Classes
5.2.1. Modeling Abstractions
5.2.2. Naming Classes
5.2.3. Extracting BottleNumber
5.2.4. Removing Arguments
5.2.5. Trusting the Process
5.3. Appreciating Immutability
5.4. Assuming Fast Enough
5.5. Creating BottleNumbers
5.6. Recognizing Liskov Violations
5.7. Summary
6. Achieving Openness
6.1. Consolidating Data Clumps
6.2. Making Sense of Conditionals
6.3. Replacing Conditionals with Polymorphism
6.3.1. Dismembering Conditionals
6.3.2. Manufacturing Objects
6.3.3. Prevailing with Polymorphism
6.4. Transitioning Between Types
6.5. Making the Easy Change
6.6. Defending the Domain
6.7. Summary
7. Manufacturing Intelligence
7.1. Contrasting the Concrete Factory with Shameless Green
7.2. Fathoming Factories
7.3. Opening the Factory
7.4. Supporting Arbitrary Class Names
7.5. Dispersing The Choosing Logic
7.6. Self-registering Candidates
7.7. Auto-registering Candidates
7.8. Summary
8. Developing a Programming Aesthetic
8.1. Appreciating the Mechanical Process
8.2. Clarifying Responsibilities with Pseudocode
8.3. Extracting the Verse
8.4. Coding by Wishful Thinking
8.5. Inverting Dependencies
8.5.1. Injecting Dependencies
8.5.2. Isolating Variants
8.5.3. Grappling with Inversion
8.6. Obeying the Law of Demeter
8.6.1. Understanding the Law
8.6.2. Curing Demeter Violations
8.7. Identifying What The Verse Method Wants
8.8. Pushing Object Creation to the Edge
8.9. Summary
9. Reaping the Benefits of Design
9.1. Choosing Which Units to Test
9.1.1. Contrasting Unit and Integration Tests
9.1.2. Foregoing Tests
9.2. Reorganizing Tests
9.2.1. Gathering BottleVerse Tests
9.2.2. Revealing Intent
9.3. Seeking Context Independence
9.3.1. Examining Bottles' Responsibilities
9.3.2. Purifying Tests With Fakes
9.3.3. Purging Redundant Tests
9.3.4. Profiting from Loose Coupling
9.4. Communicating With the Future
9.4.1. Enriching Code with Signals
9.4.2. Verifying Roles
9.4.3. Obliterating Obsolete Context
9.5. Summary
Afterword
Appendix A: Initial Exercise
Getting the exercise
Doing the exercise
Test Suite
References
Acknowledgements
