Foreword

One of the core truisms of software development is that as your code grows and requirements for the system that you are building change, additional logic will be added that is not yet present in the current system. In almost all cases, maintainability over the life of the code is more important than optimizing its present state.

The promise of using object-oriented (OO) design is that your code will be easier to maintain and evolve than otherwise. If you are new to programming, how do you unlock these secrets to maintainability using OO? The fact is that many of us have never had holistic training in writing clean object-oriented code, instead picking up our techniques through osmosis from colleagues and a myriad of older books and online sources. Or if we were given a primer in OO during school, it was done in languages such as Java or C++. (The lucky ones were taught using Smalltalk!)

Sandi Metz’s Practical Object-Oriented Design in Ruby covers all of the basics of OO using the Ruby language, meaning that it’s ready to usher countless Ruby and Rails newcomers to the next steps in their professional development as mature programmers.

Ruby itself, like Smalltalk, is a completely object-oriented (OO) language. Everything in it, even primitive data constructs such as strings and numbers, is represented by objects with behavior. When you write your own applications in Ruby, you do so by coding your own objects, each encapsulating some state and defining its own behavior. If you don’t already have OO experience, it can feel daunting to know how to start the process. This book guides you every step of the way, from the most basic questions of what to put in a class, through basic concepts such as the Single Responsibility Principle, all the way through to making tradeoffs between inheritance and composition, and figuring out how to test objects in isolation.

The best part, though, is Sandi’s voice. She’s got a ton of experience and is one of the nicest members of the community you’ll ever meet, and I think she did a great job getting that feeling across in her writing. I’ve known Sandi for several years now, and I wondered if her manuscript would live up to the pleasure of actually getting to know Sandi in real life. I’m glad to say that it does, in spades, which is why I’m glad to welcome her as our newest author to the Professional Ruby Series.

—Obie Fernandez, Series Editor

Addison Wesley Professional Ruby Series