Review of Refactoring to Patterns by Joshua Kerievsky

Wednesday, 12 April 2006

Refactoring To Patterns brings together the Patterns movement, and the practice of Refactoring commonplace in the Agile community. Whereas the original Gang of Four book told us what patterns were, what sort of problems they solved, and how the code might be structured, Refactoring To Patterns illustrates how, why and when to introduce patterns into an existing codebase.

The opening chapters cover the background, introducing both refactoring and design patterns, and the context in which the book was written. This gives the reader a clear overview of what is involved in Refactoring to Patterns, and paves the way for the refactoring catalogue which makes up the bulk of the book.

The catalogue is divided into chapters based on the type of change required — is this a refactoring to simplify code, generalize code, or increase encapsulation and protection? Each chapter has an introduction which gives an overview of the refactorings contained within that chapter, followed by the refactorings themselves. These introductions clearly illustrate the principles and choices which would lead one to follow the refactorings that follow.

Each refactoring starts with a brief one sentence summary, and before and after structure diagrams with reference to the structure diagrams for the relevant pattern in the Design Patterns book. The sections that follow then cover the Motivation for using this refactoring, step-by-step Mechanics, and a worked Example, relating back to the steps given for the Mechanics. Finally, some of the refactorings finish with Variations on the same theme. The examples are all pulled from a small sample of projects, which are introduced at the beginning of the catalogue section, and help illuminate the instructions given in the Mechanics section. The mechanics themselves are generally clear, and broken down into small steps — sometimes smaller steps than I might take in practice, but I think this is probably wise, as large steps can easily confuse. Finally, the Motivation sections do a good job of explaining why one would choose to do a particular refactoring, and any pitfalls to doing so — the "Benefits and Liabilities" tables provide a useful summary.

This book is well written, easy to read, and genuinely useful. It has helped me put some of the refactorings I do into a larger context, and given me insight into how I can integrate patterns with existing code, rather than designing them in up front. As John Brant and Don Roberts highlight in their Afterword, this is a book to study, the real benefit comes not from knowing the mechanics, but by understanding the motivation, and the process, so that one may apply the same thinking to other scenarios not covered by this book. If you are serious about software development, buy this book, inwardly digest it, and keep it by your side.

Highly Recommended.

Buy this book

Refactoring to Patterns
Joshua Kerievsky
Published by Addison-Wesley
ISBN 0-321-21335-1

Buy from Amazon.co.uk
Buy from Amazon.com

Posted by Anthony Williams
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