Introduction From the Author

Design patterns were originally delineated in the seminal work of the “Gang of Four”: Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides. They, in turn, were responding to an earlier work by the architect Christopher Alexander, The Timeless Way of Building. The Gang of Four's work, Design Patterns: Elements of Reusable Object-Oriented Software, received a lot of attention when it was originally published in 1994.

Some have suggested, however, that the industry has moved beyond the patterns in the more than two decades that have passed since its publication, and that in the age of agile processes and test-driven development, the patterns are no longer relevant. They say that patterns represent an old view, namely “big design up front.”

I disagree. Not only do I think the patterns are relevant today, I believe they are far more relevant than they were when the work was originally published. Here's why:

Patterns form a rich language of design. This is all the more important now that nearly all software projects are highly collaborative. We need reliable ways to communicate and make group decisions. Nearly all complex human activity involves specialized language: medicine, law, engineering, etc. They all have their own rich nomenclature to describe what is being proposed, or what has been done.

Patterns all follow the principles of good design in different ways. This means they are highly changeable without decay. The agile movement says, among other things, that we must embrace change. The patterns help us to do this.

This book is not meant to teach you the patterns; see the References section for books that do. This is meant to be a field guide for those who are already pattern practitioners, something to refer to as part of your day-to-day activities.