All well-structured object-oriented architectures are full of patterns. Indeed, one of the ways that I measure the quality of an object-oriented system is to judge whether or not its developers have paid careful attention to the common collaborations among its objects. Focusing on such mechanisms during a system’s development can yield an architecture that is smaller, simpler, and far more understandable than if these patterns are ignored.

The importance of patterns in crafting complex systems has been long recognized in other disciplines. In particular, Christopher Alexander and his colleagues were perhaps the first to propose the idea of using a pattern language to architect buildings and cities. His ideas and the contributions of others have now taken root in the object-oriented software community. In short, the concept of the design pattern in software provides a key to helping developers leverage the expertise of other skilled architects.

In this book, Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides introduce the principles of design patterns and then offer a catalog of such patterns. Thus, this book makes two important contributions. First, it shows the role that patterns can play in architecting complex systems. Second, it provides a very pragmatic reference to a set of well-engineered patterns that the practicing developer can apply to crafting his or her own specific applications.

I’m honored to have had the opportunity to work directly with some of the authors of this book in architectural design efforts. I have learned much from them, and I suspect that in reading this book, you will also.

Grady Booch

Chief Scientist, Rational Software Corporation