Danny

Design Patterns

Design Patterns
Elements of Reusable Object
Oriented Software

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358

Gamma – Helm - Johnson – Vlissides

Preface
This book isn't an introduction to object-oriented technology or design. Many books already do a good job of
that. This book assumes you are reasonably proficient in at least one object-oriented programming language,
and you should have someexperience in object-oriented design as well. You definitely shouldn't have to rush to
the nearest dictionary the moment we mention "types" and "polymorphism," or "interface" as opposed to
"implementation" inheritance.
On the other hand, this isn't an advanced technical treatise either. It's a book of design patterns that describes
simple and elegant solutions to specific problems inobject-oriented software design. Design patterns capture
solutions that have developed and evolved over time. Hence they aren't the designs people tend to generate
initially. They reflect untold redesign and recoding as developers have struggled for greater reuse and flexibility
in their software. Design patterns capture these solutions in a succinct and easily applied form.
The design patterns requireneither unusual language features nor amazing programming tricks with which to
astound your friends and managers. All can be implemented in standard object-oriented languages, though they
might take a little more work than ad hoc solutions. But the extra effort invariably pays dividends in increased
flexibility and reusability.
Once you understand the design patterns and have had an "Aha!" (andnot just a "Huh?") experience with them,
you won't ever think about object-oriented design in the same way. You'll have insights that can make your own
designs more flexible, modular, reusable, and understandable—which is why you're interested in objectoriented technology in the first place, right?
A word of warning and encouragement: Don't worry if you don't understand this book completely onthe first
reading. We didn't understand it all on the first writing! Remember that this isn't a book to read once and put on
a shelf. We hope you'll find yourself referring to it again and again for design insights and for inspiration.
This book has had a long gestation. It has seen four countries, three of its authors' marriages, and the birth of
two (unrelated) offspring. Many people have hada part in its development. Special thanks are due Bruce
Anderson, Kent Beck, and André Weinand for their inspiration and advice. We also thank those who reviewed
drafts of the manuscript: Roger Bielefeld, Grady Booch, Tom Cargill, Marshall Cline, Ralph Hyre, Brian
Kernighan, Thomas Laliberty, Mark Lorenz, Arthur Riel, Doug Schmidt, Clovis Tondo, Steve Vinoski, and
Rebecca Wirfs-Brock. We arealso grateful to the team at Addison-Wesley for their help and patience: Kate
Habib, Tiffany Moore, Lisa Raffaele, Pradeepa Siva, and John Wait. Special thanks to Carl Kessler, Danny
Sabbah, and Mark Wegman at IBM Research for their unflagging support of this work.
Last but certainly not least, we thank everyone on the Internet and points beyond who commented on versions
of the patterns,offered encouraging words, and told us that what we were doing was worthwhile. These people
include but are not limited to Jon Avotins, Steve Berczuk, Julian Berdych, Matthias Bohlen, John Brant, Allan
Clarke, Paul Chisholm, Jens Coldewey, Dave Collins, Jim Coplien, Don Dwiggins, Gabriele Elia, Doug Felt,
Brian Foote, Denis Fortin, Ward Harold, Hermann Hueni, Nayeem Islam, Bikramjit Kalra, PaulKeefer, Thomas
Kofler, Doug Lea, Dan LaLiberte, James Long, Ann Louise Luu, Pundi Madhavan, Brian Marick, Robert
Martin, Dave McComb, Carl McConnell, Christine Mingins, Hanspeter Mössenböck, Eric Newton, Marianne
Ozkan, Roxsan Payette, Larry Podmolik, George Radin, Sita Ramakrishnan, Russ Ramirez, Alexander Ran,
Dirk Riehle, Bryan Rosenburg, Aamod Sane, Duri Schmidt, Robert Seidl, Xin Shu, and...