Design Patterns:Elements of Rousable Object-Oriented Software Preface to Book 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 some experience 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 in object-oriented software design.Design patterns capture solutions that have developed and evolved overtime.Hence they aren't the designs people tend to generate initially.They rerlect 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 require neither unusual language features nor amazing programming tricks with which to astound your friends and managers.All can be implemented instandard object-oriented languages,though they might take a little nore work the on extra effor iably pays divider in increased flexibility and reusability. Once you understand the design patterns and have had an "Aha!"(and not just a "Huh?")experience with them,you won't ever think about obiect-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 object-oriented technology in the first place,right? A word of warning and encouragement:Don't worry if you don't understand this book completely on the 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'11 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 had a par in its development.Special thanks are due Bruce Anderson,Kent Beck,and Andre Weinand for their inspiration and advice.We also thank those who reviewed drafta ot the manuscript:Roger Bieleteld,Grady Booch,Tom Carqill,Marshall cline, Ralph Hyre,Brian Kernighan,Thomas Laliberty, nz,Arthur Riel,Doug 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, 7Design Patterns: Elements of Reusable Object-Oriented Software 7 Preface to Book 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 some experience 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 in object-oriented software design. Design patterns capture solutions that have developed and evolved overtime. 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 require neither 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!" (and not 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 object-oriented technology in the first place, right? A word of warning and encouragement: Don't worry if you don’t understand this book completely on the 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 had a 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, andRebecca Wirfs-Brock. We are also 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