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Design patterns are a cornerstone of modern software development. Using them can improve the maintainability of your Java applications and help you accomplish other tasks more effectively. Jeff Friesen's three-part introduction to design patterns takes a wide-angle view, from the GoF patterns to design patterns used in interaction design, team organization, and more. Part 1 introduces the concept and use of design patterns, offers some design pattern history and trivia, and ends with a listing of software design patterns by classification. Included with this article is a complete index of design pattern tutorials on JavaWorld.
Numerous strategies have been devised to simplify and reduce the costs of designing software, especially in the area of maintenance. Learning how to identify and work with reusable software components (occasionally referred to as software integrated circuits) is one strategy. Using design patterns is another.
This article launches a three-part series on design patterns. In this part I'll introduce the conceptual framework of design patterns and walk through a demonstration of evaluating a design pattern for a particular use case. I'll also discuss the history of design patterns, including some interesting trivia. Finally, I'll classify and summarize the most-used software design patterns that have been discovered and documented through the past couple of decades.
Designing reusable object-oriented software that models an existing system is genuinely challenging. A software developer must factor the system's entities into classes whose public interfaces aren't overly complex, establish relationships among classes, expose inheritance hierarchies, and more. Because most software remains in use long after it was written, software developers also need to address current application requirements while keeping their code and infrastructure flexible enough to meet future needs.
Experienced object-oriented developers have discovered that software design patterns facilitate the coding of stable and robust software systems. Reusing these design patterns rather than constantly developing new solutions from scratch is efficient, and it reduces some of the risk of error. Every design pattern identifies a recurring design problem in a specific application context, then offers a generalized, reusable solution that is applicable to different application scenarios.
"A design pattern describes the classes and interacting objects used to solve a general design problem in a specific context."
Some developers define a design pattern as a class-encoded entity (such as a linked list or bit vector), whereas others say that a design pattern is in the entire application or subsystem. My view is that a design pattern describes the classes and interacting objects used to solve a general design problem in a specific context. More formally, a design pattern is specified as a description that consists of four basic elements:
David Geary's Java design patterns series is an excellent first stop for learning about some of the Gang of Four patterns mentioned in this article:
Allen Holub wrote about several important concurrency patterns and other design techniques for his Java toolbox series, "Programming Java in the real world":
Brian Goetz's two JavaWorld articles about double-checked locking are essential reading for anyone inclined to put too much faith in an out-of-the-box solution:
Design patterns are discussed in JavaWorld's Java 101, Java tips and Java Q&A series:
Some patterns have become more important with time:
Others have stood the test of time:
Additional resources for learning about design patterns: