Java Design Patterns : Definition

We all talk about the way we do things in our jobs, hobbies, and home life, and we recognize repeating patterns all the time.

• Sticky buns are like dinner rolls, but I add brown sugar and nut filling to them.

• Her front garden is like mine, but I grow as till be in my garden.

• This end table is constructed like that one, but in this one, there are doors instead of drawers.

We see the same thing in programming when we tell a colleague how we accomplished a tricky bit of programming so he doesn't have to recreate it from scratch. We simply recognize effective ways for objects to communicate while maintaining their own separate existences.

Some useful definitions of design patterns have emerged as the literature in this field has expanded.

• Design patterns are recurring solutions to design problems you see over and over. (The Smalltalk Companion)

• Design patterns constitute a set of rules describing how to accomplish certain tasks in the realm of software development. (Pree 1995)

• Design patterns focus more on reuse of recurring architectural design themes, while frameworks focus on detailed design and implementation. (Coplien and Schmidt 1995)

• A pattern addresses a recurring design problem that arises in specific design situations and presents a solution to it. (Buschmann et al. 1996)

• Patterns identify and specify abstractions that are above the level of single classes and instances or of components. (Gamma, Johnson, and Vlissides, 1993)

But while it is helpful to draw analogies to architecture, cabinetmaking, and logic, design patterns are not just about the design of objects but about the interaction between objects. One possible view of some of these patterns is to consider them as communication patterns.

Some other patterns deal not just with object communication but with strategies for object inheritance and containment. It is the design of simple, but elegant, methods of interaction that makes many design patterns so important.

Design patterns can exist at many levels from very low-level, specific solutions to broadly generalized system issues. There are now hundreds of patterns in the literature. They have been discussed in articles and at conferences of all levels of granularity. Some are examples that apply widely, and a few writers have ascribed pattern behavior to class groupings that apply to just a single problem (Kurata 1998).

It has become apparent that you don't just write a design pattern off the top of your head. In fact, most such patterns are discovered rather than written. The process of looking for these patterns is called "pattern mining," and it is worthy of a book of its own.