| The concept of design patterns did not necessarily start with the need for reusable software. In fact, the seminal work on design patterns is about constructing buildings and cities. As Christopher Alexander noted in A Pattern Language: Towns, Buildings, Construction : "Each pattern describes a problem which occurs over and over again in our environment, and then describes the core of the solution to that problem, in such a way that you can use the solution a million times over, without ever doing it the same way twice." The Four Elements of a Pattern The GoF describe a pattern as having four essential elements: -
The pattern name is a handle we can use to describe a design problem, its solutions, and consequences in a word or two. Naming a pattern immediately increases our design vocabulary. It lets us design at a higher level of abstraction. Having a vocabulary for patterns lets us talk about them with our colleagues, in our documentation, and even to ourselves . It makes it easier to think about designs and to communicate them and their tradeoff to others. Finding good names has been one of the hardest parts of developing our catalog. -
The problem describes when to apply the pattern. It explains the problem and its content. It might describe specific design problems, such as how to represent algorithms as objects. It might describe class or object structures that are symptomatic of an inflexible design. Sometimes the problem will include a list of conditions that must be met before it makes sense to apply the pattern. -
The solution describes the elements that make up the design, their relationships, responsibilities, and collaborations. The solution doesn't describe a particular concrete design or implementation, because a pattern is like a template that can be applied in many different situations. Instead, the pattern provides an abstract description of a design problem, and how a general arrangement of elements (classes and objects in our case) solves it. -
The consequences are the results and tradeoffs of applying the pattern. Although consequences are often unvoiced, when we describe design decisions, they are critical for evaluating design alternatives and for understanding the costs and benefits of the applying pattern. The consequences for software often concern space and time tradeoffs. They might address language and implementation issues as well. Because reuse is often a factor in object-oriented design, the consequences of a pattern include its impact on a system's flexibility, extensibility, or portability. Listing the consequences explicitly helps you understand and evaluate them.
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