Unlike most of the patterns we reviewed in Chapter 8, the adapter pattern is designed to interact with existing code. We would not design a brand new set of objects that implement the adapter pattern. Adapters are used to allow two pre-existing objects to work together, even if their interfaces are not compatible. Like the keyboard adapters that allow USB keyboards to be plugged into PS/2 ports, an adapter object sits between two different interfaces, translating between them on the fly. The adapter object's sole purpose is to perform this translating job; translating may entail a variety of tasks, such as converting arguments to a different format, rearranging the order of arguments, calling a differently named method, or supplying default arguments.
In structure, the adapter pattern is similar to a simplified decorator pattern. Decorators typically provide the same interface that they replace, whereas adapters map between two different interfaces. Here it is in UML form...