The Adapter Pattern

Overview

Unlike most of the patterns we reviewed in the previous chapter, 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 preexisting objects to work together, even if their interfaces are not compatible. Like the display adapters that allow us to plug our Micro USB charging cable into a USB-C phone, an adapter object sits between two different interfaces, translating between them on the fly. The adapter object’s sole purpose is to perform this translation. Adapting 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. This is depicted in UML form in the following diagram: