Use this tag for questions relating to the Adapter design pattern, one of the Gang of Four's structural design patterns. Also consider using the [design-patterns] tag and a programming language tag if applicable.
According to the GoF book (page 139) the purpose of the Adapter design pattern is to,
Convert the interface of a class into another interface clients expect. The adapter lets classes work together that couldn't otherwise because of incompatible interfaces.
There are two different versions of the Adapter design pattern (page 141).
- A class adapter uses multiple inheritance to adapt one interface to another. 2. An object adapter relies on object composition.
There are three scenarios where the Adapter design pattern is applicable (page 140).
- you want to use an existing class, and its interface does not match the one you need. 2. you want to create a reusable class that cooperates with unrelated or unforeseen classes, that is, classes that don't necessarily have compatible interfaces. 3. (object adapter only) you need to use several existing subclasses, but it's unpractical to adapt their interface by subclassing every one. An object adapter can adapt the interface of its parent class.
There are different consequences of applying the Adapter design pattern depending on which version is used (page 142).
Class and object adapters have different trade-offs. A class adapter
- adapts Adaptee to Target by committing to a concrete Adaptee class. As a consequence, a class adapter won't work when we want to adapt a class and all its subclasses.
- lets Adapter override some of Adaptee's behavior, since Adapter is a subclass of Adaptee.
- introduces only one object, and no additional pointer indirection is needed to get to the adaptee.
An object adapter
- lets a single Adapter work with many Adaptees—that is, the Adaptee itself and all of its subclasses (if any). The Adapter can also add functionality to all Adaptees at once.
- makes it harder to override Adaptee behavior. It will require subclassing Adaptee and making Adapter refer to the subclass rather than the Adaptee itself.
For details about the structure and implementation of the Adapter design pattern, see the following online resources.
Note the design-patterns tag encompasses this pattern as well as the other 22 patterns from the GoF book. Consider using any of these tags in combination, as applicable.
