Summary | 203Summary
The adapter pattern allows existing classes to be used in situations that didn’t exist
or weren’t anticipated when they were developed. For example, an application can
have a lot of legacy code, code that is no longer supported or maintained, but needs
to be used for various reasons. Adaptor classes use or extend existing code to fit new
requirements. The main focus of the adapter pattern is to convert the interface of the
existing class to one that fits a new context.
Adapter patterns are very close in intent and in function to several other patterns
such as the façade, decorator (see Chapter 4), and bridge. The primary distinctions
are that the adapter requires a target interface to which the adapter provides the
implementation, there has to be an existing class that the adapter uses or extends to
provide the implementation, and there is no requirement to simplify the interface.
There are two types of adapters, and choosing between object and class adapters
highlights the advantages and disadvantages of composition versus inheritance in
implementation. They also illustrate the importance of interfaces, and how they
allow the designer to encapsulate implementation details.