Structural Patterns - Applied Java Patterns - page 89

Description

Sometimes you'd like to use a class in a new framework without recoding it to match the new environment. In
such cases, you can design an Adaptee class to act as a translator. This class receives calls from the environment
and modifies the calls to be compatible with the Adaptee class.

Typical environments where an Adapter can be useful include applications that support plug-in behavior like
graphics, text, or media editors. Web browsers are another environment where the Adapter can be useful.
Applications involving internationalization (language conversion, for example) might benefit from Adapters, as
well as those that use components (such as JavaBeans™) that are added on the fly.

A real-world example of the Adapter is a foreign language phrase book. The phrase book translates common
expressions (messages) from one language to another, enabling two otherwise incompatible individuals to
communicate with each other. This assumes, of course, that the phrase book translates the messages properly
based on the context: “I need to buy some matches,” for example, might not map correctly to “My hovercraft is
full of eels” in all circumstances.

Implementation

The Adapter class diagram interface is shown in Figure 3.1 .

Figure 3.1. Adapter class diagram interface

Implementing the Adapter pattern requires the following:

Framework - The Framework uses the Adapter . It either constructs the ConcreteAdapter or it gets set
somewhere.

Adapter - The interface that defines the methods the Framework uses.

ConcreteAdapter - An implementation of the Adapter interface. It keeps a reference to the Adaptee and
translates the method calls from the Framework into method calls on the Adaptee. This translation also possibly
involves wrapping or modifying parameters and return types.

Adaptee - The interface that defines the methods of the type that will be adapted. This interface allows the
specific Adaptee to be dynamically loaded at runtime.

ConcreteAdaptee - An implementation of the Adaptee interface. The class that needs to be adapted so that the
Framework can use this class.

For more complex communication, it can be useful to establish an action map to better understand how to manage
the communication. An action map is a table showing how the Adapter maps methods and call parameters
between the caller and adaptee. Table 3-1 shows the mapping for a single method.

Table 3-1. Example action map

Framework Adapter Action Adaptee

method1 None method2

argument1 None a rgument1

argument2 wrapper a rgument2

create a rgument3

The Unified Modeling Language (UML) can effectively represent this; typically, the sequence diagram is a useful
tool for action mapping. The Adapter sequence diagram for action mapping is shown in Figure 3.2 .