Pattern: Adapter
Synopsis
The adapter pattern is used to convert the interface
of one class into one another interface that the client expects. This allows
classes to collaborate when they may not have been originally designed to
do so. The Adapter pattern is also called a Wrapper.
Context
Suppose you acquire a class that needs to use a service
of some sort and provides an interface for it. Further, you have another class
that provides the service, but does not conform to the required interface.
The Adapter pattern can be used to make the two classes compatible.
Forces
In the typical case, we have a class that we may not
have the software for, or it is a general-purpose class that has a well-defined
interface that it uses to acquire services. In both cases, the client and
the interface cannot be changed.
You may also encounter cases where you want to dynamically
determine which methods of another object that will be called by a client
object. And, you want to do this without the client knowing anything about
the called object.
Solution
We create a class that converts, or "adapts"
the client's interface to the service provider class's interface. It performs
the conversions required to connect the client's calling interface to the
interface called in the server. One may think of an electric adapter that
takes 120V AC electric from a wall socket and converts it to 9V DC that an
electronic device might need. The device calls for 9V DC, but the electric
source supplies 120V AC.
The general pattern for the Adapter pattern is as follows:
There are two variations of the general pattern, however. The first is called
the class adapter, where the Adapter is a subclass of the Adaptee,
as shown here:
In some cases it is not possible, nor practical to have the Adapter subclass
the Adaptee. In this case we use the object adapter variation, shown
here:
Consequences
- The client and the service provider classes are independent
of each other.
- Object adapters ad an extra level of indirection to the program.
- A class adapter allows you to override some of the Adaptee's behavior since
it is a subclass of the Adaptee.
Implementation
A code example for the class and object adapters is
in this zip file.
Related Patterns
Bridge
Decorator
Proxy
Modified: 21-Jan-2004
Gary Pollice