Applied Design Patterns with Java

Structural :: Adapter (137) {C ch 9}

Collaborations

• Clients call operations on an Adapter instance. In turn, the Adapter calls Adaptee operations that carry out the request.

Consequences

Class and object adapters have different trade-offs. A class Adapter

• adapts Adaptee to Target by committing to a concrete Adapter class. As a consequence, a class adapter won't work 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 - 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.

Here are other issues to consider when using the Adapter:

1. How much adapting does Adapter do? Adapters vary in the amount of work they do to adapt Adaptee to the Target interface. The amount of work Adapter does depends on how similar the Target interface is to Adaptee's.
2. Pluggable adapters. A class is more reusable when restricting the assumptions other classes must make to use it. By building interface adaptation into a class, the assumption other classes see the same interface is eliminated. The term pluggable Adapter describes classes with built-in interface adaptation. Consider a TreeDisplay widget that can display tree structures graphically. Different tree structures will have different interfaces. In a directory hierarchy, children might be accessed with a GetSubdirectories operation, whereas in an inheritance hierarchy, the corresponding operation might be called GetSubclasses. A reusable TreeDisplay widget must be able to display both kinds of hierarchies even if they use different interfaces. In other words, the TreeDisplay should have interface adaptation built into it.
3. Using two-way adapters to provide transparency. A potential problem with adapters is that they aren't transparent to all clients. An adapted object no longer conforms to the Adaptee interface, so it can't be used as is wherever an Adaptee object can. Two-way adapters can provide such transparency. Specifically, they're useful when two different clients need to view an object differently.

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