Applied Design Patterns with Java

Structural :: Bridge (151) {C ch 10}

Applicability

Use the Bridge pattern when:

• to avoid a permanent binding between an abstraction and its implementation. This might be the case when the implementation must be selected or switched at run-time;
• both the abstractions and their implementations should be extensible by subclassing. The Bridge pattern allows combining different abstractions and implementations and extending them independently;
• changes in the implementation of an abstraction should have no impact on clients; their code should not have to be recompiled;
• there is a need to hide the implementation of an abstraction completely from clients;
• a proliferation of classes exists, and there is a need for splitting an object into two parts (Rumbaugh's "nested generalizations");
• to share an implementation among multiple objects, and this fact should be hidden from the client.

Structure

The Bridge pattern has this general organization:

Participants

• Abstraction (Window) defines the abstraction's interface and maintains a reference to an object of type Implementor.
• RefinedAbstraction (IconWindow) extends the interface defined by Abstraction.
• Implementor (WindowImp) defines the interface for implementation classes. This interface doesn't have to correspond to Abstraction's interface; in fact the interfaces can be quite different. The Implementor interface provides primitive operations, and Abstraction defines higher-level operations based on these primitives.
• ConcreteImplementor (XWindowImp, PMWindowImp) implements the Implementor interface and defines its concrete implementation.

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