Applied Design Patterns with Java

Structural :: Decorator (175) {C ch 12}

Applicability

Use the Decorator pattern when:

• to add responsibilities to individual objects dynamically and transparently without affecting other objects;
• for responsibilities that can be withdrawn;
• when extension by subclassing is impractical. Often, a large number of independent extensions are possible and would produce an explosion of subclasses to for every combination. Or a class definition may be hidden or otherwise unavailable for subclassing.

Structure

The Decorator pattern has this general organization:

Participants

• Component (VisualComponent) defines the interface for objects that can have responsibilities added to them dynamically.
• ConcreteComponent (TextView) defines an object to which additional responsibilities can be attached.
• Decorator maintains a reference to a Component object and defines an interface that conforms to Component's interface.
• ConcreteDecorator (BorderDecorator, ScrollDecorator) adds responsibilities to the component.

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