Applied Design Patterns with Java

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

Example - UML : CoolSlash

Example - Java :: Patterns\Structural\Decorator

Pattern Concept: to provide a way to dynamically modify the behavior of individual objects by attaching additional responsibilities without having to create a new derived class. Decorators provide a flexible alternative to subclassing for new functionality.

Issues and Consequences of the Decorator pattern include:

• Decorator favors object containment over object inheritance, a theme among many patterns;
• Decorator adds functionality by wrapping a class (it does not 'hide' complexity);
• Decorators allows easy extension of GUI elements, but they are not limited to to objects that enhance visual classes;
• Decorator provides an alternative to subclassing high in the inheritance hierarchy, and it also allows selectively choosing the classes to decorate;
• A Decorator and its enclosed component will NOT be of the same type, so any test for object type equivalence will fail;
• Overuse of the Decorator pattern will result in too many 'little' objects, requiring excessive maintenance.

The first set of example Java programs show decorating GUI buttons; they are named 'decoWindow', 'slashWindow', and 'borderWindow'.

The UML diagram is above, and the list of Java files is below:

The second example Java program is are 'DecoStream' and it shows the decorating of non-visual object methods (read()) by intercepting and forwarding them.

The UML diagram is above, and the list of Java files is below:

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