Presents your 
JAVA E-NEWSLETTER for September 26, 2002

<------------------------------------------->

ADD FUNCTIONALITY WITH THE DECORATOR PATTERN

You're probably already using the Decorator pattern, but as with all
patterns, it's important to know why you're using it and to learn how 
to communicate about it with other developers.

The main force governing usage of the Decorator pattern--also known as
the Wrapper pattern--is a need to add functionality to a class without
modifying its code or disturbing its inheritance. While you may want 
the class to have more functionality, using the Decorator pattern means 
that your improved functionality version doesn't have to reuse the existing
functionality by extending the class.

Given an interface named Action with two methods on it, act1() and
act2(), and a concrete version of this interface named ConcreteAction, the
Decorator would be a class that implements Action and whose constructor
takes an Action (usually ConcreteAction). So the code would look like this:

public class ActionDecorator implements Action {

    private Action action;
    
    public ActionDecorator(Action action) {
        this.action = action;
    }

    public void act1() {
        action.act1();
    }

    public void act2() {
        // do nothing
    }

}

Using the Decorator results in fewer classes than inheritance, so coding
is simpler; however, it usually results in more objects, which can make
debugging harder, especially since the added flexibility can introduce
new areas for errors.

----------------------------------------