The Corporate Code

Today I will be discussing the decorator pattern. This pattern is used to add responsibilities to your objects at run-time rather then at compile time. You can find some examples of the decorator pattern by looking at the Java I/O API or the swing table API. Below is a small example of the decorator pattern in action :

LineNumberReader lrdr = new LineNumberReader(new FileReader(filename));

As you can see we wrap the FileReader into the LineNumberReader which adds certain functionalities to the FileReader.
As always, we start with the UML scheme for this pattern :

As you can see in the scheme above, The decorator will hold a reference to the decorated object and delegate it’s methods to the object. In this way we create a chain which will go all the way into the most inner class of the pattern. Below is a use case diagram which shows you how it works :

Because most of the methods in the decorator just pass the call to the reference object, we can create an abstract class of the decorator which has a default behavior which is to delegate the call to his reference object.
Let’s take a look at a code sample for this pattern. Imagine a Textview widget. We might want the textview to have some nice borders. By implementing this with the decorator pattern, the textview object doesn’t have to be modified in any way, in fact, it doesn’t even have to know anything about the borders to function correctly.

public abstract class AView {
public abstract void draw();
}


next up, we have an implementation of the Aview :


public class Textview extends AView {
public void draw();
// draw the textview
}

Now we create the decorator, which is an abstract class :

public abstract class BorderDecorator extends AView {
public abstract void draw();
}


And now for the actual Decorators :


public class 3Dborder extends BorderDecorator {
private AView component;
public 3Dborder(AView compo) {
This.component = compo;
}
public draw() {
component.draw();
// code to draw the 3D border
}
}
public class BevelBorder extends BorderDecorator {
private AView component;
public 3Dborder(AView compo) {
This.component = compo;
}
public draw() {
component.draw();
// code to draw the bevel border
}
}


And now for the usage :


AView tv = new TextView();
Tv = new 3Dborder(tv);
Aview tv2 = new BevelBorder(new TextView());