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Decorator Design Pattern in Java

Decorator design pattern

  1. Create a "lowest common denominator" that makes classes interchangeable
  2. Create a second level base class for optional functionality
  3. "Core" class and "Decorator" class declare an "isa" relationship
  4. Decorator class "has a" instance of the "lowest common denominator"
  5. Decorator class delegates to the "has a" object
  6. Create a Decorator derived class for each optional embellishment
  7. Decorator derived classes delegate to base class AND add extra stuff
  8. Client has the responsibility to compose desired configurations
// 1. "lowest common denominator"
interface Widget {
    void draw();
}

// 3. "Core" class with "is a" relationship
class TextField implements Widget {
    private int width, height;

    public TextField(int width, int height) {
        this.width = width;
        this.height = height;
    }
    public void draw() {
        System.out.println("TextField: " + width + ", " + height);
    }
}

// 2. Second level base class with "isa" relationship
abstract class Decorator implements Widget {
    // 4. "has a" relationship
    private Widget widget;

    public Decorator(Widget widget) {
        this.widget = widget;
    }

    // 5. Delegation
    public void draw() {
        widget.draw();
    }
}

// 6. Optional embellishment
class BorderDecorator extends Decorator {
    public BorderDecorator(Widget widget) {
        super(widget);
    }
    public void draw() {
        // 7. Delegate to base class and add extra stuff
        super.draw();
        System.out.println("  BorderDecorator");
    }
}

// 6. Optional embellishment
class ScrollDecorator extends Decorator {
    public ScrollDecorator(Widget widget) {
        super(widget);
    }
    public void draw() {
        super.draw(); // 7. Delegate to base class and add extra stuff
        System.out.println("  ScrollDecorator");
    }
}

public class DecoratorDemo {
    public static void main(String[] args) {
        // 8. Client has the responsibility to compose desired configurations
        Widget widget = new BorderDecorator(new BorderDecorator(new ScrollDecorator(new TextField(80, 24))));
        widget.draw();
    }
}

Output

TextField: 80, 24
   ScrollDecorator
   BorderDecorator
   BorderDecorator

Code examples

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