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 “hasa” instance of the “lowest common denominator”
5. Decorator class delegates to the “hasa” object
6. Create a Decorator derived class for each optional embellishment
7. Decorator derived classes delegate to base class AND add extra stuf
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 w, int h ) {
    width  = w;
    height = h;
  }
  public void draw() {
    System.out.println( "TextField: " + width + ", " + height );
  }
}

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

  public Decorator( Widget w ) {
    wid = w;
  }

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

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

// 6. Optional embellishment
class ScrollDecorator extends Decorator {
  public ScrollDecorator( Widget w ) {
    super( w );
  }
  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 aWidget = new BorderDecorator(
                       new BorderDecorator(
                         new ScrollDecorator(
                           new TextField( 80, 24 ))));
    aWidget.draw();
  }
}