// Purpose.  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

interface Widget { void draw(); }             // 1. "lowest common denominator"

class TextField implements Widget {           // 3. "Core" class with "isa" rel
   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
abstract class Decorator implements Widget {    //    with "isa" relationship
   private Widget wid;                          // 4. "hasa" relationship
   public Decorator( Widget w ) { wid = w; }
   public void draw()           { wid.draw(); } // 5. Delegation
}

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

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

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();
}  }

// TextField: 80, 24
//    ScrollDecorator
//    BorderDecorator
//    BorderDecorator