Delegation and interface

Question

i m reading Grand's Patterns in java, he says that delegation and interface patterns are often used together, however i wasnot able to find any references, i cant quite see why that is?

Answer 1

GoF discusses delegation. They don't make it a pattern, rather they use it in several of their patterns. Their example of a Window delegating its area operation to a Rectangle and then being able to switch to using a Circle at runtime is dependent on Rectangle and Circle having the same "type", ie implementing the same interface. Here's a Java version of the delegation example on page 20 of "Design Patterns"
public class Window implements TwoD
     TwoD my2D = new Rectangle(1,1);     // we delegate area to this TwoD
     // switch to circular delegate
     void useCircle(double radius)
     {my2D = new Circle(radius);}
     // switch to rectangular delegate
     void useRectangle(double width, double height)
     {my2D = new Rectangle(width,height);}
     // our area method uses the delegate
     public double area()
     {return my2D.area();}
     public static void main(String[] args)
          Window w = new Window();     // construct a window
          w.useRectangle(4.0, 6.0);     // set the shape to a rectangle
          System.out.println("area of rectangular window="+w.area());
          w.useCircle(2.5);               // set the shape to a circle
          System.out.println("area of circular window="+w.area());
// 2 d shape used by Window, Circle and REctangle
interface TwoD
     double area();     // requires an area method
// rectangular shape
class Rectangle implements TwoD
     double width, height;
     Rectangle(double w, double h)
          width = w;
          height = h;
     public double area(){return width*height;}
// circular shape
class Circle implements TwoD
     double radius;
     Circle(double r)
          radius = r;
     public double area(){return Math.PI*radius*radius;}
}