Here is the Rectangle class that was presented in the Creating Objects section, rewritten to implement Relatable.

public class RectanglePlus implements Relatable {
    public int width = 0;
    public int height = 0;
    public Point origin;

    // four constructors
    public RectanglePlus() {
	origin = new Point(0, 0);
    }
    public RectanglePlus(Point p) {
	origin = p;
    }
    public RectanglePlus(int w, int h) {
	origin = new Point(0, 0);
	width = w;
	height = h;
    }
    public RectanglePlus(Point p, int w, int h) {
	origin = p;
	width = w;
	height = h;
    }

    // a method for moving the rectangle
    public void move(int x, int y) {
	origin.x = x;
	origin.y = y;
    }

    // a method for computing the area of the rectangle
    public int getArea() {
	return width * height;
    }
    
    // a method required to implement the Relatable interface
    public int isLargerThan(Relatable other) {
    	RectanglePlus otherRect = (RectanglePlus)other;
    	if (this.getArea() < otherRect.getArea()) return -1; else if (this.getArea() > otherRect.getArea())
    		return 1;
    	else
    		return 0;    		
    }
}

Because RectanglePlus implements Relatable, the size of any two RectanglePlus objects can be compared.
Note: The isLargerThan method, as defined in the Relatable interface, takes an object of type Relatable. The line of code, shown in bold in the previous example, casts other to a RectanglePlus instance. Type casting tells the compiler what the object really is. Invoking getArea directly on the other instance (other.getArea()) would fail to compile because the compiler does not understand that other is actually an instance of RectanglePlus.