创建一个接口，然后实现该接口

//时 代 J a v a - nowjava.com 提供

import java.awt.Point;


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;

    }// 来自 时代Java - N o w  J a v a . c o m


    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 to implement Relatable

    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;

    }

}


interface Relatable {


    // this (object calling isLargerThan) and

    // other must be instances of the same class

    // returns 1, 0, -1 if this is greater

    // than, equal to, or less than other

    public int isLargerThan(Relatable other);


}