java例子5：平面圆，继承圆

[root@gdc1000 java]# cat PlaneCircle.java public class PlaneCircle extends Circle {public double cx, cy;public PlaneCircle ( double r, double x, double y) {super.r = r;this.cx = x;this.cy = y;}public boolean isInside(double x, double y) {double dx = x - cx, dy = y - cy;double distance = Math.sqrt(dx*dx + dy*dy);return (distance < r);}} [root@gdc1000 java]# 

[root@gdc1000 java]# cat PlaneTest.java public class PlaneTest {public static void main(String[] args) {int input = 0;if(0 == args.length)                {System.out.println("default input");input = 0;}else{input = Integer.parseInt(args[0]);}PlaneCircle c = new PlaneCircle(input, 0, 0);System.out.println(c.area());System.out.println(c.isInside(3,5));}}

这里使用了super, this分别指代类中不同的关系，父亲，自己。

使用了extends关键词