1 interface Playable { 2 void play(); 3 } 4 interface Bounceable { 5 void play(); 6 } 7 interface Rollable extends Playable, Bounceable { 8 Ball ball = new Ball(“PingPang”); 9 } 10 class Ball implements Rollable { 11 private String name; 12 public String getName() { 13 return name; 14 } 15 public Ball(String name) { 16 this.name = name; 17 } 18 public void play() { 19 ball = new Ball(“Football”); 20 System.out.println(ball.getName()); 21 } 22 }
错。"interface Rollable extends Playable, Bounceable"没有问题。interface可继承多个interfaces，所以这里没错。问题出在interface Rollable里的"Ball ball = new Ball("PingPang");"。任何在interface里声明的interface variable (接口变量，也可称成员变量)，默认为public static final。也就是说"Ball ball = new Ball("PingPang");"实际上是"public static final Ball ball = new Ball("PingPang");"。在Ball类的Play()方法中，"ball = new Ball("Football");"改变了ball的reference，而这里的ball来自Rollable interface，Rollable interface里的ball是public static final的，final的object是不能被改变reference的。因此编译器将在"ball = new Ball("Football");"这里显示有错。