java面试题

 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。