下面代码运行时会不会出错：
interface Playable
{
void play();
}
interface Bounceable
{
void play();
}
interface Rollable extends Playable, Bounceable
{
Ball ball = new Ball("PingPang");
}
class Ball implements Rollable
{
private String name;
public String getName()
{
return name;
}
public Ball(String name)
{
this.name = name;
}
public void play()
{
ball = new Ball("Football");
System.out.println(ball.getName());
}
}
答案: 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。
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