引用对象接口时的注意点

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。