程序代码：

package test;

interface ISport {
    void run();
}

class Athelete implements ISport {
    public void run() {
        System.out.println("三级跳");
    }
}

class Students implements ISport {
    public void run() {
        System.out.println("我随便跑跑");
    }
}

class CollegeStudent extends Students {
    public void study() {
        System.out.println("好好学习，天天向上");
    }
}

public class AnyTest {
    public static void main(String[] args) {
        ISport i = new Athelete();
        i.run();
        i = new Students();
        i.run();
        i = new CollegeStudent();
        i.run();
        i.study();// 为山么i无法引用到设个方法？i=new CollegeStudent();
        CollegeStudent name = new CollegeStudent();
        name.study();

    }
}