向后兼容

 1 package com.cn;
 2 
 3 class A{
 4     public void fun1(){
 5         System.out.println("A fun1");
 6     }
 7     public void fun2(){
 8         fun1();
 9     }
10 }
11 class B extends A{
12     public void fun1(){
13         System.out.println("B fun1");
14     }
15     public void fun3(){
16         System.out.println("B fun3");
17     }
18 }
19 class Test{
20     public static void main(String [] args){
21         B b = new B();
22         callA(b);
23     }
24     public static void callA(A a){
25         a.fun2();
26     }
27 }