通过匿名内部类overrides

package com.ykp.test;

public class A {
public void printName() {
System.err.println("printName");
}
}

package com.ykp.test;

public class B {

public static void main(String[] args) {
A a1 = new A() {
@Override
public void printName() {
System.err.println("this is new printName!");
}
};
a1.printName();//实际代用的是我们Override之后的方法

A a2 = new A();
a2.printName();//这里调用的还是原来的方法
}
}

Output in console:

this is new printName!
printName