多态

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class Message {
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    public void print(){
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        System.out.println("i am messge");
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    }
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}
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class DataBaseMessage extends Message {
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    public void print(){
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        System.out.println("i am DataBaseMessage");
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    }
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}
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class WebServerMessage extends Message {
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    public void print(){
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        System.out.println("i am  WebServerMessage");
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    }
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}
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public class duotai{
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    public static void main(String args []){
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        fun(new DataBaseMessage());
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        fun(new WebServerMessage());
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    }
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    public static void fun(Message msg){
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        msg.print();
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    }
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}

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class Person{
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    public void print(){
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        System.out.println("i am just a common man");
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    }
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}
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class Superman extends Person{
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    public void fly(){
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        System.out.println("i can fly");
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    }
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}
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public class duotai{
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    public static void main(String args []){
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        Person superman = new Superman();        // 如果你在这个地方是 Person per = new Person(); 则后面无法转型成Supermanm类
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        superman.print();
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        //向下转型
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        Superman man = (Superman) superman;
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        man.fly();
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    }
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}