继承的特点、多态，

package sonandfather;
class father{
    void eat(){
        System.out.println("father eat");
    }
    void hitson(){
        System.out.println("father hit son");
    }
}
class sonXiaoming extends father{
    void eat(){
        System.out.println("sonXiaoming eat");
    }
    void play(){
        System.out.println("sonXiaoming play");
    }
}
class sonXiaogang extends father{
    void eat(){
        System.out.println("sonXiaogang eat");
    }
    void sing(){
        System.out.println("sonXiaogang sing");
    }
}
public class SonextendsFather {
    public static void main(String args[]) {
        father f=new sonXiaoming(); //父类类型引用子类对象，向上转型。
        f.eat();   //调用的是子类的方法，规则是先从子类找该方法，找不到先父类找
        f.hitson();//子类无该方法，向父类找
    //  f.play(); 报错，父类f无该方法
        father f1=new father();
        f1.eat();  //父类对象调用父类方法
        f1.hitson();
        sonXiaoming xm=new sonXiaoming();
        xm.eat();
        xm.play();
        father fa=new sonXiaoming();
        method(fa);
        method(new father());
        method(new sonXiaoming());
        method(new sonXiaogang());
    }
    public static void method(father f) {
        if(f instanceof sonXiaoming){
            sonXiaoming xm=(sonXiaoming)f; //向下转型
            xm.play();      
        }
        if(f instanceof sonXiaogang){
            sonXiaogang xg=(sonXiaogang)f; //向下转型
            xg.sing();
        }
    }
}

sonXiaoming eat
father hit son
father eat
father hit son
sonXiaoming eat
sonXiaoming play
sonXiaoming play
sonXiaoming play
sonXiaogang sing