Java多态:upcast和downcast

upcast例:

public class Test
{
    public static void main(String[] args)
    { 
        Cup aCup = new BrokenCup();
        aCup.addWater(10); // method binding
    }
}

class Cup 
{
    public void addWater(int w) 
    {
        this.water = this.water + w;
    }

    public void drinkWater(int w)
    {
        this.water = this.water - w;
    }

    private int water = 0;
}

class BrokenCup extends Cup
{
    public void addWater(int w) 
    {
        System.out.println("shit, broken cup");
    }

    public void drinkWater(int w)
    {
        System.out.println("om...num..., no water inside");
    }
}

downcast例:

public class TestJavaDemo{
    public static void main(String[] args) {
        Person p=new Student();
        Student s=(Student)p;
        s.fun1();
        s.fun2();
    }
}

class Person{
    public void fun1(){
        System.out.println("1.Person{fun1()}");
    }

    public void fun2(){
        System.out.println("2.Person{fun2()}");
    }
}

class Student extends Person{
    public void fun1(){
        System.out.println("3.Student{fun1()}");
    }
    public void fun3(){
        System.out.println("4.Student{fun3()}");
    }
}