1.向上类型转换upcast
向下类型转换downcast

public class PolyTest2
{
    public static void main(String[] args)
    {
        /*
        Animal animal = new Cat();
        Animal animal2 = new Animal();

        animal2 = animal;
        animal2.sing();
        */

        /*
        Animal animal = new Cat();
        Animal animal2 = new Animal();

        animal = animal2;
            animal.sing();
        */

        /*
        Cat cat = new Cat();
        Animal animal = cat;
        animal.sing();
        */

        /*
        Animal animal = new Animal();
        Cat cat = (Cat)animal;//可编译，不能执行。
        */

        //向上类型转换
        Cat cat = new Cat();
        Animal animal = Cat;
        animal.sing();

        //向下类型转换
        Animal a = new Cat();
        Cat cat = (Cat)a;
        cat.sing();

    }
}

class Animal
{
    public void sing()
    {
        System.out.println("animal is singing");
    }
}

class Dog extends Animal
{
    public void sing()
    {
        System.out.println("dog is singing");
    }
}

class Cat extends Animal
{
    public void sing()
    {
        System.out.println("cat is singing");
    }
}

public class PolyTest3
{
    public static void main(String[] args)
    {
        //Fruit f = new Pear();
        //f.run();

        //Pear p = (Pear)f;
        //p.run();

        System.out.println("----------------");

        //Fruit f = new Pear();
        //f.grow();//无法编译，父类Fruit 没有grow()方法
        
        Fruit f = new Pear();
        Pear p = (Pear)f;
        p.grow();
    }
}

class Fruit
{
    public void run()
    {
        System.out.println("fruit is running");
    }
}

class Pear extends Fruit
{
    public void run()
    {
        System.out.println("pear is running");
    }

    public void grow()
    {
        System.out.println("pear is growing");
    }
}

public class PolyTest4
{
    public static void main(String[] args)
    {
        A a = null;

        if(args[0].equals("1"))
        {
            a = new B();
        }

        else if(args[0].equals("2"))
        {
            a = new C();
        }

        else if(args[0].equals("3"))
        {
            a = new D();
        }

        a.method();
    }
}

class A
{
    public void method()
    {
        System.out.println("A");
    }
}

class B extends A
{
    public void method()
    {
        System.out.println("B");
    }
}

class C extends A
{
    public void method()
    {
        System.out.println("C");
    }
}

class D extends A
{
    public void method()
    {
        System.out.println("D");
    }
}