基类小于子类

class Mammal{}
class Dog extends Mammal {}
class Cat extends Mammal{}

public class TestCast
{
    public static void main(String args[])
    {
        Mammal m;
        Dog d=new Dog();
        Cat c=new Cat();
        m=d;
        //d=m;
        d=(Dog)m;
        //d=c;
        c=(Cat)m;

    }
}