abstract class Animal {  
    abstract void eat();  
}  
class Cat extends Animal {  
    public void eat() {  
        System.out.println("吃鱼");  
    }  
    public void work() {  
        System.out.println("抓老鼠");  
    }  
}  
class Dog extends Animal {  
    public void eat() {  
        System.out.println("吃骨头");  
    }  
    public void work() {  
        System.out.println("看家");  
    }  
}
public class MTest {
    public static void main(String[] args) {       
      Animal a = new Cat();      // 向上转型  
      a.eat();                   // 调用的是 Cat 的 eat
      if(a instanceof Cat) {
        Cat c = (Cat)a;         // 向下转型  
        c.work();               // 调用的是 Cat 的 work
      } else if(a instanceof Dog) {
          Dog d = (Dog)a;
          d.work();
      }
      Animal a2 = new Dog();
      a2.eat();
      if(a2 instanceof Cat) {
          Cat c = (Cat)a2;
          c.work();
      } else if(a2 instanceof Dog) {
          Dog d = (Dog)a2;
          d.work();
      }
  }  
}