1 abstract class Animala{ 2     String name; 3     abstract public void eat(); 4      5 } 6 class Dog extends Animala{ 7     public void eat(){ 8         System.out.println("狗吃骨头"); 9     }10 }11 class Cat extends Animala{12     public void eat(){13         System.out.println("猫吃鱼");14     }15 }16 class MyAnimalList{17     private Animala[] animals = new Animala[5]; //初始化数组，即这个数组里可以放5个Animala类型的类的对象18     private int nextIndex = 0; //下标19     20     public void add(Animala a){21         if(nextIndex < animals.length){22             animals[nextIndex] = a;23             System.out.println("Animal add at "+ nextIndex );24             nextIndex++;25         }26     }27 }28 public class ThreeDuo {29     public static void main(String[] args){30         MyAnimalList list = new MyAnimalList(); //定义集合的对象31         Cat a = new Cat();32         Dog c = new Dog();33         list.add(a);34         list.add(c);35     }36 }