1 package com.hxl; 
2  
3 public class Animal {} 
4  
5 class Dog extends Animal {} 
6  
7 class Cat extends Animal {}

 1 package com.hxl; 
 2  
 3 import java.util.ArrayList; 
 4 import java.util.Collection; 
 5  
 6 public class Test { 
 7  
 8     public static void main(String[] args) { 
 9         // 这里不报错的条件是，后者类型参数满足前者类型参数要求！！ 
10         // Collection类型参数若明确写，则必须与ArrayList类型参数保持一致 
11         // 可以偷个懒，写?替代ArrayList类型参数 
12         Collection<Animal> c1 = new ArrayList<Animal>(); 
13         Collection<?> c2 = new ArrayList<Cat>(); 
14         Collection<?> c3 = new ArrayList<Dog>(); 
15  
16         // ? extends E，表示E本身及它的所有子类 
17         // Collection<? extends Animal> c4 = new ArrayList<Object>(); 报错是因为Object不是Animal的子类！ 
18         Collection<? extends Animal> c5 = new ArrayList<Animal>(); 
19         Collection<? extends Animal> c6 = new ArrayList<Dog>(); 
20  
21         // ? super E，表示E本身及它的所有层次的父类 
22         Collection<? super Dog> c7 = new ArrayList<Object>(); 
23         Collection<? super Dog> c8 = new ArrayList<Animal>(); 
24         Collection<? super Dog> c9 = new ArrayList<Dog>(); 
25         //Collection<? super Dog> c9 = new ArrayList<Cat>(); 报错是因为Cat不是Dog的父类！ 
26     } 
27 }