Android（java）学习笔记92：泛型高级之通配符

 1 package cn.itcast_07;
 2 
 3 import java.util.ArrayList;
 4 import java.util.Collection;
 5 
 6 /*
 7  * 泛型高级(通配符)
 8  * ?:任意类型，如果没有明确，那么就是Object以及任意的Java类了
 9  * ? extends E:向下限定，E及其子类
10  * ? super E:向上限定，E极其父类
11  */
12 public class GenericDemo {
13     public static void main(String[] args) {
14         // 泛型如果明确的写的时候，前后必须一致
15         Collection<Object> c1 = new ArrayList<Object>();
16         // Collection<Object> c2 = new ArrayList<Animal>();
17         // Collection<Object> c3 = new ArrayList<Dog>();
18         // Collection<Object> c4 = new ArrayList<Cat>();
19 
20         // ?表示任意的类型都是可以的
21         Collection<?> c5 = new ArrayList<Object>();
22         Collection<?> c6 = new ArrayList<Animal>();
23         Collection<?> c7 = new ArrayList<Dog>();
24         Collection<?> c8 = new ArrayList<Cat>();
25 
26         // ? extends E:向下限定，E及其子类
27         // Collection<? extends Animal> c9 = new ArrayList<Object>();
28         Collection<? extends Animal> c10 = new ArrayList<Animal>();
29         Collection<? extends Animal> c11 = new ArrayList<Dog>();
30         Collection<? extends Animal> c12 = new ArrayList<Cat>();
31 
32         // ? super E:向上限定，E极其父类
33         Collection<? super Animal> c13 = new ArrayList<Object>();
34         Collection<? super Animal> c14 = new ArrayList<Animal>();
35         // Collection<? super Animal> c15 = new ArrayList<Dog>();
36         // Collection<? super Animal> c16 = new ArrayList<Cat>();
37     }
38 }
39 
40 class Animal {
41 }
42 
43 class Dog extends Animal {
44 }
45 
46 class Cat extends Animal {
47 }