java 16-8 泛型高级之通配符

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