Android（java）学习笔记31：泛型高级之通配符

1. 泛型高级之通配符：

 package cn.itcast_07;

 import java.util.ArrayList;
 import java.util.Collection;

 /*
  * 泛型高级(通配符)
  * ?:任意类型，如果没有明确，那么就是Object以及任意的Java类了
  * ? extends E:向下限定，E及其子类
  * ? super E:向上限定，E极其父类
  */
 public class GenericDemo {
     public static void main(String[] args) {
         // 泛型如果明确的写的时候，前后必须一致
         Collection<Object> c1 = new ArrayList<Object>();
         // Collection<Object> c2 = new ArrayList<Animal>();
         // Collection<Object> c3 = new ArrayList<Dog>();
         // Collection<Object> c4 = new ArrayList<Cat>();

         // ?表示任意的类型都是可以的
         Collection<?> c5 = new ArrayList<Object>();
         Collection<?> c6 = new ArrayList<Animal>();
         Collection<?> c7 = new ArrayList<Dog>();
         Collection<?> c8 = new ArrayList<Cat>();

         // ? extends E:向下限定，E及其子类
         // Collection<? extends Animal> c9 = new ArrayList<Object>();
         Collection<? extends Animal> c10 = new ArrayList<Animal>();
         Collection<? extends Animal> c11 = new ArrayList<Dog>();
         Collection<? extends Animal> c12 = new ArrayList<Cat>();

         // ? super E:向上限定，E极其父类
         Collection<? super Animal> c13 = new ArrayList<Object>();
         Collection<? super Animal> c14 = new ArrayList<Animal>();
         // Collection<? super Animal> c15 = new ArrayList<Dog>();
         // Collection<? super Animal> c16 = new ArrayList<Cat>();
     }
 }

 class Animal {
 }

 class Dog extends Animal {
 }

 class Cat extends Animal {
 }