java 16-8 泛型高级之通配符

　　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

 import java.util.ArrayList;
 import java.util.Collection;
 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 {
 }