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 {
}