【泛型高级-通配符】

package com.yjf.esupplier.common.test;

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

/**
 * @author shusheng
 * @description
 * @Email [email protected]
 * @date 2018/12/13 17:10
 */
public class GenericDemo {

    public static void main(String[] args) {

        //泛型如果是明确的写的时候,前后必须一致,否则无法通过编译
        Collection<Object> c1 = new ArrayList<Object>();

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

        //? extends E:向下限定,E及其子类,否则无法通过编译
        Collection<? extends Animal> c10 = new ArrayList<Animal>();    //正确
        Collection<? extends Animal> c11 = new ArrayList<Dog>();    //正确
        Collection<? extends Animal> c12 = new ArrayList<Cat>();    //正确

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

}

class Animal {
}

class Dog extends Animal {
}

class Cat extends Animal {
}