集合框架(泛型高级之通配符)

核心代码：

  package com.ithelei;

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
    //本身和儿子
    // 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
    //本身和父亲
    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 {
}

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