PECS原則：Producer Extends Consumer Super

頻繁往外讀取內(nèi)容的，適合用上界Extends。
經(jīng)常往里插入的，適合用下界Super。

        List <? extends Fruit> extendsList = new ArrayList();
        extendsList.add(new Object()); //ERROR
        extendsList.add(new Fruit()); //ERROR
        extendsList.add(new Apple()); //ERROR
        extendsList.add(new Pear()); //ERROR
        Object e1 = extendsList.get(0);
        Fruit e2 = extendsList.get(0);
        Apple e3 = extendsList.get(0); //ERROR
        Pear e4 = extendsList.get(0); //ERROR


        List <? super Fruit> superList = new ArrayList();
        superList.add(new Object()); //ERROR
        superList.add(new Fruit());
        superList.add(new Apple());
        superList.add(new Pear());
        Object s1 = superList.get(0);
        Fruit s2 = superList.get(0); //ERROR
        Apple s3 = superList.get(0); //ERROR
        Pear s4 = superList.get(0); //ERROR

參考：https://itimetraveler.github.io/2016/12/27/%E3%80%90Java%E3%80%91%E6%B3%9B%E5%9E%8B%E4%B8%AD%20extends%20%E5%92%8C%20super%20%E7%9A%84%E5%8C%BA%E5%88%AB%EF%BC%9F/