Java 中的逆变和继承有什么区别？

Question

我有一个父 class A 和它的子 class B。这两个片段有什么区别：

    public static void main (String[] args) {
        ArrayList<? super A> e = new ArrayList<>();
        B thing = new B();
        e.add(thing);
        System.out.println(e.get(0));
    }

    public static void main (String[] args) {
        ArrayList<A> e = new ArrayList<>();
        B thing = new B();
        e.add(thing);
        System.out.println(e.get(0));
    }

Answer 1

ArrayList<? super A>和ArrayList<A>的区别在于，前者可以赋给ArrayList<T>的对象，其中T是A的超类或者A 本身。

具体来说，这是有效的：

ArrayList<? super A> listOfSuperA = new ArrayList<Object>();

这不是：

ArrayList<A> listOfA = new ArrayList<Object>();

这意味着您可以从 listOfA 和 listOfSuperA 中获取不同类型的值：

A a = listOfA.get(0); // valid
Object o = listOfA.get(0); // valid, because A can be converted to Object.
A a2 = listOfSuperA.get(0); // invalid, because listOfSuperA could contain Objects, which are not convertible to A
Object o2 = listOfSuperA.get(0); // valid, because whatever is in listOfSuperA, it can be converted to Object

This might be useful if you want to learn more about where to use ? super T

Java 中的逆变和继承有什么区别？

What's the difference between contravariance and inheritance in Java?

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inheritance

contravariance