为什么不能将一个对象添加到列表中?超级号码&gt ;? [英] Why can't one add an Object to List<? super Number>?

问题描述

考虑以下Java代码:

Consider the following Java code:

List<? super Number> list = new ArrayList<>();
Number n = new Integer(1);
Object o = new Object();
list.add(n); // works, apparently Number super Number is alright
list.add(o); // compiler error! 

但是为什么会这样呢?由于在内部，类型擦除使列表成为保存对象的列表，为什么不允许将对象(所有类的超类，因此应满足下限通配符)添加到列表中?

But why is that the case? Since internally, type erasure makes list a list that holds Objects, why isn't it allowed to add Objects (which are superclasses of all classes, and thereby should satisfy the lower-bound wildcard) to the list?

推荐答案

编译器仅知道list包含Number的某些超类型(或Number本身)，该超类型可能是Object，也可能不是.因此必须禁止Object，因为它可能是List<Number>.

The compiler only knows that list contains some supertype of Number (or Number itself), which may or may not be Object. So it must disallow Object because it could be List<Number>.

这是一个更详尽的解释，滚动查看谈论?超级"的一半.

Here's a more thorough explanation, scroll about half way down where it talks about "? super".

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