Java

Question

我有以下 Java 代码:

List<? extends Collection<String>> attributes = new ArrayList<HashSet<String>>(nrAttributes);

...

attributes.replaceAll((in) -> {
        List<String> out = new ArrayList<>(); 
        out.addAll(in); 
        return out;
    });

但它给了我一个编译器异常(最后一个是红色的)说:

类型不匹配:无法从 List 转换为 capture#2-of ？扩展集合

Answer 1

您的扩展只是确保您将阅读 Collection<String>从你的列表中。但是你不能写它。

要获得很好的解释，请参阅此 link我从哪里获取了以下内容:

You can't add any object to List<? extends T> because you can't guarantee what kind of List it is really pointing to, so you can't guarantee that the object is allowed in that List. The only "guarantee" is that you can only read from it and you'll get a T or subclass of T.