的ArrayList< T> vs ArrayList<？> [英] ArrayList<T> vs ArrayList<?>

问题描述

我知道使用了 ArrayList< T> ，但是何时应该使用 ArrayList<> ？你能用例子来解释吗？谢谢。

I know what ArrayList<T> is used for, but when should I use ArrayList<?> ? can you explain with example? thanks.

推荐答案

据我所知， ArrayList<？> / code>基本上告诉编译器：



As far as I've been able to tell, ArrayList<?> basically tells the compiler:

是的，我知道有一个通用版本的ArrayList可供我使用，但我真的，真的不知道我期待在这一件物品中有什么样的物品。所以不要给我警告说我没有按照我应该的方式使用泛型。

Yes, I know that there is a generic version of ArrayList available to me, but I really, genuinely don't know what kind of objects I'm expecting to be in this one. So don't give me warnings that I'm not using generics the way I should be.

更新

我刚了解到使用原始类型（ ArrayList ）和带通配符的泛型（ ArrayList< ;？> ）超越避免编译器警告。显然，一旦你将某些东西声明为Raw类型，即使泛型不是基于你省略的类型，泛型也不能用于该类型的任何方法。 请参阅此处以获取示例。

Update

I just learned that there is a real difference between using Raw Types (ArrayList) and generics with wildcards (ArrayList<?>) that goes beyond avoiding compiler warnings. Apparently once you declare something as a Raw type, no generics will work on any methods of that type, even if the generics weren't based on the type you omitted. See here for an example.

因此，虽然我的原始答案通常是正确的，但我认为使用 ArrayList <？>< / code>代替 ArrayList 不仅仅是删除编译器警告的问题。

So while my original answer was generally correct, I thought it would be important to mention that using ArrayList<?> instead of ArrayList is more than just a matter of removing compiler warnings.

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