java - 列表<?> 或列表<对象>

Question

我有一个方法，其参数应该是“任何东西的列表”。该方法不会修改列表的内容。将此方法定义为更正确吗

void foo(List<?> list) {
}

或

void foo(List<Object> list) {
}

到底有什么区别？

Answer 1

简短回答，使用 List<?>将允许您接受类似 List<String> 的内容使用 List<Object> 时不会的。

这在官方泛型路径中进行了讨论，here和 here .

[...] here is a naive attempt at writing it using generics (and the new for loop syntax):

   void printCollection(Collection<Object> c) {
       for (Object e : c) {
           System.out.println(e);
       }
   }

The problem is that this new version is much less useful than the old one. Whereas the old code could be called with any kind of collection as a parameter, the new code only takes Collection<Object>, which, as we've just demonstrated, is not a supertype of all kinds of collections!
So what is the supertype of all kinds of collections? It's written Collection<?> (pronounced "collection of unknown"), that is, a collection whose element type matches anything. It's called a wildcard type for obvious reasons. We can write:

   void printCollection(Collection<?> c) {
       for (Object e : c) {
           System.out.println(e);
       }
   }

and now, we can call it with any type of collection.