Java泛型：之间有什么区别 <?> 并跳过它？

Question

Maybe it is silly question: Let say I have

abstract class A<T> {
  List<Wrapper<T>> doStuff()
}

And I have class B extends A<String> and class C extends A<Integer>

Now I want to have:

List<A> aces = list with instances of B and C;

List<Wrapper> wrapperedItems = flattened list of lists returned from doStuff() on all items in aces

At this point I don't care what type is within Wrapper . And my question is: shall I use somewhere <?> or can I skip it? What is the difference?

Answer 1

If you use List<Wrapper<?>> , you will be able to get objects out of the list, but you will not be able to add new items. The wildcard essentially correcponds to a type that is different from any other type, including other wildcards.

If you use List<Wrapper> , you will be able to both get list items and add new ones, but the onus is now up to you to guard for improper type casts.