Why use wildcards in Java generics and not generic methods?

Question

I'm currently learning Java generics, and while going through the Oracle's tutorial in Wildcard capture and helper methods it suddenly hit me: what is the benefit of:

void someMethod(List<?> list) { // .. }

over:

<T> void someMethod(List<T> list) { // .. }

?

The reason I ask is that the link provided above presents a certain compilation error that occurs while using the first form (using wildcards), which is solved by using a helper method of the second form. So the question arises - why use the first form in the first place?

You need to use the second form if you are trying to put something back into the `list` - in the first form, even if it is something you took out of the list (e.g. `list.add(list.get(0));`), the compiler doesn't recognize it as being of the correct type. The second form allows the compiler to know that the thing is of type `T`; because you can add a `T` to a `List`, the compiler allows this. — Andy Turner, Jul 31 '23 at 10:30
If you don't need to add anything to `list` (more generally, invoke a method which accepts a `T` or a type referencing `T` as a parameter), it's just unnecessary to have it there. — Andy Turner, Jul 31 '23 at 10:32

score -2 · Answer 1 · edited Jul 31 '23 at 11:27

In generic code, the question mark (?), called the wildcard, represents an unknown type. The wildcard can be used in a variety of situations: as the type of a parameter, field, or local variable; sometimes as a return type (though it is better programming practice to be more specific).

The wildcard is never used as a type argument for a generic method invocation, a generic class instance creation, or a super type.

Why use wildcards in Java generics and not generic methods?

1 Answers1