What type is <?> when making instantiating lists?

Question

I have seen in multiple different places people who instantiate a list or ArrayList like:

List<?> l = new ArrayList<>();

What type is ?? Does this mean that it can hold any types in it? If so, why would this be used instead of just and ArrayList?

Answer 1

Does this mean that it can hold any types in it?

No. It means that your l variable could be referring to a list parameterized with any type. So it's actually a restriction: you will not be allowed to add any object to l because you have no idea which items it accepts. To give a concrete example, l could be a List<String> or it could be a List<ExecutorService>.

Answer 2

As correctly pointed by Marko, its an unknown restriction on the List type.

The Java docs says that:

The unbounded wildcard type is specified using the wildcard character (?), for example, List<?>. This is called a list of unknown type. There are two scenarios where an unbounded wildcard is a useful approach:

• If you are writing a method that can be implemented using functionality provided in the Object class.
• When the code is using methods in the generic class that don't depend on the type parameter. For example, List.size or List.clear. In fact, Class<?> is so often used because most of the methods in Class do not depend on T.