How can I return a list of a certain type when calling a generic method that accepts a Class<T> and returns a T?

Question

I'm calling a method from a library with this signature:

public <T> T get(Class<T> c)

And I'd like to get a List<MyClass> as a return value. But calling it like this does not compile ("Cannot select from parameterized type"):

List<MyClass> myClasses = get(List<MyClass>.class);

This compiles, but gives a warning:

List<MyClass> myClasses = get(List.class);

The warning says "Unchecked Assignment". How can I avoid this warning and avoid casting my list?

Answer 1

You are using generics here, so the underlying type of item in the list has been erased at runtime and the compiler knows it can't check it for you at compile time thus the warning. If you use a generic type here you won't be able to avoid the warning. You can suppress it if you know for sure that the cast you are doing won't lead to an exception.

@SuppressWarnings("unchecked")

Answer 2

Note: I post this more as a point of interest than as a recommended answer. You're generally better off simply suppressing warnings.

If you really hate suppressing warnings but you also hate seeing the warnings, there is a hacky alternative to the accepted answer. You can instantiate an archetype object, such as ArrayList<MyClass>, and then pass archetype.getClass() into the generic method instead of List.class as done in the example and the accepted answer.

This would look like:

ArrayList<MyClass> archetype = new ArrayList<>();
List<MyClass> myClasses = get(archetype.getClass());

One obvious drawback here is the unnecessary cost of instantiating the archetype object, but this can be mitigated somewhat by making it a static instance. A bigger drawback, in my estimation, is that you have to use a concrete class as your archetype, which can be needlessly limiting.