Question

Suppose I have the following method, which can be used to create a collection of a given type specified.

private static Collection<?> void create(Class<? extends Collection<?>> cls) {
    return cls.newInstance();
}

This is all good if the cls argument is passed in during runtime:

List<String> list = new LinkedList<String>();
create(list.getClass());

But how do I invoke this method in code without an unchecked warning? Say I want to do something like:

create(LinkedList.class);

It’ll complain that create(Class) is not defined, which strictly speaking is correct because List is not #capture of Collection, but how do I make it work?

Many thanks!

Answer 1

Neal Gafter talks about pretty much exactly this problem here. The fix is to use super type tokens. This is what Guice uses to maintain generic type information, I believe.

Basically it’s an extra class you can use to represent a type instead of using Foo.class. You’d use it like this:

TypeReference<LinkedList<String>> x = new TypeReference<LinkedList<String>>() {};
create(x);

Note that your attempt to use the raw type LinkedList would still cause the compiler to complain, and deliberately so – when you use raw types, you’re basically opting out of type safety. But this scheme allows you to express the generic type in a way which is otherwise harder.