How to get parametrized Class instance

Question

Since generics were introduced, Class is parametrized, so that List.class produces Class<List>. This is clear.

What I am not able to figure out is how to get a instance of Class of type which is parametrized itself, i.e. Class<List<String>>. Like in this snippet:

public class GenTest {
    static <T> T instantiate(Class<T> clazz) throws Exception {
        return clazz.newInstance();
    }
    public static void main(String[] args) throws Exception {
        // Is there a way to avoid waring on the line below
        // without using  @SuppressWarnings("unchecked")?
        // ArrayList.class is Class<ArrayList>, but I would like to
        // pass in Class<ArrayList<String>>
        ArrayList<String> l = GenTest.instantiate(ArrayList.class);
    }
}

I run into variations of this problem quite often and I still don't know, if I just miss something, or if there is really no better way. Thanks for suggestions.

Answer 1

The Class class is a run-time representation of a type. Since parametrized types undergo type erasure at runtime, the class object for Class would be the same as for Class<List<Integer>> and Class<List<String>>.

The reason you cannot instantiate them using the .class notation is that this is a special syntax used for class literals. The Java Language Specification specifically forbids this syntax when the type is parametrized, which is why List<String>.class is not allowed.

Answer 2

Classes represent classes loaded by a class loader, which are raw types. To represent a parameterized type, use java.lang.reflect.ParameterizedType.