javagenericsjls

How to create a class literal of a known type: Class<List<String>>

Take the following:

public Class<List<String>> getObjectType() {
    // what can I return here?
}

What class literal expression can I return from this method which will satisfy the generics and compile? List.class won't compile, and neither will List.<String>class.

If you're wondering "why", I'm writing an implementation of Spring's FactoryBean<List<String>>, which requires me to implement Class<List<String>> getObjectType(). However, this is not a Spring question.

edit: My plaintive cries have been heard by the powers that be at SpringSource, and so Spring 3.0.1 will have the return type of getObjectType() changed to Class<?>, which neatly avoids the problem.

Solution

  • You can always cast to what you need, like

    return (Class<List<String>>) new ArrayList<String>().getClass();

    or

    return (Class<List<String>>) Collections.<String>emptyList().getClass();

    But I assume that's not what you are after. Well, it works, with a warning, but it isn't exactly "beautiful".

    I just found this:

    Why is there no class literal for wildcard parameterized types?

    Because a wildcard parameterized type has no exact runtime type representation.

    So casting might be the only way to go.