How to pass a parameterized class as an argument

Question

My goal is to develop a class that can output an object of a specified class.

public class GetMe<T> {
    public T get() {
        Object obj = generateObject();
        return (T) obj;
    }
}

Now, I know this isn't possible due to erasure. So, we can pass in a class instance and use that to cast.

public class GetMe<T> {
    public GetMe<T>(Class<T> clazz) {
        this.clazz = clazz;
    }

    public T get() {
        Object obj = generateObject();
        return clazz.cast(obj);
    }
}

This works great! As long as the class isn't parameterized. If it is, then I've got a problem.

I'm not allowed to use List<String>.class. If I pass in a ParameterizedType (which in itself is difficult to generate), there's no cast method to use.

Is there a way out of this quagmire?

Answer 1

I think super type tokens may solve this problem for you.

Answer 2

The problem with List<String> is that, because of erasure, it would at runtime indistinguishable from any other List<?>. The easiest way around this is to create a new class or interface which has the generic part "fixed", like

public interface StringList extends List<String> {
    /* nothing to see here */
}

This way you have a type token (the StringList.class object) which you can pass around at runtime and specifies exactly what you want, but without the need for generics at runtime.

Answer 3

Here is just a small idea. I'm not really sure if it will fit in your context but nevertheless:

public class GetMe<T>
{
    public List<T> getList() {
        @SuppressWarnings("unchecked")
        List<T> result = (List<T>) new LinkedList(); 
        return result;
    }
}

Cheers!