Class object type parameterization in Java

Question

Suppose the following object structure:

class Super {}

class SubA extends Super {}
class SubB extends Super {}

I want to be able to have a variable that will hold the class object for either of my subclasses. I feel like this should do it:

Class<Super> classObj;

Then, I want to be able to something like this:

classObj = SubA.class;

or:

classObj = SubB.class;

This doesn't work though. I get the following error:

Type mismatch: cannot convert from Class<SubA> to Class<Super>

Any ideas why? What do I need to fix?

Answer 1

You need a bounded wildcard:

Class<? extends Super> classObj;

See the lesson on wildcards from the Java tutorials.

Answer 2

As mmyers pointed out you could use wildcarding.

As an alternative, you could have the classes implement a common interface and then access them through that interface.

Answer 3

Try this (fails to compile):

public class ListCopy {
  public static void main(String[] args) {
    List<String> stringList = new ArrayList<String>();
    List<Object> objectList = stringList;
  }
}

This will fail to compile even though String extends Object. Java stuff is not covariant - type parameters are erased, so the compiler does not know what will be there at runtime.

It's the same with Class.