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?

Is there a reason you need to store a variable and not just use getClass() when needed? — Kathy Van Stone
– Kathy Van Stone, Commented Jun 23, 2009 at 22:04
The reason is I am using enum types in a factory. The factory method takes one of the enums, and instantiates the appropriate class. There are other things stored in the enum as well, so this seemed to make sense to me, at least. — pkaeding
– pkaeding, Commented Jun 24, 2009 at 14:57

Michael Myers · Accepted Answer · 2009-06-24 14:48:09Z

13

You need a bounded wildcard:

Class<? extends Super> classObj;

See the lesson on wildcards from the Java tutorials.

edited Jun 24, 2009 at 14:48

answered Jun 23, 2009 at 21:08

Michael Myers♦

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kgrad · Accepted Answer · 2009-06-23 21:24:22Z

1

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.

edited Jun 23, 2009 at 21:24

answered Jun 23, 2009 at 21:13

kgrad

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3 Comments

Michael Myers Over a year ago

Don't mind me; I just keep a (rough) running total of people who misspell my name. I only started a month ago and I'm up to 17 already. And one of them is Jeff Atwood--I'm practically almost sort of famous!

kgrad Over a year ago

Ah, my apologies, the situation has been rectified.

Michael Myers Over a year ago

No problem; I'm used to it by now. (My brother once got a letter which was addressed to him, except that they misspelled all three of his names. He eventually got over it.)

George · Accepted Answer · 2009-06-24 06:52:49Z

1

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.

edited Jun 24, 2009 at 6:52

answered Jun 23, 2009 at 22:35

George

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