newbie question: OJB cannot treat subclass as superclass?

[shivaken <sh...@e-can.ne.jp>]

I'm using polymorphism around my business classes in order to add new 
subclasses dose not have unique attribute.
So, I want to add persistence them with OJB
and want to store in the same table. But, it seems to be difficult.
# All subclasses don't have own field.

I created a abstract Call class, and LocalCall class as a subclass of it.

Then broker.store(localCall) causes 
 org.apache.ojb.broker.metadata.ClassNotPersistenceCapableException:
 com.mydomain.LocalCall not found in OJB Repository

I know persistent classes has to have default (no parameter) constructor.
# They seem to be constructed with that. Then, abstract class cannot be used
# with OJB.

Are there any good ways?

-- shivaken