rmi problem

maybe someone can help me with my problem
I have a core and decentralized computers
I want to get data from serial ports on the remote machines
My problem is now. We have implemented it before with a self written console server.
We have the following design:
the core on the central unit
on the decentralised computers I have a class ParentDevice
all classes which are reding data are a subclass from this
My problem is I need this class also in the core
Now I have compilation probelems with ant
I tried to copy the class over to the core and put it in the classpath but no effect
Does anyone have a solution for such a problem?