The entity bean model is a component model,
it does not directly support polymorphism.
You create bean instances on the home interface
MyBean bean = home.create(someParameters);
or find them
bean = home.findBySomething(someParameters);
The spec insists the home interface must return
exactly the remote interface.
There are some articles on the web about
how to do it. They all involve doing
some book-keeping yourself that is normally
done by the java "this" pointer.