Extending an entity bean will make your application difficult to map to a RDB source and port to other
J2EE application servers. IBM's WebSphere AS supports entity bean inheritance, but you must define your hierarchy in a IBM-specific DD file. This makes it impossible to port to a different AS. The RDB mapping on the other hand, is a choice between a denormalized table structure and introduction of a new table.
I would suggest you rethink the idea of generalizing an entity bean. Sun does discourage the use of non-standard features in the implementation of EJBs. Providing a server neutrality is one of their chief goals.
On the other hand, one can find different ways to implement the same functionality without the use of entity bean inheritance. For instance, instead of reuse by subclassing, you can consider reuse by delegation.
