you need to create an server-api-package and a server-impl-package. You Server-Api-Package contains all interfaces, but no implementation. So your client and your server-impl can include this package and use the same interfaces. If you change an interface, it will automaticly available in the server and the client, there is no code-redundance.
For smaller or learning-projects, you can create this packages by hand, for processional use, maven has good plugins.