Daniel @ RethinkDB here. We're going to ship an official Java driver very soon. We decided to focus on a small number of core drivers first while the protocol and query language were still undergoing rapid extension. Now that the protocol is stable, we're going to expand our official driver support step by step. The Java one will be first.

You can follow the progress on https://github.com/rethinkdb/rethinkdb/issues/3930