Prior to Content Manager 9.0 there was no supported Java client for the ServiceAPI, although I did publish an experimental project. As of 9.0 we are able to use the ServiceStack Java client. This video demonstrates that client.
The sample project I created in the video is on our samples site. Beware, in the video I omit the call to 'setAlwaysSendBasicAuthHeaders', you should always call this when using basic authentication.