Hi @ieugen,
apart from the external task client (which is a REST client for parts of the API), there is no official Java client. There is a project aiming to integrate swagger with the REST API, however, I think this is not up-to-date. You can find it on GitHub: https://github.com/camunda/camunda-bpm-swagger
Also, have a look into this topic.
Cheers,
Miklas