If you want to use just API of splunk, please refer to the following manual.
http://docs.splunk.com/Documentation/Splunk/latest/Developer/RESTIntro
The SDK of Java is not yet officially supported although there was unofficial SDK in google community site before.
The Java SDK will be available sometime TODAY as per psanford's answer here: http://splunk-base.splunk.com/answers/36346/universal-forwarder-as-a-language-api?page=1#36447