Hey @kanavsha , great question. Unfortunately, we do not have a Java language client for App Search today. There is a plan to have a Java Client eventually, but it has not yet been started. This should not, however, prevent you from interacting with App Search from your java applications. We have fully documented our REST API, and you can use your Java REST client of choice to interface with those JSON APIs.
While I'm not aware of any community-built Java clients for App Search, you may be able to get some ideas by looking at this community-built Java client for Workplace Search. They are different endpoints, but the same concerns (bearer auth, Enterprise Search base URL, JSON serialization, etc) are present there.
Good luck!