I have started to create an app, this one has all the frontend in java using the android studio. It occurred to me that it would be a good idea to create the backend in golang but I had some problems to start with. How do I connect what I created in golang with the android studio project? How do I define golang classes for CRUD?
You need to setup a server in the backend that speaks with a protocol that your frontend understands. If crud is all you need then REST might be a good choice.
Depending on what you need you might also work with grpc, or whatever else fits your use case. As long as the client speaks the same you are fine.