get a good security for multiplayer game

Question

I made a game using eclipse. I have a server program and a client program. The clients communicate with the server using sockets. If the client starts it sends its username

Answer 1

Keep the sensitive state in the server program and have the client request the current value. Then you control it at the server.

If someone has valid credentials, then they are able to do whatever your protocol permits the user to do within their context. This is something you must control from the server program to prevent unintended operations by the client from impacting on overall integrity.