[Eranga]

Ok, I'll explain it in simple way as follows.

Code:

public class Client{
    String username;
    
    // Get the user name through a GUI and set it to username

    // Object for class Server
    Server myServer = new Server();
    myServer.isValid(username);
}


public class Server{
   // Check that username is correct
   public void isValid(String name){
        // Check that user name is valid, and so do the process
   }
}

Is that clear now, or need more explanations.

Actually if you don't know how those basis things done, please spend some time with Java fundamentals. If you don't know such things to do, how can you handle a server-client application.

In a such a project you have to do much more than this, it's not going to be easy for you pal