Notes

We put the classes in separate files; here's the client class:

import java.awt.*; 
import javax.swing.*; 

public class Client { 

  public void registerWith(ServerInterface server) {
    System.out.println("Registering with the server now...");     
  } 
}

We already note something: server and client access each other through their public interfaces.

public interface ServerInterface {

}

The server is minimal:

public class Server implements ServerInterface {

}

We can run the setup now (if we wanted to). If we did, this would be test no. 1.