Client.java

package com.rmicallback.client;

import com.rmicallback.server.ServerInterface;
import java.rmi.NotBoundException;
import java.rmi.RMISecurityManager;
import java.rmi.RemoteException;
import java.rmi.registry.LocateRegistry;
import java.rmi.registry.Registry;

public class Client {

    private static final String SRVHOST = "localhost";
    private static Registry registry;
    private static ClientInterface callback;

    public static void main(String[] args) {

        System.setProperty("java.security.policy", "src/cli.policy");

        if(System.getSecurityManager()==null)
            System.setSecurityManager(new RMISecurityManager());

        try {
            registry = LocateRegistry.getRegistry(SRVHOST, Registry.REGISTRY_PORT);
            ServerInterface services = (ServerInterface) registry.lookup("ServerAPI");
            callback = new ClientImpl();

           int nCallback = services.setClientInterface(callback);
           services.action(nCallback);
           System.out.println(callback.getN());
           callback.setN(5);
           System.out.println(callback.getN());
           services.action(nCallback);
        } catch (RemoteException | NotBoundException ex) {
            ex.printStackTrace();
        }
    }
}