import java.rmi.*;import java.rmi.server.*;import java.rmi.registry.Registry;import java.rmi.registry.LocateRegistry;import java.net.*;import java.io.*;/** * This class represents the object server for a distributed * object of class Callback, which implements the remote  * interface CallbackInterface. * @author M. L. Liu */public class CallbackServer  {  public static void main(String args[]) {    InputStreamReader is =       new InputStreamReader(System.in);    BufferedReader br = new BufferedReader(is);    String portNum, registryURL;    try{           System.out.println(        "Enter the RMIregistry port number:");      portNum = (br.readLine()).trim();      int RMIPortNum = Integer.parseInt(portNum);      startRegistry(RMIPortNum);      CallbackServerImpl exportedObj =         new CallbackServerImpl();      registryURL =         "rmi://localhost:" + portNum + "/callback";      Naming.rebind(registryURL, exportedObj);      System.out.println("Callback Server ready.");    }// end try    catch (Exception re) {      System.out.println(        "Exception in HelloServer.main: " + re);    } // end catch  } // end main  //This method starts a RMI registry on the local host, if  //it does not already exists at the specified port number.  private static void startRegistry(int RMIPortNum)    throws RemoteException{    try {      Registry registry =         LocateRegistry.getRegistry(RMIPortNum);      registry.list( );          // This call will throw an exception        // if the registry does not already exist    }    catch (RemoteException e) {       // No valid registry at that port.      Registry registry =         LocateRegistry.createRegistry(RMIPortNum);    }  } // end startRegistry} // end class