import java.rmi.*;
import java.rmi.server.*;
import java.util.Vector;

/**
 * This class implements the remote interface 
 * CallbackServerInterface.
 * @author M. L. Liu
 */

public class CallbackServerImpl extends UnicastRemoteObject
     implements CallbackServerInterface {

   private Vector clientList;


   public CallbackServerImpl() throws RemoteException {
      super( );
     clientList = new Vector();
   }

  public String sayHello( )   
    throws java.rmi.RemoteException {
      return("hello");
  }

  public synchronized void registerForCallback(
    CallbackClientInterface callbackClientObject)
    throws java.rmi.RemoteException{
      // store the callback object into the vector
      if (!(clientList.contains(callbackClientObject))) {
         clientList.addElement(callbackClientObject);
      System.out.println("Registered new client ");
      doCallbacks();
    } // end if
  }  

// This remote method allows an object client to 
// cancel its registration for callback
// @param id is an ID for the client; to be used by
// the server to uniquely identify the registered client.
  public synchronized void unregisterForCallback(
    CallbackClientInterface callbackClientObject) 
    throws java.rmi.RemoteException{
    if (clientList.removeElement(callbackClientObject)) {
      System.out.println("Unregistered client ");
    } else {
       System.out.println(
         "unregister: clientwasn't registered.");
    }
  } 

  private synchronized void doCallbacks( ) throws java.rmi.RemoteException{
    // make callback to each registered client
    System.out.println(
       "**************************************\n"
        + "Callbacks initiated ---");
    for (int i = 0; i < clientList.size(); i++){
      System.out.println("doing "+ i +"-th callback\n");    
      // convert the vector object to a callback object
      CallbackClientInterface nextClient = 
        (CallbackClientInterface)clientList.elementAt(i);
      // invoke the callback method
        nextClient.notifyMe("Number of registered clients="
           +  clientList.size());
    }// end for
    System.out.println("********************************\n" +
                       "Server completed callbacks ---");
  } // doCallbacks

}// end CallbackServerImpl class