callback method and threads

You should use Obersver and Observable. Check out http://javaalmanac.com/egs/java.util/Obs.html

 And also here:

http://www.javaworld.com/javaworld/javatips/jw-javatip10.html
http://home.att.net/~baldwin.rg/Intermediate/Java079.htm
http://home.att.net/~baldwin.rg/Intermediate/Java078.htm
http://home.att.net/~baldwin.rg/Intermediate/Java077.htm

The approach CEHJ is suggesting is very common, and I have used it myself on several occasions.

Here is a simpler approach that might work for you;

// file: ThreadCallback.java

public interface ThreadCallback
{
   public boolean somethingHasChanged();
}

// file: MyThreadThatCallsBack.java

public class MyThreadThatCallsBack
extends Thread
{
   ThreadCallback callBackReceiver;

   public MyThreadThatCallsBack(ThreadCallback callBackReceiver)
   {
      this.callBackReceiver = callBackReceiver;
   }

   public void run()
   {
      boolean done = false;
      do
      {
         done = callBackReceiver.somethingHasChanged();
         if(!done)
         {
            // do some more work
         }
      } while(!done);
   }
}

// file: MyClassThatReceivesCallBacks.java

public class MyClassThatReceivesCallBacks
implements ThreadCallback
{
   public boolean somethingHasChanged()
   {
      // return if something has changed so that the thread should exit
   }
}

Good luck!

An outline to what I am trying to do and then some code ..

Explanation: I want to monitor a queue of objects, when something is added to the queue then I want to do some work on the object that is added then notify an object that there is a change in the queue. At the moment I am merely trying to get the callback stuff working. I am currently using the javaworld article above as a starting point but have not programmed with interface much .. so some help with the following would be useful..

//file: NetInterface.java
public interface NetInterface {

    public void interestingEvent();    
}

//file: EventNotifier.java
import java.util.*;

public class EventNotifier {

    static private NetInterface ie;

    static private boolean somethingHappened;
   
     public EventNotifier (NetInterface event) {

          ie = event;

          somethingHappened = false;
     }

    public static void doWork () {
          //check to see if something has hapenned
          if (somethingHappened) {
              // Signal the event by invoking the interface's method.
               //??
              ie.interestingEvent ();
         }
     }
     
     public static void main(String args[]) {
          HashMap newMap = new HashMap(5);
         
          int i = 0;
         
          while (i<0) {
               newMap.put((Integer.toString(i)), new String("hello "+i));
               i++;
          }
          somethingHappened = true;
          doWork();    
     }
}



//file: CallMe.java
public class CallMe implements NetInterface {

    private EventNotifier en;
   
     public CallMe () {

          // Create the event notifier and pass ourself to it
          en = new EventNotifier (this);
     }
    // Define the actual handler for the event.

    public void interestingEvent () {
          // Wow!  Something really interesting must have occurred!
          // Do something...
          System.out.println("oh something interesting has happened!");
     }
}

 What exactly is the problem?

the above does not work. when it runs it returns null pointer exception ..

Exception in thread "main" java.lang.NullPointerException
     at EventNotifier.doWork(EventNotifier.java:22)
     at EventNotifier.main(EventNotifier.java:36)

 Which one is the line 22 of the EventNotifier? This line is null and thus the programme fails.

public static void doWork () {
     //check to see if something has hapenned
     if (somethingHappened) {
         // Signal the event by invoking the interface's method.
         //??
         ie.interestingEvent ();
}

line 22 is actually the closing brace; so I pressume that ie.interestingEvent is null .. what do I need to do? How should this work?

 Are you sure you have initialized properly the "ie" variable? It seems to me it's still null.

girionis, I have posted the code and asked the question what do _I_ need to do to make it work?

 Ok... I have to go to some birthday in 5 mins. I will look at it later on tonight (or tomorrow morning) and I will let you know, if nobody else does meanwhile.

As you can see, Threads are irrelevant. This functionality is all performed within the same thread. Indeed you don't want asynchronicity here.

Ok I am making v slow progress at the moment, and would like to try and finish this asap. CEHJ I think the following follows on from the help you have provided.

A couple of questions regarding the following .. If I want to make a callback to Left from a different class that would call Right from how would I do this? what call would I make to Right ?

the following process might explain this better:

PressButton ---> right.makeCall() --- > Interface ---> left_is_notified. ----> left do stuff.


//File: CallBack.java

public interface CallBack {
     public void qchange();
}

//File: Left.java

public class Left {
     CallBack c;
     
     public Left(CallBack ca) {
          this.c = ca;
     }
     
     public void go() {
          c.qchange();
     }
}

//File: Right.java

public class Right implements CallBack {
     
     public static void main(String args[]){
          new Right();
     }
     
     public Right() {
          Left l = new Left(this);
         
          l.go();
     }
     
     public void qchange() {
          System.out.println("in Right callback made");
     }
     
}