Callbacks help.

Question

I am kinda iffy on what callbacks actually are, and I would like some verification to see if I am thinking correctly.
Definition: 'Master' class that can use the code of a 'slave' class by implementing a 'listener' when an action is made.
Heres some random code to see if i get the idea:
Listener:
public interface listener
public abstract void message(String msg);
Slave:
public class 'slave' implements 'listener'
something.addListener(this);
public vod message(String e)
if (e.equalsIgnoreCase("Something"));
do something here
Master:
public class Master
listener listens;
public void addListener(listener h)
listens = h;
public void something()
listens.message("Something");
}

Answer 1

That's one way.
But it's usual to allow multiple listeners, to supply you "Master" class with addXXXListener, removeXXXListener. Classes.
e.g.
class Master {
     ArrayList  snafuListeners;
    public void addSnafuListener(SnafuListener l) {
       if(snafuListeners == null) snafuListeners = new ArrayList(3);
       snafuListeners.add(l);
   public void removeSnafuListener(SnafuListener l) {
        if(snafuListeners != null)
           snafuListeners.remove(l);
    protected void fireSnafuListeners(SnafuEvent evt) {
         if(snafuListeners != null)
            for(Iterator it = snafuListeners.iterator(); it.hasNext();)
               ((SnafuLIstner)it.next()).message(evt);
         }It's also very common to use small, anonymous classes as listeners, which allows your code to respond differently to the same event type from different sources. e.g.
master1.addSnafuListener(new SnafuListener() {
    public void message(SnafuEvent e) {
         master1Snafued();
     });