New to Java - Custom EventHandlers

Question

In this sample(it's from the Oracle Site) :

// Notify all listeners that have registered interest for
// notification on this event type.  The event instance 
// is lazily created using the parameters passed into 
// the fire method.

protected void fireFooXXX() {
 // Guaranteed to return a non-null array
 Object[] listeners = listenerList.getListenerList();
 // Process the listeners last to first, notifying
 // those that are interested in this event
 for (int i = listeners.length-2; i>=0; i-=2) {
     if (listeners[i]==FooListener.class) {
         // Lazily create the event:
         if (fooEvent == null)
             fooEvent = new FooEvent(this);
         ((FooListener)listeners[i+1]).fooXXX(fooEvent);
     }
 }
}

What does this

listeners[i]==FooListener.class

Comparison do? It throws me off a bit, since it seems its comparing an instance of a class to a type of class. I would could understand it if it said something like

listeners[i].getClass() == Foolistener.class

But it doesn't...Can someone enlighten me here? Thanks in advance!

Community · Accepted Answer · 2020-06-20 09:12:55Z

1

Because that's what the documentation of getListenerList() says it does.

public Object[] getListenerList()

Passes back the event listener list as an array of ListenerType-listener pairs. Note that for performance reasons, this implementation passes back the actual data structure in which the listener data is stored internally! This method is guaranteed to pass back a non-null array, so that no null-checking is required in fire methods. A zero-length array of Object should be returned if there are currently no listeners. WARNING!!! Absolutely NO modification of the data contained in this array should be made -- if any such manipulation is necessary, it should be done on a copy of the array returned rather than the array itself.

The array is in pairs of Type and Instance. So index zero is the class (or superclass) of the actual listener found at index 1, index 2 is the class of the actual listener at 3, etc.

edited Jun 20, 2020 at 9:12

CommunityBot

11 silver badge

answered Oct 18, 2010 at 22:25

Affe

48.1k12 gold badges85 silver badges86 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Rene Over a year ago

Right, so it's comparing a type(in the array) to class...make's sense. So if I used the odd numbers in the array and used listeners[i].getClass() ==FooListener.class it would do the same?

Affe Over a year ago

Not necessarily. When they're added you specify the type of listener that they will be 'counted' as. The concrete type of the object at index 1 may actually be a subclass of the class at index 0.

SLaks · Accepted Answer · 2010-10-18 22:24:09Z

0

It looks like their array alternates between Class objects and FooListener objects.

For any n, listeners[2n] will contain a Class instance, and listeners[2n + 1] will contain a FooListener instance for that class.

answered Oct 18, 2010 at 22:24

SLaks

891k182 gold badges1.9k silver badges2k bronze badges

Comments

user455695 · Accepted Answer · 2010-10-18 23:08:27Z

0

I'm going to go the complete side-route and suggest not using EventListenerList. It's a terrible bit of code that is only useful if you are either in java 1.4 or earlier or you are using one list to hold listeners for several different listener classes.

I'd recommend instead using a List<FooListener> for your listener holder. You can still do the same stuff that EventListenerList allows, but your code will be easier to follow.

private final List<FooListener> myFooListeners;
...
void fireXXX() {
  FooEvent event = ....
  for (FooListener fl : myFooListeners) {
    fl.xxx(event);
  }
}

answered Oct 18, 2010 at 23:08

user455695

812 bronze badges

Collectives™ on Stack Overflow

New to Java - Custom EventHandlers

3 Answers 3

2 Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

2 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related