Questions about the observer pattern

[samir.vds]

Hi,

I'm an ongoing student in Business Information Systems.

Anyway, right now I'm trying to understand a design technique called
the observer pattern.

Java provides for the observable object the class
java.util.oberservable. If I want to notifiy my observers about an
event I call the method notifyObservers() to notifiy all my observers.
So in example I've an observable object, which has 2 registered
observers. The observable object contains a couple of events, which
notifies my observers, but some of those events only want to notify one
of the observers. So what's smartest way to do that ?


regards Samir

btw: I hope you understand my problem, because it's very concrete and
also my English is not the best

 

[hiwa]

Hi,

I'm an ongoing student in Business Information Systems.

Anyway, right now I'm trying to understand a design technique called
the observer pattern.

Java provides for the observable object the class
java.util.oberservable. If I want to notifiy my observers about an
event I call the method notifyObservers() to notifiy all my observers.
So in example I've an observable object, which has 2 registered
observers. The observable object contains a couple of events, which
notifies my observers, but some of those events only want to notify one
of the observers. So what's smartest way to do that ?


regards Samir

btw: I hope you understand my problem, because it's very concrete and
also my English is not the best

Use conditional in their update() method.
There would be no way for selective event delivery.
In other words, it is universal for observers.

 

[garethconner]

Hi Samir,

An alternative to using the java.util.observable class is to roll your
own. Create your own listener interface for the observers and build
your observable class with methods for adding and removing listeners.
This listener interface could have mutliple notifications i.e.:

notifyActionA
notifyActionB

The observer classes that are interested in these events would have to
implement the listener interface. However if a class is not interested
in ActionA, it can leave the implementation for that method empty.

The swing classes often use this pattern, for example a JFrame will
accept MouseListeners. Classes that implement the MouseListener
interface can receive various mouse events (mouseClicked, mouseEntered,
etc.), but can simply ignore mouse events that aren't needed.

Hope that helps, I'm a novice with Java so others may have better
suggestions.

-Gareth

 

[Deniz Dogan]

The observable object contains a couple of events, which
notifies my observers, but some of those events only want to notify one
of the observers. So what's smartest way to do that ?

Hello, Samir!

The way I would go about doing what you want is using the Object
parameter of the update(Observable, Object) method:

public void update(Observable o, Object arg) {
if (arg somethingsomething) {
//We know it's for Observer no. 1
}
else if (arg somethingelse) {
//It's for Observer no. 2
}
}

On the other hand, I'm not the best of Java programmers and this may in
fact be a weird solution.

btw: I hope you understand my problem, because it's very concrete and
also my English is not the best

Your English is just fine, don't worry about it.

- Deniz Dogan

 

[samir.vds]

Ok thanks, this helped me a lot guys

 

[Ben_]

How do you determine that one of the observers is not interested ?

If an observer registers, it's to be notified, and it will look at the
observed object to determine what neeeds done. It's not the observed object
than can know what the observer wants to do.