Question

I have a class MyObserver that listens to changes in Notifier. Notifier extends Observable and notify its events with notifyObservers(Object). The object passed as argument is always an instance of the same class. The problem is that each observer need to listen to diferent events. For example one observer needs to listen to state changed events and others to all types of events. How can I do this with observer pattern?

Thanks.

Answer 1

If you can change the design a bit:

interface MyObserver {
    public void stateChangeEvent();
    public void otherEvent();
}

class MyObserverAdapter implements MyObserver {
    public void stateChangeEvent() {
         // some default implementation or no implementation.
    }

    public void otherEvent() {
         // some default implementation or no implementation.
    }
}

class MyStateChangeObserver extends MyObserverAdapter {
    public void stateChangeEvent() {
         // implement behavior specific to this class.
    }
}

class MyOtherObserver extends MyObserverAdapter {
    public void otherEvent() {
         // implement behavior specific to this class.
    }
}

Usage:

MyObserver stateObserver = new MyStateChangeObserver();
MyObserver otherObserver = new MyOtherObserver();
notifier.notifyObservers(stateObserver);
notifier.notifyObservers(otherObserver);