Is n:1 observable:observer with generics possible in java? ( observer pattern )

Question

I would like to listen your opinions about applying observer pattern.

My goal is to implement one concrete observer class that can listen multiple observable.

If I develop each different observer like below, then it would be simple like below.

public class ConcreteObserver implements EventObserverA , EventObserverB {

    updateEventA(EventA event) {}
    updateEvetnB(EventB event) {}

}

In this case, I should write many different observer/observable classes/interfaces with almost same code snippet inside.

To avoid hassle like above, I'd like to generify observer/ but as you guys know multiple inheritance rule does not allow code like below.

public class ConcreteObserver implements Observer<EventA> , Observer<EventB> {

    update(EventA event) {}
    update(EventB event) {}

}

Because I have at least double digit Observable events to observe in one concrete observer, I want to avoid implementing every observer/observable pair separately if possible.

I might guess there could be a better pattern for this case than observer pattern, since observer pattern is designed for n:1 observer:observable while my case needs 1:n observer:observable .

Do you guys have any idea/suggestion for this case?

Answer 1

With the magic of Java 8, your ConcreteObserver doesn't necessarily have to actually implement Observer<EventA> and Observer<EventB>; if you write something like this:

public class ConcreteObserver {
    observeEventA(EventA event) {}
    observeEventB(EventB event) {}
}

then you can use ::updateEventA or ::updateEventB on a ConcreteObserver instance to get a method that is automatically convertible to the Observer<EventA> or Observer<EventB> functional interface. For example, you can write any of the following:

• Observer<EventA> eventAObserver = concreteObserver::updateEventA;
• Observer<EventB> eventAObserver = new ConcreteObserver()::updateEventB;
• observerRegistry.register(EventA.class, concreteObserver::updateEventA);