Alternative to the Observable Pattern?

Question

I need to perform some tasks on object1 when some state change happens on object2. I was trying to use observable pattern in android, I mean to use Observer and Observable classes, so object1 implements Observer and object2 extends Observable. Unfortunately, object2 already extends from another class, so I can't define it as an extension of Observable.

Is there another alternative to use this pattern? Maybe other classes that allow me to perform this behavior?

Thanks.

Answer 1

Simply add to object2 a field of type Observable to which observers are added and on which notifyObservers() is called when something changes.

This is what's meant by "favor composition over inheritance".

Answer 2

Extending the built-in Observable class is a rather bad choice to implement the observer pattern as it disrupts your class hierarchy, but you know that already.

A better way would be to define your own Observable interface and let your class implement it. You will also need a Listener interface for the listener classes

The methods you would need in your Observable interface are addListener(Listener listener) and removeListener(Listener listener)