Question

I am implementing the Observer / Observable pattern using Java. However, I am encountering a problem in the Observer portion of my code.

Observable

public class Model extends Observable {

   public void notify() {
       this.setChanged();
       this.notifyObservers(new ArrayList<A>());
       this.notifyObservers(new ArrayList<B>());
   }
}

Observer

public class View implements Observer {

    @Override
    public void update(Observable observable, Object object) {
        // TODO: If object is ArrayList<A>?
        System.out.println("A");
        // TODO: If object is ArrayList<B>?
        System.out.println("B");
    }
}

How would I fill in the TODO comments to check for the generic on the ArrayList? Is this even possible? (I would prefer to do it without implementing more classes, if possible.)

Answer 1

An Observable should send one and only one type of data.

public class ModelA extends Observable {

   public void notify() {
       this.setChanged();
       this.notifyObservers(new ArrayList<A>());
   }
}

public class ModelB extends Observable {

   public void notify() {
       this.setChanged();
       this.notifyObservers(new ArrayList<B>());
   }
}

Your other alternative is to put ArrayList<A> and ArrayList<B> into a class. You can notify your observers with that class.