Problem with Observer Pattern and generics in Java

Question

I have made a generic Observer interface and an Observable class, but can't compile my class due to some generics problem. I don't know precisely why what I'm trying to do is forbidden. The code follows:

public class Observable<U> {

    private List<Observer<Observable<U>, U>> _observers = 
            new ArrayList<Observer<Observable<U>, U>>();

    public void addObserver(Observer<? extends Observable<U>, U> obs) {
        if (obs == null) {
            throw new IllegalArgumentException();
        }
        if (_observers.contains(obs)) {
           return;
        }
        _observers.add(obs); // This line does not compile
    }

    public void notifyObservers(U data) {
        for (Observer<? extends Observable<U>, U> obs : _observers) {
            // After correction of the list declaration, this line will not compile
            obs.update(this, data); 
        }        
    }
}

interface Observer<T, U> {
    public void update(T entity, U arg);
}

Answer 1

Change your _observers definition to this:

private List<Observer<? extends Observable<U>, U>> _observers = 
        new ArrayList<Observer<? extends Observable<U>, U>>();

If you want to allow sublclasses you need to specify this in the declaration, not just in the place you're using it