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

Try this:

 public class Observable<U> {

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

        public void addObserver(Observer<Observable<U>, U> obs) {
            if (obs == null) {
                throw new IllegalArgumentException();
            }
            if (_observers.contains(obs)) {
               return;
            }
            _observers.add(obs);            }

        public void notifyObservers(U data) {
            for (Observer<? super Observable<U>, U> obs : _observers) {
                obs.atualizar(this, data);
            }
        }
    }

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

To explain the underlying problem here, generics are forcing an explicit downcast. So you can't take any 'Observable' for any implementation of U and put it into a collection, because that collection is defined as taking a specific type of U, not anything.

For this type of use case, generics have limits, and you may not be able to accomplish what you want in such a type safe way.

EDIT: Would this work for you?

public class Observable<U> {

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

       public void addObserver(Observer<U> obs) {
           if (obs == null) {
               throw new IllegalArgumentException();
           }
           if (_observers.contains(obs)) {
              return;
           }
           _observers.add(obs);            }

       public void notifyObservers(U data) {
           for (Observer<U> obs : _observers) {
               obs.atualizar(this, data);
           }
       }
   }

   interface Observer<U> {
       public void atualizar(Observable<U> entity, U arg);
   }

Answer 2

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