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);
}