Requirements
I am trying to write an Observer, Observable pair of classes. I want to parametrize the Observer so that a type safe update call is possible. Imagine this version:
class View implements Observer<Model> {
@Override
public void update(Model model) { render(model); } // no casting:)
}
instead of this version that needs casting:
class View implements Observer {
@Override
public void update(Object model) { render((Model) model); } // casting:(
}
Attempt
Here's what I have so far. My Observer interface:
public interface Observer<T extends Observable> {
public void update(T observable);
}
and my Observable abstract class:
import java.util.List;
public abstract class Observable {
private List<Observer<? extends Observable>> observers;
public Observable() {
System.out.println(this.getClass());
}
public void addObserver(Observer<? extends Observable> obs) {
observers.add(obs);
}
public void removeObserver(Observer<? extends Observable> obs) {
observers.remove(obs);
}
protected <E extends Observable> void updateObservers(E self) {
for (Observer<? extends Observable> obs : observers) {
obs.update(self); // <-------------------- COMPILER ERROR!!!
}
}
}
Problem
The line labeled "COMPILER ERROR" has a problem with the .update()
:
The method update(capture#4-of ? extends Observable) in the type Observer is not applicable for the arguments (E)
So even though the self
argument passed to update()
is of type E extends Observable
, it does not satisfy the interface method signature update(T observable);
where T extends Observable
. Why is that? I really expected those to be compatible.
Is there I can fix this to meet my requirements?
Thanks.
Aucun commentaire:
Enregistrer un commentaire