Java Collections: Pass collection of children as collection of parents

Question

Say I have an interface and some classes:

public interface IPanel<ComponentType extends Component> {
   public void addComponents(Set<ComponentType> components);
   public ComponentType create();
}

public class Button extends Component { }

public class LocalizedButton extends Button { }

public class ButtonsPanel implements IPanel<Button> {
    public void addComponents(Set<Button> components) { ... /* uses create() */ ; }
    public Button create() { return new Button(); }
}

public class LocalizedButtonsPanel extends ButtonsPanel {
    public Button create() { return new LocalizedButton(); }
}

Then I have a set of LocalizedButtons and when I call

final LocalizedButtonsPanel localizedButtonsPanel = new LocalizedButtonsPanel();
final Set<LocalizedButton> localizedButtonsSet = new LinkedHashSet<LocalizedButton>();
localizedButtonsPanel.addComponents(localizedButtonsSet);

I get that this method is not applicable for this arguments. If I try to overload this method as addComponents(Set<LocalizedButton> buttons) in LocalizedButtonsPanel, I get type erasure, of course.

May be some pattern is missed or the trick exists to deal with this architecture to implement correct adding the set of LocalizedButtons?

---

I've got the answer and I want to make my example more concrete - I have some validators in my implementation, so I need the collection type to be also stored as generic, that is simplified code I've got using the answer:

public interface IPanel<ComponentType extends Component, CollectionType extends Collection<? extends Component>> extends Validated<CollectionType> {
   public void addComponents(CollectionType components);
   public ComponentType create();
}

public class Button extends Component { }

public class LocalizedButton extends Button { }

public class ButtonsPanel implements IPanel<Button, Set<? extends Button>> {
    public void addComponents(Set<? extends Button> components) { ... /* uses create() */ ; }
    public Button create() { return new Button(); }
}

public class LocalizedButtonsPanel extends ButtonsPanel {
    public Button create() { return new LocalizedButton(); }
}

And in this case, it works

Answer 1

You have

public class ButtonsPanel implements IPanel<Button> {
    public void addComponents(Set<Button> components) { ... /* uses create() */ ; }
    public Button create() { return new Button(); }
}

Should be

public class ButtonsPanel implements IPanel<Button> {
    public void addComponents(Set<? extends Button> components) { ... /* uses create() */ ; }
    public Button create() { return new Button(); }
}

The list is created only for the type of button not for Object that extend that type.

Answer 2

Change the addComponents() signature to

public void addComponents(Set<? extends Button> components)

so that the methods accepts sets of subclasses of Button. This way, you can pass a Set<LocalizedButton> as an argument, because LocalizedButton extends Button and therefore matches the parameter Type of Set<? extends Button>.

Answer 3

Parameterised types are not covariant in Java. This is good, otherwise you would be able to add a Dog to a List<Cat> that had been upcast to List<Animal>. You can add covariance at the usage site, e.g., List<? extends Animal> can be assigned a List<Cat> and you can't call its add method.