Why doesn't this generic interface stack work?

Question

I have a bunch of generic interfaces and classes

public interface IElement {
// omited
}

class Element implements IElement {
// omited
}

public interface IElementList<E extends IElement>  extends Iterable {
   public Iterator<E> iterator();
}

class ElementList implements IElementList<Element> {

    public Iterator<Element> iterator() {
       // omited
       }
}


public interface IElementListGroup<E extends IElementList<? extends IElement>> {
    public E getChosenElementList();
}


class ElementListGroup implements IElementListGroup<ElementList> {
    public ElementList getChosenElementList() {
     // omited
    }
}

And then a simple code

ElementListGroup group;

for(Element e : group.getChosenElementList())
{
 // omited
}

And the line with for keyword throwe a "cannot convert from element type Object to Element" compiler error.

Thanks in advance.

Answer 1

IElementList needs to implement Iterable<E>. Otherwise, the interface specifies Iterator iterator(), not Iterator<E> iterator(). This makes the compiler think that you're iterating over Objects.

I made this change, and it compiled fine (after adding some null returns).

Answer 2

Your function returns an ElementList not an Element, and ElementList is not an iterable over Element