Return Iterator of interface type

[Toke Noer]

In my "application" I only use interface types from the domian in my GUI layer.
So I have in the domain layer:
public class Table implements ITable {
List<Section> sections; // Need to be Section, since I'm calling functions not available through ISection
}

This section list I want to export the iterator for, in a typesafe manner. So the ITable interface has:
Iterator<ISection> getSectionIterator();

Section implements ISection ofc.

But the only way I can get this to work, is to use the following code i Table:
public Iterator<ISection> getSectionIterator() {
Iterator iter = sections.iterator();
return iter;
}

But this doesn't look nice IMO, first getting the Iterator (without generics) and then return it as a ISection iterator, but I can't see any other way to return a Iterator<ISection> from my List<Section>. Anything obvious I'm missing here? Weird design maybe? I don't want to return Iterator<Section>, because I only want to export the functions known by ISection.

[Steve Luke]

Return an Iterator<? extends ISection>. You will limit callers from using any interface other than ISection, and allow your implementation to return Iterator<Section>.

[Toke Noer]

Thank you for the hint. Didn't know about that syntax for what I understand is co/contra variance.

Just to be sure I used it correctly, here is what I have:

ITable.java
public interface ITable {
    Iterator<? extends ISection> getSectionIterator();
}

Table.java
public class Table implements ITable {
    public Iterator<Section> getSectionIterator() {
        return sections.iterator();
    }
}

And in the code where I have an ITable object, I need to use this syntax, right?

Iterator<? extends ISection> sectionIter = table.getSectionIterator();

Can't say I like the syntax for getting the iterator though. I wish it was just:

Iterator<ISection> sectionIter = table.getSectionIterator();

But that won't compile for me.

[Steve Luke]

That code looks good to me. More important, it compiles :-)

Toke Noer wrote:

Iterator<? extends ISection> sectionIter = table.getSectionIterator();

Can't say I like the syntax for getting the iterator though. I wish it was just:

Iterator<ISection> sectionIter = table.getSectionIterator();

But that won't compile for me.

There has to be extra syntax, though. There is a real difference between guaranteeing every object in the collection is of the exact type X, and guaranteeing every object in the collection is either of type X or some sub-type of X. So there would need to be different syntax to indicate the two situations:

1) all objects in the collection are of the exact type X ==> Iterator<X>
2) all objects in the collection are assignable to the type X ==> Iterator<? extends X>

I guess it could have been less wordy, but on the other hand it is rather expressive so it works well in a language like Java which tends to favor readability at the cost of wordiness over pithiness at the cost of readability.

[Toke Noer]

Thank you for the help and explanation. Appreciated!