Idiomatic way to use for-each loop given an iterator?

When the enhanced for loop (foreach loop) was added to Java, it was made to work with a target of either an array or Iterable.

for ( T item : /*T[] or Iterable<? extends T>*/ ) {
    //use item
}

That works great for Collection classes that only implement one type of iteration, and thus have a single iterator() method.

But I find myself incredibly frustrated the odd time I want to use a non-standard iterator from a Collection class. For example, I was recently trying to help somebody use a Deque as a LIFO/stack but then print the elements in FIFO order. I was forced to do this:

for (Iterator<T> it = myDeque.descendingIterator(); it.hasNext(); ) {
   T item = it.next();
   //use item
}

I lose the advantages of the for-each loop. It's not just about keystrokes. I don't like exposing the iterator if I don't have to, since it's easy to make the mistake of calling it.next() twice, etc.

Now ideally I think the for-each loop should have accepted an Iterator as well. But it doesn't. So is there an idiomatic way of using the for-each loop in these circumstances? I'd also love to hear suggestions that use common collections libraries like Guava.

The best I can come up with in absense of a helper method/class is:

for ( T item : new Iterable<T>() { public Iterator<T> iterator() { return myDeque.descendingIterator(); } } ) {
    //use item
}

Which isn't worth using.

I'd love to see Guava have something like Iterables.wrap to make this idiomatic, but didn't find anything like that. Obviously I could roll my own Iterator wrapper via a class or helper method. Any other ideas?

Edit: As a side-note, can anybody give a valid reason for why the enhanced for-loop shouldn't have been able to just accept an Iterato

public class DescendingIterableDequeAdapter<T> implements Iterable<T> {
    private Deque<T> original;

    public DescendingIterableDequeAdapter(Deque<T> original) {
        this.original = original;
    }

    public Iterator<T> iterator() {
         return original.descendingIterator();
    }
}

And then

for (T item : new DescendingIterableDequeAdapter(deque)) {

}

So, for each such case, you'd need a special adapter. I don't think it is theoretically possible to do what you want, because the facility has to know what iterator-returning methods exist, so that it can call them.

As for your additional question - I believe because the for-each loop was actually meant to make things shorter for general-purpose scenarios. And calling an additional method makes the syntax more verbose. It could've supported both Iterable and Iterator, but what if the object passed implemented both? (would be odd, but still possible).

answered 2010-10-07T15:38:52.193

Idiomatic way to use for-each loop given an iterator?

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