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

Question

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 Iterator? It would probably go a long way to making me live with the current design.

Answer 1

Why doesn't the enhanced for loop just accept an iterator?

I want to gather a few of the potential reasons from the various answers as to why the for-each loop doesn't simply accept an iterator.

1. Convenience: The for-each loop was created partly for convenience for the common operation of performing an action given each element of a collection. It has no obligation or intention of replacing the explicit use of iterators (obviously if you want to remove elements, you need an explicit reference to the iterator).
2. Readability: The for-each loop for ( Row r : table ) is meant to be extremely readable as "for each row "r" in table...". Seeing for ( Row r : table.backwardsIterator() ) breaks that readability.
3. Transparency: If an object is both an Iterable and an Iterator, what will be the behaviour? Though it's easy to make a consistent rule (e.g. Iterable before Iterator) the behaviour will be less transparent to developers. Furthermore, this will have to be checked at compile time.
4. Encapsulation/Scope: This is (in my opinion) the most important reason. The for-each loop is designed to encapsulate the Iterator and limits its scope to the loop. This makes the loop "read-only" in two ways: it doesn't expose the iterator, meaning there is nothing (easily) tangible that has its state altered by the loop, nor can you alter the state of the operand in the loop (as you can by interfacing directly with an Iterator via remove()). Passing the Iterator yourself necessarily means the Iterator is exposed, making you lose both of those "read-only" attributes of the loop.

Answer 2

What I'd probably do is just make a utility class called Deques which could support this, along with other utilities if desired.

public class Deques {
  private Deques() {}

  public static <T> Iterable<T> asDescendingIterable(final Deque<T> deque) {
    return new Iterable<T>() {
      public Iterator<T> iterator() {
        return deque.descendingIterator();
      }
    }
  }
}

This is another case where it's really too bad we don't have lambdas and method references yet. In Java 8, you'll be able to write something like this given that the method reference descendingIterator() matches the signature of Iterable:

Deque<String> deque = ...
for (String s : (Iterable<String>) deque::descendingIterator) { ... }

Answer 3

Rather than create a descendingIterator, it would be better to write a descendingIterable() method to return a descending iterable based on a deque- which basically takes the place of your anonymous class. That seems pretty reasonable to me. As per Colin's suggestion, the iterable implementation returned by this method would call descendingIterator on the original deque each time its own iterator() method was called.

If you've only got an iterator and want to keep it that way, you'd have to write an implementation of Iterable<T> which wrapped the iterator and returned it exactly once, throwing an exception if iterator() is called more than once. That would work, but it would clearly be pretty ugly.

Answer 4

The idiomatic way in Java 8 (being a verbose language) is this:

for (T t : (Iterable<T>) () -> myDeque.descendingIterator()) {
  // use item
}

I.e. wrap the Iterator in an Iterable lambda. This is pretty much what you did yourself using an anonymous class, but it's a bit nicer with the lambda.

Of course, you could always just resort to using Iterator.forEachRemaining():

myDeque.descendingIterator().forEachRemaining(t -> {
  // use item
});

Answer 5

Guava users can do ImmutableList.copyOf(Iterator) to safely convert an Iterator into an Iterable. Despite the seeming simplicity of looping over an Iterator, there are concerns that foreach hides, and the safest option is to create a stable data structure like a list.

This is also discussed in the Idea Graveyard:

The biggest concern is that Iterable is generally assumed to be able to produce multiple independent iterators. The doc doesn't say this, but the Collection doc doesn't say this, either, and yet we assume it of its iterators. We have had breakages in Google when this assumption was violated.

The simplest workaround is ImmutableList.copyOf(Iterator), which is pretty fast, safe, and provides many other advantages besides.

Answer 6

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).

Answer 7

The Apache Commons Collections API has a class called IteratorIterable to do exactly this:

Iterator<X> iter;
for (X item : new IteratorIterable(iter)) {
    ...
}

Answer 8

Guava does of course have a solution for the reverse iterable scenario, but unfortunately you need two Steps. Iterables.reverse() takes a List as parameter, not an Iterable.

final Iterable<String> it = Arrays.asList("a", "b", "c");
for(final String item : Iterables.reverse(Lists.newArrayList(it))){
    System.out.println(item);
}

Output:

c
b
a