Use iterator, moveNext(), current.

sdk/lib/core/queue.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * A [Queue] is a collection that can be manipulated at both ends. One
  * can iterate over the elements of a queue through [forEach] or with
  * an [Iterator].
  */
 abstract class Queue<E> extends Collection<E> {
@@ skipping 289 matching lines @@
     Queue<E> other = new Queue<E>();
     DoubleLinkedQueueEntry<E> entry = _sentinel._next;
     while (!identical(entry, _sentinel)) {
       DoubleLinkedQueueEntry<E> nextEntry = entry._next;
       if (f(entry._element)) other.addLast(entry._element);
       entry = nextEntry;
     }
     return other;
   }
 
-  _DoubleLinkedQueueIterator<E> iterator() {
+  _DoubleLinkedQueueIterator<E> get iterator {
     return new _DoubleLinkedQueueIterator<E>(_sentinel);
   }
 
   String toString() {
     return Collections.collectionToString(this);
   }
 }
 
 class _DoubleLinkedQueueIterator<E> implements Iterator<E> {
   final _DoubleLinkedQueueEntrySentinel<E> _sentinel;
-  DoubleLinkedQueueEntry<E> _currentEntry;
+  DoubleLinkedQueueEntry<E> _currentEntry = null;
 
-  _DoubleLinkedQueueIterator(_DoubleLinkedQueueEntrySentinel this._sentinel) {
-    _currentEntry = _sentinel;
+  _DoubleLinkedQueueIterator(this._sentinel);
+
+  bool moveNext() {
+    if (identical(_currentEntry, _sentinel)) return false;

[Lasse Reichstein Nielsen, 2012/11/15 10:24:37]

The 'null' represents the initial state, right?
Documentation, please!

How about a boolean named '_done' that gets set when you move to the sentinel:
bool moveNext() {
  if (_done) return false;
  _currentEntry = _currentEntry._next;
  _done = identical(_currentEntry, _sentinel);
  return !_done;
}

Do use identical when it's what we really mean (and it is for linked lists and
sentinels in general), so I don't have to wonder what happens if the Entry class
implements operator==.
  

[floitsch, 2012/11/16 17:51:58]

in theory there is no need to use identical since the elements are
DoubleLinkedQueueEntrys.
added comment, and used identical below.
this will become different with the new approach.

+    if (_currentEntry == null) _currentEntry = _sentinel;
+    _currentEntry = _currentEntry._next;
+    return _currentEntry != _sentinel;
   }
 
-  bool get hasNext {
-    return !identical(_currentEntry._next, _sentinel);
-  }
-
-  E next() {
-    if (!hasNext) {
-      throw new StateError("No more elements");
+  E get current {
+    if (_currentEntry != null && _currentEntry != _sentinel) {
+      return _currentEntry.element;
     }
-    _currentEntry = _currentEntry._next;
-    return _currentEntry.element;
+    // TODO(floitsch): adapt error message.
+    throw new StateError("No more elements");
   }
 }