Spliterators of iterators

[Paul Sandoz]

On Jun 15, 2015, at 7:31 PM, Martin Buchholz <martinrb at google.com> wrote:
> On Mon, Jun 15, 2015 at 10:16 AM, Paul Sandoz <paul.sandoz at oracle.com> wrote:
> On Jun 15, 2015, at 6:34 PM, Martin Buchholz <martinrb at google.com> wrote:
> > Staring at LinkedTransferQueue.java, I see:
> >
> >                     return Spliterators.spliterator
> >                         (a, 0, i, (Spliterator.ORDERED |
> >                                    Spliterator.NONNULL |
> >                                    Spliterator.CONCURRENT));
> >
> > but ... that's a spliterator over an immutable array, so shouldn't we have IMMUTABLE instead of CONCURRENT?
> >
> 
> The intent here is that the sub-spliterators "inherit" certain characteristics of their top-level spliterator or source,
> 
> It makes sense for some characteristics to be inherited, like NONNULL.
> 

And in general for ORDERED, DISTINCT, SORTED.

Also, see CopyOnWriteArraySet, that internally uses CopyOnWriteArrayList, but does not report ORDERED.


> rather than from that which might be derived from an internal data structure used to hold a partitioned set of elements.
> 
> Note that the specification of Spliterator.CONCURRENT allows for sub-spliterators to report SIZED/SUBSIZED.
> 
> How is SIZED/SUBSIZED different from IMMUTABLE ?

Spliterators of ArrayList will report SIZED/SUBSIZED but not IMMUTABLE because the list can be modified.


> They both describe internal data structures, so yeah ... there is some kind of leak of implementation details, but these characteristics are there to be used by clients, so shouldn't we try to make them accurate?
> 

Since the concurrent collection can be modified it would be inaccurate for a spltierator (top or sub) obtained from that collection to report IMMUTABLE. Such spliterators are weakly consistent and may or may not (e.g. if SIZED/SUBSIZED) reflect any modifications subsequent to their construction.


> Alternatively: currently some spliterator characteristics describe "this" spliterator, and some describe the "parent", which is very confusing.

It's a subtle area. Certain Spliterator characteristics refer to the (element) source namely, NONULL, IMMUTABLE and CONCURRENT.

Paul.