RFR 8071597 Add Stream dropWhile and takeWhile operations

[Remi Forax]

On 06/04/2015 10:34 AM, Paul Sandoz wrote:
> On Jun 4, 2015, at 10:09 AM, Remi Forax <forax at univ-mlv.fr> wrote:
>
>> On 06/04/2015 09:37 AM, Paul Sandoz wrote:
>>> On Jun 4, 2015, at 9:04 AM, Remi Forax <forax at univ-mlv.fr> wrote:
>>>> Thinking a little more about dropWhile(),
>>>> it can be written using filter() more or less like this:
>>>>   default Stream<T> dropWhile(Predicate<? super T> predicate) {
>>>>      return filter(new Predicate<>() {
>>>>        private boolean noDropAnymore;
>>>>        public boolean test(T t) {
>>>>          return noDropAnymore || (noDropAnymore = !predicate.test(t));
>>>>        }
>>>>      });
>>>>   }
>>>> and i maybe wrong but implementing dropWhile with an op is not better than that in term of perf so for me dropWhile() doesn't pull its own weight.
>>>>
>>> Try running that in parallel.
> And try with an ordered parallel stream and see if it produces the same results as that produced sequentially.

It won't, you have to know how the splitting is done.

>
>
>> I'm not sure it's a good idea if you want good perf, the cost of checking a volatile flag inside the loop for each thread (I don't see how to have something more lightweight) will cost more than just keeping the stream sequential, no ?
>>
> You are imposing a restriction that this should *only* run sequentially.
>
> Stateful ops such as take/drop and limit/skip have to be used with care when executed in parallel. That does not mean we should rule it out (however, it's reasonable for default implementations of take/drop to not split, which is what is currently implemented more so for code-sharing).
>
> In general parallel streams should be used with care, we keep saying don't sprinkle parallel on your streams like pixie dust and expect it to work magic :-)

Ok.

>
>
>>> (Stream.parallel() affects the whole pipeline, so it's not possible to implement the default with sequential().filter(p) where p is a stateful predicate.)
>> it's possible,
>> the default should return a proxy in front of the result of sequential().filter(p) that return this if parallel() is called.
>>
> A sequential slight of hand. Ugh :-)
>
> Paul.

Rémi