RFR 8071597 Add Stream dropWhile and takeWhile operations

[Remi Forax]

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.

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 ?

>
> (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.

>   
> Paul.

Rémi