RFR 8071597 Add Stream dropWhile and takeWhile operations

[Paul Sandoz]

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.


> 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 :-)


>> 
>> (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.