RFR 8071597 Add Stream dropWhile and takeWhile operations

[Peter Levart]

Hi Paul,

This is a usefull addition to Stream API for sequential ordered streams. 
But does it have any utility in unordered streams at all? Wouldn't it be 
better to just throw an IllegalStateException or something if the stream 
is not ordered? I can't imagine currently a situation where I would need 
to select an arbitrary sub-set of the stream where all elements match a 
predicate and I would not be given any guarantee about how big a subset 
I will get - the same with a stream where an arbitrary sub-set of 
elements matching predicate is taken away. If I wanted something similar 
for unordered stream, I would rather say: "Give me at most N elements of 
the stream matching predicate" for example - and that can already be 
achieved by .filter().limit();

So any use of [take|drop]While on unordered stream would most probably 
be unintended and consequently could be considered a bug, don't you think?

Regards, Peter

On 06/02/2015 03:13 PM, Paul Sandoz wrote:
> Hi,
>
> Please review this webrev that adds take/dropWhile operations to streams:
>
>    http://cr.openjdk.java.net/~psandoz/jdk9/JDK-8071597-take-drop-while/webrev/
>
> I opted to weight the documentation of the operations towards ordered streams in the first paragraph. That is what makes most sense in terms of usage and what most people will read. Thus i refer to the "longest prefix" in the first paragraph then define what that means in subsequent paragraphs for ordered and unordered streams:
>
>   482     /**
>   483      * Returns a stream consisting of the longest prefix of elements taken from
>   484      * this stream that match the given predicate.
>   485      *
>   486      * <p>If this stream is ordered then the prefix is a contiguous sequence of
>   487      * elements of this stream.  All elements of the sequence match the given
>   488      * predicate, the first element of the sequence is the first element
>   489      * (if any) of this stream, and the element (if any) immediately following
>   490      * the last element of the sequence does not match the given predicate.
>   491      *
>   492      * <p>If this stream is unordered then the prefix is a subset of elements of
>   493      * this stream.  All elements (if any) of the subset match the given
>   494      * predicate.  In this case the behavior of this operation is
>   495      * nondeterministic; it is free to select any valid subset as the prefix.
>   496      *
>   497      * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
>   498      * stateful intermediate operation</a>.
>   499      *
> ...
>   528     default Stream<T> takeWhile(Predicate<? super T> predicate) {
>
>   537     /**
>   538      * Returns a stream consisting of the remaining elements of this stream
>   539      * after dropping the longest prefix of elements that match the given
>   540      * predicate.
>   541      *
>   542      * <p>If this stream is ordered then the prefix is a contiguous sequence of
>   543      * elements of this stream.  All elements of the sequence match the given
>   544      * predicate, the first element of the sequence is the first element
>   545      * (if any) of this stream, and the element (if any) immediately following
>   546      * the last element of the sequence does not match the given predicate.
>   547      *
>   548      * <p>If this stream is unordered then the prefix is a subset of elements of
>   549      * this stream.  All elements (if any) of the subset match the given
>   550      * predicate.  In this case the behavior of this operation is
>   551      * nondeterministic; it is free to select any valid subset as the prefix.
>   552      *
> ...
>   584     default Stream<T> dropWhile(Predicate<? super T> predicate) {
>
>
> After this has been reviewed i will follow up with a further issue regarding the specification of takeWhile, stateful predicates and cancellation. I avoided such specification here as it's likely to rathole :-)
>
> Basically the takeWhile operation is implemented such that one can do:
>
>       long t = System.currentTimeMillis();
>       List<BigInteger> pps = Stream
>           .generate(() -> BigInteger.probablePrime(1024, ThreadLocalRandom.current()))
>           .parallel()
>           .takeWhile(e -> (System.currentTimeMillis() - t) < TimeUnit.SECONDS.toMillis(5))
>           .collect(toList());
>
> Paul.