RFR 8071597 Add Stream dropWhile and takeWhile operations

[Stuart Marks]

Hi Paul,

Some comments on the spec.

On 6/2/15 6:13 AM, Paul Sandoz wrote:
>    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:

I see what you're trying to do here. The first sentence of a doc comment is 
special and so it needs to describe concisely but very generally what the method 
is trying to do.

The problem is that the first sentence is flatly contradicted by the third 
paragraph. If the stream is unordered "longest prefix" is completely 
ill-defined. "Prefix" is nonsensical for an unordered stream, but the third 
paragraph attempts to redefine it to fit. "Longest" doesn't apply, since any 
subset may be returned.

Maybe the thing to do just give up on the idea of the first sentence trying to 
cover the semantics for both ordered and unordered streams. The problem is that 
something like "takeWhile" is really mostly applicable to ordered streams, so in 
the first paragraph just give people what they're expecting to see. Then have 
the second paragraph cover the case of unordered streams.

Maybe something like this:

----------
If this stream is ordered, returns a stream consisting of the longest prefix of 
elements taken from this stream, where the elements all match the given 
predicate. The prefix is a contiguous sequence of elements of this stream.  The 
first element of the sequence is the first element (if any) of this stream, and 
the element (if any) immediately following the last element of the sequence does 
not match the given predicate.

If this stream is unordered, this method returns a subset of elements of this 
stream.  All elements (if any) of the subset match the given predicate.  In this 
case the behavior of this operation is nondeterministic; it is free to select 
any valid subset of elements that match the predicate.
----------

s'marks


>   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.
>