Spliterator.tryAdvance (original) (raw)

Doug Lea dl at cs.oswego.edu
Tue Feb 12 04:52:23 PST 2013

On 02/11/13 10:34, Remi Forax wrote:

There is another point, the specification should be relatex to allow tryAdvance to not always call the consumer taken as parameter.

These are all the same issue in disguise (including the one you mentioned that I didn't get :-)

The question is: Can you design Spliterators and/or related leaf-computation-level support such that none of the "basic" Stream ops require use of a lambda / inner class that needs a mutable variable?

I took this path in ConcurrentHashMap (see http://gee.cs.oswego.edu/dl/jsr166/dist/docs/java/util/concurrent/ConcurrentHashMap.html), resulting in 4 "basic" methods (plus 3 more for primitives). I think it is the right solution for CHM, but it cannot apply to Streams (CHM can rely on nullness, and imposes requirement that users pre-fuse multiple map and map-reduce ops, etc.)

And if you explore what it would take to do this for the Stream API, it gets quickly out of hand -- at least a dozen or so operations that every Collection, Map, or other Stream/Spliterator source author would have to write. Which led to the present solution of only requiring forEach, trySplit, and tryAdvance.

-Doug

If by example, I want to implements a Spliterator that filter the elements, this implementation should be legal: class FilterSpliterator implements Spliterator { private final Spliterator spliterator; private final Predicate predicate; public FilterSpliterator(Spliterator spliterator, Predicate predicate) { .... } public void tryAdvance(Consumer consumer) { spliterator.tryAdvance(element -> { if (predicate.test(element)) { consumer.accept(element); } }); } } otherwise, you have to use a while loop around spliterator.tryAdvance but because there is no way to transmit the information that the element is accepted or not (see my previous mail), you can not use a lambda here and you have to rely on an inner class. cheers, Rémi

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