On Jun 28, 2014, at 5:40 PM, Kasper Nielsen <[email protected]> wrote:
>
> > s.distinct().spliterator() -> Spliterator.DISTINCT = true
> > but limiting the number of distinct elements makes the stream non distinct
> > s.distinct().limit(10).spliterator() -> Spliterator.DISTINCT = false
>
> I don't observe that (see program below).
> Right, that was an error on my part.
>
> But still, I think some there are some cases where the flag should be
> maintained.
> For example, I think following the following program should print 4 'true'
> values but it only prints 1.
> Especially the second one puzzles me, invoking distinct() makes it
> non-distinct?
>
> static IntStream s() {
> return StreamSupport.intStream(Spliterators.spliterator(new int[] { 12, 34
> }, Spliterator.DISTINCT), false);
> }
>
> public static void main(String[] args) {
>
> System.out.println(s().spliterator().hasCharacteristics(Spliterator.DISTINCT));
>
> System.out.println(s().distinct().spliterator().hasCharacteristics(Spliterator.DISTINCT));
>
> System.out.println(s().boxed().spliterator().hasCharacteristics(Spliterator.DISTINCT));
>
> System.out.println(s().asDoubleStream().spliterator().hasCharacteristics(Spliterator.DISTINCT));
> }
>
The second is a good example as to why this is an implementation detail, here
is the implementation (some may want to close their eyes!):
public final IntStream distinct() {
// While functional and quick to implement, this approach is not very
efficient.
// An efficient version requires an int-specific map/set implementation.
return boxed().distinct().mapToInt(i -> i);
}
We could work out how to inject back in distinct but since the spliterator is
intended as an escape hatch i did not think it worth the effort.
Note if the latter source was a a long stream it would not be able to inject
DISTINCT because not all long values can be represented precisely as double
values.
>
> I am trying to implement the stream interfaces and I want to make sure that
> my implementation have similar behaviour as the default implementation in
> java.util.stream. The interoperability between streams and
> Spliterator.characteristics is the only thing I'm having serious issues with.
> I feel the current state is more a result of how streams are implemented at
> the moment then as part of a public API.
>
> I think something like a table with non-terminal stream operations as rows
> and characteristics as columns. Where each cell was either: "cleared", "set"
> or "maintained" would make sense.
>
We deliberately did not specify this aspect, the implementation could change
and we don't want to unduly constrain it based on an escape-hatch (it's not the
common case). Implementations can decide to what extent the quality is of that
escape-hatch spliterator. For your implementation you are free to provide
better quality escape-hatch spliterators.
I think we should clarify the documentation on BaseStream.spliterator() to say
something like:
The characteristics of the returned spliterator need not correlate with
characteristics of the stream source
and those inferred from intermediate operations proceeding this terminal
operation.
https://bugs.openjdk.java.net/browse/JDK-8048689
I have also logged the following issues :
Spliterator.NONNULL
https://bugs.openjdk.java.net/browse/JDK-8048690
~ORDERED & SORTED
https://bugs.openjdk.java.net/browse/JDK-8048691
Thanks,
Paul.
