Is there a more suitable mailing list for this kind of core-libs spec related
enquiry to be directed at?
Thanks,
Chris
> On Oct 9, 2018, at 10:48 AM, Chris Dennis <chris.w.den...@gmail.com> wrote:
>
> I’m attempting to clarify the meaning/intent of the language in the
> specification of the identity-finish characteristic of collectors.
>
> The javadoc reads:
>
> Indicates that the finisher function is the identity function and
> can be elided. If set, it must be the case that an unchecked cast
> from A to R will succeed.
>
> In the following code the second sentence clearly indicates that the second
> cast from A to R must succeed. What I’m trying to confirm however is that the
> first sentence is intended to require that the finisher is a true identity
> function, so the result of the collector must be one of the instances created
> through the supplier, and that therefore the cast from R to A is also safe.
>
> public static void main(String[] args) {
> Stream<String> a = Stream.of("foo", "bar", "baz");
> Stream<String> b = Stream.of("alice", "bob", "eve");
>
> System.out.println(collect(Stream.of(a, b), partitioningBy(s ->
> s.startsWith("b"), toList())));
> }
>
> public static <T, A, R> R collect(Stream<Stream<T>> streams, Collector<T, A,
> R> collector) {
> if
> (collector.characteristics().contains(Collector.Characteristics.IDENTITY_FINISH))
> {
> return streams.map(s -> s.collect(collector))
> // Indicates that the finisher function is the identity function and
> can be elided.
> .map(r -> (A) r)
> .reduce(collector.combiner())
>
> // If set, it must be the case that an unchecked cast from A to R will
> succeed.
> .map(a -> (R) a)
> .orElse(Stream.<T>empty().collect(collector));
> } else {
> return streams.flatMap(identity()).collect(collector);
> }
> }
>
> Thoughts, opinions?
>
> Thanks,
>
> Chris Dennis
