Re: RFR 8071600: Add a flat-mapping collector

[Peter Levart]

On 02/03/2015 02:48 PM, Paul Sandoz wrote:
Hi,
http://cr.openjdk.java.net/~psandoz/jdk9/JDK-8071600-Collector-flatMapping/webrev/

This patch adds a new flat mapping collector to Collectors. This can be useful 
if one needs to map 0 or more items into a downstream collector.

A CCC will be filed.

A following patch, which i plan to fold into the above patch, performs some 
renames on the collectors test to be consistent with the current naming:

http://cr.openjdk.java.net/~psandoz/jdk9/JDK-8071600-Collector-flatMapping-test-rename/webrev/

Thanks,
Paul.

Hi Paul,

Would the following "optimization" make any sense?

    public static <T, U, A, R>
    Collector<T, ?, R> flatMapping(Function<? super T, ? extends 
Stream<? extends U>> mapper,
                                   Collector<? super U, A, R> downstream) {
        BiConsumer<A, ? super U> downstreamAccumulator = 
downstream.accumulator();
        return new CollectorImpl<>(downstream.supplier(),
            (r, t) -> {
                try (Stream<? extends U> result = mapper.apply(t)) {
                    if (result != null) {
                        ((result.isParallel() &&
!downstream.characteristics().contains(Collector.Characteristics.CONCURRENT))
                            ? result.sequential() : result).forEach(u 
-> downstreamAccumulator.accept(r, u));
                    }
                }
            },
            downstream.combiner(), downstream.finisher(),
            downstream.characteristics());
    }


Regards, Peter