Collector の直積 https://twitter.com/gakuzzzz/status/674134392449581056

Collectors.java

package jp.t2v.xanadu.ds;

import lombok.experimental.UtilityClass;

import jp.t2v.xanadu.ds.Tuples.T2;

import java.util.function.BiFunction;
import java.util.stream.Collector;
import java.util.stream.Collector.Characteristics;
import java.util.stream.Stream;
import java.util.Set;
import static java.util.stream.Collector.Characteristics.CONCURRENT;
import static java.util.stream.Collector.Characteristics.UNORDERED;

@UtilityClass
public class Collectors {

    public <T, A1, A2, R1, R2> Collector<T, T2<A1, A2>, T2<R1, R2>> product(Collector<? super T, A1, ? extends R1> a, Collector<? super T, A2, ? extends R2> b) {
        return productWith(a, b, T2::of);
    }

    public <T, A1, A2, R1, R2, C> Collector<T, T2<A1, A2>, C> productWith(Collector<? super T, A1, ? extends R1> a, Collector<? super T, A2, ? extends R2> b, BiFunction<? super R1, ? super R2, ? extends C> f) {
        Set<Characteristics> cs1 = a.characteristics();
        Set<Characteristics> cs2 = b.characteristics();
        return Collector.of(
            () -> T2.of(a.supplier().get(), b.supplier().get()),
            (acc, t) -> {
                a.accumulator().accept(acc._1, t);
                b.accumulator().accept(acc._2, t);
            },
            (a1, a2) -> T2.of(
                a.combiner().apply(a1._1, a2._1),
                b.combiner().apply(a1._2, a2._2)
            ),
            (acc) -> f.apply(
                a.finisher().apply(acc._1),
                b.finisher().apply(acc._2)
            ),
            Stream.concat(
                cs1.contains(CONCURRENT) && cs2.contains(CONCURRENT) ? Stream.of(CONCURRENT) : Stream.empty(),
                cs1.contains(UNORDERED)  && cs2.contains(UNORDERED)  ? Stream.of(UNORDERED)  : Stream.empty()
            ).toArray(Characteristics[]::new)
        );
    }

}