Java 8 流 API - Java 9 Collectors.flatMapping 在 Java 8 中重写

Question

自 java-9 called Collectors.flatMapping that takes place as a downstream of grouping or partitioning. Such as (example taken from here 以来，我接触到了一项新功能：

List<List<Integer>> list = Arrays.asList(
    Arrays.asList(1, 2, 3, 4, 5, 6), 
    Arrays.asList(7, 8, 9, 10));

Map<Integer, List<Integer>> map =list.stream()
    .collect(Collectors.groupingBy(
         Collection::size,
         Collectors.flatMapping(
             l -> l.stream().filter(i -> i % 2 == 0),
             Collectors.toList())));

{4=[8, 10], 6=[2, 4, 6]}

这是一种相当优雅的方式，仅使用 3 个收集器。我需要在尚不支持的 java-8 中重写收集器。我尝试使用 6 收集器，这是一个非常广泛的用途，我无法找到一种使用更少的收集器的方法：

Map<Integer, List<Integer>> map = list.stream()
    .collect(Collectors.groupingBy(
        Collection::size,
        Collectors.collectingAndThen(
            Collectors.mapping(
                l -> l.stream().filter(i -> i % 2 == 0).collect(Collectors.toList()),
                Collectors.toList()),
            i -> i.stream().flatMap(j -> j.stream()).collect(Collectors.toList()))));

有没有更短更好的方法，只使用java-8？

Answer 1

对于这个特殊情况，我想这会是一个更简单的版本：

Map<Integer, List<Integer>> map =
        list.stream()
            .collect(Collectors.toMap(
                Collection::size,
                x -> x.stream().filter(y -> y % 2 == 0).collect(Collectors.toList())
            ));

如果涉及合并（两个大小相同的集合），我会添加一个非常简单的 merge 函数：

 Map<Integer, List<Integer>> map =
        list.stream()
            .collect(Collectors.toMap(
                Collection::size,
                x -> x.stream().filter(y -> y % 2 == 0).collect(Collectors.toCollection(ArrayList::new)),
                (left, right) -> {
                    left.addAll(right);
                    return left;
                }
            ));

否则，我同意 Michael 的评论，这并不难移植到 java-8。

Answer 2

我只想向后移植 flatMapping。它只需要 2 个方法和 1 个 class，没有其他依赖项。

此外，当需要升级到 Java 9 时，您可以弃用您的版本并将其任何用法替换为正确的版本。

The following code is taken from the JDK。不是我写的。我已经用您的示例对其进行了测试，结果 returns 相同。

class Nikollectors
{
   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.sequential().forEach(u -> downstreamAccumulator.accept(r, u));
                }
            },
            downstream.combiner(), downstream.finisher(),
            downstream.characteristics());
    }

   private static class CollectorImpl<T, A, R> implements Collector<T, A, R>
   {
        private final Supplier<A> supplier;
        private final BiConsumer<A, T> accumulator;
        private final BinaryOperator<A> combiner;
        private final Function<A, R> finisher;
        private final Set<Characteristics> characteristics;

        CollectorImpl(Supplier<A> supplier,
                      BiConsumer<A, T> accumulator,
                      BinaryOperator<A> combiner,
                      Function<A,R> finisher,
                      Set<Characteristics> characteristics) {
            this.supplier = supplier;
            this.accumulator = accumulator;
            this.combiner = combiner;
            this.finisher = finisher;
            this.characteristics = characteristics;
        }

        CollectorImpl(Supplier<A> supplier,
                      BiConsumer<A, T> accumulator,
                      BinaryOperator<A> combiner,
                      Set<Characteristics> characteristics) {
            this(supplier, accumulator, combiner, castingIdentity(), characteristics);
        }

        @Override
        public BiConsumer<A, T> accumulator() {
            return accumulator;
        }

        @Override
        public Supplier<A> supplier() {
            return supplier;
        }

        @Override
        public BinaryOperator<A> combiner() {
            return combiner;
        }

        @Override
        public Function<A, R> finisher() {
            return finisher;
        }

        @Override
        public Set<Characteristics> characteristics() {
            return characteristics;
        }
    }

   private static <I, R> Function<I, R> castingIdentity() {
       return i -> (R) i;
   }
}

示例用法：

Map<Integer, List<Integer>> map =list.stream()
    .collect(Collectors.groupingBy(
         Collection::size,
         Nikollectors.flatMapping( // <<<
             l -> l.stream().filter(i -> i % 2 == 0),
             Collectors.toList()
        )
    )
);

Java 8 流 API - Java 9 Collectors.flatMapping 在 Java 8 中重写

Java 8 Stream API - Java 9 Collectors.flatMapping rewritten in Java 8

java

java-8

java-stream

collectors

java-9