使用groupingBy的类属性的平面映射收集器

似乎没有平面映射收集器可

groupingBy

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 Collector.of(downstream.supplier(),      (r, t) -> mapper.apply(t).sequential().forEach(u -> downstreamAccumulator.accept(r, u)),      downstream.combiner(),      downstream.finisher(),      downstream.characteristics().stream().toArray(Collector.Characteristics[]::new));    }

如果使用该代码，并且还添加了一个返回的

quantities

Assignment``Stream<Quantity>

Map<Integer, List<Quantity>> result = assignments.stream()    .collect(groupingBy(Assignment::getIndex,         flatMapping(Assignment::quantities, toList())));