一种可能的解决方案是引入一个中间类来存储中间值（假设Month是enum）：

private static Map<Month, Integer> sumGroupingByMonth(final Collection<Scooter> scooters) {
    @AllArgsConstructor
    final class Intermediate {
        final int newVal;
        final Day rentalDay;
    }

    return scooters.stream()
            .flatMap(scooter -> {
                final int newVal = scooter.getMileage() * scooter.getMileageRate();
                return scooter.getRentalDays()
                        .stream()
                        .map(day -> new Intermediate(newVal, day));
            })
            .collect(Collectors.groupingBy(intermediate -> intermediate.rentalDay.getMonth(), Collectors.summingInt(intermediate -> intermediate.newVal)));
}

我发现这段代码非常复杂：闭包，显式中间状态，函数作用域，中间对象，以及以下内容可能会更简单，更快？

private static int[] sumGroupingByMonth(final Iterable<Scooter> scooters) {
    final int[] data = new int[12];
    for ( final Scooter scooter : scooters ) {
        final int newVal = scooter.getMileage() * scooter.getMileageRate();
        for ( final Day rentalDay : scooter.getRentalDays() ) {
            data[rentalDay.getMonth().ordinal()] += newVal;
        }
    }
    return data;
}