Multiple aggregate functions in Java 8 Stream API

Question

I have a class defined like

public class TimePeriodCalc {
    private double occupancy;
    private double efficiency;
    private String atDate;
}

Answer 1

Here's a way with a custom collector. It only needs one pass, but it's not very easy, especially because of generics...

If you have this method:

@SuppressWarnings("unchecked")
@SafeVarargs
static > Collector>
averagingManyDoubles(ToDoubleFunction... extractors) {

    List collectors = Arrays.stream(extractors)
        .map(extractor -> (C) Collectors.averagingDouble(extractor))
        .collect(Collectors.toList());

    class Acc {
        List averages = collectors.stream()
            .map(c -> c.supplier().get())
            .collect(Collectors.toList());

        void add(T elem) {
            IntStream.range(0, extractors.length).forEach(i ->
                collectors.get(i).accumulator().accept(averages.get(i), elem));
        }

        Acc merge(Acc another) {
            IntStream.range(0, extractors.length).forEach(i ->
                averages.set(i, collectors.get(i).combiner()
                    .apply(averages.get(i), another.averages.get(i))));
            return this;
        }

        List finish() {
            return IntStream.range(0, extractors.length)
                .mapToObj(i -> collectors.get(i).finisher().apply(averages.get(i)))
                .collect(Collectors.toList());
        }
    }
    return Collector.of(Acc::new, Acc::add, Acc::merge, Acc::finish);
}

This receives an array of functions that will extract double values from each element of the stream. These extractors are converted to Collectors.averagingDouble collectors and then the local Acc class is created with the mutable structures that are used to accumulate the averages for each collector. Then, the accumulator function forwards to each accumulator, and so with the combiner and finisher functions.

Usage is as follows:

Map> averages = list.stream()
    .collect(Collectors.groupingBy(
        TimePeriodCalc::getAtDate,
        averagingManyDoubles(
            TimePeriodCalc::getOccupancy,
            TimePeriodCalc::getEfficiency)));