Multiple aggregate functions in Java 8 Stream API

Kha Nguyễn :

I have a class defined like

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

I would like to perform the following SQL statement using Java 8 Stream API.

SELECT atDate, AVG(occupancy), AVG(efficiency)
FROM TimePeriodCalc
GROUP BY atDate

I tried :

Collection<TimePeriodCalc> collector = result.stream().collect(groupingBy(p -> p.getAtDate(), ....

What can be put into the code to select multiple attributes ? I'm thinking of using multiple Collectors but really don't know how to do so.

Eugene :

To do it without a custom Collector (not streaming again on the result), you could do it like this. It's a bit dirty, since it is first collecting to Map<String, List<TimePeriodCalc>> and then streaming that list and get the average double.

Since you need two averages, they are collected to a Holder or a Pair, in this case I'm using AbstractMap.SimpleEntry

  Map<String, SimpleEntry<Double, Double>> map = Stream.of(new TimePeriodCalc(12d, 10d, "A"), new TimePeriodCalc(2d, 16d, "A"))
            .collect(Collectors.groupingBy(TimePeriodCalc::getAtDate,
                    Collectors.collectingAndThen(Collectors.toList(), list -> {
                        double occupancy = list.stream().collect(
                                Collectors.averagingDouble(TimePeriodCalc::getOccupancy));
                        double efficiency = list.stream().collect(
                                Collectors.averagingDouble(TimePeriodCalc::getEfficiency));
                        return new AbstractMap.SimpleEntry<>(occupancy, efficiency);
                    })));

    System.out.println(map);