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

Assuming that your TimePeriodCalc class has all the necessary getters, this should get you the list you want:

List result = new ArrayList<>(
    list.stream()
    .collect(Collectors.groupingBy(TimePeriodCalc::getAtDate, 
        Collectors.collectingAndThen(Collectors.toList(), TimePeriodCalc::avgTimePeriodCalc)))
    .values()
);

Where TimePeriodCalc.avgTimePeriodCalc is this method in the TimePeriodCalc class:

public static TimePeriodCalc avgTimePeriodCalc(List list){
    return new TimePeriodCalc(
            list.stream().collect(Collectors.averagingDouble(TimePeriodCalc::getOccupancy)),
            list.stream().collect(Collectors.averagingDouble(TimePeriodCalc::getEfficiency)),
            list.get(0).getAtDate()
            );
}

---

The above can be combined into this monstrosity:

List result = new ArrayList<>(
    list.stream()
    .collect(Collectors.groupingBy(TimePeriodCalc::getAtDate, 
        Collectors.collectingAndThen(
            Collectors.toList(), a -> {
                return new TimePeriodCalc(
                        a.stream().collect(Collectors.averagingDouble(TimePeriodCalc::getOccupancy)),
                        a.stream().collect(Collectors.averagingDouble(TimePeriodCalc::getEfficiency)),
                        a.get(0).getAtDate()
                        );
            }
        )))
    .values());

With input:

List list = new ArrayList<>();
list.add(new TimePeriodCalc(10,10,"a"));
list.add(new TimePeriodCalc(10,10,"b"));
list.add(new TimePeriodCalc(10,10,"c"));
list.add(new TimePeriodCalc(5,5,"a"));
list.add(new TimePeriodCalc(0,0,"b"));

This would give:

TimePeriodCalc [occupancy=7.5, efficiency=7.5, atDate=a]
TimePeriodCalc [occupancy=5.0, efficiency=5.0, atDate=b]
TimePeriodCalc [occupancy=10.0, efficiency=10.0, atDate=c]