Group and calculate average of object array using stream in Java

Question

I have an array of PricingSamples objects :

public class PricingSample {
    private Date time;
    private Double average;

    public PricingSample(Date time, Double average) {
        this.time = time;
        this.average = average;
    }

    public Date getTime() {
        return time;
    }

    public void setTime(Date time) {
        this.time = time;
    }

    public Double getAverage() {
        return average;
    }

    public void setAverage(Double average) {
        this.average = average;
    }
}

The samples are hourly for the past month. I would like to calculate the overall average for each day. I am able to count the number of occurencies in a stream and I am also able to count the sum of all the averages for each day, I just don't know how to combine those to things into one stream.

Here is what I got to.

Summing all averages for each day (I group by the substring of the Date property so it groups by day i.e. Thu Apr 26):

 Map<String, Double> counts = Arrays.stream(samples).collect(Collectors.groupingBy((PricingSample e) -> {
            return e.getTime().toString().substring(0, 10);
        },
            Collectors.summingDouble((PricingSample e) -> {
                return e.getAverage();
            })));

Getting the number of samples for each day: Map counts =

Arrays.stream(samples).collect(Collectors.groupingBy((PricingSample e) -> {
            return e.getTime().toString().substring(0, 10);
        },
                Collectors.counting()
            ));

Answer 1

If you want the average, use averagingDouble. This way you don't need to calculate the sum and the number of samples.

Map<String, Double> averages = 
    Arrays.stream(samples)
          .collect(Collectors.groupingBy(e -> e.getTime().toString().substring(0, 10),
                                         Collectors.averagingDouble(PricingSample::getAverage)));

If you wish to sort the Map by its keys (the days), you can force the output to be a TreeMap:

Map<String, Double> averages = 
    Arrays.stream(samples)
          .collect(Collectors.groupingBy(e -> e.getTime().toString().substring(0, 10),
                                         TreeMap::new,
                                         Collectors.averagingDouble(PricingSample::getAverage)));