Averages in Java

Averages.java

import java.util.HashMap;
import java.util.List;
import java.util.LongSummaryStatistics;
import java.util.Map;
import java.util.stream.Collectors;

class Averages {
    public static void main(String[] args) {
        List<Map<String, Long>> data = List.of(
                Map.of("age", 18L, "rate", 30L),
                Map.of("age", 18L, "rate", 15L),
                Map.of("age", 50L, "rate", 35L)
                );


        System.out.println("Summary: " + avgByKey(data));

    }

    public static HashMap<Long, LongSummaryStatistics> avgByKey(List<Map<String, Long>> data) {
        return data.stream().collect(
                Collectors.groupingBy(
                        m -> m.getOrDefault("age", 0L),
                        HashMap::new,
                        Collectors.summarizingLong(m -> m.getOrDefault("rate", 0L))
                )
        );
    }
}

SQL is much better though:

select age, avg(rate) from person
group by age

Clojure is runner up for me:

(defn average [data]
(into {} (x/by-key :age :rate x/avg) data))