Streams: averagingDouble inside groupingBy

Downstream numeric collectors turn a grouping into an aggregate report without a second pass over the data.

Code

record Reading(String sensor, double value) {}
var readings = List.of(new Reading("t1", 20.5), new Reading("t1", 21.5),
                       new Reading("t2", 30.0), new Reading("t2", 32.0));

Map<String, Double> avg = readings.stream().collect(
        Collectors.groupingBy(Reading::sensor, TreeMap::new,
                Collectors.averagingDouble(Reading::value)));

avg.forEach((sensor, mean) -> System.out.println(sensor + " average = " + mean));

Output

t1 average = 21.0
t2 average = 31.0

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Published 2026-07-30