teeing and Summary Statistics

Key points

• Collectors.teeing (Java 12+) runs two collectors over the same stream in one pass, then merges their results.
• Use it when you need two answers at once: count and sum, min and max, matched and total.
• Without teeing you would stream twice, which is impossible when the source is one-shot.
• summarizingInt returns count, sum, min, max and average in a single object.
• averagingInt returns a Double and gives 0.0 for an empty stream, not an Optional.

Example

import java.util.*;
import java.util.stream.*;

public class Main {
    record Report(long count, int total, double average) {}

    public static void main(String[] args) {
        List<Integer> orders = List.of(20, 35, 15, 60, 10);

        // teeing (Java 12+) runs two collectors in ONE pass, then merges the answers.
        Report report = orders.stream().collect(Collectors.teeing(
                Collectors.counting(),
                Collectors.summingInt(Integer::intValue),
                (count, total) -> new Report(count, total, count == 0 ? 0 : (double) total / count)));

        // summarizingInt gets count, sum, min, max and average in one shot.
        IntSummaryStatistics stats = orders.stream()
                .collect(Collectors.summarizingInt(Integer::intValue));

        String range = orders.stream().collect(Collectors.teeing(
                Collectors.minBy(Comparator.<Integer>naturalOrder()),
                Collectors.maxBy(Comparator.<Integer>naturalOrder()),
                (min, max) -> min.orElse(0) + ".." + max.orElse(0)));

        System.out.println("teeing  : " + report);
        System.out.println("stats   : " + stats);
        System.out.println("average : " + orders.stream().collect(Collectors.averagingInt(Integer::intValue)));
        System.out.println("range   : " + range);
    }
}

One pass, two answers. That is teeing.