Collectors.teeing

Definition

Collectors.teeing (Java 12) runs two collectors over the same stream at once and combines their two results. It saves you from going through the data twice.

Example code

Count and total in one pass, then compute the average

import java.util.List;
import java.util.stream.Collectors;

class Demo {
    public static void main(String[] args) {
        List<Integer> marks = List.of(70, 80, 90, 100);

        double average = marks.stream().collect(Collectors.teeing(
                Collectors.counting(),                        // first result: how many
                Collectors.summingInt(Integer::intValue),     // second result: the total
                (count, total) -> (double) total / count));   // combine them

        System.out.println(average);
    }
}

// Output:
// 85.0