Java tutorial
import java.util.Arrays; import java.util.List; public class Main { public static void main(String[] args) throws Exception { List<Person> persons = Arrays.asList(new Person("Max", 18), new Person("Peter", 23), new Person("Pamela", 23), new Person("David", 12)); Integer ageSum = persons.stream().reduce(0, (sum, p) -> { System.out.format("accumulator: sum=%s; person=%s\n", sum, p); return sum += p.age; }, (sum1, sum2) -> { System.out.format("combiner: sum1=%s; sum2=%s\n", sum1, sum2); return sum1 + sum2; }); System.out.println(ageSum); } } class Person { String name; int age; Person(String name, int age) { this.name = name; this.age = age; } @Override public String toString() { return name; } }