Java Stream How to - Reduce to sum Person ages

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Question

We would like to know how to reduce to sum Person ages.

Answer

/*from w  w w.  ja  v  a2  s.  c o  m*/
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) -> sum += p.age, (sum1, sum2) -> 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;
  }
}

The code above generates the following result.

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