Java Stream How to - Sum ages with reduce

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The following code shows how to sum ages with reduce.

Example

import java.time.LocalDate;
import java.time.chrono.IsoChronology;
import java.util.ArrayList;
import java.util.List;
/*from   w ww.jav a2  s  .c o  m*/

public class Main {
    public static void main(String[] args) {
      List<Person> roster = createRoster();
      
      Integer totalAgeReduce = roster
          .stream()
          .map(Person::getAge)
          .reduce(
              0,
              (a, b) -> a + b);
          
      System.out.println("Sum of ages with reduce(identity, accumulator): " +
          totalAgeReduce);

    }

    public static List<Person> createRoster() {
        
        List<Person> roster = new ArrayList<>();
        roster.add(
            new Person(
            "Fred",
            IsoChronology.INSTANCE.date(1980, 6, 20),
            Person.Sex.MALE,
            "fred@example.com"));
        roster.add(
            new Person(
            "Jane",
            IsoChronology.INSTANCE.date(1990, 7, 15),
            Person.Sex.FEMALE, "jane@example.com"));
        roster.add(
            new Person(
            "George",
            IsoChronology.INSTANCE.date(1991, 8, 13),
            Person.Sex.MALE, "george@example.com"));
        roster.add(
            new Person(
            "Bob",
            IsoChronology.INSTANCE.date(2000, 9, 12),
            Person.Sex.MALE, "bob@example.com"));
        
        return roster;
    }
}

class Person {
  
  public enum Sex {
      MALE, FEMALE
  }

  String name; 
  LocalDate birthday;
  Sex gender;
  String emailAddress;

  Person(String nameArg, LocalDate birthdayArg,
      Sex genderArg, String emailArg) {
      name = nameArg;
      birthday = birthdayArg;
      gender = genderArg;
      emailAddress = emailArg;
  }  

  public int getAge() {
      return birthday
          .until(IsoChronology.INSTANCE.dateNow())
          .getYears();
  }

  public void printPerson() {
    System.out.println(name + ", " + this.getAge());
  }
  
  public Sex getGender() {
      return gender;
  }
  
  public String getName() {
      return name;
  }
  
  public String getEmailAddress() {
      return emailAddress;
  }
  
  public LocalDate getBirthday() {
      return birthday;
  }
  
  public static int compareByAge(Person a, Person b) {
      return a.birthday.compareTo(b.birthday);
  }

  
}

/*
 * Copyright (c) 2013, Oracle and/or its affiliates. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 *   - Redistributions of source code must retain the above copyright
 *     notice, this list of conditions and the following disclaimer.
 *
 *   - Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in the
 *     documentation and/or other materials provided with the distribution.
 *
 *   - Neither the name of Oracle or the names of its
 *     contributors may be used to endorse or promote products derived
 *     from this software without specific prior written permission
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
 * IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
 * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR
 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
 * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

The code above generates the following result.

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