Java Streams - Collectors reducing(U identity, Function mapper, BinaryOperator op) example

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Collectors reducing(U identity, Function<? super T,? extends U> mapper, BinaryOperator<U> op) returns a Collector to reduce its input elements under a specified mapping function and BinaryOperator.

Syntax

reducing has the following syntax.

public static <T,U> Collector<T,?,U> reducing(U identity,    Function<? super T,? extends U> mapper,    BinaryOperator<U> op)

Example

The following example shows how to use reducing.

import java.time.LocalDate;
import java.time.chrono.IsoChronology;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.function.IntConsumer;
import java.util.stream.Collectors;
/*ww w  . ja va  2  s .c o  m*/

public class Main {
    public static void main(String[] args) {
      List<Person> roster = createRoster();
      
      System.out.println("Total age by gender:");
      Map<Person.Sex, Integer> totalAgeByGender =
          roster
              .stream()
              .collect(
                   Collectors.groupingBy(
                       Person::getGender,                      
                       Collectors.reducing(
                           0,
                           Person::getAge,
                           Integer::sum)));
              
      List<Map.Entry<Person.Sex, Integer>>
          totalAgeByGenderList = 
          new ArrayList<>(totalAgeByGender.entrySet());
                   
      totalAgeByGenderList
          .stream()
          .forEach(e -> 
              System.out.println("Gender: " + e.getKey() +
                  ", Total Age: " + e.getValue()));
           

    }

    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 Averager implements IntConsumer
{
    private int total = 0;
    private int count = 0;
        
    public double average() {
        return count > 0 ? ((double) total)/count : 0;
    }
        
    public void accept(int i) { total += i; count++; }
    public void combine(Averager other) {
        total += other.total;
        count += other.count;
    }
}
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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