groupBy, mapping & statistics features in Java8 lambda stream (examples)

1. Given a list of Person objects (POJOs).
2. We would like to group person objects by their gender.
   • Group person objects by Male & Female.
3. We will also apply mapping & statistics features on list of person objects.

1. Apply groupBy, mapping & statistics features on person POJOs

1. Group person objects by gender i.e Male and Female
   • Get the List of person objects (Categorized by gender)
2. Group persons by gender i.e Male and Female
   • Get the Set of person objects
3. Group persons by gender i.e Male and Female
   • Get the  Set of names (of person) objects instead of complete person objects
4. Count person objects by gender
   • Number of females and males in person objects
5. Group person objects by gender and get person with max age
   • Max age of male and female persons
6. Group person objects by gender and generate age statistics
   • Generate age statistics of person objects
   • Get avg, min and max age etc of male and female objects

2. Program – groupBy, mapping & statistics features (java8 lambda stream)

2.1.) Person Class:

• Person POJO containing attributes of Person.
• Apply groupBy, mapping & statistics features on person objects.

package org.learn.GroupByJava8;

public class Person {
 public String name;
 public int age;
 public String gender;
 public Person(String name, int age, String gender) {
  this.name = name;
  this.age = age;
  this.gender = gender;
 }
 public String toString() {
     return "[" + name + " " + age + " " +gender +"]";
 }
 public String getName() {
  return name;
 }
 public void setName(String name) {
  this.name = name;
 }
 public int getAge() {
  return age;
 }
 public void setAge(int age) {
  this.age = age;
 }
 public String getGender() {
  return gender;
 }
 public void setGender(String gender) {
  this.gender = gender;
 }
 
}

2.2 GroupByJava8 Class:

• GroupByJava class will demonstrates the following features.
  • groupBy: We will group person objects by theirs attributes.
    1. Group person objects by genders.
  • mapping: We will map person objects to the set containing names of Person pojos etc.
  • statistics (IntSummaryStatistics):
    1. We will group person pojos by genders.
    2. We will generate age statistics of grouping (step1) like min, max or average age of male persons.

The GroupByJava8 example is as follows:

package org.learn.GroupByJava8;

import java.util.ArrayList;
import java.util.Comparator;
import java.util.IntSummaryStatistics;
import java.util.List;
import java.util.Map;
import java.util.Optional;
import java.util.Set;
import java.util.TreeMap;
import java.util.stream.Collectors;

public class GroupByJava8
{
    public static void main( String[] args )
    {
     List <Person> personList = new ArrayList<Person>();
  personList.add(new Person("Sharon", 21, "Female"));
  personList.add(new Person("Maria", 18,  "Female"));
  personList.add(new Person("Jack", 21 ,"Male"));
  personList.add(new Person("James", 35,  "Male"));  
  
  Map<String, List<Person>> groupByGenderList = 
    personList.stream().collect(Collectors.groupingBy(Person::getGender));
  
  //Group by gender List : Female-> Persons and Male -> Persons
  System.out.println("1. Group persons by gender - get result in List: ");
  System.out.println(groupByGenderList.toString());

  Map<String, Set<Person>> groupByGenderSet = 
    personList.stream().collect(Collectors.groupingBy(Person::getGender,Collectors.toSet()));
  
  //Group by gender Set: Female-> Persons and Male -> Persons
  System.out.println("2. Group persons by gender - get result in Set: ");
  System.out.println(groupByGenderSet.toString());
  
  Map<String, Set<String>> groupByGenderAndFirstNameSet
        = personList.stream().collect(Collectors.groupingBy(Person::getGender, TreeMap::new,
          Collectors.mapping(Person::getName, Collectors.toSet())));    
  System.out.println("3. Group person by gender and get name of person - get result in Set: ");
  System.out.println(groupByGenderAndFirstNameSet.toString());
  
  Map<String, Long> countPersonByGender = personList.stream().
    collect(Collectors.groupingBy(Person::getGender,Collectors.counting()));
  
  System.out.println("4. Count person objects by gender: ");
  System.out.println(countPersonByGender.toString());  

  Map<String, Optional<Person>> personByMaxAge = personList.stream().
    collect(Collectors.groupingBy(Person::getGender
      ,Collectors.maxBy(Comparator.comparing(Person::getAge))));  
  System.out.println("5. Group person objects by gender and get person with max age: ");
  System.out.println(personByMaxAge.toString());

  Map<String, IntSummaryStatistics> groupPersonsByAge = personList.stream().
    collect(Collectors.groupingBy(Person::getGender
      ,Collectors.summarizingInt(Person::getAge)));  
  System.out.println("6. Group person objects by gender and get age statistics: ");
  System.out.println(groupPersonsByAge.toString());
  IntSummaryStatistics malesAge = groupPersonsByAge.get("Male");
  System.out.println("Avgerage male age:"+ malesAge.getAverage());
  System.out.println("Max male age:"+ malesAge.getMax());
  System.out.println("Min male age:"+ malesAge.getMin());
    }    
}

3. Output: groupBy, mapping & IntSummaryStatistics features in java8

1. Group persons by gender - get result in List: 
{Male=[[Jack 21 Male], [James 35 Male]], Female=[[Sharon 21 Female], [Maria 18 Female]]}
2. Group persons by gender - get result in Set: 
{Male=[[James 35 Male], [Jack 21 Male]], Female=[[Maria 18 Female], [Sharon 21 Female]]}
3. Group person by gender and get name of person - get result in Set: 
{Female=[Sharon, Maria], Male=[James, Jack]}
4. Count person objects by gender: 
{Male=2, Female=2}
5. Group person objects by gender and get person with max age: 
{Male=Optional[[James 35 Male]], Female=Optional[[Sharon 21 Female]]}
6. Group person objects by gender and get age statistics: 
{Male=IntSummaryStatistics{count=2, sum=56, min=21, average=28.000000, max=35}, 

Female=IntSummaryStatistics{count=2, sum=39, min=18, average=19.500000, max=21}}
Avgerage male age:28.0
Max male age:35
Min male age:21