ArrayList<Object[]> csvArray = new ArrayList<Object[]>();

Stream<Person> people = Stream.of(new Person("Paul", 24), new Person("Mark",30), new Person("Will", 28));
 Map<Integer, List<String>> peopleByAge = people
.collect(groupingBy(p -> p.age, mapping((Person p) -> p.name, toList())));
 System.out.println(peopleByAge);

final List<Object[]> data = new ArrayList<>();
data.add(new Object[]{"NL", "Rotterdam", "Kees", 38});
data.add(new Object[]{"NL", "Rotterdam", "Peter", 54});
data.add(new Object[]{"NL", "Amsterdam", "Suzanne", 51});
data.add(new Object[]{"NL", "Rotterdam", "Tom", 17});

final Map<String, List<Object[]>> map = data.stream().collect(
        Collectors.groupingBy(row -> row[0].toString() + ":" + row[1].toString()));

for (final Map.Entry<String, List<Object[]>> entry : map.entrySet()) {
    final double average = entry.getValue().stream()
                                .mapToInt(row -> (int) row[3]).average().getAsDouble();
    System.out.println("Average age for " + entry.getKey() + " is " + average);
}

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class CSVExample {

  public static void main(String[] args) {
    ArrayList<Information> csvArray = new ArrayList<>();

    csvArray.add(new Information(new Object[] {"France", "Paris", "Pierre", 34}));
    csvArray.add(new Information(new Object[] {"France", "Paris", "Madeleine", 26}));
    csvArray.add(new Information(new Object[] {"France", "Toulouse", "Sam", 34}));
    csvArray.add(new Information(new Object[] {"Italy", "Rom", "Paul", 44}));

    Map<String, List<Information>> collect = csvArray.stream().collect(Collectors.groupingBy(s -> (s.getCountry() + " " + s.getCity())));
    collect.forEach((k, v) -> System.out.println(k + " " + v.stream().collect(Collectors.averagingInt(Information::getAge))));
  }
}

class Information {
  private String country;
  private String city;
  private String name;
  private int age;

  public Information(Object[] information) {
    this.country = (String) information[0];
    this.city = (String) information[1];
    this.name = (String) information[2];
    this.age = (Integer) information[3];

  }

  public Information(String country, String city, String name, int age) {
    super();
    this.country = country;
    this.city = city;
    this.name = name;
    this.age = age;
  }

  public String getCountry() {
    return country;
  }

  public String getCity() {
    return city;
  }

  public String getName() {
    return name;
  }

  public int getAge() {
    return age;
  }

  @Override
  public String toString() {
    return "Information [country=" + country + ", city=" + city + ", name=" + name + ", age=" + age + "]";
  }

}