Java tutorial
import java.time.LocalDate; import java.time.Month; import java.util.Arrays; import java.util.List; import java.util.Map; import java.util.stream.Collectors; public class Main { public static void main(String[] args) { Map<Gender, Map<Object, String>> personsByGenderAndDobMonth = Employee.persons().stream().collect( Collectors.groupingBy(Employee::getGender, Collectors.groupingBy(p -> p.getDob().getMonth(), Collectors.mapping(Employee::getName, Collectors.joining(", "))))); System.out.println(personsByGenderAndDobMonth); } } enum Gender { MALE, FEMALE } class Employee { private long id; private String name; private Gender gender; private LocalDate dob; private double income; public Employee(long id, String name, Gender gender, LocalDate dob, double income) { this.id = id; this.name = name; this.gender = gender; this.dob = dob; this.income = income; } public String getName() { return name; } public Gender getGender() { return gender; } public LocalDate getDob() { return dob; } public static List<Employee> persons() { Employee ken = new Employee(1, "Jame", Gender.MALE, LocalDate.of(1970, Month.MAY, 4), 6123.0); Employee jeff = new Employee(2, "Jeff", Gender.MALE, LocalDate.of(1971, Month.JULY, 5), 7112.0); Employee donna = new Employee(3, "Jane", Gender.FEMALE, LocalDate.of(1972, Month.JULY, 9), 8712.0); Employee chris = new Employee(4, "Jodd", Gender.MALE, LocalDate.of(1973, Month.DECEMBER, 6), 1823.0); Employee laynie = new Employee(5, "Janey", Gender.FEMALE, LocalDate.of(1974, Month.DECEMBER, 3), 1234.0); Employee lee = new Employee(6, "Jason", Gender.MALE, LocalDate.of(1975, Month.MAY, 8), 2412.0); // Create a list of persons List<Employee> persons = Arrays.asList(ken, jeff, donna, chris, laynie, lee); return persons; } }