Group and Aggregate Employees by Department with Streams

import java.util.*;
import java.util.stream.*;

public class GroupByDepartment {
    record Employee(String name, String department, double salary) { }

    public static void main(String[] args) {
        List<Employee> employees = List.of(
                new Employee("Asha", "IT", 90000),
                new Employee("Ben", "HR", 50000),
                new Employee("Carla", "IT", 70000),
                new Employee("Dev", "Finance", 65000),
                new Employee("Esha", "HR", 55000));

        // groupingBy(classifier, mapFactory, downstream): TreeMap keeps the departments sorted
        Map<String, List<String>> namesByDepartment = employees.stream()
                .collect(Collectors.groupingBy(Employee::department, TreeMap::new,
                        Collectors.mapping(Employee::name, Collectors.toList())));
        System.out.println(namesByDepartment);    // {Finance=[Dev], HR=[Ben, Esha], IT=[Asha, Carla]}

        Map<String, Double> averageSalary = employees.stream()
                .collect(Collectors.groupingBy(Employee::department, TreeMap::new,
                        Collectors.averagingDouble(Employee::salary)));
        System.out.println(averageSalary);        // {Finance=65000.0, HR=52500.0, IT=80000.0}

        Map<String, String> topEarner = employees.stream()
                .collect(Collectors.groupingBy(Employee::department, TreeMap::new,
                        Collectors.collectingAndThen(
                                Collectors.maxBy(Comparator.comparingDouble(Employee::salary)),
                                best -> best.get().name())));
        System.out.println(topEarner);            // {Finance=Dev, HR=Esha, IT=Asha}

        // partitioningBy always returns both keys, false and true
        Map<Boolean, Long> byPay = employees.stream()
                .collect(Collectors.partitioningBy(e -> e.salary() >= 65000, Collectors.counting()));
        System.out.println(byPay);                // {false=2, true=3}
    }
}