Assume an Employee class:

public class Employee {
    private String name;
    private String department;

    public Employee(String name, String department) {
        this.name = name;
        this.department = department;
    }

    public String getName() { return name; }
    public String getDepartment() { return department; }
}

And a list of employees:

List<Employee> employees = Arrays.asList(
    new Employee("Alice", "HR"),
    new Employee("Bob", "IT"),
    new Employee("Charlie", "IT"),
    new Employee("David", "HR")
);

Using Collectors.groupingBy + Collectors.counting

Map<String, Long> countByDept = employees.stream()
    .collect(Collectors.groupingBy(Employee::getDepartment, Collectors.counting()));

countByDept.forEach((dept, count) -> System.out.println(dept + ": " + count));