public static void main (String[] args) {
    Student daniel;
    StudentManagement manage = new StudentManagement();

    daniel = new Student("OAC", "123", "456");
    daniel.setGroup("123445");
    manage.addStudent(daniel);

    daniel = new Student("OAC", "125", "456");
    daniel.setGroup("12344");
    manage.addStudent(daniel);

    daniel = new Student("OAC", "124", "456");
    daniel.setGroup("123445");
    manage.addStudent(daniel);

    daniel = new Student();
    manage.addStudent(daniel);

    daniel = new Student();
    manage.addStudent(daniel);

    manage.removeStudent("123");

    System.out.println(manage.studentsByGroup());
}