public static void main(String[] args) {
Gradebook gb = new Gradebook();
gb.addStudent(new GradStudent("Bob", 99.0));
gb.addStudent(new UndergradStudent("Jane", 85.0));
gb.displayStudents();
// This is possible. We cannot call rentCar on s, tho.
Student s = new GradStudent("Bob", 99.0);
// This invokes the GradStudent version of toString because of dynamic binding.
s.toString();
// We can change the type of object that s refers to as long as it is in the Student
// hierarchy.
s = new UndergradStudent("Jane", 85.0);
}
