Model a Course Roster

public class CourseRoster {
    private String courseName;
    private List<Student> enrolledStudents = new List<Student>();

    public CourseRoster(String courseName) {
        this.courseName = courseName;
    }

    public void enroll(Student student) {
        enrolledStudents.add(student);
    }

    public Integer getEnrollmentCount() {
        return enrolledStudents.size();
    }
}

1. public class CourseRoster { — the class.
2. private String courseName; — a protected field.
3. private List<Student> enrolledStudents = new List<Student>(); — this is composition (Module 11) in action: CourseRoster has a list of Student objects rather than being one itself, starting empty the moment a CourseRoster is created.
4. (blank line)
5. public CourseRoster(String courseName) { — the constructor.
6. this.courseName = courseName; — sets the course name.
7. } — closes the constructor.
8. (blank line)
9. public void enroll(Student student) { — grows the roster one student at a time.
10. enrolledStudents.add(student); — the roster owns and protects its own list, exactly like Student protects its own fields.
11. } — closes the method.
12. (blank line)
13. public Integer getEnrollmentCount() { — reports the current size.
14. return enrolledStudents.size(); — returns it.
15. } — closes the method.
16. } — closes the class.

CourseRoster apexBasics = new CourseRoster('Apex Basics');

Student alice = new Student('S001', 'Alice Nkosi', 'alice@example.com');
Student ben = new Student('S002', 'Ben Dlamini', 'ben@example.com');

apexBasics.enroll(alice);
apexBasics.enroll(ben);

System.debug(apexBasics.getEnrollmentCount()); // 2

1. CourseRoster apexBasics = new CourseRoster('Apex Basics'); — creates the roster.
2. (blank line)
3. Student alice = new Student('S001', 'Alice Nkosi', 'alice@example.com'); — creates a student.
4. Student ben = new Student('S002', 'Ben Dlamini', 'ben@example.com'); — creates another.
5. (blank line)
6. apexBasics.enroll(alice); — enrolls the first student.
7. apexBasics.enroll(ben); — enrolls the second.
8. (blank line)
9. System.debug(apexBasics.getEnrollmentCount()); — logs 2 — the natural fit, since a roster isn't a kind of student, just a holder of them.