Add Business Rules

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

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

    public Boolean enroll(Student student) {
        if (enrolledStudents.size() >= maxSize) {
            System.debug('Roster full — cannot enroll ' + student.getName());
            return false;
        }
        enrolledStudents.add(student);
        return true;
    }
}

enroll() now returns a Boolean reporting whether it actually succeeded — this is Module 6's if-statement pattern doing real business work, and it means calling code can react to a full roster instead of blindly assuming every enrollment worked.

public Boolean enroll(Student student) {
    for (Student existing : enrolledStudents) {
        if (existing.getStudentId() == student.getStudentId()) {
            System.debug(student.getName() + ' is already enrolled.');
            return false;
        }
    }

    if (enrolledStudents.size() >= maxSize) {
        System.debug('Roster full — cannot enroll ' + student.getName());
        return false;
    }

    enrolledStudents.add(student);
    return true;
}

Checking for an existing enrollment before checking capacity means a genuinely duplicate enrollment gets a clear, specific message rather than a generic "roster full" — the order these checks run in actually matters for how helpful the feedback is.

It would be possible to write these checks as separate, standalone code wherever enroll gets called instead — but then every caller would need to remember to repeat them, and any caller that forgot would silently create an invalid roster. Putting the rules inside CourseRoster.enroll() itself (the same encapsulation principle from Module 11) means the rule is enforced everywhere, automatically, with no way to bypass it.