如何将数组列表说明为序列图？

Question

public boolean uploadResult(Lecturer lecturer, Student student, String result) throws NullPointerException {
    Schedule currentSchedule = scheduleService.findCurrentSchedule();
    if (currentSchedule.getWeek() != 12) {
        throw new IllegalArgumentException("Cannot upload result until the end of the semester.");
    }
    Student stu = studentService.findOne(student);
    List<Enrolment> enrolments = stu.getPerformance();
    Enrolment enrolment = null;
    for (Enrolment e : enrolments) {
        CourseOffering offering = e.getOffering();
        if (offering.getLecturer().equals(lecturer) && offering.getSchedule().equals(currentSchedule)) {
            enrolment = e;
            break;
        }
    }
    if (enrolment == null) {
        throw new NullPointerException("There is no such student in the lecturer's course");
    }
    enrolment.setResult(result);
    studentService.save(student);
    return true;
}

我试图理解上面的代码，以便在序列图中说明它，但我在理解其中一些方面遇到了一些麻烦，特别是这部分：

List<Enrolment> enrolments = stu.getPerformance();
    Enrolment enrolment = null;
    for (Enrolment e : enrolments) {
        CourseOffering offering = e.getOffering();
        if (offering.getLecturer().equals(lecturer) && 
offering.getSchedule().equals(currentSchedule)) {
            enrolment = e;
            break;

My current sequence diagram in progress(LINK)