if (title == null || summaryText == null || studentsNumber == null || isExtraLesson == null)
if (title == null || summaryText == null || isExtraLesson == null)
    private void edit(String title, String summaryText, Integer studentsNumber, Boolean isExtraLesson) {        if (title == null || summaryText == null || studentsNumber == null || isExtraLesson == null)            throw new NullPointerException();        setTitle(title);        setSummaryText(summaryText);        setStudentsNumber(studentsNumber);        setIsExtraLesson(isExtraLesson);        setLastModifiedDate(Calendar.getInstance().getTime());    }