public ArrayList<String> getSubjects() {
		if(items == null || items.isEmpty()) {
			return null;
		}
		
		ArrayList<String> subjects = new ArrayList<String>();
		Iterator<Map.Entry<String, ArrayList<String>>> i = items.entrySet().iterator();
		while(i.hasNext()) {
			Map.Entry<String, ArrayList<String>> item = i.next();
			String subject = item.getKey();
			if(subject != null && !subject.isEmpty()) {
				subjects.add(subject);
			}
		}
		return subjects;
	}