if(c1==null && c2==null) return 0;
		public int compare(Object o1, Object o2) {				Vigilant v1 = (Vigilant) o1;				Vigilant v2 = (Vigilant) o2;								Category c1 = v1.getTeacherCategory();				Category c2 = v2.getTeacherCategory();								if(c1==null) return -1;				if(c2==null) return 1;								return -c1.compareTo(c2);						}