@Override
 public Object getValueAt(int rowIndex, int columnIndex) {
   StudentModel student = students.get(rowIndex);
   int val;
   switch (Columns.values()[columnIndex]) {
     case SURNAME:
       return student.getSurname();
     case GROUP:
       val = student.getGroupId();
       return val >= 0 ? val : "";
     case MATAN:
       val = student.getMarks().get(0);
       return val >= 0 ? val : "";
     case GA:
       val = student.getMarks().get(1);
       return val >= 0 ? val : "";
     case PROGRAMMING:
       val = student.getMarks().get(2);
       return val >= 0 ? val : "";
     case AVERAGE:
       double average = student.getAverage();
       return average >= 0 ? average : "";
     default:
       return null;
   }
 }