@Override
        public Class getColumnClass(int column) {
               Class returnValue;
               if ((column >= 0) && (column < getColumnCount())) {
                 if(getValueAt(0, column)==null)
                     return String.class;
                 returnValue = getValueAt(0, column).getClass();
               } else {
                 returnValue = Object.class;
               }
               return returnValue;
             }