code: @Override
      public String[] getSheetNames()
      {
          String[] ret = null;
          if(workbook != null)
              ret = workbook.getSheetNames();
          else if(writableWorkbook != null)
              ret = writableWorkbook.getSheetNames();
          return ret;
      }
docs: Returns the list of worksheet names from the given Excel XLS file.
  @return The list of worksheet names from the given workbook