code: @Override
      public int numSheets()
      {
          int ret = -1;
          if(workbook != null)
              ret = workbook.getNumberOfSheets();
          else if(writableWorkbook != null)
              ret = writableWorkbook.getNumberOfSheets();
          return ret;
      }
docs: Returns the number of worksheets in the given workbook.
  @return The number of worksheets in the given workbook