code: public int getFormatIndex(CellValueRecordInterface cell) {
          ExtendedFormatRecord xfr = _xfRecords.get(cell.getXFIndex());
          if (xfr == null) {
              logger.log(POILogger.ERROR, "Cell " + cell.getRow() + "," + cell.getColumn()
                  + " uses XF with index " + cell.getXFIndex() + ", but we don't have that");
              return -1;
          }
          return xfr.getFormatIndex();
      }
docs: Returns the index of the format string, used by your cell, or -1 if none found
  
  @param cell the cell
  @return the index of the format string