code: public String getFormatString(int formatIndex) {
          String format = null;
          if (formatIndex >= HSSFDataFormat.getNumberOfBuiltinBuiltinFormats()) {
              FormatRecord tfr = _customFormatRecords.get(Integer.valueOf(formatIndex));
              if (tfr == null) {
                  logger.log(POILogger.ERROR, "Requested format at index " + formatIndex
                      + ", but it wasn't found");
              } else {
                  format = tfr.getFormatString();
              }
          } else {
              format = HSSFDataFormat.getBuiltinFormat((short) formatIndex);
          }
          return format;
      }
docs: Returns the format string, eg $##.##, for the given number format index.
  
  @param formatIndex the format index
  @return the format string