code: private final void zeroPaddingNumber(int value, int minDigits, int maxDigits, StringBuffer buffer)
      {
          // Optimization for 1, 2 and 4 digit numbers. This should
          // cover most cases of formatting date/time related items.
          // Note: This optimization code assumes that maxDigits is
          // either 2 or Integer.MAX_VALUE (maxIntCount in format()).
          try {
              if (zeroDigit == 0) {
                  zeroDigit = ((DecimalFormat)numberFormat).getDecimalFormatSymbols().getZeroDigit();
              }
              if (value >= 0) {
                  if (value < 100 && minDigits >= 1 && minDigits <= 2) {
                      if (value < 10) {
                          if (minDigits == 2) {
                              buffer.append(zeroDigit);
                          }
                          buffer.append((char)(zeroDigit + value));
                      } else {
                          buffer.append((char)(zeroDigit + value / 10));
                          buffer.append((char)(zeroDigit + value % 10));
                      }
                      return;
                  } else if (value >= 1000 && value < 10000) {
                      if (minDigits == 4) {
                          buffer.append((char)(zeroDigit + value / 1000));
                          value %= 1000;
                          buffer.append((char)(zeroDigit + value / 100));
                          value %= 100;
                          buffer.append((char)(zeroDigit + value / 10));
                          buffer.append((char)(zeroDigit + value % 10));
                          return;
                      }
                      if (minDigits == 2 && maxDigits == 2) {
                          zeroPaddingNumber(value % 100, 2, 2, buffer);
                          return;
                      }
                  }
              }
          } catch (Exception e) {
          }
  
          numberFormat.setMinimumIntegerDigits(minDigits);
          numberFormat.setMaximumIntegerDigits(maxDigits);
          numberFormat.format((long)value, buffer, DontCareFieldPosition.INSTANCE);
      }
docs: Formats a number with the specified minimum and maximum number of digits.