code: public static String formatNumber(long number) {
          String stringRepresentation = Long.toString(number);
          StringBuilder sb = new StringBuilder();
          int bound = stringRepresentation.length() - 1;
          String delimiter = ",";
          int counter = 0;
          for (int i = bound; i >= 0; i--) {
              char c = stringRepresentation.charAt(i);
              if (i != bound && counter % 3 == 0) {
                  sb.append(delimiter);
              }
              sb.append(c);
              counter++;
          }
          return sb.reverse().toString();
      }
docs: Returns a string representation of the number passed in where groups of three digits are delimited by comma
  
  @param number Input number
  @return formatted String