code: public static boolean isValid(final String hexString) {
          if (hexString == null) {
              throw new IllegalArgumentException();
          }
  
          int len = hexString.length();
          if (len != 24) {
              return false;
          }
  
          for (int i = 0; i < len; i++) {
              char c = hexString.charAt(i);
              if (c >= '0' && c <= '9') {
                  continue;
              }
              if (c >= 'a' && c <= 'f') {
                  continue;
              }
              if (c >= 'A' && c <= 'F') {
                  continue;
              }
  
              return false;
          }
  
          return true;
      }
docs: Checks if a string could be an {@code ObjectId}.
  
  @param hexString a potential ObjectId as a String.
  @return whether the string could be an object id
  @throws IllegalArgumentException if hexString is null