code: public static String getGravatarUrl(String emailAddress) {
          String hash = generateHash(emailAddress);
          if (hash == null) {
              return "https://www.gravatar.com/avatar/00000000000000000000000000000000" + ".jpg?d=mm";
          } else {
              return "https://www.gravatar.com/avatar/" + hash + ".jpg?d=mm";
          }
      }
docs: Generates a Gravatar URL for the specified email address. If
  the email address is blank or does not have a Gravatar, will
  fallback to usingthe mystery-man image.
  @param emailAddress the email address to generate the Gravatar URL from
  @return a Gravatar URL for the specified email address
  @since 1.0.0