code: public static void notEmpty(String message, String string) {
          if (string == null || string.trim().isEmpty()) {
              throw new IllegalArgumentException(message);
          }
      }
docs: Verifies that the given string is neither <code>null</code> nor empty and throws an
  <code>IllegalArgumentException</code> with the given message otherwise.
  
  @param message message that should be included in case of an error
  @param string  string that should be checked