code: public String mandatoryParam(String key) {
      String value = param(key);
      checkArgument(value != null && !value.isEmpty(), format(MSG_PARAMETER_MISSING, key));
      return value;
    }
docs: Returns a non-null value. To be used when parameter is required or has a default value.
  
  @throws java.lang.IllegalArgumentException is value is null or blank