public String getDefaultMessage() {
    String defaultMessage = getFieldValue(DEFAULT_MESSAGE_KEY);
    if (defaultMessage == null) {
      defaultMessage = this.toString();
    }
    return defaultMessage;
  }
@Test
  void getDefaultMessage_whenNotDefined_returnToString() {
    Notification notification = new Notification("type");
    assertThat(notification.getDefaultMessage()).isEqualTo("Notification{type='type', fields={}}");
  }