public Notification setFieldValue(String field, @Nullable String value) {
    fields.put(field, value);
    return this;
  }
@Test
  void toString_shouldReturnTypeAndFields() {
    Notification notification1 = new Notification("type");

    notification1.setFieldValue("key", "value1");

    assertThat(notification1).hasToString("Notification{type='type', fields={key=value1}}");
  }