public Notification setFieldValue(String field, @Nullable String value) {
    fields.put(field, value);
    return this;
  }
@Test
  void equals_whenFieldsDontMatch_shouldReturnFalse() {
    Notification notification1 = new Notification("type");
    Notification notification2 = new Notification("type");

    notification1.setFieldValue("key", "value1");
    notification2.setFieldValue("key", "value2");

    assertThat(notification1).isNotEqualTo(notification2);
  }