@Test
public void isEmptyTakesHiddenKeysIntoAccount() {

    assertFalse(createSubject().isEmpty());
    ProjectedJsonObject subject = new ProjectedJsonObject(new JsonObject(ImmutableMap.<String, Object>builder()
            .put("minimum", 2)
            .put("maximum", 20).build()), new HashSet<>(asList("minimum", "maximum")));
    assertTrue(subject.isEmpty());
}