@Test
public void testCeil() {

    FastSafeIterableMap<Integer, Boolean> map = new FastSafeIterableMap<>();
    assertThat(map.ceil(1), nullValue());
    map.putIfAbsent(1, false);
    assertThat(map.ceil(1), nullValue());
    map.putIfAbsent(2, false);
    assertThat(map.ceil(2).getKey(), is(1));
    map.remove(1);
    assertThat(map.ceil(2), nullValue());
}