public final void isEmpty() {
    if (!checkNotNull(actual).isEmpty()) {
      failWithActual(simpleFact("expected to be empty"));
    }
  }
@Test
  public void isEmptyWithFailure() {
    ImmutableMap<Integer, Integer> actual = ImmutableMap.of(1, 5);
    expectFailureWhenTestingThat(actual).isEmpty();
    assertFailureKeys("expected to be empty", "but was");
  }