public void isEmpty() {
    if (actual == null) {
      failWithActual(simpleFact("expected an empty string"));
    } else if (!actual.isEmpty()) {
      failWithActual(simpleFact("expected to be empty"));
    }
  }
@Test
  public void stringIsEmptyFail() {
    expectFailureWhenTestingThat("abc").isEmpty();
    assertFailureKeys("expected to be empty", "but was");
  }