}

  public void testTransform_presentToString() {
    assertEquals(Optional.of("42"), Optional.of(42).transform(Functions.toStringFunction()));
  }

  public void testTransform_present_functionReturnsNull() {
    assertThrows(NullPointerException.class, () -> Optional.of("a").transform(input -> null));
  }

  public void testTransform_absent_functionReturnsNull() {