Iterator<String> iterator = emptyIterator();
    assertThat(Iterators.<@Nullable String>getOnlyElement(iterator, null)).isNull();
  }

  public void testGetOnlyElement_withDefault_two() {
    Iterator<String> iterator = asList("foo", "bar").iterator();
    IllegalArgumentException expected =
        assertThrows(IllegalArgumentException.class, () -> getOnlyElement(iterator, "x"));