Iterator<String> iterator = singletonList("foo").iterator();
    assertEquals("foo", getOnlyElement(iterator));
  }

  public void testGetOnlyElement_noDefault_empty() {
    Iterator<String> iterator = emptyIterator();
    assertThrows(NoSuchElementException.class, () -> getOnlyElement(iterator));
  }

  public void testGetOnlyElement_noDefault_moreThanOneLessThanFiveElements() {

    Iterator<String> iterator = singletonList("foo").iterator();
    assertEquals("foo", getOnlyElement(iterator));
  }

  public void testGetOnlyElement_noDefault_empty() {
    Iterator<String> iterator = emptyIterator();
    assertThrows(NoSuchElementException.class, () -> getOnlyElement(iterator));
  }

  public void testGetOnlyElement_noDefault_moreThanOneLessThanFiveElements() {

   *     iterator is unspecified.
   */
  @ParametricNullness
  public static <T extends @Nullable Object> T getOnlyElement(
      Iterator<? extends T> iterator, @ParametricNullness T defaultValue) {
    return iterator.hasNext() ? getOnlyElement(iterator) : defaultValue;
  }

  /**
   * Copies an iterator's elements into an array. The iterator will be left exhausted: its {@code