assertThrows(NoSuchElementException.class, () -> Stream.empty().collect(onlyElement()));
  }

  public void testOnlyElementSingleton() {
    assertThat(Stream.of(1).collect(onlyElement())).isEqualTo(1);
  }

  public void testOnlyElementNull() {
    assertThat(Stream.<@Nullable Object>of((Object) null).collect(onlyElement())).isNull();
  }

  public void testOnlyElementMultiple() {

   * more elements, and a {@code NoSuchElementException} if the stream is empty.
   */
  @SuppressWarnings("unchecked")
  public static <T extends @Nullable Object> Collector<T, ?, T> onlyElement() {
    return (Collector) ONLY_ELEMENT;
  }

  /**
   * This atrocity is here to let us report several of the elements in the stream if there were more
   * than one, not just two.
   */

         * `length` array elements.
         */
        @SuppressWarnings("unchecked") // our callers put only E instances into the array
        E onlyElement = (E) requireNonNull(elements[0]);
        return of(onlyElement);
      default:
        /*
         * The suppression is safe because the callers promise to put non-null objects in the first
         * `length` array elements.
         */

  }

  /**
   * Returns the single element contained in {@code iterable}.
   *
   * <p><b>Java 8+ users:</b> the {@code Stream} equivalent to this method is {@code
   * stream.collect(MoreCollectors.onlyElement())}.
   *
   * @throws NoSuchElementException if the iterable is empty
   * @throws IllegalArgumentException if the iterable contains multiple elements
   */
  @ParametricNullness

  }

  /**
   * Returns the single element contained in {@code iterable}.
   *
   * <p><b>Java 8+ users:</b> the {@code Stream} equivalent to this method is {@code
   * stream.collect(MoreCollectors.onlyElement())}.
   *
   * @throws NoSuchElementException if the iterable is empty
   * @throws IllegalArgumentException if the iterable contains multiple elements
   */
  @ParametricNullness