Set<Object> set = new HashSet<>();
    assertThat(union(emptySet(), emptySet()).copyInto(set)).isSameInstanceAs(set);
    assertThat(intersection(emptySet(), emptySet()).copyInto(set)).isSameInstanceAs(set);
    assertThat(difference(emptySet(), emptySet()).copyInto(set)).isSameInstanceAs(set);
    assertThat(symmetricDifference(emptySet(), emptySet()).copyInto(set)).isSameInstanceAs(set);
  }

  public void testCopyInto_emptySet() {

   *       one with {@link CharSequence#charAt(int)}.
   *   <li>use {@link Appendable#append(CharSequence)} in {@link #copyTo(Appendable)} and {@link
   *       #copyTo(CharSink)}. We know this is correct since strings are immutable and so the length
   *       can't change, and it is faster because many writers and appendables are optimized for

   *       one with {@link CharSequence#charAt(int)}.
   *   <li>use {@link Appendable#append(CharSequence)} in {@link #copyTo(Appendable)} and {@link
   *       #copyTo(CharSink)}. We know this is correct since strings are immutable and so the length
   *       can't change, and it is faster because many writers and appendables are optimized for

      ReferenceEntry<Object, Object> copyTwo = map.copyEntry(entryTwo, copyOne);
      assertSame(keyTwo, copyTwo.getKey());
      assertEquals(hashTwo, copyTwo.getHash());
      assertSame(copyOne, copyTwo.getNext());
      assertSame(valueTwo, copyTwo.getValueReference().get());
      assertConnected(map, copyOne, copyTwo);
    }
  }

  private static <K, V> void assertConnected(

   * stable. There is usually no reason to retain a reference of type {@code SetView}; typically,
   * you either use it as a plain {@link Set}, or immediately invoke {@link #immutableCopy} or
   * {@link #copyInto} and forget the {@code SetView} itself.
   *
   * @since 2.0
   */
  public abstract static class SetView<E extends @Nullable Object> extends AbstractSet<E> {
    private SetView() {} // no subclasses but our own

   *
   * @param collection the collection to copy elements to
   * @return {@code collection}, for convenience
   * @since 14.0
   */
  @CanIgnoreReturnValue
  public final <C extends Collection<? super E>> C copyInto(C collection) {
    checkNotNull(collection);
    Iterable<E> iterable = getDelegate();
    if (iterable instanceof Collection) {
      collection.addAll((Collection<E>) iterable);
    } else {

   *
   * @param collection the collection to copy elements to
   * @return {@code collection}, for convenience
   * @since 14.0
   */
  @CanIgnoreReturnValue
  public final <C extends Collection<? super E>> C copyInto(C collection) {
    checkNotNull(collection);
    Iterable<E> iterable = getDelegate();
    if (iterable instanceof Collection) {
      collection.addAll((Collection<E>) iterable);
    } else {