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() {

   * 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 {

    assertFalse(writer.closed());
    source.copyTo(writer);
    assertFalse(writer.closed());
  }

  public void testClosesOnErrors_copyingToCharSinkThatThrows() {
    for (TestOption option : EnumSet.of(OPEN_THROWS, WRITE_THROWS, CLOSE_THROWS)) {
      TestCharSource okSource = new TestCharSource(STRING);
      assertThrows(IOException.class, () -> okSource.copyTo(new TestCharSink(option)));

    assertFalse(writer.closed());
    source.copyTo(writer);
    assertFalse(writer.closed());
  }

  public void testClosesOnErrors_copyingToCharSinkThatThrows() {
    for (TestOption option : EnumSet.of(OPEN_THROWS, WRITE_THROWS, CLOSE_THROWS)) {
      TestCharSource okSource = new TestCharSource(STRING);
      assertThrows(IOException.class, () -> okSource.copyTo(new TestCharSink(option)));

   *       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

    StringBuilder builder = new StringBuilder();

    assertEquals(expected.length(), source.copyTo(builder));

    assertExpectedString(builder.toString());
  }

  public void testCopyTo_charSink() throws IOException {
    TestCharSink sink = new TestCharSink();

    assertEquals(expected.length(), source.copyTo(sink));

    assertExpectedString(sink.getString());
  }

          }

          // The buffer has the data we need. Read from there and return immediately.
          val bytesToRead = minOf(byteCount, upstreamPos - sourcePos)
          buffer.copyTo(sink, sourcePos - bufferPos, bytesToRead)
          sourcePos += bytesToRead
          return bytesToRead
        }

      // Read from the file.
      if (source == SOURCE_FILE) {