Sets.symmetricDifference(friends, enemies).copyInto(new HashSet<String>());

    enemies.add("Dave");
    assertEquals(3, symmetricDifferenceFriendsFirst.size());
    assertEquals(4, immut.size());
    assertEquals(4, mut.size());

    immut = Sets.symmetricDifference(enemies, friends).immutableCopy();
    mut = Sets.symmetricDifference(enemies, friends).copyInto(new HashSet<String>());
    friends.add("Harry");

        Sets.symmetricDifference(friends, enemies).copyInto(new HashSet<String>());

    enemies.add("Dave");
    assertEquals(3, symmetricDifferenceFriendsFirst.size());
    assertEquals(4, immut.size());
    assertEquals(4, mut.size());

    immut = Sets.symmetricDifference(enemies, friends).immutableCopy();
    mut = Sets.symmetricDifference(enemies, friends).copyInto(new HashSet<String>());
    friends.add("Harry");

    assertThat(fluent(1, 3, 5).copyInto(Lists.newArrayList(1, 2)))
        .containsExactly(1, 2, 1, 3, 5)
        .inOrder();
  }

  public void testCopyInto_set() {
    assertThat(fluent(1, 3, 5).copyInto(newHashSet(1, 2))).containsExactly(1, 2, 3, 5);
  }

  public void testCopyInto_setAllDuplicates() {
    assertThat(fluent(1, 3, 5).copyInto(newHashSet(1, 2, 3, 5))).containsExactly(1, 2, 3, 5);
  }

   * 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 {

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

  public void testClosesOnErrors_copyingToByteSinkThatThrows() {
    for (TestOption option : EnumSet.of(OPEN_THROWS, WRITE_THROWS, CLOSE_THROWS)) {
      TestByteSource okSource = new TestByteSource(bytes);
      assertThrows(IOException.class, () -> okSource.copyTo(new TestByteSink(option)));

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

  public void testClosesOnErrors_copyingToByteSinkThatThrows() {
    for (TestOption option : EnumSet.of(OPEN_THROWS, WRITE_THROWS, CLOSE_THROWS)) {
      TestByteSource okSource = new TestByteSource(bytes);
      assertThrows(IOException.class, () -> okSource.copyTo(new TestByteSink(option)));

      TestCharSource failSource = new TestCharSource(STRING, option);
      TestCharSink okSink = new TestCharSink();
      assertThrows(IOException.class, () -> failSource.copyTo(okSink));
      // ensure writer was closed IF it was opened (depends on implementation whether or not it's
      // opened at all if source.newReader() throws).
      assertTrue(

    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());
  }