this.element = checkNotNull(element);
  }

  @Override
  public E get(int index) {
    Preconditions.checkElementIndex(index, 1);
    return element;
  }

  @Override
  public UnmodifiableIterator<E> iterator() {
    return singletonIterator(element);
  }

  @Override
  public Spliterator<E> spliterator() {
    return singleton(element).spliterator();
  }

  @Override
  public int size() {

   * overrides it.
   */
  abstract ImmutableSet<K> createKeySet();

  UnmodifiableIterator<K> keyIterator() {
    UnmodifiableIterator<Entry<K, V>> entryIterator = entrySet().iterator();
    return new UnmodifiableIterator<K>() {
      @Override
      public boolean hasNext() {
        return entryIterator.hasNext();
      }

      @Override

    @SuppressWarnings("unchecked")
    // requireNonNull is safe because of the contains check.
    C c = (C) requireNonNull(target);
    return (int) domain.distance(first(), c);
  }

  @Override
  public UnmodifiableIterator<C> iterator() {
    return new AbstractSequentialIterator<C>(first()) {
      final C last = last();

      @Override
      protected @Nullable C computeNext(C previous) {

      throw new UnsupportedOperationException();
    }

    /**
     * Scope the return type to {@link UnmodifiableIterator} to ensure this is an unmodifiable view.
     *
     * @since 20.0 (present with return type {@link Iterator} since 2.0)
     */
    @Override
    public abstract UnmodifiableIterator<E> iterator();

    @Override
    @SuppressWarnings("EqualsHashCode") // same semantics

 *       }
 *     };
 * }
 *
 * @author Chris Povirk
 * @since 12.0 (in Guava as {@code AbstractLinkedIterator} since 8.0)
 */
@GwtCompatible
public abstract class AbstractSequentialIterator<T> extends UnmodifiableIterator<T> {
  private @Nullable T nextOrNull;

  /**
   * Creates a new iterator with the given first element, or, if {@code firstOrNull} is null,
   * creates a new empty iterator.
   */

  ImmutableMapKeySet(ImmutableMap<K, V> map) {
    this.map = map;
  }

  @Override
  public int size() {
    return map.size();
  }

  @Override
  public UnmodifiableIterator<K> iterator() {
    return map.keyIterator();
  }

  @Override
  public Spliterator<K> spliterator() {
    return map.keySpliterator();
  }

  @Override

 *   <li>{@link Tables}
 * </ul>
 *
 * <h2>Abstract implementations</h2>
 *
 * <ul>
 *   <li>{@link AbstractIterator}
 *   <li>{@link AbstractSequentialIterator}
 *   <li>{@link UnmodifiableIterator}
 *   <li>{@link UnmodifiableListIterator}
 * </ul>
 *
 * <h2>Forwarding collections</h2>
 *
 * We provide implementations of collections that forward all method calls to a delegate collection

 *   <li>{@link Tables}
 * </ul>
 *
 * <h2>Abstract implementations</h2>
 *
 * <ul>
 *   <li>{@link AbstractIterator}
 *   <li>{@link AbstractSequentialIterator}
 *   <li>{@link UnmodifiableIterator}
 *   <li>{@link UnmodifiableListIterator}
 * </ul>
 *
 * <h2>Forwarding collections</h2>
 *
 * We provide implementations of collections that forward all method calls to a delegate collection

    }

    @Override
    Iterator<Entry<E>> entryIterator() {
      Iterator<Map.Entry<E, Integer>> backingEntries = backingMap.entrySet().iterator();
      return new UnmodifiableIterator<Multiset.Entry<E>>() {
        @Override
        public boolean hasNext() {
          return backingEntries.hasNext();
        }

        @Override
        public Multiset.Entry<E> next() {

    Iterator<String> mod = Lists.newArrayList("a", "b", "c").iterator();
    UnmodifiableIterator<String> unmod = Iterators.unmodifiableIterator(mod);
    assertNotSame(mod, unmod);
    assertSame(unmod, Iterators.unmodifiableIterator(unmod));
    assertSame(unmod, Iterators.unmodifiableIterator((Iterator<String>) unmod));
  }