}

    /**
     * Returns an iterator over the elements in this set. The elements are
     * returned in no particular order.
     *
     * @return an Iterator over the elements in this set.
     * @see ConcurrentModificationException
     */
    @Override
    public Iterator<E> iterator() {
        return map.keySet().iterator();
    }

    /**

 * {@link #computeNext} method, and invoke the {@link #endOfData} method when appropriate.
 *
 * <p>Another example is an iterator that skips over null elements in a backing iterator. This could
 * be implemented as:
 *
 * {@snippet :
 * public static Iterator<String> skipNulls(final Iterator<String> in) {
 *   return new AbstractIterator<String>() {
 *     protected String computeNext() {
 *       while (in.hasNext()) {

    this.elementEquivalence = Preconditions.checkNotNull(elementEquivalence);
  }

  @Override
  protected boolean doEquivalent(Iterable<T> iterableA, Iterable<T> iterableB) {
    Iterator<T> iteratorA = iterableA.iterator();
    Iterator<T> iteratorB = iterableB.iterator();

    while (iteratorA.hasNext() && iteratorB.hasNext()) {
      if (!elementEquivalence.equivalent(iteratorA.next(), iteratorB.next())) {
        return false;
      }
    }

        @Nonnull
        public Optional<Lifecycle> lookup(String id) {
            return getDelegate().lookup(id);
        }

        @Override
        public Iterator<Lifecycle> iterator() {
            return getDelegate().iterator();
        }

        protected LifecycleRegistry getDelegate() {
            return lifecycleRegistry;
        }

        @Override

      SortedSet<@Nullable Object> self = (SortedSet<@Nullable Object>) this;
      Iterator<?> iterator = self.tailSet(object).iterator();
      if (iterator.hasNext()) {
        Object ceiling = iterator.next();
        if (unsafeCompare(comparator(), ceiling, object) == 0) {
          iterator.remove();
          return true;
        }
      }
    } catch (ClassCastException | NullPointerException e) {

      @Override
      protected Iterator<Integer> newTargetIterator() {
        return newDoubler(1, 2);
      }
    }.test();
  }

  public void testDoubler() {
    Iterable<Integer> doubled =
        new Iterable<Integer>() {
          @Override
          public Iterator<Integer> iterator() {
            return newDoubler(2, 32);
          }
        };

   *     iterator().next()} or {@link Iterables#getFirst} instead.
   */
  @SuppressWarnings("nullness") // Unsafe, but we can't do much about it now.
  public final Optional<@NonNull E> first() {
    Iterator<E> iterator = getDelegate().iterator();
    return iterator.hasNext() ? Optional.of(iterator.next()) : Optional.absent();
  }

  /**

   *     iterator().next()} or {@link Iterables#getFirst} instead.
   */
  @SuppressWarnings("nullness") // Unsafe, but we can't do much about it now.
  public final Optional<@NonNull E> first() {
    Iterator<E> iterator = getDelegate().iterator();
    return iterator.hasNext() ? Optional.of(iterator.next()) : Optional.absent();
  }

  /**

     * @return An iterator of URL objects for the resources. If no resources are found, the iterator will be empty.
     *         Resources that the class loader does not have access to will not be included.
     * @see java.lang.ClassLoader#getResources(String)
     */
    public static Iterator<URL> getResources(final String name) {
        assertArgumentNotEmpty("name", name);

  Iterator<N> successorIterator = ImmutableSet.<N>of().iterator();

  static <N> EndpointPairIterator<N> of(BaseGraph<N> graph) {
    return graph.isDirected() ? new Directed<N>(graph) : new Undirected<N>(graph);
  }

  private EndpointPairIterator(BaseGraph<N> graph) {
    this.graph = graph;
    this.nodeIterator = graph.nodes().iterator();
  }

  /**