assertEquals(1, Iterators.size(registry.getSubscribers(1)));

    registry.register(new IntegerSubscriber());
    assertEquals(3, Iterators.size(registry.getSubscribers("")));
    assertEquals(1, Iterators.size(registry.getSubscribers(new Object())));
    assertEquals(2, Iterators.size(registry.getSubscribers(1)));
  }

  public void testGetSubscribers_returnsImmutableSnapshot() {

  @Override
  public @Nullable Entry<K, V> firstEntry() {
    return Iterators.<@Nullable Entry<K, V>>getNext(entryIterator(), null);
  }

  @Override
  public @Nullable Entry<K, V> lastEntry() {
    return Iterators.<@Nullable Entry<K, V>>getNext(descendingEntryIterator(), null);
  }

  @Override
  public @Nullable Entry<K, V> pollFirstEntry() {
    return Iterators.pollNext(entryIterator());
  }

  @Override

  protected @Nullable E standardHigher(@ParametricNullness E e) {
    return Iterators.getNext(tailSet(e, false).iterator(), null);
  }

  @Override
  public @Nullable E pollFirst() {
    return delegate().pollFirst();
  }

  /**
   * A sensible definition of {@link #pollFirst} in terms of the {@code iterator} method. If you
   * override {@link #iterator} you may wish to override {@link #pollFirst} to forward to this

      @Override
      public Iterator<T> iterator() {
        return Iterators.concat(
            /* lazily generate the iterators on each input only as needed */
            new AbstractIndexedListIterator<Iterator<? extends T>>(inputs.length) {
              @Override
              public Iterator<? extends T> get(int i) {
                return inputs[i].iterator();
              }
            });
      }

  @Override
  public @Nullable Entry<K, V> firstEntry() {
    return Iterators.<@Nullable Entry<K, V>>getNext(entryIterator(), null);
  }

  @Override
  public @Nullable Entry<K, V> lastEntry() {
    return Iterators.<@Nullable Entry<K, V>>getNext(descendingEntryIterator(), null);
  }

  @Override
  public @Nullable Entry<K, V> pollFirstEntry() {
    return Iterators.pollNext(entryIterator());
  }

  @Override

      }
    };
  }

  @Override
  public Iterator<EndpointPair<N>> incidentEdgeIterator(N thisNode) {
    checkNotNull(thisNode);

    Iterator<EndpointPair<N>> resultWithDoubleSelfLoop;
    if (orderedNodeConnections == null) {
      resultWithDoubleSelfLoop =
          Iterators.concat(
              Iterators.transform(
                  predecessors().iterator(),

        @Override
        protected Set<K> delegate() {
          return delegate.keySet();
        }

        @Override
        public Iterator<K> iterator() {
          return Iterators.unmodifiableIterator(delegate.keySet().iterator());
        }

        @Override
        public boolean removeAll(Collection<?> c) {
          return delegate.keySet().removeAll(c);
        }

        @Override

    return adjacentNodes();
  }

  @Override
  public Set<N> successors() {
    return adjacentNodes();
  }

  @Override
  public Iterator<EndpointPair<N>> incidentEdgeIterator(N thisNode) {
    return Iterators.transform(
        adjacentNodeValues.keySet().iterator(),
        (N incidentNode) -> EndpointPair.unordered(thisNode, incidentNode));
  }

  @Override
  public @Nullable V value(N node) {

    this.targetNode = checkNotNull(targetNode);
  }

  @Override
  public UnmodifiableIterator<E> iterator() {
    E connectingEdge = getConnectingEdge();
    return (connectingEdge == null)
        ? ImmutableSet.<E>of().iterator()
        : Iterators.singletonIterator(connectingEdge);
  }

  @Override
  public int size() {
    return getConnectingEdge() == null ? 0 : 1;
  }

        @Override
        protected Set<K> delegate() {
          return delegate.keySet();
        }

        @Override
        public Iterator<K> iterator() {
          return Iterators.unmodifiableIterator(delegate.keySet().iterator());
        }

        @Override
        public boolean removeAll(Collection<?> c) {
          return delegate.keySet().removeAll(c);
        }

        @Override