count = vertices.size() + vertices.size() / 2;
        }

        checkEdges(count);
    }

    private void checkEdges(int nEdges) {
        if (incidentEdges == null) {
            incidentEdges = new HashMap<>(nEdges);
        }
        if (excidentEdges == null) {
            excidentEdges = new HashMap<>(nEdges);
        }
    }

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

  @Override
  public Set<E> incidentEdges() {
    return Collections.unmodifiableSet(incidentEdgeMap.keySet());
  }

  @Override
  public Set<E> inEdges() {
    return incidentEdges();
  }

  @Override
  public Set<E> outEdges() {
    return incidentEdges();
  }

  @Override
  public N adjacentNode(E edge) {

   * Returns an {@link ElementOrder} that specifies the order of iteration for the elements of
   * {@link #edges()}, {@link #adjacentNodes(Object)}, {@link #predecessors(Object)}, {@link
   * #successors(Object)} and {@link #incidentEdges(Object)}.
   *
   * @since 29.0
   */
  ElementOrder<N> incidentEdgeOrder();

  /**
   * Returns a live view of this graph as a {@link Network} whose edges {@code E} are {@code

    return new UndirectedNetworkConnections<>(HashBiMap.<E, N>create(EXPECTED_DEGREE));
  }

  static <N, E> UndirectedNetworkConnections<N, E> ofImmutable(Map<E, N> incidentEdges) {
    return new UndirectedNetworkConnections<>(ImmutableBiMap.copyOf(incidentEdges));
  }

  @Override
  public Set<N> adjacentNodes() {
    return Collections.unmodifiableSet(((BiMap<E, N>) incidentEdgeMap).values());
  }

  @Override

  @Override
  public Set<N> adjacentNodes() {
    return Sets.union(predecessors(), successors());
  }

  @Override
  public Set<E> incidentEdges() {
    return new AbstractSet<E>() {
      @Override
      public UnmodifiableIterator<E> iterator() {
        Iterable<E> incidentEdges =
            (selfLoopCount == 0)
                ? Iterables.concat(inEdgeMap.keySet(), outEdgeMap.keySet())

    return delegate().predecessors(node);
  }

  @Override
  public Set<N> successors(N node) {
    return delegate().successors(node);
  }

  @Override
  public Set<E> incidentEdges(N node) {
    return delegate().incidentEdges(node);
  }

  @Override
  public Set<E> inEdges(N node) {
    return delegate().inEdges(node);
  }

  @Override
  public Set<E> outEdges(N node) {

    addNode(N1);
    Set<EndpointPair<Integer>> incidentEdges = graph.incidentEdges(N1);
    assertThrows(
        UnsupportedOperationException.class,
        () -> incidentEdges.add(EndpointPair.unordered(N1, N2)));
    putEdge(N1, N2);
    assertThat(incidentEdges).containsExactlyElementsIn(graph.incidentEdges(N1));
  }

  @Test
  public void predecessors_oneEdge() {
    putEdge(N1, N2);

    assume().that(graphIsMutable()).isTrue();

    addNode(N1);
    Set<EndpointPair<Integer>> incidentEdges = graph.incidentEdges(N1);
    assertThrows(
        UnsupportedOperationException.class, () -> incidentEdges.add(EndpointPair.ordered(N1, N2)));
    putEdge(N1, N2);
    assertThat(incidentEdges).containsExactlyElementsIn(graph.incidentEdges(N1));
  }

  @Test
  public void predecessors_oneEdge() {
    putEdge(N1, N2);

  public Set<E> adjacentEdges(E edge) {
    EndpointPair<N> endpointPair = incidentNodes(edge); // Verifies that edge is in this network.
    Set<E> endpointPairIncidentEdges =
        Sets.union(incidentEdges(endpointPair.nodeU()), incidentEdges(endpointPair.nodeV()));
    return edgeInvalidatableSet(
        Sets.difference(endpointPairIncidentEdges, ImmutableSet.of(edge)), edge);
  }

  @Override

    return nodeOrder;
  }

  @Override
  public ElementOrder<E> edgeOrder() {
    return edgeOrder;
  }

  @Override
  public Set<E> incidentEdges(N node) {
    return nodeInvalidatableSet(checkedConnections(node).incidentEdges(), node);
  }

  @Override
  public EndpointPair<N> incidentNodes(E edge) {
    N nodeU = checkedReferenceNode(edge);