@Override
  public Set<N> adjacentNodes() {
    return Collections.unmodifiableSet(adjacentNodesMultiset().elementSet());
  }

  private Multiset<N> adjacentNodesMultiset() {
    Multiset<N> adjacentNodes = getReference(adjacentNodesReference);
    if (adjacentNodes == null) {
      adjacentNodes = HashMultiset.create(incidentEdgeMap.values());

  }

  @Test
  public void adjacentNodes_oneEdge() {
    putEdge(N1, N2);
    assertThat(graph.adjacentNodes(N1)).containsExactly(N2);
    assertThat(graph.adjacentNodes(N2)).containsExactly(N1);
  }

  @Test
  public void adjacentNodes_noAdjacentNodes() {
    addNode(N1);
    assertThat(graph.adjacentNodes(N1)).isEmpty();
  }

  @Test

  }

  @Override
  @Test
  public void adjacentNodes_checkReturnedSetMutability() {
    assume().that(graphIsMutable()).isTrue();

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

  }

  @Override
  public Set<N> adjacentNodes() {
    return Collections.unmodifiableSet(adjacentNodeValues.keySet());
  }

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

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

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

  }

  @Override
  @Test
  public void adjacentNodes_checkReturnedSetMutability() {
    addNode(N1);
    Set<Integer> adjacentNodes = network.adjacentNodes(N1);
    UnsupportedOperationException e =
        assertThrows(UnsupportedOperationException.class, () -> adjacentNodes.add(N2));
    addEdge(N1, N2, E12);
    assertThat(network.adjacentNodes(N1)).containsExactlyElementsIn(adjacentNodes);
  }

  @Override

  }

  @Test
  public void adjacentNodes_oneEdge() {
    putEdge(N1, N2);
    assertThat(graph.adjacentNodes(N1)).containsExactly(N2);
    assertThat(graph.adjacentNodes(N2)).containsExactly(N1);
  }

  @Test
  public void adjacentNodes_noAdjacentNodes() {
    addNode(N1);
    assertThat(graph.adjacentNodes(N1)).isEmpty();
  }

  @Test

  @Override
  @Test
  public void adjacentNodes_checkReturnedSetMutability() {
    assume().that(graphIsMutable()).isTrue();

    addNode(N1);
    Set<Integer> adjacentNodes = network.adjacentNodes(N1);
    UnsupportedOperationException e =
        assertThrows(UnsupportedOperationException.class, () -> adjacentNodes.add(N2));
    addEdge(N1, N2, E12);
    assertThat(network.adjacentNodes(N1)).containsExactlyElementsIn(adjacentNodes);

  }

  @Override
  @Test
  public void adjacentNodes_checkReturnedSetMutability() {
    assume().that(graphIsMutable()).isTrue();

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

  @Override
  @Test
  public void adjacentNodes_checkReturnedSetMutability() {
    assume().that(graphIsMutable()).isTrue();

    addNode(N1);
    Set<Integer> adjacentNodes = network.adjacentNodes(N1);
    UnsupportedOperationException e =
        assertThrows(UnsupportedOperationException.class, () -> adjacentNodes.add(N2));
    addEdge(N1, N2, E12);
    assertThat(network.adjacentNodes(N1)).containsExactlyElementsIn(adjacentNodes);

  }

  @Override
  @Test
  public void adjacentNodes_checkReturnedSetMutability() {
    assume().that(graphIsMutable()).isTrue();

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