*
 * @author James Sexton
 * @param <E> Edge parameter type
 */
final class EdgesConnecting<E> extends AbstractSet<E> {

  private final Map<?, E> nodeToOutEdge;
  private final Object targetNode;

  EdgesConnecting(Map<?, E> nodeToEdgeMap, Object targetNode) {
    this.nodeToOutEdge = checkNotNull(nodeToEdgeMap);
    this.targetNode = checkNotNull(targetNode);
  }

  }

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

  @Override
  public Set<E> edgesConnecting(N node) {
    return new EdgesConnecting<>(((BiMap<E, N>) incidentEdgeMap).inverse(), node);
  }

  }

  @Override
  public @Nullable E edgeConnectingOrNull(N nodeU, N nodeV) {
    Set<E> edgesConnecting = edgesConnecting(nodeU, nodeV);
    switch (edgesConnecting.size()) {
      case 0:
        return null;
      case 1:
        return edgesConnecting.iterator().next();
      default:
        throw new IllegalArgumentException(String.format(MULTIPLE_EDGES_CONNECTING, nodeU, nodeV));
    }

  public int outDegree(N node) {
    return delegate().outDegree(node);
  }

  @Override
  public Set<E> edgesConnecting(N nodeU, N nodeV) {
    return delegate().edgesConnecting(nodeU, nodeV);
  }

  @Override
  public Set<E> edgesConnecting(EndpointPair<N> endpoints) {
    return delegate().edgesConnecting(endpoints);
  }

  @Override
  public @Nullable E edgeConnectingOrNull(N nodeU, N nodeV) {

/**
 * A class to represent the set of edges connecting an (implicit) origin node to a target node.
 *
 * <p>The {@link #outEdgeToNode} map allows this class to work on networks with parallel edges. See
 * {@link EdgesConnecting} for a class that is more efficient but forbids parallel edges.
 *
 * @author James Sexton
 * @param <E> Edge parameter type
 */
abstract class MultiEdgesConnecting<E> extends AbstractSet<E> {

  }

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

  @Override
  public Set<E> edgesConnecting(N node) {
    return new EdgesConnecting<>(((BiMap<E, N>) outEdgeMap).inverse(), node);
  }

    network.addNode(N1);
    network.addNode(N2);
    Set<String> edgesConnecting = network.edgesConnecting(N1, N2);
    assertThrows(UnsupportedOperationException.class, () -> edgesConnecting.add(E23));
    network.addEdge(N1, N2, E12);
    assertThat(networkForTest.edgesConnecting(N1, N2)).containsExactlyElementsIn(edgesConnecting);
  }

  @Test
  public void edgesConnecting_oneEdge() {

    addNode(N1);
    addNode(N2);
    Set<String> edgesConnecting = network.edgesConnecting(N1, N2);
    assertThrows(UnsupportedOperationException.class, () -> edgesConnecting.add(E23));
    addEdge(N1, N2, E12);
    assertThat(network.edgesConnecting(N1, N2)).containsExactlyElementsIn(edgesConnecting);
  }

  @Override
  @Test
  public void inEdges_checkReturnedSetMutability() {

    addNode(N1);
    addNode(N2);
    Set<String> edgesConnecting = network.edgesConnecting(N1, N2);
    assertThrows(UnsupportedOperationException.class, () -> edgesConnecting.add(E23));
    addEdge(N1, N2, E12);
    assertThat(network.edgesConnecting(N1, N2)).containsExactlyElementsIn(edgesConnecting);
  }

  @Override
  @Test
  public void inEdges_checkReturnedSetMutability() {

      adjacentNodes = HashMultiset.create(incidentEdgeMap.values());
      adjacentNodesReference = new SoftReference<>(adjacentNodes);
    }
    return adjacentNodes;
  }

  @Override
  public Set<E> edgesConnecting(N node) {
    return new MultiEdgesConnecting<E>(incidentEdgeMap, node) {
      @Override
      public int size() {
        return adjacentNodesMultiset().count(node);
      }
    };
  }

  @Override