@Immutable(containerOf = {"N"})
public abstract class EndpointPair<N> implements Iterable<N> {
  private final N nodeU;
  private final N nodeV;

  private EndpointPair(N nodeU, N nodeV) {
    this.nodeU = checkNotNull(nodeU);
    this.nodeV = checkNotNull(nodeV);
  }

  /** Returns an {@link EndpointPair} representing the endpoints of a directed edge. */
  public static <N> EndpointPair<N> ordered(N source, N target) {

@Immutable(containerOf = {"N"})
public abstract class EndpointPair<N> implements Iterable<N> {
  private final N nodeU;
  private final N nodeV;

  private EndpointPair(N nodeU, N nodeV) {
    this.nodeU = checkNotNull(nodeU);
    this.nodeV = checkNotNull(nodeV);
  }

  /** Returns an {@link EndpointPair} representing the endpoints of a directed edge. */
  public static <N> EndpointPair<N> ordered(N source, N target) {

      network.addEdge(1, 2, "1-2");
      EndpointPair<Integer> endpointPair = network.incidentNodes("1-2");
      assertThrows(IllegalArgumentException.class, () -> endpointPair.adjacentNode(3));
    }
  }

  @Test
  public void testEquals() {
    EndpointPair<String> ordered = EndpointPair.ordered("a", "b");
    EndpointPair<String> orderedMirror = EndpointPair.ordered("b", "a");

      // Graph<LinkedList>.
      @SuppressWarnings("unchecked")
      @Override
      public boolean contains(@Nullable Object obj) {
        if (!(obj instanceof EndpointPair)) {
          return false;
        }
        EndpointPair<?> endpointPair = (EndpointPair<?>) obj;
        return isOrderingCompatible(endpointPair)

  public boolean contains(@Nullable Object obj) {
    if (!(obj instanceof EndpointPair)) {
      return false;
    }
    EndpointPair<?> endpointPair = (EndpointPair<?>) obj;

    if (graph.isDirected()) {
      if (!endpointPair.isOrdered()) {
        return false;
      }

      Object source = endpointPair.source();
      Object target = endpointPair.target();

   * Visited Nodes = {N1}
   * EndpointPair [N2, N1] - skip
   * EndpointPair [N2, N3] - return
   * Visited Nodes = {N1, N2}
   * EndpointPair [N3, N1] - skip
   * EndpointPair [N3, N2] - skip
   * Visited Nodes = {N1, N2, N3}
   * EndpointPair [N4, N4] - return
   * Visited Nodes = {N1, N2, N3, N4}
   * </pre>
   */
  private static final class Undirected<N> extends EndpointPairIterator<N> {

  }

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

  @Override
  public @Nullable E edgeConnectingOrNull(N nodeU, N nodeV) {
    return delegate().edgeConnectingOrNull(nodeU, nodeV);
  }

  @Override
  public @Nullable E edgeConnectingOrNull(EndpointPair<N> endpoints) {
    return delegate().edgeConnectingOrNull(endpoints);

            .putEdgeValue(2, 3, "2-3")
            .putEdgeValue(1, 2, "1-2")
            .build();

    assertThat(graph.incidentEdges(2))
        .containsExactly(
            EndpointPair.ordered(2, 1), EndpointPair.ordered(2, 3), EndpointPair.ordered(1, 2))
        .inOrder();
  }

  @Test
  public void immutableValueGraphBuilder_incidentEdgeOrder_stable() {
    ImmutableValueGraph<Integer, String> graph =

  }

  @Override
  public boolean hasEdgeConnecting(EndpointPair<N> endpoints) {
    return delegate().hasEdgeConnecting(endpoints);
  }

  @Override
  public Optional<V> edgeValue(N nodeU, N nodeV) {
    return delegate().edgeValue(nodeU, nodeV);
  }

  @Override
  public Optional<V> edgeValue(EndpointPair<N> endpoints) {
    return delegate().edgeValue(endpoints);
  }

    assertThat(graph.nodes()).containsExactly("A", "B");
    assertThat(graph.edges()).containsExactly(EndpointPair.ordered("A", "B"));
  }

  @Test
  public void immutableGraphBuilder_putEdgeFromEndpointPair() {
    ImmutableGraph<String> graph =
        GraphBuilder.directed().<String>immutable().putEdge(EndpointPair.ordered("A", "B")).build();

    assertThat(graph.nodes()).containsExactly("A", "B");