@ElementTypesAreNonnullByDefault
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) {

    assertThat(graph.hasEdgeConnecting(EndpointPair.unordered(N1, N2))).isTrue();
    assertThat(graph.hasEdgeConnecting(EndpointPair.unordered(N2, N1))).isTrue();
  }

  @Test
  public void hasEdgeConnecting_mismatch() {
    putEdge(N1, N2);
    assertThat(graph.hasEdgeConnecting(EndpointPair.ordered(N1, N2))).isFalse();
    assertThat(graph.hasEdgeConnecting(EndpointPair.ordered(N2, N1))).isFalse();
  }

  @Test

   * 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> {

      for (String inEdge : network.inEdges(node)) {
        EndpointPair<Integer> endpointPair = network.incidentNodes(inEdge);
        assertThat(endpointPair.source()).isEqualTo(endpointPair.adjacentNode(node));
        assertThat(endpointPair.target()).isEqualTo(node);
      }

      for (String outEdge : network.outEdges(node)) {
        EndpointPair<Integer> endpointPair = network.incidentNodes(outEdge);

      for (String inEdge : network.inEdges(node)) {
        EndpointPair<Integer> endpointPair = network.incidentNodes(inEdge);
        assertThat(endpointPair.source()).isEqualTo(endpointPair.adjacentNode(node));
        assertThat(endpointPair.target()).isEqualTo(node);
      }

      for (String outEdge : network.outEdges(node)) {
        EndpointPair<Integer> endpointPair = network.incidentNodes(outEdge);

  public boolean contains(@CheckForNull 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();

    assertThat(graph.hasEdgeConnecting(EndpointPair.unordered(N1, N2))).isTrue();
    assertThat(graph.hasEdgeConnecting(EndpointPair.unordered(N2, N1))).isTrue();
  }

  @Test
  public void hasEdgeConnecting_mismatch() {
    putEdge(N1, N2);
    assertThat(graph.hasEdgeConnecting(EndpointPair.ordered(N1, N2))).isFalse();
    assertThat(graph.hasEdgeConnecting(EndpointPair.ordered(N2, N1))).isFalse();
  }

  @Test

            .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 =

                  return EndpointPair.ordered(thisNode, connection.node);
                } else {
                  return EndpointPair.ordered(connection.node, thisNode);
                }
              });
    }

    AtomicBoolean alreadySeenSelfLoop = new AtomicBoolean(false);
    return new AbstractIterator<EndpointPair<N>>() {
      @Override
      @CheckForNull

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