@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");

    populateStarShapedGraph();

    assertThat(graph.edges())
        .containsExactly(
            EndpointPair.ordered(2, 1),
            EndpointPair.ordered(1, 4),
            EndpointPair.ordered(1, 3),
            EndpointPair.ordered(1, 2),
            EndpointPair.ordered(3, 1),
            EndpointPair.ordered(5, 1))
        .inOrder();
  }

  @Test

      // 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();

    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

    public Set<E> outEdges(N node) {
      return delegate().inEdges(node); // transpose
    }

    @Override
    public EndpointPair<N> incidentNodes(E edge) {
      EndpointPair<N> endpointPair = delegate().incidentNodes(edge);
      return EndpointPair.of(network, endpointPair.nodeV(), endpointPair.nodeU()); // transpose
    }

    @Override
    public Set<E> edgesConnecting(N nodeU, N nodeV) {

    public Set<E> outEdges(N node) {
      return delegate().inEdges(node); // transpose
    }

    @Override
    public EndpointPair<N> incidentNodes(E edge) {
      EndpointPair<N> endpointPair = delegate().incidentNodes(edge);
      return EndpointPair.of(network, endpointPair.nodeV(), endpointPair.nodeU()); // transpose
    }

    @Override
    public Set<E> edgesConnecting(N nodeU, N nodeV) {

    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