@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) {

   * 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> {

    assertThat(graph.hasEdgeConnecting(EndpointPair.unordered(1, 2))).isFalse();
    assertThat(graph.hasEdgeConnecting(EndpointPair.unordered(2, 1))).isFalse();
  }

  @Test
  public void hasEdgeConnecting_undirected_correct() {
    graph = ValueGraphBuilder.undirected().build();
    graph.putEdgeValue(1, 2, "A");
    assertThat(graph.hasEdgeConnecting(EndpointPair.unordered(1, 2))).isTrue();
  }

  @Test

    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

    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

          // operations only in weird cases like checking for an EndpointPair<ArrayList> in a
          // Network<LinkedList>.
          @SuppressWarnings("unchecked")
          @Override
          public boolean contains(@CheckForNull Object obj) {
            if (!(obj instanceof EndpointPair)) {
              return false;
            }
            EndpointPair<?> endpointPair = (EndpointPair<?>) obj;

      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");

      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");

            .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 =

    assertThat(graph.hasEdgeConnecting(EndpointPair.unordered(1, 2))).isFalse();
    assertThat(graph.hasEdgeConnecting(EndpointPair.unordered(2, 1))).isFalse();
  }

  @Test
  public void hasEdgeConnecting_undirected_correct() {
    graph = ValueGraphBuilder.undirected().build();
    graph.putEdgeValue(1, 2, "A");
    assertThat(graph.hasEdgeConnecting(EndpointPair.unordered(1, 2))).isTrue();
  }

  @Test