public static <N> EndpointPair<N> ordered(N source, N target) {
    return new Ordered<>(source, target);
  }

  /** Returns an {@link EndpointPair} representing the endpoints of an undirected edge. */
  public static <N> EndpointPair<N> unordered(N nodeU, N nodeV) {
    // Swap nodes on purpose to prevent callers from relying on the "ordering" of an unordered pair.
    return new Unordered<>(nodeV, nodeU);
  }

    EndpointPair<String> ordered = EndpointPair.ordered("source", "target");
    assertThat(ordered.isOrdered()).isTrue();
    assertThat(ordered).containsExactly("source", "target").inOrder();
    assertThat(ordered.source()).isEqualTo("source");
    assertThat(ordered.target()).isEqualTo("target");
    assertThat(ordered.nodeU()).isEqualTo("source");
    assertThat(ordered.nodeV()).isEqualTo("target");

    EndpointPair<String> ordered = EndpointPair.ordered("source", "target");
    assertThat(ordered.isOrdered()).isTrue();
    assertThat(ordered).containsExactly("source", "target").inOrder();
    assertThat(ordered.source()).isEqualTo("source");
    assertThat(ordered.target()).isEqualTo("target");
    assertThat(ordered.nodeU()).isEqualTo("source");
    assertThat(ordered.nodeV()).isEqualTo("target");

    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

    IllegalArgumentException e =
        assertThrows(
            IllegalArgumentException.class, () -> graph.edgeValue(EndpointPair.ordered(1, 2)));
    assertThat(e).hasMessageThat().contains(ENDPOINTS_MISMATCH);
    e =
        assertThrows(
            IllegalArgumentException.class, () -> graph.edgeValue(EndpointPair.ordered(2, 1)));

    IllegalArgumentException e =
        assertThrows(
            IllegalArgumentException.class, () -> graph.removeEdge(EndpointPair.ordered(1, 2)));
    assertThat(e).hasMessageThat().contains(ENDPOINTS_MISMATCH);
    e =
        assertThrows(
            IllegalArgumentException.class, () -> graph.removeEdge(EndpointPair.ordered(2, 1)));

            .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 =

    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

            .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.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");