}

  @Test
  public void putEdgeValue_directed() {
    graph = ValueGraphBuilder.directed().build();

    assertThat(graph.putEdgeValue(1, 2, "valueA")).isNull();
    assertThat(graph.putEdgeValue(2, 1, "valueB")).isNull();
    assertThat(graph.putEdgeValue(1, 2, "valueC")).isEqualTo("valueA");
    assertThat(graph.putEdgeValue(2, 1, "valueD")).isEqualTo("valueB");
  }

  @Test

  }

  @Test
  public void putEdgeValue_directed() {
    graph = ValueGraphBuilder.directed().build();

    assertThat(graph.putEdgeValue(1, 2, "valueA")).isNull();
    assertThat(graph.putEdgeValue(2, 1, "valueB")).isNull();
    assertThat(graph.putEdgeValue(1, 2, "valueC")).isEqualTo("valueA");
    assertThat(graph.putEdgeValue(2, 1, "valueD")).isEqualTo("valueB");
  }

  @Test

    directedGraph.putEdgeValue(N1, N1, E11);
    directedGraph.putEdgeValue(N3, N4, E34);

    MutableValueGraph<Integer, String> expectedTranspose =
        ValueGraphBuilder.directed().allowsSelfLoops(true).build();
    expectedTranspose.putEdgeValue(N3, N1, E13);
    expectedTranspose.putEdgeValue(N1, N3, E31);
    expectedTranspose.putEdgeValue(N2, N1, E12);
    expectedTranspose.putEdgeValue(N1, N1, E11);

    directedGraph.putEdgeValue(N1, N1, E11);
    directedGraph.putEdgeValue(N3, N4, E34);

    MutableValueGraph<Integer, String> expectedTranspose =
        ValueGraphBuilder.directed().allowsSelfLoops(true).build();
    expectedTranspose.putEdgeValue(N3, N1, E13);
    expectedTranspose.putEdgeValue(N1, N3, E31);
    expectedTranspose.putEdgeValue(N2, N1, E12);
    expectedTranspose.putEdgeValue(N1, N1, E11);

    ImmutableValueGraph<Integer, String> graph =
        ValueGraphBuilder.directed()
            .<Integer, String>immutable()
            .putEdgeValue(2, 1, "2-1")
            .putEdgeValue(2, 3, "2-3")
            .putEdgeValue(1, 2, "1-2")
            .build();

    assertThat(graph.incidentEdges(2))
        .containsExactly(

 * // Building a mutable value graph
 * MutableValueGraph<String, Double> graph =
 *     ValueGraphBuilder.undirected().allowsSelfLoops(true).build();
 * graph.putEdgeValue("San Francisco", "San Francisco", 0.0);
 * graph.putEdgeValue("San Jose", "San Jose", 0.0);
 * graph.putEdgeValue("San Francisco", "San Jose", 48.4);
 *
 * // Building an immutable value graph
 * ImmutableValueGraph<String, Double> immutableGraph =

   *     ValueGraphBuilder.undirected()
   *         .<City, Distance>immutable()
   *         .putEdgeValue(PARIS, BERLIN, kilometers(1060))
   *         .putEdgeValue(PARIS, BRUSSELS, kilometers(317))
   *         .putEdgeValue(BERLIN, BRUSSELS, kilometers(764))
   *         .addNode(REYKJAVIK)
   *         .build();
   * }</pre>
   *

    ImmutableValueGraph<Integer, String> graph =
        ValueGraphBuilder.directed()
            .<Integer, String>immutable()
            .putEdgeValue(2, 1, "2-1")
            .putEdgeValue(2, 3, "2-3")
            .putEdgeValue(1, 2, "1-2")
            .build();

    assertThat(graph.incidentEdges(2))
        .containsExactly(

   * @throws IllegalArgumentException if the introduction of the edge would violate {@link
   *     #allowsSelfLoops()}
   */
  @CanIgnoreReturnValue
  @CheckForNull
  V putEdgeValue(N nodeU, N nodeV, V value);

  /**
   * Adds an edge connecting {@code endpoints} if one is not already present, and sets a value for
   * that edge to {@code value} (overwriting the existing value, if any).
   *

      checkPositive(++edgeCount);
    }
    return previousValue;
  }

  @Override
  @CanIgnoreReturnValue
  @CheckForNull
  public V putEdgeValue(EndpointPair<N> endpoints, V value) {
    validateEndpoints(endpoints);
    return putEdgeValue(endpoints.nodeU(), endpoints.nodeV(), value);
  }

  @Override
  @CanIgnoreReturnValue
  public boolean removeNode(N node) {
    checkNotNull(node, "node");