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(

    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(

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

 * // 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 =

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

  @Override
  @CanIgnoreReturnValue
  public @Nullable 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");

   *     nodeV}, or null if there was no such edge.
   * @throws IllegalArgumentException if the introduction of the edge would violate {@link
   *     #allowsSelfLoops()}
   */
  @CanIgnoreReturnValue
  @Nullable 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).
   *

      }
    }
  }

  @Test
  public void directedGraph() {
    graph = ValueGraphBuilder.directed().allowsSelfLoops(true).build();
    graph.putEdgeValue(1, 2, "valueA");
    graph.putEdgeValue(2, 1, "valueB");
    graph.putEdgeValue(2, 3, "valueC");
    graph.putEdgeValue(4, 4, "valueD");

    assertThat(graph.edgeValueOrDefault(1, 2, null)).isEqualTo("valueA");

      }
    }
  }

  @Test
  public void directedGraph() {
    graph = ValueGraphBuilder.directed().allowsSelfLoops(true).build();
    graph.putEdgeValue(1, 2, "valueA");
    graph.putEdgeValue(2, 1, "valueB");
    graph.putEdgeValue(2, 3, "valueC");
    graph.putEdgeValue(4, 4, "valueD");

    assertThat(graph.edgeValueOrDefault(1, 2, null)).isEqualTo("valueA");

    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);

  }

  @Override
  public boolean addNode(N node) {
    return backingValueGraph.addNode(node);
  }

  @Override
  public boolean putEdge(N nodeU, N nodeV) {
    return backingValueGraph.putEdgeValue(nodeU, nodeV, Presence.EDGE_EXISTS) == null;
  }

  @Override
  public boolean putEdge(EndpointPair<N> endpoints) {
    validateEndpoints(endpoints);
    return putEdge(endpoints.nodeU(), endpoints.nodeV());
  }