* users should prefer the {@link ValueGraph} interface.
 *
 * @author James Sexton
 * @param <N> Node parameter type
 * @param <V> Value parameter type
 * @since 20.0
 */
public interface MutableValueGraph<N, V> extends ValueGraph<N, V> {

  /**
   * Adds {@code node} if it is not already present.
   *
   * <p><b>Nodes must be unique</b>, just as {@code Map} keys must be. They must also be non-null.
   *

  @Test
  public void immutableValueGraph() {
    MutableValueGraph<String, Integer> mutableValueGraph = ValueGraphBuilder.directed().build();
    mutableValueGraph.addNode("A");
    ImmutableValueGraph<String, Integer> immutableValueGraph =
        ImmutableValueGraph.copyOf(mutableValueGraph);

    assertThat(immutableValueGraph).isNotInstanceOf(MutableValueGraph.class);
    assertThat(immutableValueGraph).isEqualTo(mutableValueGraph);

    newBuilder.incidentEdgeOrder = checkNotNull(incidentEdgeOrder);
    return newBuilder;
  }

  /**
   * Returns an empty {@link MutableValueGraph} with the properties of this {@link
   * ValueGraphBuilder}.
   */
  public <N1 extends N, V1 extends V> MutableValueGraph<N1, V1> build() {
    return new StandardMutableValueGraph<>(this);
  }

  ValueGraphBuilder<N, V> copy() {

  @Test
  public void immutableValueGraph() {
    MutableValueGraph<String, Integer> mutableValueGraph = ValueGraphBuilder.directed().build();
    mutableValueGraph.addNode("A");
    ImmutableValueGraph<String, Integer> immutableValueGraph =
        ImmutableValueGraph.copyOf(mutableValueGraph);

    assertThat(immutableValueGraph).isNotInstanceOf(MutableValueGraph.class);
    assertThat(immutableValueGraph).isEqualTo(mutableValueGraph);

   */
  public static class Builder<N, V> {

    private final MutableValueGraph<N, V> mutableValueGraph;

    Builder(ValueGraphBuilder<N, V> graphBuilder) {
      // The incidentEdgeOrder for immutable graphs is always stable. However, we don't want to
      // modify this builder, so we make a copy instead.
      this.mutableValueGraph =
          graphBuilder.copy().incidentEdgeOrder(ElementOrder.<N>stable()).build();

  @Test
  public void transpose_undirectedValueGraph() {
    MutableValueGraph<Integer, String> undirectedGraph = ValueGraphBuilder.undirected().build();
    undirectedGraph.putEdgeValue(N1, N2, E12);

    assertThat(transpose(undirectedGraph)).isSameInstanceAs(undirectedGraph);
  }

  @Test
  public void transpose_directedValueGraph() {
    MutableValueGraph<Integer, String> directedGraph =

   * nodes}.
   *
   * @throws IllegalArgumentException if any element in {@code nodes} is not a node in the graph
   */
  public static <N, V> MutableValueGraph<N, V> inducedSubgraph(
      ValueGraph<N, V> graph, Iterable<? extends N> nodes) {
    MutableValueGraph<N, V> subgraph =
        (nodes instanceof Collection)
            ? ValueGraphBuilder.from(graph).expectedNodeCount(((Collection) nodes).size()).build()

 * ValueGraphBuilder} class:
 *
 * {@snippet :
 * MutableValueGraph<Integer, Double> graph = ValueGraphBuilder.directed().build();
 * }
 *
 * <p>{@link ValueGraphBuilder#build()} returns an instance of {@link MutableValueGraph}, which is a
 * subtype of {@code ValueGraph} that provides methods for adding and removing nodes and edges. If

// TODO(user): Expand coverage and move to proper test suite.
@RunWith(JUnit4.class)
@NullUnmarked
public final class ValueGraphTest {
  private static final String DEFAULT = "default";

  MutableValueGraph<Integer, String> graph;

  @After
  public void validateGraphState() {
    assertStronglyEquivalent(graph, Graphs.copyOf(graph));
    assertStronglyEquivalent(graph, ImmutableValueGraph.copyOf(graph));

// TODO(user): Expand coverage and move to proper test suite.
@RunWith(JUnit4.class)
@NullUnmarked
public final class ValueGraphTest {
  private static final String DEFAULT = "default";

  MutableValueGraph<Integer, String> graph;

  @After
  public void validateGraphState() {
    assertStronglyEquivalent(graph, Graphs.copyOf(graph));
    assertStronglyEquivalent(graph, ImmutableValueGraph.copyOf(graph));