}

  private static MutableGraph<Integer> buildDirectedGraph() {
    MutableGraph<Integer> directedGraph = GraphBuilder.directed().allowsSelfLoops(true).build();
    directedGraph.putEdge(N1, N1);
    directedGraph.putEdge(N1, N2);
    directedGraph.putEdge(N2, N1);

    return directedGraph;
  }

  private static MutableGraph<Integer> buildUndirectedGraph() {

  final boolean graphIsMutable() {
    return graphAsMutableGraph != null;
  }

  @Before
  public final void init() {
    graph = createGraph();
    if (graph instanceof MutableGraph) {
      graphAsMutableGraph = (MutableGraph<Integer>) graph;
    }
  }

  @After
  public final void validateGraphState() {
    validateGraph(graph);
  }

  static <N> void validateGraph(Graph<N> graph) {

 * create an instance of one of the built-in implementations of {@code Graph}, use the {@link
 * GraphBuilder} class:
 *
 * <pre>{@code
 * MutableGraph<Integer> graph = GraphBuilder.undirected().build();
 * }</pre>
 *
 * <p>{@link GraphBuilder#build()} returns an instance of {@link MutableGraph}, which is a subtype
 * of {@code Graph} that provides methods for adding and removing nodes and edges. If you do not

 * create an instance of one of the built-in implementations of {@code Graph}, use the {@link
 * GraphBuilder} class:
 *
 * <pre>{@code
 * MutableGraph<Integer> graph = GraphBuilder.undirected().build();
 * }</pre>
 *
 * <p>{@link GraphBuilder#build()} returns an instance of {@link MutableGraph}, which is a subtype
 * of {@code Graph} that provides methods for adding and removing nodes and edges. If you do not

  public void forTree_acceptsDirectedGraph() throws Exception {
    MutableGraph<String> graph = GraphBuilder.directed().build();
    graph.putEdge("a", "b");

    Traverser.forTree(graph); // Does not throw
  }

  @Test
  public void forTree_withUndirectedGraph_throws() throws Exception {
    MutableGraph<String> graph = GraphBuilder.undirected().build();
    graph.putEdge("a", "b");

  public void forTree_acceptsDirectedGraph() throws Exception {
    MutableGraph<String> graph = GraphBuilder.directed().build();
    graph.putEdge("a", "b");

    Traverser.forTree(graph); // Does not throw
  }

  @Test
  public void forTree_withUndirectedGraph_throws() throws Exception {
    MutableGraph<String> graph = GraphBuilder.undirected().build();
    graph.putEdge("a", "b");