* <p>This class maintains a map of nodes to {@link NetworkConnections}. This class also maintains a
 * map of edges to reference nodes. The reference node is defined to be the edge's source node on
 * directed graphs, and an arbitrary endpoint of the edge on undirected graphs.
 *
 * <p>Collection-returning accessors return unmodifiable views: the view returned will reflect
 * changes to the graph (if the graph is mutable) but may not be modified by the user.
 *

    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");

    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");

 *
 * <ul>
 *   <li>Test cases related to whether the graph is directed or undirected.
 *   <li>Test cases related to the specific implementation of the {@link Graph} interface.
 * </ul>
 *
 * TODO(user): Make this class generic (using <N, E>) for all node and edge types.
 * TODO(user): Differentiate between directed and undirected edge strings.
 */
public abstract class AbstractGraphTest {

  Graph<Integer> graph;

import static org.junit.Assert.assertTrue;

import com.google.common.testing.EqualsTester;
import java.util.Set;
import org.junit.After;
import org.junit.Test;

/**
 * Abstract base class for testing undirected {@link Graph} implementations defined in this package.
 */
public abstract class AbstractStandardUndirectedGraphTest extends AbstractGraphTest {

  @After
  public void validateUndirectedEdges() {

import static org.junit.Assert.assertTrue;

import com.google.common.testing.EqualsTester;
import java.util.Set;
import org.junit.After;
import org.junit.Test;

/**
 * Abstract base class for testing undirected {@link Graph} implementations defined in this package.
 */
public abstract class AbstractStandardUndirectedGraphTest extends AbstractGraphTest {

  @After
  public void validateUndirectedEdges() {

import com.google.common.collect.ImmutableSet;
import com.google.common.testing.EqualsTester;
import java.util.Set;
import org.junit.After;
import org.junit.Test;

/**
 * Abstract base class for testing undirected {@link Network} implementations defined in this
 * package.
 */
public abstract class AbstractStandardUndirectedNetworkTest extends AbstractNetworkTest {

import com.google.common.testing.EqualsTester;
import java.util.Optional;
import java.util.Set;
import org.junit.After;
import org.junit.Test;

/**
 * Abstract base class for testing undirected {@link Network} implementations defined in this
 * package.
 */
public abstract class AbstractStandardUndirectedNetworkTest extends AbstractNetworkTest {

    ImmutableValueGraph<String, Integer> emptyGraph =
        ValueGraphBuilder.directed()
            .allowsSelfLoops(true)
            .nodeOrder(ElementOrder.<String>natural())
            .<String, Integer>immutable()
            .build();

    assertThat(emptyGraph.isDirected()).isTrue();
    assertThat(emptyGraph.allowsSelfLoops()).isTrue();

    ImmutableValueGraph<String, Integer> emptyGraph =
        ValueGraphBuilder.directed()
            .allowsSelfLoops(true)
            .nodeOrder(ElementOrder.<String>natural())
            .<String, Integer>immutable()
            .build();

    assertThat(emptyGraph.isDirected()).isTrue();
    assertThat(emptyGraph.allowsSelfLoops()).isTrue();