* also connect {@code nodeV} to {@code nodeU}.
     *
     * <p>If this network is directed, {@code edge} will be directed in this network; if it is
     * undirected, {@code edge} will be undirected in this network.
     *
     * <p>If this network is directed, {@code endpoints} must be ordered.
     *
     * <p><b>{@code edge} must be unique to this network</b>, just as a {@code Map} key must be. It

import java.util.Arrays;
import java.util.Collection;
import org.junit.runner.RunWith;
import org.junit.runners.Parameterized;
import org.junit.runners.Parameterized.Parameters;

/** Tests for a directed {@link StandardMutableGraph}. */
@AndroidIncompatible
@RunWith(Parameterized.class)
public final class StandardMutableDirectedGraphTest extends AbstractStandardDirectedGraphTest {

  /** Returns all edges in this graph. */
  @Override
  Set<EndpointPair<N>> edges();

  //
  // Graph properties
  //

  /**
   * Returns true if the edges in this graph are directed. Directed edges connect a {@link
   * EndpointPair#source() source node} to a {@link EndpointPair#target() target node}, while
   * undirected edges connect a pair of nodes to each other.
   */
  @Override

  }

  private static SuccessorsFunction<Character> createDirectedGraph(String... edges) {
    return createGraph(/* directed= */ true, edges);
  }

  private static SuccessorsFunction<Character> createUndirectedGraph(String... edges) {
    return createGraph(/* directed= */ false, edges);
  }

  /**
   * Creates a graph from a list of node pairs (encoded as strings, e.g. "ab" means that this graph

import java.util.Arrays;
import java.util.Collection;
import org.junit.runner.RunWith;
import org.junit.runners.Parameterized;
import org.junit.runners.Parameterized.Parameters;

/** Tests for a directed {@link StandardMutableGraph}. */
@AndroidIncompatible
@RunWith(Parameterized.class)
public final class StandardMutableDirectedGraphTest extends AbstractStandardDirectedGraphTest {

import java.util.Arrays;
import java.util.Collection;
import org.junit.runner.RunWith;
import org.junit.runners.Parameterized;
import org.junit.runners.Parameterized.Parameters;

/** Tests for a directed {@link StandardMutableNetwork} allowing self-loops. */
@AndroidIncompatible
@RunWith(Parameterized.class)
public class StandardMutableDirectedNetworkTest extends AbstractStandardDirectedNetworkTest {

import java.util.Arrays;
import java.util.Collection;
import org.junit.runner.RunWith;
import org.junit.runners.Parameterized;
import org.junit.runners.Parameterized.Parameters;

/** Tests for a directed {@link ImmutableNetwork}. */
@AndroidIncompatible
@RunWith(Parameterized.class)
public class StandardImmutableDirectedNetworkTest extends AbstractStandardDirectedNetworkTest {

import java.util.Arrays;
import java.util.Collection;
import org.junit.runner.RunWith;
import org.junit.runners.Parameterized;
import org.junit.runners.Parameterized.Parameters;

/** Tests for a directed {@link ImmutableNetwork}. */
@AndroidIncompatible
@RunWith(Parameterized.class)
public class StandardImmutableDirectedNetworkTest extends AbstractStandardDirectedNetworkTest {

  }

  private static SuccessorsFunction<Character> createDirectedGraph(String... edges) {
    return createGraph(/* directed= */ true, edges);
  }

  private static SuccessorsFunction<Character> createUndirectedGraph(String... edges) {
    return createGraph(/* directed= */ false, edges);
  }

  /**
   * Creates a graph from a list of node pairs (encoded as strings, e.g. "ab" means that this graph

 * NetworkBuilder}.
 *
 * <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