* subtype of {@code Network} that provides methods for adding and removing nodes and edges. If you
 * do not need to mutate a network (e.g. if you write a method than runs a read-only algorithm on
 * the network), you should use the non-mutating {@link Network} interface, or an {@link
 * ImmutableNetwork}.
 *
 * <p>You can create an immutable copy of an existing {@code Network} using {@link
 * ImmutableNetwork#copyOf(Network)}:

            .allowsParallelEdges(!network.allowsParallelEdges())
            .allowsSelfLoops(!network.allowsSelfLoops())
            .build();
    g2.addEdge(N1, N2, E12);

    assertThat(network).isEqualTo(g2);
  }

  // Node/edge sets and node/edge connections are the same, but edge order differs.
  // (In this case the networks are considered equivalent; the edge add orderings are irrelevant.)
  @Test

  private ImmutableNetwork(Network<N, E> network) {
    super(
        NetworkBuilder.from(network), getNodeConnections(network), getEdgeToReferenceNode(network));
  }

  /** Returns an immutable copy of {@code network}. */
  public static <N, E> ImmutableNetwork<N, E> copyOf(Network<N, E> network) {
    return (network instanceof ImmutableNetwork)
        ? (ImmutableNetwork<N, E>) network

  Set<N> successors();

  Set<E> incidentEdges();

  Set<E> inEdges();

  Set<E> outEdges();

  /**
   * Returns the set of edges connecting the origin node to {@code node}. For networks without
   * parallel edges, this set cannot be of size greater than one.
   */
  Set<E> edgesConnecting(N node);

  /**
   * Returns the node that is adjacent to the origin node along {@code edge}.
   *

            .allowsParallelEdges(!network.allowsParallelEdges())
            .allowsSelfLoops(!network.allowsSelfLoops())
            .build();
    g2.addEdge(N1, N2, E12);

    assertThat(network).isEqualTo(g2);
  }

  // Node/edge sets and node/edge connections are the same, but edge order differs.
  // (In this case the networks are considered equivalent; the edge add orderings are irrelevant.)
  @Test

import java.util.Collections;
import java.util.Map;
import java.util.Set;
import javax.annotation.CheckForNull;

/**
 * A base implementation of {@link NetworkConnections} for undirected networks.
 *
 * @author James Sexton
 * @param <N> Node parameter type
 * @param <E> Edge parameter type
 */
@ElementTypesAreNonnullByDefault

import javax.annotation.CheckForNull;

/**
 * A class to represent the set of edges connecting an (implicit) origin node to a target node.
 *
 * <p>The {@link #nodeToOutEdge} map means this class only works on networks without parallel edges.
 * See {@link MultiEdgesConnecting} for a class that works with parallel edges.
 *
 * @author James Sexton
 * @param <E> Edge parameter type
 */
@ElementTypesAreNonnullByDefault

import javax.annotation.CheckForNull;

/**
 * A class to represent the set of edges connecting an (implicit) origin node to a target node.
 *
 * <p>The {@link #outEdgeToNode} map allows this class to work on networks with parallel edges. See
 * {@link EdgesConnecting} for a class that is more efficient but forbids parallel edges.
 *
 * @author James Sexton
 * @param <E> Edge parameter type
 */
@ElementTypesAreNonnullByDefault

import com.google.common.collect.ImmutableBiMap;
import java.util.Collections;
import java.util.Map;
import java.util.Set;

/**
 * An implementation of {@link NetworkConnections} for directed networks.
 *
 * @author James Sexton
 * @param <N> Node parameter type
 * @param <E> Edge parameter type
 */
@ElementTypesAreNonnullByDefault

import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
import javax.annotation.CheckForNull;

/**
 * An implementation of {@link NetworkConnections} for undirected networks with parallel edges.
 *
 * @author James Sexton
 * @param <N> Node parameter type
 * @param <E> Edge parameter type
 */
@ElementTypesAreNonnullByDefault
final class UndirectedMultiNetworkConnections<N, E>