* 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)}:

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>

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

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

  final MapIteratorCache<N, NetworkConnections<N, E>> nodeConnections;

  // We could make this a Map<E, EndpointPair<N>>. It would make incidentNodes(edge) slightly
  // faster, but also make Networks consume 5 to 20+% (increasing with average degree) more memory.
  final MapIteratorCache<E, N> edgeToReferenceNode; // referenceNode == source if directed

            .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

            .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

   * network}.
   *
   * <p>The "queryable" properties are those that are exposed through the {@link Network} interface,
   * such as {@link Network#isDirected()}. Other properties, such as {@link
   * #expectedNodeCount(int)}, are not set in the new builder.
   */
  public static <N, E> NetworkBuilder<N, E> from(Network<N, E> network) {
    return new NetworkBuilder<N, E>(network.isDirected())