* more efficient implementation.
 *
 * @author James Sexton
 * @param <N> Node parameter type
 */
@ElementTypesAreNonnullByDefault
abstract class AbstractBaseGraph<N> implements BaseGraph<N> {

  /**
   * Returns the number of edges in this graph; used to calculate the size of {@link Graph#edges()}.

 *
 * @author James Sexton
 * @param <N> Node parameter type
 * @since 20.0
 */
@Beta
@ElementTypesAreNonnullByDefault
public abstract class AbstractGraph<N> extends AbstractBaseGraph<N> implements Graph<N> {

  @Override
  public final boolean equals(@CheckForNull Object obj) {
    if (obj == this) {
      return true;
    }
    if (!(obj instanceof Graph)) {
      return false;
    }

 *
 * @author James Sexton
 * @param <N> Node parameter type
 * @param <V> Value parameter type
 * @since 20.0
 */
@Beta
@ElementTypesAreNonnullByDefault
public abstract class AbstractValueGraph<N, V> extends AbstractBaseGraph<N>
    implements ValueGraph<N, V> {

  @Override
  public Graph<N> asGraph() {
    return new AbstractGraph<N>() {
      @Override
      public Set<N> nodes() {

 *
 * @author James Sexton
 * @param <N> Node parameter type
 * @param <V> Value parameter type
 * @since 20.0
 */
@Beta
@ElementTypesAreNonnullByDefault
public abstract class AbstractValueGraph<N, V> extends AbstractBaseGraph<N>
    implements ValueGraph<N, V> {

  @Override
  public Graph<N> asGraph() {
    return new AbstractGraph<N>() {
      @Override
      public Set<N> nodes() {