@Override
  public Set<K> keySet() {
    Set<K> result = keySet;
    return (result == null) ? keySet = new KeySet() : result;
  }

  @WeakOuter
  private final class KeySet extends ForwardingSet<K> {
    @Override
    protected Set<K> delegate() {
      return delegate.keySet();
    }

    @Override
    public void clear() {
      AbstractBiMap.this.clear();
    }

    @Override

import static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.collect.ForwardingMap;
import com.google.common.collect.ForwardingMapEntry;
import com.google.common.collect.ForwardingSet;
import com.google.common.collect.Iterators;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import com.google.errorprone.annotations.DoNotCall;
import java.util.HashMap;
import java.util.Iterator;

  /**
   * Returns the backing delegate instance that methods are forwarded to. Abstract subclasses
   * generally override this method with an abstract method that has a more specific return type,
   * such as {@link ForwardingSet#delegate}. Concrete subclasses override this method to supply the
   * instance being decorated.
   */
  protected abstract Object delegate();

  /**
   * Returns the backing delegate instance that methods are forwarded to. Abstract subclasses
   * generally override this method with an abstract method that has a more specific return type,
   * such as {@link ForwardingSet#delegate}. Concrete subclasses override this method to supply the
   * instance being decorated.
   */
  protected abstract Object delegate();

 * one to preserve compatibility.
 */
@SuppressWarnings("JdkObsolete")
public abstract class ForwardingSortedSet<E extends @Nullable Object> extends ForwardingSet<E>
    implements SortedSet<E> {

  /** Constructor for use by subclasses. */
  protected ForwardingSortedSet() {}

  @Override
  protected abstract SortedSet<E> delegate();

  @Override

 * one to preserve compatibility.
 */
@SuppressWarnings("JdkObsolete")
public abstract class ForwardingSortedSet<E extends @Nullable Object> extends ForwardingSet<E>
    implements SortedSet<E> {

  /** Constructor for use by subclasses. */
  protected ForwardingSortedSet() {}

  @Override
  protected abstract SortedSet<E> delegate();

  @Override

    } else {
      return new AddRejectingList<>(key);
    }
  }

  private static final class AddRejectingSet<K extends @Nullable Object, V extends @Nullable Object>
      extends ForwardingSet<V> {
    @ParametricNullness final K key;

    AddRejectingSet(@ParametricNullness K key) {
      this.key = key;
    }

    @Override
    public boolean add(@ParametricNullness V element) {

    } else {
      return new AddRejectingList<>(key);
    }
  }

  private static final class AddRejectingSet<K extends @Nullable Object, V extends @Nullable Object>
      extends ForwardingSet<V> {
    @ParametricNullness final K key;

    AddRejectingSet(@ParametricNullness K key) {
      this.key = key;
    }

    @Override
    public boolean add(@ParametricNullness V element) {

        cast(getKey(), value);
        return super.setValue(value);
      }
    };
  }

  @Override
  public Set<Entry<Class<? extends @NonNull B>, B>> entrySet() {
    return new ForwardingSet<Entry<Class<? extends @NonNull B>, B>>() {

      @Override
      protected Set<Entry<Class<? extends @NonNull B>, B>> delegate() {
        return MutableClassToInstanceMap.this.delegate().entrySet();
      }

                      @Override
                      public Set<Entry<String>> entrySet() {
                        Set<Entry<String>> backingSet = super.entrySet();
                        return new ForwardingSet<Entry<String>>() {
                          @Override
                          protected Set<Entry<String>> delegate() {
                            return backingSet;
                          }