* invoke methods, they invoke methods on the {@code ForwardingSet}.
 *
 * <p>The {@code standard} methods are not guaranteed to be thread-safe, even when all of the
 * methods that they depend on are thread-safe.
 *
 * @author Kevin Bourrillion
 * @author Louis Wasserman
 * @since 2.0
 */
@GwtCompatible
public abstract class ForwardingSet<E extends @Nullable Object> extends ForwardingCollection<E>

import org.jspecify.annotations.NullUnmarked;
import org.jspecify.annotations.Nullable;

/**
 * Tests for {@code ForwardingSet}.
 *
 * @author Robert Konigsberg
 * @author Louis Wasserman
 */
@NullUnmarked
public class ForwardingSetTest extends TestCase {
  static class StandardImplForwardingSet<T> extends ForwardingSet<T> {
    private final Set<T> backingSet;

    StandardImplForwardingSet(Set<T> backingSet) {

import org.jspecify.annotations.NullUnmarked;
import org.jspecify.annotations.Nullable;

/**
 * Tests for {@code ForwardingSet}.
 *
 * @author Robert Konigsberg
 * @author Louis Wasserman
 */
@NullUnmarked
public class ForwardingSetTest extends TestCase {
  static class StandardImplForwardingSet<T> extends ForwardingSet<T> {
    private final Set<T> backingSet;

    StandardImplForwardingSet(Set<T> backingSet) {

import com.google.common.base.Supplier;
import com.google.common.collect.ForwardingSet;
import java.util.Set;

/**
 * A subclass of `ForwardingSet` that throws `IllegalStateException` on invocation of any method
 * (except `hashCode` and `equals`) if the provided `Supplier` returns false.
 */
final class InvalidatableSet<E> extends ForwardingSet<E> {
  private final Supplier<Boolean> validator;
  private final Set<E> delegate;

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;

import org.jspecify.annotations.Nullable;

/**
 * {@link Set} implementation that asserts that a given lock is held whenever one of its methods is
 * called.
 */
@NullUnmarked
class LockHeldAssertingSet<E> extends ForwardingSet<E> implements Serializable {
  final Set<E> delegate;
  final Object mutex;

  LockHeldAssertingSet(Set<E> delegate, Object mutex) {
    checkNotNull(mutex);
    this.delegate = delegate;
    this.mutex = mutex;

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;

      this.delegate = delegate;
    }

    @Override
    protected Map<K, V> delegate() {
      return delegate;
    }

    @Override
    public Set<K> keySet() {
      return new ForwardingSet<K>() {
        @Override
        protected Set<K> delegate() {
          return delegate.keySet();
        }

        @Override
        public Iterator<K> iterator() {

  @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

      this.delegate = delegate;
    }

    @Override
    protected Map<K, V> delegate() {
      return delegate;
    }

    @Override
    public Set<K> keySet() {
      return new ForwardingSet<K>() {
        @Override
        protected Set<K> delegate() {
          return delegate.keySet();
        }

        @Override
        public Iterator<K> iterator() {