return (Collector) TO_OPTIONAL;
  }

  private static final Object NULL_PLACEHOLDER = new Object();

  private static final Collector<@Nullable Object, ?, @Nullable Object> ONLY_ELEMENT =
      Collector.<@Nullable Object, ToOptionalState<Object>, @Nullable Object>of(
          () -> new ToOptionalState<>(null),
          (state, o) -> state.add((o == null) ? NULL_PLACEHOLDER : o),
          ToOptionalState::combine,

import org.jspecify.annotations.NullMarked;
import org.jspecify.annotations.Nullable;

@GwtCompatible
@NullMarked
public class Helpers {
  // Clone of Objects.equals
  static boolean equal(@Nullable Object a, @Nullable Object b) {
    return a == b || (a != null && a.equals(b));
  }

  // Clone of Lists.newArrayList
  public static <E extends @Nullable Object> List<E> copyToList(Iterable<? extends E> elements) {

    private static final AtomicReferenceFieldUpdater<Waiter, @Nullable Thread> waiterThreadUpdater =
        AtomicReferenceFieldUpdater.<Waiter, @Nullable Thread>newUpdater(
            Waiter.class, Thread.class, "thread");
    private static final AtomicReferenceFieldUpdater<Waiter, @Nullable Waiter> waiterNextUpdater =
        AtomicReferenceFieldUpdater.<Waiter, @Nullable Waiter>newUpdater(
            Waiter.class, Waiter.class, "next");

  @CanIgnoreReturnValue
  @Override
  public @Nullable V remove(@Nullable Object key) {
    return delegate().remove(key);
  }

  @Override
  public void clear() {
    delegate().clear();
  }

  @Override
  public boolean containsKey(@Nullable Object key) {
    return delegate().containsKey(key);
  }

  @Override
  public boolean containsValue(@Nullable Object value) {
    return delegate().containsValue(value);

    LAST_BEFORE {
      @Override
      public <E extends @Nullable Object> int resultIndex(
          Comparator<? super E> comparator,
          @ParametricNullness E key,
          List<? extends E> list,
          int foundIndex) {
        return FIRST_PRESENT.resultIndex(comparator, key, list, foundIndex) - 1;
      }
    };

    abstract <E extends @Nullable Object> int resultIndex(
        Comparator<? super E> comparator,

import org.jspecify.annotations.Nullable;

/**
 * A simplistic set which implements the bare minimum so that it can be used in tests without
 * relying on any specific Set implementations. Slow. Explicitly allows null elements so that they
 * can be used in the testers.
 *
 * @author Regina O'Dell
 */
@GwtCompatible
@NullMarked
public class MinimalSet<E extends @Nullable Object> extends MinimalCollection<E> implements Set<E> {

import org.jspecify.annotations.Nullable;

/**
 * A simplistic set which implements the bare minimum so that it can be used in tests without
 * relying on any specific Set implementations. Slow. Explicitly allows null elements so that they
 * can be used in the testers.
 *
 * @author Regina O'Dell
 */
@GwtCompatible
@NullMarked
public class MinimalSet<E extends @Nullable Object> extends MinimalCollection<E> implements Set<E> {

     */
    public InvokerException(@Nullable String message) {
        super(message);
    }

    /**
     * Constructs a new {@code InvokerException} with the specified detail message and cause.
     *
     * @param message the detail message explaining the cause of the exception
     * @param cause the underlying cause of the exception
     */
    public InvokerException(@Nullable String message, @Nullable Throwable cause) {

   *
   * @since 21.0 (but only since 33.4.0 in the Android flavor)
   */
  @SuppressWarnings("NullableOptional") // Null passthrough is reasonable for type conversions
  public static <T> @Nullable Optional<T> fromJavaUtil(
      java.util.@Nullable Optional<T> javaUtilOptional) {
    return (javaUtilOptional == null) ? null : fromNullable(javaUtilOptional.orElse(null));
  }

  /**

   */
  @Override
  public boolean containsValue(@Nullable Object value) {
    return findEntryByValue(value) != ABSENT;
  }

  @Override
  public @Nullable V get(@Nullable Object key) {
    int entry = findEntryByKey(key);
    return (entry == ABSENT) ? null : values[entry];
  }

  @Nullable K getInverse(@Nullable Object value) {
    int entry = findEntryByValue(value);