import org.jspecify.annotations.Nullable;

/**
 * GWT emulation of {@link SingletonImmutableSet}.
 *
 * @author Hayward Chan
 */
final class SingletonImmutableSet<E> extends ImmutableSet<E> {
  private final E element;

  SingletonImmutableSet(E element) {
    this.element = checkNotNull(element);
  }

  @Override
  public int size() {
    return 1;
  }

@SuppressWarnings("serial") // uses writeReplace(), not default serialization
final class SingletonImmutableSet<E> extends ImmutableSet<E> {
  // We deliberately avoid caching the asList and hashCode here, to ensure that with
  // compressed oops, a SingletonImmutableSet packs all the way down to the optimal 16 bytes.

  final transient E element;

  SingletonImmutableSet(E element) {
    this.element = Preconditions.checkNotNull(element);
  }

  @SuppressWarnings({"unchecked"})
  public static <E> ImmutableSet<E> of() {
    return (ImmutableSet<E>) RegularImmutableSet.EMPTY;
  }

  public static <E> ImmutableSet<E> of(E e1) {
    return new SingletonImmutableSet<E>(e1);
  }

  @SuppressWarnings("unchecked")
  public static <E> ImmutableSet<E> of(E e1, E e2) {
    return create(e1, e2);
  }

  @SuppressWarnings("unchecked")