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

import org.checkerframework.checker.nullness.qual.Nullable;

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

  // This reference is used both by the custom field serializer, and by the

@ElementTypesAreNonnullByDefault
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 element) {
    return new SingletonImmutableSet<E>(element);
  }

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

  @SuppressWarnings("unchecked")

   * Collections#singleton} for code consistency, {@code null} rejection, and because the return
   * type conveys the immutability guarantee.
   */
  public static <E> ImmutableSet<E> of(E element) {
    return new SingletonImmutableSet<>(element);
  }

  /**
   * Returns an immutable set containing the given elements, minus duplicates, in the order each was

    return ImmutableSet.copyOf(elements);
  }

  public void testCreation_allDuplicates() {
    ImmutableSet<String> set = ImmutableSet.copyOf(Lists.newArrayList("a", "a"));
    assertTrue(set instanceof SingletonImmutableSet);
    assertEquals(Lists.newArrayList("a"), Lists.newArrayList(set));
  }

  public void testCreation_oneDuplicate() {
    // now we'll get the varargs overload
    ImmutableSet<String> set =

    return ImmutableSet.copyOf(elements);
  }

  public void testCreation_allDuplicates() {
    ImmutableSet<String> set = ImmutableSet.copyOf(Lists.newArrayList("a", "a"));
    assertTrue(set instanceof SingletonImmutableSet);
    assertEquals(Lists.newArrayList("a"), Lists.newArrayList(set));
  }

  public void testCreation_oneDuplicate() {
    // now we'll get the varargs overload
    ImmutableSet<String> set =

   * Collections#singleton} for code consistency, {@code null} rejection, and because the return
   * type conveys the immutability guarantee.
   */
  public static <E> ImmutableSet<E> of(E element) {
    return new SingletonImmutableSet<>(element);
  }

  /*
   * TODO: b/315526394 - Skip the Builder entirely for the of(...) methods, since we don't need to
   * worry that we might trigger the fallback to the JDK-backed implementation? (The varargs one