*/
    public abstract ImmutableCollection<E> build();
  }

  abstract static class ArrayBasedBuilder<E> extends ImmutableCollection.Builder<E> {
    // The first `size` elements are non-null.
    @Nullable Object[] contents;
    int size;
    boolean forceCopy;

    ArrayBasedBuilder(int initialCapacity) {
      checkNonnegative(initialCapacity, "initialCapacity");

   * elements and to build again.
   *
   * @since 2.0
   */
  public static class Builder<E> extends ImmutableCollection.ArrayBasedBuilder<E> {
    @VisibleForTesting @Nullable Object @Nullable [] hashTable;
    private int hashCode;

    /**
     * Creates a new builder. The returned builder is equivalent to the builder generated by {@link

   * multiple lists in series. Each new list contains all the elements of the ones created before
   * it.
   *
   * @since 2.0
   */
  public static final class Builder<E> extends ImmutableCollection.ArrayBasedBuilder<E> {
    /**
     * Creates a new builder. The returned builder is equivalent to the builder generated by {@link
     * ImmutableList#builder}.
     */
    public Builder() {