*/
    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");