}
  }

  @Override
  public String toString() {
    // TODO(dpb): Better toString, in the style of Futures.transform etc.
    return toStringHelper(this).add("state", state.get()).addValue(future).toString();
  }

  @SuppressWarnings("removal") // b/260137033
  @Override
  protected void finalize() {
    if (state.get().equals(OPEN)) {