implements ListenableFuture<V> {
  final boolean casListeners(@Nullable Listener expect, Listener update) {
    if (listeners == expect) {
      listeners = update;
      return true;
    }
    return false;
  }

  final @Nullable Listener gasListeners(Listener update) {
    Listener old = listeners;
    listeners = update;
    return old;
  }

    private final ExecutionList executionList = new ExecutionList();

    // This allows us to only start up a thread waiting on the delegate future when the first
    // listener is added.
    private final AtomicBoolean hasListeners = new AtomicBoolean(false);

    // The delegate future.
    private final Future<V> delegate;

    ListenableFutureAdapter(Future<V> delegate) {
      this(delegate, defaultAdapterExecutor);
    }