* @param executor The executor to run {@code callback} when the future completes.
   */
  public final void addCallback(FutureCallback<? super V> callback, Executor executor) {
    Futures.addCallback(this, callback, executor);
  }

   *   <li>If an exception propagates out of a {@code Runnable}, it is not necessarily seen by any
   *       {@code UncaughtExceptionHandler} for the thread. For example, if the callback passed to
   *       {@link Futures#addCallback} throws an exception, that exception will be typically be
   *       logged by the {@link ListenableFuture} implementation, even if the thread is configured