}

  private static final class DoNothingRunnable implements Runnable {
    @Override
    public void run() {}
  }

  // The thread executing the task publishes itself to the superclass' reference and the thread
  // interrupting sets DONE when it has finished interrupting.
  private static final Runnable DONE = new DoNothingRunnable();
  private static final Runnable PARKED = new DoNothingRunnable();