*       wait();
 *     }
 *     V result = value;
 *     value = null;
 *     notifyAll();
 *     return result;
 *   }
 *
 *   public synchronized void set(V newValue) throws InterruptedException {
 *     while (value != null) {
 *       wait();
 *     }
 *     value = newValue;
 *     notifyAll();
 *   }
 * }
 * }</pre>
 *
 * <h3>{@code ReentrantLock}</h3>
 *

        return shutdown;
      }
    }

    @Override
    public void shutdown() {
      synchronized (lock) {
        shutdown = true;
        if (runningTasks == 0) {
          lock.notifyAll();
        }
      }
    }

    // See newDirectExecutorService javadoc for unusual behavior of this method.
    @Override
    public List<Runnable> shutdownNow() {
      shutdown();