code: @Override void onShutdown() {
          BlockingQueue<Runnable> q = super.getQueue();
          boolean keepDelayed =
              getExecuteExistingDelayedTasksAfterShutdownPolicy();
          boolean keepPeriodic =
              getContinueExistingPeriodicTasksAfterShutdownPolicy();
          if (!keepDelayed && !keepPeriodic) {
              for (Object e : q.toArray())
                  if (e instanceof RunnableScheduledFuture<?>)
                      ((RunnableScheduledFuture<?>) e).cancel(false);
              q.clear();
          }
          else {
              // Traverse snapshot to avoid iterator exceptions
              for (Object e : q.toArray()) {
                  if (e instanceof RunnableScheduledFuture) {
                      RunnableScheduledFuture<?> t =
                          (RunnableScheduledFuture<?>)e;
                      if ((t.isPeriodic() ? !keepPeriodic : !keepDelayed) ||
                          t.isCancelled()) { // also remove if already cancelled
                          if (q.remove(t))
                              t.cancel(false);
                      }
                  }
              }
          }
          tryTerminate();
      }
docs: Cancels and clears the queue of all tasks that should not be run
  due to shutdown policy.  Invoked within super.shutdown.