@Override
    Listener gasListeners(AbstractFuture<?> future, Listener update) {
      synchronized (future) {
        Listener old = future.listeners;
        if (old != update) {
          future.listeners = update;
        }
        return old;
      }
    }

    /** Performs a GAS operation on the {@link #waiters} field. */
    @Override

    @Override
    Listener gasListeners(AbstractFuture<?> future, Listener update) {
      synchronized (future) {
        Listener old = future.listeners;
        if (old != update) {
          future.listeners = update;
        }
        return old;
      }
    }

    /** Performs a GAS operation on the {@link #waiters} field. */
    @Override

       * elements from each bin must either stay at same index, or move with a power of two offset.
       * We eliminate unnecessary node creation by catching cases where old nodes can be reused
       * because their next fields won't change. Statistically, at the default threshold, only
       * about one-sixth of them need cloning when a table doubles. The nodes they replace will be

   * CacheLoader#reload}
   * to obtain a future of the new value. If the returned future is already complete, it is returned
   * immediately. Otherwise, the old value is returned.
   *
   * <p><b>Note:</b> <i>all exceptions thrown during refresh will be logged and then swallowed</i>.
   *
   * @param duration the length of time after an entry is created that it should be considered

       * elements from each bin must either stay at same index, or move with a power of two offset.
       * We eliminate unnecessary node creation by catching cases where old nodes can be reused
       * because their next fields won't change. Statistically, at the default threshold, only
       * about one-sixth of them need cloning when a table doubles. The nodes they replace will be

      @Override
      public boolean hasNext() {
        /*
         * Don't store a new Iterator until we know the user can't remove() the last returned
         * element anymore. Otherwise, when we remove from the old iterator, we may be invalidating
         * the new one. The result is a ConcurrentModificationException or other bad behavior.
         *

      @Override
      public boolean hasNext() {
        /*
         * Don't store a new Iterator until we know the user can't remove() the last returned
         * element anymore. Otherwise, when we remove from the old iterator, we may be invalidating
         * the new one. The result is a ConcurrentModificationException or other bad behavior.
         *