code: private V returnValue(K key, V _value) {
      if (_value != null) {
        Duration d = expiryPolicy.getExpiryForAccess();
        if (d != null) {
          c2kCache.expireAt(key, calculateExpiry(d));
        }
        return _value;
      }
      return null;
    }
docs: Entry was accessed update expiry if value is non null.