this.map = new ConcurrentHashMap<>();
        }
    }

    private static class NegativeCacheEntry <T> extends CacheEntry<T> {

        /**
         * @param ttl
         */
        NegativeCacheEntry ( long ttl ) {
            super(ttl);
        }

    }

    private static final Logger log = LoggerFactory.getLogger(DfsImpl.class);