CacheEntry ( long ttl ) {
            this.expiration = System.currentTimeMillis() + ttl * 1000L;
            this.map = new ConcurrentHashMap<>();
        }
    }

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

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

    }