private void decrementStatistic(@Nonnull final String key)
    {
        final Lock lock = ROOM_CACHE_STATS.getLock(key);
        lock.lock();
        try {
            Long count = ROOM_CACHE_STATS.getOrDefault(key, 0L);
            count--;
            ROOM_CACHE_STATS.put(key, count);
        } finally {
            lock.unlock();
        }
    }
Decrements (-1) a number-based value of a statistic as maintained in {@link #ROOM_CACHE_STATS}.

@param key the key used to store the statistic in the cache.
xmppserver/src/main/java/org/jivesoftware/openfire/muc/spi/LocalMUCRoomManager.java