@Override
    public void removeClusteredCacheEntryListener(@Nonnull final String listenerId) {
        throw new UnsupportedOperationException();
    }
<strong>IMPORTANT:</strong> Unlike the standard {@link Map#keySet()} implementation, the set returned from
this method cannot be modified.

@return an unmodifiable set view of the keys contained in this map
xmppserver/src/main/java/org/jivesoftware/util/cache/CaffeineCache.java