@Override
    public void putAll(Map<? extends K, ? extends V> m) {
        super.putAll(m);
        // drop the transient sets; will be rebuilt when/if needed
        clearCache();
    }
This method is called from several of the mutators to keep
the group set in sync with the keys in the map. 

@param item The item to be added or removed if it is in the group set
@param keyOrValue True for keys, false for values
@param addOrRemove True to add, false to remove
@return true if the given item is a group
xmppserver/src/main/java/org/jivesoftware/openfire/group/ConcurrentGroupMap.java