@Override
    public Collection<String> getGroupNames()
    {
        return getGroupProviders().parallelStream()
            .map(GroupProvider::getGroupNames)
            .flatMap(Collection::stream)
            .collect(Collectors.toSet());
    }
Returns the Collection of all group names in each of the providers.

@return the Collection of all groups.
xmppserver/src/main/java/org/jivesoftware/openfire/group/GroupMultiProvider.java