private void initializePossibleAttributeNames() {
        this.possibleUserAttributeNames = backingMap.values().stream().map(Map::keySet).flatMap(Collection::stream).collect(Collectors.toCollection(LinkedHashSet::new));
    }
Sets backing map.

@param backingMap the backing map
support/cas-server-support-person-directory-core/src/main/java/org/apereo/cas/persondir/MappablePersonAttributeDao.java