class CatalogIdentifierDatasource extends SlingSafeMethodsServlet {

    protected List<String> flattenMapToList(Map<String, Collection<String>> mapOfCollections) {
        List<String> flatList = new ArrayList<>();

        mapOfCollections.forEach((provider, identifiers) -> {
            identifiers.stream().forEach(identifier -> flatList.add(provider + ":" + identifier));
        });

        return flatList;
    }

}