public static <T, U> List<U> streamFlatMapConvertToList(List<? extends T> list, Function<? super T, List<? extends U>> iConvert) {

if (isNullOrEmpty(list)) {
return Collections.EMPTY_LIST;
}

List<U> returnList = new ArrayList<>();

if (!isNullOrEmpty(list)) {
    Stream<List> uList = list.stream().map(iConvert);
    returnList = (List<U>) uList.flatMap(l -> {
if (l != null) return l.stream();
else return Collections.EMPTY_LIST.stream();
}).collect(Collectors.toList());
}

return returnList;
}