Get inner most values in the Map as a List in `Map<String, List<Map<String,Map<String,String>>>>` Java

Stream#flatMap returns a Stream of all values of all elements:

List<String> converted = yourComplexStructure.values().stream()
    .flatMap(List::stream)
    .map(Map::values)
    .flatMap(Collection::stream())
    .map(Map::values)
    .flatMap(Collection::stream()))
    .collect(Collectors.toList());

It takes the values of your structure as a Stream, maps it to a Stream with all elements of each List, maps it to a Stream with all values of the Maps, creates a Stream with all values of the most inner maps and collects it.

^{Note that it is basically the same as the other answer (we wrote it at the same time) but I changed this answer so that it doesn't require lambdas but uses method references.}