Java8 streams : Transpose map with values as list

Question

I have map with key as String and value as List. List can have 10 unique values. I need to convert this map with key as Integer and value as List. Example as below :

Answer 1

It's a bit scary (I generally try to break it down to make it more readable) but you could do it this way:

Map> transposeMap = new HashMap<>();

map.forEach((key, list) -> list.stream().forEach(
    elm -> transposeMap.put(elm,
        transposeMap.get(elm) == null ? Arrays.asList(key) : (Stream.concat(transposeMap.get(elm).stream(),
            Arrays.asList(key).stream()).collect(Collectors.toList())))));

Assuming Map> map is your original Map that you want to transpose. transposeMap will have transposed map that you need.