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<Integer, List<String>> 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<String, List<Integer>> map is your original Map that you want to transpose. transposeMap will have transposed map that you need.

Answer 2

You can Achieve in this way

Let suppose I have Person class with Gender and Age . I want to get it in this form

Map<SEX,List<Person>> 

I would write simply

Map<SEX,List<Person>> map =  personList.stream()

                           .collect(Collectors.groupingBy(Person::getGender));

it will get me something like below (one key against multiple values )

key:MALE
age31sexMALE
age28sexMALE

key:FEMALE
age40sexFEMALE
age44sexFEMALE

Answer 3

An idea could be to generate all value-key pairs from the original map and then group the keys by these values:

import java.util.AbstractMap.SimpleEntry;

import static java.util.stream.Collectors.groupingBy;
import static java.util.stream.Collectors.mapping;
import static java.util.stream.Collectors.toList;

...

Map<Integer, List<String>> transposeMap =
    map.entrySet()
       .stream()
       .flatMap(e -> e.getValue().stream().map(i -> new SimpleEntry<>(i, e.getKey())))
       .collect(groupingBy(Map.Entry::getKey, mapping(Map.Entry::getValue, toList())));

Answer 4

Alexis’ answer contains the general solution for this kind of task, using flatMap and a temporary holder for the combination of key and flattened value. The only alternative avoiding the creation of the temporary holder objects, is to re-implement the logic of the groupingBy collector and inserting the loop over the value list logic into the accumulator function:

Map<Integer, List<String>> mapT = map.entrySet().stream().collect(
    HashMap::new,
    (m,e) -> e.getValue().forEach(
                 i -> m.computeIfAbsent(i,x -> new ArrayList<>()).add(e.getKey())),
    (m1,m2) -> m2.forEach((k,v) -> m1.merge(k, v, (l1,l2)->{l1.addAll(l2); return l1;})));