Invert Map with list value Map<Key, List<Value>> to Map <Value, Key> in Java 8

user_1387275 0
Viewed 2233

I have a map kind of grouping values by key Map<String, List<Integer>>, i want to revert in order to map each value to the corresponding key

Example: I want to transform the code below 

Map<String, List<Integer>> mapOfIntList = new HashMap<String, List<Integer>>();

mapOfIntList.put("UNIT", Arrays.asList(1, 2, 3, 8, 7, 0, 8, 6));
mapOfIntList.put("TEN", Arrays.asList(24, 90, 63, 87));
mapOfIntList.put("HUNDRED", Arrays.asList(645, 457, 306, 762));
mapOfIntList.put("THOUSAND", Arrays.asList(1234, 3456, 5340, 9876));

to another Map(Integer, String) where i can find : (1, "UNIT"), (2, "UNIT")...(24, "TEN"), (90, "TEN")...(645, "HUNDRED")...(3456, "THOUSAND")...

java-stream java-8 java
4 Answers

You can use

Map<Integer, String> mapNumberToType = mapOfIntList.entrySet().stream()
    .collect(HashMap::new, (m,e)->e.getValue().forEach(v->m.put(v,e.getKey())), Map::putAll);

You may recognize the similarity to the forEach based code of this answer within the second function passed to collect (the accumulator) function. For a sequential execution, they do basically the same, but this Stream solution supports parallel processing. That’s why it needs the other two functions, to support creating local containers and to merge them.

See also the Mutable reduction section of the documentation.

user_27114880

Or use two nested forEach 

mapOfIntList.forEach((key, value) ->
            value.forEach(v -> {
                mapNumberToType.put(v, key);
            })
 );

as @nullpointer commented in one-liner

mapOfIntList.forEach((key, value) -> value.forEach(v -> mapNumberToType.put(v, key)));
user_36696980

I have found a solution :

Map<Integer, String> mapNumberToType = mapOfIntList
    .entrySet()
    .stream()
    .flatMap(
            entry -> entry.getValue().stream()
                    .map(number -> Pair.of(number, entry.getKey()))
                    .collect(Collectors.toList()).stream())
    .collect(
            Collectors.toMap(Pair::getLeft,
                    Pair::getRight, (a, b) -> {
                        return a;
                    }));

System.out.println("Number/Type correspondance : " + mapNumberToType);

hope this helps anyone having the same problem !

user_13872750

This would be simpler as:

 source.entrySet()
       .stream()
       .flatMap(e -> e.getValue().stream().map(s -> new AbstractMap.SimpleEntry<>(s, e.getKey())))
       .collect(toMap(Map.Entry::getKey, Map.Entry::getValue, (l, r) -> l));
user_58620710
Related