How to convert List of Map into Map of List using Java Streams

I have List of Maps as

[{"a"="10","b"="20"},{"a"="12","b"="22"},{"a"="14","b"="24"}]

And I want a Map as

{"a"=["10","12","14"],"b"=["20","22","24"]}

You can do this without having to instantiate an additional data structure. You first map the Maps to Map.Entry and then group by key.

var listOfMaps = List.of(
        Map.of("a", 10, "b", 20),
        Map.of("a", 12, "b", 22),
        Map.of("a", 14, "b", 24)
);

var mapOfLists = listOfMaps.stream()
        .map(Map::entrySet)
        .flatMap(Set::stream)
        .collect(Collectors.groupingBy(
                Map.Entry::getKey,
                Collectors.mapping(
                        Map.Entry::getValue,
                        Collectors.toList()
                )
        ));

System.out.println(mapOfLists);

Output:
{a=[10, 12, 14], b=[20, 22, 24]}

You can make use of Group By to achieve this. Here is an example.

First we take a stream of list using list.stream() and then we make a stream of each Map's Entry using flatMap. Now that we have a stream of each Entry(Key-value pair), we use groupingBy to collect them as a group.

First argument of groupingBy is functions what derive the key , Second argument is optional its a map factory which will be used to create the map, in case you want to have a sorted map you can use TreeMap::new , Now third param is reducer part where you tell what will be the grouped value and how it has to be collected(in our case we want it into a list)

Map<String, String> ma = new java.util.HashMap<>();
	ma.put("a", "10");
	ma.put("b", "20");
	Map<String, String> ma3 = new java.util.HashMap<>();
	ma3.put("a", "15");
	ma3.put("b", "17");
	ma3.put("c", "19");

	List<Map<String, String>> list = Arrays.asList(ma, ma3);

	Map<String, List<String>> lm = list.stream().flatMap(x -> x.entrySet().stream()).collect(Collectors
			.groupingBy(Entry::getKey, HashMap::new, Collectors.mapping(Entry::getValue, Collectors.toList())));

	System.out.println(lm);

Output looks like

{a=[10, 15], b=[20, 17], c=[19]}

Map<String,List<String>> map=new HashMap<>();
list.stream().flatMap(map->map.entrySet().stream()).forEach(entry->{
    map.putIfAbsent(entry.getKey(),new ArrayList<>());
    map.get(entry.getKey()).add(entry.getValue());
});

It is not a one-liner because I needed two statements in the forEach and I don't like to put something like that in one line but the OP did not ask for a one-liner.

It gets all entries of the inner Map as a Stream at first (flatMap).

After that, it iterates over them doing two actions:

• If no List exists for the entry value, a new one is added.

• The value of the entry is added to the inner list of the entry.

You can make use of the remapping function in Map#merge.

final Map<String,List<String>> result = new HashMap<>();
maps.forEach(map -> map.entrySet().forEach(entry -> 
    result.merge(entry.getKey(), Arrays.asList(entry.getValue()), 
        (a,b)->Stream.concat(a.stream(),b.stream()).collect(Collectors.toList()))));