Convert a map of lists into a list of maps

You can first iterate through map entries and represent list elements as Map<String,Integer> and get a stream of lists of maps, and then reduce this stream to a single list.

Try it online!

Map<String, List<Integer>> mapOfLists = new LinkedHashMap<>();
mapOfLists.put("a", List.of(1, 2, 3));
mapOfLists.put("b", List.of(4, 5, 6));
mapOfLists.put("c", List.of(7));

List<Map<String, Integer>> listOfMaps = mapOfLists.entrySet().stream()
        // Stream<List<Map<String,Integer>>>
        .map(entry -> entry.getValue().stream()
                // represent list elements as Map<String,Integer>
                .map(element -> Map.of(entry.getKey(), element))
                // collect a list of maps
                .collect(Collectors.toList()))
        // intermediate output
        //[{a=1}, {a=2}, {a=3}]
        //[{b=4}, {b=5}, {b=6}]
        //[{c=7}]
        .peek(System.out::println)
        // reduce a stream of lists to a single list
        // by sequentially multiplying the list pairs
        .reduce((list1, list2) -> list1.stream()
                // combinations of elements,
                // i.e. maps, from two lists
                .flatMap(map1 -> list2.stream()
                        .map(map2 -> {
                            // join entries of two maps
                            Map<String, Integer> map =
                                    new LinkedHashMap<>();
                            map.putAll(map1);
                            map.putAll(map2);
                            return map;
                        }))
                // collect into a single list
                .collect(Collectors.toList()))
        .orElse(null);

// output
listOfMaps.forEach(System.out::println);
//{a=1, b=4, c=7}
//{a=1, b=5, c=7}
//{a=1, b=6, c=7}
//{a=2, b=4, c=7}
//{a=2, b=5, c=7}
//{a=2, b=6, c=7}
//{a=3, b=4, c=7}
//{a=3, b=5, c=7}
//{a=3, b=6, c=7}

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<sup>See also:

• Generate all possible string combinations by replacing the hidden # number sign

• How to get all possible combinations from two arrays?</sup>