将 collections 的 map 扩展为一维 map 新“无法推断功能接口类型”

Question

I have some values in a map that I would like to transform to a simpler map where the key would be 'A' -> 1, 'E'-> 1,...'Z'-> 10 . Keep getting "Cannot infer functional interface type".

private Map<List<Character>, Integer> scoresMap
        = Map.of(
        List.of('A', 'E', 'I', 'O', 'U', 'L', 'N', 'R', 'S', 'T'), 1,
        List.of('D', 'G'), 2,
        List.of('B', 'C', 'M', 'P'), 3,
        List.of('F', 'H', 'V', 'W', 'Y'), 4,
        List.of('K'), 5,
        List.of('J', 'X'), 8,
        List.of('Q', 'Z'), 10
);

I tried a functional approach:

private Stream<Map.Entry<Character, Integer>> expandMap(Map.Entry<List<Character>, Integer> kv){
    return kv.getKey()
            .stream()
            .collect(
                    Stream.of(),
                    (Stream<Map.Entry<Character, Integer>> acc, char el) ->
                            Stream.concat(acc, Stream.of(Map.entry(el, kv.getValue()))),
                    (Stream<Map.Entry<Character, Integer>> acc, Stream<Map.Entry<Character, Integer>> el) -> Stream.concat(acc, el)
                    );

}

And tried forEach:

private Map<Character, Integer> scores(){
    Map<Character, Integer> scores = Map.of();
    scoresMap
    .entrySet()
    .forEach(
        (Map.Entry<List<Character>, Integer> kv) -> 
            kv.getKey().forEach(
                    (char k) ->
                            scores.put(k, kv.getValue());
                )
        )
    
    return scores;

}

I think this is not to do with ambiguous types as I tried using little helpers instead of Lambdas.

Answer 1

You can make use of flatMap :

Map<Character, Integer> simpleMap = scoresMap.entrySet()
            .stream()
            .flatMap(entry -> {
                Integer value = entry.getValue();
                return entry.getKey().stream().map(ch -> Map.entry(ch, value));
            })
            .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

Answer 2

You can use Map.forEach() directly and you don't have to bother with types, Java can figure it by itself.

private Map<Character, Integer> scores(){

    Map<Character, Integer> scores = new HashMap<>();

    scoresMap.forEach((key, value) -> 
            key.forEach((k) ->
                scores.put(k, value)));

    return scores;
}