地图 <String, Map<String, String> &gt;-使用流选择值键

Question

I have this Map:

Map<String, Map<String, String>> listMap = new HashMap<>();

I want to select all distinct Keys from Map which is value in main Map : listMap.value.key

List<String> distinct = listMap.entrySet().stream()
                                .map(e -> e.getValue()) //Map<String, String>
                                //Select key of value
                                .distinct().collect(Collectors.toList());

I don't know how to select key of value of listMap .

Answer 1

You need flatMap in order to map all the keys of all the inner Map s into a single Stream :

List<String> distinct = 
    listMap.values() // Collection<Map<String,String>>
           .stream() // Stream<Map<String,String>>
           .flatMap(map -> map.keySet().stream()) // Stream<String>
           .distinct() // Stream<String>
           .collect(Collectors.toList()); // List<String>

Answer 2

An alternate way of collecting these could be in a Set as:

Set<String> distinct = new LinkedHashSet<>(); // for a predictable order
listMap.values().forEach(e -> distinct.addAll(e.keySet()));