如何将 Map 的 Map 转换为 Java 中的列表

Question

I have a map of the map. I have to convert into a list of an object using lambda. I am trying to convert with Java 8.

 private Map<Integer, Map<Integer, Movie>> movies; // given

 List<Movie> movies // expected

Answer 1

You can use declarative solution by using stream API like that:

List<Movie> movieList = movies.entrySet().stream()
                .flatMap(s-> s.getValue().entrySet().stream())
                .map(Map.Entry::getValue)
                .collect(Collectors.toList());

Answer 2

Alternatively you could call Map#values and avoid some lambda expressions, which could make your snippet more readable:

List<Movie> movieList = movies.values()
                                .stream()
                                .map(Map::values)
                                .flatMap(Collection::stream)
                                .collect(Collectors.toList());

Answer 3

To get a single collection of Movie objects from the wrapping map, flatMap should be applied to the collection of values:

Map<Integer, Map<Integer, Movie>> movieMap; 

List<Movie> movies = movieMap.values().stream() // Stream<Map<Integer, Movie>>
        .map(Map::values)  // Stream<Collection<Movie>>
        .flatMap(Collection::stream) // Stream<Movie>
        .collect(Collectors.toList());

However, the resulting list is likely to contain duplicate movies. If the redundant duplicates need to be filtered, then distinct operation should be applied before .collect :

List<Movie> distinctMovies = movieMap.values().stream() // Stream<Map<Integer, Movie>>
        .map(Map::values)  // Stream<Collection<Movie>>
        .flatMap(Collection::stream) // Stream<Movie>
        .distinct()
        .collect(Collectors.toList());

Or the result may be collected into a set using .collect(Collectors.toSet()) :

Set<Movie> movieSet = movieMap.values().stream() // Stream<Map<Integer, Movie>>
        .map(Map::values)  // Stream<Collection<Movie>>
        .flatMap(Collection::stream) // Stream<Movie>
        .collect(Collectors.toSet());