Question

我试图想办法在不调用stream()两次但无效的情况下这样做：

List<Song> songs = service.getSongs();

List<ArtistWithSongs> artistWithSongsList = songs.stream()
    .collect(Collectors
        .groupingBy(s -> s.getArtist(), Collectors.toList()))
    .entrySet()
    .stream()
    .map(as -> new ArtistWithSongs(as.getKey(), as.getValue()))
    .collect(Collectors.toList());

根据要求：

class ArtistWithSongs {
    private Artist artist;
    private List<Song> songs;

    ArtistWithSongs(Artist artist, List<Song> songs) {
        this.artist = artist;
        this.songs = songs;
    }
}

有更优化的方法吗？

Answer 1

我认为在这种情况下使用forEach就足够了：

List<ArtistWithSongs> artistWithSongsList = new ArrayList<>();
service.getSongs().stream()
                  .collect(Collectors.groupingBy(s -> s.getArtist(), Collectors.toList()))
                  .entrySet()
                  .forEach((k, v) -> artistWithSongsList.add(new ArtistWithSongs(k, v)););

Answer 2

我认为你可以使用FlatMap：

List<Song> songs = service.getSongs();

List<ArtistWithSongs> artistWithSongsList = songs.stream()
               .collect(Collectors
               .groupingBy(s -> s.getArtist(), Collectors.toList()))
               .entrySet()
               .flatMap(as -> new ArtistWithSongs(as.getKey(), as.getValue()))
               .collect(Collectors.toList());

编辑：

很抱歉，我们不能在collect（）之后使用flatMap，因为它不会返回流。其他解决方案是：

    List<ArtistWithSongs> artistWithSongsList = new ArrayList<>();
    songs.stream()
         .collect(Collectors.groupingBy(Song::getArtist))
         .forEach((artist, songs) -> artistWithSongsList.add(new ArtistWithSongs(artist, songs)););

使用Stream来映射收集结果

2 个答案: