java：不兼容类型：推理变量T的边界不兼容等式约束：下界：java。util。列表

Question

我试图从一个流列表，但我有一个例外。

这是一个带有对象列表的电影对象。

public class Movie {

    private String example;
    private List<MovieTrans> movieTranses;

    public Movie(String example, List<MovieTrans> movieTranses){
        this.example = example;
        this.movieTranses = movieTranses;
    }
    getter and setter

以下是电影：

public class MovieTrans {

    public String text;

    public MovieTrans(String text){
        this.text = text;
    }
    getter and setter

我在列表中添加元素：

List<MovieTrans> movieTransList = Arrays.asList(new MovieTrans("Appel me"), new MovieTrans("je t'appel"));
List<Movie> movies = Arrays.asList(new Movie("movie played", movieTransList));
//return a list of MovieTrans
List<MovieTrans> movieTransList1 = movies.stream().map(Movie::getMovieTranses).collect(Collectors.toList());

我有一个编译器错误：

Error:(44, 95) java: incompatible types: inference variable T has incompatible bounds
    equality constraints: MovieTrans
    lower bounds: java.util.List<MovieTrans>

Answer 1

表达式的类型是List

改用flatMap：

List<MovieTrans> movieTransList1 = movies.stream()
    .flatMap(m -> m.getMovieTranses().stream())
    .collect(Collectors.toList());

Answer 2

地图呼叫

movies.stream().map(Movie::getMovieTranses)

转换流

获取单个列表

List<MovieTrans> movieTransList1 = 
    movies.stream()
          .flatMap(m -> m.getMovieTranses().stream())
          .collect(Collectors.toList());