java:不兼容类型:推理变量T具有不兼容的边界等式约束:下限:java.util.List <>

Question

我试图从流中获取一个列表,但我有一个例外.

这是带有对象列表的Movie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

这是MovieTrans:

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());

我有这个编译错误:

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

Answer 1

该map呼叫

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

将a转换Stream<Movie>为a Stream<List<MovieTrans>>,您可以将其转换为a ,而List<List<MovieTrans>>不是a List<MovieTrans>.

要获得单身List<MovieTrans>,请使用flatMap:

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

Answer 2

表达式的类型是List<List<MovieTrans>>:它是getMovieTranses方法结果的串联.

flatMap改为使用:

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