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

问题描述

我尝试从流中获取一个列表，但我有一个例外。

i try to get a list from a stream but i have an exception.

这是一个带有对象列表的Movie对象。

Here is the Movie object with a list of an object.

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：

Here is the 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>

推荐答案

地图打电话

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

将 Stream< Movie> 转换为 Stream< ;列出< MovieTrans>> ，您可以将其收集到列表< List< MovieTrans>> 中，而不是列表< MovieTrans> 。

converts a Stream<Movie> to a Stream<List<MovieTrans>>, which you can collect into a List<List<MovieTrans>>, not a List<MovieTrans>.

要获得单个列表< MovieTrans> ，使用 flatMap ：

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

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