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

2022-01-22 00:00:00 java-8 java java-stream

我尝试从流中获取列表，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

这里是 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());

我有这个编译器错误:

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

map调用

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

将 Stream<Movie> 转换为 Stream<List<MovieTrans>>，您可以将其收集到 List<List<MovieTrans>>，而不是 List<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 List<MovieTrans>，请使用 flatMap :

To get a single List<MovieTrans>, use flatMap :

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

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