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<>

Question

我尝试从流中获取列表，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

这里是 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

map 来电

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>。

要获得单个List<MovieTrans>，请使用flatMap：

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

Answer 2

您的表达式的类型是List<List<MovieTrans>>：它是getMovieTranses 方法的结果的串联。

改用flatMap：

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