Java 8 分组并追加到一个集合

Question

我有一个返回 Map<String, Set<String>> 的函数, java 8之前的代码:

Map<String, Set<String>> degreeMap = new HashMap<>();
for(Course  course : courses){
    Set<String> cList = degreeMap.get(course.getCourseLevel().toString());
    if(Objects.nonNull(cList)){
        cList.addAll(course.getMasterDegree()); //this is what i want to append to the old set
        degreeMap.put(course.getCourseLevel().toString(), cList);
    } else{
        degreeMap.put(course.getCourseLevel().toString(), new HashSet<>(course.getMasterDegree()));
    }
} 
return degreeMap;

它返回类(class)级别 -> 学位集的 map 。

例如，它读取所有类(class)并返回如下 map :

{"undergraduate" : ["BTech", "BSc", "BE"],
"masters": ["MTech", "MBA"],
"Executive": ["PGDBM", "EECP"]}

这是我的类(class)类:

public class Course {
    private List<String> masterDegree;
    private CourseLevel courseLevel;
}

但是我想用 Java 8 风格来写这段代码。为此，我尝试了这个:

Map<String, Set<String>> degreeMap = courses.stream().collect(
        Collectors.groupingBy(c -> c.getCourseLevel().toString(),
                Collectors.mapping(c -> c.getMasterDegree(), Collectors.toSet()))
);

这是行不通的，我收到以下编译时错误:

no instance(s) of type variable(s) exist so that List conforms to String inference variable T has incompatible bounds: equality constraints: String lower bounds: List

任何建议，如何实现这一目标？

Answer 1

未经测试，但看起来您正在寻找类似的东西:

    return courses.stream()
            .collect(Collectors.toMap(course -> course.getCourseLevel().toString(),
                    course -> new HashSet<>(course.getMasterDegree()),
                    (set1, set2) -> Stream.of(set1, set2)
                            .flatMap(Set::stream).collect(Collectors.toSet())));