当每个对象可以分为多个组时如何对对象进行分组？

我有一个List<Student>其中每个Student可能有多种爱好：

public class StudentData {
    public static List<Student> getData() {
        return Arrays.asList(
            new Student(1, "a1", 1, Arrays.asList("cricket", "football", "basketball")),
            new Student(2, "a2", 1, Arrays.asList("chess", "football")),
            new Student(3, "a3", 2, Arrays.asList("running")),
            new Student(4, "a4", 2, Arrays.asList("throwball", "football")),
            new Student(5, "a5", 3, Arrays.asList("cricket", "basketball")),
            new Student(6, "a6", 4, Arrays.asList("cricket")), new Student(7, "a7", 5, Arrays.asList("basketball")),
            new Student(8, "a8", 6, Arrays.asList("football")),
            new Student(9, "a9", 8, Arrays.asList("tennis", "swimming")),
            new Student(10, "a10", 8, Arrays.asList("boxing", "running")),
            new Student(11, "a11", 9, Arrays.asList("cricket", "football")),
            new Student(12, "a12", 11, Arrays.asList("tennis", "shuttle")),
            new Student(13, "a13", 12, Arrays.asList("swimming"))
        );
    }
}

如何根据兴趣对学生进行分组？我尝试了下面的代码：

List<Student> data = StudentData.getData();
data.stream().collect(Collectors.groupingBy(s -> s.getHobbies().stream()));

它没有给出正确的答案。

你基本上需要一个Stream这是由一个Pair（我选择AbstractMap.SimpleEntry此处），左侧部分作为爱好，右侧部分作为学生（也可能是相反的情况，没关系）。

稍后只需根据Hobby（在你的情况下这是一个字符串）。

data.stream()
    .flatMap(student -> student.getHobbies().stream().map(hobby -> new SimpleEntry<>(hobby, student)))
    .collect(Collectors.groupingBy(
            Entry::getKey,
            Collectors.mapping(Entry::getValue, Collectors.toList())
));

Entry::getKey作为获取密钥的方法引用，如果对您更有意义，您也可以将其编写为 lambda 表达式：

Collectors.groupingBy(entry -> entry.getKey())