public class LearnGroupBy {
    public static void main(String[] args) {
        List<Student> list = List.of(new Student[]{new Student(1,2,"小王",80),new Student(1,2,"小红",77),new Student(1,3,"小李",66)});
        Map<Integer, List<Student>> groups = list.stream()
                .collect(Collectors.groupingBy(s -> s.classId, Collectors.toList()));
        System.out.println(groups);
    }
}

class Student {
    int gradeId; // 年级
    int classId; // 班级
    String name; // 名字
    int score; // 分数
    public Student(int gradeId,int classId,String name,int score){
        this.classId=classId;
        this.gradeId=gradeId;
        this.name=name;
        this.score=score;
    }
    public String toString(){
        return String.format("{ gradeId: %d, classId: %d, name: %s, score: %d }",gradeId,classId,name,score);
    }
}