// lombok 插件，如果没有用该插件的，请自行get、set方法
@Data
public class Student { 
    public Student(Integer id, Integer classNo, String name, String sex, Double score) { 
        this.id = id;
        this.classNo = classNo;
        this.name = name;
        this.sex = sex;
        this.score = score;
    }

    private Integer id;
    private Integer classNo;
    private String name;
    private String sex;
    private Double score;
}

		Student student1 = new Student(5, 3, "天一", "男", 100D);
        Student student2 = new Student(4, 3, "王二", "男", 10D);
        Student student3 = new Student(1, 1, "张三", "女", 66D);
        Student student4 = new Student(2, 2, "李四", "男", 99D);
        Student student5 = new Student(3, 2, "王五", "女", 59D);

        List<Student> students = Arrays.asList(student1, student2, student3, student4, student5);

        // 各个班级，成绩最差的学生
        Map<Integer, Student> classNoStudentMap = students
                .parallelStream()
                .collect(
                        Collectors.toMap(Student::getClassNo, Function.identity(), (c1, c2) -> c1.getScore() < c2.getScore() ? c1 : c2)
                );

        System.out.println(classNoStudentMap);

        // 分性别，成绩最差的学生
        Map<String, Student> sexStudentMap = students
                .parallelStream()
                .collect(
                        Collectors.toMap(Student::getSex, Function.identity(), (c1, c2) -> c1.getScore() < c2.getScore() ? c1 : c2)
                );

        System.out.println(sexStudentMap);

相关阅读：https://blog.csdn.net/weixin_41835612/article/details/83687088
https://blog.csdn.net/kingmax54212008/article/details/102827306