public static void main(String[] args) {
        List<Student> list = new ArrayList<>();
        list.add(new Student("A",10,LocalDateTime.now()));
        list.add(new Student("B",11,LocalDateTime.now().minus(2, ChronoUnit.MONTHS)));
        list.add(new Student("C",12,LocalDateTime.now().minus(2, ChronoUnit.MONTHS)));
        list.add(
 
new Student("D",13,LocalDateTime.now().minus(3, ChronoUnit.MONTHS)));

        Map<String, List<Student>> map = list.stream().collect(Collectors.groupingBy(t -> t.getDateTime().format(DateTimeFormatter.ofPattern("yyyy-MM"))));
      
    }
    static class Student{
        private String name;
        
 
private Integer age;
        private LocalDateTime dateTime;

        public Student(String name, Integer age, LocalDateTime dateTime) {
            this.name = name;
            this.age = age;
            this.dateTime = dateTime;
        }

        public String getName() {
            return name;
        }

        
 
public void setName(String name) {
            this.name = name;
        }

        public Integer getAge() {
            return age;
        }

        public void setAge(Integer age) {
            this.age = age;
        }

        public LocalDateTime getDateTime() {
            return dateTime;
        }

        public void setDateTime(LocalDateTime dateTime) {
            this.dateTime = dateTime;
        }
    }