/按照List中对象的id属性升序
list.sort(Comparator.comparing(Stu::getId))

//按照List中对象的id属性降序
list.sort(Comparator.comparing(Stu::getId).reversed());

//多条件升序
list.sort(Comparator.comparing(Stu::getId).thenComparing(Stu::getSid));

//id升序,sid降序
list.sort(Comparator.comparing(Stu::getId).reversed().thenComparing(Stu::getSid));

//key值重复的map
MultiValueMap<Integer, String> timeMap = new LinkedMultiValueMap<>();

//集合升序排序
Collections.sort(student, new Comparator(){
      public int compare(StudentVo p1, StudentVo p2) {
      return Integer.parseInt(p1.getStudentCode()) -         
       Integer.parseInt(p2.getStudentCode());
  }
});