java如何自定义List中的sort()排序,用于日期排序

//待排序的集合
List<String> list=new ArrayList<String>();
list.add("2019-06");
list.add("2019-11");
list.add("2019-02");
list.add("2019-09");
list.add("2019-05");
//自定义list排序，集合数据(月份)按升序排序;
final SimpleDateFormat sdft = new SimpleDateFormat("yyyy-MM");
Collections.sort(list, new Comparator<String>(){
    @Override
 public int compare(String month1, String month2) {
  int mark = 1;
  try {
      Date date1 = sdft.parse(month1);
      Date date2 = sdft.parse(month2);
      if(date1.getTime() < date2.getTime()){
       mark = -1;//调整顺序,-1为不需要调整顺序;
      }
      if(month1.equals(month2)){
       mark =  0;
  }
  } catch (ParseException e) {
   LOG.error("日期转换异常", e);
   e.printStackTrace();
  }
 return mark;
 } //compare
});

Date() date1=new Date();
Date() date2=new SimpleDateFormat("yyyy-MM-dd").parse("2019-06-11");
Boolean flag;
if(date1.before(date2)){
 flag=true;
}

public  class  Test{
    public static void main(String[] args) {
        Student Student =new Student("张1","6M");
        Student Student1 =new Student("张2","1M");
        Student Student2 =new Student("张4","5M");
        Student Student3 =new Student("张5","1M");
        List<Student> list=new ArrayList<Student>();
        list.add(Student);
        list.add(Student1);
        list.add(Student2);
        list.add(Student3);
        Collections.sort(list, new Comparator<Student>() {
            @Override
            public int compare(Student o1, Student o2) {
                return  o2.getAge().compareTo(o1.getAge());
            }
        });
        System.out.println(list.toString());
    } 
}