java collection.sort()根据时间排序list

package com.butterfly.Class;

public class user {
    private String name;
    private String sex;
    private int age;
    private String birthday;
    
    public user(String name,String sex,int age,String birthday){
        this.name = name;
        this.sex = sex;
        this.age = age;
        this.birthday = birthday;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getSex() {
        return sex;
    }
    public void setSex(String sex) {
        this.sex = sex;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public String getBirthday() {
        return birthday;
    }
    public void setBirthday(String birthday) {
        this.birthday = birthday;
    }
    
    
}

package com.butterfly.Class;

import java.util.Comparator;

public class sortClass implements Comparator{
    public int compare(Object arg0,Object arg1){
        user user0 = (user)arg0;
        user user1 = (user)arg1;
        int flag = user0.getBirthday().compareTo(user1.getBirthday());
        return flag;
    }
}

package com.butterfly.Class;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class testClass {
    public static void main(String[] args){
        List userList = new ArrayList();
        userList.add(new user("吴光雷","男",25,"19871028"));
        userList.add(new user("吴桥","男",22,"19900923"));
        userList.add(new user("陆晓敏","男",25,"19870728"));
        userList.add(new user("王佳俊","男",23,"19890728"));
        userList.add(new user("秦菁","女",23,"19891212"));
        
        sortClass sort = new sortClass();
        Collections.sort(userList,sort);
        for(int i=0;i<userList.size();i++){
            user temp = (user)userList.get(i);
            System.out.println("姓名:"+temp.getName()+",生日:"+temp.getBirthday());
        }
    }
}