Date related operations in java

1. String and Date conversion
   (1) is implemented based on SimpleDateFormat:

   package com.bky.df;
   
   import java.text.ParseException;
   import java.text.ParsePosition;
   import java.text.SimpleDateFormat;
   import java.util.Date;
   
   public class DateLangUtils{
       public static void main(String[] args) throws ParseException {
          Date nowDate = new Date();
          String formatString = "yyyy-MM-dd HH:mm:ss";
          String dateString = "2018-12-02 16:33:15";
          System.out.println(getString(nowDate, formatString));
          System.out.println(getDate(dateString, formatString));
          System.out.println(getDatePos(dateString, formatString));
       }
   
       /**
        * Format Date using SimpleDateFormat
        *
        * @param date
        * @param formatString
        * @return
        */
       public static String getString(Date date, String formatString) {
           SimpleDateFormat formatter = new SimpleDateFormat(formatString);
           return formatter.format(date);
       }
       
       /**
        * Use SimpleDateFormat to parse date strings
        *
        * @param dateString
        * @param formatString
        * @return
        * @throws ParseException
        */
       public static Date getDate(String dateString, String formatString) throws ParseException {
           SimpleDateFormat formatter = new SimpleDateFormat(formatString);
           return formatter.parse(dateString);
       }
   
       /**
        * Use SimpleDateFormat to parse date strings
        *
        * @param dateString
        * @param formatString
        * @return
        */
       public static Date getDatePos(String dateString, String formatString) {
           SimpleDateFormat formatter = new SimpleDateFormat(formatString);
           return formatter.parse(dateString, new ParsePosition(0));
       }
   }

   (2) Implemented using commons components

   import java.text.ParseException;
   import java.util.Date;
   
   import org.apache.commons.lang3.time.DateFormatUtils;
   import org.apache.commons.lang3.time.DateUtils;
   
   public static void main(String[] args) throws ParseException {
       Date nowDate = new Date();
       String formatString = "yyyy-MM-dd HH:mm:ss";
       String dateString = "2018-12-02 16:33:15";
       System.out.println(DateFormatUtils.format(nowDate, formatString));
       System.out.println(DateUtils.parseDate(dateString, formatString));
   }

2. Date-related operations are
   implemented based on Calendar
   

   Calendar cal = Calendar.getInstance();
    // Set Calendar time(1.Date 2.longofmills) 
   cal.setTime( new Date());
   cal.setTimeInMillis(System.currentTimeMillis());
   
   // Set the year, month, day, hour, minute, second, millisecond 
   cal.set(Calendar.YEAR, cal.get(Calendar.YEAR));
    // Calendar.MONTH starts from 0 to indicate January, and needs to be converted to display/set 
   cal.set( Calendar.MONTH, cal.get(Calendar.MONTH));
   cal.set(Calendar.DAY_OF_MONTH, cal.get(Calendar.DAY_OF_MONTH));
   // 24小时制
   cal.set(Calendar.HOUR_OF_DAY, cal.get(Calendar.HOUR_OF_DAY));
   // 12小时制
   cal.set(Calendar.HOUR, cal.get(Calendar.HOUR));
   cal.set(Calendar.MINUTE, cal.get(Calendar.MINUTE));
   cal.set(Calendar.SECOND, cal.get(Calendar.SECOND));
   cal.set(Calendar.MILLISECOND, cal.get(Calendar.MILLISECOND));
   
   // Set the parameters related to the week
    // Set the day of the week (1 is Sunday, 7 is Saturday) 
   cal.set(Calendar.DAY_OF_WEEK, cal.get(Calendar.DAY_OF_WEEK));
    // Set the week of the month (The default area is calculated from Sunday, and the week is calculated by the calendar) 
   cal.set(Calendar.WEEK_OF_MONTH, cal.get(Calendar.WEEK_OF_MONTH));
    // Set the week of the current month (the setting is calculated by the number of days in the current month) Weeks, 7 days a week) 
   cal.set(Calendar.DAY_OF_WEEK_IN_MONTH, cal.get(Calendar.DAY_OF_WEEK_IN_MONTH));
    // Set the first week of the year 
   cal.set(Calendar.WEEK_OF_YEAR, cal.get(Calendar.WEEK_OF_YEAR ));
   
   // Set the first day of the month 
   cal.set(Calendar.DAY_OF_MONTH, cal.getActualMinimum(Calendar.DAY_OF_MONTH));
   cal.set(Calendar.HOUR_OF_DAY, 0);
   cal.set(Calendar.MINUTE, 0);
   cal.set(Calendar.SECOND, 0);
   cal.set(Calendar.MILLISECOND, 0);
   
   // Set the last day of the month 
   cal.set(Calendar.DAY_OF_MONTH, cal.getActualMaximum(Calendar.DAY_OF_MONTH));
   cal.set(Calendar.HOUR_OF_DAY, 0);
   cal.set(Calendar.MINUTE, 0);
   cal.set(Calendar.SECOND, 0);
   cal.set(Calendar.MILLISECOND, 0);