java 的Date 日期相关操作

1. String 与 Date互转
   （1）基于SimpleDateFormat实现：

   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));
       }
   
       /**
        * 使用SimpleDateFormat格式化Date
        * 
        * @param date
        * @param formatString
        * @return
        */
       public static String getString(Date date, String formatString) {
           SimpleDateFormat formatter = new SimpleDateFormat(formatString);
           return formatter.format(date);
       }
       
       /**
        * 使用SimpleDateFormat解析日期字符串
        * 
        * @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);
       }
   
       /**
        * 使用SimpleDateFormat解析日期字符串
        * 
        * @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）使用commons组件实现

   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. 对日期的相关操作
   基于Calendar实现
   

   Calendar cal = Calendar.getInstance();
   // 设置Calendar时间(1.Date 2.longofmills)
   cal.setTime(new Date());
   cal.setTimeInMillis(System.currentTimeMillis());
   
   // 设置年月日，时分秒，毫秒
   cal.set(Calendar.YEAR, cal.get(Calendar.YEAR));
   // Calendar.MONTH 从0开始表示1月，需要转换展示/设置
   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));
   
   // 设置与星期相关参数
   // 设置当周星期周几(1为星期日,7为星期六)
   cal.set(Calendar.DAY_OF_WEEK, cal.get(Calendar.DAY_OF_WEEK));
   // 设置当月第几个星期(默认地区是从星期日开始算，以日历计算第几周)
   cal.set(Calendar.WEEK_OF_MONTH, cal.get(Calendar.WEEK_OF_MONTH));
   // 设置当月第几个星期(设置按当月的天数算第几周，7天一周)
   cal.set(Calendar.DAY_OF_WEEK_IN_MONTH, cal.get(Calendar.DAY_OF_WEEK_IN_MONTH));
   // 设置一年中第几个星期
   cal.set(Calendar.WEEK_OF_YEAR, cal.get(Calendar.WEEK_OF_YEAR));
   
   // 设置当月第一天
   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);
   
   // 设置当月最后一天
   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);