Java-日期和时间

• 日期和时间
• • System.currentTimeMillis
  • Date
  • • Java.util.Date
    • Java.sql.Date
    • 相互转换
  • SimpleDateFormat
  • Calendar
  • LocalDate、LocalTime、LocalDateTime

package com.time;

public class TimeTest {

    public static void main(String[] args) {
        //时间戳
        //返回当前时间与1970年1月1日0时0分0秒之间以毫秒为单位的时间差
        long currentTimeMillis = System.currentTimeMillis();
        System.out.println(currentTimeMillis);//1635668404513
    }

}

package com.time;

import java.util.Date;

public class TimeTest {

    public static void main(String[] args) {
       dateTest();
    }

    public static void dateTest(){
        //Java.util.Date创建当前时间
        Date date = new Date();

        //显示年月日
        System.out.println(date);//Sun Oct 31 16:29:28 CST 2021
        //获取时间戳
        System.out.println(date.getTime());//1635669012760

        //创建指定的时间戳对象
        Date date1 = new Date(1635669012760L);
        System.out.println(date1);//Sun Oct 31 16:30:12 CST 2021
    }

}

对应数据库中的日期类型

    public static void test(){
        java.sql.Date date = new java.sql.Date(1635669012760L);
        System.out.println(date);//2021-10-31
    }

public static void utilToSql(){
        Date date = new java.util.Date(1635669012760L);

        //java.util.Date转为java.sql.Date对象步骤
        //1、转为毫秒数
        long time = date.getTime();
        //2、用时间戳创建java.sql.Date的对象
        java.sql.Date date1 = new java.sql.Date(time);
        System.out.println(date1);//2021-10-31
    }

对日期Date类的格式化与解析。

@Test
    public  void test2(){
        Date date = new Date();
        System.out.println(date);//Sun Oct 31 17:03:52 CST 2021

        //实例化
        SimpleDateFormat simpleDateFormat = new SimpleDateFormat();
        //格式化
        String format = simpleDateFormat.format(date);
        System.out.println(format);//21-10-31 下午5:04

        //解析
        String str = "21-10-31 下午5:04";
        SimpleDateFormat simpleDateFormat1 = new SimpleDateFormat();
        try {
        		//要求字符串必须是符合SimpleDateFormat 识别的格式
            Date parse = simpleDateFormat1.parse(str);//Sun Oct 31 17:04:00 CST 2021
            System.out.println(parse);
        } catch (ParseException e) {
            e.printStackTrace();
        }

        //常用格式
        SimpleDateFormat simpleDateFormat2 = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss");
        String format1 = simpleDateFormat2.format(date);
        System.out.println(format1);//2021-10-31 05:11:06
    }

java.util.Calendar，日历类

@Test
    public void calendarTest(){
        //实例化
        //方式一
        Calendar calendar = new GregorianCalendar();
        //方式二
        Calendar instance = Calendar.getInstance();

        //常用方法
        //get()
        int days = instance.get(Calendar.DAY_OF_MONTH);
        System.out.println(days);//这个月的第31天

        //set()
        instance.set(Calendar.DAY_OF_MONTH,21);//设置为这个月的第21天
        int days2 = instance.get(Calendar.DAY_OF_MONTH);
        System.out.println(days2);//21

        //add()
        instance.add(Calendar.DAY_OF_MONTH,3);//在原有的基础上+3天
        int days3 = instance.get(Calendar.DAY_OF_MONTH);
        System.out.println(days3);//24

        //getTime()，转换为Date对象
        Date time = calendar.getTime();
        System.out.println(time);//Sun Oct 31 17:38:36 CST 2021

        //setTime():Date转为日历类
        Date date = new Date();
        instance.setTime(date);
        int days4 = instance.get(Calendar.DAY_OF_MONTH);
        System.out.println(days4);//31
    }

@Test
    public void test3(){
        //实例化方式一
        //now：获取当前日期、时间、日期+时间
        LocalDate localDate = LocalDate.now();
        LocalTime localTime = LocalTime.now();
        LocalDateTime localDateTime = LocalDateTime.now();

        System.out.println(localDate);//2021-10-31
        System.out.println(localTime);//17:53:56.515
        System.out.println(localDateTime);//2021-10-31T17:53:56.515

        //实例化方式二
        //of()：设置指定的年月日时分秒
        LocalDateTime localDateTime1 = LocalDateTime.of(2021, 10, 31, 15, 56);
        System.out.println(localDateTime1);//2021-10-31T15:56

        //getXxx():获取各种属性信息
        System.out.println(localDateTime.getDayOfMonth());//31
        System.out.println(localDateTime.getHour());//17

        //withXxx():修改属性值，生成新的对象，原对象不会被改变
        LocalDateTime localDateTime2 = localDateTime.withDayOfMonth(22);
        System.out.println(localDateTime);//2021-10-31T18:00:54.167
        System.out.println(localDateTime2);//2021-10-22T18:00:54.167

        //plusXxx():在原基础上相加，得到新对象，原对象不变
        LocalDateTime localDateTime3 = localDateTime.plusDays(2);
        System.out.println(localDateTime);//2021-10-31T18:04:44.357
        System.out.println(localDateTime3);//2021-11-02T18:04:44.357

        //minusXxx():在原基础上相减，得到新对象，原对象不变
        LocalDateTime localDateTime4 = localDateTime.minusDays(2);
        System.out.println(localDateTime);//2021-10-31T18:04:44.357
        System.out.println(localDateTime4);//2021-10-29T18:06:53.532
    }