日期的使用

Calendar 类

参考文档：https://blog.csdn.net/weixin_44259720/article/details/86669177

概述

public abstract class Calendar
	extends Objectimplements Serializable, Cloneable, Comparable<Calendar>{}

该类由 abstract 修饰，是一个抽象类，因此无法通过 new 来实现。它提供了 getInstance()方法来获取由当前日期及时间初始化的子类。

常用方法

获取时间

import java.text.SimpleDateFormat;
import java.util.Calendar;
public class CalenderTest {
	//获取时间：以目前系统时间 ：2019:01:27 16:27:27 为例
	@Test
	public void testGetCalender() {
		Calendar cal = Calendar.getInstance();    							//Calendar类的实例化
		SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");	//时间的格式化

		//当前系统时间
		Date date = cal.getTime();								//date=Sun Jan 27 16:27:27 CST 2019
		String nowTime = sdf.format(cal.getTime());				//nowTime=2019-01-27 16:27:27

		//当前年
		int year = cal.get(Calendar.YEAR);						//year=2019

		//当前月 Calendar.MONTH从0开始 ,使用时通常会+1
		int month = cal.get(Calendar.MONTH) + 1;				//month=1

		//当前日：两种方法等价
		int day_of_month = cal.get(Calendar.DAY_OF_MONTH);		//day_of_month=27
		int day = cal.get(Calendar.DATE);  						//day=27

		//获取当月day的最大值！！
		int max_day_of_month = cal.getActualMaximum(Calendar.DAY_OF_MONTH);	//max_day_of_month=31

		//当前时钟：24小时制
		int hour24 = cal.get(Calendar.HOUR_OF_DAY);				//hour24=16

		//当前时钟：12小时制
		int hour12 = cal.get(Calendar.HOUR); 					//hour12=4

		//当前:分钟
		int minute = cal.get(Calendar.MINUTE);   				//minute=27

		//当前秒
		int second = cal.get(Calendar.SECOND);  				//second=27

		//星期几：用数字（1~7）表示（星期日~星期六），使用时通常会-1
		int day_of_week = cal.get(Calendar.DAY_OF_WEEK) - 1;	//day_of_week=0

		//上午-0；下午-1
		int amOrPm = cal.get(Calendar.AM_PM);  					//amOrPm=1

		//当前年的第几周
		int week_of_year = cal.get(Calendar.WEEK_OF_YEAR);		//week_of_year=5

		//当前月的星期数
		int week_of_month = cal.get(Calendar.WEEK_OF_MONTH);	//week_of_month=5

		//当前月中的第几个星期
		int day_of_week_in_month = cal.get(Calendar.DAY_OF_WEEK_IN_MONTH); //day_of_week_in_month=4

		//当前年的第几天
		int day_of_year = cal.get(Calendar.DAY_OF_YEAR);		//day_of_year=27
	}
}

设置时间

import java.text.SimpleDateFormat;
import java.util.Calendar;
public class CalenderTest {
	//设置时间：以目前系统时间 ：2019:01:27 16:27:27 为例
	@Test
	public void testSetCalender() {
		Calendar cal = Calendar.getInstance();    							//Calendar类的实例化
		SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");	//时间的格式化

		//1.当前系统时间
		Date date = cal.getTime();							//data=Sun Jan 27 16:27:27 CST 2019
		String nowTime = sdf.format(cal.getTime());			//nowTime=2019-01-27 16:27:27

		//2.设置时间：不设置项，默认Calendar已经存在日期
		cal.set(Calendar.HOUR_OF_DAY, 11);
		cal.set(Calendar.MINUTE, 30);
		cal.set(Calendar.SECOND, 30);
		String date1 = sdf.format(cal.getTime());			//time=2019-01-27 11:30:30

		//3.设置时间：全部设置，则输出自定义时间
		cal.set(Calendar.YEAR, 2018);
		cal.set(Calendar.MARCH, 7);
		cal.set(Calendar.DATE, 21);
		String date2 = sdf.format(cal.getTime());			//time=2018-08-21 11:30:30

		//4.时间重置：Date date = new date();
		String newDate1 = sdf.format(cal.getTime());		//newDate1=2018-08-21 11:30:30
		String newDate2 = sdf.format(new Date());			//newDate2=2019:01:27 16:27:27
	}
}

时间运算

import java.text.SimpleDateFormat;
import java.util.Calendar;
public class CalenderTest {
	//时间运算：以目前系统时间 ：2019:01:27 16:27:27 为例
	@Test
	public void testAddCalender() {
		Calendar cal = Calendar.getInstance();    							//Calendar类的实例化
		SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");	//时间的格式化

		String nowTime = sdf.format(cal.getTime());		//nowTime=2019-01-27 16:27:27

		cal.add(Calendar.YEAR, 1);
		String time1 = sdf.format(cal.getTime());		//年:2020-01-27 16:27:27

		cal.add(Calendar.MONTH, 2);
		String time2 = sdf.format(cal.getTime());		//月:2020-03-27 16:27:27

		cal.add(Calendar.DATE, 5);
		String time3 = sdf.format(cal.getTime());		//日:2020-04-01 16:27:27

		cal.add(Calendar.HOUR_OF_DAY, -1);
		String time4 = sdf.format(cal.getTime());		//时:2020-04-01 15:27:27

		cal.add(Calendar.MINUTE, 1);
		String time5 = sdf.format(cal.getTime());		//分:2020-04-01 15:28:27

		cal.add(Calendar.SECOND, 1);
		String time6 = sdf.format(cal.getTime());		//秒:2020-04-01 15:27:28
	}
}