获取简单的一些日期和日期格式转换

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.time.LocalDateTime;
import java.util.Calendar;
import java.util.Date;
public class test {public static void main(String[] args) throws ParseException {//日期里面的MM要大写，时间里的HH大写是24小时制，小写是12小时制SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:dd");//1.获取当前系统日期，转成String类型String data1 = sdf.format(new Date());System.out.println(data1);//2.String类型转成Date类型Date date2 = sdf.parse(data1);System.out.println(date2);//3.获取当前系统时间的后多少天LocalDateTime endTime = LocalDateTime.now().plusDays(15L);System.out.println(endTime);//获取当前系统时间的前多少天LocalDateTime begin = LocalDateTime.now().plusDays(-15L);System.out.println(begin);//4.获取当前系统时间的后多少天Calendar calendar = Calendar.getInstance();int day=calendar.get(Calendar.DATE);//此处修改为+1则是获取后一天calendar.set(Calendar.DATE,day+3);String lastDay = sdf.format(calendar.getTime());Date san=sdf.parse(lastDay);System.out.println(san);//5.获取系统当前月份的第一天和最后一天//获取当前月份的第一天Calendar calendars = Calendar.getInstance();calendars.set(Calendar.DAY_OF_MONTH,calendars.getActualMinimum(Calendar.DAY_OF_MONTH));SimpleDateFormat firstDay = new SimpleDateFormat("yyyy-MM-dd");String begins = firstDay.format(calendars.getTime());System.out.println(begins);//获取当前月份的最后一天calendars.set(Calendar.DAY_OF_MONTH,calendars.getActualMaximum(Calendar.DAY_OF_MONTH));SimpleDateFormat lastDays= new SimpleDateFormat("yyyy-MM-dd");String end = lastDays.format(calendars.getTime());System.out.println(end);}
}

String类型的日期相互比较

    public static void main(String[] args) throws ParseException {//判断某个日期是否在两个日期范围之内SimpleDateFormat simpleDateFormat =new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");//用户先择的时间String date = "2020-05-19";String time = "11:23:22";String da = date.concat(" "+time);Date dae=simpleDateFormat.parse(da);//当前的时间的后三天Calendar calendar = Calendar.getInstance();int day=calendar.get(Calendar.DATE);//此处修改为+1则是获取后一天calendar.set(Calendar.DATE,day+3);String lastDay = simpleDateFormat.format(calendar.getTime());Date san=simpleDateFormat.parse(lastDay);//当前时间的后十五天calendar.set(Calendar.DATE,day+15);String shiWu = simpleDateFormat.format(calendar.getTime());Date shiWus =simpleDateFormat.parse(shiWu);if(dae.getTime()>=san.getTime() && dae.getTime()<=shiWus.getTime()){System.out.println("在里面");}else{System.out.println("不在里面");}

LocalDateTime 类型的日期相互比较

    public static void main(String[] args) {//用户先择的时间String date = "2020-05-29";String time = "11:23:22";LocalDateTime endTime = LocalDateTime.now().plusDays(15L);LocalDateTime beginTime = LocalDateTime.now().plusDays(3L);LocalDateTime userTime = LocalDateTime.from(DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss").parse(date + " " + time));if (userTime.isAfter(beginTime) && userTime.isBefore(endTime)){System.out.printf("在这之间");}else{System.out.printf("不在这之间");}}