Java日期

Java8之前的传统日期API

Date转换成字符串

import java.text.SimpleDateFormat;
import java.util.Date;

/**
 * @author Pickle
 * @version V1.0
 * @date 2024/3/7 14:43
 */
public class Demo {
    public static void main(String[] args) {
        //创建Date类，构造的时候填入当前的日期
        Date date = new Date();
        System.out.println(date);
        //获得秒
        final long time = date.getTime();
        System.out.println(time);
        //将Date转换成更好识别的日期字符串
        final SimpleDateFormat format = new SimpleDateFormat("yyyy年MM月dd日 HH:mm:ss E a");
        final String format1 = format.format(date);
        System.out.println(format1);
        //将秒转换成更好识别的日期字符串
        final String format2 = format.format(time);
        System.out.println(format2);
    }
}

字符串解析成Date对象

    //将字符串的时间解析成日期对象
    String str = "2024-03-07 15:46:50";
    final SimpleDateFormat format3 = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    try {
        final Date parse = format3.parse(str);
        System.out.println(parse);
    } catch (ParseException e) {
        e.printStackTrace();
    }

Calendar对象

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;

/**
 * @author Pickle
 * @version V1.0
 * @date 2024/3/7 14:43
 */


public class Demo {
    public static void main(String[] args) {
        final Calendar calendar  = Calendar.getInstance();
        System.out.println(calendar);
        //输出今天时今年的第几天
        final int dayOfYear = calendar.get(Calendar.DAY_OF_YEAR);
        System.out.println(dayOfYear);
        //修改年份
        calendar.set(Calendar.YEAR, 2025);
        //获取输出年份
        final int year = calendar.get(Calendar.YEAR);
        System.out.println(year);
    }
}

Java开始新增的日期和时间

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;

/**
 * @author Pickle
 * @version V1.0
 * @date 2024/3/7 14:43
 */


public class Demo {
    public static void main(String[] args) {
        final Date date = new Date();
        final int year = date.getYear();
        //1.设计不合理，使用不方便
        System.out.println(year + 1900);    //这里得到的年份还要-1900
        Calendar cal = Calendar.getInstance();
        final int month = cal.get(Calendar.MONTH);  //这里得到月份还要自己记忆Calendar.MONTH
        System.out.println(month);
        //2.传统的日期都是可变的对象，修改后丢失原始的信息
        //3.传统的日期是线程不安全的
    }
}

分类

LocalDate

import java.time.DayOfWeek;
import java.time.LocalDate;

/**
 * @author Pickle
 * @version V1.0
 * @date 2024/3/7 16:42
 */
public class LocalDateDemo {
    public static void main(String[] args) {
        //获取本地日期对象（不可变对象）
        final LocalDate now = LocalDate.now();
        //1.获取对象中的信息
        System.out.println(now);    //2024-03-07
        final int year = now.getYear();
        System.out.println(year);    //2024
        final int month = now.getMonthValue();
        System.out.println(month);  //3
        final int day = now.getDayOfMonth();
        System.out.println(day);  //7
        final int dayOfYear = now.getDayOfYear();
        System.out.println(dayOfYear);//67
        final DayOfWeek dayOfWeek = now.getDayOfWeek();
        System.out.println(dayOfWeek); //THURSDAY
        //2.直接修改某个信息
        final LocalDate now1 = now.withYear(2099);
        final LocalDate now2 = now.withMonth(12);
        System.out.println(now);    //2024-03-07
        System.out.println(now1);   //2099-03-07
        System.out.println(now2);   //2024-12-07

        final LocalDate now3 = now.plusYears(2);    //2026-03-07
        System.out.println(now3);

        final LocalDate now4 = now.minusYears(2);
        System.out.println(now4);                   //2022-03-07

        final LocalDate now5 = LocalDate.of(2008, 12, 12);
        final LocalDate now6 = LocalDate.of(2008, 12, 22);
        System.out.println(now5);   //2008-12-12
        System.out.println(now6);   //2008-12-22

        System.out.println(now5.equals(now6));      //false
        System.out.println(now5.isBefore(now6));    //true
        System.out.println(now5.isAfter(now6));     //false

    }
}

LocalTime

import java.time.LocalTime;
/**
 * @author Pickle
 * @version V1.0
 * @date 2024/3/7 18:38
 */
public class LocalTimeDemo {
    public static void main(String[] args) {
        final LocalTime now = LocalTime.now();
        System.out.println(now);    //18:38:47.606559700

        //1.获取事件中的信息
        System.out.println(now.getHour());      //时
        System.out.println(now.getMinute());    //分
        System.out.println(now.getSecond());    //秒
        System.out.println(now.getNano());      //纳秒

        //2.修改时间 withHour,withMinute,withSecond,withNano

        //3.加时间 plusHours,plusMinutes,plusSecond,plusNanos

        //4.减多少 
        
        //...同LocalDate
    }
}

LocalDateTime

和LocalDate、LocalDateTime类似
但是LocalDateTime可以分成LocalDate和LocalTime，LocalTime和LocalDate也可以合成LocalDateTime