今天主要学习JDK8新增日期工具类

1. LocalDate:获取本地日期

获取系统日期和时间

public class Test1 {public static void main(String[] args) {LocalDate date=LocalDate.now();System.out.println(date);LocalTime time=LocalTime.now();System.out.println(time);LocalDateTime dateTime=LocalDateTime.now();System.out.println(dateTime);}
}

运行结果：

可以看到，最后日期和时间中间有个T，这就需要去瞅瞅LocalDateTime的toString方法了

public String toString() {return date.toString() + 'T' + time.toString();}

自定义设置系统日期和时间

public class Test1 {public static void main(String[] args) {LocalDate date1=LocalDate.of(2023,12,22);LocalTime time1=LocalTime.of(11,33,35);LocalDateTime dateTime1=LocalDateTime.of(date1,time1);System.out.println(dateTime1);}
}

和上面得到的结果是一样的

2. 单独获取日期时间类中的每个值

获取年份

public class Test2 {public static void main(String[] args) {//获取年份LocalDateTime localDateTime=LocalDateTime.now();int year = localDateTime.getYear();System.out.println("year = " + year);//2023}
}

获取月份-------------此处有需要注意的地方

public class Test2 {public static void main(String[] args) {LocalDateTime localDateTime=LocalDateTime.now();//获取月份Month month = localDateTime.getMonth();//枚举类型System.out.println("month = " + month);//方式一：int monthValue = month.getValue();System.out.println("monthValue = " + monthValue);//方式二：int monthValue1 = localDateTime.getMonthValue();System.out.println("monthValue1 = " + monthValue1);}
}

运行结果：

可以看到 moth的值为DECEMBER，是枚举类型，我们去瞅一下源码吧

public enum Month implements TemporalAccessor, TemporalAdjuster {/*** The singleton instance for the month of January with 31 days.* This has the numeric value of {@code 1}.*/JANUARY,/*** The singleton instance for the month of February with 28 days, or 29 in a leap year.* This has the numeric value of {@code 2}.*/FEBRUARY,/*** The singleton instance for the month of March with 31 days.* This has the numeric value of {@code 3}.*/MARCH,/*** The singleton instance for the month of April with 30 days.* This has the numeric value of {@code 4}.*/APRIL,/*** The singleton instance for the month of May with 31 days.* This has the numeric value of {@code 5}.*/MAY,/*** The singleton instance for the month of June with 30 days.* This has the numeric value of {@code 6}.*/JUNE,/*** The singleton instance for the month of July with 31 days.* This has the numeric value of {@code 7}.*/JULY,/*** The singleton instance for the month of August with 31 days.* This has the numeric value of {@code 8}.*/AUGUST,/*** The singleton instance for the month of September with 30 days.* This has the numeric value of {@code 9}.*/SEPTEMBER,/*** The singleton instance for the month of October with 31 days.* This has the numeric value of {@code 10}.*/OCTOBER,/*** The singleton instance for the month of November with 30 days.* This has the numeric value of {@code 11}.*/NOVEMBER,/*** The singleton instance for the month of December with 31 days.* This has the numeric value of {@code 12}.*/DECEMBER;

        //获取第几天int dayOfMonth = localDateTime.getDayOfMonth();System.out.println("dayOfMonth = " + dayOfMonth);

与月份一样的还有星期几 同样是枚举类型

        //获取星期几DayOfWeek dayOfWeek = localDateTime.getDayOfWeek();System.out.println("dayOfWeek = " + dayOfWeek);//FRIDAYint dayOfWeekValue = dayOfWeek.getValue();System.out.println("dayOfWeekValue = " + dayOfWeekValue);//5

        //获取小时int hour = localDateTime.getHour();System.out.println("hour = " + hour);//获取分钟int minute = localDateTime.getMinute();System.out.println("minute = " + minute);//获取秒钟int second = localDateTime.getSecond();System.out.println("second = " + second);

3. 使用给定值修改日期

		LocalDate date=LocalDate.now();System.out.println("date = " + date);//2023-12-22LocalDate date1 = date.withYear(2022);System.out.println("date1 = " + date1);//2022-12-22LocalDate date2 = date.withMonth(2);System.out.println("date2 = " + date2);//2023-02-22LocalDate date3 = date.withDayOfMonth(5);System.out.println("date3 = " + date3);//2023-12-05

		LocalDate date=LocalDate.now();System.out.println("date = " + date);//2023-12-22LocalDate date4 = date.withDayOfMonth(32);System.out.println("date4 = " + date4);

此时会报错：因为设置的天数超出范围

		LocalDate date5 = date.withYear(2021)//修改年份.withMonth(6)//修改月份.withDayOfMonth(16);//修改天数System.out.println("date5 = " + date5);//2021-06-16

4. 设置日期和时间的偏移量

public class Test4 {public static void main(String[] args) {LocalDate date=LocalDate.now();System.out.println("date = " + date);//2023-12-22//5天后LocalDate date1 = date.plusDays(5);System.out.println("date1 = " + date1);//2023-12-27//3天前LocalDate date2 = date.minusDays(3);System.out.println("date2 = " + date2);//2023-12-19//1个月之后LocalDate date3 = date.plusMonths(1);System.out.println("date3 = " + date3);//2024-01-22//7个月之前LocalDate date4 = date.minusMonths(7);System.out.println("date4 = " + date4);//2023-05-22//2周之后LocalDate date5 = date.plusWeeks(2);System.out.println("date5 = " + date5);//2024-01-05//3年之前LocalDate date6 = date.minusYears(3);System.out.println("date6 = " + date6);//2020-12-22}
}

5. Instant类

Instant类：代表时间点，获取日期变更子午线
其实这个类和Date类很相似

public class Test5 {public static void main(String[] args) {//获取当前时间LocalDateTime localDateTime=LocalDateTime.now();System.out.println("localDateTime = " + localDateTime);//获取日期变更子午线时间Instant instant=Instant.now();System.out.println("instant = " + instant);}
}

运行结果：

我们可以看到，两者的时间表示是一样的，但是得到的时间差8个小时，这是因为Instant类是获取日期变更子午线时间，而我们在东八区呀

获取最大值和最小值

//获取最大值和最小值Instant max = Instant.MAX;System.out.println("max = " + max);Instant min = Instant.MIN;System.out.println("min = " + min);

执行结果：

获取时间戳

//获取时间戳long l = instant.toEpochMilli();System.out.println("l = " + l);System.out.println(System.currentTimeMillis());

执行结果：

可以看到，这两者得到的时间是一样的

获取北京时间 ---- 具有偏移量的日期时间对象

public class Test5 {public static void main(String[] args) {//获取当前时间LocalDateTime localDateTime=LocalDateTime.now();System.out.println("localDateTime = " + localDateTime);//获取日期变更子午线时间Instant instant=Instant.now();System.out.println("instant = " + instant);//具有偏移量的日期时间对象//获取北京时间//方式一：OffsetDateTime offsetDateTime = instant.atOffset(ZoneOffset.ofHours(8));//东八区System.out.println("offsetDateTime = " + offsetDateTime);//方式二：OffsetDateTime offsetDateTime1 = instant.atOffset(ZoneOffset.of("+8"));//东八区System.out.println("offsetDateTime1 = " + offsetDateTime1);}
}

将offsetDateTime转为localDateTime
将时间戳转为 Instant对象

//将offsetDateTime转为localDateTimeInstant instant=Instant.now();OffsetDateTime offsetDateTime = instant.atOffset(ZoneOffset.ofHours(8));LocalDateTime dateTime = offsetDateTime.toLocalDateTime();System.out.println("dateTime = " + dateTime);//将时间戳转为 Instant对象Instant instant1 = Instant.ofEpochMilli(System.currentTimeMillis());System.out.println("instant1 = " + instant1);//此时拿到的时间是日期变更线的时间

执行结果：

6. DateTimeFormatter格式化和解析

6.1 将LocalDate转换成字符串String格式

public class Test6 {public static void main(String[] args) {//将LocalDate转换成字符串String格式//1.定义好要转换的格式DateTimeFormatter formatter=DateTimeFormatter.ofPattern("yyyy-MM-dd");DateTimeFormatter formatter1=DateTimeFormatter.ofPattern("yyyy年MM月dd日");DateTimeFormatter formatter2=DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");LocalDate date=LocalDate.now();LocalDateTime dateTime = LocalDateTime.now();//2.进行格式转换String s = formatter.format(date);String s1 = formatter1.format(date);String s2 = formatter2.format(dateTime);System.out.println("s = " + s);System.out.println("s1 = " + s1);System.out.println("s2 = " + s2);}
}

运行结果：

6.2 将时间戳转换成字符串String格式

//将时间戳转换成字符串String格式long l = Instant.now().toEpochMilli();LocalDateTime dateTime = LocalDateTime.ofInstant(Instant.ofEpochMilli(l), ZoneId.of("Asia/Shanghai"));DateTimeFormatter formatter=DateTimeFormatter.ofPattern("yyyy-MM-dd");String s = formatter.format(dateTime);System.out.println("s = " + s);//s = 2023-12-23

在转换时，可以发现，需要我们输入时区
那这个时区该怎样获取呢？

获取系统默认时区

        //获取系统默认时区ZoneId zoneId = ZoneId.systemDefault();System.out.println("zoneId = " + zoneId);//zoneId = Asia/Shanghai

6.3 将字符串解析成日期

        //将字符串解析成日期String str="2023-09-25";DateTimeFormatter formatter=DateTimeFormatter.ofPattern("yyyy年MM月dd日");LocalDate parse = LocalDate.parse(str, formatter);System.out.println("parse = " + parse);

运行上面代码，我们会发现报错了：

这是因为字符串格式和要解析成的格式不同
这也是我们需要特别注意的地方，两者格式一定要相同才能进行解析
下面是正确的代码：

	//将字符串解析成日期String str="2023-09-25";DateTimeFormatter formatter=DateTimeFormatter.ofPattern("yyyy-MM-dd");LocalDate parse = LocalDate.parse(str, formatter);System.out.println("parse = " + parse);

6.4 将字符串解析成日期和时间

//将字符串解析成日期和时间String str="2023年09月26日 12:30:40";DateTimeFormatter formatter=DateTimeFormatter.ofPattern("yyyy年MM月dd日 HH:mm:ss");LocalDateTime parse = LocalDateTime.parse(str, formatter);System.out.println("parse = " + parse);//parse = 2023-09-26T12:30:40

7. Period类 计算日期之间的间隔

Period类计算两个“日期”之间的间隔，得到的是年月日，假设计算2022年10月1日和2023年9月3日之间相差多少天，得到相差0年11月2天，没有办法知道具体多少天

public class Test7 {public static void main(String[] args) {LocalDate date1=LocalDate.of(2022,10,1);LocalDate date2=LocalDate.of(2023,9,3);Period period=Period.between(date1,date2);int years = period.getYears();//获取相差年数System.out.println("years = " + years);//0   System.out.println(period.getMonths());//11   获取相差月数System.out.println(period.getDays());//2   获取相差天数}
}

如果想知道具体相差多少天，可以使用ChronoUnit类和toEpochDay()方法
但是感觉还是ChronoUnit类好用一点，它是一个枚举类

//计算两个日期相差的天数//方法一：LocalDate date1=LocalDate.of(2022,10,1);LocalDate date2=LocalDate.of(2023,9,3);long days = ChronoUnit.DAYS.between(date1, date2);System.out.println("days = " + days);//days = 337long months = ChronoUnit.MONTHS.between(date1, date2);System.out.println("months = " + months);//months = 11//方法二：long days1 = date2.toEpochDay() - date1.toEpochDay();System.out.println("days1 = " + days1);//days1 = 337

8. Duration 计算两个日期时间的间隔

方式一：

public class Test8 {public static void main(String[] args) {//计算两个日期时间和间隔LocalDateTime dateTime=LocalDateTime.of(2023,12,10,12,35,40);LocalDateTime now = LocalDateTime.now();//计算方式1：Duration duration = Duration.between(dateTime, now);long days = duration.toDays();System.out.println("days = " + days);//13long hours = duration.toHours();System.out.println("hours = " + hours);//312long seconds = duration.toSeconds();System.out.println("seconds = " + seconds);//1125091}
}

方式二：

		LocalDateTime dateTime=LocalDateTime.of(2022,12,10,12,35,40);LocalDateTime now = LocalDateTime.now();long years = dateTime.until(now, ChronoUnit.YEARS);System.out.println("years = " + years);//1long months = dateTime.until(now, ChronoUnit.MONTHS);System.out.println("months = " + months);//12long days = dateTime.until(now, ChronoUnit.DAYS);System.out.println("days = " + days);//378

方式三：

        LocalDateTime dateTime=LocalDateTime.of(1923,12,10,12,35,40);LocalDateTime now = LocalDateTime.now();long years = ChronoUnit.YEARS.between(dateTime, now);//相差年数System.out.println("years = " + years);//100long centuries = ChronoUnit.CENTURIES.between(dateTime, now);//相差的世纪System.out.println("centuries = " + centuries);//1

9. 时间调节器

//时间调节器 TemporalAdjusters
public class Test9 {public static void main(String[] args) {LocalDate date=LocalDate.now();//获取本周一时间LocalDate date1 = date.with(DayOfWeek.MONDAY);System.out.println("date1 = " + date1);//获取下周一时间LocalDate date2 = date.with(TemporalAdjusters.next(DayOfWeek.MONDAY));System.out.println("date2 = " + date2);//获取本月第一天LocalDate date3 = date.with(TemporalAdjusters.firstDayOfMonth());System.out.println("date3 = " + date3);//获取本月最后一天LocalDate date4 = date.with(TemporalAdjusters.lastDayOfMonth());System.out.println("date4 = " + date4);//获取本年第一天LocalDate date5 = date.with(TemporalAdjusters.firstDayOfYear());System.out.println("date5 = " + date5);}
}

10. Date类和LocalDate(LocalTime / LocalDateTime)转换

10.1 LocalDate转Date

public class Test10 {public static void main(String[] args) {//LocalDate转DateLocalDate localDate=LocalDate.now();ZonedDateTime zonedDate = localDate.atStartOfDay().atZone(ZoneId.systemDefault());Instant instant = zonedDate.toInstant();Date date = Date.from(instant);String s = new SimpleDateFormat("yyyy-MM-dd").format(date);System.out.println("s = " + s);//s = 2023-12-23}
}

10.2 LocalDateTime转Date

public class Test10 {public static void main(String[] args) {//LocalDateTime转DateLocalDateTime localDateTime=LocalDateTime.now();ZonedDateTime zonedDateTime = localDateTime.atZone(ZoneId.systemDefault());Instant instant1 = zonedDateTime.toInstant();Date dateTime=Date.from(instant1);String s1=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").format(dateTime);System.out.println("s1 = " + s1);//s1 = 2023-12-23 13:50:42   }
}

10.3 Date转LocalDate

public class Test10 {public static void main(String[] args) {//Date转LocalDateDate date=new Date();ZonedDateTime zonedDateTime = date.toInstant().atZone(ZoneId.systemDefault());LocalDate localDate = zonedDateTime.toLocalDate();System.out.println("localDate = " + localDate);//localDate = 2023-12-23}
}

10.4 Date转LocalDateTime

public class Test10 {public static void main(String[] args) {   //Date转LocalDateTimeDate date=new Date();ZonedDateTime zonedDateTime = date.toInstant().atZone(ZoneId.systemDefault());LocalDateTime localDateTime = zonedDateTime.toLocalDateTime();//localDateTime = 2023-12-23T13:58:07.449System.out.println("localDateTime = " + localDateTime);DateTimeFormatter formatter=DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");String s = formatter.format(localDateTime);// s = 2023-12-23 13:59:17System.out.println("s = " + s);}
}