4. java.time.Instant：Instant类是用在机器可读的时间格式上的，它以Unix时间戳的形式存储日期时间，我们来看一个简单的程序。
package com.journaldev.java8.time; import java.time.Duration; import java.time.Instant; public class InstantExample { public static void main(String[] args) { //Current timestamp Instant timestamp = Instant.now(); System.out.println("Current Timestamp = "+timestamp); //Instant from timestamp Instant specificTime = Instant.ofEpochMilli(timestamp.toEpochMilli()); System.out.println("Specific Time = "+specificTime); //Duration example Duration thirtyDay = Duration.ofDays(30); System.out.println(thirtyDay); } } |
<span style="font-family: Georgia, 'Times New Roman', 'Bitstream Charter', Times, serif; font-size: 13px; line-height: 19px;">上述程序的输出是：</span>
Current Timestamp = 2014-04-28T23:20:08.489Z
Specific Time = 2014-04-28T23:20:08.489Z
PT720H
5. 日期API工具：我们早些时候提到过，大多数日期/时间API类都实现了一系列工具方法，如：加/减天数、周数、月份数，等等。还有其他的工具方法能够使用TemporalAdjuster调整日期，并计算两个日期间的周期。
package com.journaldev.java8.time; import java.time.LocalDate; import java.time.LocalTime; import java.time.Period; import java.time.temporal.TemporalAdjusters; public class DateAPIUtilities { public static void main(String[] args) { LocalDate today = LocalDate.now(); //Get the Year, check if it's leap year System.out.println("Year "+today.getYear()+" is Leap Year? "+today.isLeapYear()); //Compare two LocalDate for before and after System.out.println("Today is before 01/01/2015? "+today.isBefore(LocalDate.of(2015,1,1))); //Create LocalDateTime from LocalDate System.out.println("Current Time="+today.atTime(LocalTime.now())); //plus and minus operations System.out.println("10 days after today will be "+today.plusDays(10)); System.out.println("3 weeks after today will be "+today.plusWeeks(3)); System.out.println("20 months after today will be "+today.plusMonths(20)); System.out.println("10 days before today will be "+today.minusDays(10)); System.out.println("3 weeks before today will be "+today.minusWeeks(3)); System.out.println("20 months before today will be "+today.minusMonths(20)); //Temporal adjusters for adjusting the dates System.out.println("First date of this month= "+today.with(TemporalAdjusters.firstDayOfMonth())); LocalDate lastDayOfYear = today.with(TemporalAdjusters.lastDayOfYear()); System.out.println("Last date of this year= "+lastDayOfYear); Period period = today.until(lastDayOfYear); System.out.println("Period Format= "+period); System.out.println("Months remaining in the year= "+period.getMonths()); } } |
上述程序的输出是：
Year 2014 is Leap Year? false
Today is before 01/01/2015? true
Current Time=2014-04-28T16:23:53.154
10 days after today will be 2014-05-08
3 weeks after today will be 2014-05-19
20 months after today will be 2015-12-28
10 days before today will be 2014-04-18
3 weeks before today will be 2014-04-07
20 months before today will be 2012-08-28
First date of this month= 2014-04-01
Last date of this year= 2014-12-31
Period Format= P8M3D
Months remaining in the year= 8
6. 解析和格式化：将一个日期格式转换为不同的格式，之后再解析一个字符串，得到日期时间对象，这些都是很常见的。我们来看一下简单的例子。
package com.journaldev.java8.time; import java.time.Instant; import java.time.LocalDate; import java.time.LocalDateTime; import java.time.format.DateTimeFormatter; public class DateParseFormatExample { public static void main(String[] args) { //Format examples LocalDate date = LocalDate.now(); //default format System.out.println("Default format of LocalDate="+date); //specific format System.out.println(date.format(DateTimeFormatter.ofPattern("d::MMM::uuuu"))); System.out.println(date.format(DateTimeFormatter.BASIC_ISO_DATE)); LocalDateTime dateTime = LocalDateTime.now(); //default format System.out.println("Default format of LocalDateTime="+dateTime); //specific format System.out.println(dateTime.format(DateTimeFormatter.ofPattern("d::MMM::uuuu HH::mm::ss"))); System.out.println(dateTime.format(DateTimeFormatter.BASIC_ISO_DATE)); Instant timestamp = Instant.now(); //default format System.out.println("Default format of Instant="+timestamp); //Parse examples LocalDateTime dt = LocalDateTime.parse("27::Apr::2014 21::39::48", DateTimeFormatter.ofPattern("d::MMM::uuuu HH::mm::ss")); System.out.println("Default format after parsing = "+dt); } } |
当运行以上程序时，可以看到如下输出。
Default format of LocalDate=2014-04-28
28::Apr::2014
20140428
Default format of LocalDateTime=2014-04-28T16:25:49.341
28::Apr::2014 16::25::49
20140428
Default format of Instant=2014-04-28T23:25:49.342Z
Default format after parsing = 2014-04-27T21:39:48
7. 旧的日期时间支持：旧的日期/时间类已经在几乎所有的应用程序中使用，因此做到向下兼容是必须的。这也是为什么会有若干工具方法帮助我们将旧的类转换为新的类，反之亦然。我们来看一下简单的例子。
package com.journaldev.java8.time; import java.time.Instant; import java.time.LocalDateTime; import java.time.ZoneId; import java.time.ZonedDateTime; import java.util.Calendar; import java.util.Date; import java.util.GregorianCalendar; import java.util.TimeZone; public class DateAPILegacySupport { public static void main(String[] args) { //Date to Instant Instant timestamp = new Date().toInstant(); //Now we can convert Instant to LocalDateTime or other similar classes LocalDateTime date = LocalDateTime.ofInstant(timestamp, ZoneId.of(ZoneId.SHORT_IDS.get("PST"))); System.out.println("Date = "+date); //Calendar to Instant Instant time = Calendar.getInstance().toInstant(); System.out.println(time); //TimeZone to ZoneId ZoneId defaultZone = TimeZone.getDefault().toZoneId(); System.out.println(defaultZone); //ZonedDateTime from specific Calendar ZonedDateTime gregorianCalendarDateTime = new GregorianCalendar().toZonedDateTime(); System.out.println(gregorianCalendarDateTime); //Date API to Legacy classes Date dt = Date.from(Instant.now()); System.out.println(dt); TimeZone tz = TimeZone.getTimeZone(defaultZone); System.out.println(tz); GregorianCalendar gc = GregorianCalendar.from(gregorianCalendarDateTime); System.out.println(gc); } } |
当运行以上程序时，可以看到如下输出。
Date = 2014-04-28T16:28:54.340
2014-04-28T23:28:54.395Z
America/Los_Angeles
2014-04-28T16:28:54.404-07:00[America/Los_Angeles]
Mon Apr 28 16:28:54 PDT 2014
sun.util.calendar.ZoneInfo[id="America/Los_Angeles",offset=-28800000,dstSavings=3600000,useDaylight=true,transitions=185,lastRule=java.util.SimpleTimeZone[id=America/Los_Angeles,offset=-28800000,dstSavings=3600000,useDaylight=true,startYear=0,startMode=3,startMonth=2,startDay=8,startDayOfWeek=1,startTime=7200000,startTimeMode=0,endMode=3,endMonth=10,endDay=1,endDayOfWeek=1,endTime=7200000,endTimeMode=0]]
java.util.GregorianCalendar[time=1398727734404,areFieldsSet=true,areAllFieldsSet=true,lenient=true,zone=sun.util.calendar.ZoneInfo[id="America/Los_Angeles",offset=-28800000,dstSavings=3600000,useDaylight=true,transitions=185,lastRule=java.util.SimpleTimeZone[id=America/Los_Angeles,offset=-28800000,dstSavings=3600000,useDaylight=true,startYear=0,startMode=3,startMonth=2,startDay=8,startDayOfWeek=1,startTime=7200000,startTimeMode=0,endMode=3,endMonth=10,endDay=1,endDayOfWeek=1,endTime=7200000,endTimeMode=0]],firstDayOfWeek=2,minimalDaysInFirstWeek=4,ERA=1,YEAR=2014,MONTH=3,WEEK_OF_YEAR=18,WEEK_OF_MONTH=5,DAY_OF_MONTH=28,DAY_OF_YEAR=118,DAY_OF_WEEK=2,DAY_OF_WEEK_IN_MONTH=4,AM_PM=1,HOUR=4,HOUR_OF_DAY=16,MINUTE=28,SECOND=54,MILLISECOND=404,ZONE_OFFSET=-28800000,DST_OFFSET=3600000]
你可以看到，旧的TimeZone和GregorianCalendar类的toString()方法太啰嗦了，一点都不友好。
这就是所有的Java 8 日期/时间API的内容，我非常喜欢这个API，它易于使用，同时它采取了某项工作，使相似的方法也易于寻找，虽然从旧的类转移到新的日期时间类需要消耗一定的时间，但我相信这是值得的。
