Java 简明教程

Java 8 - New Date-Time API

借助 Java 8，引入了一个新的日期时间 API 来解决旧日期时间 API 的以下缺点。

  1. Not thread safe − java.util.Date 不是线程安全的，因此开发人员在使用日期时必须处理并发问题。新的日期时间 API 是不可变的，并且没有 setter 方法。

  2. Poor design − 默认日期从 1900 年开始，月份从 1 开始，日期从 0 开始，因此不统一。旧 API 的日期操作方法较少。新的 API 为此类操作提供了大量实用方法。

  3. Difficult time zone handling − 开发人员必须编写大量代码来处理时区问题。新的 API 是在考虑领域特定设计的背景下开发的。

Java 8 在 java.time 包下引入了新的日期时间 API。以下是 java.time 包中引入的一些重要类。

  1. Local − 简化的日期时间 API，没有时区处理的复杂性。

  2. Zoned − 处理不同时区的专业日期-时间 API。

Java Local Date-Time API

LocalDate/{@s8} 和 LocalDateTime 类简化了不需要时区的开发过程。让我们看看实际应用。

Example: Local Date-Time API

import java.time.LocalDate;
import java.time.LocalTime;
import java.time.LocalDateTime;
import java.time.Month;

public class Java8Tester {

   public static void main(String args[]) {
      Java8Tester java8tester = new Java8Tester();
      java8tester.testLocalDateTime();
   }

   public void testLocalDateTime() {
      // Get the current date and time
      LocalDateTime currentTime = LocalDateTime.now();
      System.out.println("Current DateTime: " + currentTime);

      LocalDate date1 = currentTime.toLocalDate();
      System.out.println("date1: " + date1);

      Month month = currentTime.getMonth();
      int day = currentTime.getDayOfMonth();
      int seconds = currentTime.getSecond();

      System.out.println("Month: " + month +"day: " + day +"seconds: " + seconds);

      LocalDateTime date2 = currentTime.withDayOfMonth(10).withYear(2012);
      System.out.println("date2: " + date2);

      //12 december 2014
      LocalDate date3 = LocalDate.of(2014, Month.DECEMBER, 12);
      System.out.println("date3: " + date3);

      //22 hour 15 minutes
      LocalTime date4 = LocalTime.of(22, 15);
      System.out.println("date4: " + date4);

      //parse a string
      LocalTime date5 = LocalTime.parse("20:15:30");
      System.out.println("date5: " + date5);
   }
}

它应该产生以下输出 −

Current DateTime: 2014-12-09T11:00:45.457
date1: 2014-12-09
Month: DECEMBERday: 9seconds: 45
date2: 2012-12-10T11:00:45.457
date3: 2014-12-12
date4: 22:15
date5: 20:15:30

Java Zoned Date-Time API

Zoned date-time在考虑时区时，要使用 API。让我们看看实际应用。

Example: Zoned Date-Time API

import java.time.ZonedDateTime;
import java.time.ZoneId;

public class Java8Tester {

   public static void main(String args[]) {
      Java8Tester java8tester = new Java8Tester();
      java8tester.testZonedDateTime();
   }

   public void testZonedDateTime() {
      // Get the current date and time
      ZonedDateTime date1 = ZonedDateTime.parse("2007-12-03T10:15:30+05:30[Asia/Karachi]");
      System.out.println("date1: " + date1);

      ZoneId id = ZoneId.of("Europe/Paris");
      System.out.println("ZoneId: " + id);

      ZoneId currentZone = ZoneId.systemDefault();
      System.out.println("CurrentZone: " + currentZone);
   }
}

它应该产生以下输出 −

date1: 2007-12-03T10:15:30+05:00[Asia/Karachi]
ZoneId: Europe/Paris
CurrentZone: Etc/UTC

Java Chrono Units Enum

java.time.temporal.ChronoUnit enum 添加到 Java 8 中，用于替换旧 API 中用于表示天、月等整数。我们来看看它们是如何运作的。

Example: Chrono Units Enum

import java.time.LocalDate;
import java.time.temporal.ChronoUnit;

public class Java8Tester {

   public static void main(String args[]) {
      Java8Tester java8tester = new Java8Tester();
      java8tester.testChromoUnits();
   }

   public void testChromoUnits() {
      //Get the current date
      LocalDate today = LocalDate.now();
      System.out.println("Current date: " + today);

      //add 1 week to the current date
      LocalDate nextWeek = today.plus(1, ChronoUnit.WEEKS);
      System.out.println("Next week: " + nextWeek);

      //add 1 month to the current date
      LocalDate nextMonth = today.plus(1, ChronoUnit.MONTHS);
      System.out.println("Next month: " + nextMonth);

      //add 1 year to the current date
      LocalDate nextYear = today.plus(1, ChronoUnit.YEARS);
      System.out.println("Next year: " + nextYear);

      //add 10 years to the current date
      LocalDate nextDecade = today.plus(1, ChronoUnit.DECADES);
      System.out.println("Date after ten year: " + nextDecade);
   }
}

它应该产生以下结果 −

Current date: 2014-12-10
Next week: 2014-12-17
Next month: 2015-01-10
Next year: 2015-12-10
Date after ten year: 2024-12-10

Java Period and Duration

在 Java 8 中，引入了两个专用类来处理时间差。

  1. Period − 它处理基于日期的时间量。

  2. Duration − 它处理基于时间的持续时间。

我们来看看它们是如何运作的。

Example: Period and Duration

import java.time.temporal.ChronoUnit;

import java.time.LocalDate;
import java.time.LocalTime;
import java.time.Duration;
import java.time.Period;

public class Java8Tester {

   public static void main(String args[]) {
      Java8Tester java8tester = new Java8Tester();
      java8tester.testPeriod();
      java8tester.testDuration();
   }

   public void testPeriod() {
      //Get the current date
      LocalDate date1 = LocalDate.now();
      System.out.println("Current date: " + date1);

      //add 1 month to the current date
      LocalDate date2 = date1.plus(1, ChronoUnit.MONTHS);
      System.out.println("Next month: " + date2);

      Period period = Period.between(date2, date1);
      System.out.println("Period: " + period);
   }

   public void testDuration() {
      LocalTime time1 = LocalTime.now();
      Duration twoHours = Duration.ofHours(2);

      LocalTime time2 = time1.plus(twoHours);
      Duration duration = Duration.between(time1, time2);

      System.out.println("Duration: " + duration);
   }
}

它应该产生以下输出 −

Current date: 2014-12-10
Next month: 2015-01-10
Period: P-1M
Duration: PT2H

Java Temporal Adjusters

TemporalAdjuster 用于执行日期数学。例如，获取“本月的第二个星期六”或“下个星期二”。我们来看看它们是如何运作的。

Example: Temporal Adjusters

import java.time.LocalDate;
import java.time.temporal.TemporalAdjusters;
import java.time.DayOfWeek;

public class Java8Tester {

   public static void main(String args[]) {
      Java8Tester java8tester = new Java8Tester();
      java8tester.testAdjusters();
   }

   public void testAdjusters() {
      //Get the current date
      LocalDate date1 = LocalDate.now();
      System.out.println("Current date: " + date1);

      //get the next tuesday
      LocalDate nextTuesday = date1.with(TemporalAdjusters.next(DayOfWeek.TUESDAY));
      System.out.println("Next Tuesday on : " + nextTuesday);

      //get the second saturday of next month
      LocalDate firstInYear = LocalDate.of(date1.getYear(),date1.getMonth(), 1);
      LocalDate secondSaturday = firstInYear.with(TemporalAdjusters.nextOrSame(
         DayOfWeek.SATURDAY)).with(TemporalAdjusters.next(DayOfWeek.SATURDAY));
      System.out.println("Second Saturday on : " + secondSaturday);
   }
}

它应该产生以下结果 −

Current date: 2014-12-10
Next Tuesday on : 2014-12-16
Second Saturday on : 2014-12-13

Backward Compatibility

toInstant() 方法已添加到原始日期和日历对象中，可以用来将它们转换为新的 Date-Time API。使用 ofInstant(Insant,ZoneId) 方法获取 LocalDateTime 或 ZonedDateTime 对象。我们来看看它们是如何运作的。

Example of Backward Compatibility

import java.time.LocalDateTime;
import java.time.ZonedDateTime;

import java.util.Date;

import java.time.Instant;
import java.time.ZoneId;

public class Java8Tester {

   public static void main(String args[]) {
      Java8Tester java8tester = new Java8Tester();
      java8tester.testBackwardCompatability();
   }

   public void testBackwardCompatability() {
      //Get the current date
      Date currentDate = new Date();
      System.out.println("Current date: " + currentDate);

      //Get the instant of current date in terms of milliseconds
      Instant now = currentDate.toInstant();
      ZoneId currentZone = ZoneId.systemDefault();

      LocalDateTime localDateTime = LocalDateTime.ofInstant(now, currentZone);
      System.out.println("Local date: " + localDateTime);

      ZonedDateTime zonedDateTime = ZonedDateTime.ofInstant(now, currentZone);
      System.out.println("Zoned date: " + zonedDateTime);
   }
}

它应该产生以下输出 −

Current date: Wed Dec 10 05:44:06 UTC 2014
Local date: 2014-12-10T05:44:06.635
Zoned date: 2014-12-10T05:44:06.635Z[Etc/UTC]