为什么使用新的java日期/时间API?
1.Java的日期/时间类的定义并不一致，在java.util和java.sql的包中都有日期类，此外用于格式化和解析的类在java.text包中定义。
2.java.util.Date同时包含日期和时间，而java.sql.Date仅包含日期，将其纳入java.sql包并不合理。另外这两个类都有相同的名字，这本身就是一个非常糟糕的设计。
3.对于时间，时间戳、格式化以及解析，并没有一些明确定义的类。对于格式化和解析用到的是java.text.DateFormat抽象类，使用SimoleDateFormat类
4.所有的日期类都是可变的，在多线程中是不安全的。**
5.日期类不提供国际化，没有失去支持，所以引入了java.util.Canlendar和java.util.TimeZone类。
java 8日期/时期API
1.不变性：新的日志/时间API中，所有的类都是不可变，对多线程环境有好处
2.新的API将人可读的日期时间和机器时间（unix timestamp）明确分离，它为日期（Date）、时间（Time）、日期时间（DateTime）、时间戳（unix timestamp）以及时区定义了不同的类。
3.清晰：在所有的类中，方法都被明确定义用以完成相同的行为。举个例子，要拿到当前实例我们可以使用now()方法，在所有的类中都定义了format()和parse()方法，而不是像以前那样专门有一个独立的类。为了更好的处理问题，所有的类都使用了工厂模式和策略模式，一旦你使用了其中某个类的方法，与其他类协同工作并不困难。
4.实用操作：所有新的日期/时间API类都实现了一系列方法用以完成通用的任务，如：加、减、格式化、解析、从日期/时间中提取单独部分，等等。
1. java.time.LocalDate：LocalDate是一个不可变的类，它表示默认格式(yyyy-MM-dd)的日期，我们可以使用now()方法得到当前时间，也可以提供输入年份、月份和日期的输入参数来创建一个LocalDate实例。该类为now()方法提供了重载方法，我们可以传入ZoneId来获得指定时区的日期。该类提供与java.sql.Date相同的功能，对于如何使用该类，我们来看一个简单的例子。
package com.journaldev.java8.time;
import java.time.LocalDate;
import java.time.Month;
import java.time.ZoneId;
/**
* LocalDate Examples
* @author pankaj
*
*/
public class LocalDateExample {
public static void main(String[] args) {
//Current Date
LocalDate today = LocalDate.now();
System.out.println("Current Date="+today);
//Creating LocalDate by providing input arguments
LocalDate firstDay_2014 = LocalDate.of(2014, Month.JANUARY, 1);
System.out.println("Specific Date="+firstDay_2014);
//Try creating date by providing invalid inputs
//LocalDate feb29_2014 = LocalDate.of(2014, Month.FEBRUARY, 29);
//Exception in thread "main" java.time.DateTimeException:
//Invalid date 'February 29' as '2014' is not a leap year
//Current date in "Asia/Kolkata", you can get it from ZoneId javadoc
LocalDate todayKolkata = LocalDate.now(ZoneId.of("Asia/Kolkata"));
System.out.println("Current Date in IST="+todayKolkata);
//java.time.zone.ZoneRulesException: Unknown time-zone ID: IST
//LocalDate todayIST = LocalDate.now(ZoneId.of("IST"));
//Getting date from the base date i.e 01/01/1970
LocalDate dateFromBase = LocalDate.ofEpochDay(365);
System.out.println("365th day from base date= "+dateFromBase);
LocalDate hundredDay2014 = LocalDate.ofYearDay(2014, 100);
System.out.println("100th day of 2014="+hundredDay2014);
}
}
结果：
Current Date=2014-04-28
Specific Date=2014-01-01
Current Date in IST=2014-04-29
365th day from base date= 1971-01-01
100th day of 2014=2014-04-10
2. java.time.LocalTime：LocalTime是一个不可变的类，它的实例代表一个符合人类可读格式的时间，默认格式是hh:mm:ss.zzz。像LocalDate一样，该类也提供了时区支持，同时也可以传入小时、分钟和秒等输入参数创建实例，我们来看一个简单的程序，演示该类的使用方法。
package com.journaldev.java8.time;
import java.time.LocalTime;
import java.time.ZoneId;
/**
* LocalTime Examples
* @author pankaj
*
*/
public class LocalTimeExample {
public static void main(String[] args) {
//Current Time
LocalTime time = LocalTime.now();
System.out.println("Current Time="+time);
//Creating LocalTime by providing input arguments
LocalTime specificTime = LocalTime.of(12,20,25,40);
System.out.println("Specific Time of Day="+specificTime);
//Try creating time by providing invalid inputs
//LocalTime invalidTime = LocalTime.of(25,20);
//Exception in thread "main" java.time.DateTimeException:
//Invalid value for HourOfDay (valid values 0 - 23): 25
//Current date in "Asia/Kolkata", you can get it from ZoneId javadoc
LocalTime timeKolkata = LocalTime.now(ZoneId.of("Asia/Kolkata"));
System.out.println("Current Time in IST="+timeKolkata);
//java.time.zone.ZoneRulesException: Unknown time-zone ID: IST
//LocalTime todayIST = LocalTime.now(ZoneId.of("IST"));
//Getting date from the base date i.e 01/01/1970
LocalTime specificSecondTime = LocalTime.ofSecondOfDay(10000);
System.out.println("10000th second time= "+specificSecondTime);
}
}
结果：Current Time=15:51:45.240
Specific Time of Day=12:20:25.000000040
Current Time in IST=04:21:45.276
10000th second time= 02:46:40
3. java.time.LocalDateTime：LocalDateTime是一个不可变的日期-时间对象，它表示一组日期-时间，默认格式是yyyy-MM-dd-HH-mm-ss.zzz。它提供了一个工厂方法，接收LocalDate和LocalTime输入参数，创建LocalDateTime实例。我们来看一个简单的例子。
package com.journaldev.java8.time;
import java.time.LocalDate;
import java.time.LocalDateTime;
import java.time.LocalTime;
import java.time.Month;
import java.time.ZoneId;
import java.time.ZoneOffset;
public class LocalDateTimeExample {
public static void main(String[] args) {
//Current Date
LocalDateTime today = LocalDateTime.now();
System.out.println("Current DateTime="+today);
//Current Date using LocalDate and LocalTime
today = LocalDateTime.of(LocalDate.now(), LocalTime.now());
System.out.println("Current DateTime="+today);
//Creating LocalDateTime by providing input arguments
LocalDateTime specificDate = LocalDateTime.of(2014, Month.JANUARY, 1, 10, 10, 30);
System.out.println("Specific Date="+specificDate);
//Try creating date by providing invalid inputs
//LocalDateTime feb29_2014 = LocalDateTime.of(2014, Month.FEBRUARY, 28, 25,1,1);
//Exception in thread "main" java.time.DateTimeException:
//Invalid value for HourOfDay (valid values 0 - 23): 25
//Current date in "Asia/Kolkata", you can get it from ZoneId javadoc
LocalDateTime todayKolkata = LocalDateTime.now(ZoneId.of("Asia/Kolkata"));
System.out.println("Current Date in IST="+todayKolkata);
//java.time.zone.ZoneRulesException: Unknown time-zone ID: IST
//LocalDateTime todayIST = LocalDateTime.now(ZoneId.of("IST"));
//Getting date from the base date i.e 01/01/1970
LocalDateTime dateFromBase = LocalDateTime.ofEpochSecond(10000, 0, ZoneOffset.UTC);
System.out.println("10000th second time from 01/01/1970= "+dateFromBase);
}
}
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);
}
}
结果：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());
}
}
结果：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