Java 简明教程

Java - Date and Time

Java 提供了 Date class 可用于 java.util package ，这个类封装了当前的日期和时间。

Java provides the Date class available in java.util package, this class encapsulates the current date and time.

Date 类支持两种构造函数，如下表所示。

The Date class supports two constructors as shown in the following table.

Sr.No.

Constructor & Description

1

Date( ) This constructor initializes the object with the current date and time.

2

Date(long millisec) This constructor accepts an argument that equals the number of milliseconds that have elapsed since midnight, January 1, 1970.

以下是日期类的各个方法。

Following are the methods of the date class.

Sr.No.

Method & Description

1

boolean after(Date when)This method tests if this date is after the specified date.

2

boolean before(Date when)This method tests if this date is before the specified date.

3

Object clone()This method return a copy of this object.

4

int compareTo(Date anotherDate)This method compares two Dates for ordering.

5

boolean equals(Object obj)This method compares two dates for equality.

6

static Date from​(Instant instant)This method obtains an instance of Date from an Instant object.

7

long getTime()This method returns the number of milliseconds since January 1, 1970, 00:00:00 GMT represented by this Date object.

8

int hashCode()This method returns a hash code value for this object.

9

void setTime(long time)This method sets this Date object to represent a point in time that is time milliseconds after January 1, 1970 00:00:00 GMT.

10

Instant toInstant()This method converts this Date object to an Instant.

11

String toString()This method converts this Date object to a String of the form.

Getting Current Date and Time

这是一种非常简单的方法，可以在 Java 中获取当前日期和时间。您可以使用带 toString() 方法的简单 Date 对象来打印当前日期和时间，如下所示：

This is a very easy method to get current date and time in Java. You can use a simple Date object with toString() method to print the current date and time as follows −

Example

import java.util.Date;
public class DateDemo {

   public static void main(String args[]) {
      // Instantiate a Date object
      Date date = new Date();

      // display time and date using toString()
      System.out.println(date.toString());
   }
}

这会产生以下结果 −

This will produce the following result −

on May 04 09:51:52 CDT 2009

Date Comparison

以下是比较两个日期的三种方法：

Following are the three ways to compare two dates −

  1. You can use getTime( ) to obtain the number of milliseconds that have elapsed since midnight, January 1, 1970, for both objects and then compare these two values.

  2. You can use the methods before( ), after( ), and equals( ). Because the 12th of the month comes before the 18th, for example, new Date(99, 2, 12).before(new Date (99, 2, 18)) returns true.

  3. You can use the compareTo( ) method, which is defined by the Comparable interface and implemented by Date.

Date Formatting Using SimpleDateFormat

SimpleDateFormat 是一个用于以区域敏感方式格式化和解析日期的具体类。通过 SimpleDateFormat，您可以首先为日期时间格式化选择任何用户定义的模式。

SimpleDateFormat is a concrete class for formatting and parsing dates in a locale-sensitive manner. SimpleDateFormat allows you to start by choosing any user-defined patterns for date-time formatting.

Example

import java.util.*;
import java.text.*;

public class DateDemo {

   public static void main(String args[]) {
      Date dNow = new Date( );
      SimpleDateFormat ft =
      new SimpleDateFormat ("E yyyy.MM.dd 'at' hh:mm:ss a zzz");

      System.out.println("Current Date: " + ft.format(dNow));
   }
}

这会产生以下结果 −

This will produce the following result −

Current Date: Sun 2004.07.18 at 04:14:09 PM PDT

Simple DateFormat Format Codes

要指定时间格式，请使用时间模式字符串。在此模式中，所有 ASCII 字母都保留为模式字母，定义如下：

To specify the time format, use a time pattern string. In this pattern, all ASCII letters are reserved as pattern letters, which are defined as the following −

Character

Description

Example

G

Era designator

AD

y

Year in four digits

2001

M

Month in year

July or 07

d

Day in month

10

h

Hour in A.M./P.M. (1~12)

12

H

Hour in day (0~23)

22

m

Minute in hour

30

s

Second in minute

55

S

Millisecond

234

E

Day in week

Tuesday

D

Day in year

360

F

Day of week in month

2 (second Wed. in July)

w

Week in year

40

W

Week in month

1

a

A.M./P.M. marker

PM

k

Hour in day (1~24)

24

K

Hour in A.M./P.M. (0~11)

10

z

Time zone

Eastern Standard Time

'

Escape for text

Delimiter

"

Single quote

`

Date Formatting Using printf

可以使用 printf 方法非常轻松地完成日期和时间格式化。您使用一个两位格式，从 t 开始，以表格中列出的一个字母结尾，如下面的代码所示。

Date and time formatting can be done very easily using printf method. You use a two-letter format, starting with t and ending in one of the letters of the table as shown in the following code.

Example

import java.util.Date;
public class DateDemo {

   public static void main(String args[]) {
      // Instantiate a Date object
      Date date = new Date();

      // display time and date
      String str = String.format("Current Date/Time : %tc", date );

      System.out.printf(str);
   }
}

这会产生以下结果 −

This will produce the following result −

Current Date/Time : Sat Dec 15 16:37:57 MST 2012

如果您必须多次提供日期以格式化每个部分，那会有点愚蠢。因此，格式字符串可以指示要格式化的参数的索引。

It would be a bit silly if you had to supply the date multiple times to format each part. For that reason, a format string can indicate the index of the argument to be formatted.

索引必须紧跟 %，并且必须以 $ 结尾。

The index must immediately follow the % and it must be terminated by a $.

Example

import java.util.Date;
public class DateDemo {

   public static void main(String args[]) {
      // Instantiate a Date object
      Date date = new Date();

      // display time and date
      System.out.printf("%1$s %2$tB %2$td, %2$tY", "Due date:", date);
   }
}

这会产生以下结果 −

This will produce the following result −

Due date: February 09, 2004

或者，您可以使用 < 标志。它表示应再次使用与前一个格式规范中相同的参数。

Alternatively, you can use the < flag. It indicates that the same argument as in the preceding format specification should be used again.

Example

import java.util.Date;
public class DateDemo {

   public static void main(String args[]) {
      // Instantiate a Date object
      Date date = new Date();

      // display formatted date
      System.out.printf("%s %tB %<te, %<tY", "Due date:", date);
   }
}

这会产生以下结果 −

This will produce the following result −

Due date: February 09, 2004

Date and Time Conversion Characters

Character

Description

Example

c

Complete date and time

Mon May 04 09:51:52 CDT 2009

F

ISO 8601 date

2004-02-09

D

U.S. formatted date (month/day/year)

02/09/2004

T

24-hour time

18:05:19

r

12-hour time

06:05:19 pm

R

24-hour time, no seconds

18:05

Y

Four-digit year (with leading zeroes)

2004

y

Last two digits of the year (with leading zeroes)

04

C

First two digits of the year (with leading zeroes)

20

B

Full month name

February

b

Abbreviated month name

Feb

m

Two-digit month (with leading zeroes)

02

d

Two-digit day (with leading zeroes)

03

e

Two-digit day (without leading zeroes)

9

A

Full weekday name

Monday

a

Abbreviated weekday name

Mon

j

Three-digit day of year (with leading zeroes)

069

H

Two-digit hour (with leading zeroes), between 00 and 23

18

k

Two-digit hour (without leading zeroes), between 0 and 23

18

I

Two-digit hour (with leading zeroes), between 01 and 12

06

l

Two-digit hour (without leading zeroes), between 1 and 12

6

M

Two-digit minutes (with leading zeroes)

05

S

Two-digit seconds (with leading zeroes)

19

L

Three-digit milliseconds (with leading zeroes)

047

N

Nine-digit nanoseconds (with leading zeroes)

047000000

P

Uppercase morning or afternoon marker

PM

p

Lowercase morning or afternoon marker

pm

z

RFC 822 numeric offset from GMT

-0800

Z

Time zone

PST

s

Seconds since 1970-01-01 00:00:00 GMT

1078884319

Q

Milliseconds since 1970-01-01 00:00:00 GMT

1078884319047

还有其他与日期和时间相关的实用类。如需了解更多详细信息，请参阅 Java 标准文档。

There are other useful classes related to Date and time. For more details, you can refer to Java Standard documentation.

Parsing Strings into Dates

SimpleDateFormat 类有一些其他方法，尤其是 parse( )，它尝试根据给定 SimpleDateFormat 对象中存储的格式解析字符串。

The SimpleDateFormat class has some additional methods, notably parse( ), which tries to parse a string according to the format stored in the given SimpleDateFormat object.

Example

import java.util.*;
import java.text.*;

public class DateDemo {

   public static void main(String args[]) {
      SimpleDateFormat ft = new SimpleDateFormat ("yyyy-MM-dd");
      String input = args.length == 0 ? "1818-11-11" : args[0];

      System.out.print(input + " Parses as ");
      Date t;
      try {
         t = ft.parse(input);
         System.out.println(t);
      } catch (ParseException e) {
         System.out.println("Unparseable using " + ft);
      }
   }
}

上述程序的运行示例会生成以下结果：

A sample run of the above program would produce the following result −

1818-11-11 Parses as Wed Nov 11 00:00:00 EST 1818

Sleeping for a While

您可以从一毫秒到计算机的生命周期中的任意时间段内休眠。例如，以下程序将休眠 3 秒：

You can sleep for any period of time from one millisecond up to the lifetime of your computer. For example, the following program would sleep for 3 seconds −

Example

import java.util.*;
public class SleepDemo {

   public static void main(String args[]) {
      try {
         System.out.println(new Date( ) + "\n");
         Thread.sleep(5*60*10);
         System.out.println(new Date( ) + "\n");
      } catch (Exception e) {
         System.out.println("Got an exception!");
      }
   }
}

这会产生以下结果 −

This will produce the following result −

Sun May 03 18:04:41 GMT 2009
Sun May 03 18:04:51 GMT 2009

Measuring Elapsed Time

有时，您可能需要以毫秒为单位测量时间点。所以让我们再次重写上面的示例：

Sometimes, you may need to measure point in time in milliseconds. So let’s re-write the above example once again −

Example

import java.util.*;
public class DiffDemo {

   public static void main(String args[]) {
      try {
         long start = System.currentTimeMillis( );
         System.out.println(new Date( ) + "\n");

         Thread.sleep(5*60*10);
         System.out.println(new Date( ) + "\n");

         long end = System.currentTimeMillis( );
         long diff = end - start;
         System.out.println("Difference is : " + diff);
      } catch (Exception e) {
         System.out.println("Got an exception!");
      }
   }
}

这会产生以下结果 −

This will produce the following result −

Sun May 03 18:16:51 GMT 2009
Sun May 03 18:16:57 GMT 2009
Difference is : 5993

GregorianCalendar Class

GregorianCalendar 是 Calendar 类的具体实现，它实现了您熟悉的标准公历。我们在本教程中没有讨论过 Calendar 类，您可以查阅标准 Java 文档。

GregorianCalendar is a concrete implementation of a Calendar class that implements the normal Gregorian calendar with which you are familiar. We did not discuss Calendar class in this tutorial, you can look up standard Java documentation for this.

Calendar 的 getInstance( ) 方法返回一个 GregorianCalendar，该方法使用默认区域设置和时区初始化当前日期和时间。GregorianCalendar 定义了两个字段：AD 和 BC。它们表示公历定义的两个纪元。

The getInstance( ) method of Calendar returns a GregorianCalendar initialized with the current date and time in the default locale and time zone. GregorianCalendar defines two fields: AD and BC. These represent the two eras defined by the Gregorian calendar.

还有多个 GregorianCalendar 对象的构造函数：

There are also several constructors for GregorianCalendar objects −

Sr.No.

Constructor & Description

1

GregorianCalendar() Constructs a default GregorianCalendar using the current time in the default time zone with the default locale.

2

GregorianCalendar(int year, int month, int date) Constructs a GregorianCalendar with the given date set in the default time zone with the default locale.

3

GregorianCalendar(int year, int month, int date, int hour, int minute) Constructs a GregorianCalendar with the given date and time set for the default time zone with the default locale.

4

GregorianCalendar(int year, int month, int date, int hour, int minute, int second) Constructs a GregorianCalendar with the given date and time set for the default time zone with the default locale.

5

GregorianCalendar(Locale aLocale) Constructs a GregorianCalendar based on the current time in the default time zone with the given locale.

6

GregorianCalendar(TimeZone zone) Constructs a GregorianCalendar based on the current time in the given time zone with the default locale.

7

GregorianCalendar(TimeZone zone, Locale aLocale) Constructs a GregorianCalendar based on the current time in the given time zone with the given locale.

以下列出了 GregorianCalendar 类提供的几个有用的支持方法：

Here is the list of few useful support methods provided by GregorianCalendar class −

Sr.No.

Method & Description

1

void add(int field, int amount) Adds the specified (signed) amount of time to the given time field, based on the calendar’s rules.

2

protected void computeFields() Converts UTC as milliseconds to time field values.

3

protected void computeTime() Overrides Calendar Converts time field values to UTC as milliseconds.

4

boolean equals(Object obj) Compares this GregorianCalendar to an object reference.

5

int get(int field) Gets the value for a given time field.

6

int getActualMaximum(int field) Returns the maximum value that this field could have, given the current date.

7

int getActualMinimum(int field) Returns the minimum value that this field could have, given the current date.

8

int getGreatestMinimum(int field) Returns highest minimum value for the given field if varies.

9

Date getGregorianChange() Gets the Gregorian Calendar change date.

10

int getLeastMaximum(int field) Returns lowest maximum value for the given field if varies.

11

int getMaximum(int field) Returns maximum value for the given field.

12

Date getTime() Gets this Calendar’s current time.

13

long getTimeInMillis() Gets this Calendar’s current time as a long.

14

TimeZone getTimeZone() Gets the time zone.

15

int getMinimum(int field) Returns minimum value for the given field.

16

int hashCode() Overrides hashCode.

17

boolean isLeapYear(int year) Determines if the given year is a leap year.

18

void roll(int field, boolean up) Adds or subtracts (up/down) a single unit of time on the given time field without changing larger fields.

19

void set(int field, int value) Sets the time field with the given value.

20

void set(int year, int month, int date) Sets the values for the fields year, month, and date.

21

void set(int year, int month, int date, int hour, int minute) Sets the values for the fields year, month, date, hour, and minute.

22

void set(int year, int month, int date, int hour, int minute, int second) Sets the values for the fields year, month, date, hour, minute, and second.

23

void setGregorianChange(Date date) Sets the GregorianCalendar change date.

24

void setTime(Date date) Sets this Calendar’s current time with the given Date.

25

void setTimeInMillis(long millis) Sets this Calendar’s current time from the given long value.

26

void setTimeZone(TimeZone value) Sets the time zone with the given time zone value.

27

String toString() Returns a string representation of this calendar.

Example

import java.util.*;
public class GregorianCalendarDemo {

   public static void main(String args[]) {
      String months[] = {"Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep",
         "Oct", "Nov", "Dec"};

      int year;
      // Create a Gregorian calendar initialized
      // with the current date and time in the
      // default locale and timezone.

      GregorianCalendar gcalendar = new GregorianCalendar();

      // Display current time and date information.
      System.out.print("Date: ");
      System.out.print(months[gcalendar.get(Calendar.MONTH)]);
      System.out.print(" " + gcalendar.get(Calendar.DATE) + " ");
      System.out.println(year = gcalendar.get(Calendar.YEAR));
      System.out.print("Time: ");
      System.out.print(gcalendar.get(Calendar.HOUR) + ":");
      System.out.print(gcalendar.get(Calendar.MINUTE) + ":");
      System.out.println(gcalendar.get(Calendar.SECOND));

      // Test if the current year is a leap year
      if(gcalendar.isLeapYear(year)) {
         System.out.println("The current year is a leap year");
      }else {
         System.out.println("The current year is not a leap year");
      }
   }
}

这会产生以下结果 −

This will produce the following result −

Date: Apr 22 2009
Time: 11:25:27
The current year is not a leap year

有关 Calendar 类中可用的常量列表，可以参考 Java 标准文档。

For a complete list of constant available in Calendar class, you can refer the standard Java documentation.