Summarize the time classes.

Tag: time; date; Java; dateformat

This afternoon, I was idle, and suddenly saw some usage of the time class. I felt that several classes were a bit complicated. I would like to summarize them:

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The specific classes mainly include:

Date calendar simpledateformat

1. Date class

Most of the methods in this class are outdated.

This can be used as follows:

Date date = new Date();System.out.println(date.toString());

You can print the current time: Mon Jun 30 12:50:09 CST 2014

But the format is not very nice. It doesn't matter. I'll wear nice clothes for them later.

One method is not outdated:

Date date = new Date();System.out.println(date.geTime());

This method returnsDateThe number of milliseconds the object represents.

It can be used to test the total number of milliseconds for a program to run, or the number of seconds.

You can also directly use:

long now = System.currentTimeMillis()；

This method returns the same result as the above method.

2. Calendar abstract class

This class is very powerful, basically replacing the previous date class.CalendarProvides a class MethodgetInstance（）To obtain a common object of this type.CalendarOfgetInstanceMethod returnsCalendarObject whose calendar field has been initialized by the current date and time. Therefore, this class cannot be new and can only be passed through

Calendar now = Calendar.getInstance();

To instantiate an object.

Let's take a look at an example to understand how to use a class:

Import Java. util. calendar; public class testdate {public static void main (string [] ARGs) {// obtain the current date calendar = calendar ar. getinstance (); int year = calendar. get (calendar. year); system. out. println ("year =" + year); // 2014 int month = calendar. get (calendar. month); // obtain the current month, 0 .. 11 indicates jan .. decsystem. out. println ("month =" + month); // 5 // obtain the day of the month. Int day = calendar. get (calendar. day_of_month); system. out. println ("day of month =" + day); // 30 // obtain the number of days in the week, Sun = 1, MON = 2... sta = 7int day_of_week = calendar. get (calendar. day_of_week); system. out. println ("day of week =" + day_of_week); // 1 // when the current time is 12, the value is calendar. hourint hour12 = calendar. get (calendar. hour); system. out. println ("hour 12 =" + hour12); // 1 // obtain AM, PM, 0 = am, 1 = pmint ampm = calendar. get (calendar. am_pm); // 1system. out. println ("am PM =" + ampm); // 0 = am, 1 = PM // calendar is generated when the current time is 24. hour_of_dayint hour24 = calendar. get (calendar. hour_of_day); system. out. println ("hour 24 =" + hour24); // 13 complete display time: String date = calendar ar. get (calendar. year) + "-" + calendar. get (calendar. month) + "-" + calendar. get (calendar. day_of_month) + "" + calendar. get (calendar. hour_of_day) + ":" + calendar. get (calendar. minute) + ":" + calendar. get (calendar. second); system. out. println (date );}}

Well, the last "complete real time" seems a bit complicated, so I generally don't need this. There are still better methods.

There are a lot of member variables in the calendar class. The above program only lists some of them. Others can refer to Java APIs. As for the APIS, you can download them from this site, with all the resources available.

3. simpledateformat class

Let's talk about simpledateformat. Let's talk about its father:DateFormat。

We can usegetTimeInstance,getDateInstanceOrgetDateTimeInstanceTo create a date-time formatter. Every such method can return a date/time formatter initialized in the default format.

The following uses dateformat to return a time or date format:

Returns a default time format:

DateFormat format = DateFormat.getTimeInstance();Date date = new Date();System.out.println(format.format(date));   //14:05:11

Returns a default date format:

DateFormat format = DateFormat.getDateInstance();Date date = new Date();System.out.println(format.format(date)); //2014-6-30

Returns a default date-time format:

DateFormat format = DateFormat.getDateTimeInstance();Date date = new Date();System.out.println(format.format(date)); //2014-6-30 14:08:08

Simpledateformat is used to format a date or time more freely.

        SimpleDateFormatFormat and parse a date in a language environment-related manner. It allows formatting (date-> text), parsing (text-> date), and normalization.

The date and time formats are as follows:Date and Time ModesString.

The following pattern letters are defined (all other characters‘A‘To‘Z‘And‘a‘To‘z‘Are retained ):

Letter	Date or time element	Indicates	Example
G	Era identifier	Text	AD
y	Year	Year	1996;96
M	Month in Year	Month	July;Jul;07
w	Week in Year	Number	27
W	The number of weeks in the month	Number	2
D	Days in years	Number	189
d	Days in a month	Number	10
F	Week in the month	Number	2
E	Days in a week	Text	Tuesday;Tue
a	AM/PM mark	Text	PM
H	Hours in a day (0-23)	Number	0
k	Hours per day (1-24)	Number	24
K	Hours in AM/PM (0-11)	Number	0
h	Hours in AM/PM (1-12)	Number	12
m	Minutes in an hour	Number	30
s	Seconds in minutes	Number	55
S	Milliseconds	Number	978
z	Time Zone	General Time Zone	Pacific Standard Time;PST;GMT-08:00
Z	Time Zone	RFC 822 Time Zone	-0800

For example:

Date date = new Date();SimpleDateFormat format1 = new SimpleDateFormat("yyyy-MM-dd");SimpleDateFormat format2 = new SimpleDateFormat("yyyy/MM/dd");System.out.println(format1.format(date));    //  2014-06-30System.out.println(format2.format(date));    //  2014/06/30

The "-" in the string is ignored during formatting, and the output is in the format specified by you.

It's much easier than the one above.

Another example:

Simpledateformat format3 = new simpledateformat ("HH: mm: SS"); // 14: 30: 32 simpledateformat format4 = new simpledateformat ("HH: mm: ss "); // 14:30:32 pm

Format the date object with format3.format (date. Make it output in the specified format.

If you want to see the format we use, there is a way to help us implement it, that is

System.out.println(format3.toPattern());    //  HH:mm:ss

It's easy...

I just briefly introduced some simple usage to meet our general requirements. I will discuss it in detail later.

Hope to help you.

This article is from the "Focus on Java, Linux technology" blog, please be sure to keep this source http://wuqinglong.blog.51cto.com/9087037/1432423