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This article explains the API for using Calendar, Date and Time in Java and how to format the output of a date.
Table of Contents
The Java language provides direct support for time-based objects. This article gives a few examples how this API can be used.
The java.util.Date
and the java.util.Calendar
classes provide access to storing and manipulating dates.
It is recommended to use Calendar
if possible. Existing API may required that you convert from Date
to Calendar
and vice versa.
To format a date, you can use the SimpleDateFormat
class. The following snippet gives several example for its usage.
// use dd/MM/yy as format DateFormat df1 = new SimpleDateFormat("dd/MM/yy"); String formattedDate1 = df1.format(new Date()); // or use yyyy/MM/dd as format DateFormat df2 = new SimpleDateFormat("yyyy/MM/dd"); String formattedDate2 = df2.format(theDate);
The java.util.Calendar
class is an abstract encapsulation of the Date
object.
Calendar provides getter and setter for the date fields.
public final int get(int field) public final void set(int field, int value)
Table 1. Calendar field access
Field Explanation Calendar.YEAR Identifies the year Calendar.MONTH Identifies the month Calendar.DAY_OF_MONTH Identifies the day Calendar.HOUR Identifies the hour Calendar.MINUTE Identifies the minute Calendar.SECOND Identifies the second
Create a new Java project called JavaIntroCalendar. Create the following class for testing.
package test; import java.text.SimpleDateFormat; import java.util.Calendar; import java.util.GregorianCalendar; public class CalendarTest { public static void main(String[] args) { // constructor allows to set year, month and date Calendar cal1 = new GregorianCalendar(2008, 01, 01); // constructor could also be empty // calendar cal2 = new GregorianCalendar(); // change the month cal1.set(Calendar.MONTH, Calendar.MAY); System.out.println("Year: " + cal1.get(Calendar.YEAR)); System.out.println("Month: " + (cal1.get(Calendar.MONTH) + 1)); System.out.println("Days: " + cal1.get(Calendar.DAY_OF_MONTH)); // format the output with leading zeros for days and month SimpleDateFormat date_format = new SimpleDateFormat("yyyyMMdd"); System.out.println(date_format.format(cal1.getTime())); } }
Use the following commands to convert to a Date from various formats.
package conversion; import java.text.ParseException; import java.text.SimpleDateFormat; import java.util.Calendar; import java.util.Date; import java.util.GregorianCalendar; public class ConversionExamplesDate { // convert from String to date private void stringToDate() { try { Date date1; date1 = new SimpleDateFormat("MM/dd/yy").parse("05/18/05"); System.out.println(date1); Date date2 = new SimpleDateFormat("MM/dd/yyyy").parse("05/18/2007"); System.out.println(date2); } catch (ParseException e) { e.printStackTrace(); } } // convert from millisecs to a String with a defined format private void calcDate(long millisecs) { SimpleDateFormat date_format = new SimpleDateFormat("MMM dd,yyyy HH:mm"); Date resultdate = new Date(millisecs); System.out.println(date_format.format(resultdate)); } private void writeActualDate(){ Calendar cal = new GregorianCalendar(); Date creationDate = cal.getTime(); SimpleDateFormat date_format = new SimpleDateFormat("MMM dd,yyyy HH:mm"); System.out.println(date_format.format(creationDate)); } public static void main(String[] args) { ConversionExamplesDate convert = new ConversionExamplesDate(); convert.stringToDate(); convert.calcDate(System.currentTimeMillis()); convert.writeActualDate(); } }
Java Date, Calendar and Time API - Tutorial
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原文地址:http://www.cnblogs.com/hephec/p/4580018.html