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
原文：http://www.cnblogs.com/hephec/p/4580018.html