This section provides a tutorial example on how to use calendar related classes: java.util.Calendar, java.util.GregorianCalendar, java.util.TimeZone, and java.util.SimpleTimeZone.
The Calendar class, java.util.Calendar, is an abstract base class, providing
a foundation for subclasses to represent a specific instance in time,
in a specific calendar system.
The GregorianCalendar class, java.util.GregorianCalendar, is a concrete
subclass of the Calendar class, representing
a specific instance in time in Gregorian Calendar system with time zone
and daylight saving adjustments.
This abstract base class and concrete subclass structure is very useful
for future implementations of other calendar systems, like the
Chinese lunar calendar system.
The TimeZone class, java.util.TimeZone, is an abstract base class, providing
a foundation for subclasses to represent a particular time zone system.
The SimpleTimeZone class, java.util.SimpleTimeZone, is a concrete subclass of the
TimeZone class, representing a time zone for use with the GregorianCalendar
class.
I don't know why we need this base class and subclass structure to implement
the time zone system. How many time zone systems are there? I only know one.
The following program shows some features of calendar related classes:
/**
* DateTest.java
* Copyright (c) 2002 by Dr. Herong Yang
*
* This program shows you some basic features of date related classes.
*/
import java.util.*;
class DateTest {
public static void main(String[] a) {
showDate();
showCalendar();
showTimeZone();
}
public static void showDate() {
Date now = new Date(); // the current time
long t = now.getTime();
System.out.println("Time since 01-Jan-1970 00:00:00 GMT: " +
t + " milliseconds.");
}
public static void showCalendar() {
GregorianCalendar c = new GregorianCalendar();
// the current time represented in the Gregorian calendar
// in local time zone and daylight saving adjustments
System.out.println("Year: "+c.get(Calendar.YEAR));
System.out.println("Month: "+c.get(Calendar.MONTH));
System.out.println("Date: "+c.get(Calendar.DATE));
System.out.println("Day of year: "+c.get(Calendar.DAY_OF_YEAR));
System.out.println("Day of week: "+c.get(Calendar.DAY_OF_WEEK));
System.out.println("AM or PM: "+c.get(Calendar.AM_PM));
System.out.println("Hour: "+c.get(Calendar.HOUR));
System.out.println("Hour of day: "+c.get(Calendar.HOUR_OF_DAY));
System.out.println("Minute: "+c.get(Calendar.MINUTE));
System.out.println("Second: "+c.get(Calendar.SECOND));
System.out.println("Millisecond: "+c.get(Calendar.MILLISECOND));
System.out.println("Zone offset: "+
c.get(Calendar.ZONE_OFFSET)/(60*60*1000));
System.out.println("Daylight saving offset: "+
c.get(Calendar.DST_OFFSET)/(60*60*1000));
}
public static void showTimeZone() {
GregorianCalendar c = new GregorianCalendar();
TimeZone tz = c.getTimeZone();
System.out.println("My time: "+c.get(Calendar.HOUR_OF_DAY)+
":"+c.get(Calendar.MINUTE));
System.out.println("My time zone ID: "+tz.getID());
tz = TimeZone.getTimeZone("America/Los_Angeles");
c.setTimeZone(tz); // changing the time zone
System.out.println("Los Angeles time: "+
c.get(Calendar.HOUR_OF_DAY)+":"+c.get(Calendar.MINUTE));
}
}
Output:
Time since 01-Jan-1970 00:00:00 GMT: 1035236526708 milliseconds.
Year: 2002
Month: 9
Date: 21
Day of year: 294
Day of week: 2
AM or PM: 1
Hour: 5
Hour of day: 17
Minute: 42
Second: 6
Millisecond: 738
Zone offset: -5
Daylight saving offset: 1
My time: 17:42
My time zone ID: America/New_York
Los Angeles time: 14:42
The GregorianCalendar is a tool that takes three inputs: an instance of time,
a time zone system, and daylight saying adjustment, and converts them into
calendar related information, such as the month, the date, the day of week,
and the hour of day. But in this program, I am using the default constructor,
GregorianCalendar(), to take the current time, and the time zone and daylight
saving adjustment settings on the operating system, where this program was
executed.
From the output of the program, you can see that, my computer is set to
east coast time zone, with one hour daylight saving offset.
