java.util.Calendar - The Abstract Calendar Class

JDK Tutorials - Herong's Tutorial Examples

∟java.util.Calendar - The Abstract Calendar Class

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) HerongYang.com. All Rights Reserved.
 */
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: 1708374849660 milliseconds.
Year: 2024
Month: 1
Date: 19
Day of year: 50
Day of week: 2
AM or PM: 1
Hour: 5
Hour of day: 17
Minute: 34
Second: 9
Millisecond: 693
Zone offset: -3
Daylight saving offset: 0
My time: 17:34
My time zone ID: America/Argentina/Buenos_Aires
Los Angeles time: 12:34

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.