Java GregorianCalendar class & TimeZone

GregorianCalendar class is the concrete sub-class of the Calendar class.
This class supports both the Julian and Gregorian calendar systems.

Difference between Calendar and GregorianCalendar class :
The calendar class is an abstract class. So, the instance of this class can not be instantiated. Therefore, we need to use the static method Calendar.getInstance() to initialize the object of the Calendar class :

Calendar c = Calendar.getInstance();

Since the GregorianCalendar class is a concrete subclass, it can be initialized as :

GregorianCalendar gcal = new GregorianCalendar();

Constructors of the GregorianCalendar class :

1. GregorianCalendar(): This constructor is used to initialize an object with the current time in the default time zone.
GregorianCalendar(int year, int month, int day): This constructor is used to initialize an object with the date-set specified as parameters in the default time zone and default locale.

2. GregorianCalendar(int year, int month, int day, int hours, int minutes): This constructor initializes an object with the given date and time set in the default locale and time zone.

3. GregorianCalendar(int year, int month, int day, int hours, int minutes, int seconds): This constructor initializes an object with the more specific time and date-set passed as a parameter in the default locale and time zone.

4. GregorianCalendar(Locale locale): Initializes a GregorianCalendar object with the current date and time in the default time zone and the specified locale.

5. GregorianCalendar(TimeZone timeZone): Initializes a GregorianCalendar object with the current date and time in the default locale and the specified time zone.

6. GregorianCalendar(TimeZone timeZone, Locale locale): Initializes an object with the locale and timezone passed as parameters.

Methods of the Gregorian class :

1. isLeapYear(int year) :

Checks if the year passed as a parameter is a leap year or not.
This method returns a boolean value.
Example :

import java.util.*;

public class CWH extends Thread{

    public static void main(String[] args) {

        GregorianCalendar cal = new GregorianCalendar();
        System.out.println(cal.isLeapYear(2000));
        System.out.println(cal.isLeapYear(2021));

    }
}

Output

true
false

2. roll(int field, boolean up) :

This method adds/subtracts a single unit of time from the specified time field.
true = rolls up the value by 1.
false = rolls down the value by 1.
Example :

import java.util.*;
public class CWH extends Thread{
    public static void main(String[] args) {
        GregorianCalendar c = new GregorianCalendar();
        System.out.println("Date before rolling : " + c.getTime());

        c.roll(Calendar.MONTH, true);
        c.roll(Calendar.DATE, false);
        c.roll(Calendar.YEAR, true);

        System.out.println("Date after rolling : " + c.getTime());
    }
}

Output

Date before rolling : Wed May 26 07:53:24 IST 2021
Date after rolling : Sat Jun 25 07:53:24 IST 2022

3. hashcode():

This method returns the hashcode of the calendar object.
Example :

import java.util.*;
public class CWH extends Thread{
    public static void main(String[] args) {
        GregorianCalendar c = new GregorianCalendar();
        System.out.println("Calendar : " + c.getTime());

        System.out.println("The hashcode for this calendar is : "+ c.hashCode());
    }
}

Output

Calendar : Wed May 26 08:08:33 IST 2021
The hashcode for this calendar is : 1358707903

Example

package com.company;

import java.util.Calendar;
import java.util.GregorianCalendar;
import java.util.TimeZone;

public class cwh_99_gregorian {
    public static void main(String[] args) {
        Calendar c = Calendar.getInstance();
        System.out.println(c.getTime());
        System.out.println(c.get(Calendar.DATE));
        System.out.println(c.get(Calendar.SECOND));
        System.out.println(c.get(Calendar.HOUR));
        System.out.println(c.get(Calendar.HOUR_OF_DAY) + ":" + c.get(Calendar.MINUTE) + ":" + c.get(Calendar.SECOND));
        GregorianCalendar cal = new GregorianCalendar();
        System.out.println(cal.isLeapYear(2018));
        System.out.println(TimeZone.getAvailableIDs()[0]);
        System.out.println(TimeZone.getAvailableIDs()[1]);
        System.out.println(TimeZone.getAvailableIDs()[2]);
    }
}

Java GregorianCalendar class & TimeZone

Constructors of the GregorianCalendar class :

Methods of the Gregorian class :

1. isLeapYear(int year) :

Output

2. roll(int field, boolean up) :

Output

3. hashcode():

Output

Example

Conclusion