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Unlike the Date class, that just gives us a package of information that contains the date, month, year and time without providing any methods to access any of that information individually. Calendar class gives us methods to not only retrieve an individual information about the date and time but also provide us many more methods to give us information about the hour, minute, second, day of the month, day of the week, week of the year, week of the month etc.






Note :


Calendar class is an abstract class and that's why we cannot create its object by calling its constructor.






Methods of Calendar class




Methods	Description
static Calendar getInstance()	Returns an object of Calendar class with default locale and time zone.
int get(int calendarField)	Gets the specific information regarding the mentioned calendarField.
Date getTime()	Returns a Date object converted from invoked Calendar object.
void set(int calenderField, int value)	Sets a calendarField of Calendar object with a specified value
void set(int year, int month, int dayOfMonth)	Sets a Calendar object with specified year, month, dayOfMonth
void set(int year, int month, int dayOfMonth, int hours, int minutes)	Sets a Calendar object with specified year, month, dayOfMonth,hours,minutes
void setTime(Date d)	Sets the invoked Calendar object with the value of a Date object.
void add(int calendarField, int value)	Adds a value to the specific calendarField of Calendar object.








A simple example of Calendar class.




import java.util.*;


class A
{
public static void main(String... ar)
{
Calendar c = Calendar.getInstance();
System.out.println(c);
}
}





Output




java.util.GregorianCalendar[time=1500973726436,areFieldsSet=true,areAllFieldsSet
=true,lenient=true,zone=sun.util.calendar.ZoneInfo[id="Asia/Calcutta",offset=198
00000,dstSavings=0,useDaylight=false,transitions=6,lastRule=null],firstDayOfWeek
=1,minimalDaysInFirstWeek=1,ERA=1,YEAR=2017,MONTH=6,WEEK_OF_YEAR=30,WEEK_OF_MONT
H=5,DAY_OF_MONTH=25,DAY_OF_YEAR=206,DAY_OF_WEEK=3,DAY_OF_WEEK_IN_MONTH=4,AM_PM=1
,HOUR=2,HOUR_OF_DAY=14,MINUTE=38,SECOND=46,MILLISECOND=436,ZONE_OFFSET=19800000,
DST_OFFSET=0]





Program Analysis


In the preceding code, we have created an object of Calendar class by calling its static method getInstance(). This object is actually an object of subclass of Calendar, i.e. GregorianCalendar class. By printing this object we have been presented with a lot of information about our time zone, date, month, year, hour, minute, second, day of the month, day of the year, day of the week etc, which is a little hard to read. Let's extract each of these specific information individually in a more presentable format.




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Getting the date, time etc contained in the Calendar object.


Here, we are getting the date, time contained in a Calendar object using method -


• int get(int calendarField)



import java.util.*;
class A
{
public static void main(String... ar)
{
Calendar c = Calendar.getInstance();
System.out.println(c);


System.out.println("Day of the year : "+c.get(Calendar.DAY_OF_YEAR));
System.out.println("Day of the month : "+ c.get(Calendar.DAY_OF_MONTH));
System.out.println("Day of the week : " +c.get(Calendar.DAY_OF_WEEK));

//Getting Hour and minutes of the day
System.out.println("Getting Hour, minutes, AM/PM of the day");
System.out.println("Hour of the day : "+c.get(Calendar.HOUR));
System.out.println("Minute : "+c.get(Calendar.MINUTE));
System.out.println("AM or PM? : "+c.get(Calendar.AM_PM));


//Getting current date, month and year 
System.out.println("Getting the current date, month and year");
System.out.println("Date : "+c.get(Calendar.DATE));
System.out.println("Month : "+c.get(Calendar.MONTH));
System.out.println("Year: "+c.get(Calendar.YEAR));

//Getting the extra information about the week
System.out.println("Getting the extra information about the week");
System.out.println("Week of the month : "+c.get(Calendar.WEEK_OF_MONTH));
System.out.println("Week of the year : "+c.get(Calendar.WEEK_OF_YEAR));
}
}





Output




Day of the year : 206
Day of the month : 25
Day of the week : 3
Getting Hour, minutes, AM/PM of the day
Hour of the day : 2
Minute : 39
AM or PM? : 1
Getting the current date, month and year
Date : 25
Month : 6
Year: 2017
Getting the extra information about the week
Week of the month : 5
Week of the year : 30


In the last code, we have extracted a more presentable and less cluttered specific piece information(one by one) about our day and time etc using the Calendar object.






Note :


Calendar.Month field starts counting month from zero, hence it gives us current number of month, which is one less than number of month we know.






Setting/updating the date, time etc contained in a Calendar object.


In the next code we are getting and setting the date, time contained in a Calendar object.

import java.util.*;

class A
{
public static void main(String... ar)
{
Calendar c = Calendar.getInstance();

//Getting modified current date, month and year 
System.out.println("Getting the current date, month and year");
System.out.println("Date : "+c.get(Calendar.DATE)); //calling get(int calendarField) method
System.out.println("Month : "+c.get(Calendar.MONTH));
System.out.println("Year: "+c.get(Calendar.YEAR));

c.set(2018, 10, 27);		//calling set(int year, int month, int dayOfMonth) method

//Getting current date, month and year 
System.out.println("Getting the modified current date, month and year");
System.out.println("Modified Date : "+c.get(Calendar.DATE));
System.out.println("Modified Month : "+c.get(Calendar.MONTH));
System.out.println("Modified Year: "+c.get(Calendar.YEAR));


//Another way to set the date in a Calendar object
c.set(Calendar.DATE,10);	//calling set(int calendarField, int value) method
c.set(Calendar.MONTH,2);
c.set(Calendar.YEAR,5);


//Getting modified current date, month and year 
System.out.println("Getting the modified current date, month and year");
System.out.println("Modified Date : "+c.get(Calendar.DATE));
System.out.println("Modified Month : "+c.get(Calendar.MONTH));
System.out.println("Modified Year: "+c.get(Calendar.YEAR));


//Getting the hour, minutes and seconds of the day
System.out.println("Getting the hour, minutes, second of the day");
System.out.println("Hour of the day : "+c.get(Calendar.HOUR));
System.out.println("Minute : "+c.get(Calendar.MINUTE));
System.out.println("Second : "+c.get(Calendar.SECOND));

//Setting the hour, minutes and seconds of the day
c.set(Calendar.HOUR,7);
c.set(Calendar.MINUTE,50);
c.set(Calendar.SECOND,23);

//Getting the modified hour, minutes and seconds of the day
System.out.println("Getting Modified hour, minutes and second of the day");
System.out.println("Modified Hour of the day : "+c.get(Calendar.HOUR));
System.out.println("Modified Minute : "+c.get(Calendar.MINUTE));
System.out.println("Modified Second : "+c.get(Calendar.SECOND));
}
}





Output




Getting the current date, month and year
Date : 25
Month : 6
Year: 2017
Getting the modified current date, month and year
Modified Date : 27
Modified Month : 10
Modified Year: 2018
Getting the modified current date, month and year
Modified Date : 10
Modified Month : 2
Modified Year: 5
Getting the hour, minutes and second of the day
Hour of the day : 2
Minute : 43
Second : 15
Getting modified hour, minutes and second of the day
Modified Hour of the day : 7
Modified Minute : 50
Modified Second : 23


We have twice modified the current date, month and year contained in a Calender object, by-
• First using- void set(int year, int month, int dayOfMonth) method
• Secondly, using void set(int calendarField, int value) method







Adding to and subtracting from the current date, year, month




import java.util.*;

class A
{
public static void main(String... ar)
{
Calendar c = new GregorianCalendar(); //Creating an object of Calendar class by calling construtor of GregorianCalendar

//Getting current date, month and year 
System.out.println("Getting the current date, month and year");
System.out.println("Date : "+c.get(Calendar.DATE));
System.out.println("Month : "+c.get(Calendar.MONTH));
System.out.println("Year: "+c.get(Calendar.YEAR));


//Another way to set the date in a Calendar object
c.add(Calendar.DATE,4);		 //adding 4 days to the current date.
c.add(Calendar.MONTH,-2);	//subtracting 2 days from the current month.
c.add(Calendar.YEAR,-5);	//subtracting 5 years from the current year.

//Getting current date, month and year 
System.out.println("Getting the modified current date, month and year");
System.out.println("Modified Date : "+c.get(Calendar.DATE));
System.out.println("Modified Month : "+c.get(Calendar.MONTH));
System.out.println("Modified Year: "+c.get(Calendar.YEAR));
}
}




Output-

Getting the current date, month and year
Date : 25
Month : 6
Year: 2017
Getting the modified current date, month and year
Modified Date : 29
Modified Month : 4
Modified Year: 2012








Initializing a Calendar object using Date object.

import java.util.*;
class A
{
public static void main(String... ar)
{
Date d= new Date(1000000000000l);
System.out.println("Date object" + d);

Calendar c = Calendar.getInstance();
c.setTime(d);
System.out.println("Calendar object initialized to Date object: ")
System.out.println("DATE : "+ c.get(Calendar.DATE));
System.out.println("MONTH : "+c.get(Calendar.MONTH));
System.out.println("YEAR : "+ c.get(Calendar.YEAR));
System.out.println("HOUR : "+ c.get(Calendar.HOUR));
System.out.println("MINUTE : "+ c.get(Calendar.MINUTE));
System.out.println("SECOND : "+ c.get(Calendar.SECOND));
}
}

Output -

Date object : Sun Sep 09 07:16:40 IST 2001
Calendar Object:
DATE : 9
MONTH : 8
YEAR : 2001
HOUR : 7
MINUTE : 16
SECOND : 40

• At first, we have created a Date object
• Secondly, we have initialized a Calendar object using the date and time information in the existing Date object.

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