# Java Calendar Class
## Overview
Java's calendar class is an abstract class that gives you access to numerous calendar instances and methods for manipulating dates and times.

To address several issues with java. util.Date class, the java.util.Calendar class was added to Java with JDK 1.4. It did simplify some tasks; for example, it is now simpler to create arbitrary dates with the new GregorianCalendar(2016, Calendar.JUNE, 11) constructor than with the Date class, where the year starts from 1900 and the month starts from zero.
## Introduction to Java Calendar Class
The Java Calendar class is an abstract class that offers methods for converting dates between a specific point in time and a collection of calendar fields like MONTH, YEAR, HOUR, and so on. It derives from an Object and implements a Comparable interface.
Because it is an Abstract class, we cannot create an instance with a constructor. To instantiate and implement a sub-class, we must utilize the static method Calendar.getInstance().
## Declaration

public abstract class Calendar extends Object   
implements Serializable, Cloneable, Comparable<Calendar>  

## Fields for Java Calendar Class
The fields for the java.util.Calendar class are listed below.

| Fields| Details |
| -------- | -------- |
| static int ALL_STYLES | This specifies names in all styles, including "January" and "Jan," for getDisplayNames.|
| static int AM | The AM PM field value represents the time of day between midnight and shortly before noon. |
| static int AM_PM | This is the get and set field number that indicates if the HOUR is before or after noon. |
| static int APRIL |In the Gregorian and Julian calendars, this is the value of the MONTH field, indicating the fourth month of the year. |
| protected boolean areFieldsSet |This is true if the fields[] match the time currently specified. |
| static int AUGUST | In the Gregorian and Julian calendars, this is the value of the MONTH field, representing the eighth month of the year. |
| static int DATE | This is the get and set field number that indicates the month's day. |
| static int DAY_OF_MONTH |This is the get and set field number that indicates the day of the month. |
| static int DAY_OF_WEEK |This is the get and set field number that indicates the weekday.|
| static int DAY_OF_WEEK_IN_MONTH | This is the get and set field number that represents the ordinal number of the current month's day of the week. |
| static int DAY_OF_YEAR | This is the get and set field number that indicates the current year's day number. |
| static int DECEMBER | In the Gregorian and Julian calendars, this is the value of the MONTH field, signifying the twelfth month of the year. |
| static int DST_OFFSET |This is the get and set field number that shows the milliseconds of daylight savings time. |
| static int ERA | This is the get and set field number that denotes the Julian calendar era, such as AD or BC. |
| static int FEBRUARY |In the Gregorian and Julian calendars, this is the value of the MONTH field, indicating the second month of the year. |
| static int FIELD_COUNT |This is the number of unique fields that get and set recognise. |
| protected int[] fields | This is the calendar field values for the current time on the calendar. |
| static int FRIDAY | This is the value of the DAY OF WEEK field for Friday. |
| static int HOUR | This is the get and set field number for the hour of the morning or afternoon. |
| static int HOUR_OF_DAY | This is the get and set field number that represents the time of day. |
| protected boolean[] isSet | These are the flags that indicate whether or not a certain calendar field is set for the calendar.|
| protected boolean isTimeSet |If the time value is right, this is true.
|static int JANUARY |In the Gregorian and Julian calendars, this is the value of the MONTH field, signifying the first month of the year.|
|static int JULY |In the Gregorian and Julian calendars, this is the value of the MONTH field, representing the seventh month of the year.
|static int JUNE|In the Gregorian and Julian calendars, this is the value of the MONTH field, indicating the sixth month of the year.|
|static int LONG|When a long name, such as "January," is supplied, this is the style specifier for getDisplayName and getDisplayNames.|
|static int MARCH|In the Gregorian and Julian calendars, this is the value of the MONTH field, indicating the third month of the year.|
|static int MAY|In the Gregorian and Julian calendars, this is the value of the MONTH field, indicating the fifth month of the year.|
|static int MILLISECOND|This is the get and set field number, which represents a millisecond inside a second.|
|static int MINUTE |This is the get and set field number that indicates the minute inside an hour.|
|static int MONDAY |This is the value of the DAY OF WEEK field for Monday.|
|static int MONTH |This is the field number for the month for get and set.|
|static int NOVEMBER|In the Gregorian and Julian calendars, this is the value of the MONTH field, representing the eleventh month of the year.
|static int OCTOBER |In the Gregorian and Julian calendars, this is the MONTH field value for the tenth month of the year.
|static int PM |The AM PM field value represents the time of day between noon and shortly before midnight.|
|static int SATURDAY|This is the Saturday value for the DAY OF WEEK field.
|static int SECOND |This is the get and set field number for the second inside the minute.|
|static int SEPTEMBER |In the Gregorian and Julian calendars, this is the value of the MONTH field, representing the ninth month of the year.|
|static int SHORT |When a short name, such as "Jan," is supplied, this is the style specifier for getDisplayName and getDisplayNames.|
|static int SUNDAY|This is the value of the DAY OF WEEK field for Sunday.|
|static int THURSDAY |This is the Thursday value for the DAY OF WEEK field.|
|protected long time |After January 1, 1970, 0:00:00 GMT, this is the calendar's current time, reported in milliseconds.|
|static int TUESDAY |This is the value of the DAY OF WEEK field for Tuesday.|
|static int UNDECIMBER| This is the value for the MONTH field for the thirteenth month of the year..|
static int WEDNESDAY| This is the value of the DAY OF WEEK field for Wednesday.|
static int WEEK_OF_MONTH | This is the get and set field number that indicates the current month's week number.|
static int WEEK_OF_YEAR −|This is the Field number for get and set, which returns the week number for the current year.|
static int YEAR |This is the get and set field number that reflects the year.|
|static int ZONE_OFFSET |This is the field number used to retrieve and set the raw offset from GMT in milliseconds.## Constructors of Java Calendar Class
|Constructor|Descrption|
|-----------|-----------|
|protected Calendar()|This  constructor creates a Calendar with the default time zone and locale.|
|protected Calendar(TimeZone zone, Locale aLocale)|This constructor constructs a calendar with the specified time zone and locale.|
## Methods of Java Calendar Class

|Method|Description|
|------|------------|
|abstract void add(int field, int amount)|Based on the calendar's rules, this function adds or subtracts the provided amount of time from the supplied calendar field.|
boolean after(Object when)|This method returns whether this Calendar represents a time after the time represented by the specified Object.|boolean before(Object when)|This function returns if this Calendar reflects a time before the supplied Object's time.|
void clear()|This method renders this Calendar's calendar field values and time value (millisecond offset from the Epoch) undefined.|void clear(int field)|This method makes the specified calendar field value and this Calendar's time value (millisecond offset from the Epoch) unknown.|
Object clone()|This function copies this object and returns it.|
int compareTo(Calendar anotherCalendar)|This function compares two Calendar objects' time values (millisecond offsets from the Epoch).|
protected void complete()|This function populates any calendar fields that have unset values.|
protected abstract void computeFields()|The current millisecond time value time is converted to calendar field values in fields[].|
protected abstract void computeTime()|The current calendar field values in fields[] are converted to the millisecond time value time using this procedure.|
boolean equals(Object obj)|This function compares this Calendar to the Object given.|
int get(int field)|This function returns the value of the calendar field specified.|
int getActualMaximum(int field)|Given the time value of this Calendar, this function returns the greatest value that the specified calendar field might have.|
int getActualMinimum(int field)|Given the time value of this Calendar, this function returns the smallest value that the specified calendar field might have.|
static Locale[] getAvailableLocales()|This method produces an array containing all locales for which this class's getInstance methods may provide localised instances.|
String getDisplayName(int field, int style, Locale locale)| The method returns a string representation of the calendar field value in the style and location specified.|
Map<String,Integer> getDisplayNames(int field, int style, Locale locale)|This function produces a Map containing all calendar field names in the provided style and location, as well as their associated field values.
|int getFirstDayOfWeek()|This approach determines the first day of the week, such as SUNDAY in the United States and MONDAY in France.|
abstract int getGreatestMinimum(int field)|This function returns the Calendar instance's maximum minimum value for the provided calendar field.|
static Calendar getInstance()|This function retrieves a calendar based on the current time zone and location.|
static Calendar getInstance(Locale aLocale)|This function retrieves a calendar using the default time zone and locale.|
static Calendar getInstance(TimeZone zone)|This function retrieves a calendar based on the time zone and locale given.|
static Calendar getInstance(TimeZone zone, Locale aLocale|This function returns a calendar with the time zone and locale supplied.|
abstract int getLeastMaximum(int field)|This method returns the Calendar instance's lowest maximum value for the provided calendar field.|
abstract int getMaximum(int field)|This method returns the maximum value for the provided Calendar instance's calendar property.|
int getMinimalDaysInFirstWeek()|This function returns the minimum number of days necessary in the first week of the year; for example, if the first week is defined as one that includes the first day of the first month of the year, this method returns 1.|
abstract int getMinimum(int field)|This method returns the smallest value for the provided Calendar instance's calendar property.
|Date getTime()|This function returns a Date object with the time value of this Calendar (millisecond offset from the Epoch").|
long getTimeInMillis()|This function returns the time value of this Calendar in milliseconds.|
TimeZone getTimeZone()|The time zone is obtained using this way.|
int hashCode()|This technique This calendar's hash code is returned.|
|protected int internalGet(int field)|This function returns the value of the calendar field specified.|
boolean isLenient()|This technique indicates if date/time interpretation should be lax.|
boolean isSet(int field)|This method detects if a value has been set for the supplied calendar field, including circumstances where the value has been set by internal field computations triggered by a get method call.|
abstract void roll(int field, boolean up)|This method adds or subtracts (up/down) a single unit of time from the provided time field while leaving the bigger fields alone.|
void roll(int field, int amount)|This method inserts the provided (signed) amount to the chosen calendar field while leaving the bigger fields alone.|
void set(int field, int value)|This method changes the value of the specified calendar field.|
void set(int year, int month, int date)|This method configures the calendar fields YEAR, MONTH, and DAY OF MONTH.|
|void set(int year, int month, int date, int hourOfDay, int minute)|This method populates the calendar fields YEAR, MONTH, DAY OF MONTH, HOUR OF DAY, and MINUTE with values.|
void set(int year, int month, int date, int hourOfDay, int minute, int second)|This method populates the fields YEAR, MONTH, DAY OF MONTH, HOUR, MINUTE, and SECOND with values.|
|void setFirstDayOfWeek(int value)|This method determines the first day of the week, such as SUNDAY in the United States and MONDAY in France.|
void setLenient(boolean lenient)|This method indicates whether date/time interpretation should be lax or strict.
|void setMinimalDaysInFirstWeek(int value)|This method specifies the minimum number of days necessary in the first week of the year; for example, if the first week is defined as one that includes the first day of the first month of the year, call this method with value.|
void setTime(Date date)|This function sets the time on this Calendar to the supplied Date.|
void setTimeInMillis(long millis)|This method sets the current time of this Calendar to the specified long value.|
void setTimeZone(TimeZone value)|This method sets the time zone to the specified value.|
String toString()|This function returns the calendar's string representation.
## Java Calendar Class Examples
### Example1
In this example we are going to see how to get instance of calendar class and get current date and time.

import java.util.*;  
public class calendar_demo{  
   public static void main(String[] args) {  
      // getinstance of calendar class
   Calendar calendar = Calendar.getInstance();  
   // getTime() to get the current date and time
   System.out.print("present Date And Time Is: " + calendar.getTime());  
   }  
}

**Output**

present Date And Time Is: Wed Oct 26 16:43:14 IST 2022

**Code Explanation**
In above example we have used `getInstance()` method to get the Instance of calendar class.
`getTime()` is used to get the current date and time.

### Example2
In this example we are going to create instance of calendar class and use `add()` method.

import java.util.Calendar;  
public class Calendar_demo {  
   public static void main(String[] args) {  
      // getinstance of calendar class
   Calendar calendar = Calendar.getInstance();  
   // getTime() to get the current time
   System.out.println("The current date is : " + calendar.getTime()); 
   // Subtract 15 days from current date 
   calendar.add(Calendar.DATE, -15);  
   System.out.println("15 days ago: " + calendar.getTime());  
   calendar.add(Calendar.MONTH, 4);  
   // Add 4 month to current calendar 
   System.out.println("4 months later: " + calendar.getTime());  
   // Add 2 year to current calendar 
   calendar.add(Calendar.YEAR, 2);  
   System.out.println("2 years later: " + calendar.getTime());  
   }  
}   

**Output**

The current date is : Wed Oct 26 16:33:05 IST 2022
15 days ago: Tue Oct 11 16:33:05 IST 2022
4 months later: Sat Feb 11 16:33:05 IST 2023
2 years later: Tue Feb 11 16:33:05 IST 2025

**Code Explanation**
In Above example we have created a instance of calendar class using `getinstance()` method.
We have used `add()` method to with field parameter as `Calender.DATE` and amount parameter as -15 this will allow us to get 15 days before current time.

### Example 3
In this example we are going to use `getMaximum()` method.

import java.util.*;  
public class calendar_demo {  
   public static void main(String[] args) {  
      // get instance of calendar class
   Calendar calendar = Calendar.getInstance();  
   // get maximum number of day in week
   int maximum = calendar.getMaximum(Calendar.DAY_OF_WEEK);  
   System.out.println("Maximum number of days in week: " + maximum);
   // get maximum week in year  
   maximum = calendar.getMaximum(Calendar.WEEK_OF_YEAR);  
   System.out.println("Maximum number of weeks in year: " + maximum);  
   }  
}  

**Output**

Maximum number of days in week: 7
Maximum number of weeks in year: 53
	

**Code Explanation**
In this example we have used `getMaximum()` to get the maximum value of days in week and weeks in year.

## Conclusion
* The Java Calendar class is an abstract class that offers methods for converting dates between a certain point in time and a collection of calendar fields like MONTH, YEAR, HOUR, and so on.
* Because it is an Abstract class, we cannot create an instance with a constructor
* To instantiate and implement a sub-class, we must instead utilise the static method Calendar.getInstance()
* `add()` function adds or subtracts the provided amount of time from the supplied calendar field.
* `getTime()` returns a Date object with the time value of this Calendar (millisecond offset from the Epoch")