The Calendar class is an abstract class that provides methods for converting between a specific instant in time and a set of calendar fields such as YEAR, MONTH, DAY_OF_MONTH, HOUR, and so on, and for manipulating the calendar fields, such as getting the date of the next week. An instant in time can be represented by a millisecond value that is an offset from the Epoch, January 1, 1970 00:00:00.000 GMT (Gregorian).

The class also provides additional fields and methods for implementing a concrete calendar system outside the package. Those fields and methods are defined as protected.

Like other locale-sensitive classes, Calendar provides a class method, getInstance, for getting a generally useful object of this type. Calendar's getInstance method returns a Calendar object whose calendar fields have been initialized with the current date and time:

     Calendar rightNow = Calendar.getInstance();
 

A Calendar object can produce all the calendar field values needed to implement the date-time formatting for a particular language and calendar style (for example, Japanese-Gregorian, Japanese-Traditional). Calendar defines the range of values returned by certain calendar fields, as well as their meaning. For example, the first month of the calendar system has value MONTH == JANUARY for all calendars. Other values are defined by the concrete subclass, such as ERA. See individual field documentation and subclass documentation for details.

Getting and Setting Calendar Field Values

The calendar field values can be set by calling the set methods. Any field values set in a Calendar will not be interpreted until it needs to calculate its time value (milliseconds from the Epoch) or values of the calendar fields. Calling the get, getTimeInMillis, getTime, add and roll involves such calculation.

Leniency

Calendar has two modes for interpreting the calendar fields, lenient and non-lenient. When a Calendar is in lenient mode, it accepts a wider range of calendar field values than it produces. When a Calendar recomputes calendar field values for return by get(), all of the calendar fields are normalized. For example, a lenient GregorianCalendar interprets MONTH == JANUARY, DAY_OF_MONTH == 32 as February 1.

When a Calendar is in non-lenient mode, it throws an exception if there is any inconsistency in its calendar fields. For example, a GregorianCalendar always produces DAY_OF_MONTH values between 1 and the length of the month. A non-lenient GregorianCalendar throws an exception upon calculating its time or calendar field values if any out-of-range field value has been set.

First Week

Calendar defines a locale-specific seven day week using two parameters: the first day of the week and the minimal days in first week (from 1 to 7). These numbers are taken from the locale resource data when a Calendar is constructed. They may also be specified explicitly through the methods for setting their values.

When setting or getting the WEEK_OF_MONTH or WEEK_OF_YEAR fields, Calendar must determine the first week of the month or year as a reference point. The first week of a month or year is defined as the earliest seven day period beginning on getFirstDayOfWeek() and containing at least getMinimalDaysInFirstWeek() days of that month or year. Weeks numbered ..., -1, 0 precede the first week; weeks numbered 2, 3,... follow it. Note that the normalized numbering returned by get() may be different. For example, a specific Calendar subclass may designate the week before week 1 of a year as week n of the previous year.

Calendar Fields Resolution

When computing a date and time from the calendar fields, there may be insufficient information for the computation (such as only year and month with no day of month), or there may be inconsistent information (such as Tuesday, July 15, 1996 (Gregorian) -- July 15, 1996 is actually a Monday). Calendar will resolve calendar field values to determine the date and time in the following way.

If there is any conflict in calendar field values, Calendar gives priorities to calendar fields that have been set more recently. The following are the default combinations of the calendar fields. The most recent combination, as determined by the most recently set single field, will be used.

For the date fields:

 YEAR + MONTH + DAY_OF_MONTH
 YEAR + MONTH + WEEK_OF_MONTH + DAY_OF_WEEK
 YEAR + MONTH + DAY_OF_WEEK_IN_MONTH + DAY_OF_WEEK
 YEAR + DAY_OF_YEAR
 YEAR + DAY_OF_WEEK + WEEK_OF_YEAR
 

For the time of day fields:

 HOUR_OF_DAY
 AM_PM + HOUR
 

If there are any calendar fields whose values haven't been set in the selected field combination, Calendar uses their default values. The default value of each field may vary by concrete calendar systems. For example, in GregorianCalendar, the default of a field is the same as that of the start of the Epoch: i.e., YEAR = 1970, MONTH = JANUARY, DAY_OF_MONTH = 1, etc.

Note: There are certain possible ambiguities in interpretation of certain singular times, which are resolved in the following ways:

1. 23:59 is the last minute of the day and 00:00 is the first minute of the next day. Thus, 23:59 on Dec 31, 1999 < 00:00 on Jan 1, 2000 < 00:01 on Jan 1, 2000.
2. Although historically not precise, midnight also belongs to "am", and noon belongs to "pm", so on the same day, 12:00 am (midnight) < 12:01 am, and 12:00 pm (noon) < 12:01 pm

The date or time format strings are not part of the definition of a calendar, as those must be modifiable or overridable by the user at runtime. Use java.text.DateFormat to format dates.

Field Manipulation

The calendar fields can be changed using three methods: set(), add(), and roll().

set(f, value) changes calendar field f to value. In addition, it sets an internal member variable to indicate that calendar field f has been changed. Although calendar field f is changed immediately, the calendar's time value in milliseconds is not recomputed until the next call to get(), getTime(), getTimeInMillis(), add(), or roll() is made. Thus, multiple calls to set() do not trigger multiple, unnecessary computations. As a result of changing a calendar field using set(), other calendar fields may also change, depending on the calendar field, the calendar field value, and the calendar system. In addition, get(f) will not necessarily return value set by the call to the set method after the calendar fields have been recomputed. The specifics are determined by the concrete calendar class.

Example: Consider a GregorianCalendar originally set to August 31, 1999. Calling set(Calendar.MONTH, Calendar.SEPTEMBER) sets the date to September 31, 1999. This is a temporary internal representation that resolves to October 1, 1999 if getTime()is then called. However, a call to set(Calendar.DAY_OF_MONTH, 30) before the call to getTime() sets the date to September 30, 1999, since no recomputation occurs after set() itself.

add(f, delta) adds delta to field f. This is equivalent to calling set(f, get(f) + delta) with two adjustments:

Add rule 1. The value of field f after the call minus the value of field f before the call is delta, modulo any overflow that has occurred in field f. Overflow occurs when a field value exceeds its range and, as a result, the next larger field is incremented or decremented and the field value is adjusted back into its range.

Add rule 2. If a smaller field is expected to be invariant, but it is impossible for it to be equal to its prior value because of changes in its minimum or maximum after field f is changed or other constraints, such as time zone offset changes, then its value is adjusted to be as close as possible to its expected value. A smaller field represents a smaller unit of time. HOUR is a smaller field than DAY_OF_MONTH. No adjustment is made to smaller fields that are not expected to be invariant. The calendar system determines what fields are expected to be invariant.

In addition, unlike set(), add() forces an immediate recomputation of the calendar's milliseconds and all fields.

Example: Consider a GregorianCalendar originally set to August 31, 1999. Calling add(Calendar.MONTH, 13) sets the calendar to September 30, 2000. Add rule 1 sets the MONTH field to September, since adding 13 months to August gives September of the next year. Since DAY_OF_MONTH cannot be 31 in September in a GregorianCalendar, add rule 2 sets the DAY_OF_MONTH to 30, the closest possible value. Although it is a smaller field, DAY_OF_WEEK is not adjusted by rule 2, since it is expected to change when the month changes in a GregorianCalendar.

roll(f, delta) adds delta to field f without changing larger fields. This is equivalent to calling add(f, delta) with the following adjustment:

Roll rule. Larger fields are unchanged after the call. A larger field represents a larger unit of time. DAY_OF_MONTH is a larger field than HOUR.

Example: See GregorianCalendar.roll(int, int).

Usage model. To motivate the behavior of add() and roll(), consider a user interface component with increment and decrement buttons for the month, day, and year, and an underlying GregorianCalendar. If the interface reads January 31, 1999 and the user presses the month increment button, what should it read? If the underlying implementation uses set(), it might read March 3, 1999. A better result would be February 28, 1999. Furthermore, if the user presses the month increment button again, it should read March 31, 1999, not March 28, 1999. By saving the original date and using either add() or roll(), depending on whether larger fields should be affected, the user interface can behave as most users will intuitively expect.

Field Summary
static int	ALL_STYLES A style specifier for getDisplayNames indicating names in all styles, such as "January" and "Jan".
static int	AM Value of the AM_PM field indicating the period of the day from midnight to just before noon.
static int	AM_PM Field number for get and set indicating whether the HOUR is before or after noon.
static int	APRIL Value of the MONTH field indicating the fourth month of the year in the Gregorian and Julian calendars.
protected boolean	areFieldsSet True if fields[] are in sync with the currently set time.
static int	AUGUST Value of the MONTH field indicating the eighth month of the year in the Gregorian and Julian calendars.
static int	DATE Field number for get and set indicating the day of the month.
static int	DAY_OF_MONTH Field number for get and set indicating the day of the month.
static int	DAY_OF_WEEK Field number for get and set indicating the day of the week.
static int	DAY_OF_WEEK_IN_MONTH Field number for get and set indicating the ordinal number of the day of the week within the current month.
static int	DAY_OF_YEAR Field number for get and set indicating the day number within the current year.
static int	DECEMBER Value of the MONTH field indicating the twelfth month of the year in the Gregorian and Julian calendars.
static int	DST_OFFSET Field number for get and set indicating the daylight savings offset in milliseconds.
static int	ERA Field number for get and set indicating the era, e.g., AD or BC in the Julian calendar.
static int	FEBRUARY Value of the MONTH field indicating the second month of the year in the Gregorian and Julian calendars.
static int	FIELD_COUNT The number of distinct fields recognized by get and set.
protected int[]	fields The calendar field values for the currently set time for this calendar.
static int	FRIDAY Value of the DAY_OF_WEEK field indicating Friday.
static int	HOUR Field number for get and set indicating the hour of the morning or afternoon.
static int	HOUR_OF_DAY Field number for get and set indicating the hour of the day.
protected boolean[]	isSet The flags which tell if a specified calendar field for the calendar is set.
protected boolean	isTimeSet True if then the value of time is valid.
static int	JANUARY Value of the MONTH field indicating the first month of the year in the Gregorian and Julian calendars.
static int	JULY Value of the MONTH field indicating the seventh month of the year in the Gregorian and Julian calendars.
static int	JUNE Value of the MONTH field indicating the sixth month of the year in the Gregorian and Julian calendars.
static int	LONG A style specifier for getDisplayName and getDisplayNames indicating a long name, such as "January".
static int	MARCH Value of the MONTH field indicating the third month of the year in the Gregorian and Julian calendars.
static int	MAY Value of the MONTH field indicating the fifth month of the year in the Gregorian and Julian calendars.
static int	MILLISECOND Field number for get and set indicating the millisecond within the second.
static int	MINUTE Field number for get and set indicating the minute within the hour.
static int	MONDAY Value of the DAY_OF_WEEK field indicating Monday.
static int	MONTH Field number for get and set indicating the month.
static int	NOVEMBER Value of the MONTH field indicating the eleventh month of the year in the Gregorian and Julian calendars.
static int	OCTOBER Value of the MONTH field indicating the tenth month of the year in the Gregorian and Julian calendars.
static int	PM Value of the AM_PM field indicating the period of the day from noon to just before midnight.
static int	SATURDAY Value of the DAY_OF_WEEK field indicating Saturday.
static int	SECOND Field number for get and set indicating the second within the minute.
static int	SEPTEMBER Value of the MONTH field indicating the ninth month of the year in the Gregorian and Julian calendars.
static int	SHORT A style specifier for getDisplayName and getDisplayNames indicating a short name, such as "Jan".
static int	SUNDAY Value of the DAY_OF_WEEK field indicating Sunday.
static int	THURSDAY Value of the DAY_OF_WEEK field indicating Thursday.
protected long	time The currently set time for this calendar, expressed in milliseconds after January 1, 1970, 0:00:00 GMT.
static int	TUESDAY Value of the DAY_OF_WEEK field indicating Tuesday.
static int	UNDECIMBER Value of the MONTH field indicating the thirteenth month of the year.
static int	WEDNESDAY Value of the DAY_OF_WEEK field indicating Wednesday.
static int	WEEK_OF_MONTH Field number for get and set indicating the week number within the current month.
static int	WEEK_OF_YEAR Field number for get and set indicating the week number within the current year.
static int	YEAR Field number for get and set indicating the year.
static int	ZONE_OFFSET Field number for get and set indicating the raw offset from GMT in milliseconds.

Constructor Summary
protected	Calendar() Constructs a Calendar with the default time zone and locale.
protected	Calendar(TimeZone zone, Locale aLocale) Constructs a calendar with the specified time zone and locale.

Method Summary
abstract void	add(int field, int amount) Adds or subtracts the specified amount of time to the given calendar field, based on the calendar's rules.
boolean	after(Object when) Returns whether this Calendar represents a time after the time represented by the specified Object.
boolean	before(Object when) Returns whether this Calendar represents a time before the time represented by the specified Object.
void	clear() Sets all the calendar field values and the time value (millisecond offset from the Epoch) of this Calendar undefined.
void	clear(int field) Sets the given calendar field value and the time value (millisecond offset from the Epoch) of this Calendar undefined.
Object	clone() Creates and returns a copy of this object.
int	compareTo(Calendar anotherCalendar) Compares the time values (millisecond offsets from the Epoch) represented by two Calendar objects.
protected void	complete() Fills in any unset fields in the calendar fields.
protected abstract void	computeFields() Converts the current millisecond time value time to calendar field values in fields[].
protected abstract void	computeTime() Converts the current calendar field values in fields[] to the millisecond time value time.
boolean	equals(Object obj) Compares this Calendar to the specified Object.
int	get(int field) Returns the value of the given calendar field.
int	getActualMaximum(int field) Returns the maximum value that the specified calendar field could have, given the time value of this Calendar.
int	getActualMinimum(int field) Returns the minimum value that the specified calendar field could have, given the time value of this Calendar.
static Locale[]	getAvailableLocales() Returns an array of all locales for which the getInstance methods of this class can return localized instances.
String	getDisplayName(int field, int style, Locale locale) Returns the string representation of the calendar field value in the given style and locale.
Map<String,Integer>	getDisplayNames(int field, int style, Locale locale) Returns a Map containing all names of the calendar field in the given style and locale and their corresponding field values.
int	getFirstDayOfWeek() Gets what the first day of the week is; e.g., SUNDAY in the U.S., MONDAY in France.
abstract int	getGreatestMinimum(int field) Returns the highest minimum value for the given calendar field of this Calendar instance.
static Calendar	getInstance() Gets a calendar using the default time zone and locale.
static Calendar	getInstance(Locale aLocale) Gets a calendar using the default time zone and specified locale.
static Calendar	getInstance(TimeZone zone) Gets a calendar using the specified time zone and default locale.
static Calendar	getInstance(TimeZone zone, Locale aLocale) Gets a calendar with the specified time zone and locale.
abstract int	getLeastMaximum(int field) Returns the lowest maximum value for the given calendar field of this Calendar instance.
abstract int	getMaximum(int field) Returns the maximum value for the given calendar field of this Calendar instance.
int	getMinimalDaysInFirstWeek() Gets what the minimal days required in the first week of the year are; e.g., if the first week is defined as one that contains the first day of the first month of a year, this method returns 1.
abstract int	getMinimum(int field) Returns the minimum value for the given calendar field of this Calendar instance.
Date	getTime() Returns a Date object representing this Calendar's time value (millisecond offset from the Epoch").
long	getTimeInMillis() Returns this Calendar's time value in milliseconds.
TimeZone	getTimeZone() Gets the time zone.
int	hashCode() Returns a hash code for this calendar.
protected int	internalGet(int field) Returns the value of the given calendar field.
boolean	isLenient() Tells whether date/time interpretation is to be lenient.
boolean	isSet(int field) Determines if the given calendar field has a value set, including cases that the value has been set by internal fields calculations triggered by a get method call.
abstract void	roll(int field, boolean up) Adds or subtracts (up/down) a single unit of time on the given time field without changing larger fields.
void	roll(int field, int amount) Adds the specified (signed) amount to the specified calendar field without changing larger fields.
void	set(int field, int value) Sets the given calendar field to the given value.
void	set(int year, int month, int date) Sets the values for the calendar fields YEAR, MONTH, and DAY_OF_MONTH.
void	set(int year, int month, int date, int hourOfDay, int minute) Sets the values for the calendar fields YEAR, MONTH, DAY_OF_MONTH, HOUR_OF_DAY, and MINUTE.
void	set(int year, int month, int date, int hourOfDay, int minute, int second) Sets the values for the fields YEAR, MONTH, DAY_OF_MONTH, HOUR, MINUTE, and SECOND.
void	setFirstDayOfWeek(int value) Sets what the first day of the week is; e.g., SUNDAY in the U.S., MONDAY in France.
void	setLenient(boolean lenient) Specifies whether or not date/time interpretation is to be lenient.
void	setMinimalDaysInFirstWeek(int value) Sets what the minimal days required in the first week of the year are; For example, if the first week is defined as one that contains the first day of the first month of a year, call this method with value 1.
void	setTime(Date date) Sets this Calendar's time with the given Date.
void	setTimeInMillis(long millis) Sets this Calendar's current time from the given long value.
void	setTimeZone(TimeZone value) Sets the time zone with the given time zone value.
String	toString() Return a string representation of this calendar.

Methods inherited from class java.lang.Object

finalize, getClass, notify, notifyAll, wait, wait, wait