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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

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

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
 

 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 DateFormat to format dates.

Field Manipulation

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 java.util.GregorianCalendar.roll(intint) .

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.

The default implementation of this method uses an iterative algorithm to determine the actual minimum value for the calendar field.

This field reflects the correct daylight saving offset value of the time zone of this Calendar if the TimeZone implementation subclass supports historical Daylight Saving Time schedule changes.

This field reflects the correct GMT offset value of the time zone of this Calendar if the TimeZone implementation subclass supports historical GMT offset changes.

add(Calendar.DAY_OF_MONTH, -5).
Parameters:
  field - the calendar field.
Parameters:
  amount - the amount of date or time to be added to the field.
See Also:   Calendar.roll(int,int)
See Also:   Calendar.set(int,int)

 compareTo(when) > 0
 

 compareTo(when) < 0
 

The Calendar.HOUR_OF_DAY , Calendar.HOUR and Calendar.AM_PM fields are handled independently and the the resolution rule for the time of day is applied. Clearing one of the fields doesn't reset the hour of day value of this Calendar. Use Calendar.set(int,int) set(Calendar.HOUR_OF_DAY, 0) to reset the hour value.
Parameters:
  field - the calendar field to be cleared.
See Also:   Calendar.clear()

The Calendar parameters are the values represented by the isLenient, getFirstDayOfWeek, getMinimalDaysInFirstWeek and getTimeZone methods. If there is any difference in those parameters between the two Calendars, this method returns false.

Use the Calendar.compareTo(Calendar) compareTo method to compare only the time values.
Parameters:
  obj - the object to compare with. true if this object is equal to obj;false otherwise.

The default implementation of this method uses an iterative algorithm to determine the actual maximum value for the calendar field. Subclasses should, if possible, override this with a more efficient implementation.
Parameters:
  field - the calendar field the maximum of the given calendar field for the timevalue of this Calendar
See Also:   Calendar.getMinimum(int)
See Also:   Calendar.getMaximum(int)
See Also:   Calendar.getGreatestMinimum(int)
See Also:   Calendar.getLeastMaximum(int)
See Also:   Calendar.getActualMinimum(int)
since:
   1.2

The default implementation of this method uses an iterative algorithm to determine the actual minimum value for the calendar field. Subclasses should, if possible, override this with a more efficient implementation - in many cases, they can simply return getMinimum().
Parameters:
  field - the calendar field the minimum of the given calendar field for the timevalue of this Calendar
See Also:   Calendar.getMinimum(int)
See Also:   Calendar.getMaximum(int)
See Also:   Calendar.getGreatestMinimum(int)
See Also:   Calendar.getLeastMaximum(int)
See Also:   Calendar.getActualMaximum(int)
since:
   1.2

For example, if this Calendar is a GregorianCalendar and its date is 2005-01-01, then the string representation of the Calendar.MONTH field would be "January" in the long style in an English locale or "Jan" in the short style. However, no string representation would be available for the Calendar.DAY_OF_MONTH field, and this method would return null.

The default implementation supports the calendar fields for which a DateFormatSymbols has names in the given locale.
Parameters:
  field - the calendar field for which the string representationis returned
Parameters:
  style - the style applied to the string representation; one ofCalendar.SHORT or Calendar.LONG.
Parameters:
  locale - the locale for the string representation the string representation of the givenfield in the given style, ornull if no string representation isapplicable.
exception:
  IllegalArgumentException - if field or style is invalid,or if this Calendar is non-lenient and anyof the calendar fields have invalid values
exception:
  NullPointerException - if locale is null
since:
   1.6

The values of other calendar fields may be taken into account to determine a set of display names. For example, if this Calendar is a lunisolar calendar system and the year value given by the Calendar.YEAR field has a leap month, this method would return month names containing the leap month name, and month names are mapped to their values specific for the year.

The default implementation supports display names contained in a DateFormatSymbols . For example, if field is Calendar.MONTH and style is Calendar.ALL_STYLES , this method returns a Map containing all strings returned by DateFormatSymbols.getShortMonths and DateFormatSymbols.getMonths .
Parameters:
  field - the calendar field for which the display names are returned
Parameters:
  style - the style applied to the display names; one of Calendar.SHORT, Calendar.LONG, or Calendar.ALL_STYLES.
Parameters:
  locale - the locale for the display names a Map containing all display names instyle and locale and theirfield values, or null if no display namesare defined for field
exception:
  IllegalArgumentException - if field or style is invalid,or if this Calendar is non-lenient and anyof the calendar fields have invalid values
exception:
  NullPointerException - if locale is null
since:
   1.6

roll(Calendar.DATE, true). When rolling on the year or Calendar.YEAR field, it will roll the year value in the range between 1 and the value returned by calling getMaximum(Calendar.YEAR). When rolling on the month or Calendar.MONTH field, other fields like date might conflict and, need to be changed. For instance, rolling the month on the date 01/31/96 will result in 02/29/96. When rolling on the hour-in-day or Calendar.HOUR_OF_DAY field, it will roll the hour value in the range between 0 and 23, which is zero-based.
Parameters:
  field - the time field.
Parameters:
  up - indicates if the value of the specified time field is to berolled up or rolled down. Use true if rolling up, false otherwise.
See Also:   Calendar.add(intint)
See Also:   Calendar.set(intint)

NOTE: This default implementation on Calendar just repeatedly calls the version of Calendar.roll(int,boolean) roll() that rolls by one unit. This may not always do the right thing. For example, if the DAY_OF_MONTH field is 31, rolling through February will leave it set to 28. The GregorianCalendar version of this function takes care of this problem. Other subclasses should also provide overrides of this function that do the right thing.
Parameters:
  field - the calendar field.
Parameters:
  amount - the signed amount to add to the calendar field.
since:
   1.2
See Also:   Calendar.roll(int,boolean)
See Also:   Calendar.add(int,int)
See Also:   Calendar.set(int,int)

This method supports the calendar fields resolution as described in the class description. If the bit mask for a given field is on and its field has not been set (i.e., isSet(field) is false), then the default value of the field has to be used, which case means that the field has been selected because the selected combination involves the field. a bit mask of selected fields
See Also:   Calendar.isExternallySet(int)
See Also:   Calendar.setInternallySetState(int)

Note: Calling setTime() with Date(Long.MAX_VALUE) or Date(Long.MIN_VALUE) may yield incorrect field values from get().
Parameters:
  date - the given Date.
See Also:   Calendar.getTime()
See Also:   Calendar.setTimeInMillis(long)