Calendar

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java.util
Class Calendar

java.lang.Object   
  java.util.Calendar

All Implemented Interfaces:: Serializable , Cloneable , Comparable <Calendar >

Direct Known Subclasses:: GregorianCalendar

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

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:

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.
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

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.

Since:: JDK1.1
See Also:: System.currentTimeMillis() , Date , GregorianCalendar , TimeZone , DateFormat , Serialized Form

Field Summary
`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.
`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.
`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.
`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.
`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.
`static int`	`JULY` Value of the `MONTH` field indicating the seventh month of the year.
`static int`	`JUNE` Value of the `MONTH` field indicating the sixth month of the year.
`static int`	`MARCH` Value of the `MONTH` field indicating the third month of the year.
`static int`	`MAY` Value of the `MONTH` field indicating the fifth month of the year.
`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.
`static int`	`OCTOBER` Value of the `MONTH` field indicating the tenth month of the year.
`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.
`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.