This class represents the Gregorian calendar, that is used in most
countries all over the world. It does also handle the Julian calendar
for dates smaller than the date of the change to the Gregorian calendar.
The Gregorian calendar differs from the Julian calendar by a different
leap year rule (no leap year every 100 years, except if year is divisible
by 400).
This change date is different from country to country, and can be changed with
setGregorianChange
. The first countries to adopt the Gregorian
calendar did so on the 15th of October, 1582. This date followed October
the 4th, 1582 in the Julian calendar system. The non-existant days that were
omitted when the change took place are interpreted as Gregorian dates.
Prior to the changeover date, New Year's Day occurred on the 25th of March.
However, this class always takes New Year's Day as being the 1st of January.
Client code should manually adapt the year value, if required, for dates
between January the 1st and March the 24th in years prior to the changeover.
Any date infinitely forwards or backwards in time can be represented by
this class. A
proleptic calendar system is used, which allows
future dates to be created via the existing rules. This allows meaningful
and consistent dates to be produced for all years. However, dates are only
historically accurate following March the 1st, 4AD when the Julian calendar
system was adopted. Prior to this, leap year rules were applied erraticly.
There are two eras available for the Gregorian calendar, namely BC and AD.
Weeks are defined as a period of seven days, beginning on the first day
of the week, as returned by
getFirstDayOfWeek()
, and ending
on the day prior to this.
The weeks of the year are numbered from 1 to a possible 53. The first week
of the year is defined as the first week that contains at least the minimum
number of days of the first week in the new year (retrieved via
getMinimalDaysInFirstWeek()
). All weeks after this are numbered
from 2 onwards.
For example, take the year 2004. It began on a Thursday. The first week
of 2004 depends both on where a week begins and how long it must minimally
last. Let's say that the week begins on a Monday and must have a minimum
of 5 days. In this case, the first week begins on Monday, the 5th of January.
The first 4 days (Thursday to Sunday) are not eligible, as they are too few
to make up the minimum number of days of the first week which must be in
the new year. If the minimum was lowered to 4 days, then the first week
would instead begin on Monday, the 29th of December, 2003. This first week
has 4 of its days in the new year, and is now eligible.
The weeks of the month are numbered from 0 to a possible 6. The first week
of the month (numbered 1) is a set of days, prior to the first day of the week,
which number at least the minimum number of days in a week. Unlike the first
week of the year, the first week of the month only uses days from that particular
month. As a consequence, it may have a variable number of days (from the minimum
number required up to a full week of 7) and it need not start on the first day of
the week. It must, however, be following by the first day of the week, as this
marks the beginning of week 2. Any days of the month which occur prior to the
first week (because the first day of the week occurs before the minimum number
of days is met) are seen as week 0.
Again, we will take the example of the year 2004 to demonstrate this. September
2004 begins on a Wednesday. Taking our first day of the week as Monday, and the
minimum length of the first week as 6, we find that week 1 runs from Monday,
the 6th of September to Sunday the 12th. Prior to the 6th, there are only
5 days (Wednesday through to Sunday). This is too small a number to meet the
minimum, so these are classed as being days in week 0. Week 2 begins on the
13th, and so on. This changes if we reduce the minimum to 5. In this case,
week 1 is a truncated week from Wednesday the 1st to Sunday the 5th, and week
0 doesn't exist. The first seven day week is week 2, starting on the 6th.
On using the
clear()
method, the Gregorian calendar returns
to its default value of the 1st of January, 1970 AD 00:00:00 (the epoch).
The day of the week is set to the correct day for that particular time.
The day is also the first of the month, and the date is in week 0.
add
public void add(int field,
int amount)
Adds the specified amount of time to the given time field. The
amount may be negative to subtract the time. If the field overflows
it does what you expect: Jan, 25 + 10 Days is Feb, 4.
- add in interface Calendar
field
- one of the time field constants.amount
- the amount of time to add.
IllegalArgumentException
- if field
is
ZONE_OFFSET
, DST_OFFSET
, or invalid; or
if amount
contains an out-of-range value and the calendar
is not in lenient mode.
computeFields
protected void computeFields()
Converts the milliseconds since the epoch UTC
(time
) to time fields
(fields
).
- computeFields in interface Calendar
computeTime
protected void computeTime()
Converts the time field values (fields
) to
milliseconds since the epoch UTC (time
).
- computeTime in interface Calendar
IllegalArgumentException
- if any calendar fields
are invalid.
equals
public boolean equals(Object o)
Compares the given calendar with this. An object, o, is
equivalent to this if it is also a GregorianCalendar
with the same time since the epoch under the same conditions
(same change date and same time zone).
- equals in interface Calendar
o
- the object to that we should compare.
- true, if the given object is a calendar, that represents
the same time (but doesn't necessarily have the same fields).
IllegalArgumentException
- if one of the fields
ZONE_OFFSET
or DST_OFFSET
is
specified, if an unknown field is specified or if one
of the calendar fields receives an illegal value when
leniancy is not enabled.
getActualMaximum
public int getActualMaximum(int field)
Gets the actual maximum value that is allowed for the specified field.
This value is dependent on the values of the other fields. Note that
this calls complete()
if not enough fields are set. This
can have ugly side effects. The value given depends on the current time
used by this instance; thus, leap years have a maximum day of month value of
29, rather than 28.
- getActualMaximum in interface Calendar
field
- the time field. One of the time field constants.
- the actual maximum value.
getActualMinimum
public int getActualMinimum(int field)
Gets the actual minimum value that is allowed for the specified field.
This value is dependent on the values of the other fields. Note that
this calls complete()
if not enough fields are set. This
can have ugly side effects. The value given depends on the current
time used by this instance.
- getActualMinimum in interface Calendar
field
- the time field. One of the time field constants.
- the actual minimum value.
- 1.2
getGreatestMinimum
public int getGreatestMinimum(int field)
Gets the greatest minimum value that is allowed for the specified field.
This is the largest value returned by the getActualMinimum(int)
method.
- getGreatestMinimum in interface Calendar
field
- the time field. One of the time field constants.
- the greatest minimum value.
getActualMinimum(int)
getGregorianChange
public final Date getGregorianChange()
Gets the date of the switch from Julian dates to Gregorian dates.
- the date of the change.
getLeastMaximum
public int getLeastMaximum(int field)
Gets the smallest maximum value that is allowed for the
specified field. This is the smallest value returned
by the getActualMaximum(int)
. For example,
this is 28 for DAY_OF_MONTH (as all months have at least
28 days).
- getLeastMaximum in interface Calendar
field
- the time field. One of the time field constants.
- the least maximum value.
- 1.2
getActualMaximum(int)
getMaximum
public int getMaximum(int field)
Gets the biggest value that is allowed for the specified field.
- getMaximum in interface Calendar
field
- one of the time field constants.
- the biggest value.
getMinimum
public int getMinimum(int field)
Gets the smallest value that is allowed for the specified field.
- getMinimum in interface Calendar
field
- one of the time field constants.
- the smallest value for the specified field.
isLeapYear
public boolean isLeapYear(int year)
Determines if the given year is a leap year. The result is
undefined if the Gregorian change took place in 1800, so that
the end of February is skipped, and that year is specified.
(well...).
To specify a year in the BC era, use a negative value calculated
as 1 - y, where y is the required year in BC. So, 1 BC is 0,
2 BC is -1, 3 BC is -2, etc.
year
- a year (use a negative value for BC).
- true, if the given year is a leap year, false otherwise.
roll
public void roll(int field,
boolean up)
Rolls the specified time field up or down. This means add one
to the specified field, but don't change the other fields. If
the maximum for this field is reached, start over with the
minimum value.
Note: There may be situation, where the other
fields must be changed, e.g rolling the month on May, 31.
The date June, 31 is automatically converted to July, 1.
This requires lenient settings.
- roll in interface Calendar
field
- the time field. One of the time field constants.up
- the direction, true for up, false for down.
IllegalArgumentException
- if one of the fields
ZONE_OFFSET
or DST_OFFSET
is
specified, if an unknown field is specified or if one
of the calendar fields receives an illegal value when
leniancy is not enabled.
roll
public void roll(int field,
int amount)
Rolls the specified time field by the given amount. This means
add amount to the specified field, but don't change the other
fields. If the maximum for this field is reached, start over
with the minimum value and vice versa for negative amounts.
Note: There may be situation, where the other
fields must be changed, e.g rolling the month on May, 31.
The date June, 31 is automatically corrected to June, 30.
- roll in interface Calendar
field
- the time field. One of the time field constants.amount
- the amount by which we should roll.
IllegalArgumentException
- if one of the fields
ZONE_OFFSET
or DST_OFFSET
is
specified, if an unknown field is specified or if one
of the calendar fields receives an illegal value when
leniancy is not enabled.
setGregorianChange
public void setGregorianChange(Date date)
Sets the date of the switch from Julian dates to Gregorian dates.
You can use new Date(Long.MAX_VALUE)
to use a pure
Julian calendar, or Long.MIN_VALUE
for a pure Gregorian
calendar.
date
- the date of the change.
java.util.GregorianCalendar
Copyright (C) 1998, 1999, 2001, 2002, 2003, 2004
Free Software Foundation, Inc.
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