I know add() adds the specified (signed) amount of time to the given time field, based on the calendar's rules.
And roll() adds the specified (signed) single unit of time on the given time field without changing larger fields.
I can't think of an everyday usage of roll() I would do everything by add().
Can you help me out with examples when do we use roll() and when add()?
EDIT 1
Joda answers are not accepted!
add() - almost always, as you said
roll() - for example you want to "dispense" events in one month. The algorithm may be to proceed a number of days and place the event, then proceed further. When the end of the month is reached, it should start over from the beginning. Hence roll().
Found in jGuru
Calendar.roll()
Changes a specific unit
and leaves 'larger' (in terms of time-month
is 'larger' than day) units unchanged. The API example is that
given a date of August 31, 1999,
rolling by (Calendar.MONTH, 8) yields
April 30, 1999. That is, the DAY was
changed to meet April's maximum, but
the 'larger' unit, YEAR, was
unchanged.
roll(): Rolls up 8 months here i.e., adding 8 months to Aug will result in Apr but year remains unchanged(untouched).
Calendar.add()
Will cause the
next 'larger' unit to change, if
necessary. That is, given a date of
August 31, 1999, add(Calendar.MONTH,
8) yields April 30, 2000. add() also
forces a recalculation of milliseconds
and all fields.
add(): Adds months to the current date i.e., adding 8 months to Aug will give Apr of Next Year, hence forces the Year change.
I was just asking the same question (which is how I found this page) and someone at my work place (well done, DCK) came up with a suggestion:
The date selectors on many smart phones (and other similar interfaces) will "roll" the day from the 31st to the 1st without altering the month, similarly for the month field.
I can't think of another use ATM and this one could be implemented in other ways, but at least it's an example!
Tim
Here is an example that will not work. The condition in the loop will never be satisfied, because the roll, once reaching January 31, 2014, will go back to January 1, 2014.
Calendar start=new GregorianCalendar();
start.set(Calendar.YEAR, 2014);
start.set(Calendar.MONTH, 0);
start.set(Calendar.DAY_OF_MONTH, 1);
//January 2, 2014
Calendar end=new GregorianCalendar();
end.set(Calendar.YEAR, 2014);
end.set(Calendar.MONTH, 1);
end.set(Calendar.DAY_OF_MONTH, 2);
//February 2, 2014
while (start.getTime().before(end.getTime())){
start.roll(Calendar.DATE, 1);
}
Related
I was studying the old Calendar API to see how bad it was, and I found out that Calendar has a roll method. Unlike the add method, roll does not change the values of bigger calendar fields.
For example, the calendar instance c represents the date 2019-08-31. Calling c.roll(Calendar.MONTH, 13) adds 13 to the month field, but does not change the year, so the result is 2019-09-30. Note that the day of month changes, because it is a smaller field.
Related
I tried to find such a method in the modern java.time API. I thought such a method has to be in LocalDate or LocalDateTime, but I found nothing of the sort.
So I tried to write my own roll method:
public static LocalDateTime roll(LocalDateTime ldt, TemporalField unit, long amount) {
LocalDateTime newLdt = ldt.plus(amount, unit.getBaseUnit());
return ldt.with(unit, newLdt.get(unit));
}
However, this only works for some cases, but not others. For example, it does not work for the case described in the documentation here:
Consider a GregorianCalendar originally set to Sunday June 6, 1999.
Calling roll(Calendar.WEEK_OF_MONTH, -1) sets the calendar to Tuesday
June 1, 1999, whereas calling add(Calendar.WEEK_OF_MONTH, -1) sets the
calendar to Sunday May 30, 1999. This is because the roll rule imposes
an additional constraint: The MONTH must not change when the
WEEK_OF_MONTH is rolled. Taken together with add rule 1, the resultant
date must be between Tuesday June 1 and Saturday June 5. According to
add rule 2, the DAY_OF_WEEK, an invariant when changing the
WEEK_OF_MONTH, is set to Tuesday, the closest possible value to Sunday
(where Sunday is the first day of the week).
My code:
System.out.println(roll(
LocalDate.of(1999, 6, 6).atStartOfDay(),
ChronoField.ALIGNED_WEEK_OF_MONTH, -1
));
outputs 1999-07-04T00:00, whereas using Calendar:
Calendar c = new GregorianCalendar(1999, 5, 6);
c.roll(Calendar.WEEK_OF_MONTH, -1);
System.out.println(c.getTime().toInstant());
outputs 1999-05-31T23:00:00Z, which is 1999-06-01 in my timezone.
What is an equivalent of roll in the java.time API? If there isn't one, how can I write a method to mimic it?
First, I cannot remember having seen any useful application of Calendar.roll. Second, I don’t think that the functionality is very well specified in corner cases. And the corner cases would be the interesting ones. Rolling month by 13 months would not be hard without the rollmethod. It may be that similar observations are the reasons why this functionality is not offered by java.time.
Instead I believe that we would have to resort to more manual ways of rolling. For your first example:
LocalDate date = LocalDate.of(2019, Month.JULY, 22);
int newMonthValue = 1 + (date.getMonthValue() - 1 + 13) % 12;
date = date.with(ChronoField.MONTH_OF_YEAR, newMonthValue);
System.out.println(date);
Output:
2019-08-22
I am using the fact that in the ISO chronology there are always 12 months in the year. Since % always gives a 0-based result, I subtract 1 from the 1-based month value before the modulo operation and add it back in afterwards And I am assuming a positive roll. If the number of months to roll may be negative, it gets slightly more complicated (left to the reader).
For other fields I think that a similar approach will work for most cases: Find the smallest and the largest possible value of the field given the larger fields and do some modulo operation.
It may become a challenge in some cases. For example, when summer time (DST) ends and the clock is turned backward from 3 to 2 AM, so the day is 25 hours long, how would you roll 37 hours from 6 AM? I’m sure it can be done. And I am also sure that the functionality is not built in.
For your example with rolling the week of month, another difference between the old and the modern API comes into play: a GregorianCalendar not only defines a calendar day and time, it also defines a week scheme consisting of a first day of the week and a minimum number of days in the first week. In java.time the week scheme is defined by a WeekFields object instead. So while rolling the week of month may be unambiguous in GregorianCalendar, without knowing the week scheme it isn’t with LocalDate or LocalDateTime. An attempt may be to assume ISO weeks (start on Monday, and the first week is the on that has at least 4 days of the new month in it), but it may not always be what a user had intended.
Week of month and week of year are special since weeks cross month and year boundaries. Here’s my attempt to implement a roll of week of month:
private static LocalDate rollWeekOfMonth(LocalDate date, int amount, WeekFields wf) {
LocalDate firstOfMonth = date.withDayOfMonth(1);
int firstWeekOfMonth = firstOfMonth.get(wf.weekOfMonth());
LocalDate lastOfMonth = date.with(TemporalAdjusters.lastDayOfMonth());
int lastWeekOfMonth = lastOfMonth.get(wf.weekOfMonth());
int weekCount = lastWeekOfMonth - firstWeekOfMonth + 1;
int newWeekOfMonth = firstWeekOfMonth
+ (date.get(wf.weekOfMonth()) - firstWeekOfMonth
+ amount % weekCount + weekCount)
% weekCount;
LocalDate result = date.with(wf.weekOfMonth(), newWeekOfMonth);
if (result.isBefore(firstOfMonth)) {
result = firstOfMonth;
} else if (result.isAfter(lastOfMonth)) {
result = lastOfMonth;
}
return result;
}
Try it out:
System.out.println(rollWeekOfMonth(LocalDate.of(1999, Month.JUNE, 6), -1, WeekFields.SUNDAY_START));
System.out.println(rollWeekOfMonth(LocalDate.of(1999, Month.JUNE, 6), -1, WeekFields.ISO));
Output:
1999-06-01
1999-06-30
Explanation: The documentation you quote assumes that Sunday is the first day of the week (it ends “where Sunday is the first day of the week”; it was probably written in the USA) so there is a week before Sunday June 6. And rolling by -1 week should roll into this week before. My first line of code does that.
In the ISO week scheme, Sunday June 6 belong to the week from Monday May 31 through Sunday June 6, so in June there is no week before this week. Therefore my second line of code rolls into the last week of June, June 28 through July 4. Since we cannot go outside June, June 30 is chosen.
I have not tested whether it behaves the same as GregorianCalendar. For comparison,the GregorianCalendar.roll implementation uses 52 code lines to handle the WEEK_OF_MONTH case, compared to my 20 lines. Either I have left something out of consideration, or java.time once again shows it superiority.
Rather my suggestion for the real world is: make your requirements clear and implement them directly on top of java.time, ignoring how the old API behaved. As an academic exercise, your question is a fun and interesting one.
TL;DR
There is no equivalent.
Think about whether you really need the behavior of roll of java.util.Calendar:
/**
* Adds or subtracts (up/down) a single unit of time on the given time
* field without changing larger fields. For example, to roll the current
* date up by one day, you can achieve it by calling:
* 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.
*
* #param field the time field.
* #param up indicates if the value of the specified time field is to be
* rolled up or rolled down. Use true if rolling up, false otherwise.
* #see Calendar#add(int,int)
* #see Calendar#set(int,int)
*/
public void roll(int field, boolean up);
When using java.time in Scala I experienced a strange behavior. I want to calculate the number of months between two dates like this:
import java.time._
Period.between(LocalDate.parse("2015-03-31"), LocalDate.parse("2015-04-30"))
// java.time.Period = P30D
// I would expect java.time.Period = P1M
Period.between(LocalDate.parse("2015-03-31"), LocalDate.parse("2015-05-01"))
// java.time.Period = P1M1D
Is this a bug or do I have got it all wrong?
org.joda.time works as I would expect it:
import org.joda.time.DateTime
import org.joda.time.Months
Months.monthsBetween( new DateTime().withDate(2015, 3, 31), new DateTime().withDate(2015, 4, 30))
//org.joda.time.Months = P1M
When adding months to a java.time.LocalDate it works fine:
java.time.LocalDate.parse("2015-03-31").plusMonths(1)
// java.time.LocalDate = 2015-04-30
This is not a bug, and it is behaving like expected (see also JDK-8152384 and JDK-8037392, which were closed as "Not An Issue"). Joda Time and the Java Time API have different behaviour regarding this. Quoting Stephen Colebourne from the previous bug report:
The OP appears to want a rule where the days are calculated based on the original month length, not the one that results once the month-year difference is applied. The OP is not wrong, its just that its not how we choose to make the calculation in java.time.
Indeed, from Period.between:
The period is calculated by removing complete months, then calculating the remaining number of days, adjusting to ensure that both have the same sign. [...] A month is considered to be complete if the end day-of-month is greater than or equal to the start day-of-month.
Between the 31st of March, and the 30th of April, no complete month has elapsed. As such, you have a period containing the number of days between the two dates, which is 30. To have the complete month of April elapsed, you need to add one day to the end date, and make it the 1st of June.
Joda has a different way of calculating the month period. From Months.monthsBetween:
This method calculates by adding months to the start date until the result is past the end date. As such, a period from the end of a "long" month to the end of a "short" month is counted as a whole month.
Joda explicitly takes the variable number of days in a month into account when calculating the number of months between the two dates. Java Time doesn't.
I agree that it is a bit unexpected, but it is the correct result if you take into account the javadoc.
From the javadoc
The start date is included, but the end date is not. The period is calculated by removing complete months, then calculating the remaining number of days, adjusting to ensure that both have the same sign. The number of months is then split into years and months based on a 12 month year. A month is considered if the end day-of-month is greater than or equal to the start day-of-month. For example, from 2010-01-15 to 2011-03-18 is one year, two months and three days.
The difference comes from what a "complete month" means.
In this case 1st April to 1st May (exclusive) is considered a complete month while 31st March to 30th April (exclusive) is not.
I believe the Period.between is returning P30D in the first example because the second parameter is exclusive. This is according to https://docs.oracle.com/javase/8/docs/api/java/time/Period.html#between-java.time.LocalDate-java.time.LocalDate-
public static Period between(LocalDate startDateInclusive, LocalDate endDateExclusive)
So let's say I have this code:
//someParameterizedDate = New Years Eve 2011
Calendar cal = new GregorianCalendar();
cal.setTime(someParameterizedDate);
cal.roll(Calendar.DAY_OF_YEAR, 1);
Will the calendar now be equal to january 1, 2012? I found all the JavaDocs a little confusing.
java.util.Calendar
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.
You roll with DAY_OF_YEAR which means it will not affect MONTH or YEAR which are larger units
So basically, you should get you to December 1st 2011
You can use add if you want it to go to January 1st 2012
I am having the following code
Calendar calendar = Calendar.getInstance();
calendar.set(2011, 7, 29); //case 1
// calendar.set(2011, 7, 30); //case 2
// calendar.set(2011, 7, 31); //case 3
System.out.println("===After seting the date== "+calendar.getTime());
System.out.println("=================================================================");
calendar.add(Calendar.MONTH, 6);
System.out.println("===result after adding 6 months== "+calendar.getTime());
and for the case 2 and case 3 also i am getting the same results. it should overflow to next month and show the new date. but its not happening.
It's not clear whether you're using "should" to mean "this is what I want it to do" or to mean "this is what I believe it's documented to do". The documentation actually backs up the behaviour given:
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.
The second part of the example is exactly the situation you're facing: the DAY_OF_MONTH is expected to be invariant, but has to change in order to stay within the right month, so it's adjusted to the closest possible value (29 in your case).
So it looks like the behaviour is consistent to me - in what way do you believe it's inconsistent?
I found the following to be the simplest method to achieve by using gregorian calendars.
set the day of month to the last day of the next month for preventing it to become the 1st day of next to next month.
Try following.
cal.set(Calendar.DAY_OF_MONTH, Math.min(Calendar.DAY_OF_MONTH, cal.getActualMaximum(Calendar.DAY_OF_MONTH)));
We are using Calendar.roll to either move the dates up or down. The javadoc mentions that the larger fields are not modified (i.e. if we move the date by 5 to the left starting on the first day of the month, unfortunately the calendar.getTime() doesn't get me a value from the previous month). The month value remains unchanged, how do I change this behavior. I really would like to move the date value as appropriate. (e.g. If I moved 5 days to the left on Aug 1st, 2010 - I would want to see Jun 27th, 2010 instead of Aug 27th, 2010). What am I missing here?
You can use Calendar.add with a negative amount.
You will need to use add(Calendar.DATE, -5) method from Calendar because of roll rule check.
roll method is described as :
Add to field a signed amount without
changing larger fields. A negative
roll amount means to subtract from
field without changing larger fields.
Example: Consider a GregorianCalendar
originally set to August 31, 1999.
Calling roll(Calendar.MONTH, 8) sets
the calendar to April 30, 1999. Using
a GregorianCalendar, the DAY_OF_MONTH
field cannot be 31 in the month April.
DAY_OF_MONTH is set to the closest
possible value, 30. The YEAR field
maintains the value of 1999 because it
is a larger field than MONTH.
Example: Consider a GregorianCalendar
originally set to Sunday June 6, 1999.
Calling roll(Calendar.WEEK_OF_MONTH,
-1) sets the calendar to Tuesday June 1, 1999, whereas calling
add(Calendar.WEEK_OF_MONTH, -1) sets
the calendar to Sunday May 30, 1999.
This is because the roll rule imposes
an additional constraint: The MONTH
must not change when the WEEK_OF_MONTH
is rolled. Taken together with add
rule 1, the resultant date must be
between Tuesday June 1 and Saturday
June 5. According to add rule 2, the
DAY_OF_WEEK, an invariant when
changing the WEEK_OF_MONTH, is set to
Tuesday, the closest possible value to
Sunday (where Sunday is the first day
of the week).