javajava-timelocaldatejava-calendar

Calendar behavior is not compatible with LocalDate?

Hi buddies I'm in a trouble trying to migrate a behavior from calendar to localdate.

payDate.set(Calendar.DAY_OF_MONTH,payDay)

Lets imagine that payDate had the current date, 2020-01-29

for business reasons payDay can had the value of 0, so, when the previous code line is executed with the previous scenario, the result is that payDate update the date to 2019-12-31, that is to say the the date back to the last day of the past month.

I'm not sure, the technical reason of this, if someone can explain to me this I'll be so thankful, I tried checking the java doc but it was not helpful.

So I need to replicate that behavior with LocalDate java library. From my point of view; the similar of set method from Calendar with the value of DAY_OF_MONTH in LocalDate is:

payDate.withDayOfMonth(payDay)

But when the below scenario is presented and payDay is equal to 0 I get an error:

java.time.DateTimeException: Invalid value for DayOfMonth (valid values 1 - 28/31): 0

Also I had some ideas about how can I get the same result of calendar in localDate when the rule comes on (if payDay is 0, return to the last day of previous month), but are too verbose.

If you know a similar behavior on LocalDate please help me. Thanks.

Solution

  • TL;DR: Use payDate = payDate.plusDays(payDay - payDate.getDayOfMonth());

    The behavior of Calendar you're describing is documented in the javadoc:

    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.

    To show the effect of this, try setting the date of a Calendar to January 70, 2020:

    Calendar cal = Calendar.getInstance();
cal.clear();
cal.set(2020, Calendar.JANUARY, 70);
System.out.println(new SimpleDateFormat("yyyy-MM-dd").format(cal.getTime()));

    Output

    2020-03-10

    You would get the same result if you did:

    cal.set(2020, Calendar.JANUARY, 1);
cal.add(Calendar.DAY_OF_MONTH, 69);

    LocalDate is always non-lenient, so you can't set the day-of-month value to a value that is out-of-range. You can however get the same result as what Calendar does, by changing the operation to "add" instead of "set".

    So, if you have a particular date, e.g. the 2020-01-29 date mentioned in the question, and you want to "set" the day-of-month value to 70 or 0, with same lenient overflow logic as Calendar has, do this:

    LocalDate date = LocalDate.parse("2020-01-29");
date = date.plusDays(70 - date.getDayOfMonth());
System.out.println(date);

    LocalDate date = LocalDate.parse("2020-01-29");
date = date.plusDays(0 - date.getDayOfMonth());
System.out.println(date);

    Output

    2020-03-10

    2019-12-31

    As you can see, date.plusDays(dayToSet - date.getDayOfMonth()) will give you the desired result.