oby1 oby1 - 2 months ago 8
Java Question

Recommended use for Joda-Time's DateMidnight

The javdoc for

LocalDate#toDateMidnight
reads as follows:


As from v1.5, you are recommended to avoid DateMidnight and use
toDateTimeAtStartOfDay() instead because of the exception detailed
below.

This method will throw an exception if the default time zone switches
to Daylight Savings Time at midnight and this LocalDate represents
that switchover date. The problem is that there is no such time as
midnight on the required date, and as such an exception is thrown.


The fact that midnight does not exist in certain time zones seems like reason enough to avoid using
DateMidnight
entirely (assuming your code is not using a fixed time zone that is known not to have this DST situation and will never need to use different time zones in the future).

However,
DateMidnight
is not deprecated and there is no similar recommendation or warning in the javadoc for the
DateMidnight
class itself. Furthermore, the
DateMidnight
constructor happily accepts an instant and time zone such that midnight does not exist on the given day, rather than throwing an
IllegalArgumentException
like
LocalDate#toDateMidnight
. The resulting
DateMidnight
behaves like a
DateTime
with time at start of day.

When midnight does not exist on a given day, why does
LocalDate#toDateMidnight
throw an exception while the
DateMidnight
constructor does not? What is the recommended use case for
DateMidnight
if any?

Answer

There is no good reason to use DateMidnight. LocalDate is the better option. Thats because midnight does not occur once a year in certain time-zones, completely messing up the usability of the class, and creating bugs in applications.

The constructor was fixed to avoid the worst problem, however seeing a DateMidnight object with the internal millisecond value pointing at 01:00 isn't exactly great.