Sunday, February 22, 2009

Adjusting Date and Time with javax.time

Java's JSR-310 date and time API, co-lead by Michael Nascimento Santos and Stephen Colebourne is a natural spinoff from the venerable joda time. The implementation has many advantages over java util's Date and Calendar. Compared to java.util.Date the API has many types of Date and Time including LocalDate, LocalTime, LocalDateTime, OffsetDate, OffsetTime, OffsetDateTime, and ZonedDateTime.

The new JSR-310 date, time, and date/time classes are immutable and thread safe, unlike java.util.Calendar, but at the same time they offer many basic math, adjusters, and matchers that enable date and time calculations missing in java.util.Date. They are created from a TimeSource that can be tied to the System clock, fixed, or offset in time. This makes the classes extremely test friendly.

This entry discusses how the basic date/time math work and how the date and time adjusters can be used to solve common problems. Lets look first at the basic math.

Look Ma, no Setters: The JSR-310 date/time classes are immutable and thread safe. To accomplish this, the API doesn't include any setXX methods. Days, Hours, Years, etc are manipulated through "plus", "minus", and "with" methods that return new objects of the same type. Here are a few examples, first with date then time:

// tomorrow and 5 years from now
clock = Clock.systemDefaultZone()
today =
assert clock.tomorrow() == today.plusDays(1)
fiveYearsFromNow = today.plusYears( 5 )
assert today.plusMonths( 60 ).year == fiveYearsFromNow.year

// two hours ago
now = clock.timeToSecond()
twoHoursAgo = now.minusHours( 2 )
assert now.minusMinutes( 120 ) == twoHoursAgo

// use offset datetime to get today at noon
noon = clock.offsetDateTime().withTime( 12, 0, 0 )
assert noon.hourOfDay == 12
assert noon.minuteOfHour == 0
assert noon.secondOfMinute == 0

The Adjusters: Here is a tricky problem: how do you compute the specific date of a week numbered day of the month, for example the 3rd friday or 4th tuesday. Bay area residents know that not being able to calculate these simple problems can cost real money (street sweep adys). So here is how JSR-310 handles this:

dt = clock.offsetDate()

thirdFridayAdjuster = DateAdjusters.dayOfWeekInMonth( 3, DayOfWeek.FRIDAY )
fourthTuesdayAdjuster = DateAdjusters.dayOfWeekInMonth( 4, DayOfWeek.TUESDAY )

thirdFriday = dt.with( thirdFridayAdjuster )
println "third friday -> ${thirdFriday}, ${thirdFriday.toDayOfWeek()}, ${thirdFridayAdjuster}"
assert DayOfWeek.FRIDAY == thirdFriday.toDayOfWeek()

fourthTuesday = dt.with( fourthTuesdayAdjuster )
println "fourth tuesday -> ${fourthTuesday}, ${fourthTuesday.toDayOfWeek()}, ${fourthTuesdayAdjuster}"
assert DayOfWeek.TUESDAY == fourthTuesday.toDayOfWeek()

These examples just scratch the surface of the many date/time manipluation methods for JSR-310's javax.time package. The next entry will discuss how JSR-310 matchers help determine if a date lands on a leap year, leap day, last day of the month, etc. and how to use this in groovy.

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