java.util.Date inconsistent increment in scala

Question

I am tried to print 30 consecutive dates. The following is my code

val myDate: Long = LocalDate.parse("2017-07-01").atStartOfDay()
  .toInstant(ZoneOffset.of("+0")).getEpochSecond * 1000
val sdf: SimpleDateFormat = new SimpleDateFormat("yyyy-MM-dd")

(0 to 29).foreach(i => println(sdf.format(new Date(myDate + 24 * 3600 * 1000 * i))))

However output does not consecutive:

2017-07-01
2017-07-02
2017-07-03
2017-07-04
2017-07-05
2017-07-06
2017-07-07
2017-07-08
2017-07-09
2017-07-10
2017-07-11
2017-07-12
2017-07-13
2017-07-14
2017-07-15
2017-07-16
2017-07-17
2017-07-18
2017-07-19
2017-07-20
2017-07-21
2017-07-22
2017-07-23
2017-07-24
2017-07-25
2017-06-06 <--- !!!!!!
2017-06-07
2017-06-08
2017-06-09
2017-06-10

What is the reason of this behavior and how to fix it?

int math overflow: 24 * 3600 * 1000 * 25 = -2134967296. You need to force long math. — Andreas
– Andreas, Commented Aug 3, 2018 at 2:28

Elliott Frisch · Accepted Answer · 2018-08-03 02:51:59Z

6

Your math must overflow somewhere (integer math). Coerce the terms to long before you begin multiplication (you can append L) like

(0 to 29).foreach(i => println(sdf.format(new Date(myDate + 24L * 3600 * 1000 * i))))

Or, use TimeUnit.DAYS which can handle this correctly; something like

import java.util.concurrent.TimeUnit

and then

(0 to 29).foreach(i => println(sdf.format(new Date(myDate + TimeUnit.DAYS.toMillis(i)))))

But, since we're using LocalDate anyway; it's probably better to write it like

import java.time.{Instant, LocalDate, ZoneOffset}
import java.time.format.DateTimeFormatter
import java.time.temporal.ChronoUnit

val dtf: DateTimeFormatter = DateTimeFormatter.ISO_DATE
val ld: LocalDate = LocalDate.parse("2017-07-01")
(0 to 29).foreach(i => println(dtf.format(ld.plusDays(i))))

edited Aug 3, 2018 at 2:51

answered Aug 3, 2018 at 2:21

Elliott Frisch

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2 Comments

Andreas Over a year ago

"Prefer TimeUnit.DAYS"??? Wouldn't your preference be to stay way, waayyy clear of SimpleDateFormat if you have LocalDate support (see line 1 of question code)? Because even your code will produce incorrect result when it crosses Daylight Savings Time.

Andreas Over a year ago

You're right about int math overflow: 24 * 3600 * 1000 * 25 = -2134967296

jwvh · Accepted Answer · 2018-08-03 03:56:33Z

1

Such a simple task really doesn't need such complicated calculations.

import java.time.LocalDate
import java.time.temporal.ChronoUnit.DAYS

Seq.iterate(LocalDate.parse("2017-07-01"), 30)(DAYS.addTo(_, 1))
   .foreach(println)

answered Aug 3, 2018 at 3:56

jwvh

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1 Comment

som-snytt Over a year ago

Deserves a symbolic extension method! Maybe +@ for plus days.

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