将字符串解析为日期的异常

Question

I cannot see why this code is failing:

@Test
public void test() {
    final SimpleDateFormat sdf= new SimpleDateFormat("yyyyMMddHHmm");
    try {
        sdf.setLenient(false);
        Date d = sdf.parse("202003290230");
    } catch (ParseException e){
        e.printStackTrace();
    }
}

I get the exception java.text.ParseException: Unparseable date .

If I set sdf.setLenient(true) then it works, but the time in the object Date returned is '03:30' and not '02:30'.

Can someone explain me what's going on in here? Thanks.

Answer 1

As OH GOD SPIDERS noted in a comment your real problem is that that time doesn't exist in you default time zone because of the spring forward , the transition to summer time (DST).

java.time

I recommend that you use java.time, the modern Java date and time API, for your date and time work. The short-sighted answer is: use LocalDateTime from java.time.

    DateTimeFormatter formatter = DateTimeFormatter.ofPattern("uuuuMMddHHmm");
    String dateTimeString = "202003290230";
    LocalDateTime ldt = LocalDateTime.parse(dateTimeString, formatter);
    System.out.println(ldt);

Output is:

2020-03-29T02:30

A LocalDateTime is a date and time without time zone. So it doesn't discover that the time doesn't exist in your time zone. The result is 2:30 as in the string.

Detecting a non-existing time

It seems you're in a Central European time zone or some other time zone where summer time began on the last Sunday in March, and the clocks were turned forward from 2 to 3 (AM). So there was no 2:30. Supposing that you want to know, you may do:

    ZoneId zone = ZoneId.of("Europe/Paris");
    ZonedDateTime zdt = ldt.atZone(zone);
    if (zdt.toLocalDateTime().equals(ldt)) {
        System.out.println("The time " + zdt + " exists");
    } else {
        System.out.println("The time " + ldt + " does not exist in " + zone);
    }

The time 2020-03-29T02:30 does not exist in Europe/Paris

java.time too gives you 3:30 instead of 2:30 when given a time in the spring gap. So when converting back from ZonedDateTime to LocalDateTime in this case we don't get the same time again, which we use for detecting the non-existing time.

I further recommend that you use ZonedDateTime for past dates and times, not LocalDateTime .

Java 6?

This project is made with Java6 and the company doesn't want to update it, …

java.time has been backported, and I have tested the code above with the backport, ThreeTen Backport, see the link at the bottom.

• In Java 8 and later and on newer Android devices (from API level 26) the modern API comes built-in.
• In non-Android Java 6 and 7 get the ThreeTen Backport, the backport of the modern classes (ThreeTen for JSR 310). I'm unsure whether the very latest release of ThreeTen Backport works with Java 6 or only with Java 7(+). If this is an issue, go back a few releases and find one that works with Java 6 too.
• On older Android either use desugaring or the Android edition of ThreeTen Backport. It's called ThreeTenABP. In the latter case make sure you import the date and time classes from org.threeten.bp with subpackages.

Don't use SimpleDateFormat

The date and time classes you were trying to use, Date and SimpleDateFormat , are poorly designed and long outdated. In your case they showed a behaviour that depends on a time zone that isn't present in the code at all, which is quite confusing. I recommend that you don't use those classes.

Links

• Oracle tutorial: Date Time explaining how to use java.time.
• Java Specification Request (JSR) 310 , where java.time was first described.
• ThreeTen Backport project , the backport of java.time to Java 6 and 7 (ThreeTen for JSR-310).
• Java 8+ APIs available through desugaring
• ThreeTenABP , Android edition of ThreeTen Backport
• Question: How to use ThreeTenABP in Android Project , with a very thorough explanation.