Java date handling in improper format

Question

Firstly, I'm a C# dev learning Java. I'm converting a program I wrote in C# as an exercise and am having problems with parsing a date being submitted from an html form. The form is sent as an email and the java program reads the emails and parses the body. I have a drop down calendar for my peeps to select a date from but there's always some jerk who has to type it in and mess everything up. Currently I am doing this in my code:

public void SetDatePlayed(String datePlayed)
{
    this.datePlayed = LocalDate.parse(datePlayed);
}

datePlayed being passed in is a string usually formatted as yyyy-MM-dd but of course someone typed in 3/7 instead of using the calendar drop down on the form. this.datePlayed is a LocalDate. In C# I would just end up with a date that assumed 2020 for the year - no problem. LocalDate really wants it in the yyyy-MM-dd format and I don't know what the best practice here is with Java. I've been googling it all morning and haven't come across this as being an issue for anyone else. I don't care if I'm using LocalDate but I do need it to be a date datatype so I can do date checks, sorts, searches, etc later on.

"In C# I would just end up with a date that assumed 2020 for the year" What C# API handles both yyyy-MM-dd and 3/7 formats? I don't recall such a thing... — Sweeper
– Sweeper, Commented Apr 11, 2020 at 12:07
I'd look inside DateTimeFormatter and see how it does the parsing. Especially bits that use java.time.format.ResolverStyle#SMART — bobah
– bobah, Commented Apr 11, 2020 at 12:15
@Sweeper sure try DateTime.Parse("3/7") and you'll see you get a value of 3/7/2020 12:00:00AM. 99% of my input coming through my website uses yyyy-MM-dd so I know it's able to deal with both formats. — user1572
– user1572, Commented Apr 11, 2020 at 13:32
@user1572 Oh, you meant that... I never dared to let it guess the format like that... — Sweeper
– Sweeper, Commented Apr 11, 2020 at 14:51

Andreas · Accepted Answer · 2020-04-11 12:51:38Z

You can use DateTimeFormatterBuilder and parseDefaulting() to supply default value for the year.

Building on the answer by Sweeper, it can be done like this:

static LocalDate parseLoosely(String text) {
    DateTimeFormatter fmt = new DateTimeFormatterBuilder()
            .appendPattern("[uuuu-M-d][M/d/uuuu][M/d]")
            .parseDefaulting(ChronoField.YEAR, Year.now().getValue())
            .toFormatter();
    return LocalDate.parse(text, fmt);
}

^{Warning: Do not cache the formatter in e.g. a static field, since it snapshots the year, if the program might be running across New Year's Eve, which a webapp would, unless you add logic to make the cache auto-refresh on year change.}

Test

System.out.println(parseLoosely("2019-04-07"));
System.out.println(parseLoosely("2019-4-7"));
System.out.println(parseLoosely("4/7/2019"));
System.out.println(parseLoosely("4/7"));

Output

Sweeper · Accepted Answer · 2020-04-11 12:30:58Z

2

I see two possible interpretations of your question. I'm not sure which one it is, so I'll answer both.

How do I parse a date string in a format that has no year, such as M/d (3/7), to a LocalDate?

You don't. A LocalDate by definition must have year, month, and day. If you only have a month and a day, that's a MonthDay:

MonthDay md = MonthDay.parse("3/7", DateTimeFormatter.ofPattern("M/d"));

If you want the current year added to it, you can do it later:

LocalDate ld = md.atYear(Year.now(/*optionally insert time zone*/).getValue());

How do I handle both yyyy-MM-dd and M/d patterns?

Here's one way: create a DateTimeFormatter that recognises both patterns, parse the string to a TemporalAccessor, check if the TemporalAccessor supports the "year" field:

TemporalAccessor ta = DateTimeFormatter.ofPattern("[M/d][yyyy-MM-dd]").parse("3/7");
if (ta.isSupported(ChronoField.YEAR_OF_ERA)) { // yyyy-MM-dd
    LocalDate ld = LocalDate.from(ta);
} else if (ta.isSupported(ChronoField.MONTH_OF_YEAR)) { // M/d
    MonthDay md = MonthDay.from(ta);
} else {
    // user has entered an empty string, handle error...
}

answered Apr 11, 2020 at 12:30

Sweeper

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2 Comments

Andreas Over a year ago

You actually can parse a date string in a format that has no year. See my answer, which builds on the solution you have at the bottom.

Sweeper Over a year ago

@Andreas Ahh~ parseDefaulting! I knew there has to be a method like this after writing my answer, I was going to add it at the end as an alternative way, but couldn't find what the method was called...

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