Prevent more common DateTimeFormatter issues

[codylerum]

Description of the problem / feature request:

Create new bug pattern which alerts if a java.time.format.DateTimeFormatter is created using capital Y (like YYYY-dd-MM) without a w indicating that it is using week based year.

https://docs.oracle.com/javase/8/docs/api/java/time/format/DateTimeFormatter.html

Feature requests: what underlying problem are you trying to solve with this feature?

Prevent users from making a mistake with formatting for example the following code

System.out.println("Pattern: yyyy-MM-dd = " + DateTimeFormatter.ofPattern("yyyy-MM-dd").format(LocalDate.of(2019,12,31)));
System.out.println("Pattern: YYYY-MM-dd = " + DateTimeFormatter.ofPattern("YYYY-MM-dd").format(LocalDate.of(2019,12,31)));

Will Print

Pattern: yyyy-MM-dd = 2019-12-31
Pattern: YYYY-MM-dd = 2020-12-31

This can be a common mistake.

It may also be useful to alert on other patterns such as

D (day of year) in combination with M (month) where

DateTimeFormatter.ofPattern("YYYY-MM-DD").format(LocalDate.of(2019,12,31) will produce 2020-12-365

Bugs: what's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

DateTimeFormatter.ofPattern("YYYY-MM-dd").format(LocalDate.of(2019,12,31)

What version of Error Prone are you using?

2.3.4

Have you found anything relevant by searching the web?

Somewhat of a common issue. Jodatime used x for week based year to limit confusion.

https://www.reddit.com/r/java/comments/ehucj3/important_at_this_time_of_the_year_beware_of_yyyy/

[tbroyer]

Isn't that already taken care of by https://errorprone.info/bugpattern/MisusedWeekYear?

[codylerum]

@tbroyer Yes, I missed that and it handles the Week Based Year. Thanks.

Less common I'm sure, but maybe there should be a MisuedDayOfYear around using D in combination with M

DateTimeFormatter.ofPattern("yyyy-MM-DD").format(LocalDate.of(2019, 1, 31) Produces 2019-01-31
DateTimeFormatter.ofPattern("yyyy-MM-DD").format(LocalDate.of(2019, 2, 1) Produces 2019-02-32
DateTimeFormatter.ofPattern("yyyy-MM-dd").format(LocalDate.of(2019, 2, 1) Produces 2019-02-01

I confirmed that the following does not throw a compile warning with errorprone.

System.out.println(DateTimeFormatter.ofPattern("yyyy-MM-DD").format(LocalDate.of(2019, 2, 1)));

[graememorgan]

Nice suggestion, thanks! This has been implemented and sent out for review internally. Should be in a future version.