When creating a DateTimeFormatter using the WeekFields.weekBasedYear() temporal field, the resulting year number may be
off by 1 at the beginning of a new year (when the date to format is in a week that is shared by two consecutive years).
Using this year number in combination with an incompatible week temporal field yields a result that may be confused with the first week of the
previous year.
Instead, when paired with a week temporal field, the week-based year should only be used with the week of week-based year temporal field
WeekFields.weekOfWeekBasedYear().
Alternatively the temporal field ChronoField.ALIGNED_WEEK_OF_YEAR can be used together with a regular year (but not the week based
year).
Noncompliant code example
new DateTimeFormatterBuilder()
.appendValue(ChronoField.YEAR, 4) // Noncompliant: using week of week-based year with regular year
.appendLiteral('-')
.appendValue(WeekFields.ISO.weekOfWeekBasedYear(), 2)
.toFormatter();
new DateTimeFormatterBuilder()
.appendValue(ChronoField.YEAR_OF_ERA, 4) // Noncompliant: using week of week-based year with regular year
.appendLiteral('-')
.appendValue(WeekFields.ISO.weekOfWeekBasedYear(), 2)
.toFormatter();
new DateTimeFormatterBuilder()
.appendValue(WeekFields.ISO.weekBasedYear(), 4) // Noncompliant: using aligned week of year with week-based year
.appendLiteral('-')
.appendValue(ChronoField.ALIGNED_WEEK_OF_YEAR, 2)
.toFormatter();
Here the first two formatters would wrongly format the 1st of January 2016 as "2016-53" while the last one would format it as "2015-01"
Compliant solution
new DateTimeFormatterBuilder()
.appendValue(WeekFields.ISO.weekBasedYear(), 4)
.appendLiteral('-')
.appendValue(WeekFields.ISO.weekOfWeekBasedYear(), 2)
.toFormatter();
new DateTimeFormatterBuilder()
.appendValue(ChronoField.YEAR, 4)
.appendLiteral('-')
.appendValue(ChronoField.ALIGNED_WEEK_OF_YEAR, 2)
.toFormatter();
new DateTimeFormatterBuilder()
.appendValue(ChronoField.YEAR_OF_ERA, 4)
.appendLiteral('-')
.appendValue(ChronoField.ALIGNED_WEEK_OF_YEAR, 2)
.toFormatter();
Here the first formatter would format the 1st of January 2016 as "2015-53" while the last two would produce "2016-01", both of which are correct
depending on how you count the weeks.
Exceptions
No issue is raised when week-based year is not used in combination with a week temporal field.
Similarly, no issue is raised if week of week-based year is not used in combination with a year temporal field.
