Java LocalDate How to get last week of year as 53rd week instead of 1st week of new year?

Question

If you consider that week will start from Jan 01 of every year & week start is SUNDAY then there will be 53 weeks in 2019. Following above Jan 29,30,31 2019 will be into Week-53 of 2019.

As given in documentation of IsoFields for WEEK_OF_WEEK_BASED_YEAR that all three fields are validated against their range of valid values. The week-of-week-based-year field is validated from 1 to 52 or 53 depending on the week-based-year.

So I'm assuming that following code should give the output as: WEEK_OF_WEEK_BASED_YEAR 53 & WEEK_BASED_YEAR 2019.

But it's giving output as: 1 & 2020

import java.time.LocalDate;
import java.time.chrono.IsoChronology;
import java.time.format.DateTimeFormatter;
import java.time.format.ResolverStyle;
import java.time.temporal.IsoFields;

public class WeekStartDemo {
    public static void main(String args[]) {
        DateTimeFormatter DATE_FORMATTER = DateTimeFormatter
                .ofPattern("uuuu-MM-dd")
                .withChronology(IsoChronology.INSTANCE)
                .withResolverStyle(ResolverStyle.STRICT);

        LocalDate updatedDate = LocalDate.parse("2019-12-30", DATE_FORMATTER);

        System.out.println(updatedDate.toString());

        System.out.println(updatedDate.get(IsoFields.WEEK_OF_WEEK_BASED_YEAR));
        System.out.println(updatedDate.get(IsoFields.WEEK_BASED_YEAR));
    }
}

If I pass the date as 2019-12-28 then it's returning WEEK_OF_WEEK_BASED_YEAR 52 & WEEK_BASED_YEAR 2019. But doesn't work for last week of 2019 (which is 53rd week)

Let me know what I'm missing in above code.

Answer 1

As I mentioned in the comments, and from your Javadoc link for IsoFields, The week-based-year itself is defined relative to the standard ISO proleptic year. It differs from the standard year in that it always starts on a Monday (not a Sunday). It should be easy enough to find the years with 53 weeks using your posted code, iterate from 1900 to 2300 and parse the WEEK_OF_WEEK_BASED_YEAR for the last day of the given year and print the values where it is 53. Like,

DateTimeFormatter DATE_FORMATTER = DateTimeFormatter.ofPattern("uuuu-MM-dd")
        .withChronology(IsoChronology.INSTANCE)
        .withResolverStyle(ResolverStyle.STRICT);

for (int i = 1900; i < 2300; i++) {
    LocalDate updatedDate = LocalDate.parse(String.format("%d-12-31", i), DATE_FORMATTER);
    if (updatedDate.get(IsoFields.WEEK_OF_WEEK_BASED_YEAR) == 53) {
        System.out.println(i);
    }
}

The first few values I get are

1903
1908
1914
1920
1925
1931
1936
1942

skipping ahead a bit...

2009
2015
2020
2026

So this year (2020) has 53 weeks, and 2019 does not.