CODEWITHSUNDEEP
javaweek-numberdate
Tom
Tom

Reputation: 6342

How to calculate the days belonging to an ISO based week number

I have an ISO based week number that is calculated using the following Java 8 LocalDate API

int weekNumOfYear = LocalDate#get(IsoFields.WEEK_OF_WEEK_BASED_YEAR)

Given the year, and the week number (eg, 201927),How could I calculate the start day and end day for this week(201917)?

I am Using Calendar class for this problem, but not sure it is correct(especially, whether is has followed the ISO format)

Update： The following code doesn't work correctly for 201953, there is no 53th week for 2019

@Test
    public void testGetWeekDays() {
        Integer year = 2019;
        Integer week = 27;
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
        Calendar cal = Calendar.getInstance();

        //Use ISO format
        cal.setMinimalDaysInFirstWeek(4);

        cal.setFirstDayOfWeek(Calendar.MONDAY);
        cal.set(Calendar.YEAR, year);
        cal.set(Calendar.WEEK_OF_YEAR, week);
        cal.set(Calendar.DAY_OF_WEEK, cal.getFirstDayOfWeek());
        String beginDate = sdf.format(cal.getTime());
        cal.add(Calendar.DAY_OF_WEEK, 6);
        String endDate = sdf.format(cal.getTime());
        System.out.println(beginDate);
        System.out.println(endDate);
    }

Upvotes: 1

Views: 766

Answers (2)

Basil Bourque
Basil Bourque

Reputation: 338730

tl;dr

Given the year, and the week number (eg, 201927),I could I calculate the start day and end day for this week(201917)?

YearWeek              // Represent a particular week of a week-based year.
.of(                  // Factory method, rather than calling `new`. 
    2019 ,            // The number of the desired week-based year ( *not* calendar year! ).
    26                // Week of the year, 1 through 52 or 53. 
)
.atDay(               // Generate a `LocalDate` object for a day within this week.
    DayOfWeek.MONDAY  // Specify the day-of-week.
)                     // Generate a `LocalDate` for the first day of this week.
.plusDays( 6 )        // Generate a `LocalDate` representing the last day of this week.

ThreeTen-Extra — YearWeek

Working with standard weeks is much easier with the org.threeten.extra.YearWeek class provided in the ThreeTen-Extra project. That project adds functionality to the modern java.time classes built into Java 8 and later.

If you have an ISO 8601 compliant string representing a standard week, parse.

String input = "2019-W27";
YearWeek yw = YearWeek.parse( input);

yw.toString(): 2019-W27

A standard week begins with Monday. The YearWeek object can generate a LocalDate for any day in the week. Specify a day using the DayOfWeek enum.

LocalDate ld = yw.atDay( DayOfWeek.MONDAY);

ld.toString(): 2019-07-01

To get the rest of the week, add a day over and over.

LocalDate nextDay = ld.plusDays( 1 ) ;

ld.toString(): 2019-07-02

To get the end of the week, add six.

LocalDate nextDay = ld.plusDays( 6 ) ;

ld.toString(): 2019-07-07

Or get fancy using Java streams. Generate a stream by calling LocalDate::datesUntil.

Stream < LocalDate > stream = ld.datesUntil ( ld.plusWeeks ( 1 ) );
List < LocalDate > dates = stream.collect ( Collectors.toList () );

dates.toString(): [2019-07-01, 2019-07-02, 2019-07-03, 2019-07-04, 2019-07-05, 2019-07-06, 2019-07-07]

By the way, the standard way to represent a day-of-week within a standard week is to append a hyphen and number 1-7.

2019-W26-1 = Monday

2019-W26-2 = Tuesday

…

2019-W26-7 = Sunday

Upvotes: 2

Ryuzaki L
Ryuzaki L

Reputation: 40058

You can also use with to get start day and end day of week

System.out.println(localDate.with(DayOfWeek.MONDAY));
System.out.println(localDate.with(DayOfWeek.SUNDAY));

If you want to get start day and end day from week number, then use ISO_WEEK_DATE

LocalDate startDay = LocalDate.parse("2019-W26-1", DateTimeFormatter.ISO_WEEK_DATE);

LocalDate endDay = LocalDate.parse("2019-W26-7", DateTimeFormatter.ISO_WEEK_DATE);

One digit for the day-of-week. The value run from Monday (1) to Sunday (7).

Upvotes: 4

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