How to find total weeks of a year in Java?

Question

I am working on a project. There I should find the total weeks of a year. I tried with the following code, but I get the wrong answer: 2020 has 53 weeks, but this code gives 52 weeks.

Where have I gone wrong in this code?

package com.hib.mapping;

import java.time.LocalDate;
import java.time.temporal.WeekFields;
import java.util.Calendar;
import java.util.GregorianCalendar;

import org.joda.time.DateTime;

public class TestWeek {

    public static void main(String args[]) {
        System.out.println(getWeeks());
    }

    public static int getWeeks() {

        Calendar cal = Calendar.getInstance();
        cal.set(Calendar.YEAR, 2020);
        cal.set(Calendar.MONTH, Calendar.JANUARY);
        cal.set(Calendar.DAY_OF_MONTH, 1);
        GregorianCalendar gregorianCalendar = new GregorianCalendar();

        int weekDay = cal.get(Calendar.DAY_OF_WEEK) - 1;
        if (gregorianCalendar.isLeapYear(2020)) {
            if (weekDay == Calendar.THURSDAY || weekDay == Calendar.WEDNESDAY)
                return 53;
            else
                return 52;
        } else {
            if (weekDay == Calendar.THURSDAY)
                return 53;
            else
                return 52;
        }

    }

}

Output:

52

Answer 1

The flexible solution

This should work for any week numbering scheme that can be represented in a WeekFields object.

public static int noOfWeeks(WeekFields wf, int year) {
    LocalDate lastDayOfYear = YearMonth.of(year, Month.DECEMBER).atEndOfMonth();
    if (lastDayOfYear.get(wf.weekBasedYear()) > year) { // belongs to following week year
        return lastDayOfYear.minusWeeks(1).get(wf.weekOfWeekBasedYear());
    }
    else {
        return lastDayOfYear.get(wf.weekOfWeekBasedYear());
    }
}

The idea is to find the week number of the last week of the week based year. I try first with 31 December, but that may be in the first week of the following year. If so, I go one week back.

I have tested pretty thoroughly with WeekFields.ISO, not so much with other WeekFields objects, but as I said, I believe it works.

If you know for a fact that you will always need ISO 8601 weeks, I think you should go with one of the good answers by Sweeper and by Basil Bourque. I posted this in case you needed a more flexible solution that would work with other week numbering schemes too.

Use java.time

The code in your question is funny in that it imports classes both from Joda-Time and from java.time, yet uses the old Calendar and GregorianCalendar from Java 1.1. These classes were poorly designed and are now long outdated, you should not use them. Joda-Time is in maintenance mode, java.time has taken over after it. Which is what I use and recommend that you use.

Answer 2

Using the Wikipedia definition here. A year has 53 weeks if 1st Jan is a Thursday, or 31st Dec is a Thursday, otherwise it has 52 weeks. This definition is equivalent to the one you used. I think this is a way easier condition to check for, as you don't need to check for leap years.

Using the Java 8 java.time APIs:

int year = 2020;
boolean is53weekYear = LocalDate.of(year, 1, 1).getDayOfWeek() == DayOfWeek.THURSDAY ||
        LocalDate.of(year, 12, 31).getDayOfWeek() == DayOfWeek.THURSDAY;
int weekCount = is53weekYear ? 53 : 52;

Answer 3

tl;dr

For a standard ISO 8601 week, use the YearWeek class from ThreeTen-Extra library with a ternary statement.

YearWeek          // Represents an entire week of a week-based-year.
.of( 2020 , 1 )   // Pass the number of the week-based-year (*not* calendar year), and a week number ranging from 1 to 52 or 1 to 53.
.is53WeekYear()   // Every standard week-based-year has either 52 or 52 complete weeks.
? 53              // Ternary statement returns 53 if the predicate returns True, …
: 52              // … otherwise returns 52. 

That is, YearWeek.of( 2020 , 1 ).is53WeekYear() ? 53 : 52

Define “week”

You need to define a week. In your code sample, the definition of week varies by the JVM’s current default Locale. So your results may vary at runtime.

Your code also uses terrible date-time classes that were supplanted years ago by the modern java.time classes. Stop using GregorianCalendar & Calendar; they were replaced for good reasons.

ISO 8601 week

The ISO 8601 standard defines a week as:

• Weeks start on Monday, end on Sunday.
• Week # 1 has the first Thursday of the calendar-year.

That definition means:

• The first and last few days of a week-based-year may be the trailing/leading days of the previous/following calendar-year.
• The week-based-year has either 52 or 53 complete weeks.

If your definition differs, see the Answer by Ole V.V..

YearWeek:is53WeekYear

If this matches your definition, then add the ThreeTen-Extra library to your project to extend the java.time functionality built into Java 8 and later. You then have access to the YearWeek class.

ZoneId z = ZoneId.of( "America/Montreal" ) ;
YearWeek yearWeekNow = YearWeek.now( z ) ;
boolean is53WeekYear = yearWeekNow.is53WeekYear() ;

int weeksLong = yearWeekNow.is53WeekYear() ? 53 : 52 ;

To ask about a particular week-based-year, just arbitrarily pick any week of the year. For example, for the week-based year 2020 we ask for week # 1.

int weeksLong = YearWeek.of( 2020 , 1 ).is53WeekYear() ? 53 : 52 ;
LocalDate weekStart = YearWeek.of( 2020 , 1 ).atDay( DayOfWeek.MONDAY ) ;

weeksLong = 53

weekStart = 2019-12-30

Notice how the first day of the week-based-year of 2020 is from the calendar-year 2019.

Answer 4

I think this should work just fine as well:

int year = 2020;
long numOfWeeks = LocalDate.of(year, 1, 1).datesUntil(LocalDate.of(year, 12, 31), Period.ofDays(7)).count();
System.out.println("Weeks: " + numOfWeeks);