How to parse from week/year to dd/mm/yyyy

Question

I'm trying to parse a String that contains week/year information to a normal dd-mm-yyyy format

import java.util.Calendar;
import java.util.*;
import java.text.*;
import java.util.GregorianCalendar;

public class FromWeektoDate {

public static void main(String[] args) {
    Calendar gregorianCalendar = new GregorianCalendar();
    gregorianCalendar.setFirstDayOfWeek(Calendar.SUNDAY);
    gregorianCalendar.setMinimalDaysInFirstWeek(4);

     String SemaineYear[] = "20/2018".split("/");


         int s = Integer.parseInt(SemaineYear[0]);
         int a = Integer.parseInt(SemaineYear[1]);
      int numWeekofYear = s;  //INPUT
       int year = a;         //INPUT

    gregorianCalendar.set(Calendar.YEAR , year);
    gregorianCalendar.set(Calendar.WEEK_OF_YEAR , numWeekofYear);

  Date date = new Date();

     date.setDay(gregorianCalendar.get(Calendar.DAY_OF_MONTH) );   
     date.setMonth(gregorianCalendar.get(Calendar.MONTH)  + 1 );
     date.setYear(gregorianCalendar.get
                        (Calendar.YEAR));

    System.out.println(date);
}
}

but I'm getting this error :

 /tmp/java_5kSPR0/FirstDayofWeek.java:26: error: cannot find symbol date.setDay(gregorianCalendar.get(Calendar.DAY_OF_MONTH) ); ^ symbol: method setDay(int) location: variable date of type Date /tmp/java_5kSPR0/FirstDayofWeek.java:27: warning: [deprecation] setMonth(int) in Date has been deprecated 

Answer 1

If you don't care what day within the week you want, you can simply start from New Year's Day and add as many weeks as you like:

LocalDate date = LocalDate.of(year, 1, 1).plusWeeks(weekOfYear - 1)

For your example (20th week of 2018), this gives me 2018-05-14 .

Answer 2

java.time

java.time, the modern Java date and time API, can do that.

    // In Malta weeks begin on Sunday, and there must be at least 4 days
    // of a week in a year for it to be week 1 of that year
    WeekFields wf = WeekFields.of(Locale.forLanguageTag("mt"));

    DateTimeFormatter yearWeekFormatter = new DateTimeFormatterBuilder()
            .appendValue(wf.weekOfWeekBasedYear(), 2)
            .appendLiteral('/')
            .appendValue(wf.weekBasedYear(), 4)
            .parseDefaulting(ChronoField.DAY_OF_WEEK, wf.getFirstDayOfWeek().getValue())
            .toFormatter();

    String weekString = "20/2018";
    LocalDate sundayOfWeek20 = LocalDate.parse(weekString, yearWeekFormatter);
    System.out.println(sundayOfWeek20);

Output from this snippet is:

2018-05-13

To format into 13/05/2018 use another DateTimeFormatter :

    DateTimeFormatter dateFormatter = DateTimeFormatter.ofPattern("dd/MM/uuuu");
    System.out.println(sundayOfWeek20.format(dateFormatter));

13/05/2018

I interpreted from your code that your require a week scheme where the week starts on Sunday and there are at least 4 days in the first week. The WeekFields class is used for defining such a scheme. If your requirement comes out of a specific locale (Malta or Ireland), I recommend using that locale for defining the WeekFields object to use, as I do in the snippet above. If, on the other hand, your requirement doesn't come from some locale, it's better to specify it more directly:

    WeekFields wf = WeekFields.of(DayOfWeek.SUNDAY, 4);

I am using java.time's built-in parsing mechanism through LocalDate.parse . No need to hand parse or otherwise reinvent the wheel. To have the formatter understand our week scheme I pass TemporalField objects that I get from the WeekFields to DateTimeFormatterBuilder.appenValue . To parse into a LocalDate we need to add a day of week (even though you said you don't care which). I use the call to parseDefaulting for that.

Frankly I was surprised to see how flexible java.time was to meet your requirements. Even though I knew already that it is a pleasure to work with it.

The date-time classes that you used — Calendar , GregorianCalendar and Date — are poorly designed and long outdated. I recommend you don't use them.

Link: Oracle tutorial: Date Time explaining how to use java.time.

Answer 3

Just use getTime to get Date object

Date date =  gregorianCalendar.getTime();

Returns a Date object representing this Calendar's time value (millisecond offset from the Epoch").

Or use SimpleDateFormat for parsing specific format

SimpleDateFormat dateFormat = new SimpleDateFormat("dd/MM/yyyy");
dateFormat.setTimeZone(gregorianCalendar.getTimeZone());