How to get Date from Week Number, Year and dayOfWeek in java?

tl;dr

LocalDate localDate = 
        YearWeek               // Represent an entire week of a week-based year per the ISO 8601 standard definition of a week.
        .of(                   // Instantiate a `YearWeek` object.
            2019 ,             // Specify the week-based year number, NOT the calendar year.
            51                 // Specify the week number, 1-52 or 1-53. 
        )
        .atDay(
            DayOfWeek.of( 1 )  // The value 1 yields a `DayOfWeek.MONDAY` object.
        )
;

org.threeten.extra.YearWeek

The Answer by Sweeper looks correct. But there is a more specialized class for this.

If doing much work with weeks of week-based years per the ISO 8601 definition of week, use the YearWeek class found in the ThreeTen-Extra library. This library adds extra functionality to the java.time classes built into Java 8 and later.

Determine the week.

YearWeek yearWeek = YearWeek.of( 2019 , 51 ) ;

Get a LocalDate for the day-of-week within that week.

LocalDate localDate = yearWeek.atDay( DayOfWeek.MONDAY ) ;

For the day-of-week, you should be using DayOfWeek enum objects in your code rather than mere integer numbers. To get a DayOfWeek from an original number 1-7 for Monday-Sunday, call DayOfWeek.of( x ).

DayOfWeek dow = DayOfWeek.of( 1 ) ;  // 1 = Monday, 7 = Sunday.

Putting that all together we get this one-liner.

LocalDate localDate = YearWeek.of( 2019 , 51 ).atDay( DayOfWeek.of( 1 ) ) ;
 

To be clear… The ISO 8601 definition of a week is:

• Week # 1 contains the first Thursday of the year.
• Weeks start on a Monday, ending on a Sunday.
• A year has either 52 or 53 complete 7-day weeks.
• The first/last weeks of the week-based year may contain the trailing/leading days of the previous/next calendar years. Thus, the calendar year of those days differ from the week-based year.

It also depends on the Locale.

Note that the first day of the week is Locale-dependent e.g. it is Monday in the UK while Sunday in the US. As per the ISO 8601 standards, it is Monday. For comparison, check the US calendar and the UK calendar. Accordingly, the date will vary as shown in the example below:

import java.time.DayOfWeek;
import java.time.LocalDate;
import java.time.temporal.WeekFields;
import java.util.Locale;

public class Main {
    public static void main(String[] args) {
        // Test
        int weekNumber = 34;
        int year = 2021;
        
        System.out.println(getLocalDate(weekNumber, DayOfWeek.TUESDAY, year, Locale.UK));
        System.out.println(getLocalDate(weekNumber, DayOfWeek.TUESDAY, year, Locale.US));
        
        System.out.println(getLocalDate(weekNumber, DayOfWeek.SUNDAY, year, Locale.UK));
        System.out.println(getLocalDate(weekNumber, DayOfWeek.SUNDAY, year, Locale.US));        
    }

    static LocalDate getLocalDate(int weekNumber, DayOfWeek dow, int year, Locale locale) {
        return LocalDate.of(year, 2, 1)
                .with(dow)
                .with(WeekFields.of(locale).weekOfWeekBasedYear(), weekNumber);
    }
}

Output:

2021-08-24
2021-08-17
2021-08-29
2021-08-15

ONLINE DEMO

Learn more about the modern Date-Time API from Trail: Date Time.

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^{* For any reason, if you have to stick to Java 6 or Java 7, you can use ThreeTen-Backport which backports most of the java.time functionality to Java 6 & 7. If you are working for an Android project and your Android API level is still not compliant with Java-8, check Java 8+ APIs available through desugaring and How to use ThreeTenABP in Android Project.}

Instead of adding a number of days to wkstart, use with again to set the day of week. For example:

LocalDate date = LocalDate.now()
    .with(WeekFields.ISO.weekBasedYear(), 2018) // year
    .with(WeekFields.ISO.weekOfWeekBasedYear(), 51) // week of year
    .with(WeekFields.ISO.dayOfWeek(), DayOfWeek.MONDAY.getValue()); // day of week