Calendar c = Calendar.getInstance();
DateFormat sdf = new SimpleDateFormat("dd/MM/yyyy");
c.setTime( sdf.parse("31/12/2010"));
out.println( c.get( Calendar.WEEK_OF_YEAR ) );

public static void main(String[] args) throws ParseException {

    DateFormat sdf = new SimpleDateFormat("dd/MM/yyyy");
    Date lastDec2010 = sdf.parse("31/12/2010");

    Calendar calUs = Calendar.getInstance(Locale.US);
    calUs.setTime(lastDec2010);

    Calendar calDe = Calendar.getInstance(Locale.GERMAN);
    calDe.setTime(lastDec2010);

    System.out.println( "us: " + calUs.get( Calendar.WEEK_OF_YEAR ) );
    System.out.println( "de: " + calDe.get( Calendar.WEEK_OF_YEAR ) );
}

us: 1
de: 52

Calendar c = Calendar.getInstance();
c.setMinimalDaysInFirstWeek(7);//anything more than 1 will work in this year
DateFormat sdf = new SimpleDateFormat("dd/MM/yyyy");
c.setTime( sdf.parse("31/12/2010"));
System.out.println( c.get( Calendar.WEEK_OF_YEAR ) );

52

java.time.LocalDate.parse(
    "31/12/2010" ,
    DateTimeFormatter.ofLocalizedDate( FormatStyle.SHORT ).withLocale( Locale.UK )
)
.get( IsoFields.WEEK_OF_WEEK_BASED_YEAR )

org.threeten.extra.YearWeek.from(     // Convert from a `LocalDate` object to a `YearWeek` object representing the entire week of that date’s week-based year.
    java.time.LocalDate.parse( "31/12/2010" , DateTimeFormatter.ofLocalizedDate( FormatStyle.SHORT ).withLocale( Locale.UK )
).getWeek()                           // Extract an integer number of that week of week-based-year, either 1-52 or 1-53 depending on the year.

ZoneId zoneId = ZoneId.of ( "America/Montreal" );
ZonedDateTime now = ZonedDateTime.now ( zoneId );

int week = now.get ( IsoFields.WEEK_OF_WEEK_BASED_YEAR );
int weekYear = now.get ( IsoFields.WEEK_BASED_YEAR );

LocalDate ld = LocalDate.parse( "2010-12-31" ) ;
int weekOfWeekBasedYear = ld.get( IsoFields.WEEK_OF_WEEK_BASED_YEAR ) ;
int yearOfWeekBasedYear = ld.get( IsoFields.WEEK_BASED_YEAR ) ;

String outputWeek = String.format( "%04d" , yearOfWeekBasedYear ) + "-W" + String.format( "%02d" , weekOfWeekBasedYear ) ;
String outputDate = outputWeek + "-" + ld.getDayOfWeek().getValue() ;

YearWeek yw = YearWeek.from( ld ) ;  // Determine ISO 8601 week of a `LocalDate`.

String output = yw.toString() ;

YearWeek yearWeek = YearWeek.parse( "2010-W52" ) ;

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