Java 8 Date API - Get total number of weeks in a month

user_1033686 0
Viewed 1870

I have a Kotlin function to get the total number of weeks in a month

Code

fun getTotalWeeksInMonth(instant: Instant): Int {
    val calendar = Calendar.getInstance()
    calendar.time = Date.from(instant)

    return calendar.getActualMaximum(Calendar.WEEK_OF_MONTH)
}

However this is using a mix of the old Java date/time APIs (Date and Calendar) and the new APIs (Instant)

How would I achieve the same result, using just the new APIs?

java-8 kotlin date datetime java
4 Answers

You can try something like this pair of lines:

YearMonth currentYearMonth = 
    YearMonth.now( 
        ZoneId.systemDefault() 
    )
;
int weeks = 
    currentYearMonth
    .atEndOfMonth()
    .get(
        WeekFields.ISO.weekOfMonth()
    )
;
user_79468070

You can evaluate the "week of month" of last day of this month, in java:

static int getTotalWeeksInMonth(Instant instant) {
    LocalDate localDate = LocalDate.ofInstant(instant, ZoneId.systemDefault());
    LocalDate lastDayOfMonth = localDate.withDayOfMonth(localDate.lengthOfMonth());
    int lastWeekOfMonth = lastDayOfMonth.get(WeekFields.ISO.weekOfMonth());
    return lastWeekOfMonth;
}
user_66902000

See if this fits you, be careful about what Zone you are actually passing, and about WeekFields.ISO, in some regions it may work fine, but in others it may not:

Instant now = Instant.now();

ZonedDateTime zonedNow = now.atZone(ZoneId.systemDefault());
ZonedDateTime monthEnd = zonedNow.with(TemporalAdjusters.lastDayOfMonth());

System.out.println(monthEnd.get(WeekFields.ISO.weekOfMonth()));
user_48966510

Having an Instant I would convert it to date first:

val date = LocalDateTime.ofInstant(instant, ZoneId.systemDefault())

Then go with either 

YearMonth.from(date).atEndOfMonth().get(ChronoField.ALIGNED_WEEK_OF_MONTH)

or

YearMonth.from(date).atEndOfMonth().get(WeekFields.ISO.weekOfMonth())

Complete example:

fun getTotalWeeksInMonth(instant: Instant): Int {
    val date = LocalDateTime.ofInstant(instant, ZoneId.systemDefault())
    return YearMonth.from(date).atEndOfMonth().get(ChronoField.ALIGNED_WEEK_OF_MONTH)
}
user_105846050
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