64. Converting between Date and YearMonth
Converting a java.util.Date to JDK 8 java.time.YearMonth can be done based on YearMonth.from(TemporalAccessor temporal). A TemporalAccessor is an interface (more precisely, a framework-level interface) that exposes read-only access to any temporal object including date, time, and offset (a combination of these is also allowed). So, if we convert the given java.util.Date to java.time.LocalDate then the result of the conversion can be passed to YearMonth.from() as follows:
public static YearMonth toYearMonth(Date date) {
return YearMonth.from(date.toInstant()
.atZone(ZoneId.systemDefault())
.toLocalDate());
}
The vice-versa can be obtained via Date.from(Instant instant) as follows:
public static Date toDate(YearMonth ym) {
return Date.from(ym.atDay(1).atStartOfDay(
ZoneId.systemDefault()).toInstant());
}
Well, that was easy, isn’t it?
65. Converting between int and YearMonth
Consider that we have YearMonth.now(), and we want to convert it to an integer (for example, this can be useful for storing a year/month date in a database using a numeric field). Check out the solution:
public static int to(YearMonth u) {
return (int) u.getLong(ChronoField.PROLEPTIC_MONTH);
}
The proleptic-month is a java.time.temporal.TemporalField which basically represents a date-time field such as month-of-year (our case) or minute-of-hour. The proleptic-month starts from 0 and counts the months sequentially from year 0. So, getLong() returns the value of the specified field (here, the proleptic-month) from this year-month as a long. We can cast this long to int since the proleptic-month cannot go beyond the int domain (for instance, for 2023/2 the returned int is 24277).The vice-versa can be accomplished as follows:
public static YearMonth from(int t) {
return YearMonth.of(1970, 1)
.with(ChronoField.PROLEPTIC_MONTH, t);
}
You can start from any year/month. The 1970/1 (known as the epoch and the starting point for java.time.Instant) choice was just an arbitrary choice.
66. Converting week/year to Date
Let’s consider the year 2023, week 10. The corresponding date is Sun Mar 05 15:15:08 EET 2023 (of course, the time component is relative). Converting the year/week to java.util.Date can be done via the Calendar API as in the following self-explanatory snippet of code:
public static Date from(int year, int week) {
Calendar calendar = Calendar.getInstance();
calendar.set(Calendar.YEAR, year);
calendar.set(Calendar.WEEK_OF_YEAR, week);
calendar.set(Calendar.DAY_OF_WEEK, 1);
return calendar.getTime();
}
If you prefer to obtain a LocalDate instead of a Date then you can easily perform the corresponding conversion or you can rely on java.time.temporal.WeekFields. This API exposes several fields for working with week-of-year, week-of-month, and day-of-week. This being said, here is the previous solution written via WeekFields to return a LocalDate:
public static LocalDate from(int year, int week) {
WeekFields weekFields = WeekFields.of(Locale.getDefault());
return LocalDate.now()
.withYear(year)
.with(weekFields.weekOfYear(), week)
.with(weekFields.dayOfWeek(), 1);
}
On the other hand, if we have a java.util.Date and we want to extract from it the year and the week then we can use the Calendar API quite straightforward. Here, we extract the year:
public static int getYear(Date date) {
Calendar calendar = Calendar.getInstance();
calendar.setTime(date);
return calendar.get(Calendar.YEAR);
}
And, here we extract the week:
public static int getWeek(Date date) {
Calendar calendar = Calendar.getInstance();
calendar.setTime(date);
return calendar.get(Calendar.WEEK_OF_YEAR);
}
Getting the year and the week from a LocalDate is easy thanks to ChronoField.YEAR and ChronoField.ALIGNED_WEEK_OF_YEAR:
public static int getYear(LocalDate date) {
return date.get(ChronoField.YEAR);
}
public static int getWeek(LocalDate date) {
return date.get(ChronoField.ALIGNED_WEEK_OF_YEAR);
}
Of course, getting the week can be accomplished via WeekFields as well:
return date.get(WeekFields.of(
Locale.getDefault()).weekOfYear());
Challenge yourself to obtain week/month and day/week from a Date/LocalDate.