All 15 Java Date & Time API (java.time) questions
More in Java Date & Time API (java.time)
How would you get the current date and time, and how does specifying an explicit ZoneId (versus relying on the system default) affect the result of LocalDate.now() or LocalDateTime.now()? What is the difference between LocalDate, LocalTime, LocalDateTime, and ZonedDateTime, and when would you use each? How would you calculate the difference between two dates or times using Period and Duration, and what's the distinction between these two classes? How do you parse and format date/time values using DateTimeFormatter, and how does this replace the older, notoriously thread-unsafe SimpleDateFormat class? How would you calculate a person's age or the number of days until a recurring annual event (like a birthday) using ChronoUnit, and what's a common pitfall with leap-year birthdays (February 29)? What is the significance of storing date-time values in UTC in a database or when transmitting between systems, rather than storing them in a specific local timezone? How would you determine whether a given year is a leap year, and calculate the number of days in a specific month, using the java.time API's built-in support rather than hand-writing this classic calendrical logic? How does the Instant class represent a point in time, and how does it differ fundamentally from ZonedDateTime in terms of what it actually represents and how it should be used? How would you correctly handle daylight saving time (DST) transitions when performing date-time arithmetic with ZonedDateTime, and what specific pitfalls exist around the 'spring forward' and 'fall back' transitions? How would you convert between the legacy java.util.Date/Calendar classes and the modern java.time API when working with older libraries or APIs that still require the legacy types?
Advertisement
Java › Java Date & Time API (java.time)

How would you use TemporalAdjusters to compute dates like 'the next Monday' or 'the last day of the month' relative to a given LocalDate?

Intermediate

Answer

TemporalAdjusters provides a collection of pre-built, common date-adjustment strategies (nextOrSame(DayOfWeek), lastDayOfMonth(), firstDayOfNextMonth(), and several others) usable via LocalDate's with(TemporalAdjuster) method, expressing common calendrical calculations declaratively and readably rather than hand-writing the underlying day-counting/comparison logic yourself, which is both more concise and considerably less error-prone (avoiding subtle off-by-one or month-boundary bugs that manual date arithmetic is notoriously prone to).
LocalDate today = LocalDate.of(2024, 6, 15);  // a Saturday

LocalDate nextMonday = today.with(TemporalAdjusters.next(DayOfWeek.MONDAY));
LocalDate lastDayOfMonth = today.with(TemporalAdjusters.lastDayOfMonth());
LocalDate firstDayOfNextMonth = today.with(TemporalAdjusters.firstDayOfNextMonth());

System.out.println(nextMonday);        // 2024-06-17
System.out.println(lastDayOfMonth);    // 2024-06-30
Real-world example A billing system calculating each customer's next invoice date (always the last day of the current month) uses TemporalAdjusters.lastDayOfMonth() rather than hand-writing month-length-aware logic (which would need to separately account for February, leap years, and months with 30 versus 31 days), eliminating an entire category of manual calendar-arithmetic bugs.

Common follow-ups: How would you write a completely custom TemporalAdjuster for a business-specific date rule not covered by the built-in ones (like 'next business day, skipping weekends')?;What's the difference between TemporalAdjusters.next() and nextOrSame() for a date that already falls on the target day of week?

Design Patterns in Java;Functional Interfaces & Method References
All 15 Java Date & Time API (java.time) questions