All 15 Java Date & Time API (java.time) questions
More in Java Date & Time API (java.time)
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 use TemporalAdjusters to compute dates like 'the next Monday' or 'the last day of the month' relative to a given LocalDate? 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 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? How would you implement a custom TemporalAdjuster to calculate business-specific date logic (like 'the next business day, skipping weekends and holidays')? How would you correctly serialize and deserialize java.time types (like LocalDateTime or ZonedDateTime) to/from JSON using a library like Jackson, and what common configuration pitfalls arise? How would you implement a recurring event scheduling system that correctly handles both fixed-instant recurrence (every exactly 24 hours) and calendar-based recurrence (every day at 9 AM local time), given these have meaningfully different behavior across DST transitions?
Advertisement
Back to results for “Fundamentals”
Java › Java Date & Time API (java.time)

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?

Intermediate

Answer

Year.isLeap(yearValue) (a static method) or an existing LocalDate/YearMonth instance's isLeapYear() method correctly determines leap-year status following the standard Gregorian calendar rule (divisible by 4, except centuries not divisible by 400) without requiring you to hand-implement this classic but easy-to-get-subtly-wrong rule yourself; YearMonth.lengthOfMonth() returns the correct number of days for a specific year/month combination, automatically accounting for leap-year February correctly, both being simple, well-tested utility methods that eliminate an entire category of manual calendrical-logic bugs.
boolean isLeap = Year.isLeap(2024);  // true
boolean isLeap2 = Year.of(2024).isLeap();  // equivalent, true

YearMonth february2024 = YearMonth.of(2024, 2);
System.out.println(february2024.lengthOfMonth());  // 29, correctly leap-year-aware

YearMonth february2023 = YearMonth.of(2023, 2);
System.out.println(february2023.lengthOfMonth());  // 28, correctly non-leap-year-aware
Real-world example A calendar-rendering feature displaying the correct number of day cells for each month uses YearMonth.lengthOfMonth() rather than hand-writing the classic (and easy to get subtly wrong, especially the century-divisible-by-400 exception) leap-year calculation logic, trusting the well-tested standard library implementation instead.

Common follow-ups: What's the specific Gregorian calendar rule exception (century years) that a naive 'divisible by 4' leap-year check would incorrectly get wrong?;How does YearMonth differ from simply using LocalDate with a placeholder day value for representing a year-month combination?

Diagnostics & Performance;Java Fundamentals: Syntax Data Types & Operators
All 15 Java Date & Time API (java.time) questions