interview-preparation-day-003

1. List out Java 8 Date/Time API - Practical Formulas & Methods (Included this question to make the preparation easy)
2. What is Java 8 Date/Time API? Why better than old Date/Calendar?
3. Key classes: LocalDate, LocalTime, LocalDateTime, ZonedDateTime, Instant.
4. Common operations: plus/minus, Period, Duration, formatting/parsing.
5. Time zones and ZoneId — examples.
6. Difference between Period and Duration.
7. How to handle legacy Date with new API (toInstant, from).
8. Best practices for Date/Time in production (immutability, thread-safety).

List out Java 8 Date/Time API - Practical Formulas & Methods

Organized the Java 8 Date/Time APIs into the following key categories for easy memorization 

1. Date, Time, DateTime – Classes/Methods (No Zone)
2. DateTime with Zone – Classes/Methods
3. Formatting/Parsing
4. Common Operations – plus/minus
5. Periods (Date-based differences)
6. Duration (Time-based differences)
7. Convert Legacy Date to New API
8. Convert New API to Legacy Date

1. Date, Time, DateTime – Classes/Methods (No Zone)

Class	Method/Example	Output (DD-MM-YYYY)	Use Case
LocalDate	LocalDate.now()	31-01-2026	Current date only (birthday, holiday)
	LocalDate.of(2026, 1, 31)	31-01-2026	Specific date
LocalTime	LocalTime.now()	14:30:25	Current time only (meeting time)
	LocalTime.of(14, 30)	14:30	Specific time
LocalDateTime	LocalDateTime.now()	31-01-2026T14:30:25	Current date + time (no zone)
	LocalDateTime.of(2026, 1, 31, 14, 30)	31-01-2026T14:30	Specific date-time

2. Date/Time with Zone – Classes/Methods

Class	Method/Example	Output (DD-MM-YYYY)	Use Case
ZoneId	ZoneId.of("Asia/Kolkata")	Asia/Kolkata	Define timezone
ZonedDateTime	ZonedDateTime.now()	31-01-2026T14:30:25+05:30[Asia/Kolkata]	Current date-time with zone
	ZonedDateTime.now(ZoneId.of("America/New_York"))	31-01-2026T04:00:25-05:00[America/New_York]	Specific zone time
	ZonedDateTime.of(localDateTime, ZoneId.of("Asia/Kolkata"))	31-01-2026T14:30+05:30[Asia/Kolkata]	Attach zone to LocalDateTime
Instant	Instant.now()	2026-01-31T09:00:25Z	UTC machine timestamp

3. Formatting/Parsing

Class/Method	Example Code	Output / Result	Use Case
DateTimeFormatter	DateTimeFormatter.ofPattern("dd-MM-yyyy HH:mm")	(formatter object)	Custom pattern
format	localDateTime.format(formatter)	31-01-2026 14:30	Date → String
parse	LocalDateTime.parse("31-01-2026 14:30", formatter)	LocalDateTime object	String → DateTime
Pre-defined	DateTimeFormatter.ISO_LOCAL_DATE.format(localDate)	2026-01-31	Standard ISO format

4. Common Operations – plus/minus

Operation	Example Code	Output (DD-MM-YYYY)	Use Case
plus	date.plusDays(10)	10-02-2026	Add days
	dateTime.plusMonths(2)	31-03-2026T14:30	Add months
minus	date.minusYears(1)	31-01-2025	Subtract years
with (replace)	date.withDayOfMonth(15)	15-01-2026	Change day

5. Periods (Date-based)

Class/Method	Example Code	Output	Use Case
Period	Period.between(LocalDate.of(1990, 5, 15), LocalDate.of(2026, 1, 31))	P35Y8M16D	Calculate age
	Period.ofMonths(3)	P3M	Fixed period
	date.plus(period)	30-04-2026	Add period

6. Duration (Time-based)

Class/Method	Example Code	Output	Use Case
Duration	Duration.between(LocalTime.of(10, 0), LocalTime.of(15, 30))	PT5H30M	Time difference
	Duration.ofHours(8)	PT8H	Fixed duration
	time.plus(duration)	22:30	Add duration

7. Convert Legacy Date to New API

Method	Example Code	Result	Use Case
Date → New	Date legacy = new Date(); Instant instant = legacy.toInstant(); LocalDateTime ldt = LocalDateTime.ofInstant(instant, ZoneId.systemDefault());	LocalDateTime (e.g., 31-01-2026T14:30)	Migrate old code to new API

8. Convert New API to Legacy Date

Method	Example Code	Result	Use Case
New → Date	LocalDateTime ldt = LocalDateTime.of(2026, 1, 31, 14, 30); Instant instant = ldt.atZone(ZoneId.systemDefault()).toInstant(); Date legacy = Date.from(instant);	java.util.Date object	Use with legacy libraries

Memory Tip
=> Local = no zone
=> Zoned = with zone
=> Instant = UTC machine time
=> Period = date diff (years/months/days)
=> Duration = time diff (hours/minutes/seconds)

What is Java 8 Date/Time API? Why better than old Date/Calendar?

=> Java 8 Date/Time API a immutable and thread safe API for handling dates, time, duration and periods to address the long standing flaws in the legacy classes java.util.date, java.util.calendar. 

=> java.time is designed for production — immutable and thread-safe by default. 

Java 8 Date/Time API	Old Date, Calander classes
Package : java.time	Package : java.util.Date, Java.util.Calander
All classes are immutable. ie. Operations return new instances	Date and Calnder are mutable. ie. modifying one instance can affect others unexpectedly
Thread safe and so ideal for concurrent applications	Concurrent modifications lead to bugs
=> Separate classes for date, time, date-time, with consistent methods => Better time zone support. Ie ZoneId and ZonedDateTime handle DST and offsets correctly (DST – Daylight Saving Time) => Built-in support for periods, durations, formatting/parsing	=> No separate methods for date, time, timezone concepts. => Date represents both date and time but confusing methods (eg. months 0-based) => Timezone handling is poor, confusing and buggy => Calander has many deprecated methods, poor internationalization
Use case : Always prefer java.time over legacy classes	Cannot prefer over Java 8 Date/Time API
Example : // New way (immutable, fluent) LocalDate date = LocalDate.now(); LocalDate newDate = date.plusDays(10); // Original date unchanged	Example : // Old way (mutable, error-prone) Date date = new Date(); Calendar cal = Calendar.getInstance(); cal.setTime(date); cal.add(Calendar.DAY_OF_MONTH, 10); // Modifies cal

Key classes: LocalDate, LocalTime, LocalDateTime, ZonedDateTime, Instant.

=> The java.time package has specialized classes for different needs

=> LocalDate: Date without time or timezone (year-month-day).
Example: Birthday, holiday.

LocalDate date = LocalDate.of(2025, 12, 26);

=> LocalTime: Time without date or timezone (hour-minute-second-nano).
Example: Meeting time in local context.

LocalTime time = LocalTime.of(14, 30);

=> LocalDateTime: Combination of LocalDate + LocalTime (no timezone).
Example: Event timestamp without zone.

LocalDateTime dt = LocalDateTime.now();

=> ZonedDateTime: LocalDateTime + timezone (handles DST, offsets).
Example: Global meeting time.

ZonedDateTime zdt = ZonedDateTime.now(ZoneId.of("Asia/Kolkata"));

=> Instant: Machine timestamp (UTC, seconds + nanos since epoch).
Example: Logging, timestamps in databases.

Instant instant = Instant.now();

=> Best Practice: Use the most specific class needed — LocalDateTime for most cases, ZonedDateTime for zone-aware, Instant for storage.

Common operations: plus/minus, Period, Duration, formatting/parsing.

=> plus/minus: Chainable methods to add/subtract units.

LocalDate date = LocalDate.now();
LocalDate future = date.plusDays(10).plusMonths(2).minusYears(1);

 => Period: Date-based amount (years, months, days).

Period period = Period.between(LocalDate.of(2025, 1, 1), LocalDate.of(2026, 1, 1)); // P1Y
LocalDate newDate = date.plus(period);

=> Duration: Time-based amount (hours, minutes, seconds, nanos).

Duration duration = Duration.ofHours(5);
LocalTime newTime = time.plus(duration);

=> Formatting/Parsing:

DateTimeFormatter formatter = DateTimeFormatter.ofPattern("dd-MM-yyyy HH:mm");
String text = zdt.format(formatter);
ZonedDateTime parsed = ZonedDateTime.parse(text, formatter);

=> Best Practice: Use DateTimeFormatter for custom formats; pre-defined ISO_LOCAL_DATE for standard.

Time zones and ZoneId — examples.

=> ZoneId: Represents timezone (e.g., "Asia/Kolkata", "America/New_York").

ZoneId kolkata = ZoneId.of("Asia/Kolkata");
ZoneId ny = ZoneId.of("America/New_York");

=> With ZonedDateTime:

ZonedDateTime kolkataTime = ZonedDateTime.now(kolkata);
ZonedDateTime nyTime = kolkataTime.withZoneSameInstant(ny);  // Convert to NY time

=> ZoneOffset: Fixed offset (e.g., +05:30 for IST)

ZoneOffset ist = ZoneOffset.of("+05:30");

=> Best Practice: Store in UTC (Instant), display in user's ZoneId.

Difference between Period and Duration.

Aspect	Period	Duration
Unit	Date-based  (years, months, days)	Time-based  (hours, minutes, seconds, nanos)
Use Case	Human dates (birthdays, anniversaries)	Machine time (timers, intervals)
Example	Period.ofYears(1) = P1Y	Duration.ofHours(24) = PT24H
Add to	LocalDate, LocalDateTime	LocalTime, LocalDateTime, Instant
Precision	Calendar differences (handles months/years)	Exact nanos

Code Example : 

Period p = Period.ofDays(30);  // 30 days
Duration d = Duration.ofDays(30);  // Exactly 30*24 hours 

=> Best Practice: Use Period for date differences, Duration for time.

How to handle legacy Date with new API (toInstant, from).

Convert between legacy and new API

=> Legacy Date/Calendar to new:

Date oldDate = new Date();
Instant instant = oldDate.toInstant();
ZonedDateTime zdt = instant.atZone(ZoneId.systemDefault());
LocalDateTime ldt = LocalDateTime.ofInstant(instant, ZoneId.systemDefault());

=> New to legacy:

LocalDateTime ldt = LocalDateTime.now();
Instant instant = ldt.atZone(ZoneId.systemDefault()).toInstant();
Date legacyDate = Date.from(instant);

=> Best Practice: Convert at boundaries (e.g., DB/API) — use new API internally.

Best practices for Date/Time in production (immutability, thread-safety).

=> Immutability: All java.time classes are immutable — never modify, always create new instances.
=> Thread-Safety: Safe for concurrent use (no synchronization needed).
=> Store in UTC: Use Instant or OFFSET for storage.
=> Display in User Zone: Convert with ZoneId for UI.
=> Avoid Legacy: Don't mix Date/Calendar unless forced.
=> Use DateTimeFormatter Carefully: Pre-defined ISO formats or thread-safe custom (not shared mutable formatter).
=> Handle Nulls: Use Optional or default values.
=> Code Example (Thread-Safe):
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd");
String text = localDate.format(formatter);  // Safe in multi-thread

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Practice the following coding exercises

1. Reduce to sum of integers in list.
2. Reduce to product of integers.
3. Reduce to concatenate strings with delimiter.
4. FlatMap: Flatten List<List<String>> to List<String>.
5. FlatMap: Flatten List<Optional<String>> to List<String>.
6. Generate 10 random numbers using Stream.generate + limit.
7. Generate even numbers sequence using Stream.iterate + limit.
8. Create LocalDate for today and print.
9. Add 10 days to today's date using plusDays.
10. Calculate age from birth date using Period.between.
11. Parse string to LocalDateTime using DateTimeFormatter.
12. Format LocalDateTime to custom pattern (e.g., "dd-MM-yyyy HH:mm").
13. Get current time in different time zones (ZoneId.of("Asia/Kolkata"), "America/New_York").
14. Convert old Date to LocalDateTime.
15. Duration between two LocalDateTime.
16. Find max/min date in list of LocalDate.
17. Group dates by month using groupingBy.
18. Infinite stream of Fibonacci numbers (limit 10).
19. Stream of primes (generate + filter + limit).
20. Debug Stream with peek (print intermediate values).