# Dates & Times with java.time — Core Java Fundamentals: Syntax, OOP and Collections

Source: https://www.geekswithgeeks.com/en/core-java/java-time-api

> Work with dates, times, durations and time zones using the immutable java.time API instead of Date and Calendar.

## LocalDate, LocalDateTime and formatting

The `java.time` classes are immutable and thread-safe. Use `LocalDate` for dates, `LocalDateTime` for date plus time and `DateTimeFormatter` to print or parse.

```java
LocalDate today = LocalDate.now();
LocalDate due = today.plusDays(14);
DateTimeFormatter f = DateTimeFormatter.ofPattern("dd MMM yyyy");
System.out.println(due.format(f));
System.out.println(LocalDate.parse("2026-10-01").getDayOfWeek());
```

## Durations, periods and zones

`Duration` measures time-based amounts, `Period` date-based amounts and `ZonedDateTime` carries a time zone.

```java
Duration d = Duration.between(start, end);
System.out.println(d.toMinutes());
Period p = Period.between(LocalDate.of(2026, 1, 1), LocalDate.now());
ZonedDateTime tokyo = ZonedDateTime.now(ZoneId.of("Asia/Tokyo"));
```

Store timestamps as `Instant` (UTC) and convert to a zone only for display. Avoid the legacy `Date`, `Calendar` and `SimpleDateFormat`.

Quick check

**Quiz:** Which class represents a point on the UTC timeline?

- [x] Instant
- [ ] LocalDate
- [ ] Period
- [ ] SimpleDateFormat

*Answer:* Instant. Instant is a machine timestamp; convert it to a zone for display.

## Final quiz 1 of 8

Final quiz

**Quiz:** What does javac produce?

- [ ] Native machine code
- [ ] An .exe file
- [x] Platform-neutral bytecode
- [ ] Nothing

*Answer:* Platform-neutral bytecode. The JVM runs the bytecode.

## Final quiz 2 of 8

Final quiz

**Quiz:** How should you compare String contents?

- [ ] ==
- [ ] compare with +
- [ ] hashCode()
- [x] equals()

*Answer:* equals(). == compares references.

## Final quiz 3 of 8

Final quiz

**Quiz:** Which class avoids O(n²) string concatenation in loops?

- [x] StringBuilder
- [ ] String
- [ ] Scanner
- [ ] Objects

*Answer:* StringBuilder. StringBuilder mutates one buffer.

## Final quiz 4 of 8

Final quiz

**Quiz:** What must you do when overriding equals()?

- [ ] Make the class final only
- [x] Override hashCode() consistently
- [ ] Return null
- [ ] Nothing

*Answer:* Override hashCode() consistently. Equal objects must share a hash code.

## Final quiz 5 of 8

Final quiz

**Quiz:** Which collection stores unique values with fast membership checks?

- [ ] ArrayList
- [ ] LinkedList
- [x] HashSet
- [ ] int[]

*Answer:* HashSet. HashSet offers average O(1) contains.

## Final quiz 6 of 8

Final quiz

**Quiz:** Which base class should an unchecked custom exception extend?

- [ ] Exception
- [ ] Throwable
- [ ] Error
- [x] RuntimeException

*Answer:* RuntimeException. Callers are not forced to catch it.

## Final quiz 7 of 8

Final quiz

**Quiz:** What does try-with-resources do?

- [x] Closes AutoCloseable resources automatically
- [ ] Skips exceptions
- [ ] Starts threads
- [ ] Compiles faster

*Answer:* Closes AutoCloseable resources automatically. Resources close even if an exception occurs.

## Final quiz 8 of 8

Final quiz

**Quiz:** Which java.time class suits a date with no time?

- [ ] Instant
- [x] LocalDate
- [ ] Duration
- [ ] ZonedDateTime

*Answer:* LocalDate. LocalDate holds year, month and day only.