Reasoning correctly about dates and times seems easy but is not. Designing a good date/time API is even harder. We will be looking at how Java got it wrong so far, and how things will change with the upcoming JDK 8. Presentation for the Iasi JUG, 18 September 2013

1. Java
Date and Time
Past and Future
Iulian Dogariu
Ness Technologies
Iași Java User Group
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2. APIs that make me happy

5. Timestamp

6. “System.currentTimeMillis() should be
enough for anybody”
-- Bill Gates,
after seeing the JDK 1.0 Javadocs
(well, not really)

7. java.util.Date

8. java.util.Calendar

9. POLA !

11. java.util.Calendar, as of JDK 7
• Missing abstractions
• Full of bad surprises
– Unsound design – violates Liskov
– Violates POLA
– Getter methods with side effects!
– Not thread safe
– Untestable

14. Instants

15. Instants
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16. Civil time

17. Instants ↔ Civil time

18. Let’s Practice !

19. Java 8 Time vs Joda Time

20. We love .NET too
http://nodatime.org/

22. Java 8 Time – Recap
• Instants
• Civil time (Days, Months, Weeks)
• Zones and Offsets
• Choose your abstractions well,
and the rest is simple

23. Instants
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java.time.Instant
java.time.ZonedDateTime
java.time.OffsetDateTime

24. Civil time java.time.LocalDateTime
java.time.Period

25. Q, and, hopefully, A
?

26. Links
• Java 8 EA https://jdk8.java.net/download.html
– Current version: b106
• Javadocs: http://download.java.net/jdk8/docs/api/
• ISO 8601: http://en.wikipedia.org/wiki/ISO_8601
• Olson TZ aka Zoneinfo
– http://www.iana.org/time-zones
– http://en.wikipedia.org/wiki/Tz_database
• Stephen Colebourne - author of Joda Time

27. Thank you!