Java 8 date & time api
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The document summarizes the new Java 8 Date & Time API. It discusses problems with the old Date and Calendar classes, including inconsistent behavior and lack of thread safety. It then overviews the new API, which was influenced by Joda-Time and aims to distinguish machine and human views of time. Key aspects covered include Instant, LocalDate, Duration, and formatting/parsing utilities. The new API provides a more clear and consistent model for working with dates and times in Java.
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Java 8 date & time api
- 1. Java 8 Date & Time API! Rasheed Amir - Aurora Solutions
- 2. www.aurorasolutions.io Full Stack Developer, Software Architect & Agile Coach @ Aurora - Provides REMOTE Teams specializing in JVM languages and Angular + Ember About: Rasheed LinkedIn: https://se.linkedin.com/in/rasheedwaraich 2 Email: [email protected] Co-founder of: ➔ www.aurorasolutions.io Board of directors: ➔ www.simsamcareers.com Organizer: Spring Meetup Summary: ❏ 12+ years of hands on “full stack” development experience ❏ Love DDD, TDD, CD, Agility!
- 3. Example How many bugs in this code? Date date = new Date(2010, 12, 13, 16, 40); TimeZone zone=TimeZone.getTimeZone("Asia/HongKong"); Calendar cal = new GregorianCalendar(date, zone); DateFormat fm = new SimpleDateFormat("HH:mm Z"); String str = fm.format(cal);
- 4. Example 6 bugs in the code! Date date = new Date(2010, 12, 13, 16, 40); TimeZone zone=TimeZone.getTimeZone("Asia/HongKong"); Calendar cal = new GregorianCalendar(date, zone); DateFormat fm = new SimpleDateFormat("HH:mm Z"); String str = fm.format(cal);
- 5. Overview ● Review the current date and time API ● Understand date and time concepts ● Take a look at the new date and time API
- 6. Problems Getting a Date Several problems here: 1. Which 12 is for which date field? 2. Month 12 is December, right? No. January 3. Year 12 is 12 CE, right? Wrong. 1913 4. Wait - there is a time in a date? 5. More than that, there is a time zone
- 7. A Sorry Implementation ● Conceptually an instant, not a date ● Properties have random offsets ○ Some zero-based, like month and hours ○ Some one-based, like day of the month ○ Year has an offset of 1900 ● Mutable, not thread-safe ● Not internationalizable ● Millisecond granularity ● Does not reflect UTC
- 8. Backstory ● Date was the work of James Gosling and Arthur van Hoff ● Added in JDK 1.0, mostly deprecated in JDK 1.1, never removed ● IBM donated Calendar code to Sun
- 10. Problems Getting a Date Several problems here: 1. Which 12 is for which date field? 2. Month 12 is December, right? No. January 3. They got the year right! Almost. 13 CE 4. Wait - there is a time in a calendar? 5. More than that, there is a time zone
- 11. Calendar ● “Calendar” represents a date, time and time-zone ● Defaults to Gregorian calendar ● In Thailand only, you get a Buddhist calendar ● You can ask specifically ask for a Japanese calendar Calendar
- 12. Not Much Improvement ● Conceptually an instant, not a calendar ● But, can’t create a Calendar from a Date ● Can’t format a Calendar ● Zero-based offsets ● Stores internal state in two different ways ○ milliseconds from epoch ○ set of fields ● Has bugs and performance issues ● Mutable, not thread-safe
- 13. JAVA 8 Date and Time API ● 2002 - Stephen Colebourne starts open source Joda- Time project ● 2005 - Release of Joda-Time 1.0 ● 2007 - JSR 310, for inclusion in Java ● 2011 - Release of Joda-Time 2.0 ● 2014 - Finally, the date and time API is in Java 8
- 14. No Problem Getting a Date No problems: 1. ISO 8601 order of fields - year, month, day. 2. Month 12 is December. 3. Year is 12 CE. 4. No time component. 5. No time zone
- 15. Bad Arguments
- 16. Concepts Most importantly, the Java 8 date and time API forces you to think carefully about what you are doing.
- 17. Epoch ● Reference point to measure time ● May be based on religious or political milestones ● Divides the timeline into eras ● Start of a particular era
- 18. Computer System Epochs ● January 0, 0 - MATLAB ● January 1, 1 - Symbian, .NET ● January 1, 1601 - COBOL, Windows ● January 1, 1900 - LISP, SNTP ● January 1, 1904 – Old Mac OS ● January 1, 1970 - Unix Epoch (Linux, Mac OS X), Java, C, JavaScript, Perl, PHP, Python, Ruby
- 19. Calendar System ● Organizes days for social, religious, commercial or administrative purposes ● Names periods like days, weeks, months, and years ● Periods may follow cycles of the sun or moon ● A date is a specific day in the system ● May be based on an epoch
- 20. UTC ● GMT is Greenwich Mean Time ● Mean solar time at the Royal Observatory in Greenwich ● UTC is Coordinated Universal Time ● Precisely defined with atomic time Does not change with seasons ● Replaced GMT as reference time scale on 1 January 1972
- 21. ISO 8601 ● International standard for representation of dates and times ● Uses the Gregorian calendar system ● Ordered from most to least significant: year, month, day, hour, minute ● Each date and time value has a fixed number of digits with leading zeros ● Uses four-digit year at minimum, YYYY
- 22. Machine and Human Timelines ● Machines have one view of time ○ discrete points corresponding to the smallest measurement possible ○ a single, ever increasing number ● Humans have a different view of time ○ continuous timelines ○ calendar systems ○ arbitrary units like years, months, days, hours ○ time zones, and daylight savings rules
- 23. Design Principles ● Distinguish between machine and human views ● Well-defined and clear purpose ● Immutable, thread-safe ● Reject null and bad arguments early ● Extensible, by use of strategy pattern ● Fluent interface with chained methods
- 24. Instant ● Point on a discretized time-line ● Stored to nanosecond resolution ○ long for seconds since epoch, and ○ int for nanosecond of second ● Convert to any date time field using a Chronology ● Use for event time-stamps
- 25. Partial ● An indication of date or time that cannot identify a specific, unique instant ● Definition uses fields such as year, month, day of month, and time of day ● Commonly used partials, such as LocalDate and LocalTime are available ● Others like MonthDay, YearMonth (card expiration?) are also available
- 26. Duration ● Precise length of elapsed time, in nanoseconds ● Does not use date-based constructs like years, months, and days ● Can be negative, if end is before start
- 27. Period ● A length of elapsed time ● Defined using calendar fields - years, months, and days (not minutes and seconds) ● Takes time zones into account for calculation
- 28. Time Zone ● Region with uniform standard time for legal, commercial, social, and political purposes ● Some countries use daylight saving time for part of the year ● Offset from UTC (UTC-12 to UTC+14) ● UTC is sometimes denoted by Z (Zulu) ● JDK time zone data is updated with JDK releases
- 29. Clock ● Gets the current instant using a time-zone ● Use instead of System.currentTimeMillis() ● Use an alternate clock for testing Clock
- 30. Chronology ● Pluggable calendar system ● Provides access to date and time fields ● Built-in ○ ISO8601 (default): IsoChronology ○ Chinese: MinguoChronology ○ Japanese: JapaneseChronology ○ Thai Buddhist: ThaiBuddhistChronology ○ Islamic: HijrahChronology
- 31. New Packages ● java.time - instants, durations, dates, times, time zones, periods ● java.time.format - formatting and parsing ● java.time.temporal - field, unit, or adjustment access to temporals ● java.time.zone – support for time zones ● java.time.chrono - calendar systems other than ISO- 8601
- 32. Commonly Used Classes ● LocalDate ○ ISO 8601 date without time zone and time ○ Corresponds to SQL DATE type ○ Example: birthdate or employee hire-date ● LocalTime ○ ISO 8601 time without time zone and date ○ Corresponds to SQL TIME type ○ Example: the time that an alarm clock goes off ● LocalDateTime ○ ISO 8601 date and time without time zone ○ Corresponds to SQL TIMESTAMP type
- 34. Consistent Operations ● of - static factory, validates input ● from - static factory, converts to instance of target class ● get - returns part of the state is - queries the state ● with - immutable copy with elements changed ● to - converts to another object type ● plus, minus - immutable copy after operation
- 35. Staying Constant ● Day of week, for example DayOfWeek.SUNDAY ● Month , for example LocalDate.of(2014, Month.MAY, 20); ● Time units, for example Instant.now().plus(1, ChronoUnit.DAYS) ● Other useful constants ○ LocalTime.MIDNIGHT // 00:00 ○ LocalTime.NOON // 12:00
- 36. Formatting ● Format with a DateTimeFormatter instance ● Internationalization is supported ● Custom formats can be used, including am/ pm for time
- 37. Parsing ● Parse with a DateTimeFormatter instance ● parse(…) methods return a temporal ● Use from(…) to convert to a known date or time type
- 39. Temporal Adjusters Java 8 style
- 40. Temporal Queries ● Strategy for extracting information from temporals ● Externalize the process of querying ● Examples ○ get the time zone in a temporal ○ check if date is February 29 in a leap year ○ calculate days until your next birthday ● TemporalQueries class has implementations of common queries
- 41. Summary ● Existing date-related APIs can be error- prone and tedious ● Separate concepts of computer-related times and human-related times Need to manipulate dates and times? Use Joda-Time or the Java 8 date and time API
- 42. Resources ● JSR 310: A New Java Date/Time API ● Joda-Time ● Why JSR-310 isn't Joda-Time ● Java 101: The next generation: It's time for a change
- 43. Code Code used in this presentation on GitHub: