This post is an extract of a Code Kata presentation I had hosted back in November 2015. Java 8 Date Time API was relatively new to many of the attendees.

This Kata-based style of teaching includes sharing a set of failing / non-functional JUnit tests with TODO comments indicating actions that need to be taken to resolve/pass the JUnit tests. Solving the JUnit tests repeatedly helps developers gain muscle-memory into using the feature (similar to the karate kata).

Link to the code kata is on Github, located at the bottom of the post.

Background: Why a new API

First, some background on why there was a need to create a new API.

Issues with java.util.Date

java.util.Date:

• is mutable-only – Date instances are not immutable.
• has concurrency problems – Date instances are not thread-safe.
• has incorrect naming – Date is not a “date” but a “timestamp”.
• lacks convention – Days start with 1, months with 0 and years with 1900.
• lacks fluency – Cannot create durations (a quarter, 5 minutes) OR combos(year+month, date without seconds) etc.

Additional observations about the pre-Java 8 Date API

• System.currentTimeInMillis() is not accurate and can return same value for multiple sequential calls.
• java.util.Date vs. java.sql.Date – SQL flavor is only a DATE with no time.
• java.sql.Timestamp – SQL flavor replicating java.util.Date but additionally storing nanoseconds.

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Issues with java.util.Calendar

java.util.Calendar:

• lacks clarity – Mix of dates and times.
• has confusing timezone support – Not very easy to switch timezones, offsets etc.
• has severe formatting hurdles – SimpleDateFormat and Calendar do not interoperate well.
• presents extension hardships – New calendar systems are created by extending Calendar (therefore all it’s problems).

“Calendar is the Ravenous Bugblatter Beast of Traal for Date APIs” – Chandra Guntur (circa 2000).

What was proposed to fix this?

JSR-310: Date and Time API
Excerpts from JSR-310 (https://jcp.org/en/jsr/detail?id=310)

• “The main goal is to build upon the lessons learned from the first two APIs (Date and Calendar) in Java SE, providing a more advanced and comprehensive model for date and time manipulation.”
• “The new API will be targeted at all applications needing a data model for dates and times. This model will go beyond classes to replace Date and Calendar, to include representations of date without time, time without date, durations and intervals.“
• “The new API will also tackle related date and time issues. These include formatting and parsing, taking into account the ISO8601 standard and its implementations, such as XML.”
• “The final goal of the new API is to be simple to use. The API will need to contain some powerful features, but these must not be allowed to obscure the standard use cases. Part of being easy to use includes interaction with the existing Date and Calendar classes …”

Origins of the JSR-310

• JSR-310 is “inspired from” the very popular Joda-Time library by Stephen Colebourne, who is also the lead on JSR-310.
• JSR-310 was a means to both overcome the shortcomings as well as refactor portions of the Joda-Time. http://blog.joda.org/2009/11/why-jsr-310-isn-joda-time_4941.html.
• Check out the cheatsheet from converting from Joda-Time to java.time apis: http://blog.joda.org/2014/11/convertingfrom-joda-time-to-javatime.html
• Also check out Meno Hochschild’s (author of the Time4J library) response at stack overflow: http://stackoverflow.com/questions/24631909/differences-between-java-8-date-time-apijava-time-and-joda-time

Java 8 Date Time API

The Java classes in lavender below are all linked to the JDK 8 API. Please feel free to click.

Date Time Classes

Dates and Times: Simple Date and Time ‘containers’

• Instant stores a numeric timestamp from Java epoch, + nanoseconds.
• LocalDate stores a date without a time portion (calendar date).
• LocalTime stores a time without a date portion (wall clock).
• LocalDateTime stores a date and time (LocalDate + Local Time).
• ZonedDateTime stores a date and time with a time-zone.
• OffsetTime stores a time and offset from UTC without a date.
• OffsetDateTime stores a date with time and offset from UTC.

Ranges and Partials: Spans and ranges of temporality

• Duration models time in nanoseconds for time intervals. (e.g. 5 mins)
• Period models amount of time in years, months and/or days. (e.g. 2 Days)
• Month stores a month on its own. (e.g. MARCH)
• MonthDay stores a month and day without a year or time (e.g. date of birth)
• Year stores a year on its own. (e.g. 2015)
• YearMonth stores a year and month without a day or time. (e.g. credit card expiry)
• DayOfWeek stores a day-of-week on its own. (e.g. WEDNESDAY)

Chronology: A calendar system to organize and identify dates

• Chronology is a factory to create or fetch pre-built calendar systems.
  Default is IsoChronology (e.g. ThaiBuddhistChronology).
• ChronoLocalDate stores a date without a time in an arbitrary chronology.
• ChronoLocalDateTime stores a date and time in an arbitrary chronology.
• ChronoZonedDateTime stores a date, time and timezone in an arbitrary chronology.
• ChronoPeriod models a span on days/time for use in an arbitrary chronology.
• Era stores a timeline [typically two per Chronology, but some have more eras].

Date Time Common API – Reference Table

Date and Time

Type	Y	M	D	H	m	S(n)	Z	ZId	toString
Instant									1999-01-12T12:00:00.747Z
LocalDate									1999-01-12
LocalTime									12:00:00.747
LocalDateTime									1999-01-12T12:00:00.747
ZonedDateTime									1999-01-12T12:00:00.747-05:00 [America/New_York]
OffsetTime									12:00:00.747-05:00
OffsetDateTime									1999-01-12T12:00:00.747-05:00

Ranges

Type	Y	M	D	H	m	S(n)	Z	ZId	toString
Duration									P22H
Period									P15D

Partials

Type	Y	M	D	H	m	S(n)	Z	ZId	toString
Month		*							JANUARY
MonthDay									-01-12
Year									1999
YearMonth									1999-01
DayOfWeek			*						TUESDAY

Fluency and Symmetry in the new Date Time API

The Java 8 Date Time API introduces a certain symmetry in operations that make for a pleasant developer experience. Below is a list of prefixes for methods across the API that perform in a similar pattern where encountered.

• of {static factory method prefix} – Factory method to obtain an instance with provided parameters – validates and builds with no conversion.example usage: LocalDate.of(...) or Instant.ofEpochSecond(...)“
• from {static factory method prefix} – Factory method to obtain an instance with provided parameters – validates, converts and builds.example usage: LocalDateTime.from(...) or OffsetTime.from(...)
• parse {static factory method prefix} – Factory method to obtain an instance with provided CharSequence parameters, by parsing the content.example usage: LocalDate.parse(...) or OffsetDateTime.parse(...)
• format {instance method prefix} – Formats the instance with provided formatter.example usage: localDate.format(formatter)
• get {instance method prefix} – Return part of the state of the target temporal object.example usage: localDate.getDayOfWeek()
• is {instance method prefix} – Queries a part of the state of the target temporal objectexample usage: localTime.isAfter(...)
• with {instance method prefix} – Returns a copy of the immutable temporal object with a portion altered. Alternate for a set operation.example usage: offsetTime.withHour(...)
• plus {instance method prefix} – Return a copy of the temporal object with an added time.example usage: localDate.plusWeeks(...)
• minus {instance method prefix} – Return a copy of the temporal object with subtracted time.example usage: localTime.minusSeconds(...)
• to {instance method prefix} – Convert the temporal object into a new temporal object of another type.example usage: localDateTime.toLocalDate(...)
• at {instance method prefix} – Combine the temporal object into a new temporal object with supplied parameters.example usage: localDate.atTime(...)

KATA TIME !!!

The Code for the Kata is located at: https://github.com/c-guntur/javasig-datetime

Your mission, should you choose to accept it, is to:

1. setup the kata using your favorite IDE
2. launch each JUnit test (Exercise1Test through Exercise5Test) and fix the tests, so all execute as instructed in the TODO above any commented lines.
3. repeat as often as necessary to build muscle memory of using the awesome Date Time API

There are solutions, but looking at solutions may limit your learning different ways of solving the same problem.

Link to solutions: https://github.com/c-guntur/javasig-datetime-solutions

I hope you found this post fun!

Happy coding!