Java 8 Date and Time API

Sualeh Fatehi
Java 8 Date and
Time API
This work by Sualeh Fatehi is licensed under
a Creative Commons Attribution-
NonCommercial 4.0 International License.

 Review the current date and time API
 Understand date and time concepts
 Take a look at the new date and time API
Overview

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.
Problems Getting a Date
System.out.println(new Date(12, 12, 12));
// Sun Jan 12 00:00:00 EST 1913

 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
A Sorry Implementation

 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
Back Story

System.out.println(new
GregorianCalendar(12, 12, 12));
Revisited Examples
java.util.GregorianCalendar[time=?,areFieldsSet=false,areA
llFieldsSet=false,lenient=true,zone=sun.util.calendar.Zone
Info[id="America/New_York",offset=-
18000000,dstSavings=3600000,useDaylight=true,transitions=2
35,lastRule=java.util.Simpletime
zone[id=America/New_York,offset=-
18000000,dstSavings=3600000,useDaylight=true,startYear=0,s
tartMode=3,startMonth=2,startDay=8,startDayOfWeek=1,startT
ime=7200000,startTimeMode=0,endMode=3,endMonth=10,endDay=1
,endDayOfWeek=1,endTime=7200000,endTimeMode=0]],firstDayOf
Week=1,minimalDaysInFirstWeek=1,ERA=?,YEAR=12,MONTH=12,WEE
K_OF_YEAR=?,WEEK_OF_MONTH=?,DAY_OF_MONTH=12,DAY_OF_YEAR=?,
DAY_OF_WEEK=?,DAY_OF_WEEK_IN_MONTH=?,AM_PM=0,HOUR=0,HOUR_O
F_DAY=0,MINUTE=0,SECOND=0,MILLISECOND=?,ZONE_OFFSET=?,DST_
OFFSET=?]

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.
Problems Getting a Date
System.out.println(dtFmt.format(new
GregorianCalendar(12,12,12).getTime()));
// January 12, 0013 12:00:00 AM EST

 “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

 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
Not Much Improvement

 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
Java 8 Date and Time API

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.
No Problem Getting a Date
System.out.println(
LocalDate.of(12, 12, 12));
// 0012-12-12

System.out.println(
LocalDate.of(13, 13, 13));
Bad Arguments
java.time.DateTimeException: Invalid value for MonthOfYear
(valid values 1 - 12): 13
at java.time.temporal.ValueRange.checkValidValue(Unknown
Source)
at
java.time.temporal.ChronoField.checkValidValue(Unknown Source)
at java.time.LocalDate.of(Unknown Source)
…

Most importantly, the Java 8 date and time
API forces you to think carefully about what
you are doing.
Concepts
// Don’t code like this again
Calendar calendar = new GregorianCalendar();
Date date = calendar.getTime();

 Reference point to measure time
 May be based on religious or political
milestones
 Divides the timeline into eras
 Start of a particular era
Epoch

 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
Computer System Epochs

 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
Calendar System

 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
UTC

 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
ISO 8601

http://xkcd.com/1179/

 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
Machine and Human Timelines

 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
Design Principles

 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
Instant

 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
Partial

 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
Duration

 A length of elapsed time
 Defined using calendar fields - years,
months, and days (not minutes and
seconds)
 Takes time zones into account for
calculation
Period

 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
Time Zone

 Gets the current instant using a time-zone
 Use instead of System.currentTimeMillis()
 Use an alternate clock for testing
Clock
public class SomeBean {
@Inject private Clock clock;
public void process() {
LocalDate date = LocalDate.now(clock);
...
}
}

 Pluggable calendar system
 Provides access to date and time fields
 Built-in
 ISO8601 (default): IsoChronology
 Chinese: MinguoChronology
 Japanese: JapaneseChronology
 Thai Buddhist: ThaiBuddhistChronology
 Islamic: HijrahChronology
Chronology

 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
New Packages

 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
Commonly Used Classes

Class or Enum Year Month Day Hours Minutes Nanos
Zone
Offset
Zone
ID toString Output
Instant ✓ 2013-08-20T15:16:26.355Z
LocalDate ✓ ✓ ✓ 2013-08-20
LocalDateTime ✓ ✓ ✓ ✓ ✓ ✓ 2013-08-20T08:16:26.937
ZonedDateTime ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ 2013-08-21T00:16:26.941+09:00[Asia/Tokyo]
LocalTime ✓ ✓ ✓ 08:16:26.943
MonthDay ✓ ✓ --08-20
Year ✓ 2013
YearMonth ✓ ✓ 2013-08
Month ✓ AUGUST
OffsetDateTime ✓ ✓ ✓ ✓ ✓ ✓ ✓ 2013-08-20T08:16:26.954-07:00
OffsetTime ✓ ✓ ✓ ✓ 08:16:26.957-07:00
Duration ✓ PT20H
Period ✓ ✓ ✓ P10D
Commonly Used Classes
http://docs.oracle.com/javase/tutorial/datetime/iso/overview.html

 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
Consistent Operations

 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
Staying Constant

 Format with a DateTimeFormatter instance
 Internationalization is supported
 Custom formats can be used, including
am/ pm for time
Formatting

 Parse with a DateTimeFormatter instance
 parse(…) methods return a temporal
 Use from(…) to convert to a known date or
time type
Parsing

TemporalAdjuster fourMinutesLater =
new TemporalAdjuster() {
@Override
public Temporal adjustInto(Temporal
temporal) {
return temporal.plus(4,
ChronoUnit.MINUTES);
}
};
LocalTime time = LocalTime.of(12, 0, 0);
LocalTime later = time.with(fourMinutesLater);
Temporal Adjusters

LocalTime time = LocalTime.of(12, 0, 0);
time.with(temporal ->
temporal.plus(4, ChronoUnit.MINUTES)));
Temporal Adjusters
Java 8 Style

 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
Temporal Queries

 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.
Summary

 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
Resources

Code used in this presentation on GitHub:
https://github.com/sualeh/java8-timeapi-examples
Code

Java 8 Date and Time API

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