Java 8 Date and Time API

Java 8: Date Time API
Ganesh Samarthyam
ganesh@codeops.tech

–Craig Larman
"The critical design tool for software development
is a mind well educated in design principles"

Discussion Example
// using java.util.Date
Date today = new Date();
System.out.println(today);
$ java DateUse
Wed Dec 02 17:17:08 IST 2015
Why should we get the
time and timezone details
if I only want a date? Can
I get rid of these parts?
No!

So What?!
Date today = new Date();
Date todayAgain = new Date();
System.out.println(todayAgain);
System.out.println(today.compareTo(todayAgain) == 0);
Thu Mar 17 13:21:55 IST 2016
Thu Mar 17 13:21:55 IST 2016
false
What is going
on here?

Joda API
JSR 310: Java Date and Time API
Stephen Colebourne

Refactoring for Date
Replace inheritance
with delegation

java.time package!
Date, Calendar, and TimeZone
Java 8 replaces
these types

Refactored Solution
LocalDate today = LocalDate.now();
LocalDate todayAgain = LocalDate.now();
System.out.println(todayAgain);
System.out.println(today.compareTo(todayAgain) == 0);
2016-03-17
2016-03-17
true
Works ﬁne
now!

Refactored Example …
You can use only date,
time, or even timezone,
and combine them as
needed!
LocalDate today = LocalDate.now();
LocalTime now = LocalTime.now();
System.out.println(now);
ZoneId id = ZoneId.of("Asia/Tokyo");
System.out.println(id);
LocalDateTime todayAndNow = LocalDateTime.now();
System.out.println(todayAndNow);
ZonedDateTime todayAndNowInTokyo = ZonedDateTime.now(ZoneId.of("Asia/Tokyo"));
System.out.println(todayAndNowInTokyo);
2016-03-17
13:28:06.927
Asia/Tokyo
2016-03-17T13:28:06.928
2016-03-17T16:58:06.929+09:00[Asia/Tokyo]

“Fluent interfaces”
❖ Code is more readable and easier to use:
❖ Classes in this package have numerous static methods
(many of them factory methods)
❖ Methods in the classes follow a common naming
convention (for example, they use the preﬁxes plus
and minus to add or subtract date or time values)

java.time Sub-packages
❖ java.time.temporal —Accesses date/time ﬁelds and
units
❖ java.time.format —Formats the input and output of
date/time objects
❖ java.time.zone —Handles time zones
❖ java.time.chrono —Supports calendar systems such as
Japanese and Thai calendars

ISO-8601 Calendar System Format
❖ The Java 8 date and time API uses ISO 8601 as the
default calendar format.
❖ In this internationally accepted format, the date and
time values are sorted from the largest to the smallest
unit of time: year, month/week, day, hour, minute,
second, and millisecond/nanosecond.
❖ Example: LocalDate is represented in the in a year-
month-day format (YYYY-MM-DD), as in, 2015-10-26.

java.time.LocalDate
LocalDate newYear2016 = LocalDate.of(2016, 1, 1);
System.out.println("New year 2016: " + newYear2016);
New year 2016: 2016-01-01

java.time.LocalDate
LocalDate valentinesDay = LocalDate.of(2016, 14, 2);
System.out.println("Valentine's day is on: " + valentinesDay);
Exception in thread "main"
java.time.DateTimeException: Invalid value
for MonthOfYear(valid values 1 - 12): 14

java.time.LocalDate
long visaValidityDays = 180L;
LocalDate currDate = LocalDate.now();
System.out.println("My Visa expires on: " + currDate.plusDays(visaValidityDays));
My Visa expires on: 2016-04-23

Important Methods in LocalDate

java.time.LocalTime
LocalTime currTime = LocalTime.now();
System.out.println("Current time is: " + currTime);
Current time is: 12:23:05.072

java.time.LocalTime
System.out.println(LocalTime.of(18,30));
prints: 18:30

java.time.LocalTime
long hours = 6;
long minutes = 30;
LocalTime currTime = LocalTime.now();
System.out.println("Current time is: " + currTime);
System.out.println("My meeting is at: " +
currTime.plusHours(hours).plusMinutes(minutes));
Current time is: 12:29:13.624
My meeting is at: 18:59:13.624

Important Methods in LocalTime

java.time.LocalDateTime
LocalDateTime currDateTime = LocalDateTime.now();
System.out.println("Today's date and current time is: " + currDateTime);
Today's date and current time is:
2015-10-29T21:04:36.376

LocalDateTime christmas = LocalDateTime.of(2015, 12, 25, 0, 0);
LocalDateTime newYear = LocalDateTime.of(2016, 1, 1, 0, 0);
System.out.println("New Year 2016 comes after Christmas 2015”
+ newYear.isAfter(christmas));
New Year 2016 comes after
Christmas 2015? true

LocalDateTime dateTime = LocalDateTime.now();
System.out.println("Today's date and current time: " + dateTime);
System.out.println("The date component is: " + dateTime.toLocalDate());
System.out.println("The time component is: " + dateTime.toLocalTime());
Today's date and current time:
2015-11-04T13:19:10.497
The date component is: 2015-11-04
The time component is: 13:19:10.497

java.time.Instant
import java.time.Instant;
public class UsingInstant {
public static void main(String args[]){
// prints the current timestamp with UTC as time zone
Instant currTimeStamp = Instant.now();
System.out.println("Instant timestamp is: "+ currTimeStamp);
// prints the number of seconds as Unix timestamp from epoch time
System.out.println("Number of seconds elapsed: " + currTimeStamp.getEpochSecond());
// prints the Unix timestamp in milliseconds
System.out.println("Number of milliseconds elapsed: " + currTimeStamp.toEpochMilli());
}
}
Instant timestamp is: 2015-11-02T03:16:04.502Z
Number of seconds elapsed: 1446434164
Number of milliseconds elapsed: 1446434164502

java.time.Period
LocalDate manufacturingDate = LocalDate.of(2016, Month.JANUARY, 1);
LocalDate expiryDate = LocalDate.of(2018, Month.JULY, 18);
Period expiry = Period.between(manufacturingDate, expiryDate);
System.out.printf("Medicine will expire in: %d years, %d months, and %d days (%s)n",
expiry.getYears(), expiry.getMonths(), expiry.getDays(), expiry);
Medicine will expire in: 2 years, 6 months, and 17
days (P2Y6M17D)

The Java 8 date and time API differentiates how humans
and computers use date- and time-related information.
For example, the Instant class represents a Unix
timestamp and internally uses long and int variables.
Instant values are not very readable or usable by
humans because the class does not support methods
related to day, month, hours, and so on (in contrast, the
Period class supports such methods).

java.time.Duration
LocalDateTime comingMidnight =
LocalDateTime.of(LocalDate.now().plusDays(1), LocalTime.MIDNIGHT);
LocalDateTime now = LocalDateTime.now();
Duration between = Duration.between(now, comingMidnight);
System.out.println(between);
PT7H13M42.003S

Summary of Instant, Period and Duration

TemporalUnit
import java.time.temporal.ChronoUnit;
public class ChronoUnitValues {
public static void main(String []args) {
System.out.println("ChronoUnit DateBased TimeBased Duration");
System.out.println("---------------------------------------");
for(ChronoUnit unit : ChronoUnit.values()) {
System.out.printf("%10s t %b tt %b tt %s %n”,
unit, unit.isDateBased(), unit.isTimeBased(), unit.getDuration());
}
}
}

ZoneId
System.out.println("My zone id is: " + ZoneId.systemDefault());
My zone id is: Asia/Kolkata
ZoneId AsiaKolkataZoneId = ZoneId.of("Asia/Kolkata");

ZonedDateTime
LocalDate currentDate = LocalDate.now();
LocalTime currentTime = LocalTime.now();
ZoneId myZone = ZoneId.systemDefault();
ZonedDateTime zonedDateTime = ZonedDateTime.of(currentDate,
currentTime, myZone);
System.out.println(zonedDateTime);
2015-11-05T11:38:40.647+05:30[Asia/Kolkata]

ZonedDateTime
ZoneId myZone = ZoneId.of("Asia/Kolkata");
LocalDateTime dateTime = LocalDateTime.now();
ZonedDateTime zonedDateTime = dateTime.atZone(myZone);
ZoneOffset zoneOffset = zonedDateTime.getOffset();
System.out.println(zoneOffset);
+05:30

ZonedDateTime
ZoneId singaporeZone = ZoneId.of(“Asia/Singapore");
ZonedDateTime dateTimeInSingapore =
ZonedDateTime.of(LocalDateTime.of(2016, Month.JANUARY, 1, 6, 0), singaporeZone);
ZoneId aucklandZone = ZoneId.of("Paciﬁc/Auckland");
ZonedDateTime sameDateTimeInAuckland =
dateTimeInSingapore.withZoneSameInstant(aucklandZone);
Duration timeDifference = Duration.between(
dateTimeInSingapore.toLocalTime(),
sameDateTimeInAuckland.toLocalTime());
System.out.printf("Time difference between %s and %s zones is %d hours”,
singaporeZone, aucklandZone, timeDifference.toHours());
Time difference between Asia/Singapore and
Paciﬁc/Auckland zones is 5 hours

Daylight Savings
ZoneId kolkataZone = ZoneId.of("Asia/Kolkata");
Duration kolkataDST = kolkataZone.getRules().getDaylightSavings(Instant.now());
System.out.printf("Kolkata zone DST is: %d hours %n", kolkataDST.toHours());
ZoneId aucklandZone = ZoneId.of("Paciﬁc/Auckland");
Duration aucklandDST = aucklandZone.getRules().getDaylightSavings(Instant.now());
System.out.printf("Auckland zone DST is: %d hours", aucklandDST.toHours());
Kolkata zone DST is: 0 hours
Auckland zone DST is: 1 hours

DateTimeFormatter
Predefined formatters:
• ISO_DATE (2015-11-05)
• ISO_TIME (11:25:47.624)
• RFC_1123_DATE_TIME (Thu, 5 Nov 2015 11:27:22 +0530)
• ISO_ZONED_DATE_TIME (2015-11-05T11:30:33.49+05:30[Asia/Kolkata])

DateTimeFormatter
Wake up time: 06:00:00
LocalTime wakeupTime = LocalTime.of(6, 0, 0);
System.out.println("Wake up time: " + DateTimeFormatter.ISO_TIME.format(wakeupTime));
01 Jan 2016
DateTimeFormatter customFormat = DateTimeFormatter.ofPattern("dd MMM yyyy");
System.out.println(customFormat.format(LocalDate.of(2016, Month.JANUARY, 01)));

Uppercase and lowercase letters can have similar or
different meanings when used in format strings for
dates and times. Read the Javadoc for these patterns
carefully before trying to use these letters. For example,
in dd-MM-yy, MM refers to month; however, in dd-mm-
yy, mm refers to minutes !

Formatting Dates
• G (era: BC, AD)
• y (year of era: 2015, 15)
• Y (week-based year: 2015, 15)
• M (month: 11, Nov, November)
• w (week in year: 13)
• W (week in month: 2)
• E (day name in week: Sun, Sunday)
• D (day of year: 256)
• d (day of month: 13)

Custom Date Patterns
public class CustomDatePatterns {
// patterns from simple to complex ones
String [] dateTimeFormats = {
"dd-MM-yyyy", /* d is day (in month), M is month, y is year */
"d '('E')' MMM, YYYY", /*E is name of the day (in week), Y is year*/
"w'th week of' YYYY", /* w is the week of the year */
"EEEE, dd'th' MMMM, YYYY" /*E is day name in the week */
};
LocalDateTime now = LocalDateTime.now();
for(String dateTimeFormat : dateTimeFormats) {
System.out.printf("Pattern "%s" is %s %n", dateTimeFormat,
DateTimeFormatter.ofPattern(dateTimeFormat).format(now));
}
}
} Pattern "dd-MM-yyyy" is 05-11-2015
Pattern "d '('E')' MMM, YYYY" is 5 (Thu) Nov, 2015
Pattern "w'th week of' YYYY" is 45th week of 2015
Pattern "EEEE, dd'th' MMMM, YYYY" is Thursday, 05th November, 2015

Formatting Times
• a (marker for the text a.m./p.m. marker)
• H (hour: value range 0–23)
• k (hour: value range 1–24)
• K (hour in a.m./p.m.: value range 0–11)
• h (hour in a.m./p.m.: value range 1–12)
• m (minute)
• s (second)
• S (fraction of a second)
• z (time zone: general time-zone format)

Custom Time Patterns
class CustomTimePatterns {
// patterns from simple to complex ones
String [] timeFormats = {
"h:mm", /* h is hour in am/pm (1-12), m is minute */
"hh 'o''clock'", /* '' is the escape sequence to print a single quote */
"H:mm a", /* H is hour in day (0-23), a is am/pm*/
"hh:mm:ss:SS", /* s is seconds, S is milliseconds */
"K:mm:ss a" /* K is hour in am/pm(0-11) */
};
LocalTime now = LocalTime.now();
for(String timeFormat : timeFormats) {
System.out.printf("Time in pattern "%s" is %s %n", timeFormat,
DateTimeFormatter.ofPattern(timeFormat).format(now));
}
}
}
Time in pattern "h:mm" is 12:27
Time in pattern "hh 'o''clock'" is 12 o'clock
Time in pattern "H:mm a" is 12:27 PM
Time in pattern "hh:mm:ss:SS" is 12:27:10:41
Time in pattern "K:mm:ss a" is 0:27:10 PM

Flight Travel - Time Calculation - Example
DateTimeFormatter dateTimeFormatter =
DateTimeFormatter.ofPattern("dd MMM yyyy hh.mm a");
// Leaving on 1st Jan 2016, 6:00am from "Singapore"
ZonedDateTime departure = ZonedDateTime.of(
LocalDateTime.of(2016, Month.JANUARY, 1, 6, 0),
ZoneId.of("Asia/Singapore"));
System.out.println("Departure: " + dateTimeFormatter.format(departure));
// Arrival on the same day in 10 hours in "Auckland"
ZonedDateTime arrival =
departure.withZoneSameInstant(ZoneId.of("Paciﬁc/Auckland")).plusHours(10);
System.out.println("Arrival: " + dateTimeFormatter.format(arrival));
Departure: 01 Jan 2016 06.00 AM
Arrival: 01 Jan 2016 09.00 PM

Summary
❖ The Java 8 date and time API uses ISO 8601 as the default calendar format.
❖ The java.time.LocalDate class represents a date without time or time zones;
the java.time.LocalTime class represents time without dates and time
zones; the java.time.LocalDateTime class represents both date and time
without time zones.
❖ The java.time.Instant class represents a Unix timestamp.
❖ The java.time.Period is used to measure the amount of time in terms of
years, months, and days.
❖ The java.time.Duration class represents time in terms of hours, minutes,
seconds, and fraction of seconds.

Summary
❖ ZoneId identiﬁes a time zone; ZoneOffset represents time zone offset
from UTC/Greenwich.
❖ ZonedDateTime provides support for all three aspects: date, time,
and time zone.
❖ You have to account for daylight savings time (DST) when working
with different time zones.
❖ The java.time.format.DateTimeFormatter class provides support for
reading or printing date and time values in different formats.
❖ The DateTimeFormatter class provides predeﬁned constants (such as
ISO_DATE and ISO_TIME ) for formatting date and time values.

Summary
❖ You encode the format of the date or time using case-
sensitive letters to form a date or time pattern string
with the DateTimeFormatter class.
❖ The enumeration java.time.temporal.ChronoUnit
implements the java.time.temporal.TemporalUnit
interface.
❖ Both TemporalUnit and ChronoUnit deal with time unit
values such as seconds, minutes, and hours and date
values such as days, months, and years.

Quiz Question #1
Choose the correct option based on this code segment:
LocalDate babyDOB = LocalDate.of(2015, Month.FEBRUARY, 20);
LocalDate now = LocalDate.of(2016, Month.APRIL, 10);
System.out.println(Period.between(now, babyDOB).getYears()); //
PERIOD_CALC
A. The code segment results in a compiler error in the line marked with the
comment PERIOD_CALC
B. The code segment throws a DateTimeException
C. The code segment prints: 1
D. The code segment prints: -1

Quiz Question #1 - Answer
The code segment prints: - 1
Here are the arguments to the between() method in the Period class:
Period between(LocalDate startDateInclusive, LocalDate
endDateExclusive)
The ﬁrst argument is the start and the second argument is the end, and
hence the call Period.between(now, babyDOB) results in -1 (not +1).

Quiz Question #2
Which one of the following classes is best suited for storing
timestamp values of application events in a ﬁle?
A. java.time.ZoneId class
B. java.time.ZoneOffset class
C. java.time.Instant class
D. java.time.Duration class
E. java.time.Period class

C . Instant class
The Instant class stores the number of seconds elapsed since the start of
the Unix epoch (1970-01-01T00:00:00Z). The Instant class is suitable for
storing a log of application events in a ﬁle as timestamp values.
The ZoneId and ZoneOffset classes are related to time zones and hence
are unrelated to storing timestamp values. The Duration class is for
time-based values in terms of quantity of time (such as seconds, minutes,
and hours). The Period class is for date-based values such as years,
months, and days.

Quiz Question #3
Given this code segment:
ZoneId zoneId = ZoneId.of("Asia/Singapore");
ZonedDateTime zonedDateTime =
ZonedDateTime.of(LocalDateTime.now(), zoneId);
System.out.println(zonedDateTime.getOffset());
assume that the time-offset value for the Asia/Singapore time zone from UTC/Greenwich is
+08:00. Choose the correct option.
A. This code segment results in throwing DateTimeException
B. This code segment results in throwing UnsupportedTemporalTypeException
C. The code segment prints: Asia/Singapore
D. The code segment prints: +08:00
E. This code segment prints: +08:00 [Asia/Singapore]

D. The code segment prints: + 08:00
Given a ZonedDateTime object, the getOffset() method returns a
ZoneOffset object that corresponds to the offset of the time zone from
UTC/Greenwich. Given that the time-offset value for the Asia/
Singapore zone from UTC/Greenwich is +08:00, the toString() method of
ZoneOffset prints the string “+08:00” to the console.

Quiz Question #4
Choose the correct option based on this code segment:
DateTimeFormatter dateFormat = DateTimeFormatter.ISO_DATE; // DEF
LocalDate dateOfBirth = LocalDate.of(2015, Month.FEBRUARY, 31);
System.out.println(dateFormat.format(dateOfBirth)); // USE
A. The program gives a compiler error in the line marked with the comment DEF
B. The program gives a compiler error in the line marked with the comment USE
C. The code segment prints: 2015-02-31
D. The code segment prints: 2015-02-03
E. This code segment throws java.time.DateTimeException with the message "Invalid
date 'FEBRUARY 31'"

E. This code segment throws java.time.DateTimeException with the
message "Invalid date 'FEBRUARY 31'" .
The date value 31 passed in the call LocalDate.of(2015, 2, 3 1); is invalid
for the month February, and hence the of() method in the LocalDate class
throws DateTimeException.
One of the predeﬁned values in DateTimeFormatter is ISO_DATE.
Hence, it does not result in a compiler error for the statement marked
with the comment DEF. The statement marked with the comment USE
compiles without errors because it is the correct way to use the format()
method in the DateTimeFormatter class.

Quiz Question #5
Consider this code segment:
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("EEEE", Locale.US);
System.out.println(formatter.format(LocalDateTime.now()));
Which of the following outputs matches the string pattern "EEEE" given
in this code segment?
A. F
B. Friday
C. Sept
D. September

B. Friday
E is the day name in the week; the pattern "EEEE" prints the name of
the day in its full format. “Fri” is a short form that would be printed by
the pattern “E", but "EEEE" prints the day of the week in full form: for
example, “Friday”. Because the locale is Locale.US, the result is printed
in English. The output “Sept” or “September” is impossible because E
refers to the name in the week, not in a month.

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ganesh@codeops.tech @GSamarthyam
www.codeops.tech slideshare.net/sgganesh
+91 98801 64463 bit.ly/ganeshsg

Java 8 Date and Time API

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