Modern Programming in Java 8 - Lambdas, Streams and Date Time API

Modern Programming in Java 8
Lambdas, Streams and Date/Time API
GANESH & HARI
CODEOPS TECHNOLOGIES
ganesh@codeops.tech
hari@codeops.tech

Adapt: Learn functional
programming

Agenda
• Introduction & Overview
• Lambdas
• Functional interfaces
• Streams
• Parallel Streams
• Date & Time package
• Refactoring to Java 8

Java meets functional
programming (with lambdas)

Java is not your grandma’s
language anymore!

He he, but lambdas
are fun, not scary

List<String> strings = Arrays.asList("eeny", "meeny", "miny", "mo");
Consumer<String> printString = string -> System.out.println(string);
strings.forEach(printString);
Lambda
functions!

Lambdas is just a fancy
name for functions
without a name!

What are lambda functions?
❖ (Java 8) One way to think about lambdas is
“anonymous function” or “unnamed function” - they
are functions without a name and are not associated
with any class
❖ They don’t change external state

What is functional programming?
❖ Functional languages view programs as an entity—
called a function—that accepts inputs and produces
output
❖ Functions are connected together by their outputs to
other functions’ inputs
❖ Underlying approach: “Evaluate an expression. Then
use the results for something else.”

Perspective - for loops!
for(String string : strings) {
System.out.println(string);
}
External Iteration

strings.forEach(string -> System.out.println(string));
Internal Iteration

Internal Iteration
}
External Iteration

Internal Iteration
}
External Iteration
Procedural
thinking
Functional
thinking

You can use lambdas for
some amazing stuff

sedime
nt
pre-
carbon
ultra-
ﬁlter
post-
carbon
Filtered
water
E.g., you can compose lambda
functions as in pipes-and-ﬁlters

$ cat limerick.txt
There was a young lady of Niger
Who smiled as she rode on a tiger.
They returned from the ride
With the lady inside
And a smile on the face of the tiger.

$ cat limerick.txt | tr -cs "[:alpha:]" "n" | awk '{print length(), $0}' | sort | uniq
1 a
2 as
2 of
2 on
3 And
3 Who
3 she
3 the
3 was
4 They
4 With
4 face
4 from
4 lady
4 ride
4 rode
5 Niger
5 There
5 smile
5 tiger
5 young
6 inside
6 smiled
8 returned

List<String> lines
= Files.readAllLines(Paths.get("./limerick.txt"), Charset.defaultCharset());
Map<Integer, List<String>> wordGroups
= lines.stream()
.map(line -> line.replaceAll("W", "n").split("n"))
.flatMap(Arrays::stream)
.sorted()
.distinct()
.collect(Collectors.groupingBy(String::length));
wordGroups.forEach( (count, words) -> {
words.forEach(word -> System.out.printf("%d %s %n", count, word));
});
1 a
2 as
2 of
2 on
3 And
3 Who
3 she
3 the
3 was
4 They
4 With
4 face
4 from
4 lady
4 ride
4 rode
5 Niger
5 There
5 smile
5 tiger
5 young
6 inside
6 smiled
8 returned

Lambdas & streams help in
productive programming!

public static void main(String []file) throws Exception {
// process each file passed as argument
// try opening the file with FileReader
try (FileReader inputFile = new FileReader(file[0])) {
int ch = 0;
while( (ch = inputFile.read()) != -1) {
// ch is of type int - convert it back to char
System.out.print( (char)ch );
}
}
// try-with-resources will automatically release FileReader object
}
public static void main(String []file) throws Exception {
Files.lines(Paths.get(file[0])).forEach(System.out::println);
}
Existing APIs are enriched with
lambdas and streams support

Java 8 is the new Groovy ;-)
import java.io.*;
class Type {
public sta7c void main(String []files) {
// process each file passed as argument
for(String file : files) {
// try opening the file with FileReader
try (FileReader inputFile = new FileReader(file)) {
int ch = 0;
while( (ch = inputFile.read()) != -1) {
// ch is of type int - convert it back to char
System.out.print( (char)ch );
}
} catch (FileNotFoundExcep7on fnfe) {
System.err.prinR("Cannot open the given file %s ", file);
}
catch(IOExcep7on ioe) {
System.err.prinR("Error when processing file %s; skipping it", file);
}
// try-with-resources will automa7cally release FileReader object
}
}
}
args.each { println new File(it).getText() }

Agenda
• Introduction & Overview
• Lambdas
• Functional interfaces
• Streams
• Parallel streams
• Date & Time package
• Refactoring to Java 8

Java 8 lambdas - “Hello world!”
interface LambdaFunction {
void call();
}
class FirstLambda {
public static void main(String []args) {
LambdaFunction lambdaFunction = () -> System.out.println("Hello world");
lambdaFunction.call();
}
}

void call();
}
class FirstLambda {
}
}
Functional interface - provides
signature for lambda functions
Lambda function/expression
Call to the lambda
Prints “Hello world” on the console when executed

Parts of a lambda expression
() -> System.out.println("Hello world");
No parameters, i.e., ()
Arrow operator that separates
parameters and the body
The lambda body
Return type “void” inferred from the body

Method references
Method references - “syntactic sugar” for lambda
functions
They “route” function parameters
arg -> System.out.println(arg)
System.out::println

Method references
Consumer<String> printString = System.out::println;
Method
reference

Method references
Cannot use method references when lambda functions do
more than“routing” function parameters
strings.forEach(string -> System.out.println(string.toUpperCase()));
More processing here than just
“routing” parameters

Method references
public static void printUpperCaseString(String string) {
System.out.println(string.toUpperCase());
}
strings.forEach(MethodReference::printUpperCaseString);

“Effectively final” variables
import java.util.Arrays;
import java.util.List;
class PigLatin {
String suffix = "ay";
List<String> strings = Arrays.asList("one", "two", "three", "four");
strings.forEach(string -> System.out.println(string + suffix));
}
} Accessing “local variable” sufﬁx
here; hence it is considered
“effectively ﬁnal”

“Effectively final” variables
class PigLatin {
String suffix = "ay";
List<String> strings = Arrays.asList("one", "two", "three", “four");
suffix = "e"; // assign to suffix variable
}
}
PigLatinAssign.java:9: error: local variables referenced from a
lambda expression must be final or effectively final
^
1 error

Functional interfaces
@FunctionalInterface
void call();
}
Functional interface
Abstract method providing the signature of the
lambda function
Annotation to explicitly state that it is a functional
interface

void call();
}
class FirstLambda {
}
}
Functional interface - provides
signature for lambda functions
Lambda function/expression
Call to the lambda
Prints “Hello world” on the console when executed

Older Single Abstract Methods (SAMs)
// in java.lang package
interface Runnable { void run(); }
// in java.util package
interface Comparator<T> { boolean compare(T x, T y); }
// java.awt.event package:
interface ActionListener { void actionPerformed(ActionEvent e) }
// java.io package
interface FileFilter { boolean accept(File pathName); }

Functional interfaces: Single abstract methods
void call();
// Single Abstract Method (SAM)
}

Using built-in functional interfaces
// within Iterable interface
default void forEach(Consumer<? super T> action) {
Objects.requireNonNull(action);
for (T t : this) {
action.accept(t);
}
}
// in java.util.function package
public interface Consumer<T> {
void accept(T t);
// the default andThen method elided
}

Using built-in functional interfaces

Default methods in interfaces
public interface Iterator<E> {
boolean hasNext();
E next();
default void remove() {
throw new UnsupportedOperationException("remove");
}
default void forEachRemaining(Consumer<? super E> action) {
Objects.requireNonNull(action);
while (hasNext())
action.accept(next());
}
}

“Diamond” inheritance problem?

interface Interface1 {
default public void foo() { System.out.println("Interface1’s foo"); }
}
}
public class Diamond implements Interface1, Interface2 {
new Diamond().foo();
}
}
Error:(9, 8) java: class Diamond inherits unrelated defaults for foo()
from types Interface1 and Interface2

}
}
public class Diamond implements Interface1, Interface2 {
public void foo() { Interface1.super.foo(); }
}
}
Add this deﬁnition
to resolve the
ambiguity

class BaseClass {
public void foo() { System.out.println("BaseClass’s foo"); }
}
interface BaseInterface {
default public void foo() { System.out.println("BaseInterface’s foo”); }
}
public class Diamond extends BaseClass implements BaseInterface {
}
}
Compiles cleanly; Java 8
rules help deal with the
diamond problem

Built-in functional interfaces

Built-in functional interfaces are a
part of the java.util.function
package (in Java 8)

Built-in interfaces
Predicate<T> Checks a condition and returns a
boolean value as result
In filter() method in
java.util.stream.Stream
which is used to remove elements
in the stream that don’t match the
given condition (i.e., predicate) asConsumer<T> Operation that takes an argument but
returns nothing
In forEach() method in
collections and in
java.util.stream.Stream; this
method is used for traversing all
the elements in the collection orFunction<T,
R>
Functions that take an argument and
return a result
In map() method in
java.util.stream.Stream to
transform or operate on the passed
value and return a result.
Supplier<T> Operation that returns a value to the
caller (the returned value could be
same or different values)
In generate() method in
java.util.stream.Stream to
create a inﬁnite stream of
elements.

Predicate interface
Stream.of("hello", "world")
.filter(str -> str.startsWith("h"))
.forEach(System.out::println);
The ﬁlter() method takes a Predicate
as an argument (predicates are
functions that check a condition and
return a boolean value)

Predicate interface
A Predicate<T> “afﬁrms” something as true or
false: it takes an argument of type T, and returns a
boolean value. You can call test() method on a
Predicate object.
public interface Predicate<T> {
boolean test(T t);
// other methods elided
}

Predicate interface: Example
import java.util.function.Predicate;
public class PredicateTest {
Predicate<String> nullCheck = arg -> arg != null;
Predicate<String> emptyCheck = arg -> arg.length() > 0;
Predicate<String> nullAndEmptyCheck = nullCheck.and(emptyCheck);
String helloStr = "hello";
System.out.println(nullAndEmptyCheck.test(helloStr));
String nullStr = null;
System.out.println(nullAndEmptyCheck.test(nullStr));
}
}
Prints:
true
false

Predicate interface: Example
import java.util.ArrayList;
public class RemoveIfMethod {
List<String> greeting = new ArrayList<>();
greeting.add("hello");
greeting.add("world");
greeting.removeIf(str -> !str.startsWith("h"));
greeting.forEach(System.out::println);
}
}
Prints:
hello

Consumer interface
Stream.of("hello", "world")
// void forEach(Consumer<? super T> action);
Prints:
hello
world

Consumer interface
A Consumer<T> “consumes” something: it takes
an argument (of generic type T) and returns
nothing (void). You can call accept() method on a
Consumer object.
public interface Consumer<T> {
void accept(T t);
// the default andThen method elided
}

Consumer interface: Example
Consumer<String> printUpperCase =
str -> System.out.println(str.toUpperCase());
printUpperCase.accept("hello");
Prints:
HELLO

Consumer interface: Example
import java.util.stream.Stream;
import java.util.function.Consumer;
class ConsumerUse {
Stream<String> strings = Stream.of("hello", "world");
}
}
Prints:
hello
world

Function interface
public class FunctionUse {
Arrays.stream("4, -9, 16".split(", "))
.map(Integer::parseInt)
.map(i -> (i < 0) ? -i : i)
}
}
Prints:
4
9
16

Function interface
A Function<T, R> “operates” on something and
returns something: it takes one argument (of
generic type T) and returns an object (of generic
type R). You can call apply() method on a Function
object.
public interface Function<T, R> {
R apply(T t);
// other methods elided
}

Function interface: Example
Function<String, Integer> strLength = str -> str.length();
System.out.println(strLength.apply("supercalifragilisticexpialidocious"));
Prints:
34

Function interface: Example
import java.util.function.Function;
public class CombineFunctions {
Function<String, Integer> parseInt = Integer:: parseInt ;
Function<Integer, Integer> absInt = Math:: abs ;
Function<String, Integer> parseAndAbsInt = parseInt.andThen(absInt)
Arrays.stream("4, -9, 16".split(", "))
.map(parseAndAbsInt)
.forEach(System. out ::println);
}
}
Prints:
4
9
16

Supplier interface
import java.util.stream.Stream;
import java.util.Random;
class GenerateBooleans {
Random random = new Random();
Stream.generate(random::nextBoolean)
.limit(2)
}
}
Prints two boolean
values “true” and “false”
in random order

Supplier interface
A Supplier<T> “supplies” takes nothing but
returns something: it has no arguments and
returns an object (of generic type T). You can call
get() method on a Supplier object
public interface Supplier<T> {
T get();
// no other methods in this interface
}

Supplier interface: Example
Supplier<String> currentDateTime = () -> LocalDateTime.now().toString();
System.out.println(currentDateTime.get());
Prints current time:
2015-10-16T12:40:55.164

Summary of built-in interfaces in
java.util.function interface
❖ There are only four core functional interfaces in this
package: Predicate, Consumer, Function, and Supplier.
❖ The rest of the interfaces are primitive versions, binary
versions, and derived interfaces such as
UnaryOperator interface.
❖ These interfaces differ mainly on the signature of the
abstract methods they declare.
❖ You need to choose the suitable functional interface
based on the context and your need.

Java 8 streams (and parallel streams):
Excellent example of applying functional
programming in practice

Arrays.stream(Object.class.getMethods())
.map(method -> method.getName())
.distinct()
wait
equals
toString
hashCode
getClass
notify
notifyAll

Method[] objectMethods = Object.class.getMethods();
Stream<Method> objectMethodStream = Arrays.stream(objectMethods);
Stream<String> objectMethodNames
= objectMethodStream.map(method -> method.getName());
Stream<String> uniqueObjectMethodNames = objectMethodNames.distinct();
uniqueObjectMethodNames.forEach(System.out::println);
Arrays.stream(Object.class.getMethods())
.map(method -> method.getName())
.distinct()
Breaking up into
separate (looong)

stream pipeline
Stream
source
Intermediate
opera1ons
Terminal
opera1on
stream
stream
Examples:
IntStream.range(),
Arrays.stream()
Examples:
map(), ﬁlter(),
dis1nct(), sorted()
Examples:
sum(), collect(),
forEach(), reduce()

DoubleStream.
of(1.0, 4.0, 9.0)
map(Math::sqrt)
.peek(System.out::
println)
Stream
Source (with
elements 1.0,
4.0, and 9.0)
Intermediate
Opera=on 1
(maps to
element values
1.0, 2.0, and 3.0)
Intermediate
Opera=on 2
(prints 1.0, 2.0,
and 3.0)
.sum();
Terminal
Opera=on
(returns the
sum 6.0)
DoubleStream.of(1.0, 4.0, 9.0)
.map(Math::sqrt)
.peek(System.out::println)
.sum();

IntStream.range(1, 6)
You can use range or iterate
factory methods in the
IntStream interface
IntStream.iterate(1, i -> i + 1).limit(5)

1 2 3 4 5
1 4 9 16 25
map(i -> i * i)
IntStream.range(1, 5).map(i -> i * i).forEach(System.out::println);
Using streams instead of imperative for i = 1 to 5, print i * i

Stream.of (1, 2, 3, 4, 5)
.map(i -> i * i)
.peek(i -> System.out.printf("%d ", i))
.count();
prints: 1 4 9 16 25

stream can be
inﬁnite
IntStream.iterate(0, i -> i + 2).forEach(System.out::println);
This code creates inﬁnite stream of even numbers!

IntStream
.iterate(0, i -> i + 2)
.limit(5)
Using the “limit” function to limit the stream to 5 integers

IntStream chars = "bookkeep".chars();
System.out.println(chars.count());
chars.distinct().sorted().forEach(ch -> System.out.printf("%c ", ch));
Cannot “reuse” a stream; this code
throws IllegalStateException

Files.lines(Paths.get("FileRead.java")).forEach(System.out::println);
This code prints the contents of
the ﬁle “FileRead.java” in the
current directory

Pattern.compile(" ").splitAsStream("java 8 streams").forEach(System.out::println);
This code splits the input string “java 8
streams” based on whitespace and hence
prints the strings “java”, “8”, and
“streams” on the console

new Random().ints().limit(5).forEach(System.out::println);
Generates 5 random integers and prints
them on the console

"hello".chars().sorted().forEach(ch -> System.out.printf("%c ", ch));
Extracts characters in the string “hello”,
sorts the chars and prints the chars

I really really hate
concurrency problems

Sometimes, dreams do come
true even at 86 :-)

So, keep dreaming till you
become 86!

long numOfPrimes = LongStream.rangeClosed(2, 100_000)
.ﬁlter(PrimeNumbers::isPrime)
.count();
System.out.println(numOfPrimes);
Prints 9592
2.510 seconds

Parallel code
Serial code
Let’s ﬂip the switch by
calling parallel() function

long numOfPrimes = LongStream.rangeClosed(2, 100_000)
.parallel()
.ﬁlter(PrimeNumbers::isPrime)
.count();
System.out.println(numOfPrimes);
Prints 9592
1.235 seconds

Wow! That’s an awesome ﬂip
switch!

Internally, parallel streams make
use of fork-join framework

class StringConcatenator {
public static String result = "";
public static void concatStr(String str) {
result = result + " " + str;
}
}
class StringSplitAndConcatenate {
String words[] = "the quick brown fox jumps over the lazy dog".split(" ");
Arrays.stream(words).forEach(StringConcatenator::concatStr);
System.out.println(StringConcatenator.result);
}
}
Gives wrong results with
with parallel() call

–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 {
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

Examples of refactorings (to Java 8)
❖ Convert anonymous inner
classes to lambda expressions
(when they are functional
interfaces)
❖ Convert for/foreach loops to
streams (i.e., external iteration
to internal iteration)

Refactoring loops to streams
❖ Replace if conditions with ‘ﬁlter’ and calls to methods
that return a boolean value (Predicate)
❖ Replace accumulation operations with reduce (or its
special forms like sum, count, etc).

Source: LAMBDAFICATOR: From Imperative to Functional Programming through Automated Refactoring. Lyle Franklin; Alex
Gyori; Jan Lahoda; Danny Dig. 35th International Conference on Software Engineering (ICSE), 2013.

Tool support for refactoring
❖ Most Java IDEs provide
suggestions to automatically
refactor to lambdas and
streams
❖ IDEs that support Java 8
refactoring include: Eclipse,
IntelliJ Idea and NetBeans

Refactoring suggestions in Netbeans
Image source: http://refactoring.info/tools/LambdaFicator/

Java 8 Migration Aids in IntelliJ IDEA
Image source: https://www.jetbrains.com/help/img/idea/ij_java_8_inspection_results_migration_runnable.png

Java 8 Refactorings in IntelliJ IDEA
Image source: https://www.jetbrains.com/help/img/idea/ij_java_8_replace_method_reference_with_lambda.png
https://www.jetbrains.com/help/img/idea/ij_java_8_can_be_replaced_with_method_ref.png

Suggested Reading
❖ Refactoring with Loops and Collection Pipelines
(Martin Fowler, 2015)
❖ Pragmatic Functional Refactoring with Java 8 (Raoul-
Gabriel Urma & Richard Warburton, 2015)
❖ Migrating to Java 8 (IntelliJ IDEA, 2016)

Meetups
hYp://www.meetup.com/JavaScript-Meetup-Bangalore/
hYp://www.meetup.com/Container-Developers-Meetup-Bangalore/
hYp://www.meetup.com/So^ware-Cra^smanship-Bangalore-Meetup/
hYp://www.meetup.com/Core-Java-Meetup-Bangalore/
hYp://www.meetup.com/Technical-Writers-Meetup-Bangalore/
hYp://www.meetup.com/CloudOps-Meetup-Bangalore/
hYp://www.meetup.com/Bangalore-SDN-IoT-NetworkVirtualiza7on-Enthusiasts/
hYp://www.meetup.com/So^wareArchitectsBangalore/

Image credits
❖ http://mayhemandmuse.com/wp-content/uploads/2013/04/This-optical-illusion-drawing-by-WE-
Hill-shows-both-his-wife-and-his-mother-in-law.jpg
❖ http://www.webtrafficroi.com/wp-content/uploads/2012/10/mahatma-gandhi-apple-think-
different.jpg
❖ http://rexx-language-association-forum.44760.x6.nabble.com/file/n2236/Ruby-lambda-function.jpg
❖ http://www.ibm.com/developerworks/library/j-jn16/figure1.png
❖ http://www.ibm.com/developerworks/library/j-jn16/figure2.png
❖ http://img.viralpatel.net/2014/01/java-lambda-expression.png
❖ http://www.codercaste.com/wp-content/uploads/2011/01/animals.png
❖ http://blog.takipi.com/wp-content/uploads/2014/03/blog_lambada_2.png
❖ http://quotespictures.com/wp-content/uploads/2014/01/it-is-not-the-strongest-of-the-species-that-
survive-nor-the-most-intelligent-but-the-one-most-responsive-to-change-charles-darwin.jpg
❖ http://7-themes.com/data_images/out/27/6859733-surfing-wallpaper.jpg

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

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