Functional Programming in Java 8 - Lambdas and Streams

Functional Programming

Lambdas & Streams
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!

Greek characters

are scary!

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 clas
s

❖ 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’ input
s

❖ 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
;

for(String string : strings)
{

System.out.println(string);

}

External Iteration

;

;

Internal Iteration
;

for(String string : strings)
{

System.out.println(string);

}

External Iteration
Procedural
thinking
Functional
thinking

You can use lambdas for
some amazing stuff

sedime
nt
pre-
carbon
ultra-
fi
lter
post-
carbon
Filtered

water
E.g., you can compose lambda
functions as in pipes-and-
fi
lters

$ 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

}

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 sta
ti
c void main(String []
fi
les) {

// process each
fi
le passed as argument

for(String
fi
le :
fi
les) {

// try opening the
fi
le with FileReader

try (FileReader inputFile = new FileReader(
fi
le)) {

int ch = 0;




}

} catch (FileNotFoundExcep
ti
on fnfe) {

System.err.prin
tf
("Cannot open the given
fi
le %s ",
fi
le);

}

catch(IOExcep
ti
on ioe) {

System.err.prin
tf
("Error when processing
fi
le %s; skipping it",
fi
le);

}

// try-with-resources will automa
ti
cally release FileReader object

}

}

}

args.each { println new File(it).getText() }

Java library “enhanced” with streams

Agenda

• Lambdas


• Streams

• Parallel streams



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
;


strings.forEach(printString);

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

;

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

;

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 suf
fi
x = "ay";

List<String> strings = Arrays.asList("one", "two", "three", "four")
;

strings.forEach(string -> System.out.println(string + suf
fi
x));

}

}

Accessing “local variable” suf
fi
x
here; hence it is considered
“effectively
fi
nal”

“Effectively final” variables
;

import java.util.List;

class PigLatin
{

{

String suf
fi
x = "ay";

List<String> strings = Arrays.asList("one", "two", "three", “four")
;

suf
fi
x = "e";

// assign to suf
fi
x variable

strings.forEach(string -> System.out.println(string + suf
fi
x));

}

}

PigLatinAssign.java:9: error: local variables referenced from a
lambda expression must be final or effectively final

strings.forEach(string -> System.out.println(string + suffix));

^

1 error

Functional interfaces
@FunctionalInterfac
e

{

void call();

}

Functional interface
Abstract method providing the signature of the
lambda function
Annotation to explicitly state that it is a functional
interface

@FunctionalInterfac
e

{

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
@FunctionalInterfac
e
{

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

@FunctionalInterfac
e

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 Interface
2

{

}

}

{

}

}

public class Diamond implements Interface1, Interface2 {

public void foo() { Interface1.super.foo();
}

{

new Diamond().foo()
;

}

}
Add this de
fi
nition
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
{

{

new Diamond().foo()
;

}

}
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
fi
lter() 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 in
fi
nite stream of
elements.

Predicate interface
Stream.of("hello", "world"
)

.
fi
lter(str -> str.startsWith("h")
)

.forEach(System.out::println);
The
fi
lter() method takes a Predicate
as an argument (predicates are
functions that check a condition and
return a boolean value)

Predicate interface
A Predicate<T> “af
fi
rms” 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.
@FunctionalInterfac
e
public interface Predicate<T>
{

boolean test(T t)
;

// other methods elide
d

}

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:

tru
e

false

Predicate interface: Example
import java.util.List
;

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"
)

.forEach(System.out::println)
;

// void forEach(Consumer<? super T> action);
Prints:

hell
o

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.
@FunctionalInterfac
e
public interface Consumer<T>
{

void accept(T t)
;

// the default andThen method elide
d

}

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");

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

;

}

}
Prints:

hell
o

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.
@FunctionalInterfac
e
public interface Function<T, R>
{

R apply(T t)
;

// other methods elide
d

}

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
@FunctionalInterfac
e
public interface Supplier<T>
{

T get()
;

// no other methods in this interfac
e

}

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.

Agenda

• Lambdas


• Stream
s

• Parallel streams



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
in
fi
nite
IntStream.iterate(0, i -> i + 2).forEach(System.out::println)
;

This code creates in
fi
nite stream of even numbers!

IntStrea
m

.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
fi
le “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)

.
fi
lter(PrimeNumbers::isPrime)

.count();

System.out.println(numOfPrimes);
Prints 9592
2.510 seconds

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

long numOfPrimes = LongStream.rangeClosed(2, 100_000)

.parallel()

.
fi
lter(PrimeNumbers::isPrime)

.count();

System.out.println(numOfPrimes);
Prints 9592
1.235 seconds

Wow! That’s an awesomefl ip
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
fi
ne
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 pre
fi
xes plus
and minus to add or subtract date or time values)

java.time Sub-packages
❖ java.time.temporal —Accesses date/time
fi
elds and
unit
s

❖ java.time.format —Formats the input and output of
date/time object
s

❖ java.time.zone —Handles time zone
s
❖ 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: 12:29:13.62
4

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:
2015-11-04T13:19:10.49
7

The date component is: 2015-11-0
4
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: 2015-11-02T03:16:04.502
Z

Number of seconds elapsed: 144643416
4

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("Paci
fi
c/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
Paci
fi
c/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("Paci
fi
c/Auckland");

Duration aucklandDST = aucklandZone.getRules().getDaylightSavings(Instant.now());

System.out.printf("Auckland zone DST is: %d hours", aucklandDST.toHours());
Kolkata zone DST is: 0 hour
s

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 fo
r

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 05-11-201
5

Pattern "d '('E')' MMM, YYYY" is 5 (Thu) Nov, 201
5

Pattern "w'th week of' YYYY" is 45th week of 201
5

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 12:2
7

Time in pattern "hh 'o''clock'" is 12 o'cloc
k

Time in pattern "H:mm a" is 12:27 P
M

Time in pattern "hh:mm:ss:SS" is 12:27:10:4
1

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("Paci
fi
c/Auckland")).plusHours(10);

System.out.println("Arrival: " + dateTimeFormatter.format(arrival));
Departure: 01 Jan 2016 06.00 A
M

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
)

❖ Replace nested if conditions and
loops and instead use enhanced
stream methods in collections

Refactoring loops to streams
❖ Replace if conditions with stream functions that return
boolean value (
fi
lter,
fi
ndFirst, anyMatch, …)

❖ Replace for, while, and do-while loops with stream
operations (especially forEach)

❖ Replace accumulation operations with collect, or reduce
(or its special forms such as sum, count, etc)


// process each file passed as argument

// try opening the file with FileReader

try (FileReader inputFile = new FileReader(file[0])) {

int ch = 0;




}

}

// try-with-resources will automatically release FileReader object

}

Files.lines(Paths.get(file[0])).forEach(System.out::println);

}

Exploit enriched Java APIs with
lambdas and streams support
External iteration (for loop) to internal
iteration (forEach in sterams)

// Imperative code in Java

String integers = "4, -9, 16";

String []intStrings = integers.split(", ");

for(String anInteger: intStrings) {

int intValue = Integer.parseInt(anInteger);

int absIntValue = (intValue < 0) ? -intValue : intValue;

System.out.println(absIntValue);

}
// Equivalent functional code (using streams library) in Java

Arrays.stream("4, -9, 16".split(", "))

.map(Integer::parseInt)

.map(intValue -> (intValue < 0) ? -intValue: intValue)


Function calls and
inline code
Composing functions
(using lambdas)

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
h
tt
p://www.meetup.com/CloudOps-Meetup-Bangalore/

(10th June, Citrix)

h
tt
ps://www.meetup.com/Bot-builder-Bangalore/

(17th June, Microso
ft
)

h
tt
p://www.meetup.com/Mobile-App-Developers-Bangalore-Meetup/

(24th June, Sedin)

h
tt
p://www.meetup.com/Container-Developers-Meetup-Bangalore/

(8th July, Citrix)

h
tt
p://www.meetup.com/JavaScript-Meetup-Bangalore/

(15th July, IG Group)

h
tt
p://www.meetup.com/Core-Java-Meetup-Bangalore/

(22nd July, PayPal)

h
tt
ps://www.meetup.com/Serverless-Bangalore/

(??)

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.webtraf
fi
croi.com/wp-content/uploads/2012/10/mahatma-gandhi-apple-think-
different.jpg

❖ http://rexx-language-association-forum.44760.x6.nabble.com/
fi
le/n2236/Ruby-lambda-function.jpg

❖ http://www.ibm.com/developerworks/library/j-jn16/
fi
gure1.png

❖ http://www.ibm.com/developerworks/library/j-jn16/
fi
gure2.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-sur
fi
ng-wallpaper.jpg

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

Functional Programming in Java 8 - Lambdas and Streams

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