Functional Thinking - Programming with Lambdas in Java 8

Functional Thinking: Introduction to Lambdas
Ganesh Samarthyam
ganesh.samarthyam@gmail.com
www.designsmells.com

Adapt: Learn functional
programming

Programming paradigms
Langauge
Paradigm
Declarative
Languages
Imperative
Languages
Logic
Languages
Functional
Languages
Procedural
Languages
Object-Oriented
Languages
e.g. Prolog e.g. Haskell e.g. C e.g. Java

Perspective - for loops!
List<String> strings = Arrays.asList("eeny", "meeny", "miny", "mo");
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

Lambdas
Consumer<String> printString = string -> System.out.println(string);
strings.forEach(printString);
Lambda
functions!

Lambdas
Capture in a
variable
Execute later

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.”

Functional languages
Haskell
Common Lisp
Scheme
Clojure
F#
OCaml
Erlang Hope
Scala
R

Languages that support functional programming
C# (3.0)
Python
JavaScript
D
Fortran 95
Visual Basic
PHP Java (8)
C++11

“Success is very largely a matter of adjusting one's self to the
ever-varying and changing environments of life”
- Napoleon Hill

Popular Languages
http://www.tiobe.com/index.php/content/paperinfo/tpci/index.html

Productive programming with lambdas
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() }

Workshop overview
❖ Assumes that you already
know Java
❖ You’ll know how to use Java 8
lambdas and streams after this
session
❖ Try out the programs in your
machine

Code examples & images from my upcoming book:
Oracle Certified Professional Java SE 8 Programmer Exam
1Z0-809 by S G Ganesh, Hari Kiran Kumar and Tushar Sharma,
Apress, December 2015

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();
}
}

Java 8 lambdas - “Hello world!”
void call();
}
class FirstLambda {
LambdaFunction lambdaFunction = () -> System.out.println("Hello world");
lambdaFunction.call();
}
}
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

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

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
@FunctionalInterface
public interface Consumer<T> {
void accept(T t);
// the default andThen method elided
}

Using built-in functional interfaces

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
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 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
import java.util.List;
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

One-liner #1
Files.lines(Paths.get("FileRead.java")).forEach(System.out::println);
This code prints the contents of
the ﬁle “FileRead.java” in the
current directory

One-liner #2
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

One-liner #3
new Random().ints().limit(5).forEach(System.out::println);
Generates 5 random integers and prints
them on the console

One-liner #4
"hello".chars().sorted().forEach(ch -> System.out.printf("%c ", ch));
Extracts characters in the string “hello”,
sorts the chars and prints the chars

First stream example
"hello".chars().sorted().forEach(ch -> System.out.printf("%c ", ch));
The chars() method in String
results in a Stream; sorted() sorts
the entries in the stream
// prints e h l l o

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()

Stream pipeline example
Arrays.stream(Object.class.getMethods()) // source
.map(method -> method.getName()) // intermediate op
.distinct() // intermediate op
.forEach(System.out::println); // terminal operation

Stream pipeline example
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);

Stream pipeline illustration
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();

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

Stream sources
Arrays.stream(new int[] {1, 2, 3, 4, 5})
Arrays.stream(new Integer[] {1, 2, 3, 4, 5})
You can use the stream() method in
java.util.Arrays class to create a
stream from a given array

Stream sources
Stream.of(1, 2, 3, 4, 5)
Stream.of(new Integer[]{1, 2, 3, 4, 5})
We can also create streams using factories
and builders (e..g, of() and build() methods
in the Stream interface)
Stream.builder().add(1).add(2).add(3).add(4).add(5).build()

Stream sources
• The lines() method in java.nio.file.Files class
• The splitAsStream() method in java.util.regex.Pattern class
• The ints() method in java.util.Random class
• The chars() method in java.lang.String class
There are numerous classes/interfaces in
the Java library that return a stream

Intermediate operations
Stream<T> filter(Predicate<? super T>
check) Removes the elements for which the check predicate returns false.
<R> Stream<R> map(Function<? super T,?
extends R> transform)
Applies the transform() function for each of the elements in the
stream.
Stream<T> distinct() Removes duplicate elements in the stream; it uses the equals()
method to determine if an element is repeated in the stream.
Stream<T> sorted()
Stream<T> sorted(Comparator<? super T>
compare)
Sorts the elements in its natural order. The overloaded version takes
a Comparator – you can pass a lambda function for that.
Stream<T> peek(Consumer<? super T>
consume)
Returns the same elements in the stream, but also executes the
passed consume lambda expression on the elements.
Stream<T> limit(long size)
Removes the elements if there are more elements than the given
size in the stream.

Terminal operations
void forEach(Consumer<?
super T> action)
Calls the action for every element in the
stream.
Object[] toArray()
Returns an Object array that has the elements
in the stream.
Optional<T> min(Comparator<?
super T> compare)
Returns the minimum value in the stream
(compares the objects using the given compare
function).
Optional<T> max(Comparator<?
super T> compare)
Returns the maximum value in the stream
(compares the objects using the given compare
function).
long count() Returns the number of elements in the stream.

Using “range” instead of “for” loop
IntStream.range(1, 10).map(i -> i * i).forEach(System.out::println);
Using streams instead of imperative for i = 1 to 1, print i * i
Prints:
1
4
9
16
25
36
49
64
81

“Mapping” elements in a stream
1 2 3 4 5
1 4 9 16 25
map(i -> i * i)

“Generating” even numbers
IntStream.iterate(0, i -> i + 2).forEach(System.out::println);
The problem is it creates inﬁnite number of even numbers!

“Generating” limited even numbers
IntStream
.iterate(0, i -> i + 2)
.limit(5)
.forEach(System.out::println);
Using the “limit” function to limit the stream to 5 integers

“Generating” limited even numbers
IntStream
.iterate(0, i -> i + 1)
.filter(i -> (i % 2) == 0)
.limit(5)
You can also use the “ﬁlter” method

“Reduction” using “sum” function
System.out.println(IntStream.rangeClosed(0, 10).sum());
Sum of integers from 1 to 10 using “implicit reduction”

“Reduction” using “sum” function
System.out.println(
IntStream
.rangeClosed(1, 5)
.reduce((x, y) -> (x * y))
.getAsInt());
Factorial of 5 using “explicit reduce”

Using “map” function
strings.stream().map(value -> value.toUpperCase()).forEach(System.out::println);
public static void printUpperCaseString(String string) {
System.out.println(string.toUpperCase());
}
strings.forEach(MethodReference::printUpperCaseString);

Using “collect” function
String boxedString =
Arrays
.asList("eeny", "meeny", "miny", "mo")
.stream()
.collect(Collectors.joining(“ ,", "[", "]"));
System.out.println(boxedString);
Prints: [eeny, meeny, miny, mo]

Using Files.list()
Files.list(Paths.get("."))
.map(path -> path.toAbsolutePath())

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.

Important stream interfaces
<<interface>>
java.u0l.stream.BaseStream
<<interface>>
java.u0l.stream.IntStream
<<interface>>
java.u0l.stream.LongStream
<<interface>>
java.u0l.stream.DoubleStream
<<interface>>
java.u0l.stream.Stream<T>

map vs. flatMap methods
1 2 3 4 5
1 4 9 16 25
map(i -> i * i)
1 3 5
1 3 5 2 4
ﬂatMap(ints -> ints.stream())
2 4

sorted()
1 2 3 4 5
1 4 9 16 25
map(i -> i * i)

String limerick = "There was a young lady named Bright " +
"who traveled much faster than light " +
"She set out one day " +
"in a relative way " +
"and came back the previous night ";
IntSummaryStatistics wordStatistics =
Pattern.compile(" ")
.splitAsStream(limerick)
.mapToInt(word -> word.length())
.summaryStatistics();
System.out.printf(" Number of words = %d n Sum of the length of the words = %d n" +
" Minimum word size = %d n Maximum word size %d n " +
" Average word size = %f n", wordStatistics.getCount(),
wordStatistics.getSum(), wordStatistics.getMin(),
wordStatistics.getMax(), wordStatistics.getAverage());
Data calculation methods in stream

Fork-join framework
forkJoinAlgorithm() {
fork (split) the tasks;
join the tasks;
compose the results;
}
doRecursiveTask(input) {
if (the task is small enough to be handled by a thread) {
compute the small task;
if there is a result to return, do so
}
else {
divide (i.e., fork) the task into two parts
call compute() on first task, join() on second task, return combined results
}
}

Stream - for primes
import java.util.stream.LongStream;
class PrimeNumbers {
private static boolean isPrime(long val) {
for(long i = 2; i <= val/2; i++) {
if((val % i) == 0) {
return false;
}
}
return true;
}
long numOfPrimes = LongStream.rangeClosed(2, 100_000)
.filter(PrimeNumbers::isPrime)
.count();
System.out.println(numOfPrimes);
}
}

Stream - for primes
import java.util.stream.LongStream;
class PrimeNumbers {
private static boolean isPrime(long val) {
for(long i = 2; i <= val/2; i++) {
if((val % i) == 0) {
return false;
}
}
return true;
}
long numOfPrimes = LongStream.rangeClosed(2, 100_000)
.parallel()
.filter(PrimeNumbers::isPrime)
.count();
System.out.println(numOfPrimes);
}
}

Be careful with parallel streams
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);
}
}

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

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