Functional Thinking for Java Developers (presented in Javafest Bengaluru)

Lets Get Funky with Functional Progamming
Ganesh Samarthyam
ganesh@konfhub.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?
❖ 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.”

Productive programming with lambdas
import java.io.*;
class Type {
public static 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 (FileNotFoundException fnfe) {
System.err.printf("Cannot open the given file %s ", file);
}
catch(IOException ioe) {
System.err.printf("Error when processing file %s; skipping it", file);
}
// try-with-resources will automatically 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
lambdas and streams after this
session
❖ Try out the programs in your
machine

Java 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 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

`
() -> 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.Arrays;
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

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 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);

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

Stream pipeline illustration
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

“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) as
argument.
Consumer<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 or
stream.
Function<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
In generate() method in
java.util.stream.Stream to

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

Final (longer) example
import java.util.stream.*;
import java.nio.file.*;
import java.util.*;
import java.nio.charset.Charset;
import java.io.IOException;
class ReadFile {
public static void main(String []args) throws IOException {
List<String> lines = Files.readAllLines(Paths.get("./Ulysses.txt"), Charset.def
Map<Integer, List<String>> wordGroups
= lines.parallelStream()
.map(line -> line.replaceAll("W", " ").split(" "))
.flatMap(Arrays::stream)
.filter(str -> str.length() > 7)
.distinct()
.sorted()
.collect(Collectors.groupingBy(String::length));
wordGroups.forEach( (count, words) -> {
System.out.printf("word(s) of length %d %n", count);
words.forEach(System.out::println); });
}
}
Lists words organised by
their length in the novel
“Ulysses.txt”

Ganesh Samarthyam
ganesh@konfhub.com

Functional Thinking for Java Developers (presented in Javafest Bengaluru)

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