Functional programming in Java

Functional Programming
Haim Michael
May 14th
, 2019
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© 2008 Haim Michael 20151026
Introduction

© 2008 Haim Michael 20151026
Introduction
 The main programming languages paradigms include
the following.
Imperative Paradigm
The program includes statements executed one by one that change the
state.
Structured Paradigm
Based on Imperative. The code has a more logical structure. Avoiding the
goto statement.

© 2008 Haim Michael 20151026
Introduction
Procedural Paradigm
Based on Structured. The code is split into procedures been called in
order to have a shorted code to maintain.
Object Oriented Paradigm
During the execution of our code objects are created in order to represent
the things our code deals with. Objects are connected with each other.
Methods can be invoked on these objects.

© 2008 Haim Michael 20151026
Introduction
Event Driven Paradigm
The control flow is determined mainly by the events (e.g. mouse clicks,
loading of data ends).
Declarative Paradigm
The code defines what we want to get without getting into the logic of it nor
the details.

© 2008 Haim Michael 20151026
 Functional programming is a programming paradigm
that emphasizes the use of expressions and their
evaluation and especially through the definition of
functions that are treated as expressions. In addition, it
avoids the complexity involved with state changes as
the one when using objects and variables.

© 2008 Haim Michael 20151026
 The use of functions as expressions enable us getting
more expressive code. In many cases we will exploit
the power of recursion in order to get expressive
succinct (expressed in few words) code.

© 2008 Haim Michael 20151026
Pure Functions

© 2008 Haim Michael 20151026
Pure Functions
 When we define a function that always returns the same
value for the very same arguments, and it doesn't depend
on any hidden information or state and its evaluation of the
result does not cause any observable side effects nor
output then it is a pure function.
 Pure functions are usually simpler and much easier to test
and are very popular in functional programming.

© 2008 Haim Michael 20151026
Pure Functions
 In order to write a pure function we should make sure that we
write local only code.
 Writing a lambda expression as a pure function is the common
approach.

© 2008 Haim Michael 20151026
Lambda Expressions

© Haim Michael 2017. All Rights Reserved.
Introduction
 When dealing with an interface that includes one
method only and the sole purpose of that interface is to
allow us passing over functionality to another part of
our program we can use the lambda expression as an
alternative for the anonymous inner class.

Syntax
 The lambda expression includes a comma separated
list of formal parameters enclosed within parentheses
(the data type of the parameters can be usually
omitted). If there is only one parameter then we can
omit the parentheses.
ob -> ob.age<40

Syntax
 The body includes a single expression or a statement
block. We can include the return statement.

Simple Demo
package com.lifemichael.samples;
public class SimpleLambdaExpressionDemo {
interface MathOperation {
int execute(int a, int b);
}
public int calculate(int a, int b, MathOperation op) {
return op.execute(a, b);
}

Simple Demo
public static void main(String... args) {
SimpleLambdaExpressionDemo myApp =
new SimpleLambdaExpressionDemo();
MathOperation addition = (a, b) -> a + b;
MathOperation subtraction = (a, b) -> a - b;
int num = addition.execute(40, 2);
System.out.println("40 + 2 = " + num);
System.out.println("20 – 10 = " +
myApp.calculate(20, 10, subtraction));
}
}

Simple Demo
The Output

Methods References
 There are four types of methods references:
reference to static method, reference to an instance
method of a particular object, reference to an
instance method of an arbitrary object of a specific
type and reference to constructor.

Reference to Static Method
 We can refer a static method by specifying the name of
the class following with "::" and the name of the static
method.

}

MathOperation op = Utils::calcu;
int num = op.execute(40, 2);
System.out.println("40 * 2 = " + num);
}
}
class Utils
{
public static int calcu(int a,int b)
{
return a*b;
}
}

The Output

Reference to Instance Method
 We can refer a specific instance method by specifying
the reference for the object following with "::" and the
name of the instance method.

}

public static void main(String... args)
{
Utils utils = new Utils();
MathOperation op = utils::calcu;
int num = op.execute(40, 2);
System.out.println("40 * 2 = " + num);
}
}
class Utils
{
public int calcu(int a,int b)
{
System.out.println("calcu is called");
return a*b;
}
}

The Output

of an Arbitrary Object
 We can refer a specific instance method without been
specific about the object on which it should be invoked.
 Instead of specifying the reference for a specific object we
should specify the name of the class followed by :: and the
name of the function.

Student[] students = {
new Student(123123,98,"dave"),
new Student(234233,88,"ron"),
new Student(452343,82,"ram"),
new Student(734344,64,"lara")
};
Arrays.sort(students,Student::compareTo);
for(Student std : students) {
System.out.println(std);
}
}
}

class Student implements Comparable<Student>
{
private double average;
private int id;
private String name;
Student(int id, double avg, String name)
{
this.average = avg;
this.id = id;
this.name = name;
}

@Override
public String toString() {
return id+" "+average+" "+name;
}
@Override
public int compareTo(Student o) {
if(this.average>o.average) {
return 1;
}
else if(this.average<o.average) {
return -1;
}
else return 0;
}
}

The Output

Reference to Constructor
 We can refer the default constructor in a class by
writing the name of the class following with ::new.

public class ReferenceToConstructor
{
Student []students = new Student[8];
populate(students, Student::new);
for(int i=0; i<students.length; i++) {
System.out.println(students[i]);
}
}
public static void populate(
Student[] vec,
StudentsGenerator ob) {
for(int i=0; i<vec.length; i++) {
vec[i] = ob.create();
}
}
}

interface StudentsGenerator {
public Student create();
}
class Student implements Comparable<Student>
{
private static int counter = 1;
private static String[] names ={"david","moshe","anat","jake"};
private double average;
private int id;
private String name;
Student() {
id = counter++;
name = names[(int)(names.length*Math.random())];
average = (int)(100*Math.random());
}

Student(int id, double avg, String name)
{
this.average = avg;
this.id = id;
this.name = name;
}
@Override
public String toString()
{
return id+" "+average+" "+name;
}

@Override
public int compareTo(Student o) {
if(this.average>o.average) {
return 1;
}
else if(this.average<o.average) {
return -1;
}
else return 0;
}
}

The Output

© 2008 Haim Michael 20151026
Higher Order Functions

© 2008 Haim Michael 20151026
Higher Order Functions
 When the function we define receives another function (or
functions) as an argument(s) or when its returned value is
another function it will called an higher order function.

© 2008 Haim Michael 20151026
Streams

Introduction
 Java 8 introduces the possibility to represent a
sequence of objects as a stream.
 Using streams we can process the sequence of
objects in a declarative.

Stream of Collection
 The stream() method returns a sequential
stream composed of elements coming from a
collection.

import java.util.function.*;
import java.util.*;
import java.util.stream.Stream;
public class Program {
public static void main(String[] args) {
List<Integer> numbers = new LinkedList<Integer>();
numbers.add(24);
numbers.add(13);
numbers.add(43);
numbers.add(45);
numbers.add(12);
Stream<Integer> stream = numbers.stream();
stream.filter(num->num%2==0).
forEach(num->System.out.println(num));
}
}

Stream of Array
 The stream() method that was defined in the
Arrays class allows us to create a stream out
of an array we hold.

Stream of Array
String[] cities = new String[]{"Rome",
"London",
"Moscow", "Ashdod", "Haifa", "Eilat",
"Zermatt"};
Stream<String> full = Arrays.
stream(cities);
Stream<String> partial = Arrays.
stream(cities, 1, 3);
full.forEach(str->System.out.println(str));

Building Streams
 Calling the builder static method in Stream
we get a Stream.Builder<T> object. Calling
builder we should specify the desired type,
otherwise we will get a builder for stream of
objects.

Building Streams
Stream.Builder<String> builder =
Stream.<String>builder();
Stream<String> stream = builder.
add("london").
add("Paris").
add("Zurich").build();
stream.forEach(str -> System.out.println(str));

Infinite Streams
 We can easily create infinite streams. We can
do it either using the Stream.generate() or
the Stream.iterate() functions.

Infinite Streams
 The Stream.generate static method receives a
Supplier<T> object, that generates endless
number of objects.
 If we don't want the Supplier<T> object to work
endlessly you better use the limit function in
order to limit the size of the generated stream.

Infinite Streams
Stream<String> stream =
Stream.generate(() -> String.
valueOf((int(100*Math.random()))).
limit(10);
stream.forEach(str -> System.out.println(str));

Infinite Streams
 The Stream.iterate static method provides
us with an alternative way for creating endless
streams. The first argument this method
receives is the first element of the stream it is
going to generate. The second argument is an
UnaryOperator object.

Infinite Streams
 If we don't want to get endless stream till the
memory ends we better use the limit function
in order to limit it.

Infinite Streams
Stream<Integer> stream = Stream.
iterate(10, n -> n+5).
limit(10);
stream.forEach(data -> System.out.println(data));

Empty Streams
 The empty() method was defined as a static
method in Stream.
 Calling this method we will get an empty stream.
It is a useful method when developing a method
that should return a reference for a Stream
object. Instead of returning null we can easily
return an empty stream.

Empty Streams
public class Main {
List<String> ob = null;
Stream stream = streamOf(ob);
}
public static Stream<String> streamOf(List<String> list) {
return list == null || list.isEmpty() ? Stream.empty() :
list.stream();
}
}

Stream of Primitives
 We can create streams out of three primitive types: int,
long and double. In order to allow that, three new
special interfaces were created: IntStream,
LongStream, DoubleStream.
 Each one of these three interfaces include the definition
of static methods that generates various streams of the
specific primitive type the interface serves .

Stream of Primitives
IntStream ints = IntStream.range(1, 12);
ints.forEach(num->System.out.println(num));

Stream of Random Numbers
 The Random class has the doubles method that
generates randomly generates values of the type
double.

Stream of Random Numbers
Random random = new Random();
DoubleStream stream = random.doubles(10);
stream.forEach(num->System.out.println("number="+num));

Stream of Chars
 Strings can also be used as a source for creating
a stream. Using the chars() method of the
String class we can get a stream of chars.
Since the Java API doesn't have the
CharStream, we will use the IntStream instead.

Stream of Chars
IntStream stream = "abc".chars();
stream.forEach((tav->System.out.print((char)tav)));

Stream of File
 The Files class allows to generate a
Stream<String> of a text file using the
lines() method. Every line of the text
becomes an element of the stream.

Stream of File
Path path = Paths.get("./file.txt");
Stream<String> stream = Files.lines(path);
stream.forEach(str->System.out.println(str));

Streams Cannot Be Reused
 Once a terminal operation was executed the stream will
become inaccessible. It won't be possible to iterate it
again.
 This behavior makes sense. Streams were designed to
provide an ability to apply a finite sequence of operations
on a series of elements coming from a specific store.
Streams weren't created for storing elements.

The forEach() Method
 Using this method we can iterate the elements our
stream includes.

import java.util.function.*;
import java.util.*;
import java.util.stream.Stream;
new Random().
ints().limit(10).forEach(System.out::println);
}
}

The map() Method
 Using this method we map each element with a
new one calculated based on the first.

The map() Method
List<Integer> list = new LinkedList();
list.add(12);
list.add(54);
list.add(53);
list.add(65);
list.add(72);
list.stream().map(n->n*n).
filter(num->num%2==0).distinct().
forEach(System.out::println);
}
}

The map() Method

The filter() Method
 Using this method we can filter our stream
taking out all elements that don't meet our
criteria.

The filter() Method
public class FilterDemo {
list.add(12);
list.add(54);
list.add(53);
list.add(65);
list.add(72);
list.stream().
filter(num->num%2==0).
}
}

The filter() Method

The sorted() Method
 Using this method we can sort our stream.
There are two versions for this method. The
first one sorts the elements in according with
their natural order. The second one has a
second parameter of the Comparator type.

The sorted() Method
list.add(12);
list.add(54);
list.add(53);
list.add(65);
list.add(72);
list.add(8);
list.stream().sorted().
}
}

The sorted() Method

Streams Pipeline
 In order to perform a sequence of operations
over the elements coming from a specific data
source and aggregate their results, three parts
are needed. The source, the intermediate
operation(s) and the terminal operation.
 The intermediate operations return a new
modified stream.

Streams Pipeline
 If more than one modification is needed,
intermediate operations can be chained with
each other.
 Only one terminal operation can be used per
stream. The forEach method is a terminal
operation.

Streams Pipeline
IntStream ints = IntStream.range(-100, 101);
ints.filter(number->number%2==0).
filter(number->number>0).
filter(number->number%3==0).
map(number->2*number).
forEach(num->System.out.println(num));

Lazy Invocation
 The intermediate operations are lazy. They will
be invoked only if necessary in order to allow
the terminal operation to execute.

Lazy Invocation
System.out.println("counter="+counter);
IntStream ints = IntStream.range(-100, 101);
IntStream ints2 = ints.filter(number->number%2==0).
filter(number->{
counter++;
return number>0;
}).
filter(number->number%3==0).
map(number->2*number);
System.out.println("number of elements is "+ints2.count());

Lazy Invocation

Order of Execution
 The right order is one of the most important
aspects of chaining operations in the stream
pipeline.The intermediate operations that
reduce the size of the stream should be placed
before the operations that apply each element.
Methods, such as filter(), skip() and
distinct() better be in the beginning of the
chain. This way the performance will improve.

Functional programming in Java

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