The document provides a comprehensive overview of Kotlin, detailing its syntax, features, and capabilities, including strong typing, object-oriented and functional programming paradigms. It discusses various programming constructs such as functions, classes, inheritance, and operators, and illustrates them with code examples. Additionally, it covers Kotlin's interoperability with Java and its applications in JavaScript and Kotlin Native development.

1. Welcome!

2. Your Skills?
•Java/Android
•C#
•Groovy

3. Kotlin
Java C#
Groovy

4. About Me
Ing. Marco Vasapollo
C. E. O. & Founder @ MetaRing
C. E. O. & Founder @ Aequalitam Foundation
Mail: ceo@metaring.com
Skype: marco.vasapollo
https://www.metaring.com
https://www.aequalitam.org

5. About Kotlin
•Strongly typed
•Both OOP & Functional
•Lightweight Syntax
•On the top of JVM

6. Warnings
•Kotlin is NOT Java
•It has its own APIs (for cross-platform reasons)
•Kotlin understands Java because of Plugins
•Plugins convert Kotlin APIs into Java APIs*
*when needed

7. How it works
HelloWorld.class
kotlin-compiler
HelloWorld.kt
import java.util.List;
import java.util.Date;
import ……
Access to all Java
libraries
org.jetbrains.kotlin.cli.jvm.K2JVMCompiler
• Via command line
• Via official/unofficial IDE plugins (Eclipse, IntelliJ Idea,Visual
Studio Code)
• Via Packages Managers (Maven, Gradle)
Plain Classic Java ByteCode (No Java source needed)

8. Online IDE
https://try.kotlinlang.org

9. HelloWorld.kt
fun main(args : Array<String>) {
println(“Hello, world!”)
}
class Greeter(val name : String) {
fun greet() {
println("Hello, $name");
}
}
fun main(args : Array<String>) {
Greeter(args[0]).greet()
}
The simple way The OO Way
VS

10. Variables
var name : String = “Marco”
var name = “Marco”; //; can be omitted
val name = “Marco” //readonly, like Java final
val name : String
//0, 1, or more instructions later…
name = “Marco”
name = “Giuseppe” //cannot do this, once it’s assigned, value cannot be changed
var name : String? = null; //vars and vals are NullSafe by default, to allow
null you have to insert ‘?’ at the end

11. If
var a = 11
var b : String
if(a > 10) {
b = “foo”
} else {
b = “bar”
}
var a = 11
var b = if(a > 10) {
“foo”
} else {
“bar”
}
var a = 11
var b = if(a > 10) “foo” else “bar”
In Kotlin, if is an expression, not just a simple construct
Ternary operator does NOT exists, so you cannot do:
var b = a > 10 ? “foo” : “bar”

12. When
var a = 11
var b = when {
a > 10 -> “foo”
a < 10 -> “bar”
else -> “default case”
}
No need of switch construct. when operator is extremely powerful
var a = 11
var b = when(a) {
10 -> “foo”
11 -> “bar”
else -> “default case”
}
var a = 11
when {
a > 10 -> println(“foo”)
a < 10 -> println(“bar”)
else -> println(“default case”)
}

13. Nullables
var a : String? = “foo” //Can be null
var b = a.length //Syntax ERROR: a can be null and b is IMPLICITLY NullSafe
var b = if (a != null) a.length else -1 //Correct form
var b = a?.length //Same as above. But this time b is implicitly Int?
var name = bob?.department?.head?.name //Safe calls can be Concatenated
a?.let {println(a.length)} //If a is not null, execute this lambda expression
var b = a?.length ?: -1 //Elvis operator, if value to the left of ?: is null,
then use the one to the right. In this case b is Int
var b = a!!.length //a.length or NullPointerException

14. Equality
var eq = a === b //referential equality
var eq = a.equals(b) //Structural equality
var eq = a == b //syntactic sugar for a?.equals(b) ?: (b === null)

15. Strings
var name = “Marco”
var message = “My name is ” + name + “, pleasure to meet you”
message = “My name is ${name}, pleasure to meet you”//$name is correct too
var user = //get user data object from somewhere...
message = “My name is ”${user.name}${‘”’} and my bank account has ${user.cash}${‘$’}”
//Prints: My name is “Marco” and my bank account has 500$
message = “My name is ${user.name}, my bank account has ${user.cash + 300} ${‘$’}”
message = “My bank account has ${user.cash + getOtherSavings(user.id)} ${‘$’}”

16. Strings
var json =
“{nt”name”:”Marco”,nt”surname”:”Vasapollo”,nt”company”:”MetaRing”
n}”
json = “““
{
“name” : “Marco”,
“surname” : “Vasapollo”,
“company” : “Metaring”,
“age” : ${user.age}
}
”””

17. Arrays
var array1 = arrayOf("a", "b", "c", “d”) //Array<String> -> String[]
var array2 = arrayOf("a", "b", "c", 5) //Object[]
var array3 = arrayOfNulls<String>(10) //Array<String?>
var array4 = Array<String>(5) { "it = $it" } //for each element, execute this lambda
var list = array1.toList() //READONLY, NOT java.util.List
var mutableList = array1.toMutableList() //or list.toMutableList()
for(i in array1.indices) {//available on lists too
println(i) //prints 0,1,2,3
println(array1[i]) //prints a,b,c,d
}
for(i in array1) {
println(i) //prints a,b,c,d
}

18. Ranges
for(i in 1..4) print(i) //Prints “1234”
for(i in 1 until 4) print(i) //Prints “123”
for(i in 4 downTo 1) print(i) //Prints "4321"
for(i in 1..4 step 2) print(i) //Prints "13"
for(i in 4 downTo 1 step 2) print(i) //Prints "42“
var array1 = arrayOf("a", "b", "c", “d”)
var result = “c” in array1 //corresponds to array1.contains(“c”)
when("c") {
in array2 -> println(“foo”)
else -> println(“bar”)
}

19. Functions
fun sayMyName(name : String) { //default modifier is public, also for classes
println(“Your name is $name”)
}
fun foo() = println(“Hello!”) //Inline
fun bar() : Unit = println(“Ciao!”) //Unit is similar to Java void, can be omitted
fun main(args : Array<String>) {
var fooResult = foo() //returns Unit. Unlike Java void, it can be stored in a var
var barResult = bar() //returns Unit
//Unit is a Singleton, there is only a unique instance in all the System, so:
println(fooResult === barResult) //true
println(fooResult == barResult) //true
println(fooResult.equals(barResult)) //true
}

20. Functions
fun sum1(x : Int, y : Int) : Int {
return x + y
}
fun sum2(x : Int, y : Int) : Int = x + y //Inline, Groovy Style
var sum3 = fun(x : Int, y : Int) : Int = x + y //Anonymous function
var sum4 : (Int, Int) -> Int = { x, y -> x + y} //Type is same as sum3
fun logOperation(x : Int, y : Int, operation : (Int, Int) -> Int) {
println("Operation result between $x and $y is ${operation(x,y)}")
}
fun main(args : Array<String>) {
logOperation(3, 5, ::sum1) //references a function
logOperation(1, 4, sum3) //references an anonymous function
}

21. Packages
/* File src/Math.kt
* (match between path and package
* declaration is not mandatory) */
package marco.math
fun fantasticAbs() …
fun amazingPow() …
fun luxurySqrt() …
//File MyCode.kt
import marco.math.*
fun foo(args : Array<String>) {
fantasticAbs(...)
amazingPow(...)
luxurySqrt(...)
}
//File MyOtherCode.kt
import marco.math.amazingPow
import marco.math.luxurySqrt as thatCuteFunction
fun bar(args : Array<String>) {
amazingPow(...)
thatCuteFunction(...)
}

22. Classes
class User class User {} class User() class User() {} //All empty classes
class User(n : String) { //primary Constructor
var name = n
}
class User constructor(n : String) { //keyword constructor is omittable
var name = n
}
class User private constructor(n : String) { //needed when use modifiers or annotations
var name = n
}
class User(var name : String) //name is now a field of the User class

23. Classes
import java.util.Date
class User(var name : String) {
var minAge = 18
println(“User $name created”) //ERROR, primary constructor can only assign vars
init {
println(“User $name created”) //Use init block to add behavior to primary constructor
}
val creationTime = Date() //To instantiate an object, new keyword isn’t needed
init { //There can be more, non consecutive init blocks, they are executed sequentially
println(“Creation time: $creationTime”)
//Note that creationTime cannot be used in the first init block
}
}

24. Classes
class User(var name : String) {
constructor(age : Int) : this(“Marco”) { //Secondary constructor
// : this(...) is mandatory if primary constructor is explicitly declared
//primary constructor and init blocks are executed always and before secondary
//secondary constructors inputs like age cannot be used as class fields
//... Behavior of the secondary constructor ...
userAge = age
}
var userAge : Int
}

25. Inheritance
abstract class GeometricFigure {
abstract fun getArea() : Double
}
open class Rectangle : GeometricFigure { //classes are final by default
val x : Double
val y : Double
constructor(x : Double, y : Double) {
this.x = x
this.y = y
}
override final fun getArea() : Double = x * y
}
class Square(side : Double) : Rectangle(side, side) //super light!

26. Properties
class Rectangle(var x : Double, var y : Double) {
val area
get() = x * y //getter only, look at val
}
Class fields are Properties by default (Finally!!!!! ç_ç )
class User {
var fiscalCode : String
get
private set //getter is accessible, setter is not
//Properties value is mandatory. Use a default value, constructor input or lateinit
constructor(fiscalCode : String) = this.fiscalCode = fiscalcode
lateinit var name : String //Mandatory init before read!
var surname : String? = null
}

27. Data Classes
class User( //Common pattern: use default values in primary construction
var name : String = “”,
var surname : String = “”,
var fiscalCode : String = “”)
data class User(var name : String = “”, var surname : String = “”)
//data keyword auto generates a specific behavior for toString and equals, based on
//properties. E.G. toString -> “User(name=Marco, surname=)”
//plus, a useful copy method that can be used to clone instances in another ref
//To init (both non-data and data)
var user = User (
surname = “Rossi”, //order is not important
name = “Mario”)
var copy = user.copy(surname=“Bianchi”) //data only

28. Operators
class Point(var x = 0, var y = 0) {
operator fun inc() : Point {x++; y++; return this} //point++ on a Point var
operator fun dec() : Point {x--; y--; return this} //point–- on a Point var
operator fun plusAssign(n : Int) {x+=n; y+=n} //point += 5
}
fun main(args : Array<String>) {
var point = Point()
println(point) //Point(x=0, y=0)
point++
println(point) //Point(x=1, y=1)
point+=10 //x = 11, y = 11
}
https://kotlinlang.org/docs/reference/operator-overloading.html

29. Any
package kotlin
public open class Any {
public open operator fun equals(other: Any?): Boolean
public open fun hashCode(): Int
public open fun toString(): String
val <T : Any> T.javaClass: Class<T>
}
Any is the Kotlin superclass

30. Delegates
import kotlin.reflect.KProperty
class Foo {
var bar by LoggablePropertyDelegate<String>()
}
class LoggablePropertyDelegate <T> (var value : T? = null) {
operator fun getValue(thisRef: Any?, property: KProperty<*>): T? = value
operator fun setValue(thisRef: Any?, property: KProperty<*>, value: T?) {
var oldValue = this.value
this.value = value
println("${property.name} in $thisRef has been changed from $oldValue to $value")
}
}

31. Delegates
interface Base {
fun printInfo()
}
class BaseImpl(val x : Int) : Base {
override fun printInfo() = print(x)
}
class Derived(b : Base) : Base by b
fun main(args: Array<String>) {
val b = BaseImpl(10)
b.printInfo() //prints 10
Derived(b).printInfo() //prints 10
}
https://kotlinlang.org/docs/reference/delegation.html
https://kotlinlang.org/docs/reference/delegated-properties.html

32. Extension Methods
abstract class Employee(var name : String = "", var surname : String = "")
class Developer(name : String = "", surname : String = "", var language : String =
"Kotlin"): Employee(name, surname)
class Architect(name : String = "", surname : String = ""): Employee(name, surname)
class Manager(name : String = "", surname : String = ""): Employee(name, surname)
//If a want a toJson() Method?
import com.google.gson.Gson;
abstract class Employee(var name : String = "", var surname : String = "") {
fun toJson() = Gson().toJson(this)
}

33. Extension Methods
import com.google.gson.Gson;
fun Employee.toJson() = Gson().toJson(this) //In a module, or in a class
fun Any.toJson() = Gson().toJson(this) //Better
//Even better, as a readonly property
val Employee.json
get() = Gson().toJson(this)
fun main(args : Array<String>) {
var d = Developer()
println(d.toJson())
}
What If Employee, Developer, Architect and Manager are read-only (in jar, in maven package, etc)?

34. Android

35. Android

36. Android

37. Android

38. Android

39. Android
EXAMPLE

40. JavaScript
<build>
<sourceDirectory>src/main/kotlin</sourceDirectory>
<plugins>
<plugin>
<groupId>org.jetbrains.kotlin</groupId>
<artifactId>kotlin-maven-plugin</artifactId>
<version>1.1.51</version>
<executions>
<execution>
<id>compile</id>
<phase>compile</phase>
<goals>
<goal>js</goal>
</goals>
</execution>
</executions>
</plugin>
</plugins>
</build>
<dependencies>
<dependency>
<groupId>org.jetbrains.kotlin</groupId>
<artifactId>kotlin-stdlib-js</artifactId>
<version>1.1.51</version>
</dependency>
</dependencies>

41. JavaScript
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-dependency-plugin</artifactId>
<executions>
<execution>
<id>unpack</id>
<phase>compile</phase>
<goals>
<goal>unpack</goal>
</goals>
<configuration>
<artifactItems>
<artifactItem>
<groupId>org.jetbrains.kotlin</groupId>
<artifactId>kotlin-stdlib-js</artifactId>
<version>${kotlin.version}</version>
<outputDirectory>${project.build.directory}/js/lib</outputDirectory>
<includes>*.js</includes>
</artifactItem>
</artifactItems>
</configuration>
</execution>
</executions>
</plugin>

42. JavaScript
EXAMPLE

43. Kotlin + jQuery
npm install ts2kt
ts2kt -d headers jquery.d.ts
Download jquery.d.ts from
https://github.com/DefinitelyTyped/DefinitelyTyped/blob/types/jquery/jquery.d.ts
@JsName("$")
public external fun jQuery(selector : String) : JQuery
$(“#dataTable”).asDynamic().datatable()
Of course you have to import your jQuery and lib scripts into html

44. Kotlin-Native
• Kotlin Source -> Compiler -> Platform Binary (NO VM Needed)
• Code Sharing Between Platforms
• Windows, iOS, Android, Linux, OS X, RaspberryPI
• Still Early Beta (0.3)
git clone https://github.com/JetBrains/kotlin-native.git
cd kotlin-native
.gradlew.bat dependencies-update
.gradlew.bat dist
.gradlew.bat cross_dist
.gradlew.bat backend.native:tests:run <- Optional takes a veeery long time to end
.distbinkotlinc HelloWorld.kt –o HelloWorld -opt

45. Questions?