Intro to Kotlin

● Properties (val, var)
● String templates
● Null Safety
● Smart Cast
● OOP
● Lambdas
● Collections
● Extensions
● Infix Notation
● Operator Overloading
Agenda

What is Kotlin?
● Statically typed language
● From Jetbrains
● Inspired by Java, Scala, C#, Groovy etc.
● Targets: JVM and Javascript

1.0
Release
1.1
Release
Kotlin is
born
Gradle
Android
Official
Spring
Timeline (Kotlin 1.1.4 is out)
Chart source: https://github.com/jetbrains/kotlin-workshop

Conventions
• The same conventions like on Java
• Uppercase for types
• Lower camelCase for methods and properties
• Semicolons are optional
• Reverse notation for packages
• A file could contain multiple classes
• The folder names not have to match the package name

Development tools
• JDK
• Version 6, 7, 8 and 9
• Kotlin Compiler
• Editor or IDE
• IntelliJ IDEA, Android Studio, NetBeans, Eclipse

Build tools
• Maven
• Gradle
• Kobalt
• Ant
• Command Line

Basic Data Types
Numbers
Characters
Booleans
Arrays
Strings
Any
Nothing

Properties: val and var
• val is immutable (read-only) and you can only assign a value to them exactly
one time. This is the recommended type to use.
• var is mutable and can be reassigned.
• The type inference is more powerful than Java
• Unit in Kotlin corresponds to the void in Java. Unit is a real class (Singleton)
with only one instance.
• Nothing is a type in Kotlin that represents “a value that never exists”, that
means just “no value at all”.

private final int a = 1;
private int b = 2;
private final int c = 3;
private int d = 4;
public int getA() { return a; }
public int getB() { return b; }
public void setB(int b) { this.b = b; }
public int getC() { return c; }
public int getD() { return d; }
public void setD(int d) { this.d = d; }
val a: Int = 1
var b: Int = 2
val c = 3
var d = 4
Properties: val and var

String templates
• Kotlin supports template expressions
• Simple reference uses $
• Complex references uses ${}
• Raw Strings

class KotlinClass {
fun main(args: Array<String>) {
val first = "Intro to"
val second = "Koltin"
var both = "$first $second"
println("$both has ${both.length}")
println(""""$both" has ${both.length}""")
}
}
/*
1. Menu>Tools>Kotlin>Show Kotlin Bytecode
2. Click on the Decompile button
3. See the java code */
String templates
Public final class JavaClass {
public final void main(@NotNull String[]
args) {
Intrinsics.checkParameterIsNotNull(args,
"args");
String first = "Intro to";
String second = "Koltin";
String both = "" + first + ' ' + second;
String var5 = "" + both + " has " +
both.length();
System.out.println(var5);
var5 = '"' + both + "" has " +
both.length();
System.out.println(var5);
}
}

Null safety
• No more NPEs
• Possible causes for NPE:
• !!
• throw NullPointerException();
• External Java code
• Data inconsistency with regard to initialization

Null safety
• No more NPEs
• Possible causes for NPE:
• !!
• throw NullPointerException();
• External Java code
• Data inconsistency with regard to initialization
class KotlinClass {
val notNullString: String = null //error!
val nullableString: String? = null
//condition checks
var theString: String? = "abc"
val length1 = if (theString != null)
theString.length else -1
//safe calls
val length2 = theString?.length
//elvis operator
val length3 = theString?.length ?: -1
//use !!
val length4 = theString!!.length
//safe casts
val aInt: Int? = theString as? Int
}

Class
• Many classes in the same file
• Classes and methods are final by default. Use open for extensibility.
• Only properties(public by default) and functions(fun)
• No new keyword on instantiation
• lateinit for properties (not null) and they should be mutable
• init block is equal with the constructor in Java
• inner keyword to be able to access members of the surrounding class

class KotlinClass(var name: String) {
lateinit var gdg: GDGPitesti
val language: String
init {
language = "Kotlin"
}
fun initializeName (name: String) {
gdg = GDGPitesti(name)
}
inner class GDGPitesti(val text: String)
fun sayItFromGDG(): String = "${gdg.text}
$name $language"
}
var a = KotlinClass("loves")
a.initializeName("GDG Pitesti")
print(a.sayItFromGDG())//GDG Pitesti loves Kotlin
}
Class
public final class JavaClass {
@NotNull
public KotlinClass.GDGPitesti gdg;
@NotNull
private final String language;
@NotNull
private String name;
@NotNull
public final KotlinClass.GDGPitesti getGdg() {
KotlinClass.GDGPitesti var10000 = this.gdg;
if(this.gdg == null) {
Intrinsics.throwUninitializedPropertyAccessExcept
ion("gdg");
}
return var10000;
}
…

Data Class
• data class contains:
• properties
• equals
• hashCode
• toString
• copy
• component (Destructuring Declarations)

data class Coordinate(var lat: Double, var lon:
Double)
val city1 = Coordinate(24.5, 48.5)
//copy
val city2 = city1.copy(lat = 25.7)
//component
val (theLat, theLon) = city2
println(city1)
println(city2)
println(theLat)
println(theLon)
}
Data Class
public final class CoordinateJava {
private double lat;
private double lon;
public final double getLat() {
return this.lat;
}
public final void setLat(double var1) {
this.lat = var1;
}
public final double getLon() {
return this.lon;
}
public final void setLon(double var1) {
this.lon = var1;
}
….

Interface
• Could contain abstract methods and/or properties
• They need to be initialized in the implementing class in order to respect the
contract of the interface
• override keyword is mandatory
• A property declared in an interface can either be abstract, or it can provide
implementations for accessors.

interface GDG {
val name: String
fun getCity(): String
}
class GDGPitesti: GDG{
override val name: String = "GDG"
override fun getCity(): String = "$name
Pitesti"
}
println(GDGPitesti().getCity())
}
Interface

Object
• The object keyword creates singleton in Kotlin
• An object declaration inside a class can be marked with the companion keyword
• Members of the companion object can be called by using simply the class name
as the qualifier (like static on Java)
• The name of the companion object can be omitted, in which case the name
Companion will be used

object SingletonClass {
fun getHello(): String = "Hello Singleton"
}
class CompaniedClass(val str: String) {
companion object Printer {
fun getHello(): String = "Hello Companion"
}
}
class NoNameCompaniedClass(val str: String) {
companion object {
fun getHello(): String = "Hello No Name
Companion"
}
}
SingletonClass.getHello() // Hello Singleton
CompaniedClass.getHello() // Hello Companion
NoNameCompaniedClass.getHello() // Hello No
Name Companion
}
Object

val a = 4; val b = 7
val max = if (a > b) {
println("$a is grater than $b"); a
} else {
println("$b is grater than $a"); b
}
println(max)
println(describe("GDG"))
for (i in 1..10 step 2) {
print("$i ")
}}
fun describe(obj: Any): String =
when (obj) {
1 -> "One"
"Hello" -> "Greeting"
is Long -> "Long"
!is String -> "Not a string"
else -> "Awesome"
}
It’s all about expressions...

Collections and Lambdas
• Kotlin distinguishes between
mutable and immutable
collections
• The supported operations on
Collections are: map,
flatMap, forEach, fold,
reduce, filter, zip etc.
• to is in infix function
listOf(1, 2, 3); mutableListOf("a", "b", "c")
setOf(1, 2, 3); mutableSetOf("a", "b", "c")
mapOf(1 to "a", 2 to "b", 3 to "c")
mutableMapOf("a" to 1, "b" to 2, "c" to 3)
val aList = listOf(1, 2, 4)
println(aList.map { elem ->
elem + 1
})
println(aList)
println(aList.filter { it != 1 })
fun sum(a: Int, b: Int) = a + b
println(aList.reduce(::sum))
}

Extensions functions
• Add operators to the classes by
using the operator
• Create infix functions with the
infix keyword
• Extension functions are normal
static methods bearing no
connection with the class they are
extending, other than taking an
instance of this class as a
parameters
val x = "Kotlin"
println(x.last())
val m1 = Matrix(1, 2, 3, 4)
val m2 = Matrix(1, 1, 1, 1)
val m3 = m1 + m2
println("${m3.a} ${m3.b} ${m3.c} ${m3.d}")
}
fun String.last(): Char {
return this[length - 1]
}
class Matrix(val a: Int, val b: Int, val c: Int,
val d: Int) {
operator fun plus(matrix: Matrix): Matrix {
return Matrix(a + matrix.a, b + matrix.b,
c + matrix.c, d + matrix.d)
}
}

Resources
● Images source: buzzfeed.com
● http://jussi.hallila.com/Kollections/
● https://try.kotlinlang.org
● https://kotlinlang.org/docs/books.html

Intro to Kotlin

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