Welcome to Kotlin

Introduction for Programmers
Rainer Kern 2019

Hello World

  • Statically typed C-like programming language

  • object-oriented and functional hybrid

  • Runs on the JVM with full bidirectional interoperability

  • Runs natively on Android (generates 1.6 Bytecode)

  • Possibility to compile to Javascript using LLVM

  • Kotlin Native (beta) for iOS, MacOs and Windows

  • Open source compiler and tools

 

YAJL - Yet another JVM Language!

  • Jetbrains is well-known and offers good tooling support
  • Nothing really new - just the best of all of them
  • It’s about the Java ecosystem, not the language
  • Feels like Java 2.0 (Java on steroids)
 
// Kotlin is

(1..6).forEach{ println("very") }

// easy to learn

log -pretty --date=short

2010     Start of Development by JetBrains

2011     Unveiling of Project Kotlin

2012     Open Source under Apache 2

2016     Release of v 1.0 in Feb

2017     Android Native

2019     v. 1.3.52

 
 

Kotlin is also an Island

Why Kotlin

  • Syntax similar to JAVA / C#

  • Expressiveness makes more readable.

  • Conciseness makes      more understandable.

  • Flexibility -  It allows developers to declare the

  • functions at the top level

  • No static declaration (can cause problems to Java devs)

  • Has Coroutines

     

Basic Syntax

kotlinlang.org/docs/reference

// Java
System.out.println("bar");

// Kotlin
println("bar")
fun main() {    
  println("Hello World")
}
package com.systemkern.demo

fun main(args: Array<String>) {
  val name = 
      if (args.length > 0) args[0]
      else ”Kotlin”
    
  println("Hello $name")
}
// immediate assignment
val a: Int = 1

// 'Int' type is inferred
val b = 2 	

// ininitialized 'Int'
val c: Int
c = 3
// variable of type int
val one = 1
val million = 1_000_000

// variable of type string
val str = "foo"

//explicit declaration
val d: Double = 13.37

val strList = listOf("foo", "bar")
// 'Int' type is inferred
var x = 5
x += 1
// top level variables

val PI = 3.14
var x = 0

fun main() {
    x += 1
}
// traditional usage
var max = a

if (a > b) max = b

// with else
var max: Int = 
  if (a > b) max = a
  else (a > b) max = b
// when replaces the switch operator
when (x) {
  1 -> print ("x == 1")
  2 -> print ("x == 2")
  else -> {
    print ("x is neither 1 nor 2")
  }
}
// for loops
for (item in collection) {
  println(item)
}

for (i in 1..10)
  println (i)

for (i in array.indices)
  println (array[i])
// while loops (and do..while)
while (x > 0) {
  x--
}

do {
  val y = retreiveData()
} while (y != null)

Language Features

kotlinlang.org/docs/reference

Nullability

 
//nullability types checked by the compiler
val sure: String = null // error
val maybe: String? = null

if (maybe == null)
  println("nullable is null")

if (maybe != null)
  println("nullable is not null")
val nullable: String? = null

nullable.length // compiler error
val l1: Int  = nullable!!.length
val l2: Int? = nullable?.length
val l3: Int  = nullable?.length ?: -1
// smart casting
var local: String? = null
// maybe = "can be reassigned"

if (local != null)
  println("length: " + local.length)

print (local ?: "Elvis operation")

if (ext != null) 
  println("length: " + ext?.length)

Class Declaration

 
// single ctor initializing prop
open class MyClazz(
  private val member: String
) {
//  optional init =^= constructor
//  init {
//  }
  fun alert() {
    println("Member is: $member")
  }
}

Multiple Classes per File

 

Extensions

 
// Javascript like top level functions
fun add(x: Int, y: Int): Int {
  return x + y
}

// expression functions
fun add(x: Int, y: Int): Int = x + y

// extension functions
fun Int.third():Int = (this / 3)
val res = 18.third() // 6
// extension properties
val big1 = BigDecimal(100) // this is tedous

val Int.bd.BigDecimal
  get() = BigDecimal(this)

val Double.i
  get() = this.toInt()

big2 = 100_000.bd
int1 = 13.37.i
// extension properties
// cannot modify the class
// so no backing field
val globStore =
  mutableMapOf<Int, String>()
var Int.description: String
  get() = globStore[this] ?: ""
  set(value) {
    globStore[this] = value
  }
fun main() {
  42.description = """
    | The answer to the ultimate
    | question of life,
    | the universe and everything
    """.trimMargin()
    
  println(42.description)
}

// insanely powerful

The Collection API

kotlinlang.org/docs/reference

listOf().firstOrNull()       // null
val list = mutableListOf(2,1,null,4,3)
  .filterNotNull()           // 2,1,4,3

list.first()                 // 2
list.first { it > 2 } 	     // 4
list.filter { it > 1}        // 2,4,3
list.firstOrNull { it > 10 } // null
  ?: -1                      // -1
list.forEach { println(it) }
list.map { "_$it_" } // list of strings

Higher Order Functions

kotlinlang.org/docs/reference

fun higher(
  str: String = "",
  transform: (String) -> (String)
): String = transform(str)
  
higher("baz") { "$it postfix"}

// compare to java
higher(baz,(String str) -> {
  str + "postfix"
});

More Language Features

 

kotlinlang.org/docs/reference

//Pojos

public class Pojo {
  private String id;
  private int amount;
  
  public Pojo (String id, int amount) {
    this.id = id;
    this.amount = amount;
  }
  
  public String getId() { retrun id; }
  
  public void setId(String id) { this.id = id }
  
  public int getAmount() { return amount; }
  
  public void setAmount(int amount) { this.amount = amount; }
}
//Pojos

public class Pojo (
  var id: String,
  var amount: Int
)
//Pojos "Java Style"

public class Pojo (
  var id: String,
  var amount: Int
)

val myPojo = Pojo("foo", 100)
myPojo.amount = 50
//Pojos Kotlin Idiomatic

public class Pojo (
  val id: String,
  val amount: Int
)

Named & Default Args

kotlinlang.org/docs/reference

fun printGreeting(
  name: String = "World",
  str: String = "Hello",
  t: Int = 1
) {
  for (i in 1..t) println("$str $name")
}

printGreeting("Vanessa")
printGreeting("Angie", str = "Hi")
printGreeting(t = 7, name ="Gloria")

Kotlin @ Google IO 2017
   http://youtube.com/watch?

   v=X1RVYt2QKQE

Try Kotlin Online
   try.kotlinlang.org

Learn Kotlin
   kotlinlang.org

Kotlin Vienna User Group
   kotlin.wien

Talking Kotlin podcast
   talkingkotlin.com

kotlinlang.slack.com
   https://surveys.jetbrains.com/s3/

   kotlin-slack-sign-up

 

Welcome to Kotlin

By Rainer Kern

Welcome to Kotlin

Introduction to the Kotlin programming languages. Its design concepts and features. Target Audience: Programmers with no Kotlin experience

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