This document provides an introduction to the Kotlin programming language. It highlights several key features of Kotlin including static typing, null safety, type inference, immutability, fun syntax, classes, data classes, delegates, extensions, lambdas, and domain-specific languages (DSLs). It also summarizes new features being added in Kotlin 1.1 such as direct Java 8/9 support, type aliases, delegated properties everywhere, and coroutines with async/await. The document encourages the reader to start their journey with Kotlin and provides some links for additional resources.

1. Let’s fly to the Kotlin
island
short and comprehensive introduction
Aliaksei Zhynhiarouski
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2. http://try.kotl.in
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3. Kotlin Island
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4. Why is Kotlin
• Static Typing
• Java + Kotlin = ❤.
Effortless mixing both in one project
• Java Interoperability.
100% interoperable with Java.
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5. Why is Kotlin
• Null Safety
• Type Inference
• Immutability in the mind
• fun
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6. fun main(args : Array<String>) {
println("Hello, MO 2016!")
}
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7. val from_value : Type
var from_variable : Type
val i_am_string = “mobile”
var i_am_long = 666L
val i_am_double = 3.14e10
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val & var

8. Null Safety
// can’t be null
val foo: String = “mobile”
// need always perform the check
val bar: String? = null
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Type? = Type or null

9. Null Safety
val bar: String? = “mobile”
var a = bar?.length // 6
var b = bar?.length?.inc() // 7
var c = bar!!.length // can be NPE
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10. Null Safety
val bar: String? = null
var a = if (bar != null) bar.length
else 0 // 0
var b = bar?.length ?: 0 // 0
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11. Type Definition
class Phone(val brand: String)
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• concise syntax for constructor
• properties – not fields
• final by default. Use open to open
• simple old java class under the hood

12. Type Definition
class Phone(val brand: String){
var version: Int = 0
}
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get() {…}
private set
val phone = Phone(“Apple”)
phone.brand == “Apple” // true

13. Value Object
data class Phone(val brand: String)
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Lombok – no more

14. Get ready to be inspired
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15. Delegates
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class Foo<T> : List<T> {
private val innerList = arrayListOf<T>()
override val size: Int get() = innerList.size
override fun isEmpty() = innerList.isEmpty()
…
…
…
}

16. Delegates – right way
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class Foo<T>
( innerList : List<T> = ArrayList<T>() )
: List<T> by innerList
All functions invocation delegated to innerList

17. Delegates – right way
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class View {
}
val lazyProp by lazy { “mobile” }
val ops by observable(“”) {
prop, old, new ->
print("$old to $new")
}

18. Delegates – wow way
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class Activity {
}
private val btn: Button by lazy {
findViewById(R.id.btn) as Button
}
override fun onCreate(savedBundle: Bundle?) {
btn.setText(“Optimize!”)
}

19. Delegates – wow way
19
class SMS(val props: Map<String, String>) {
}
val date by props
val address by props
val info: String
get() {
“$date - $address”
}

20. Delegates – wow way
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class SMS(val props: Map<String, String>) {
}
val date by props
val address by props
…
val props = mapOf(“date” to “128932”,
“address” to “Sweden”)
val sms = SMS(props)
print(sms.info) // “128932 - Sweden”

21. Get ready to be inspired
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22. Extensions
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// Example
“Mobile”.lastChar() // “e”
// Definition
fun String.lastChar():Char
= this.get(length - 1)

23. Extensions
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// Java under the hood
@file:JvmName(“StringUtils")
StringUtils.lastChar(“Mobile”);

24. Extensions
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Collections.max(list)
↓
list.max()

25. Extensions
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operator fun BigDecimal.inc()
= this + BigDecimal.ONE
// Example
var counter = BigDecimal.ZERO
print(++counter) // 1

26. Fun
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Named Parameters
// Bad
ContentResolver.query(URI, new String[]{ },
null, null, null);
// Good
ContentResolver.query(URI,
projection = new String[]{ },
selection = null,
selectionArgs = null,
sortOrder = null)

27. Fun
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Named Parameters & Default Values
// Bad
IN_PROGRESS(true, false, false, false, false, false),
FAILED (false, true, true, false, false, false),
COMPLETE (true, false, true, false, false, true)
// Good
COMPLETE(stop = true,
cancel = true,
disable = true)

28. Fun
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Named Parameters & Default Values
// Awesome
class COMPLETE(
stop: Boolean = false,
cancel: Boolean = false,
…
…)

29. Lambdas
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val boys = listOf(
Boy(“Alex”), Boy(“Magnus”), Boy(“Nils”))
// 1
boys.filter({ b: Boy -> b.age > 18 })
// 2
boys.filter({ it.age > 18 })
// 3
boys.filter { it.age > 18 }

30. λ
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boys.filter { it.age > 18 }
.map { Pair(it, Boy("Eugene")) }
.forEach { dance(it) }
// Under the hood
inline fun <T> Iterable<T>.filter
(predicate: (T) -> Boolean)

31. DSL
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• Type-safe
• Express your code by flexible syntax
• Ideal to build own query engine
• if/for/when just inside DSL

32. DSL
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"(#C1 = :active) AND (#C2 = :type) AND " +
"(attribute_not_exists(#C1) OR #C1 IN
(:model, :allModels)) AND " +
"...";
if (smth)
append("(#C2 = :active) AND NOT
contains(#C3, :brandAll)");
…
// wait, oh shi~

33. DSL
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val expr = filterExpr {
group {
eq("#1", ":2") and eq("#1", ":2")
or group {
eq("#2", ":2")
if (smth) {
and eq("#2", ":2")
}
}
and ……
}

34. One more DSL thing
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Gradle meets Kotlin

35. Kotlin 1.1
• Direct Java 8/9 support
• Type aliases
• Delegated properties everywhere
• Coroutines with async/await
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bit.ly/kotlin_11

36. Kotlin 1.1
typealias Callback<T> = (T) -> T
// And now
fun alias(cb: Callback<String>) {}
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Type aliases

37. Kotlin 1.1
typealias Table<K> =
MutableMap<K,MutableList<String>>
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Type aliases

38. Kotlin 1.1
fun (cb: Callback<String>) {
val callonce by lazy { cb("MO 2016") }
println(callonce)
}
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Local delegated properties

39. Kotlin 1.1
val future = async<String> {
(1..5).map {
await(loooongAsyncOperation(it))
}.joinToString("n")
}
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Coroutines with async/await

40. Kotlin friends
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jprof.by
Belarus Kotlin User Group
bkug.by

41. Links
Awesome Kotlin kotlin.link
Slack kotlinlang.slack.com
Local Chat gitter.im/JavaBy/Kotlin
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42. Start your journey
with
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twitter: @a_lithium