Introduction to Kotlin at "Mobile Optimized 2016" Conference. twitter: @a_lithium

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
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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