Short introduction to Kotlin, with theory and practice examples. Presented on Dev Day MiniQ 2016, Vitebsk, Belarus

1. Why Kotlin is your next
language?
theory and practice
Aliaksei Zhynhiarouski
1
Dev Day MiniQ, 2016

2. Kotlin Island
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3. Why is Kotlin
• Static Typing bit.ly/st-vs-dyn
• Java + Kotlin = ❤.
Effortless mixing both in one project
• Java Interoperability.
100% interoperable with Java.
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4. Why is Kotlin
• Null Safety
• Type Inference
• Immutability in the mind
• fun
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5. fun main(args : Array<String>) {
println("Hello, MiniQ!")
}
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6. 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

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

8. Null Safety
val bar: String? = “Dev Day”
var a = bar?.length // 6
var b = bar?.length?.inc() // 7
var c = bar!!.length // can be NPE
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9. 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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10. 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

11. 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

12. Value Object
data class Phone(val brand: String)
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toString()
hashCode()
equals()
copy()

13. Get ready to be inspired
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14. Extensions
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// Usage Example
“MiniQ”.lastChar() // “Q”
// Definition
fun String.lastChar():Char
= this.get(length - 1)

15. Extensions
15
// Java under the hood
@file:JvmName(“StringUtils")
StringUtils.lastChar(“Mobile”);

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

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

18. Extensions
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“25”.toInt() // Int
“8.$minor.5”.toOsVersion() // to object

19. Android Extensions
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val text = find<TextView>(R.id.text)
Just extend Activity and have fun
inline fun <reified T : View>
Activity.find(id: Int): T = this.findViewById(id) as T
final TextView text = (TextView) v.findViewById(R.id.text);

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

21. λ
21
boys.filter { it.age > 18 }
.map { Pair(it, Boy("Eugene")) }
.forEach { dance(it) }
// Definition
inline fun <T> Iterable<T>.filter
(predicate: (T) -> Boolean)

22. λ
22
db.inTransaction {
delete(“users”, “name = ?”, arrayOf(“Alex”))
}
fun SQLiteDatabase.inTransaction(func: SQLiteDatabase.() -> Unit) {
beginTransaction()
try {
func()
setTransactionSuccessful()
} finally {
endTransaction()
}
}

23. Delegates – right way
23
class View {
}
val lazyProp by lazy { “Dev Day” }
val ops by observable(“”) {
prop, old, new ->
print("$old to $new")
}

24. Delegates – wow way
24
class Activity {
}
private val btn: Button by lazy {
findViewById(R.id.btn) as Button
}
override fun onCreate(savedBundle: Bundle?) {
btn.setText(“MiniQ!”)
}

25. Delegates – wow way
25
class Query(val props: Map<String, String>) {
}
val category by props
val model by props
val info: String
get() {
return “$category - $model”
}

26. Delegates – wow way
26
class Query(val props: Map<String, String>) {
}
val category by props
val model by props
…
val props = mapOf(“category” to “XXX”,
“model” to “YYY”)
val query = Query(props)
print(query.info) // “XXX - YYY”

27. 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

28. 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~

29. DSL
29
val expr = filterExpr {
group {
eq("#1", ":2") and eq("#1", ":2")
or group {
eq("#2", ":2")
if (smth) {
and eq("#2", ":2")
}
}
and ……
}

30. One more DSL thing
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Gradle 3 meets Kotlin

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

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

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

34. Why is Kotlin
• Use all existing Java frameworks and
libraries
• Code reviews are not a problem
• Suitable for enterprise Java
• Adopting is low risk
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35. Why is Kotlin
• Android
• Gradle
• Can be learned in a few hours
• Kotlin also compiles to JavaScript ;)
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36. http://try.kotl.in
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37. Links
Awesome Kotlin kotlin.link
Slack kotlinlang.slack.com
Local’s Chat gitter.im/JavaBy/Kotlin
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38. Links just for you
Kotlin compilation speed
Performance: Kotlin Bytecode
https://www.youtube.com/watch?v=eQ4YHpAXdp4
Experience of Square with Kotlin. Good analysis
Android Development advanced
https://vimeo.com/144877458
Kotlin in real project with a lot of nuances
https://www.youtube.com/watch?v=CABN2r4GPpQ
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39. Kotlin friends
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jprof.by
bkug.by

40. Go v Minsk, ja sozdal