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Why Kotlin is your next language? theory and practice
Aliaksei Zhynhiarouski
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Dev Day MiniQ, 2016
Kotlin Island2
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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Why is Kotlin• Null Safety
• Type Inference
• Immutability in the mind
• fun
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fun main(args : Array<String>) {
println("Hello, MiniQ!")
}
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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
Null Safety
// can’t be nullval foo: String = “Dev Day”
// need always perform the check val bar: String? = null
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Type? = Type or null
Null Safetyval bar: String? = “Dev Day”
var a = bar?.length // 6var b = bar?.length?.inc() // 7var c = bar!!.length // can be NPE
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Null Safetyval bar: String? = null
var a = if (bar != null) bar.length else 0 // 0var b = bar?.length ?: 0 // 0
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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
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
Value Object
data class Phone(val brand: String)
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toString() hashCode() equals()
copy()
Get ready to be inspired13
Extensions
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// Usage Example“MiniQ”.lastChar() // “Q”
// Definitionfun String.lastChar():Char = this.get(length - 1)
Extensions
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// Java under the hood@file:JvmName(“StringUtils") StringUtils.lastChar(“Mobile”);
Extensions
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Collections.max(list)↓
list.max()
Extensions
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operator fun BigDecimal.inc() = this + BigDecimal.ONE
// Examplevar counter = BigDecimal.ZEROprint(++counter) // 1
Extensions
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“25”.toInt() // Int
“8.$minor.5”.toOsVersion() // to object
Android Extensions
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val text = find<TextView>(R.id.text)
Just extend Activity and have fun
inline fun <reified T : View> Activity.find(id: Int): T = this.findViewById(id) as T
final TextView text = (TextView) v.findViewById(R.id.text);
Lambdas
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val boys = listOf( Boy(“Alex”), Boy(“Petr”), Boy(“Oleg”))
// 1 boys.filter({ b: Boy -> b.age > 18 })// 2boys.filter({ it.age > 18 })// 3boys.filter { it.age > 18 }
λ
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boys.filter { it.age > 18 } .map { Pair(it, Boy("Eugene")) }.forEach { dance(it) }
// Definitioninline fun <T> Iterable<T>.filter (predicate: (T) -> Boolean)
λ
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db.inTransaction { delete(“users”, “name = ?”, arrayOf(“Alex”)) }
fun SQLiteDatabase.inTransaction(func: SQLiteDatabase.() -> Unit) { beginTransaction() try {
func()
setTransactionSuccessful() } finally { endTransaction() }}
Delegates – right way
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class View {
}
val lazyProp by lazy { “Dev Day” }
val ops by observable(“”) { prop, old, new ->
print("$old to $new")
}
Delegates – wow way
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class Activity {
}
private val btn: Button by lazy { findViewById(R.id.btn) as Button} override fun onCreate(savedBundle: Bundle?) { btn.setText(“MiniQ!”) }
Delegates – wow way
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class Query(val props: Map<String, String>) {
}
val category by propsval model by propsval info: String get() { return “$category - $model” }
Delegates – wow way
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class Query(val props: Map<String, String>) {
}
val category by propsval model by props…
val props = mapOf(“category” to “XXX”, “model” to “YYY”) val query = Query(props)
print(query.info) // “XXX - YYY”
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
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~
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 …… }
One more DSL thing
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Gradle 3 meets Kotlin
Kotlin 1.1
typealias Callback<T> = (T) -> T
// And nowfun alias(cb: Callback<String>) {}
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Type aliases
Kotlin 1.1
typealias Table<K> = MutableMap<K, MutableList<String>>
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Type aliases
Kotlin 1.1
val future = async<String> {
(1..5).map {
await(loooongAsyncOperation(it)) }.joinToString("\n")
}
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Coroutines with async/await
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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Why is Kotlin• Android
• Gradle
• Can be learned in a few hours
• Kotlin also compiles to JavaScript ;)
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http://try.kotl.in
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LinksAwesome Kotlin kotlin.link
Slack kotlinlang.slack.com
Local’s Chat gitter.im/JavaBy/Kotlin
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Links just for youKotlin compilation speed
Performance: Kotlin Bytecode https://www.youtube.com/watch?v=eQ4YHpAXdp4
Experience of Square with Kotlin. Good analysis
Android Development advancedhttps://vimeo.com/144877458
Kotlin in real project with a lot of nuanceshttps://www.youtube.com/watch?v=CABN2r4GPpQ
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Go v Minsk, ja sozdal
