Kotlin a better Java Language

Kongkow IT Medan Meetup

Introduction

@desdulianto

What is Kotlin

• a static programming language
• runs on top of JVM (and Android)
• and Javascript and Native (LLVM)
• 100% interoperable with Java
• developed by JetBrains since 2011
• official Android programming language since May 2017

Why use Kotlin

• Industrial-strength programming language
• 100% Java interop
• Expressive & concise codes
• Safe
• Functional programming features

100% Java Interop

data class KotlinPerson @JvmOverloads constructor(val name: String,
                                                  val gender: String = "Pria",
                                                  var age: Int)

public class TestPersonJava {
    public static void main(String []args) {
        JavaPerson jvPerson = new JavaPerson("Budi", 25);
        KotlinPerson ktPerson = new KotlinPerson("Budi", 25);

        System.out.println("Java Person");
        System.out.println("Name: " + jvPerson.getName());
        System.out.println("Gender: " + jvPerson.getGender());
        System.out.println("Age: " + jvPerson.getAge());
        System.out.println("Java Person: " + jvPerson.toString());

        System.out.println("Kotlin Person");
        System.out.println("Name: " + ktPerson.getName());
        System.out.println("Gender: " + ktPerson.getGender());
        System.out.println("Age: " + ktPerson.getAge());
        System.out.println("Kotlin Person: " + ktPerson.toString());
    }
}

Expressive & Concise Codes

• Less boilerplate
• String interpolation
• Default parameter
• Type inference

data class KotlinPerson @JvmOverloads constructor(val name: String,
                                                  val gender: String = "Pria",
                                                  var age: Int)

fun main(args: Array<String>) {
    val jvPerson = JavaPerson("Budi", 25)
    val ktPerson = KotlinPerson(name = "Budi", age = 25)

    println("Java Person")
    println("Name: ${jvPerson.name}")
    println("Gender: ${jvPerson.gender}")
    println("Age: ${jvPerson.age}")
    println("Java Person: $jvPerson")

    println("Kotlin Person")
    println("Name: ${ktPerson.name}")
    println("Gender: ${ktPerson.gender}")
    println("Age: ${ktPerson.age}")
    println("Kotlin Person: $ktPerson")
}

Expressive & Concise Codes

Destructuring, Smart casting, If, when & try expression, Expression function, Infix function, Extension function

infix fun KotlinPerson.sayHello(other: KotlinPerson) = 
        println("${this.name}: Hello, ${other.name}")

fun main(args: Array<String>) {
    val people = listOf(KotlinPerson(name = "Budi", age = 25),
                        KotlinPerson(name = "Susi", gender = "Wanita", age = 25),
                        KotlinPerson(name = "Tuti", gender = "Wanita", age = 20),
                        KotlinPerson(name = "Ali", gender = "Pria", age = 30),
                        KotlinPerson(name = "Rudi", gender = "Pria", age = 27))

    // destructuring
    for ((name, gender, age) in people) {
        val genderString = if (gender == "Wanita") "woman" else "man"
        println("$name is $age years old $genderString")
    }

    val budi = people[0]
    val susi = people[1]

    budi.sayHello(susi)
    susi sayHello budi
}

Safe

• var (mutable) vs val (immutable)
• Null safety

data class KotlinPerson @JvmOverloads constructor(val name: String,
                                                  val gender: String = "Pria",
                                                  var age: Int)

fun prettyPrint(person: KotlinPerson?) {
    person?.let {
        println("Name: ${it.name}")
        println("Gender: ${it.gender}")
        println("Age: ${it.age}")
    } ?: println("No person found!")
}

fun main(args: Array<String>) {
    val budi = KotlinPerson("Budi", "Pria", 25)

    // cannot change name, because it is declared as val
    // budi.name = "Rudi"
    // can change age, because it is declared as var
    budi.age = 27

    prettyPrint(budi)
    prettyPrint(null)
}

Functional Programming Features

• Lambda
• Higher-order function

fun displayPeople(people: List<KotlinPerson>, label: String = "",
                  filter: (KotlinPerson) -> Boolean) {

    println(if (label == "") "Kotlin Person" else label)

    for (person in people) {
        if (filter(person)) {
            println(person)
        }
    }
}

fun main(args: Array<String>) {
    val people = listOf(...)

    displayPeople(people, "All Pria", {p -> p.gender == "Pria"})

    displayPeople(people, "Equal or above 25 years old") { it -> it.age >= 25 }

    displayPeople(people, "All Wanita above 20 years old") {
        it -> it.gender == "Wanita" && it.age > 20
    }
}

StdLib Collection Extension Functions

fun main(args: Array<String>) {
    val people = listOf(...)

    // print all person name and age
    people.forEach { println("Name: ${it.name}, age: ${it.age} years old") }
    // all people sorted by age (ascending)
    people.sortedBy { it.age }
    // all Pria
    people.filter { it.gender == "Pria" }
    // first wanita
    people.first { it.gender == "Wanita"}
    // oldest wanita
    people.filter {it.gender == "Wanita" }.maxBy { it.age }
    // group people by age
    people.groupBy { it.age }.maxBy { it.value.count() }
    // age 10 years from now
    people.map { it.copy(age = it.age + 10) }
    // partition people with age below and equal or above 25
    people.partition { it.age <= 25}
    // each gender count
    people.groupingBy { it.gender }.eachCount()
    // all people where age below average age
    val averageAge = people.map { it.age }.average()
    people.filter { it.age <= averageAge }
}

What else

• Delegation
• Sealed Class (Algebraic Data Type)
• Object
• Coroutine
• many more...

How to use Kotlin

• Using IDE
  • IntelliJ IDEA
  • Netbeans
  • Eclipse
• Using SDKMan (command line)

Learning Resources

• https://try.kotlinlang.org/
• Kotlin Koans (IDE version)
• Kotlin ID User Group:
  • Telegram: @kotlinid (https://t.me/KotlinID)
  • Facebook group: https://www.facebook.com/groups/395469687469099/
  • Github: https://github.com/KotlinID
• Kongkow IT Medan: @kongkowITMedan (https://t.me/KongkowITMedan)

Thank You!