Learn Kotlin in 30 Min

4 min readMar 1, 2022

1. Function

fun function() {
    print("")
}

fun functionWithParam(param: Int) {
    print(param)
}

fun functionWithReturn(): Int {
    return 100
}

fun functionWithParamReturn(param: Int): Int {
    return param + 100
}

fun shortFunction(param: Int): Int = param

2. var vs. val

fun varExample() {
    var variable = 1
    variable += 1
    variable = 10

    var variable2: Int
    variable2 = 10
    variable2 = 20
}

fun valExample() {
    val value = 1
    // value += 1 Error occur
    // value = 10 Error occur

    val value2: Int
    value2 = 10
    // value2 = 20 Error occur
}

3. String template

fun stringTemplate() {
    val name = "name"
    print("My name is " + name)
    print("My name is $name.")
    print("My name is ${name}.")
}

4. If statement

fun ifStatement() {
    val age = 19
    if (age < 19) {
        print("kids")
    } else {
        print("adult")
    }
}fun inlineIfStatement(age: Int) = if (age < 19) "kids" else "adult"

fun whenStatement() {
    var age = 19
    when (age) {
        in 0..18 -> "kids"
        in 19..120 -> "adult"
        else -> "hero"
    }
}

5. Array, List, ArrayList

// Array
fun array() {
    var array = arrayOf(1, 2, 3, 4)

    print(array[0])
    print(array.get(0))

    array[0] = 1

    // array.add(5) Error occur
}

// Immutable List
fun list() {
    var list = listOf(1, 2, 3, 4)

    print(list[0])
    print(list.get(0))
}

// Mutable list
fun mutableList() {
    var mutableList = mutableListOf(1, 2, 3, 4)
    mutableList[0]
    mutableList.get(0)
    mutableList.add(5)
    mutableList.removeAt(0)
    mutableList.remove(5) // returns true
    mutableList.remove(6) // returns false

    var arrayList = arrayListOf(1, 2, 3, 4)
    arrayList[0]
    arrayList.get(0)
    arrayList.add(5)
    arrayList.removeAt(0)
    arrayList.remove(5) // returns true
    arrayList.remove(6) // returns false
}

6. Loop

fun loop() {
    var alphabets = listOf("A", "B", "C", "D")
    for (alphabet in alphabets) {
        print(alphabet)
    }

    for ((index, alphabets) in alphabets.withIndex()) {
        print("${index}, ${alphabets}")
    }

    for (i in 1..10) {
        print(i)
    }

    var i = 0
    while (i < 10) {
        print(i)
        i++
    }
}

7. Non-nullable vs. Nullable

fun nullable() {
    var nonNullableString: String = "string"
    nonNullableString.uppercase()
    nonNullableString[0]

    // var nullableString: String = null Error Occur
    var nullableString: String? = null
    // nullableString.uppercase() Error
    // nullableString[0] Error

    // ?. operator
    nonNullableString?.uppercase() // returns "STRING"
    nonNullableString?.get(0) // returns "s"
    nullableString?.uppercase() // returns null
    nullableString?.get(0) // returns null

    // ?: operator (aka. Elvis operator)
    nonNullableString ?: "default" // returns "string"
    nonNullableString?.uppercase() ?: "default" // returns "STRING"
    nullableString ?: "default" // returns "default"
    nullableString?.uppercase() ?: "default" // returns default

    // Throws Index out of bound Error
    // nullableString?.get(0) ?: throw IndexOutOfBoundsException()

    // prints "The value is string"
    nonNullableString?.let { value ->
        print("The value is ${value}")
    }
    // does not print
    nullableString?.let { value ->
        print("The value is ${value}")
    }

    // value-parameter can be omitted
    nullableString?.let {
        print("The value is ${it}")
    }
}

8. Class

class Person(var name: String, var age: Int) {
    lateinit var hobby: String // recommended to use private

    init {
        print("Printed this when a class is instantiated")
    }

    constructor(
        name: String,
        age: Int,
        hobby: String
    ) : this(name, age) {
        this.hobby = hobby
    }

    fun nextYear() {
        age++
    }
}

fun createPerson() {
    var person = Person("chris", 20)
    print(person.age)
    // print(person.hobby) // Not Initialized error occur

    var person2 = Person("chris", 20, "debugging")
    print(person2.age)
    print(person2.hobby)
    person2.nextYear()

9. Lambda

fun lambdaExample() {
    var result: (name: String, age: Int) -> (String) =
        { name, age ->
            "My name is ${name}, and I'm ${age} years old"
        }

    // Inference type
    var result2 = { name: String, age: Int ->
        "My name is ${name}, and I'm ${age} years old"
    }

    var printPretty: String.() -> String = {
        "My name is ${this}"
    }
    "chris".printPretty() // returns "My name is chris"

    var printPretty2: String.(age: Int) -> String = {
        "My name is ${this}, and I'm ${it} years old"
    }
    "chris".printPretty2(20) // returns "My name is chris, and I'm 20 years old"

    var printPretty3: String.(age: Int, hobby: String) -> String =
        { age, hobby ->
            "My name is ${this}, and I'm ${age} years old, and hobby is ${hobby}"
        }
    "chris".printPretty3(20, "debugging")
    // returns "My name is chris, and I'm 20 years old, and hobby is debugging"

    fun invokeLambda(lambda: (Int) -> Boolean): Boolean {
        return lambda(1)
    }

    var result3 = invokeLambda {
        it > 10
    }

    fun invokeLambda2(
        score: Int,
        lambda: (Int) -> Boolean
    ): Boolean {
        return lambda(score)
    }

    var result4 = invokeLambda2(1) {
        it > 10
    }
}

10. Data class

fun dataClassExample() {
    data class PersonData(
        var name: String,
        var age: Int
    )

    var person = PersonData("chris", 20)
    person.toString() // returns "PersonData(name=chris, age=20)"
    person.age++
    person.name = "Anonymous"
}

11. Companion object

class PersonCompanion(var name: String, var age: Int) {
    companion object {
        private const val MAX_AGE = 120
        fun create(
            name: String,
            age: Int = MAX_AGE
        ): Person {
            return Person(name, age)
        }
    }
}

fun companionExample() {
    var person =
        PersonCompanion.Companion.create("chris", 20)
    var person2 = PersonCompanion.create("chris", 20)
    var person3 = PersonCompanion.create("chris")
    person3.age // returns 120
}

12. Object

data class Ticket(val price: Int)

object TicketFactory {
    val tickets = arrayListOf<Ticket>()
    fun addTicket(price: Int) {
        tickets.add(Ticket(price))
    }
}

fun objectExample() {
    TicketFactory.addTicket(1)
    TicketFactory.addTicket(1)
    TicketFactory.addTicket(1)
    TicketFactory.tickets.size // returns 3
}