Kotlin Quick Guide for Android Basics in Kotlin

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This Kotlin Quick Guide summarizes the topics covered in the Android Basics in Kotlin course in the form of code snippets.

Learn more

  • See the Kotlin Language Documentation for full reference.
  • See the Kotlin Koans for more snippets to practice with. Note: This brings you to an external kotlinlang.org site that is developed by JetBrains.

Unit 1

Kotlin programs

The smallest main() program for printing text

fun main() {
    println("Hello, world!")
}

Printing a line of text

println("This is the text to print!")

Inline comments

// This is a comment line.
// This is another comment.

Variables

// Assign once, cannot change.
val age = "5"
val name = "Rover"

// Assign and change as needed.
var roll = 6
var rolledValue: Int = 4

Printing variables with string templates

println("You are already ${age}!")
println("You are already ${age} days old, ${name}!")

Data types

Int       // Whole positive and negative numbers and zero
String    // Piece of text between quotes
IntRange  // Setting a range such as 1..6
Boolean   // Can only be true or false

Function without arguments

// Define the function.
fun printHello () {
    println ("Hello Kotlin")
}

// Call the function.
printHello()

Function with arguments

fun printBorder(border: String, timesToRepeat: Int) {
    repeat(timesToRepeat) {
        print(border)
    }
    println()
}

Function that returns a value

fun roll(): Int {
     val randomNumber = (1..6).random()
     return randomNumber
}

Basic operators

*     // multiplication
/     // division
+     // addition
-     // subtraction
=     // assignment

Logic operators

>    // greater than
<    // less than
==   // equal
>=   // greater or equal
<=   // less or equal
!=   // not equal

Creating random numbers

val randomNumber = diceRange.random()

fun roll() {
     val anotherRandomNumber = (1..6).random()
     println(randomNumber)
}

Controlling program flow

Repeating an action with repeat()

fun printBorder() {
    repeat(23) {
        print("=")
    }
}

Nesting repeat() loops

fun printCakeBottom(age: Int, layers: Int) {
    repeat(layers) {
        repeat(age + 2) {
            print("@")
        }
        println()
    }
}

Conditional statements with if/else

fun main() {
   val num = 4
   if (num > 4) {
       println("The variable is greater than 4")
   } else if (num == 4) {
       println("The variable is equal to 4")
   } else {
       println("The variable is less than 4")
   }
}

Conditional statements with when

when (rollResult) {
    luckyNumber -> println("You won!")
    1 -> println("So sorry! You rolled a 1. Try again!")
    2 -> println("Sadly, you rolled a 2. Try again!")
    3 -> println("Unfortunately, you rolled a 3. Try again!")
    4 -> println("No luck! You rolled a 4. Try again!")
    5 -> println("Don't cry! You rolled a 5. Try again!")
    6 -> println("Apologies! you rolled a 6. Try again!")
}

Assigning the result of a when statement to a variable

// Determine which drawable resource ID to use based on the dice roll.
val drawableResource = when (diceRoll) {
    1 -> R.drawable.dice_1
    2 -> R.drawable.dice_2
    3 -> R.drawable.dice_3
    4 -> R.drawable.dice_4
    5 -> R.drawable.dice_5
    else -> R.drawable.dice_6
}

Classes

Simple class with property and method

class Dice {
    var sides = 6

    fun roll() {
        val randomNumber = (1..6).random()
        println(randomNumber)
    }
}

Class with parameter

class Dice (val numSides: Int) {

    fun roll(): Int {
        val randomNumber = (1..numSides).random()
        return randomNumber
    }
}

Creating instances

val myFirstDice = Dice(6)

Unit 2

Classes

Abstract class

abstract class Dwelling() {
}

Abstract class with an abstract property

abstract class Dwelling() {
    abstract val buildingMaterial: String
}

Abstract class with an abstract method

abstract class Dwelling() {
    abstract fun floorArea(): Double
}

Mark a class open so it can be subclassed

open class RoundHut(residents: Int) {
}

Create a subclass by extending a parent class

class SquareCabin : Dwelling()

Override a function from a superclass

override fun floorArea(): Double {
}

Call the superclass implementation of a function

override fun floorArea(): Double {
    return super.floorArea() * floors
}

Lists

Define a read-only list

val numbers = listOf(1, 2, 3, 4, 5, 6)

Get the size of the list

numbers.size

Get the first item of a list

numbers[0]

Get a copy of a list in reverse order

listOf("red", "blue", "green").reversed()

Define a mutable list of strings

val entrees = mutableListOf<String>()

Add an item to a mutable list

entrees.add("spaghetti")

Modify an item in a mutable list

entrees[0] = "lasagna"

Remove an item from a mutable list

entrees.remove("lasagna")

Loops

Use for loop to iterate over items in a list

for (element in myList) {
    // Perform an operation with each item
    println(element)
}

Use while loop to iterate over items in a list

var index = 0
while (index < myList.size) {
    println(myList[index])
    index++
}

Strings

Number of characters in a String

val name = "Android"
println(name.length)

Use a variable in a String template

val number = 10
println("$number people")

Use an expression in a String template

val number = 10
val groups = 5
println("${number * groups} people")

Miscellaneous

Augmented assignments

a += b   // a = a + b
a -= b   // a = a - b
a *= b   // a = a * b
a /= b   // a = a / b

Use with to simplify access to an object

with(squareCabin) {
    println("Capacity: ${capacity}")
    println("Material: ${buildingMaterial}")
    println("Has room? ${hasRoom()}")
}

Import from the Kotlin math library

import kotlin.math.PI

Use fully qualified name for a constant in the Kotlin math library (no import statement needed)

kotlin.math.PI * radius * radius

Chain calls together (for accessing properties and calling methods)

val stringInTextField = binding.costOfService.text.toString()

Variable number of function arguments

fun addToppings(vararg val toppings: String)

Package declaration

package com.example.affirmations.model

Unit 3

Sets

Create a set from a list

val numbers = listOf(0, 3, 8, 4, 0, 5, 5, 8, 9, 2)
val setOfNumbers = numbers.toSet()

Define a set

val set1 = setOf(1,2,3)
val set2 = mutableSetOf(3, 4, 5)

Set operations

set1.intersect(set2) // 3
set1.union(set2) // 1, 2, 3, 4, 5

Maps

Define a mutable map

val peopleAges = mutableMapOf<String, Int>(
    "Fred" to 30,
    "Ann" to 23
)

Set a value in a mutable map

peopleAges.put("Barbara", 42)
peopleAges["Joe"] = 51

Collection Operations

Iterate over a collection

peopleAges.forEach { print("${it.key} is ${it.value}, ") }
// Fred is 31, Ann is 23, Barbara is 42, Joe is 51,

Transform each item in a collection

println(peopleAges.map { "${it.key} is ${it.value}" }.joinToString(", ") )
// Fred is 31, Ann is 23, Barbara is 42, Joe is 51

Filter the items in a collection

val filteredNames = peopleAges.filter { it.key.length < 4 }
println(filteredNames)
// {Ann=23, Joe=51}

Other collection operations

val words = listOf("about", "acute", "balloon", "best", "brief", "class")
val filteredWords = words.filter { it.startsWith("b", ignoreCase = true) }
    .shuffled() // [brief, balloon, best]
    .take(2) // [brief, balloon]
    .sorted() // [balloon, brief]

Scope functions

let

arguments?.let {
    letterId = it.getString(LETTER).toString()
}

apply

binding?.apply {
    ...
    flavorFragment = this@FlavorFragment
}

Miscellaneous

Backing property

private var _currentScrambledWord = "test"
val currentScrambledWord: String
    get() = _currentScrambledWord

Safe calls

val letterId = intent?.extras?.getString("letter").toString()

Lambda functions

val triple: (Int) -> Int = { a: Int -> a * 3 }
println(triple(5))

Companion objects


class DetailActivity: AppCompatActivity() {

    ...

    companion object {
        const val LETTER = "letter"
    }

    ...

}

// accessible outside DetailActivity
DetailActivity.LETTER

Property delegation

// syntax
var <property-name> : <property-type> by <delegate-class>()
// example
private val viewModel: GameViewModel by viewModels()

Late initialization

private var wordsList: MutableList<String> = mutableListOf() // has a value at initialization
private lateinit var currentWord: String // needs to be assigned after initialization

Elvis operator

var quantity : Int? = null
quantity ?: 0 // 0
quantity = 4
quantity ?: 0 // 4

Unit 4

Coroutines

Declare a suspend function

suspend fun getValue(): Double {
    // long running work or calls to other suspend functions
}

Run a suspend function in the GlobalScope

GlobalScope.launch {
    val output = getValue()
}

Call suspend function from another suspend function.

suspend fun processValue() {
    val value = getValue()
    // modify the value
}

Access a coroutine Job

val job: Job = GlobalScope.launch {
    val output = getValue()
}

Cancel a coroutine Job

job.cancel()

Run a suspend function and block the current thread until the function completes

runBlocking {
    val output = getValue()
}

Use async to make a suspend function deferable

runBlocking {
    val output = await { getValue() }

    println("Output is ${output.await()}")
}

Miscellaneous

Declare an object

object DataProviderManager {

}

Catch an exception

try {
    // code that may throw an error
} catch (exception: Exception) {
    // handle the thrown exception
}

Create an enum class

enum class Direction {
    NORTH, SOUTH, WEST, EAST
}

Access an enum class value

val direction = Direction.NORTH

Check enum values

when (direction) {
    Directon.NORTH -> {

    }
    Direction.SOUTH -> {

    }
    Direction.WEST -> {

    }
    Direction.EAST -> {

    }
}