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