Kotlin

I made these notes using the Developer Android Course

We can use the Kotlin playground here to practice.

Hello World in Kotlin

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

Style guide

Function names should be in camel case and should be verbs or verb phrases.
Each statement should be on its own line.
The opening curly brace should appear at the end of the line where the function begins.
There should be a space before the opening curly brace.
The function body should be indented in by 4 spaces. Do not use a tab character to indent your code, type in 4 spaces.
The closing curly brace is on its own line after the last line of code in the function body. The closing brace should line up with the fun keyword at the beginning of the function.
See the full Kotlin style guide here

Variables

Data Types

Source Developer Android

Declaration

Source Developer Android

Variables that can be reassigned use the var keyword.

var x = 5 // `Int` type is inferred

Constants have to be declared using val
val keyword - Use when you expect the variable value will not change.
var keyword - Use when you expect the variable value can change.
Constants in Kotlin here

Comments

// this is a comment
/* 
* This is a multiple
* lines
* comment
*/

Functions

Function declaration

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Example

fun birthdayGreeting(): String {
    println("Happy Birthday, Rover!")
    println("You are now 5 years old!")
}

Declaring a function with a return type

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Example

fun birthdayGreeting(): String {
    val nameGreeting = "Happy Birthday, Rover!"
    val ageGreeting = "You are now 5 years old!"
    return "$nameGreeting\n$ageGreeting"
}
// call it
birthdayGreeting()

The Unit type

By default, if you don't specify a return type, the default return type is Unit. Unit means the function doesn't return a value. Unit is equivalent to void return types in other languages

Store a function in a var

fun main() {
    val greeting = birthdayGreeting()
}

Function with parameter(s)

Declaration

Source Developer Android

Example with one parameter

fun birthdayGreeting(name: String): String {
    val nameGreeting = "Happy Birthday, $name!"
    val ageGreeting = "You are now 5 years old!"
    return "$nameGreeting\n$ageGreeting"
}
// call it and print it
println(birthdayGreeting("Rex"))

Example with multiple parameters

fun birthdayGreeting(name: String, age: Int): String {
    val nameGreeting = "Happy Birthday, $name!"
    val ageGreeting = "You are now $age years old!"
    return "$nameGreeting\n$ageGreeting"
}
// call it and print it
println(birthdayGreeting("Rex", 2))
// or
println(birthdayGreeting(name = "Rex", age = 2))
// or
println(birthdayGreeting(age = 2, name = "Rex"))

⚠️ Warning: Unlike in some languages, such as Java, where a function can change the value passed into a parameter, parameters in Kotlin are immutable. You cannot reassign the value of a parameter from within the function body.

Function with default arguments

Example

fun birthdayGreeting(name: String = "Rover", age: Int): String {
    return "Happy Birthday, $name! You are now $age years old!"
}
// multiple ways to print it and call it
println(birthdayGreeting(age = 5))
println(birthdayGreeting("Rex", 2))
println(birthdayGreeting(age = 5))
println(birthdayGreeting(age = 2))

Resources

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Last updated 2 years ago