- Basics
- Variables
-
var - integer, float, string, char, boolean
- predefined type ->
var someInt : Int = 5
-
- Constants
-
val
-
- Comments
- Array (limited capacity)
- List (no capacity limits)
- Set (unique items)
- Map (key value)
- Operators
- String Templates
- Conditionals
- If statement
- Switch
- Loops
- For
- While
- Functions
- Range
- 'in' operator
- Collections
- filter
- Package
- Class
- 'is' operator
- Inheritance
- Variables
| Name | Description |
|---|---|
| kotlinlang.org |
| Name | Description |
|---|---|
| Kotlin Course - Tutorial for Beginners |
- [Comments][#Comments]
- Varibles
- Input
- Array
- ArrayList
- LinkedList
- Set
- Hashmap
- Operators
- Strings
- Conditionals
- Collections
- Classes
- Calendar
- Package
- Constructor
- Conversion
fun main() {
// this is a comment
println("Hello world!")
}
// This is a single line comment in Kotlin
/*
This is a
multiline comment
*/
println("Print with a new line at the end")
print("Print without a new line at the end")
val a: Int = 1
val b = 2
val c: Int
c = 3
d = null
/* When using `val` you can't change the variable later.
If you need to change it, use `var` */
var x = 3
x += 1
var y:String? // Without setting a value
// Increment count variable value by one
count += 1
// Decrement count variable value by two
count -= 2
val name:String = readLine()!!.toString()
val age:Int = readLine()!!.toInt()
val grade = readLine()!!.toDouble()
var someArray = arrayOfNulls<String>(4) // An array with the length of 4
var anotherArray:Array<String> = Array(4){""}
someArray[0] = "Mario"
someArray[1] = "Luigi"
var anotherArray = intArrayOf(1, 2, 3)
println(anotherArray.size)
anotherArray.set(1, 100) // first param is index, second param is the new value in this index
anotherArray.get(2) // result is 3
println(anotherArray[2]) // same as above
var listOfStrings:Array<String> = Array(3){""}
for(i in 0..2){
listofStrings[i] = readLine()!!.toString()
}
for(i in 0..4){
println("String $i: ${listOfStrings[i]}")
}
The problem with Array = limited capacity. Use instead lists.
val myList = ArrayList<String>()
myList.add("Mario")
myList.add("Luigi") // ["Mario", "Luigi"]
myList.add(1, "Peach") // ["Mario", "Peach", "Luigi"]
// Array Size
myList!!.size
val myList = LinkedList<String>()
myList.add("Mario")
myList.add("Luigi") // ["Mario", "Luigi"]
myList.add(1, "Peach") // ["Mario", "Peach", "Luigi"]
Unique values
val mySet = HashSet<String>()
mySet.add("item") // ["item"]
mySet.add("item") // ["item"]
mySet.size // 1
Store key value items. Also called in other languages "Dictionary".
val myMap = HashMap<String, String>()
myMap.put("one", "1")
myMap.put("two", "2")
println(myMap["one"]) // "1"
// The following will have the same effect as the code above
myMap["one"] = "1"
myMap["two"] = "2"
println("${myMap["one"]}") // "1"
// Print each key,value in the HashMap
for (key in myMap.keys)
println("$key: ${myMap[key]}")
var x = 1
x = x + 1 // x -> 2
x++ // x -> 3
x-- // x -> 2
var y = 0
println(x > y) // true
println(y > x) // false
var message = "Hello World"
var anotherMessage = "Have a nice day"
println("The message is: $message")
val y = "The message is: $message"
val newMessage = message + anotherMessage
val newMessage = "$message,$anotherMessage"
println(newMessage[0]) // will print "H"
println(newMessage[1]) // will print "e"
println(newMessage.toLowerCase())
println(newMessage.toUpperCase())
println(newMessage.trim())
val tokens = message.split(" ")
var statement = (10>2) // statement = true
println(10>2) // ture
println(10!=2) // true
println(10==5) // false
val num = 5
println(num>=2 && num <=7) // true
println(num==2 || num <=3) // false
println(!(num==10)) // true
println(num in 1..10)) // true
val num = 10
if (num > 1){
println("num is greater than 1")
}
if (num in 1..10){
printlin("It's somewhere between 1 to 10")
}
else if(num in 10..20){
printlin("It's somewhere between 10 to 20")
}
else {
print("It's not between 1 to 20")
}
fun getMax(a: Int, b: Int): Int {
if (a > b) {
return a
} else if (b > a) {
return b
} else {
print("equal")
return a
}
}
Or as an expression:
fun getMax(a: Int, b: Int) = if (a > b) a else b
val x = true
val y = 2
val z = if (x==true && y>=1) 1 else 0 // 1
Return null if x is not an Integer
fun get(x: String): Int? {
// ...
}
if (some_string!!.contains("some_word")){
print("It contains it")
}
// if a is true
if (a) {
...
}// if a is null
if (a == null) {
...
}"when" is the equivalent of switch operator in Kotlin
val number = readLine()!!.toInt()
when (num) {
1 -> number = "1"
2 -> number = "2"
3 -> number "3"
else -> number = 0
}
fun get(obj: Any): String =
when (obj) {
1 -> "One"
"2" -> "Two"
!is String -> "Not a string"
else -> "Unknown"
}
val x = readLine()!!.toInt()
when(x){
1 -> {
print("You got 1")
print("Exciting")
}
5 -> {
print("You got 5!")
}
else -> {
print("Not 1, neither 5")
}
val x = 1
val y = when(x){
1 -> true
2 -> false
}
fun test() {
val x = 5 * 4
println(x)
}
- Sum of two numbers
fun sum(a: Int, b: Int): Int {
return a + b
}
OR
fun sum(a: Int, b: Int) = a + b
val someNumbers = intArrayOf(1, 2, 3)
for (number in someNumbers) {
val x = number * 2
println(x)
}
val someNumbers = intArrayOf(1, 2, 3)
for (i in someNumbers.indices) {
val y = someNumbers[i] * 2
println(y)
}
for (x in 0..5) {
val y = x * 2
println(y)
}
for (x in 1..100 step 2){
printlin("Number: $x")
}
for (x in 100 downTo 0 step 2){
printlin("Number: $x")
}
val items = listOf("x", "y", "z")
for (item in items) {
println(item)
}
var x = 0
while (x < 10) {
val y = x * 2
println(y)
x++
}
val items = listOf("x", "y", "z")
var i = 0
while (i < items.size) {
println("item is ${items[i]}")
i++
}
i = 1
do{
printlin("Number: $i")
i += 3
}while (i<=20)
for (num in 3..6){
if(num == 4){
continue // if num is 4, it will not be printed and the loop will continue to the next number
}
println(num)
}
for (num in 3..6){
if(num == 4){
break // if num is 4, the loop will break and will not continue to the next number
}
println(num)
}
loop@ for (num in 3..6){
println("Outer loop number: $num")
for (innerNum in 7..10){
println("Inner loop number: $innerNum")
if(innerNum==8){
break@loop // break outer loop
}
}
}
- Check if number in range
if (5 in 1..10) {
println("in range")
}
- Check if out of range
if (11 !in 1..10) {
println("11 is out of range")
}
- Iterating over a range with 2 steps
for (x in 1..10 step 2) {
print(x)
}- Iterating over a collection
for (item in items) {
println(item)
}
- Check if a collection contains an item
when {
"witcher" in items -> println("good game")
"smb" in items -> println("smb is the best")
}
- Filter and map collections
val games = listOf("smb", "starcraft", "witcher")
games
.filter { it.startsWith("g") }
.sortedBy { it }
.map { it.toUpperCase() }
.forEach { println(it) }
- Create a class
class Person(var name:String, var age:Int) {
}- Create an instance
val person = Person("Arie", 52)
prrintln(person.name)- Class with a method
class Person(var name:String, var surname:String, var age:Int) {
init {
println("Created an object class")
}
fun getFullName():String{
return "$this.name $this.surname"
}
}
fun main() {
val person = Person("John Lon", 29)
println(person.getFullName())
}import java.util.*
Calendar.getInstnace().get(Calendar.YEAR) // Get current year
Calendar.getInstnace().get(Calendar.MONTH) // Get current month
Calendar.getInstnace().get(Calendar.DAY) // Get current day// To create one in the IDE
// Click on src directory -> New -> Package -> Name: com.box.a
// Next you can create a class
// Click on the package -> New -> Class -> Name: Box
// In the class write the following
package com.box.a
class Box {
init {
println("Created an object class")
}
}
fun main () {
val box = Box()
}class Person(){
var name:String?=null
var surname:String?=null
var age:Int?=null
constructor(name:String, surname:String, age:Int):this(){
this.name = name
this.surname = surname
this.age = age
}
constructor(name:String, surname:String):this(){
this.name = name
this.surname = surname
}
}// String to Integer (variable)
x.toString().toInt()
// String to Integer (String)
Integer.parse("some text")Use pull requests to contribute to this project.
