kotlin-cheatsheet.md

Kotlin Cheatsheet for Developers

Table of Contents

• Basic
• Variables
• Print
• Strings
• Booleans
• Floats
• If Else
• While Loop
• Classes
• Functions
• Collection

Basic

• You do not need ; to break statements since it is an interpreted language
• The keyword void common in Java or C# is called Unit in Kotlin
• Unlike Java or C#, you declare the type of a variable after the name, e.g. var firstName : String

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Variables

• There are two keywords for variable declaration, var and val
• var : var is like a general variable and can be assigned multiple times and is known as the mutable variable in Kotlin
• val : val is a constant variable and can not be assigned multiple times and can be Initialized only a single time and is known as the immutable variable in Kotlin

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Print

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

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Strings

fun main(args : Array<String>) {
    val name = "Adam"
    val helloName = "Hello, $name"
    val upercaseName = "Hello".uppercase()
}

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Booleans

val trueBoolean = true
val falseBoolean = false
val andCondition = trueBoolean && falseBoolean
val orCondition = trueBoolean || falseBoolean

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Floats

val intNumber = 10
val doubleNumber = 10.0
val longNumbe = 10L
val floatNum = 10.0F

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If Else

Kotlin supports the usual logical conditions from mathematics:

• Less than: a < b
• Less than or equal to: a <= b
• Greater than: a > b
• Greater than or equal to: a >= b
• Equal to a == b
• Not Equal to: a != b

Kotlin has the following conditionals:

• Use if to specify a block of code to be executed, if a specified condition is true
• Use else to specify a block of code to be executed, if the same condition is false
• Use else if to specify a new condition to test, if the first condition is false
• Use when to specify many alternative blocks of code to be executed

val time = 20
if (time < 18) {
  println("Hello")
} else {
  println("Bye")
}
//output - "Bye"

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While Loop

The while loop loops through a block of code as long as a specified condition is true

var i = 0
while (i < 5) {
  println(i)
  i++
} 
//Will print i from 0 t0 4 (i<5)

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Classes

Classes in Kotlin are declared using the keyword class class Student{ /*...*/ }

Primary Constructor

class Student(val name: String, val age: Int)
val student1 = Student("Peter", 11)

Secondary Constructor

class Student(val name: String) {
    private var age: Int? = null
    constructor(name: String, age: Int) : this(name) {
        this.age = age
    }
}
// Above can be replaced with default params
class Student(val name: String, val age: Int? = null)

Enum Class

enum class Color(val rgb: Int) {
    RED(0xFF0000),
    GREEN(0x00FF00),
    BLUE(0x0000FF)
}

Data Class

data class Student(val name: String, val age: Int)

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Functions

Kotlin functions are declared using the fun keyword

fun printStudentName() {
    print("Richa")
}

Parameters & Return Types

fun printStudentName(student: Student) {
    print(student.name)
}

Default Parameters

fun getStudentName(student: Student, intro: String = "Hello,") {
    return "$intro ${student.name}"
}

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Collections

A collection is a group of related items

Collections	Syntax
List	val numbers = listOf("one", "two", "three", "four")
Set	val numbers = setOf(1, 2, 3, 4)
Map	val numbersMap = mapOf("key1" to 1, "key2" to 2, "key3" to 3, "key4" to 1)

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