Kotlin Programming Language - Part 1

Kotlin language website is at https://kotlinlang.org.

All the codes here can be copied and run on Kotlin online editor.

Basics

You do not need ; to break statements.
Comments are similar to Java or C#, /* This is comment */ for multi line comments and // for single line comment.
Unlike Java, you do not need to match your file name to your class name.
Like JavaScript, you can create functions outside classes. So there is no need to stuff your functions as static members of classes like what you do in C# or Java.
Kotlin has string templates, which is awesome. e.g. "$firstName $lastName" for simple variable name or "${person.name} is ${1 * 2}" for any expressions. You can still do the string concatenation if you like e.g. "hello " + "world", but that means being stupid.
It has no tuple although Kotlin's data classes is an option to use in place of tuple.

Variables

There are two keywords for variable declaration, var and val.
Use var when the variable value is to be modified and val where the variable value will not change after first assigned.
This val is similar to readonly keyword in C# or final keyword in Java.
val variable must be initialized at declaration.
Unlike Java or C#, you declare the type of a variable after the name, e.g. var firstName : String
Number primitive types are as follows: Double, Float, Long, Int, Short, Byte. There is no automatic conversion between types. You have to explicitly convert them.
More primitive types: Char, String, Boolean.
All variable declarations in Kotlin must be initialized.
The keyword void common in Java or C# is called Unit in Kotlin.

Null

In Kotlin you have to decide whether a variable can be assigned null or not. This applies to both primitives or class types. A nullable variable is marked by assigning ? after the type, e.g. var firstName: String?

You can assign a value from not-nullable to nullable type without any problem.

fun main(args : Array<String>) { 
    val firstName : String = "Adam"
    val name : String? = firstName 
    print("$name") 
}

The other way around though requires that you declare that this nullable variable does not contain null at the point of assignment with !! operator (which pretty much declares : "I am sure this nullable variable is not null at this point")

fun main(args : Array<String>) { 
    val name : String? = "Adam" 
    val firstName : String = name!! 
    print("$firstName") 
}

Type inference

Kotlin is pretty smart about inferring what type a variable is, whether it is primitives or class. This is similar to the var keyword in C#.

fun main(args : Array<String>) { 
    val firstName = "Adam" 
    val middle = 'c' 
    val lastName = "Brown" 
    val age = 15 
    println("$firstName $middle $lastNameis $age") 
}

You will encounter in further examples of more capabilities of Kotlin's type inference.

Functions

We are going to spend a considerable time in discussing function because it has many different forms and subtleties. Here is a list of facilities that Kotlin provides for functions

Single expression function.
Optional parameter.
Positional argument and named argument.
Variable argument.
Single expression function.
Function type.
Function literals.
Callable references.
Extension functions.
Infix function call.
Local function.
Closure.
Generic function.
Operator overloading.

Below is an example of functions

fun main(args : Array<String>) { 
    greet(englishGreeting()) 
    greet(italianGreeting())
} 

fun greet(msg : String){ 
    println(msg) 
} 
    
fun englishGreeting() : String = "Hello world" 
    
fun italianGreeting() : String{ 
    return "bon giorno" 
}

Functions can exists on their own.
It is marked by fun keyword.
If a function returns value, you declare it after the function name.
englishGreeting() is a single expression function.
A void function such as greet() returns Unit type but you are not required to declare it.
All parameters in a Kotlin function are read only. You are actually not allowed to mark it with either val or var keyword.

Single expression function

This is a shorthand form in defining a function when you only have a single expression to be executed.

fun main(args : Array<String>) {
   val res = add(1,1)
   show("$res")
}

fun add(a : Int, b : Int) = a + b
fun show(msg : String) = println("$msg")

As you can see above, in a single expression function, the function return type is inferred. You can declare the return type if you want to such as below.

fun main(args : Array<String>) {
   val res = add(1,1)
   show("$res")
}

fun add(a : Int, b : Int) : Int = a + b
fun show(msg : String) : Unit = println("$msg")

Optional parameters

Kotlin allows you to assign default values for your parameters, making them optional.

fun main(args : Array<String>) {
  show()
  show("Good morning")
}

fun show (msg : String = "Hello World"){
    println("$msg") 
}

If you are mixing mandatory parameter and optional parameter, the mandatory parameters must be listed first.

Arguments

fun main(args : Array<String>) { 
    greet(firstName = "Frasensco", lastName = "Merini") 
    greet(lastName = "John", firstName = "Stamos") 
    greet("Borat", "Ismail") 
    greet("Crystal", lastName = "Stamos") 
    call("Xavier", age = 20, location = "Portugal") 
} 
    
fun greet(firstName : String, lastName : String){
    println("Good morning $firstName $lastName") 
} 
    
fun call(name : String, location : String, age : Int){ 
    println("Call $name who lives at $location and he is $age old") 
}

Kotlin allows positional argument, named argument and the mix between the two. When you mix named and positional argument, you must start with positional argument.

Variable arguments

Use the keyword vararg.

fun main(args : Array<String>) {
  names("John", "Adam", "Joy")
}

fun names(vararg  names : String){
  for(n in names){
    println("$n")
  }
}

If vararg parameter is not the last parameter, named argument must be used to supply the function argument.

fun main(args : Array<String>) {
  names("John", "Adam", "Joy", age = 20)
}

fun names(vararg  names : String, age : Int){
  for(n in names){
    println("$n is $age old")
  }
}

vararg produces array of argument

fun main(args : Array<String>) {
  names("John", "Adam", "Joy")
}

fun names(vararg  names : String){
  println("Argument length is ${names.size}")
  println("${names[0]}")
  val nns : Array<String> = names
  println("${nns[1]}")
}

Using array to supply variable arguments

Use the * operator in front of the array variable

fun main(args : Array<String>) {
  val n = array("John", "Adam", "Joy")
  names(*n)
}

fun names(vararg  names : String){
  println("Argument length is ${names.size}")
  println("${names[0]}")
  val nns : Array<String> = names
  println("${nns[1]}")
}

Passing one varargs argument to another

fun main(args : Array<String>) {
  val n = array("John", "Adam", "Joy")
  fugitives(*n)
}

fun fugitives(vararg escapees: String){
  names(*escapees) 
}

fun names(vararg  names : String){
  println("Argument length is ${names.size}")
  println("${names[0]}")
  val nns : Array<String> = names
  println("${nns[1]}")
}

Since vararg creates an array, you simply use the * operator to pass one vararg to another.

Function Types and Function Literals

A function type is a type consisted of a function signature and function return type that are separated by -> operator. In its simplest form, it looks as follows:

() -> Unit

Above is a type for a function that takes no parameter and returns a Unit (void in other language parlance)

() -> String

Above is a type for a function that takes no parameter and return a String

(String) -> Unit

Above is a type for a function that takes a string and returns nothing.

(String, Float) -> Unit

Above is a type for a function that takes two parameters (String and Float) and returns nothing.

Because a function type is just a type, it means that you can assign it to a variable, you can pass it as a function argument and you can return it from a function.

Different ways to write function literals

val m = { (x : String) -> println("$x") } 
val n : (String) -> Unit = { x -> println("$x") } 
val o : (String) -> Unit = { (x : String) -> println("$x") } 

fun main(args : Array<String>) { 
    m("good morning")
    n("good morning") 
    o("good morning") 
}

Above code is an example of function literals. All m, n and o represent the same function.

Below is a function that returns a function type

fun main(args : Array<String>) { 
    val greet = greetingFrom("Cairo, Egypt") 
    greet("Brown") 
} 

fun greetingFrom(location : String) : (String) -> Unit{ 
    return { name -> println ("Hello $name from $location")}
}

Below shows that you can specify a function type as an argument and supply it with function literal with corresponding function signature and return type.

fun evening(): String = "Good Evening" 
fun main(args : Array<String>){ 
    say({ "good morning"}) 
    say { val msg = "good afternoon" msg } 
    say({evening()})
} 

fun say(greet : () -> String){ 
    println("${greet()}") 
}

Callable references

How about if you already have a function that you want to pass as a parameter? You prefix the function name with '::'

fun main(args : Array<String>) {
 calcAndShow(10,10, ::add) //20
 calcAndShow(10,10, ::multiply) /100
 calcAndShow(10,19, { x, y -> x - y }) //-9
}

fun calcAndShow(a : Int, b : Int,  func : (a : Int, b : Int) -> Int){
 val result = func (a, b)
 println("$result")
}

fun add(a : Int, b : Int) : Int = a + b
fun multiply (a : Int, b : Int) : Int = a * b

Function expansion

When you call a function which has a function type as the last argument, you can expand it by { }

fun main(args : Array<String>) {
    val a =  calculate(1) { x -> 10 + x } //11
    val b = calculate(2) { x -> 20 * x } //40

    println("a = $a, b = $b")
}

fun calculate(a : Int,  calc : (Int) -> Int) : Int{
    return calc(a)
}

Closure

Kotlin support Closure as highlighted by the example below

fun main(args : Array<String>) {
    val total = add(1)(2)
    println("Total value is $total")
}

fun add(a : Int) : (Int) -> Int{
    return { x -> a + x }
}

Local function

You can declare a function inside a function. It will have access to the local variable at the parent function.

fun main(args : Array<String>){ 
    accumulate() 
} 

fun accumulate(){
    var i = 0 

    fun add(){ 
        i++ 
    } 

    for (i in 1..10){
        add() 
    } 

    println("i is now $i") 
}

//It prints "i is now 10"

Extension function

Extension function enables a function to be accessed from the type function. It works in the form of type.function Inside the function, the keyword this refers to the instance.

For example:

fun Int.show(){
    println("This number is $this")
}
    
fun main(args : Array<String>){
    3.show()
}

Above example shows how the Int built in type has been enriched by show extension function. Notice the use of this keyword that refers to the 3 number.

Notice You can extend a function on a nullable type and it will be accessible for both nullable and non nullable type. The reverse though does not apply.

fun Int?.show(){
    println("This number is $this")
}
 
fun Int.show2(){
    println("This number is $this")
}

fun main(args : Array<String>){
    var number : Int? = null
    number.show()
    5.show()
    //number.show2() will not compile
}

Extension function expressed in function literals

val show = { Int.() -> println("This is number $this") }
val add = { Int.(number : Int) : Int -> 
    val now = this + number
    now
}

fun main(args : Array<String>){
    5.add(10).show()
}

Both show and add extension functions are expressed in literal format. Please notice that add function returns an Int.

Extension function in infix form

fun main(args : Array<String>) {
   val res = 1 add 2
   println("$res")
}

fun Int.add (one : Int) : Int = this + one

If the extension function only takes one argument, you can call them in infix form (you drop the . between the type and the function). So instead of 1.add(2), you can call it in the form of 1 add 2. This makes certain constructs looks natural (more like an operator than a function call) and especially useful in construction DSL in Kotlin.

Variable arguments and function type argument

vararg parameter can also be naturally combined with a function type parameter.

fun main(args : Array<String>) {
  names("John", "Adam", "Joy"){ 
    name  -> println ("$name")
  }
}

fun names(vararg  names : String, print : (String) -> Unit){
  for(n in names){
   print(n)
  }
}

above code can also be expressed in this matter (using named argument)

fun main(args : Array<String>) {
  names("John", "Adam", "Joy", print = {name  -> println ("$name")})
}

fun names(vararg  names : String, print : (String) -> Unit){
  for(n in names){
   print(n)
  }
}

Control Structures

If statement

Kotlin if statement should look familiar with other language

fun main(args : Array<String>) {
  val total = 10
  
  if (total > 5) {
      println("$total is greater than 5") 
  } else if (total > 10) {
      println("$total is greater than 10")
  } else {
      println("$total is less than 6")
  }
}

jiffle/kotlin-language-cheatsheet-part1.md