MostafaNasiri

fun main() {
    var city: String = "New York"

    // You can omit the type if you declare and initialize a variable at the same time
    var city = "New York"

    // But you can't do something like this:
    var city // Error: Either specify the variable type or initialize it
    city = "New York"

MostafaNasiri

fun main() {
    val anInt = 2
    val aLong: Long = anInt // Error: Type mismatch
    aLong = anInt.toLong() // This is correct

    val stringNum = "123"
    val num = stringNum.toInt()
}

MostafaNasiri

interface Processor<T> {
    fun process(): T
}

class ExampleProcessor : Processor<Unit> {
    override fun process() {
        // Do something
    }
}

MostafaNasiri

fun main() {
    var firstName: String = null // Error: You can't assign null to a non-null type


    // The question mark tells the compiler that this variable can hold null:
    val lastName: String? = null


    // You can't assign a nullable variable to a non-null type 
    // even though they're both Strings:

MostafaNasiri

fun main() {
    var name: String? = "joey"

    if (name != null) {
        val nameLength = name.length
    }
    

    // The same thing can be done in a much shorter way:
    var nameLength = name?.length // nameLength = 4

MostafaNasiri

fun main() {
    var name: String? = null
    
    // name?.length evaluates to null so the elvis operator
    // chooses the second value:
    val nameLength = name?.length ?: 0 // nameLength = 0
}

MostafaNasiri

fun main() {
    var name: String? = "Arthur"
    var nameLength = name!!.length // nameLength = 6

    name = null
    nameLength = name!!.length // KotlinNullPointerException
}

MostafaNasiri

fun main() {
    // Example 1
    val x = 2
    // Just a simple if expression
    if (x % 2 == 0) {
        println("Even")
    }
    

    // Example 2

MostafaNasiri

fun main() {
    // Example 1
    val firstNum = 1
    val message = when (firstNum) {
        1 -> "One"
        2 -> "Two"
        3 -> "Three"
        else -> "Unknown Number" // "else" is mandatory here
    }

MostafaNasiri

fun main() {
    var x = 1
    while (x <= 5) {
        println(x)
        x++
    }

    val y = false
    do {
        println("do while loops execute at least once")