First program, val and var, and types

Your first program

fun main() {
    println("Namaste, Kotlin!")
}

fun is short for function, a named block of code. The curly braces hold its body. Unlike Java, you do not need a class or semicolons. Text in double quotes is a String.

val and var

A variable is a name for a value. Kotlin has two kinds. val (value) is read-only: once given a value, it cannot be reassigned. var (variable) can be reassigned. Professional Kotlin code uses val by default and var only when a value really changes, because values that never change make programs easier to understand.

fun main() {
    val city = "Surat"
    var visitors = 120
    visitors = visitors + 30      // fine: var can change
    // city = "Vadodara"          // error: Val cannot be reassigned
    println(city)
    println(visitors)             // 150
}

Basic types

Kotlin uses type inference: it works out the type from the value, so val seats = 72 is an Int. You can also write the type yourself after a colon: val seats: Int = 72. Once a variable has a type, it keeps it; you cannot later put text into an Int variable.

Numbers and division

fun main() {
    println(7 / 2)       // 3: Int divided by Int gives an Int (the decimal part is dropped)
    println(7 % 2)       // 1: the remainder
    println(7 / 2.0)     // 3.5: one Double makes the result a Double
    println(7.toDouble() / 2)   // 3.5
}

String templates

A string template puts a value inside text. Write $name for a simple variable and ${ } around any expression. This is easier to read than joining text with +.

fun main() {
    val item = "Masala dosa"
    val price = 80
    val qty = 3
    println("$item x $qty = Rs ${price * qty}")
    println("Name has ${item.length} characters")
}

Converting text to numbers

fun main() {
    val typed = "42"
    val n = typed.toInt()           // 42
    val bad = "4two".toIntOrNull()  // null: not a number
    println(n + 1)
    println(bad)
}

Comments

A comment is a note for humans that the compiler ignores. // starts a one-line comment; /* ... */ wraps several lines. Use comments to explain why, not to repeat what the code obviously does.

Common mistakes

• Using var everywhere. Start with val; change to var only if the compiler says the value is reassigned.
• Expecting 7 / 2 to be 3.5. Integer division gives 3; use 7 / 2.0.
• Writing "Total: " + price + qty and getting "Total: 803" instead of 83. Use a template: "Total: ${price + qty}".
• Using toInt() on user input. One bad character crashes the app; use toIntOrNull().