Variables

Variables in Kotlin come in two flavors: val and var. And once you understand the difference, you'll write safer code without even thinking about it.

val — read-only (immutable)

A val can only be assigned once. Think of it as a constant — once you set it, it's locked.

fun main() {
  val name = "Alice"
  println(name)
  // name = "Bob"  // This would cause an error!
}
    

Use val by default. Your future self will thank you.

var — mutable

A var can be changed as many times as you want. Use it when you really need a value to change.

fun main() {
  var score = 0
  println(score)
  score = 10
  println(score)
  score = score + 5
  println(score)
}
    

Type inference is real

Notice I didn't tell Kotlin that name is a String or score is an Int. Kotlin figures it out from the value you assign. That's called type inference.

You can still be explicit if you want:

fun main() {
  val name: String = "Alice"
  var score: Int = 0
  println("$name has $score points")
}
    

Naming rules

• Can contain letters, numbers, and underscores
• Must start with a letter or underscore
• Are case-sensitive (so name and Name are different)
• Use camelCase by convention: myVariableName