Variables

Kotlin has powerful type inference. While you can explicitly declare the type of a variable, you'll usually let the compiler do the work by inferring it. Kotlin does not enforce immutability, though it is recommended. In essence use val over var.

fun main() {
    var a: String = "initial"  // 1
    println(a)
    val b: Int = 1             // 2
    val c = 3                  // 3
}

1. Declares a mutable variable and initializes it.
2. Declares an immutable variable and initializes it.
3. Declares an immutable variable and initializes it without specifying the type. The compiler infers the type Int.

fun main() {
    var e: Int  // 1
    println(e)  // 2
}

{validate="false"}

1. Declares a variable without initialization.
2. An attempt to use the variable causes a compiler error: Variable 'e' must be initialized.

You're free to choose when you initialize a variable, however, it must be initialized before the first read.

fun someCondition() = true 

fun main() {
    val d: Int  // 1
    
    if (someCondition()) {
        d = 1   // 2
    } else {
        d = 2   // 2
    }
    
    println(d) // 3
}

1. Declares a variable without initialization.
2. Initializes the variable with different values depending on some condition.
3. Reading the variable is possible because it's already been initialized.