val and var

val and var

val and var Versus const and let

val declares a read-only reference, like const, and var a reassignable one, like let; prefer val and reach for var only when a value must change. Kotlin also fixes each variable's type at compile time:

vals.kt: reassigning a val, and changing a var's typeKotlin

const val CURRENCY = "USD"          // compile-time constant, top level only
fun main() {
    val title = "The Quiet Harbor"  // read-only reference, like const
    var stock = 3                   // reassignable, like let
    stock -= 1
    title = "Salt and Saffron"
    stock = "two"
    val cart = mutableListOf(title) // the reference is fixed, the list is not
    cart.add("Gardens in Glass")
    println("$cart, $stock left, prices in $CURRENCY")
}

Output

vals.kt:6:5: error: 'val' cannot be reassigned.
...
vals.kt:7:11: error: assignment type mismatch: actual type is 'String', but 'Int' was expected.
...

Without lines 6 and 7 it prints [The Quiet Harbor, Gardens in Glass], 2 left, prices in USD. As with a const array, val cart fixes the reference, not the contents; the type says whether the list can change, since listOf() returns a read-only List and mutableListOf() a MutableList (Collections). const val goes further: a compile-time primitive or String, copied into every place that uses it.

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