Null Safety

Nullable vs non-null

In Kotlin a normal type can never be null. Add ? to allow null, and the compiler forces you to handle it.

var a: String = "hi"
// a = null            // ❌ won't compile

var b: String? = "hi"
b = null               // ✅ allowed

// Safe call: returns null instead of crashing
println(b?.length)

// Elvis operator: provide a default
val len = b?.length ?: 0

Smart casts

fun printLength(text: String?) {
    if (text != null) {
        // inside here, Kotlin knows text is not null
        println(text.length)
    }
}