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Kotlin's standout feature is null safety — types are non-nullable by default. To allow null, suffix the type with ?:

val name: String = "Alice"        // never null
val label: String? = null         // can be null

// label.length      // ✗ compile error
label?.length         // safe call — returns null if label is null
label?.length ?: 0    // Elvis operator — default value
label!!.length        // !! — assert non-null (throws if null)

This eliminates the NullPointerException class of bugs from your code (mostly — !! and Java interop can still trigger it). The compiler tracks nullability through your code automatically.

fun greet(name: String?) {
    if (name != null) {
        println(name.length)   // ✓ smart-cast — compiler knows it's non-null here
    }
}