!! Non-Null Assertion

// Use !! to convert a Nullable type to a non-null type.
val name: String? = "Alice"
val length: Int = name!!.length  // Treated as String? → String.

// If the value is null, KotlinNullPointerException is thrown.
val nullValue: String? = null
val len = nullValue!!.length  // Exception thrown

// Demo of the !! operator
fun getUser(id: Int): String? = if (id > 0) "User-$id" else null

fun main() {
    // Works normally when the value is not null.
    val name: String? = "Alice"
    println(name!!.length)   // 5
    println(name!!.uppercase())  // ALICE

    // Example of using !! on the result of a function where smart cast is not available
    val user = getUser(1)!!  // When you know the result is not null
    println("User: $user")  // User: User-1

    // Comparison with safer alternatives to !!
    val nullable: String? = "Kotlin"

    // Option 1: !! (throws if null)
    println(nullable!!.length)  // 6

    // Option 2: ?.let (safe)
    nullable?.let { println(it.length) }  // 6

    // Option 3: ?: (fallback value if null)
    println(nullable?.length ?: 0)  // 6

    // Using !! on a null value causes an exception.
    try {
        val nullStr: String? = null
        println(nullStr!!.length)  // Exception thrown here
    } catch (e: NullPointerException) {
        println("Exception caught: ${e::class.simpleName}")  // KotlinNullPointerException
    }
}

The !! operator bypasses Kotlin's null safety features, so its use should be kept to a minimum. Overusing it in production code can lead to the same kinds of issues as Java's NullPointerException.

Warning: Using !! anywhere other than places where you are certain the value cannot be null is dangerous. Consider using the ?. operator, the ?: Elvis operator, or smart casting with ?.let instead.

For the basics of nullable types, see Nullable Types / ?. Operator. For setting default values, see ?: Elvis Operator.