Not Null Assertion

The safe call operator and Elvis operator handle nulls gracefully, but sometimes you're absolutely certain a nullable value isn't null at a specific point in your code. The not-null assertion operator !! lets you tell Kotlin: "I guarantee this isn't null."

fun main() {
    val name: String? = "Alice"
    val length = name!!.length  // Converts String? to String
    println(length)  // Prints: 5
}

The !! operator converts a nullable type to its non-nullable counterpart. However, if the value actually is null, your program will crash with a NullPointerException:

fun main() {
    val name: String? = null
    val length = name!!.length  // Crashes here!
}

When to use it: Only use !! when you have logic that guarantees the value cannot be null, but the compiler can't verify it. In most cases, prefer safe calls ?. or the Elvis operator ?: instead. The not-null assertion should be your last resort, not your first choice.