Non-Null Assertion

Using the !! Operator

Appending !! to a nullable expression asserts to the compiler that the value is definitely not null, allowing direct, non-nullable-style access -- but it throws immediately at runtime if that assumption is wrong.

Example: Using the !! Operator

fun main() {
    val name: String? = "Kotlin"
    val length = name!!.length
    println("Length: $length")
}

The Danger of !! on Actual Nulls

If the asserted value really is null at runtime, !! throws a NullPointerException immediately, which is exactly the kind of crash Kotlin's null safety was designed to prevent.

Example: The Danger of !! on Actual Nulls

fun main() {
    val name: String? = "Kotlin"
    try {
        val other: String? = null
        println(other!!.length)
    } catch (e: NullPointerException) {
        println("Caught NPE: as expected when forcing a null value")
    }
    println("Safe name length: ${name!!.length}")
}

Preferring Safer Alternatives

In almost every case, ?., ?:, or an explicit if (value != null) check communicates intent more clearly and avoids crashing, making !! unnecessary.

Note: Reach for !! only when you can prove, from the surrounding logic, that the value cannot possibly be null.

Example: Preferring Safer Alternatives

fun main() {
    val name: String? = null
    // Prefer this over `name!!.length`
    val length = name?.length ?: 0
    println("Safe length: $length")
}

When !! Can Be Justified

A rare acceptable use is right after a null check performed in a way the compiler cannot smart-cast, such as through a separately stored Boolean flag, where the developer has genuinely already ruled out null.

Example: When !! Can Be Justified

fun main() {
    val values = mutableMapOf("count" to 5)
    if (values.containsKey("count")) {
        println("Count is ${values["count"]!!}")
    }
}