What are two exclamation marks used for in programming?

The two exclamation marks !! in Kotlin are used for unsafe cast (type casting without checks).

// Example of unsafe cast
val value: Any? = "Hello"
val str: String = value as String // Regular cast, may throw ClassCastException
val anotherStr: String = value!! as String // Unsafe cast. If value is not String, it will also throw ClassCastException, but the !! syntax applies to nullability

The main purpose of !! is to convert a nullable type to a non-nullable type, explicitly stating that the developer is sure that the variable will not be null at this point in execution.

// Example of using !! to convert nullable to non-nullable
var name: String? = "Kotlin"
val length: Int = name!!.length // Explicitly stating that name is not null

If, during code execution, the variable to which !! is applied turns out to be null, a KotlinNullPointerException will be thrown.

This should be used cautiously, only when you are 100% sure there is no null. In most cases, it is preferable to use safer mechanisms such as:

• Safe call operator: ?.
• Elvis operator: ?:
• Null check (if (variable != null))
• Functions let, run, also, apply with null check

// Examples of safe null handling
var surname: String? = null
val lenSafeCall: Int? = surname?.length // Safe call operator, returns null if surname is null

val lenElvis: Int = surname?.length ?: 0 // Elvis operator, returns 0 if surname is null

if (surname != null) {
    val lenIf: Int = surname.length // Smart cast after null check
}

surname?.let {
    // Code executes only if surname is not null
    val lenLet: Int = it.length
}

Thus, !! is a tool for converting a nullable type to a non-nullable one, carrying the risk of NullPointerException and used for explicit assertion of non-nullity at the call site.