What is the Kotlin double-bang (!!) operator?

This is unsafe nullable type (T?) conversion to a non-nullable type (T), !! will throw NullPointerException if the value is null.

It is documented here along with Kotlin means of null-safety.

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Here is an example to make things clearer. Say you have this function

fun main(args: Array<String>) {
    var email: String
    email = null
    println(email)
}

This will produce the following compilation error.

Null can not be a value of a non-null type String

Now you can prevent that by adding a question mark to the String type to make it nullable.

So we have

fun main(args: Array<String>) {
    var email: String?
    email = null
    println(email)
}

This produces a result of

null

Now if we want the function to throw an exception when the value of email is null, we can add two exclamations at the end of email. Like this

fun main(args: Array<String>) {
    var email: String?
    email = null
    println(email!!)
}

This will throw a KotlinNullPointerException

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Kotlin's double-bang operator is an excellent sample for fans of NullPointerException (NPE).

The not-null assertion operator !! converts any value to a non-null type and throws an exception if the value is null.

val nonNull = str!!.length

If you write str!!, it'll return a non-null value of str (str is a String? here) or throw an NPE if str is null. This operator should be used in cases where the developer is guaranteeing – the value will never be null. If you want an NPE, you have to ask for it explicitly.

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!! is an assertion that it is not null. Two exclamation marks after a nullable value convert it to a non-nullable type. At the same time, before the conversion, it is not checked in any way that the value really does not contain null. Therefore, if during the execution of the program it turns out that the value that the !! operator is trying to convert is still null, then there will be only one way out - to throw a NullPointerException.

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