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

In Kotlin, "!!" is an operator that is known as the double-bang operator. This operator is also known as "not-null assertion operator". This operator is used to convert any value to a non-NULL type value and it throws an exception if the corresponding value is NULL. In the following example, we will see how to use this double-bang operator.

Example 1

In this example, we will consider a variable "name" and as a programmer, we want to throw a NULL pointer exception whenever the value of "name" is NULL. Now, execute the following code

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

Output

It will generate the following output −

null

This code is not actually solving our requirement statement. We wanted to throw a NULL pointer exception whenever the value of "name" is NULL. Let’s modify our previous example with the help of double-bang operator (!!).

Example 2

fun main(args: Array<String>) {
   var name: String?
   name = null

   // Nothing has been used to resolve overload ambiguity exception
   println(name!! is Nothing?)
}

Output

It will throw a NULL Pointer Exception at runtime −

Exception in thread "main" java.lang.NullPointerException
at MainKt.main(main.kt:6)