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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)