Not Null Assertion(!!) Operator in Kotlin

Definition

• This operator converts the type of a boxed variable to not-null type.
• This is represented by ”!!”
• When the asserted variable holds a null value, Null Pointer Exception occurs.

Syntax

//"value" variable is asserted to a not null value.
val variable_name = value!!.length

Not Null Assertion Operator(!!) Example Program

//Not Null Assertion Operator Example Program in Kotlin
//Operator Kotlin Programs, Basic Kotlin Program

fun main(args: Array<String>) {
    //Declaring boxed String
    var name: String? = "Little Drops"
    //Printing the length of the asserted value
    println("Length of $name is ${name!!.length}")
    //Setting null to the name variable
    name = null
    //This statement will create Null Pointer Exception
    println("Length of $name is ${name!!.length}")
}

Sample Output

Length of Little Drops is 12
Exception in thread "main" kotlin.KotlinNullPointerException
	at NotNullAssertionOperatorKt.main(NotNullAssertionOperator.kt:20)

How It Works

Here is a step-by-step walkthrough of how the Kotlin program for Not Null Assertion(!!) Operator in Kotlin runs, line by line:

1. fun main(args: Array<String>) { — defines a function used by the program.
2. var name: String? = "Little Drops" — declares or assigns a value the program uses.
3. println("Length of $name is ${name!!.length}") — writes a line of output shown in the sample output.
4. name = null — declares or assigns a value the program uses.

After running it, compare your console output with the Sample Output above. Try changing the values and re-running the program to see how the result changes — experimenting is the fastest way to understand the logic.