[Solved] Kotlin Error: Null can not be a value of a non-null type String

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

This Kotlin Error occurs when a String variable is declared as non-nullable, but assigned a null value in the due course of program. We shall recreate the Error and provide necessary technique to handle the “Kotlin Error Null can not be a value of a non-null type String”

Let us first recreate the Error scenario using the following example, where we shall declare a variable str as non-nullable, but try to assign a null value.

example.kt

fun main(args: Array){
    var str: String    // variable is declared as non-null
    str = "Hello !"
    println("Value of string is : $str")
	// try assigning a null value to non-null String variable
    str = null // this causes compilation error
    println("Value of string is : $str")
}

Output

Error:(6, 10) Null can not be a value of a non-null type String

How to Fix Kotlin Compilation Error: “Null can not be a value of a non-null type String”

There are two ways to handle this exception. They are :

1. Avoid assigning a null value to the non-nullable String variable.

example.kt

fun main(args: Array){
    val str: String    // variable is declared as non-null
    str = "Hello !"
    println("Value of string is : $str")
}

Output

Compilation completed successfully
Value of string is : Hello !

2. Declare String variable to allow null using  ?  operator. In this scenario, caution has to be exercised to handle the most adverse exception ‘NullPointerException’. It is advised you follow the null safety in kotlin provided by Kotlin type system.

example.kt

fun main(args: Array){
    var str: String?    // variable is declared as nullable
    str = "Hello !"
    println("Value of string is : $str")
    str = null
    println("Value of string is : $str")
}

Output

Compilation completed successfully
Value of string is : Hello !
Value of string is : null

Conclusion

In this Kotlin Tutorial, we have learnt how to handle the compilation error caused during assignment of null value to a non-null declared String variable.