Kotlin - why do I get a KotlinNullPointerException

Question

Consider the following code:

Example

fun main(args: Array<String>) {
    maybeWriteMessage()
}

fun maybeWriteMessage(message: String? = null) {
    writeMessage(message!!)
}

fun writeMessage(message: String) {
    println("Hello World")
}

Output

Exception in thread "main" kotlin.KotlinNullPointerException at com.series0ne.ApplicationKt.maybeWriteMessage(Application.kt:8) at com.series0ne.ApplicationKt.maybeWriteMessage$default(Application.kt:7) at com.series0ne.ApplicationKt.main(Application.kt:4)

This is because I'm passing message!! (null, damn it!) to a non-nullable parameter, however the parameter is never accessed.

Question

Why does Kotlin forcibly throw KotlinNullPointerException even when the null reference isn't accessed?

Answer 1

message: String? is indicating that message may or may not be null.

Since your function maybeWriteMessage has a default value of null for message and you are calling maybeWriteMessage() without specifying message - the default value (null) will be used when calling writeMessage(message!!).

As written in the documentation the !!-operator throws an exception when the value is null.

One way to trigger writeMessage safely would be to use .let:

fun maybeWriteMessage(message: String? = null) {
    message?.let { 
      writeMessage(it)
    }
}