Kotlin - why do I get a KotlinNullPointerException

Question

Consider the following code:

Example

fun main(args: Array) {
    maybeWriteMessage()
}

fun maybeWriteMessage(message:         


        

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)
    }
}