Kotlin - why do I get a KotlinNullPointerException

Question

Consider the following code:

Example

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

fun maybeWriteMessage(message:         


        

Answer 1

In the following code:

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

you're asserting that the nullable parameter message is not null by using !! (the not-null assertion operator). If message is null though, this will throw a null pointer.

TL;DR: Do not use !! unless you exactly know what you're doing. There's almost always a better way.

The !! Operator

The third option is for NPE-lovers: the not-null assertion operator (!!) converts any value to a non-null type and throws an exception if the value is null. We can write b!!, and this will return a non-null value of b (e.g., a String in our example) or throw an NPE if b is null:

val l = b!!.length

Thus, if you want an NPE, you can have it, but you have to ask for it explicitly, and it does not appear out of the blue.