The Beauty Of The KotlinNullPointerException

Cody Engel

It seems like ever day I’m finding more reasons to enjoy Kotlin development. It first started when I heard Kotlin eradicates the possibility of the dreaded NullPointerException (false information, but hey it was something to get excited about). Then I saw how elegant it was to model information using the data class and as an RxJava user I found sealed classes an easy way to model streams of data. As of late though I’ve found myself diving deeper into the intricacies of the language and stumbling upon ways of writing concise code that is safer than the verbose Java code I was writing just a year ago.

Today I think I finally found a legitimate reason to use the !! operator. For those unfamiliar, this is the operator to use when you want to ask for a KotlinNullPointerException, it is the only way (aside from calling platform code) that you can get such an exception. In the time I’ve been writing Kotlin it is also frowned upon to use, but I hope this small example will change a couple minds.

First though, let’s look at what I was going to write today while running on auto-pilot trying to avoid any trace of an NPE:

val classWithNullableProperties = ClassWithNullableProperties()
return if (classWithNullableProperties.propertyOne != null &&
    classWithNullableProperties.propertyTwo != null &&
    classWithNullableProperties.propertyThree != null &&
    classWithNullableProperties.propertyFour != null &&
    classWithNullableProperties.propertyFive != null &&
    classWithNullableProperties.propertySix != null) {
} else {
    RepositoryResponse.Error("Uh Oh, Something Was Null")

The code above is a whole lot of ugly, couldn’t Kotlin do any better? I’d like to think it could, thought it might be interesting to catch the KotlinNullPointerException and just return an error if that happens. Since conditions are evaluated as expressions this is possible and seems to be supported by the language creators. By doing just that we are able to clean things up substantially:

val classWithNullableProperties = ClassWithNullableProperties()
return try {
} catch (ex: KotlinNullPointerException) {
    RepositoryResponse.Error("Uh Oh, Something Was Null")

This solution seems to go against all of my old beliefs on dealing with null pointer exceptions as I have actively tried avoiding them by checking if something is null before using them. This of course is nothing new if you are coming from Java, you can always just catch a NullPointerException and have the same outcome. I think the difference between Kotlin and Java comes down to explicitly asking for a NullPointerException on each property that you know can be null. This difference is really only possible because nullability is built into the type system of Kotlin.

That’s it. This was a fairly short example that I wanted to share because I think it was a bit out there and the conventional null safety mechanisms wouldn’t have been sufficient, or at least they would have lead to some very verbose Java-ese looking code.

Having the freedom to work with Kotlin and avoid Java almost entirely has been a breath of fresh air for me. I feel very lucky to be working at a company like ActiveCampaign that is so forward thinking. We’re also growing like crazy, if you’re as excited about Kotlin as I am then check out our current job postings.

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