Best language features in Kotlin (for me)

I have been doing Java and Scala for many years. Recently my team switched to Kotlin and everybody is enjoying it so far. In this blog post I want to share a personal selection of language features that I really like.

Smart casts on polymorphic collections

Kotlin has a super concise and safe way to narrow down a polymorphic list to a specific sub-type using the as? syntax.

	interface Animal {
	val name: String
	}
	data class Monkey(override val name: String): Animal
	data class Fish(override val name: String): Animal
	val animals: List<Animal> = listOf(Monkey("gorilla"), Fish("trout"), Monkey("oranutan"))
	val monkeys: List<Monkey> = animals.mapNotNull { it as? Monkey }

view raw SmartCastList.kt hosted with ❤ by GitHub

Logging return values

There are situations, where we want to log the return value of a function before actually returning it. Usually, this is done by storing the return value into a variable, doing the log statement and finally returning that variable. Kotlin makes this is a bit simpler with the .also keyword for side-effects. Using .also, we don't have to introduce this artificial variable.

	class Service(dependency: Dependency) {
	private val logger = KotlinLogging.logger {}
	fun doSomething(userId: String): Int {
	return dependency.evaluate(userId = userId).also { result ->
	logger.trace { "UserId '$userId' was evaluated to: $result" }
	}
	}
	}

view raw LoggingBeforeReturn.kt hosted with ❤ by GitHub

... more to come

DynamoDBLocal and UnsatisfiedLinkError in Gradle

This week I started to work on a new project using DynamoDB. As always, I’d like to write some integration tests, to verify my datastore integration works as intended. I know one cheap way to test DynamoDB, is using containers and LocalStack. However, I decided to go even simpler and give DynamoDBLocal a spin. This is just a library, for your tests to depend on. Super easy to integrate in gradle or maven projects.

Unfortunately using DynamoDBLocal is not as straightforward. Relatively soon you might hit a UnsatisfiedLinkError related to SQLLite - similar to:

To fix this in a gradle build, we modified our test task, to copy some binaries around and also make a system property available to the tests.

	dependencies {
	...
	testCompile "com.amazonaws:DynamoDBLocal:1.11.477"
	}
	task copyDependencies(type: Copy) {
	from(configurations.compile + configurations.testCompile) {
	include '*.dll'
	include '*.dylib'
	include '*.so'
	}
	into 'build/libs'
	}
	test {
	dependsOn copyDependencies
	systemProperty 'sqlite4java.library.path', 'build/libs'
	useJUnitPlatform()
	}

view raw build.gradle hosted with ❤ by GitHub

One last pitfall might be your IntelliJ IDE. Delete all existing test configurations and make sure to run your tests using Gradle Runner.