Kotlin first impressions


Recently while starting work on a project we’ve (team of two ;)) decided to try something new and use Kotlin. The project is far from ready and still under active development, but after writing some code in Kotlin I’ve got few observations I’d like to share.

What I like about kotlin

Immutability by default

A lot of things is immutable by default. Collections are immutable, classes are final, fields are usually immutable (val). Those all are great design assumptions (final classes are tricky while testing or in java frameworks, but there are workarounds) which should pay off in long-term.

Extensions methods

This one is great. I’ve written my share of helpers and stuff for things that weren’t so useful out of the box, or required to pass the same arguments all over the application. With extensions methods, I’m not forced to create static methods and other hacks to work around the API of external vendor. Now I can create extension method and fix the API myself :) What I particularly like about it is that under the hood it is a static method, not bytecode manipulation.

Here is what we did to avoid repeating the same code in all of the tests:

private fun MockHttpServletRequestBuilder.withJsonContent(jsonBody: String) = this

fun MockHttpServletRequestBuilder.withJsonContent(vararg entries: Pair<String, Any?>) = this

fun MockHttpServletRequestBuilder.withJsonContent(map: Map<String, Any?>) = this

Ok(ish) spring and hibernate support

When we were starting we decided to use some of the technologies that are familiar to us (we want to deliver not just play with new toys) because of that we decided to use spring and hibernate. At first, I was little afraid of how it will work out with Kotlin. Turns out it works ok. Sure there are some plugins you’ll need to add but once you are done with the configuration you are good to go. It is worth pointing out that spring 5 supports Kotlin (didn’t test it) and you can write build scripts in Kotlin and Gradle.

To make Kotlin work with spring and hiberante just drop few plugins which change behaviour of Kotlin’s compiler:

buildscript {
  ext {
    kotlinVersion = "1.1.60"
  repositories {
  dependencies {
    classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlinVersion"
    classpath "org.jetbrains.kotlin:kotlin-allopen:$kotlinVersion"
    classpath "org.jetbrains.kotlin:kotlin-noarg:$kotlinVersion"

apply plugin: "kotlin"
apply plugin: "kotlin-spring"
apply plugin: "kotlin-jpa"

Great integration with java

I’ve mixed Java and Kotlin both ways (Java using Kotlin, Kotlin using Java) and it worked just fine. Luckily until now, there was only single thing I needed to do in Java because it was impossible in Kotlin.

Named parameters

This one is awesome :) Even if you have 2 or 3 parameters it is nice to have a possibility to be verbose about the stuff you do. I like my code to be easy to read and verbosity is one way of achieving it ;)

Null handling

With Kotlin null checks are basically enforced during compilation. Sure you can get NPE or IllegalArgumentException while trying to pass null to a not nullable field but the compiler will help you out a lot. What is the most important from my perspective (it aligns with my private policy) is that Kotlin forces you to think about null values ahead of time not just after NPE is thrown on production.

Statically typed language with a lot of compiler help to figure out types of data

Kotlin’s compiler is great in figuring out the types of your data. Support of this "dynamic typing" in IntelliJ is really good. Just say your field is val and compiler will know what you had in mind.

Compact syntax

With few exceptions, I like the syntax of Kotlin language. It is very compact and easy to read. I like the way how you can implement functions. For example when using spring very often controller’s only responsibility is to have MVC annotation and call the service. With Kotlin syntax is very simple:

fun updateTeamPreferences(
  @PathVariable projectId: Id,
  @RequestBody teamPreferences: TeamPreferencesDto)
  = projectService.updateTeamPreferences(projectId, teamPreferences)

No need for lombok for data transfer objects

Lombok is great library to avoid writing a lot of boilerplate code. In Kotlin you don’t need it. In Kotlin you have data classes which are perfect for handling DTOs. You have immutable data structures with equals and hashCode implemented for free. As an extra, you’ve got something like builder (named parameters are awesome) and all the getters in just a few lines of code. This is great for simple data structures without any logic inside.

data class TeamPreferencesDto(
  val extraRequirements: String,
  val preferredTechnologies: Set<Id>,
  val requiredTechnologies: Set<Id>,
  val minTeamSize: Int?,
  val maxTeamSize: Int?)

What I don’t like about Kotlin

Everything is either private or public

What it basically means is that either you cannot access me or anyone can do whatever they want with me. I like my code to be more organized than this. I create packages which are responsible for handling one particular thing, and usually, a bunch of those small things is building something bigger. With everything public or private (I’m not going to create separate Gradle/Maven module with 4 classes just to use internal visibility) I cannot say what is public API of my package. I’m not going to put everything in one file either - I will not create a big file just to share private stuff inside it.

Hacks to create getter for field which doesn’t exist or of different type

Everything in Kotlin is property which means unless it is private you can access it. What it means is that in case when field you’ve created is of type X but you want to return Y as result of getter you’ll have to to write hacks to make it work.

private var _preferredTechnologies: MutableSet<Technology> =  = mutableSetOf()
val preferredTechnologies: Set<Technology>
  get () = _preferredTechnologies.toSet()

It’s easy to create global functions and global variables

I’m still wrapping my head around this one. I mean I’ve seen this in javascript or python, but JVM is not node ;) It might be very nice thing in some cases, but on the other hand, it is scary how easy it is :)

Hacks to make it working with Mockito and other Java frameworks/libraries

Every class is final by default. Idea behind is great but down to earth work with this might be troublesome. It is especially true when you avoid writing interfaces just to have them with a single implementation. Luckily there is a solution to this:

To make mockito stub final classes add file src/test/resources/mockito-extensions/org.mockito.plugins.MockMaker with content:


Java annotations are not fully supported

More like an observation than real problem :) You can live with it mixing java and Kotlin code. You can not annotate attributes of your annotation with other annotation KT-11475 (for example @AliasFor in spring). Luckily you can create this annotation in pure java and use it in Kotlin.


The thing that bothers me a lot is no convention on how to writer getters (properties) and that it is enforced by the way how you write your code (syntax). Because of this, you have to write ugly hacks to create APIs which are consistent with Kotlin look’n’feel. It usually happens in the domain so workaround might be very simple - create models in plain old java which can look nice and clean with a bit of Lombok :)

The most painful thing (worse than getters hacks), for me this is not acceptable for a really big project (we might live in the microservices era right now, but sometimes micro is not so micro after some time…​) is that Kotlin has basically only private and public visibility and nothing else. There is also protected and private but those are different things. I feel uncomfortable creating a public class which I know should not be used by anyone from outside my package. This is not good. Working with this kind of code in bigger projects requires a lot of self-discipline which might be hard to keep in check under pressure and/or in project lifespan…​

If in the future Kotlin will introduce something like package scope and figure out better conventions for writing getters instead of syntax then it migt become my language of choice :) Until then I can recommend it for small projects where the domain is simple (or not existing) and code base will not grow too big. Otherwise, it might require a lot of self-discipline to keep it clean and tidy. I think that in a long-term the best way of keeping yourself disciplined is a compilation error and that’s what I’d like to see in Kotlin :)

See Also

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29 Nov 2017 #development #interesting