Kotlin Nitpicker’s guide

October 14, 2020


Code review is an essential tool for a development team, which helps ensure high-quality standards of code. While reviewing others’ code one might find bugs, design issues, and so on. One another part of reviewing is getting used to the codebase and following team’s code style for better maintenance. Though it is pretty cool in theory, in practice team might face few issues, one of which is nitpicking. When review is bloated with dozens of similar comments related to e.g. how beautifully code looks like. Common code style is important, but having a lot of similar comments in each review doesn’t help. Instead of trying to understand what code is doing, nitpicker writes a lot of similar comments on the style. Back and forth discussions or fixes of such slow down development process and overall make team’s morale worse.

Kotlin is a great language, saves a lot of time on some common tasks. But with “idiomatic” code introduced the language becomes the dream of the nitpicker. “Concise” often is misused with “Short”, which ends with “can be shorter” comments. But while it is possible to write something shorter, doesn’t mean that it is automatically better.

In this article, I’d like to go through some features of Kotlin, which you can point at the code review to make your reviews useless. In the beginning of each section there will be some anti-suggestion.


If you see !! in the code - immediately say that it has to be removed.
This is a code smell, we might have a crash!

Yes, !! in code should warn you, because the operation is unsafe. But in some situations, you might be fine with that. Sometimes Kotlin compiler can’t infer that the value is not null in some given moment. And you are 100% sure that it is safe. For example let’s take a look at the reverse list function:

fun <T> Node<T>.reverse(): Node<T> {
    if (this.next == null) {
        return this

    var previousNode: Node<T>? = null
    var currentNode: Node<T>? = this
    var nextNode: Node<T>?

    while (currentNode != null) {
        nextNode = currentNode.next
        currentNode.next = previousNode

        previousNode = currentNode
        currentNode = nextNode


    return previousNode!!

As variable is nullable, I get nullable result. It is possible to see that if I got to the last return, then previousNode can’t be null. Compiler can’t infer that, but I, as a developer, can.

This is the case when I think !! is totally legit.

What are other options:

val prev = previousNode
if (prev == null) {
    throw IllegalStateException()

Yes, we now even more explicit in our intentions, but we have more bytecode for our solution now.

return previousNode ?: throw IllegalStateException()

And probably there are also a lot of many other ways on how to solve that. Imagine how much time could be spent on discussing various options, especially if all these would be discussed in written form in the review.

Personally, I was also felt pretty negative about !!.
I think main issue was that a few years ago while we were converting projects from Java to Kotlin, converter automatically added many !! in various places.
So, if one saw !! in code the first thought was that some compiler “errors” were missed and need to be addressed.
Currently, I see no issues with !! while writing code. But one should be careful with usages.


If you can remove curly braces with = then do that.
Less lines, less code, better!

In Kotlin it is allowed to omit function body and write =.
For example these are equivalents:

fun hello(): String {
    return "hello"

fun hello(): String = "hello"

Using the second option saves you two lines! Let’s use it everywhere!
But there are some caveats.
First is that it is not required to use = only for one-liners:

fun hello(): String =

And this variant might look not that awesome.

Second is that we’re reading code from top to bottom, right to left. And if we need to “scan” code to find something we usually go top to bottom first and then when we need details we go to the right. Having return might allow you to see relevant information while going in the “scanning mode”, which is not available if you used one-liner with =.

I think that it is a matter of taste and agreement in the team. At the same time, I think that there is nothing bad or wrong in using explicit returns.

One good usage of not-one-liner = I see in maintaining exhaustive when:

fun resolve(orientation: Orientation): Int = when (orientation) {
    VERTICAL -> 1

Here we save some space and at the same time support our “scanning mode” because the right part with when here is not that important.

Implicit return types

If you can omit return type then do that.
Less code is better.

Also in Kotlin it is possible to omit return type if we used =. If we consider previous example we could write:

fun hello() = "hello"

It is clear that return result will be String, so why type it?
In such simple cases, it might be good, but there are also caveats here. If we have non-“primitive” return type we might face a situation like:

fun getFactory() = FactoryImpl()

Return type will be inferred as FactoryImpl while we might want to have Factory interface instead. It becomes even worse if this method is part of our public API and we exposed implementation instead of an interface, which might lead to issues with later maintenance.

Again, I see nothing bad in using explicit return types everywhere.
It is fine to omit return type for private or internal functions, but for public API explicit return types are a must.

Implicit variable types

If you can omit variable type, then do.
Less code is better.

Kotlin compiler can infer types. So when in java we have to write:

final Object object = new Object();

in Kotlin it is just:

val object = Object()

First note on this is similar to previous note on implicit return type: if you create implementation on the right side, then it might be a good move to declare variable type explicitly. Especially, if that variable is part of public API.

Second note is about primitive numbers initialization.
Unlike Java in Kotlin there is no auto-convert between primitive numbers. When in Java it is possible to write:

float a = 0.4;
int b = a;

in Kotlin explicit conversion is required:

val a: Float = 0.4f
val b: Int = a.toInt()

And here we get to the caveat: it is so tempting to omit variable type here, because value is 0.4f - it is float. If it was 0.0 then it would be double, if 0L then long and if just 0 then Int. It is clear and idiomatic!
But there is nothing wrong with using explicit variable types:

val long1 = 1_000_000L
val long2: Long = 1_000_000

val float1 = 0.004304939340f
val float2: Float = 0.004304939340f

If the number is big and you use implicit types then reader have to look at the whole line till the very end to infer type instead of looking at the declaration.
Remember, we write code not for compiler.

forEach vs for

If you need a for-loop then use forEach.
We’re doing functional programming, not imperative!

For collections, arrays, and basically, all objects which implement Iterable interface (or even Strings) it is possible to use forEach method instead of explicit for loop. In many cases, it has a decisive advantage. Compare:

for (i in 0 until list.size) {

list.forEach { println(it) }

Though at the same time it is possible to use:

for (item in list) {

Where the difference is not that significant. But we write functional code, right? Functions should be our primary option?

I see no issues in using for loops instead of forEach.
In some cases using forEach would be bad:

(1..10).forEach { ... }

will create additional object for IntRange, which we won’t have if we use just for loop.

Another important thing is that lambda declares scope. And in such a case Kotlin compiler might not be able to infer types. Let’s look at the example of finding max value in an array:

var max: Int? = null
val array = intArrayOf(1, 2, 3, 4)

for (i in array) {
    if (max == null || i > max) {
        max = i

array.forEach { i ->
    if (max == null || i > max) { // <- Smart cast to 'Int' is impossible, because 'max' is a local variable that is captured by a changing closure
        max = i

Version with for works well because compiler was able to smart cast from Int? to Int. In case of forEach smart cast was impossible.

it vs method reference

If you can use method reference, then do.
Remember, functional programming.

When we use function which accepts lambda as a parameter (inside which we call some function) it might look better to use method reference:

list.forEach { println(it) }


Second example is shorter and generally looks better.
But there is nothing wrong with using lambda with it.


If you can use with/apply, then do. You can group everything so that it might become a one-liner.
Then you can remove curly braces and return type. Cool!

Using with/apply and other similar methods allow grouping code into logically coupled statements:

recyclerView.apply {
    adapter = ItemAdapter()
    layoutManager = LinearLayoutManager(context)

And while it is a good option, and I use it almost all the time, there is nothing wrong with:

recyclerView.adapter = ItemAdapter()
recyclerView.layoutManager = LinearLayoutManager(context)

It seems bloated, but at the same time, it is shorter ;)

Usage of apply/with/etc. also might lead to some issues with declaring what is this in given scope, in case where we use some nesting:

object1.apply {
    object2.apply {
        value = 0
        this@object1.value = this.value

If you use labeled this you should point yourself that code could look better without apply:

object2.value = 0
objet1.value = object2.value


Code style is important. Code review is important.
Discuss among the team what you expect to get from code review: find code style issues or bugs, architectural decisions, and so on.
It is generally better to discuss code style topics in the team and set up some code style checking tool and do not spend a lot of time in review fixing nitpicks.
Don’t try to use in your review arguments like “this is not a kotlin idiomatic code”, “can be shorter” and so.
Your code should solve problems, your code should be in one style among team.
Work in a team.

Happy coding.