A few weeks ago, I had to create an extension function - a usual task for any Kotlin developer. But there were a few limitations:

  • The receiver was a common type, polluted with too many methods.
  • The extension function was only relevant to my feature package.
  • Creating a Gradle module was out of scope.
  • Introducing a new type to hold the function felt like too much.
  • Kotlin doesn’t support package-private, yet.

So how can I have the advantages of using extension functions but avoid these issues?

One way is to use objects as namespace for extension functions.

Here is an example:

// feature/MyFeatureActivityManagers.kt

// Namespace object.
internal object MyFeatureActivityManagers {

  // Extension functions that are only interested to my feature.
  fun ActivityManager.doSomethingThatOnlyYouCare() = TODO("")
  fun ActivityManager.doSomethingThatOnlyYouCareToo() = TODO("")

And here is the usage:

// feature/MyFeature.kt

import MyFeatureActivityManagers.doSomethingThatOnlyYouCare

fun myFeature(activityManager: ActivityManager) {

It didn’t change much but the approach has a few advantages:

  • It ensures that code in the same package can only access the extension function with a direct import;
  • It helps humans, and tools, to identify what file the extension is coming from;
  • But most important, it has the right level of discoverability*.

What I mean with discoverability is that IntelliJ IDEA seems to have a hierarchy for suggesting auto-complete in the following order:

  • Member functions
  • Global extensions
  • Object extensions

Hence, IDEA will only suggest to other developers your shiny function if they are actively looking for it.

What’s up with the naming?

I want to indicate why these particular functions are related, given the infinite number of possible extensions I could write.

A pattern I find useful is: {Context}{Receiver}s, where:

  • Context groups the functions based on their relevance.
  • Receiver refers to the type of the receiver for these functions.
  • s represents the collection of extensions.

Wrapping up

If you find yourself creating extensions that should be limited in access, consider creating a namespace object for them. That will help your project with organization and readability.


Special thanks to Maria Chietera, and Jacob Rein proofread review! 🔍

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