Kotlin extension members are great. When using a type that you do not control, you can paper over the awkwardness of calling a static utility method and promote the member to appear as an instance member of the type, with full discoverability in your IDE’s autocomplete.

Kotlin extension functions have a use even for types that you do control!

Consider this case:

abstract class TypeAdapter<T> { // Implicit <T: Any?>
  abstract fun adapt(string: String): T

  fun nonNull(): TypeAdapter<T> {
    val delegate = this
    return object: TypeAdapter<T>() {
      override fun adapt(string: String): T {
        return delegate.adapt(string)!!
      }
    }
  }
}

This is unfortunate for the API.

object IdentityTypeAdapter: TypeAdapter<String>() {
  override fun adapt(string: String) = string
}

With the current API of TypeAdapter, we can call IdentityTypeAdapter.nonNull(), even though IdentityTypeAdapter’s adapt function never returns null, anyway! IdentityTypeAdapter.nonNull() is a useless factory call.

An extension function can improve the API by limiting the scope of the generic type argument for the nonNull function.

abstract class TypeAdapter<T> { // Implicit <T: Any?>
  abstract fun adapt(string: String): T
}

fun <T: Any> TypeAdapter<T?>.nonNull(): TypeAdapter<T> {
  val delegate = this
  return object: TypeAdapter<T>() {
    override fun adapt(string: String): T {
      return delegate.adapt(string)!!
    }
  }
}

This new API uses an extension function on its own type to constrain the generic type argument’s bounds further for just this nonNull function. Now, IdentityTypeAdapter.nonNull() will not compile. nonNull() is only a valid function for TypeAdapters with nullable type arguments.

Of course, we could add similar extension functions in our API for other bounds like

fun TypeAdapter<Number>.adaptToSuccessOrFailure(string: String): Boolean {
  return adapt(string) == 0
}

just as we could in Java with static methods like

static boolean adaptToSuccessOrFailure(TypeAdaper<Number> adapter, String string) {
  return adapter.adapt(string) == 0;
}

The Java API is hidden and awkward.
With Kotlin, add the extension function to your API!