How to verify no null elements in list, Kotlin

Question

I'd like to check a List<Int?> for nulls and if no nulls exist, then allow the List to proceed as a List<Int>, otherwise use an emptyList()

This is what I came up with:

var noNullElements: List<Int> = emptyList()
if (listWithPossibleNullElements.all { it != null }) {
    noNullElements = listWithPossibleNullElements as List<Int>
}
//do something with noNullElements list

I'd be shocked if this was the best, most idiomatic way.

Solution

There are many acceptable solutions below, but the one I chose was a combination of the solutions

fun getSafeList(list: List<Int?>): List<Int> =if (null in list) emptyList() else list.requireNoNulls()

Answer 1

Here's a one liner extension function:

inline fun <reified T : Any> Iterable<T?>.filterNoneNull(): Iterable<T>? =
  takeIf { null !in it }?.requireNoNulls()

• takeIf {} - a scope function that will return null if the lambda evaluates to false
• null !in it - use the in operator function to determine if the list contains null
• finally, if takeIf {} returns a non-null value, filterNotNull() will convert the element type from the nullable T? to non-null T

Note that it will return null if the provided Iterable<T?> contains null, else List<T>, in case it's important to differentiate between the provided list being empty, or containing null. An alternative can be provided using the elvis operator.

Example usage - run in Kotlin Playground:

fun main() {
  val listA: List<Int?> = listOf(1, 2, 3, null)

  // since listA contains 'null', the result is 'null'
  println(listA.filterNoneNull())
  // prints: null

  // provide an alternative, using the elvis operator
  println(listA.filterNoneNull() ?: emptyList<Int>())
  // prints: []

  val listB: List<Int> = listOf(1, 2, 3)

  // since listB does not contain null, the same list is returned
  println(listB.filterNoneNull())
  // prints: [1, 2, 3]
}

inline fun <reified T : Any> Iterable<T?>.filterNoneNull(): Iterable<T>? =
  takeIf { null !in it }?.requireNoNulls()

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Note: I edited the example and replaced filterNotNull() with requireNoNulls(), as the latter will not instantiate a new list