CODEWITHSUNDEEP
typesfilternullablekotlin
GlenPeterson
GlenPeterson

Reputation: 5206

Kotlin: eliminate nulls from a List (or other functional transformation)

Problem

What is the idiomatic way of working around this limitation of the null-safety in the Kotlin type system?

val strs1:List<String?> = listOf("hello", null, "world")

// ERROR: Type Inference Failed: Expected Type Mismatch:
// required: List<String>
// round:    List<String?>
val strs2:List<String> = strs1.filter { it != null }

This question is not just about eliminating nulls, but also to make the type system recognize that the nulls are removed from the collection by the transformation.

I'd prefer not to loop, but I will if that's the best way to do it.

Work-Around

The following compiles, but I'm not sure it's the best way to do it:

fun <T> notNullList(list: List<T?>):List<T> {
    val accumulator:MutableList<T> = mutableListOf()
    for (element in list) {
        if (element != null) {
            accumulator.add(element)
        }
    }
    return accumulator
}
val strs2:List<String> = notNullList(strs1)

Upvotes: 40

Views: 21473

Answers (3)

Hun
Hun

Reputation: 3798

val strs1 = listOf("hello", null, "world") // [hello, null, world]
val strs2 = strs1.filter { !it.isNullOrBlank() } // [hello, world]

Upvotes: 0

gderaco
gderaco

Reputation: 2362

you can also use

mightContainsNullElementList.removeIf { it == null }

Upvotes: 2

Sergey Tselovalnikov
Sergey Tselovalnikov

Reputation: 6036

You can use filterNotNull

Here is a simple example:

val a: List<Int?> = listOf(1, 2, 3, null)
val b: List<Int> = a.filterNotNull()

But under the hood, stdlib does the same as you wrote

/**
 * Appends all elements that are not `null` to the given [destination].
 */
public fun <C : MutableCollection<in T>, T : Any> Iterable<T?>.filterNotNullTo(destination: C): C {
    for (element in this) if (element != null) destination.add(element)
    return destination
}

Upvotes: 91

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