What's the best way to take a `Map<String?, Int>` and convert it to `Map<String, Int>`? ...

# getting-started

[Joshua Hansen]

What's the best way to take a Map<String?, Int> and convert it to Map<String, Int>?

I tried filterKeys { it != null } but the compiler doesn't seem to be smart enough to change the resulting type, so I have to cast after filtering.

val myMap: Map<String?, Int> = mapOf(
    null to 0,
    "one" to 1,
    "two" to 2,
)

val withoutNullKeys = myMap.filterKeys { it != null } // Still is Map<String?, Int>

[Joshua Hansen]

Looks like there's an issue for this.
https://youtrack.jetbrains.com/issue/KT-4734

[Daniel Pitts]

Not the prettiest work around, but you could do:

val map = mapOf(
        null to 0,
        "One" to 1,
        "Two" to 2,
    )
    val result: Map<String, Int> = map.mapNotNull { (key, value) ->
        key?.let { key to value }
    }.toMap()

[Shawn]

could simplify that to key?.to(value) but otherwise yeah, I think that's the move until they add a dedicated method to the stdlib

[Daniel Pitts]

Good call on the ?.to. To be honest, I let github CoPilot write that bit for me. 😁

[ephemient]

build the map directly, skipping the intermediate list of pairs

buildMap { for ((key, value) in map) put(key ?: continue, value) }

[Klitos Kyriacou]

Clever use of continue there!

[Daniel Pitts]

Or, just tell the compiler its wrong:

@Suppress("UNCHECKED_CAST")
val result = (myMap - null) as Map<String, Int>

[Daniel Pitts]

Or more generally:

@Suppress("UNCHECKED_CAST")
fun <K : Any, V> Map<K?, V>.onlyNonNullKeys(): Map<K, V> = (this - null) as Map<K, V>