If I have

val data =
    hashMapOf(
            "firstName" to firstName,
            "lastName" to lastName)

but I wanted to not add firstName or lastName to the hashmap if they are null. What would be the best way to do that?

must it be a HashMap or are other maps allowed?

Do you really need an explicit

java.util.HashMap

as output? In general it's more common to stick with

mapOf

or

mutableMapOf

. When building a read-only map, you can use

buildMap

with conditions inside:

val map = buildMap {
    firstName?.let { put("firstName", it) }
    lastName?.let { put("lastName", it) }
}

If what you need is a mutable map, you can also add stuff conditionally with `apply`:

val map = mutableMapOf<String, String>().apply {
    firstName?.let { put("firstName", it) }
    lastName?.let { put("lastName", it) }
}

I'm following firebases docs which have a hashMapOf, but I suppose I can try other maps? Shouldn't be an issue.

also options,

buildMap {
    firstName?.let { put("firstName", it) }
    lastName?.let { put("lastName", it) }
}

@Suppress("UNCHECKED_CAST")
fun <K, V> Map<K, V?>.filterNotNullValues(): Map<K, V> = filterValues { it != null } as Map<K, V>

mapOf("firstName" to firstName, "lastName" to lastName).filterNotNullValues()

Thanks. I like the conciseness of just having the ext func. But TIL about buildMap!

You can always make a helper function:

fun <K: Any, V: Any> nonNullHashMapOf(
    vararg entries: Pair<K?, V?>
): HashMap<K, V> {
    val map = HashMap<K, V>()
    for ((key, value) in entries) {
        map[key ?: continue] = value ?: continue
    }
    return map
}

And use it

val data = nonNullHashMapOf(
    "firstName" to firstName,
    "lastName" to lastName,
)

I would name it as

mapOfNotNull

just to match

listOfNotNull

better, but yeah that can also be done

Oooh boy! So many options!

@ephemient That's probably a good idea. I'm not great at naming (like most programmers...)

I'm not great either, I'm just copying stdlib here :)

If I use

fun <K: Any, V: Any> nonNullHashMapOf(vararg entries: Pair<K?, V?>) {
    val map = HashMap<K, V>()
    for ((key, value) in entries) {
        map[key ?: continue] = value ?: continue
    }
}

should I try to use a Regular map internally, or is HashMap alright?

Use a hash map if you need a hash map for some reason, otherwise just use

mutableMapOf<K, V>()

if your function is named

hashMapOf

it should probably return a

HashMap

to reduce confusion 😛

(By the way, I update my comment to include the return value, which I forgot the first time.)

in practice

MutableMap

is backed by a

LinkedHashMap

, so it is a

HashMap

in a sense

Okay, so I'm using @Ruckus method you outlined, but @ephemient naming (stdlib naming). Anything else I should do to improve? (the only reason I asked about converting this to a regular Map instead of HashMap was because that was in the opening comment.

fun <K: Any, V: Any> mapOfNotNull(
    vararg entries: Pair<K?, V?>
): HashMap<K, V> {
    val map = HashMap<K, V>()
    for ((key, value) in entries) {
        map[key ?: continue] = value ?: continue
    }
    return map
}

If you don't need a hash map:

fun <K: Any, V: Any> mapOfNotNull(
    vararg entries: Pair<K?, V?>
): Map<K, V> = buildMap {
    for ((key, value) in entries) {
        this[key ?: continue] = value ?: continue
    }
}

@ephemient I'll file a bug fore firebase docs to just be mapOf instead of hashMapOf

With the new definitely non null types, this could also be

fun <K, V> mapOfNotNull(
    vararg entries: Pair<K, V>
): Map<K & Any, V & Any> = buildMap {
    for ((key, value) in entries) {
        this[key ?: continue] = value ?: continue
    }
}

Though I don't know if that's any better... https://blog.jetbrains.com/kotlin/2022/04/kotlin-1-6-20-released/#definitely-non-nullable-types

TIL "Definitely non-nullable types"

Just in case it’s helpful, GitLive has a library that lets you read and write kotlinx.serialization objects to/from firebase instead of using the HashMap based API. https://firebaseopensource.com/projects/gitliveapp/firebase-kotlin-sdk/