How can I convert a kotlin Map<String, String?> to Map<String, String> in a clear way?

Question

I'd like to find the clearest and most elegant way to convert a Map<String, String?> to a Map<String, String>, filtering out the pairs with null values.

I have a contrived solution below, but I don't like how I have to do an unsafe !!. Is there a better way to do this?

fun Map<String, String?>.filterNonNull() = this
            .filter { it.value != null }
            .map { it.key to it.value!! }
            .toMap()

I'd argue that there is nothing bad about !! in this particular situation: it's pretty simple function, and it's clear that there is no NPE. So I like your solution (it can be simplified/optimized by using kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/…) — user2956272
– user2956272, Commented Jul 23, 2019 at 20:38
if you've already filtered out all the null results then I don't see how your !! is unsafe — Quinn
– Quinn, Commented Jul 23, 2019 at 20:45
It's actually much more questionable that you have Map<..., ...?> in the first place. When I write your_map[k] and receive null, I would usually expect it to means that the key is not present in the map. Maybe it's possible to avoid creating such map at all? — user2956272
– user2956272, Commented Jul 23, 2019 at 20:59

Alexey Romanov · Accepted Answer · 2019-07-23 20:16:43Z

2

mapNotNull serves as a combination of map and filter, but returns a List and not the Map you want so

fun <K, V> Map<K, V?>.filterNonNull(): Map<K, V> = 
    this.mapNotNull { 
        (key, value) -> if (value == null) null else Pair(key, value) 
    }.toMap()

edited Jul 23, 2019 at 20:16

answered Jul 23, 2019 at 20:10

Alexey Romanov

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blackr4y · Accepted Answer · 2019-07-23 20:38:36Z

2

Based on the discussion here you can also use something like this:

fun <K, V> Map<K, V?>.filterNotNullValues(): Map<K, V> =
    mutableMapOf<K, V>().apply { 
        for ((k, v) in this@filterNotNullValues) if (v != null) put(k, v) 
    }

answered Jul 23, 2019 at 20:38

blackr4y

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