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()

Answer 1

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()

Answer 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) 
    }