Issue
I'm trying to think of a function that would allow a Map<String, Any?>
object to be treated as Map<String,Any>
through type inference through applying a single function.
I am pretty new to the transformation functions in Kotlin and have tried the various filter
and filterValues
filterNot
on the map like so:
val input = mapOf(Pair("first",null))
val filtered: Map<String,Any> = input.filter { it.value!=null }
it also fails to compile with any of these
input.filterValues { it!=null }
input.filterNot { it.value==null }
input.filterNot { it.value is Nothing }
The closest I can seem to get is applying multiple steps or having an Unchecked cast warning. I would have thought that filtering the values to be !=null
would suffice. My only other thought is that it's due to the generics?
Solution
The filter functions return a Map with the same generic types as the original map. To transform the type of the value, you need to map the values from Any? to Any, by doing a cast. The compiler can't know that the predicate you pass to filter() makes sure all the values of the filtered map are non-null, so it can't use type inference. So your best et is to use
val filtered: Map<String, Any> = map.filterValues { it != null }.mapValues { it -> it.value as Any }
or to define a function doing the filtering and the transformation in a single pass, and thus be able to use smart casts:
fun filterNotNullValues(map: Map<String, Any?>): Map<String, Any> {
val result = LinkedHashMap<String, Any>()
for ((key, value) in map) {
if (value != null) result[key] = value
}
return result
}
Answered By - JB Nizet
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