Take a look at this kotlin one liner:
val nonNullArr : List<NonNullType> = nullArray.filter {it != null}
The compiler gives a type error at this line, saying that a list of nullables can't be assigned to a list of non-nulls. But the filter conditional makes sure that the list will only contain non null values. Is there something similar to !! operator that I can use in this situation to make this code compile?
It seems logical to assume that the compiler would take into account the predicate
it != null
and infer the type as
List<NonNullType>
but it does not.
There are 2 solutions:
val nonNullList: List<NonNullType> = nullableArray.filterNotNull()
or
val nonNullList: List<NonNullType> = nullableArray.mapNotNull { it }
As far as I know, you cannot convert nullable types into nonNull types by just verifying that they are not null. To achieve what you want, you need to manually map nullable values to non-null type by simply creating NonNull type object. For this you can use map extension function.
val nullableArray: Array<String?> = arrayOf("abc", "xyz", null, "efg")
val nonNullList: List<String> = nullableArray.filter { it != null }.map {
it.toString()
}
Or you can use filterNotNull() method as @forpas suggested in comments
val nonNullList: List<String> = nullableArray.filterNotNull()
Hope it helps you!
You can't assign a nullable type to a non-nullable type of value.
The type-matching maybe works when you assign a value, not after filter operation called.
// the type-matching works before `.filter` is called
val nonNullArr : List<NonNullType> = nullArray//.filter {it != null}
instead, if you want to do this without an error or without concerning the type. Remove the type from the val, so it goes like this
val nonNullArr = nullArray.filter {it != null}
Hope it helps