Kotlin: How to work with List casts: Unchecked Cast: kotlin.collections.List<Kotlin.Any?> to kotlin.colletions.List<Waypoint>
Solution 1
In Kotlin, there's no way to check the generic parameters at runtime in general case (like just checking the items of a List<T>
, which is only a special case), so casting a generic type to another with different generic parameters will raise a warning unless the cast lies within variance bounds.
There are different solutions, however:
-
You have checked the type and you are quite sure that the cast is safe. Given that, you can suppress the warning with
@Suppress("UNCHECKED_CAST")
.@Suppress("UNCHECKED_CAST") val waypointList = list as? List<Waypoint> ?: return null
-
Use
.filterIsInstance<T>()
function, which checks the item types and returns a list with the items of the passed type:val waypointList: List<Waypoint> = list.filterIsInstance<Waypoint>() if (waypointList.size != list.size) return null
or the same in one statement:
val waypointList = list.filterIsInstance<Waypoint>() .apply { if (size != list.size) return null }
This will create a new list of the desired type (thus avoiding unchecked cast inside), introducing a little overhead, but in the same time it saves you from iterating through the
list
and checking the types (inlist.foreach { ... }
line), so it won't be noticeable. -
Write a utility function that checks the type and returns the same list if the type is correct, thus encapsulating the cast (still unchecked from the compiler's point of view) inside it:
@Suppress("UNCHECKED_CAST") inline fun <reified T : Any> List<*>.checkItemsAre() = if (all { it is T }) this as List<T> else null
With the usage:
val waypointList = list.checkItemsAre<Waypoint>() ?: return null
Solution 2
To improve @hotkey's answer here's my solution:
val waypointList = list.filterIsInstance<Waypoint>().takeIf { it.size == list.size }
This gives you the List<Waypoint>
if all the items can be casted, null otherwise.
Solution 3
In case of generic classes casts cannot be checked because type information is erased in runtime. But you check that all objects in the list are Waypoint
s so you can just suppress the warning with @Suppress("UNCHECKED_CAST")
.
To avoid such warnings you have to pass a List
of objects convertible to Waypoint
. When you're using *
but trying to access this list as a typed list you'll always need a cast and this cast will be unchecked.
Solution 4
I made a little variation to @hotkey answer when used to check Serializable to List objects :
@Suppress("UNCHECKED_CAST")
inline fun <reified T : Any> Serializable.checkSerializableIsListOf() =
if (this is List<*> && this.all { it is T })
this as List<T>
else null
Solution 5
Instead of
myGenericList.filter { it is AbstractRobotTurn } as List<AbstractRobotTurn>
I like doing
myGenericList.filter { it is AbstractRobotTurn }.map { it as AbstractRobotTurn }
Not sure how performant this is, but no warnings at least.
Lukas Lechner
Freelance Android and Kotlin Developer from Austria.
Updated on September 09, 2021Comments
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Lukas Lechner over 2 years
I want to write a function that returns every item in a
List
that is not the first or the last item (a via point). The function gets a genericList<*>
as input. A result should only be returned if the elements of the list are of the typeWaypoint
:fun getViaPoints(list: List<*>): List<Waypoint>? { list.forEach { if(it !is Waypoint ) return null } val waypointList = list as? List<Waypoint> ?: return null return waypointList.filter{ waypointList.indexOf(it) != 0 && waypointList.indexOf(it) != waypointList.lastIndex} }
When casting the
List<*>
toList<Waypoint>
, I get the warning:Unchecked Cast: kotlin.collections.List to kotlin.colletions.List
I can't figure out a way to implement it otherwise. What's the right way to implement this function without this warning?
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Lukas Lechner about 8 yearsGreat Answer! I choose list.filterIsInstance<Waypoint>() solution because I think it's the cleanest solution.
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mfulton26 about 8 yearsNote that if you use
filterIsInstance
and the original list contains elements of a different type your code will silently filter them out. Sometimes this is what you want but sometimes you might rather have anIllegalStateException
or similar thrown. If the later is the case then you can create your own method to check and then cast:inline fun <reified R> Iterable<*>.mapAsInstance() = map { it.apply { check(this is R) } as R }
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bj0 over 6 yearsNote that
.apply
does not return the return value of the lambda, it returns the receive object. You probably want to use.takeIf
if you want the option to return a null. -
daviscodesbugs over 4 yearsWas looking for a solution like this, but this errors:
Cannot access 'Serializable': it is internal in 'kotlin.io'
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Fortran over 3 yearssolution: In android to pass a user-defined object around, your class should implements Parcelable instead of Serializable interface.