Kotlin: How to work with List casts: Unchecked Cas

问题:

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 generic List<*> as input. A result should only be returned if the elements of the list are of the type Waypoint:

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<*> to List<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?

回答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 (in list.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
  

回答2:

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 Waypoints 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.