Avoid casting immutable collection types to mutable collection types KT-E1007

Casting to a mutable collection (such as MutableList) should be avoided as it poses inherent safety risks. A common design pattern in classes involves exposing read-only views of mutable collections as immutable collection instances to safeguard them from external mutations. Kotlin adopts this approach to prevent crashes when dealing with Lists, Maps, etc., that must remain unmodified. When this design pattern is circumvented through casting, it may succeed at runtime if the underlying list isn't a Java immutable list implementation. However, doing so can lead to hard-to-track bugs and therefore, should be avoided.

Bad Practice

val list : List<Int> = getAList()
(list as MutableList).add(42)

Recommended

Consider using toMutableLists() instead.

val list : List<Int> = getAList()
val mutableList = list.toMutableList().add(42)

References

• Stackoverflow - Casting List to MuableList in Kotlin