Multi tool use

Casting Kotlin ArrayLists ClassCastException

I have a MutableList<Card> called cards that I am sorting based on one of the properties, using the sortedWith function. This returns a sorted generic list type, so a cast is necessary. However, when I cast the list, it crashes with a ClassCastException:

MutableList<Card>

cards

sortedWith

private var cards: MutableList<Card> = ArrayList()
...
cards = cards.sortedWith(compareBy{it.face}) as ArrayList<Card>



java.lang.ClassCastException: java.util.Arrays$ArrayList cannot be cast to java.util.ArrayList


EDIT: I just realized I need to use the more generic type of cards for the cast, MutableList<Card>. Now, can someone explain why the cast with ArrayList fails?

MutableList<Card>

2 Answers
2

The cast fails because the list returned by sortedWith function is not an instance of java.util.ArrayList.

sortedWith

java.util.ArrayList

Moreover it's unsafe to cast it to MutableList too, because the implementation of sortedWith can be changed later, so that it returns List that's no longer a MutableList.

MutableList

sortedWith

List

MutableList

If you have a MutableList and you want to sort it, you have two options:

MutableList

either sort it in-place with the sortWith function (not sortedWith):

sortWith

sortedWith

cards.sortWith(compareBy{it.face})
// now cards list is sorted


or sort it to a new list and then copy it to a mutable list, if you need to mutate it after

cards = cards.sortedWith(compareBy{it.face}).toMutableList()


sortBy { }

sortWith(compareBy { })

There are easier ways to sort a list in place without creating a new one and assign back to the original. The simplest is cards.sortBy { it.face }

cards.sortBy { it.face }

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