Casting Kotlin ArrayLists ClassCastException

Question

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:

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?

Ilya · Accepted Answer · 2018-07-01 23:27:03Z

5

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

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

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

edited Jul 1, 2018 at 23:27

answered Jul 1, 2018 at 23:21

Ilya

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2 Comments

Gus W Over a year ago

Thanks, sorting it in place is a much better option. What is the difference between sorting in place with sortWith and sortBy?

Ilya Over a year ago

@GusW sortBy { } is a shortcut for sortWith(compareBy { }). Better to ask as a separate question, and you'll get a more detailed answer.

user8959091 · Accepted Answer · 2018-07-01 23:27:38Z

0

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 }

answered Jul 1, 2018 at 23:27

user8959091

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