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kotlinlang

How expensive is `MutableList::toList`? Is it just basically a toggle or something, or does it have to copy values into a new object? I imagine that perhaps mutable list is a wrapper for list, but i'm not sure

Mutable list is not a wrapper, it's interface and actual implementatation may be different, by default it is Java ArrayList

so dang, it is as expensive as `List::toMutableList`, then

Yes, it's exactly the same, all those extensions create new list so possible to use for protective copy

Do you have some specific usecase? Because if you don't need a copy, just use MuyableList directly where you need List

I think the convention is that methods starting with `to` will copy all the content to a new structure, while methods starting with `as` will just create a view over the original structure.

Depending on your use case, you can cast it to `List` or return it as a `List` from a function so the caller only sees the ‘immutable’ interface. I believe it’s an ArrayList (by default) under the covers regardless, and Kotlin merely relies on the interface to ‘control’ access.

```
SomeClass {
    private val _myList: MutableList&lt;String&gt;? = null
    val myList: List&lt;String&gt;? = _myList
    // do things
}
```

It's not always ArrayList, for example listOf(1) will create SingletoneList under the hood and emptyList() doesn't create new instance, but mostly it's true and ArrayList will be used as implementation of List