Why use MutableList<T> instead of ArrayList<T> in Kotlin? What was the idea standing behind the creation of that class in Kotlin when mostly the rest of the collections are used as declared in Java? If you have any docs/articles that might help me understand it, it would be very helpful. Thanks 👋🏼

In Java the

List

interface implies mutability - it has

add

and

remove

etc. methods. If you make an immutable List implementation you have to keep all of those methods and make them throw

UnsupportedOperationException

. In Kotlin

List

does not have those methods, they are only added in

MutableList

. Which is much better - you have to explicitly type for mutability, and if a method returns

List

rather than

MutableList

you know you are not intended to mutate it.

ArrayList

in Kotlin, as in Java, is a particular way of implementing a List (mutable or otherwise)- via a backing memory array. You can also implement it as a

LinkedList

, or in other ways.

Interesting how ArrayList extends MutableList

MutableList is indeed an interface, but there is also a standalone function with the same name. So when you write

val x = MutableList<String>(10) { it.toString() }

, although it looks as if you're calling a constructor, you are in fact calling a standalone function MutableList which returns a MutableList object implemented as an ArrayList. The interface

MutableList

is indeed preferred to be used as an argument rather than

ArrayList

. If you want to distinguish between ArrayList and LinkedList, for example, to use a better performing algorithm when you know that the list features fast random access, you can use the fact that such a list may implement the

RandomAccess

marker interface.