It allows you to directly manipulate the memory allocated by the object (trimToSize). The ArrayList cares about the 'structure' of the object. It just returns 'something that acts like a MutableList'. The difference, philosophically, is that the MutableList only cares about the behaviour of the object being returned. The difference, in practice, is that ArrayList has a few methods that are not part of the MutableList interface (trimToSize and ensureCapacity). mutableListOf() returns a MutableList, so the actual ArrayList is 'disguised' as just the parts that are described by the MutableList interface. The only difference is that arrayListOf() returns the ArrayList as an actual ArrayList. ArrayList is a class that happens to implement the MutableList interface. Under the covers, both mutableListOf() and arrayListOf() create an instance of ArrayList.

Then you probably want to either use ArrayList directly, or use the arrayListOf factory function (an ArrayList-specific analogue to mutableListOf). On the other hand, maybe you spent a bunch of time thinking about your problem, and figured that ArrayList really is the best fit for your problem, and you don't want to risk getting moved to something sub-optimal. Go with mutableListOf if that sounds like your case. mutableListOf would then have you using that new list implementation transparently, and you'd get that better behaviour for free.

Likelier than not, the Kotlin team would only make that change if they figure the new implementation works better for most use cases. Now, let's say that a future version of Kotlin changes mutableListOf to return a different type of list. In practice, in the current implementation of Kotlin compiling to the JVM, calling mutableListOf will produce an ArrayList, and there's no difference in behaviour: once the list is built, everything will behave the same. When you write, instead, val a = ArrayList(), you're saying 'I specifically want an ArrayList'. When you write val a = mutableListOf(), you're saying 'I want a mutable list, and I don't particularly care about the implementation'. The only difference between the two is communicating your intent.

Arraylist kotlin