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Why are you casting to `ArrayList` in the first place? It is not defined which list implementation is returned by these methods.

yeah, basically I have a method that expects the output to be ArrayList

Then you will have to explicitly do `ArrayList(&lt;existing list&gt;)`, which creates another copy.

However if possible you should adjust those methods, code against interfaces (`List`, `MutableList`) whenever possible.

If you'll look at the response of `l2.distnct()` with your sample data then it is a String "A" and String cannot be casted directly to `ArrayList&lt;String&gt;`

Yes, you're right. But why is returned type is different based on output, ideally it should be same irrespective of output, right?

I want to understand the reason behind this implementation in the language itself.

The stdlib optimizes small lists so that not the whole arraylist needs to be kept around for a small result

```However if possible you should adjust those methods, code against interfaces (`List`, `MutableList`) whenever possible.``` why should I refactor to List and MutableList? <@U589QURRQ>

It's good OOP practice. You should not rely on a specific implementation, just specify which interface contract needs to be fulfilled

You see the reason right here: Your method demands an `ArrayList`, even though it really only needs a `MutableList` or even just a `List`. That limits your ability to use this method

you may try using `l2.distinct().toList()`

You are _required_ to convert everything to an `ArrayList`, even if its already a `List`

<@UQELRUSH3> No. That returns a `List`, not `ArrayList`.

<@U589QURRQ> `l2.distinct()` can't be casted directly then `a.distinct().toList()` will be available for casting without any exception; despite this all I still wonder the return type of `distinct` is `List&lt;T&gt;` and should work directly.

The return type of `distinct` is `List`. That means it can return something other than `ArrayList`.

`toList()` does not guarantee the underline implementation is `Arraylist`. it might work today, but the contract is to just have a `List`, kotlin stdlib can change implementation any time and the cast would fail

that, or <https://kotlinlang.slack.com/archives/C0922A726/p1573550525218200?thread_ts=1573550408.215700&amp;cid=C0922A726>