Hi there slightly smiling face why does Kotlin need `Array`

Question

Hi there slightly smiling face why does Kotlin need `Array` in the first place Why not use `Collection` and let the compiler optimize them away as JVM Arrays where possible

voddan · Answer

Arrays are here because Java has them and thus kotlin has to express this concept

voddan · Answer

You are correct it is strongly not recommended to use arrays unless you know what you are doing and you need that extra little bit of performance

voddan · Answer

Instead one should use lists in the code which in most cases are `ArrayList`

horse_badorties · Answer

not sure I got what you said sorry The concept of java Arrays could be expressed by a kotlin Collection and the kotlin compiler could provide the necessary magic And `ArrayList` is a java class not a kotlin class so I m not sure why that would be relevant

diesieben07 · Answer

You can t add interfaces to object by compiler magic You can t make Java s array objects implement `Collection` by compiler magic

voddan · Answer

Now I am not sure what it is we are talking about `Collection` is an interface `Array` is a class one can t replace the other

voddan · Answer

There is no real distinction between kotlin classes and java classes I mentioned `ArrayList` as an example of a `List` which is indeed very optimized by JVM

horse_badorties · Answer

what I meant is getting rid of `Array` altogether and only provide the `Collection` type hierarchy an Array being essentially a mutable fixed sized List I have yet to understand your Arrays are here because Java has them and thus kotlin has to express this concept I m not saying you are wrong but I still don t really understand the why slightly smiling face

diesieben07 · Answer

Arrays are the only real way to create a data structure on the JVM linked lists aside Everything HashMap ArrayList etc is ultimately based on them If Kotlin does not provide means to access this mechanic you cannot write any data structures in Kotlin

horse_badorties · Answer

yeah but ArrayList for instance does already all the array access and kotlin just uses it Why would kotlin have to have his own `Array` type

diesieben07 · Answer

Because `ArrayList` is a high level implementation You need low level building blocks like arrays for when you need performance or exact control

diesieben07 · Answer

For example something like `AtomicReferenceArray` is not possible if you don t have arrays

horse_badorties · Answer

Yes I understand and I don t doubt that the JVM needs arrays I don t understand why kotlin has to expose them Performance is largely a compiler optimization thing no

horse_badorties · Answer

and `Array` is only one dimensional so it doesn t even express the complete concept of JVM Java Arrays

diesieben07 · Answer

JVM arrays are one dimensional too Multi dimensional arrays are just arrays of pointers to other arrays

diesieben07 · Answer

Kotlin arrays work the same way `Array lt Array lt String gt gt ` works fine

horse_badorties · Answer

I see thanks

diesieben07 · Answer

It s not just about performance You can do things with arrays e g do CAS on the elements at least when we get varhandles in java 9 before that you need Unsafe that you cannot do with ArrayList

horse_badorties · Answer

CAS

diesieben07 · Answer

compare and swap an operation you need for high efficiency concurrent algorithms

voddan · Answer

gt Arrays are here because Java has them and thus kotlin has to express this concept Consider if java exposes overloads `test String x ` and `test List lt String gt y ` how do you call one over the other if you can t provide an array