Hi all. How can I create a List with initial capacity in Kotlin similar to Java? It seems the mutabl...

# getting-started

[dineshbob]

Hi all. How can I create a List with initial capacity in Kotlin similar to Java? It seems the mutableListOf method only has elements parameter.

[voddan]

dineshbob: MutableList<String>(n, {""})

[dineshbob]

Thanks Daniil. And it looks like the list has to be initialized inside the lamda itself, right?

[voddan]

Not sure what you meant by initializing, here are the docs: http://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/-mutable-list.html

[jk]

You would have to use the ArrayList constructor: val list = ArrayList(size)

[jk]

I think the reason is because you can't make an assumption about what implementation mutableListOf gives you

[jk]

E.g if it gives you a linked list which doesn't have a size initialiser

[voddan]

@jk `MutableList`function was added in 1.1 and works the same as ArrayList

[jk]

My point is that the name MutableList just indicates "a list, that is mutable". If they wanted to change the underlying implementation in the future they could do

[jk]

Although MutableList<String>(n, {""}) will work fine - @dineshbob it's just initialising the first n as "", you can still mutate it after initialisation

[dineshbob]

Thanks @jk @voddan . If the ArrayList is created with an initial size, we can just call add() to add the elements. But if we create MutableList<String>(n, {""} then the List is initialized with empty strings. If we call add() to add the elements now, they are added after the empty strings. I see that's one big difference.

[dineshbob]

So is it ok to use the Java's ArrayList(size) directly in the Kotlin world? I thought that's not advised in Kotlin.

[muhil]

@dineshbob i don't think it is a bad practice. You could instantiate an ArrayList without any issue 😉

[jk]

Kotlin has it's own ArrayList that implements MutableList: https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/-array-list/index.html

[dineshbob]

whoa. looks like there are two ArrayList. One for JVM which is type alias to Java's ArrayList, and another one for JS which implements MutableList

[jk]

Oh I didn't notice that! So on JVM it doesn't implement MutableList?

[dineshbob]

In this document they say to use the methods such as mutableListOf to create the Lists: https://kotlinlang.org/docs/reference/collections.html @muhil

[jk]

Yes, using MutableList should be the default when you need a mutable list

[jk]

But since you seem to specifically want to use an ArrayList type structure where you specify an initial capacity of the underlying array, using the a specific ArrayList is fine

[dineshbob]

Okay. that makes sense. Thanks 🙂

[voddan]

on JVM it doesn't implement MutableList?

It does. The type hierarchy does not depend on the target platform. The types are mapped. Details: http://kotlinlang.org/docs/reference/java-interop.html#mapped-types