Unrelated question. How is

MutableList

both an interface and a class in the stdlib without their names clashing? Do they have separate jvmNames?

can you link to the doc page for the class? I wasn’t aware that there was duplication here

All I know is I can write this:

class foo<T>(data: MutableList<T>): MutableList<T> by data {
}

And you can only delegate to interfaces. And I can also write this:

val foo = MutableList(10) { it + 2 }

What is going on here? Is the second not an invocation?

it looks like a top-level function defined in

Collections.kt

Yeah… Here’s the body of the function:

public inline fun <T> MutableList(size: Int, init: (index: Int) -> T): MutableList<T> {
    val list = ArrayList<T>(size)
    repeat(size) { index -> list.add(init(index)) }
    return list
}

Maybe I just never noticed or does IDEA make class instantiation calls look the same as function calls? How did i get that mixed up?

Well without a

new

operator constructor calls are indistinguishable from function calls.

on my machine, IDEA renders the

MutableList

invocation in italics, much like static methods in java or like extension functions (without the orange text color anyhow)

I want to avoid using

open

classes whenever possible, so I'm trying to use delegation to implement wrapper classes succinctly.

Behind the scenes, when you create a kotlin list, it creates a java collection. The kotlin interfaces is how we wind up with List and MutableList.

Yeah i think

MutableList

is just an extension of Java's

ArrayList

with the kotlin stdlib collections functions added

Here's a quote from the JetBrains course: "_Under the hood, both these interfaces are compiled to java.util.List, to the same interface. And also by default when you create an instance of ArrayList, java.util.ArrayList will be created and Kotlin somehow pretends that java.util.ArrayList implements Kotlin MutableList. There is some kind of compiler magic when a Java interface is substituted by Kotlin interface._"

Just Ctrl+Click in IDEA and you’ll get to the source.

@elizarov all I see when I do that is the interface.