The "Builder-style usage of methods that return Unit" on Kotlin website confuse me

Question

The Idioms section of the official Kotlin docs contains this example:

Builder-style usage of methods that return Unit

fun arrayOfMinusOnes(size: Int): IntArray {
    return IntArray(size).apply { fill(-1) }
}

As the function apply returns the generic type, and I thought Unit is as same as void in Java, this section is suggesting we can use a void method in builder-style? That doesn't make sense to me - what's it trying to say?

Answer 1

The point it's trying to make is that if you just did traditional Java builder style, like this:

return IntArray(size)
    .fill(-1)

then it wouldn't compile, because it's of type Unit, not IntArray. So traditionally, you'd have to do something like this:

val ret = IntArray(size)
ret.fill(-1)
return ret

apply enables you to avoid this, because the return type is still of type IntArray (or T, in general).