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?
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).
Take this one:
class X {
var a: Int? = null
var b: Int? = null
fun first(a: Int) = apply { this.a = a }
fun second(b: Int) = apply { this.b = b }
}
X().first(2).second(3)
The apply functions are used to return the instance of X after setting the property. This enables builder-style call of both methods. If apply were removed, the function would return Unit.
