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?

Oliver Charlesworth · Accepted Answer · 2017-12-23T19:03:45.677

6

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).

edited Dec 23 '17 at 19:03

answered Dec 23 '17 at 18:47

Oliver Charlesworth

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So it means use "apply" function as a wrapper of any void method, so than the builder can keep? – ShangXubin Dec 23 '17 at 18:50
@ShangXubin - `apply` can be used to wrap any number of methods, but it always returns the object. – Oliver Charlesworth Dec 23 '17 at 18:52

score 1 · Answer 2 · answered Dec 23 '17 at 19:02

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.

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

Builder-style usage of methods that return Unit

2 Answers2

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