问题:

Kotlin Website Link

The website said
"Builder-style usage of methods that return Unit"

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

As function "apply" returns the generic type, and I thought Unit is as same as void in Java, so use a void method as a builder-style, it doesn't make sense.

回答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).

回答2:

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.