Kotlin: pass array to varargs method with spread operator

Question

I spent far too long trying to work out how to pass a number of Migration instances to a Room database builder for a Dagger2 module.

The Room database builder requires context and the database name.

Room.databaseBuilder(context, Database::class.java, dbName)
                .build()

Migrations can be added to the builder with the addMigrations method which takes vararg migrations: Migration!

One option is to create the builder.

val builder = Room.databaseBuilder(context, Database::class.java, dbName)

And then forEach through the migrations

migrations.forEach { builder.addMigrations(it) }

however this is messy and unnecessary, and the spread operator should be used instead.

I managed to miss this as the only documentation I found for the spread operator was a single sentence in the docs.

When we call a vararg-function, we can pass arguments one-by-one, e.g. asList(1, 2, 3), or, if we already have an array and want to pass its contents to the function, we use the spread operator (prefix the array with *)

Possible duplicate of Varargs Kotlin Java interop not working properly — Ruckus T-Boom
– Ruckus T-Boom, Commented Jul 10, 2017 at 20:30

Theo Pearson-Bray · Accepted Answer · 2017-07-10 20:13:42Z

4

In order to pass the array to the function. The array just needs to be prefixed with an asterisk *.

val migrations: Array<Migration>
Room.databaseBuilder(context, Database::class.java, dbName)
                .addMigrations(*migrations)
                .build()

In general if we have

fun foo(vararg things: Type) {
    //Do something with the things
}

We can call foo with arrayOf, or with an Array.

foo(*Array(n, { it.toType()}))
foo(*arrayOf(instance1, instance2, instance3))
val array = Array<Type>(n, {it.toType()})
foo(*array)

answered Jul 10, 2017 at 20:13

Theo Pearson-Bray

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5 Comments

holi-java Over a year ago

the documentation already mentioned spread operator here and here. so publish it here is unnecessary.

Theo Pearson-Bray Over a year ago

I saw the second link. What I was looking for, and didn't see originally, was an explanation like stackoverflow.com/documentation/kotlin/5835/… this, explaining what the spread is doing rather than just saying "stick an asterisk before your" argument. I also wanted to find out if there is a particular reason for having the operator, rather than automatically spreading any array passed to a vararg method.

holi-java Over a year ago

Hi, there is a demo about asList at the bottom in the second link. :)

Theo Pearson-Bray Over a year ago

I did see it. Thinking about the reason for the operator, I suppose if I have vararg things: Any and pass an array to it, it is not clear whether the array is a single argument, or the array contents are individual arguments, which is why an explicit * operator is required. Thanks for the help.

holi-java Over a year ago

Hi, the asList function signature is already tell you that will spreads all array elements into the parameters one-by-one. in short: arrayOf(1,*arrayOf(2),3) is equivalent to arrayOf(1,2,3).

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