Kotlin asterisk operator before variable name or Spread Operator in Kotlin

Question

I want to know what exactly an asterisk does before a variable name in Kotlin. I saw this (*args) in a Spring boot Kotlin example:

@SpringBootApplication
open class Application {

    @Bean
    open fun init(repository: CustomerRepository) = CommandLineRunner {
        repository.save(Customer("Jack", "Bauer"))
        repository.save(Customer("Chloe", "O'Brian"))
        repository.save(Customer("Kim", "Bauer"))
        repository.save(Customer("David", "Palmer"))
        repository.save(Customer("Michelle", "Dessler"))
    }
}

fun main(args: Array<String>) {
    SpringApplication.run(Application::class.java, *args)
}

Answer 1

The * operator is known as the Spread Operator in Kotlin.

From the Kotlin Reference:

When you call a vararg-function, you can pass arguments individually, for example asList(1, 2, 3). if you already have an array and want to pass its contents to the function, use the spread operator (prefix the array with *):

It can be applied to an Array before passing it into a function that accepts vararg.

For Example...

If you have a function that accepts a varied number of arguments...

fun sumOfNumbers(vararg numbers: Int): Int {
    return numbers.sum()
}

Use the spread operator to pass an array's elements as the arguments:

val numbers = intArrayOf(2, 3, 4)
val sum = sumOfNumbers(*numbers)
println(sum) // Prints '9'

---

Notes:

• The * operator is also the multiplication operator (of course).
• The operator can only be used when passing arguments to a function. The result of the operation cannot be stored since it yields no value (it is purely syntactic sugar).
• The operator may confuse some C/C++ programmers at first because it looks like a pointer is being de-referenced. It isn't; Kotlin has no notion of pointers.
• The operator can be used in-between other arguments when calling a vararg-function. This is demonstrated in the example here.
• The operator is similar to the apply function in various functional programming languages.

Answer 2

In addition to the answers that were directly towards "what is this thing!?!", you often have the case where you have a List and want to pass it to a function that is expecting a vararg. For this, the conversion is:

someFunc(x, y, *myList.toTypedArray())

Assuming that last parameter of someFunc is vararg of the same type as the elements in the list.

Answer 3

As described in the documentation this is a spread operator:

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 *):

val a = arrayOf(1, 2, 3) 
val list = asList(-1, 0, *a, 4)

Answer 4

In Java you can pass an array as is but an advantage of unpacking an array with spread operator * is that spread operator lets you combine the values from an array and some fixed values in a single call. Java doesn't support this.

Answer 5

If a function which accept a vararg(Variable number of arguments) parameter like:

fun sum(vararg data:Int)
{
   // function body here         
}

Now to call this method, we can do:

sum(1,2,3,4,5)

But what if we have these value in an array, like:

val array= intArrayOf(1,2,3,4,5)

then, to call this method we have to use spread operator, like:

 sum(*array)

Here, *(spread operator) will pass all content of that array.

*array is equivalent to 1,2,3,4,5

But wait a minute, what if we call it like this: sum(array) it will give us Type Mismatch compile time error:

Type mismatch.
Required:Int
Found:IntArray

The problem is sum function accept a vararg Int parameter(which accept value like: 1,2,3,4,5) and if we pass array, it will be passed as IntArray.