Kotlin asterisk operator before variable name or Spread Operator in Kotlin

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

From the Kotlin Reference…

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 *):

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

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.

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