Kotlin Crash Course for Programmers/

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Default Values & Named Parameters

Default Values & Named Parameters

Discover how to use default parameter values to increase the flexibility of your functions and how to call functions using named parameters.

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In Kotlin, you can define default values for function parameters to make them optional.

Default Parameter Values #

Let’s say you have a function that joins a collection of strings into a single string:

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fun join(strings: Collection<String>, delimiter: String) = strings.joinToString(delimiter)

For instance, join(listOf("Kotlin", "Java"), " > ") would return "Kotlin > Java".

With this function definition, you always have to pass in a delimiter. But most commonly, you’ll want to have a comma as delimiter, so it makes sense to set this as the default value:

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fun join(strings: Collection<String>, delimiter: String = ", ") = strings.joinToString(delimiter)

Default values are defined by simply adding an assignment to the parameter definition. This way, you now have different ways to call this function:

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fun join(strings: Collection<String>, delimiter: String = ", ") = strings.joinToString(delimiter)
fun main() {
  val planets = listOf("Saturn", "Jupiter", "Earth", "Uranus")
  val joined1 = join(planets, " - ")
  val joined2 = join(planets)
  println(joined1)
  println(joined2)
}

You’re still free to use a custom delimiter by passing in a value, but you can now omit the delimiter to simplify the function call. Think of these as ...