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Kotlin Default Arguments

A default argument is a value a parameter uses when a call leaves that argument out. A call can still pass a real value and replace the default.

A greeting with a default name

greet carries the default name friend in its definition. The first call leaves the name out and the second call passes a name.

fun greet(name: String = "friend") {
    println("Hello, $name")
}

fun main() {
    greet()
    greet("Ana")
}

A default tax rate

total adds a tax of 5 when the call gives no tax. The second call passes 20 and replaces the default.

fun total(price: Int, tax: Int = 5): Int {
    return price + tax
}

fun main() {
    println(total(100))
    println(total(100, 20))
}

Replacing only the default by name

The price stays positional here and the tax is named. Only the default changes in this call.

fun total(price: Int, tax: Int = 5): Int {
    return price + tax
}

fun main() {
    println(total(200, tax = 10))
}

Example

fun makeLabel(name: String, label: String = "guest"): String {
    return "$name is a $label"
}

fun main() {
    println(makeLabel("Ana"))
    println(makeLabel("Bo", "member"))
    println(makeLabel("Cy", label = "host"))
}

Key points

  • Write a default with an = sign after the parameter name in the definition.
  • Use a default to make an argument optional at a call.
  • A call that passes no value for a parameter uses its default.
  • A call can replace a default by passing a value for that parameter.

In this example, makeLabel takes a name and a label with the default guest. The first call passes only a name, so the default fills the label. The second call passes a label in the normal position. The third call names the label and replaces the default that way.

Note

A parameter with no default must still get a value at every call.

Try it Yourself

Edit the code, then press Run.

Running is simulated · Kotlin Playground

Exercise

A parameter has the default value 10. A call passes no value for it. Which value does the function use?