Default parameters

Learn to define parameters with default values.

Default parameters: default values

You can give a parameter a default value: fun greet(name: String, msg: String = "Hi"). If the caller omits it, the default is used; if provided, it overrides.

fun greet(name: String, msg: String = "Hi") {
    println("$msg $name")
}
greet("Ava")          // Hi Ava
greet("Ava", "Hello") // Hello Ava

Default parameters let one definition support many usage forms; they reduce the need for overloads.

Order of default parameters

Default parameters usually sit at the end of the parameter list. This lets the caller pass the leading required parameters by position and omit the rest.

fun order(item: String, qty: Int = 1, gift: Boolean = false)
order("book")           // qty=1, gift=false
order("book", 3)        // qty=3, gift=false
order("book", 3, true)  // qty=3, gift=true

If a non-default parameter comes after a default one, the caller must use a named argument to skip the default one.

Exercise Define greet with a default parameter

Define a fun greet(name: String, msg: String = "Hello") whose body is println("$msg $name"). From main call greet("Ava").

Show example solution

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

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

Test yourself

Fill in the blank: default value

fun greet(name: String, msg: String ____ "Hello")

Show answer

fun greet(name: String, msg: String = "Hello")

Match call form to outcome

msg: String = "Hi"

Parameter with default
name: String

Required parameter
greet("Ava")

Default is used
greet("Ava", "Hi")

Default is overridden
fun greet(name: String, msg: String = "Merhaba"); what does greet("Ayşe") print?
- Merhaba Ava
- Ava
- Throws an exception
- Hi Ava
Show answer

Merhaba Ava — Since msg is not given, the default "Merhaba" is used.

Next lesson: Named arguments

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