companion extensions

Learn to add extensions to companion objects.

companion extension

A companion extension adds an extension function to a class's companion object. It is written as fun ClassName.Companion.method(). It is called by class name like a static method.

Example: fun User.Companion.default(): User { return User("guest") }. This extension is called as User.default(). You can add factory methods and helpers without changing the class's own companion object. It is especially useful when you cannot touch the class source. Companion is the default name of the companion object.

The default factory method

A companion extension can add a factory method to a class from outside. The Companion reference is written as the receiver type.

Example: fun User.Companion.default(): User { return User("guest") }. Calling User.default() returns User("guest"). This is a way to add factory methods without changing the class definition. It is especially useful for third-party library classes. Inside a companion extension, this refers to the companion object.

Exercise The User.Companion.default extension

Define a class User(val name: String) with a companion object block (it may be empty). Define an extension function fun User.Companion.default(): User whose body returns User("guest"). In main print User.default().name with println.

Show example solution

class User(val name: String) {
    companion object
}

fun User.Companion.default(): User {
    return User("guest")
}

fun main() {
    println(User.default().name)
}

Test yourself

How is a <strong>companion extension</strong> called?
- Through a class instance
- Only from a subclass
- Directly via the class name
- With a new object
Show answer

Directly via the class name — A companion extension is called directly via the class name.
Match the companion extension parts

ClassName.Companion

Receiver reference
fun User.Companion.default

Extension definition
User.default()

Call
User("guest")

Return value

Complete the companion extension definition

fun User.____.default(): User {
    return User("guest")
}

Show answer

fun User.Companion.default(): User {
    return User("guest")
}

Next lesson: Standard library

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