companion extensions
- Kotlin course
- Intermediate
- Lesson 39/116
- 5 min
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.
Practice 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.
Starter code
// Define the User class with a class-level singleton block
// Companion reference and default extension definition
// call default in main and print nameSample solution
class User(val name: String) {
companion object
}
fun User.Companion.default(): User {
return User("guest")
}
fun main() {
println(User.default().name)
}Expected output
guest
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 How is a <strong>companion extension</strong> called?
- Through a class instance
- Only from a subclass
- Directly via the class name Correct answer
- With a new object
Explanation: A companion extension is called directly via the class name.
2 Match the companion extension parts
| Term | Meaning |
|---|---|
ClassName.Companion |
Receiver reference |
fun User.Companion.default |
Extension definition |
User.default() |
Call |
User("guest") |
Return value |
3 Complete the companion extension definition
fun User.____.default(): User {
return User("guest")
}fun User.Companion.default(): User {
return User("guest")
}Explanation: The Companion reference points to the companion object.