companion object
- Kotlin course
- Intermediate
- Lesson 5/116
- 5 min
Learn class-level members with companion object.
Companion object
The companion object defines class-level members. Kotlin has no static keyword; instead, members inside a companion object are accessed through the class name.
Example: class Counter { companion object { var total = 0; fun create() = Counter() } }. You call Counter.total and Counter.create(). Each class can have only one companion object. Members are accessed through the class name, so they are usable without an instance.
Factory method
A method inside a companion object can act as a factory: it centralizes instance creation. It is useful to enforce constraints, keep counters or return different subtypes.
Example: companion object { var total = 0; fun create(): Counter { total += 1; return Counter() } }. Each call to create increments total. The companion object tracks how many instances have been produced. This is a clean way to manage setup logic without leaking it outside the class.
Practice Add a companion object to the Counter class
Define a class named Counter with a primary constructor taking val id: Int. Add a companion object containing var total: Int = 0 and a fun create(): Counter method. The create method increments total by 1 and returns Counter(total). In main call Counter.create() twice, then print Counter.total and the second instance's id.
Starter code
// Define the Counter class
// primary: val id: Int
// companion object: var total and fun create()
// create increments total and returns Counter(total)
// call create twice in main, print total and idSample solution
class Counter(val id: Int) {
companion object {
var total: Int = 0
fun create(): Counter {
total += 1
return Counter(total)
}
}
}
fun main() {
val a = Counter.create()
val b = Counter.create()
println(Counter.total)
println(b.id)
}Expected output
2 2
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which statement about companion object is correct?
- A class can have multiple companion objects
- Companion object members are called through the class name Correct answer
- Companion object members are called on an instance
- A companion object can contain only val
Explanation: Companion object members are accessed through the class name.
2 Match the companion object parts
| Term | Meaning |
|---|---|
companion object |
Block keyword |
var total = 0 |
Class-level state |
fun create() |
Factory method |
Counter.total |
Class-name access |
3 Complete the companion object block
class Counter {
____ object {
var total = 0
}
}class Counter {
companion object {
var total = 0
}
}Explanation: The companion object block defines class-level members.