Kotlin Objects
Creating single instances and singletons
🎯 What are Objects?
Objects in Kotlin are single instances that exist only once in your program. Unlike classes that can create multiple instances, objects are singletons - perfect for utilities and shared resources.
// Object declaration - only one instance exists
object MathUtils {
fun add(a: Int, b: Int): Int {
return a + b
}
fun multiply(a: Int, b: Int): Int {
return a * b
}
}
Usage:
val result = MathUtils.add(5, 3)
println(result) // 8
Key Object Concepts
Singleton
Only one instance exists
object Database { }Utility Functions
Shared helper methods
object Utils { fun format() }Companion Objects
Objects inside classes
companion object { }Anonymous Objects
Objects without names
object : Interface { }🔹 Object Declaration
Create a singleton object that can be used throughout your program:
object Logger {
private var logCount = 0
fun log(message: String) {
logCount++
println("[$logCount] $message")
}
fun getLogCount(): Int {
return logCount
}
}
fun main() {
Logger.log("Application started")
Logger.log("User logged in")
Logger.log("Data saved")
println("Total logs: ${Logger.getLogCount()}")
}Output:
[1] Application started
[2] User logged in
[3] Data saved
Total logs: 3
🔹 Companion Objects
Objects inside classes that can be called without creating an instance:
class Person(val name: String, val age: Int) {
companion object {
private var personCount = 0
fun createChild(name: String): Person {
personCount++
return Person(name, 0)
}
fun createAdult(name: String): Person {
personCount++
return Person(name, 18)
}
fun getTotalPeople(): Int {
return personCount
}
}
fun introduce() {
println("Hi, I'm $name, age $age")
}
}
fun main() {
val child = Person.createChild("Tommy")
val adult = Person.createAdult("Sarah")
child.introduce()
adult.introduce()
println("Total people created: ${Person.getTotalPeople()}")
}Output:
Hi, I'm Tommy, age 0
Hi, I'm Sarah, age 18
Total people created: 2
🔹 Anonymous Objects
Create objects on-the-fly without declaring a class:
interface ClickListener {
fun onClick()
}
fun main() {
// Anonymous object implementing an interface
val button = object : ClickListener {
override fun onClick() {
println("Button was clicked!")
}
}
// Anonymous object with custom properties
val config = object {
val appName = "MyApp"
val version = "1.0"
fun getInfo() = "$appName v$version"
}
button.onClick()
println(config.getInfo())
}Output:
Button was clicked!
MyApp v1.0
🔹 Object vs Class
Understanding when to use objects vs classes:
// Use CLASS when you need multiple instances
class Car(val brand: String) {
fun start() = println("$brand starting...")
}
// Use OBJECT when you need only one instance
object CarFactory {
private var carsProduced = 0
fun createCar(brand: String): Car {
carsProduced++
println("Producing car #$carsProduced")
return Car(brand)
}
}
fun main() {
// Multiple car instances
val car1 = CarFactory.createCar("Toyota")
val car2 = CarFactory.createCar("Honda")
car1.start()
car2.start()
// Only one CarFactory exists
}Output:
Producing car #1
Producing car #2
Toyota starting...
Honda starting...