Careeroza

Object Declarations (Singletons)

One Instance Only

An object declaration defines a class and creates its single instance at the same time. It is created lazily on first use and is thread-safe. Use it for things the whole app shares: a logger, a config, an in-memory cache.

Example

object AppSession {
    var userName: String? = null
    private var loginCount = 0

    fun login(name: String) {
        userName = name
        loginCount++
        println("Logged in as $name (login #$loginCount)")
    }
}

fun main() {
    AppSession.login("Asha")
    AppSession.login("Ravi")
    println(AppSession.userName)
    println(AppSession === AppSession)   // always the same object
}

Output

Logged in as Asha (login #1)
Logged in as Ravi (login #2)
Ravi
true

Objects Can Inherit

Example

interface Comparator2<T> { fun compare(a: T, b: T): Int }

object ByLength : Comparator2<String> {
    override fun compare(a: String, b: String) = a.length - b.length
}

fun main() {
    val words = listOf("banana", "kiwi", "apple")
    println(words.sortedWith { a, b -> ByLength.compare(a, b) })
}

Output

[kiwi, apple, banana]

Note: Avoid storing mutable global state in objects in large apps - it makes testing hard. For shared services, dependency injection (Hilt/Koin) usually provides the single instance instead.