Singletons

Singletons

Object Declarations and Singletons

An object declaration defines a class and its only instance in one step. The instance is created lazily on first access, and that initialization is thread-safe, so Java's double-checked locking is unnecessary. It resembles a JavaScript module that exports one shared object.

A singleton cart store and a data objectKotlin

object CartStore {
  private val titles = mutableListOf<String>()
  init { println("CartStore initialized") }
  fun add(title: String) { titles += title }
  val count get() = titles.size
}
data object EmptyCart
fun main() {
  println("BookNest started")
  CartStore.add("The Quiet Harbor")
  println("Items: ${CartStore.count}, then $EmptyCart")
}

Output

BookNest started
CartStore initialized
Items: 1, then EmptyCart

CartStore initialized prints after BookNest started, at the first call to add. The data object prints its name, where a plain object prints something like PlainCart@6bc7c054, and it suits stateless cases such as data object Loading in a sealed hierarchy (Sealed and Enum Classes). The related object expression, object : Runnable { ... }, creates an anonymous instance each time it runs, like a JavaScript object literal. Never hold an Activity or Context in a singleton: it outlives the screen and leaks it.

MENU