Why is this an issue?
In Kotlin, object declarations and object expressions are the designed way to implement the singleton pattern. Because this is a built-in language
feature, it should be used as an idiom instead of resorting to custom idioms.
An alternative is to declare private constructors and to provide a single class instance within a companion object. However, this idiom is adopted
from other languages like Java which do not feature the direct declaration of objects. It should not be used in Kotlin.
What is the potential impact?
Readability and Understanding
This change makes it easier to understand the code because this is how singletons are intended to be used in Kotlin. When developers share common
standards and idioms, they need to spend less effort on understanding each other’s code.
Code Redundancy
Using a built-in language feature or a standard API is always better than a custom implementation, because the reimplementation of something that
already exists is unnecessary.
How to fix it
Remove the private constructor of the class and remove the companion object or the variable that provides the single instance from the class.
Replace the class declaration with an object declaration if a name is required for the singleton. Replace the class declaration with an object
expression if the singleton instance can be anonymous.
Code examples
Noncompliant code example
class DeviceManager private constructor() {
fun instanceMethod() {
// ...
}
companion object { // Noncompliant, explicit class instance provided
val instance = DeviceManager()
}
}
Compliant solution
object DeviceManager { // Compliant, object declaration used
fun instanceMethod() {
// ...
}
}
Noncompliant code example
interface LayoutStrategy {
val instanceProperty: String
fun instanceMethod()
}
class CustomLayoutStrategy private constructor(): LayoutStrategy {
override val instanceProperty = "Hello, world!"
override fun instanceMethod() {
// ...
}
companion object { // Noncompliant, explicit class instance provided
private val instance by lazy {
CustomLayoutStrategy()
}
fun getInstance(): LayoutStrategy = instance
}
}
fun createUI() {
val component = Container()
component.setLayoutStrategy(CustomLayoutStrategy.getInstance())
}
Compliant solution
fun createUI() {
val component = Container()
component.setLayoutStrategy(object: LayoutStrategy { // Compliant, object expression used
override val instanceProperty = "Hello, world!"
override fun instanceMethod() {
// ...
}
})
}
Resources
Documentation
Articles & blog posts
