[FIXED] Singleton class in Kotlin

Issue

I want to know how to create a singleton class in Kotlin, so that my Util class instantiates it only once per app execution. However, when I converted my Java class to kotlin, the code below was generated.

Is this correct?

companion object {
    private var utilProject: UtilProject? = null

    val instance: UtilProject
        get() {
            if (utilProject == null) utilProject = UtilProject()
            return utilProject!!
        }
} 

I could find a related question, but it is with parameters, and I am not getting it convert without params.

Solution

Just

companion object {
    val instance = UtilProject()
} 

will do the job because the companion object itself is a language-level singleton.
(The instance will be created when the companion object is first called.)

-- Updated --

If you need to control when the singleton object is initialized, you can create one object for each class.

class UtilProject {
    ....
    companion object {
        val instance = UtilProject()
    }
}

class AnotherClass {
    ...
    companion object {
        val instance = AnotherClass()
        const val abc = "ABC"
    }
}

fun main(args: Array<String>) {
    val a = UtilProject.instance // UtilProject.instance will be initialized here.
    val b = AnotherClass.abc // AnotherClass.instance will be initialized here because AnotherClass's companion object is instantiated.
    val c = AnotherClass.instance
}

Here, AnotherClass.instance is initialized before AnotherClass.instance is actually called. It is initialized when AnotherClass's companion object is called. To prevent it from being initialized before when it is needed, you can use like this:

class UtilProject {
    ....
    companion object {
        fun f() = ...
    }
}

class AnotherClass {
    ...
    companion object {
        const val abc = "ABC"
    }
}

object UtilProjectSingleton {
    val instance = UtilProject()
}

object AnotherClassSingleton {
    val instance = AnotherClass()
}

fun main(args: Array<String>) {
    UtilProject.f()
    println(AnotherClass.abc)

    val a = UtilProjectSingleton.instance // UtilProjectSingleton.instance will be initialized here.
    val b = AnotherClassSingleton.instance // AnotherClassSingleton.instance will be initialized here.

    val c = UtilProjectSingleton.instance // c is a.
}

If you don't care when each singleton is initialized, you can also use it like this:

class UtilProject {
    ....
    companion object {
        fun f() = ...
    }
}

class AnotherClass {
    ...
    companion object {
        const val abc = "ABC"
    }
}

object Singletons {
    val utilProject = UtilProject()
    val anotherClass = AnotherClass()
}

fun main(args: Array<String>) {
    val a = Singletons.utilProject
    val b = Singletons.anotherClass 
}

In summary,
an object or a companion object is one singleton object in Kotlin.
You can assign variables in an object or objects, and then use the variables just like they were singletons.

object or companion object is instantiated when it is first used. vals and vars in an object are initialized when the object is first instantiated (i.e., when the object is first used).

EDIT: William Hu said in the comment that "a companion object is when the class is loaded."

Answered By - Naetmul
Answer Checked By - Willingham (JavaFixing Volunteer)