Singleton class in Kotlin

Solution 1:

There is a special keyword object for singletons in Kotlin. You can just type something as simple as this to get working singleton class:

object MySingleton

or when you want some member functions:

object MySingleton {
    fun someFunction(...) {...}
}

And then use it:

MySingleton.someFunction(...)

there is a reference: https://kotlinlang.org/docs/reference/object-declarations.html#object-declarations

EDIT:

In your case, you just need to replace in your definition of class UtilProject to this:

object UtilProject {

    // here you put all member functions, values and variables
    // that you need in your singleton Util class, for example:

    val maxValue: Int = 100

    fun compareInts(a: Int, b: Int): Int {...}
}

And then you can simply use your singleton in other places:

UtilProject.compareInts(1, 2)
//or
var value = UtilProject.maxValue

Solution 2:

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).