Please help why below code snippet throws `NullPointerException` when calling `createInstance()` met...

# getting-started

[pavankumar]

Please help why below code snippet throws NullPointerException when calling createInstance() method. Is there any solution to make it work

fun main() {
    val a = A.getInstance()
    println("value of ${a.num}")
}

class A {
    val num: Int = 10
    
    companion object : SingletonHolderWithoutArgs<A>(::createInstance) {
        private fun createInstance(): A {
            return A()
        }
    }
}

open class SingletonHolderWithoutArgs<out T>(private val func: () -> T) {

    @Volatile
    private var instance: T? = null

    fun getInstance(): T =
        synchronized(this) {
            instance ?: func().also { instance = it }
        }
}

[Sam]

I think what’s happening here is that you’re creating a bound reference to the A.createInstance function inside A’s constructor, i.e. before A has actually been initialised. Although A has been initialised by the time you call the function, the reference was created before that happened.

[Sam]

I’m not sure about this though 🤔 it’s confusing

[Luke Armitage]

some sort of convoluted Singleton-constructor pattern?

[pavankumar]

If i put inline keyword before createInstance method then problem is solved but I don't know why inline keyword fixes the problem.

[Sam]

Here: https://youtrack.jetbrains.com/issue/KT-13469

[Sam]

That’s the cause of the NPE, though it doesn’t help you fix it. Why do you need your singleton creation to be so complicated? Does something prevent you just making A an object?

[Sam]

The normal way to make a singleton like that in Kotlin would just be

fun main() {
    val a = A
    println("value of ${a.num}")
}

object A {
    val num: Int = 10
}

[mkrussel]

Even if you cannot make it an object, you could still easily just create one in the companion, it can even use the lazy delegate.

[pavankumar]

can you give me example

[pavankumar]

This is the class i'm trying to create.

internal class Starter private constructor(
    initializer: Initializer,
    creator: Creator,
    unlocker: Unlocker
) : Initializer by initializer,
    Creator by creator,
    Unlocker by unlocker {

    companion object :
        SingletonHolderWithoutArgs<Starter>(Starter::createInstance) {

        private inline fun createInstance(): Starter {
            val job = Components.from(AComponent::class.java).Job()
            return Starter(
                SdkDataVaultInitializerImpl(job),
                SdkDataVaultCreatorImpl(job),
                SdkDataVaultUnlockerImpl(job)
            )
        }
    }
}

[mkrussel]

class A {
    val num: Int = 10

    companion object {
        val instance by lazy { A() }
    }
}

fun main() {
    val a = A.instance
    println("value of ${a.num}")
}

[mkrussel]

Seems like you are just trying to make a lazy singleton, if so just declare Starter as an object class.

Are these singleton holders are not needed (they weren't really need in Java either)

[mkrussel]

The createInstance function just becomes your init block.

[pavankumar]

Ok

[pavankumar]

If i replace createInstance() with init block then i'm making my class not easily testable right?
What i mean is the private constructor can be @VisibleForTesting constructor(...) in future. So init block might give me trouble at that time.

[pavankumar]

In brief,
I want to simplify my class construction process in production (i.e all objects needed for my class are instantiated inside the class. )
But this class should be easily testable (So want to expose a constructor that accepts all required parameters)

[Luke Armitage]

hmm, sounds like you need some dependency injection - it’s generally not considered a good idea for a class to instantiate its dependencies, as (like you’ve pointed out) it’s tricky to test, and it can be inflexible.

[Kevin Del Castillo]

Btw @Volatile is useless