Please help why below code snippet throws `NullPointerExcept

Question

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 lt A gt createInstance private fun createInstance A return A open class SingletonHolderWithoutArgs lt out T gt private val func gt T Volatile private var instance T = null fun getInstance T = synchronized this instance func also instance = it ```

Sam · Answer

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 · Answer

I m not sure about this though thinking face it s confusing

Luke Armitage · Answer

some sort of convoluted Singleton constructor pattern

pavankumar · Answer

If i put `inline` keyword before `createInstance` method then problem is solved but I don t know why `inline` keyword fixes the problem

Sam · Answer

Here <https youtrack jetbrains com issue KT 13469>

Sam · Answer

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 · Answer

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 · Answer

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 · Answer

can you give me example

pavankumar · Answer

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 lt Starter gt Starter createInstance private inline fun createInstance Starter val job = Components from AComponent class java Job return Starter SdkDataVaultInitializerImpl job SdkDataVaultCreatorImpl job SdkDataVaultUnlockerImpl job ```

mkrussel · Answer

```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 · Answer

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 · Answer

The `createInstance` function just becomes your `init` block

pavankumar · Answer

Ok

pavankumar · Answer

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 · Answer

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 · Answer

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 · Answer

Btw ` Volatile` is <https kotlinlang org docs shared mutable state and concurrency html volatiles are of no help|useless >