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kotlinlang

> <@UHAJKUSTU> I am facing this issue where it says
> java.lang.NullPointerException: Parameter specified as non-null is null:
Code in thread :thread:

Lets say I have this abstract class
```abstract class FeedComponent(
    componentContext: ComponentContext,
) : BaseComponent(componentContext) {

    private val pagesNavigation = PagesNavigation<FeedPagesConfig>()
    
    internal val pages: List<FeedPagesConfig> = listOf(
        FeedPagesConfig.Trending,
        FeedPagesConfig.Latest
    )


    internal val childPages = childPages(
        source = pagesNavigation,
        initialPages = { Pages(pages, 0) },
        serializer = FeedPagesConfig.serializer(),
        childFactory = ::createChild
    )

    internal abstract fun createComponent(
        config: FeedPagesConfig,
        componentContext: ComponentContext
    ): FeedPageComponent

    private fun createChild(
        config: FeedPagesConfig,
        componentContext: ComponentContext
    ): FeedPages {
        val component = createComponent(config, componentContext)
        return when(config) {
            FeedPagesConfig.Trending -> FeedPages.Trending(component)
            FeedPagesConfig.Latest -> FeedPages.Latest(component)
        }
    }


    internal fun onPageSelected(index: Int) {
        pagesNavigation.select(index)
    }

}```
And this is the child class
```internal class FeedTabComponent(
    componentContext: ComponentContext,
    private val onTabReselected: Flow<Unit>,
    val onNavEvent: (AppNavEvents) -> Unit
) : FeedComponent(componentContext) {

    override fun createComponent(
        config: FeedPagesConfig,
        componentContext: ComponentContext
    ) = FeedTabPageComponent(
        feedCategory = config.feedCategory,
        componentContext = componentContext,
        onTabReselected = onTabReselected,
        onNavEvent = onNavEvent
    )
}```
This is the leaf component

```internal class FeedTabPageComponent (
    feedCategory: FeedCategory,
    componentContext: ComponentContext,
    val onTabReselected: Flow<Unit>,
    override val onNavEvent: (AppNavEvents) -> Unit
): FeedPageComponent(componentContext) {

    override val component = this

    override val feed = feedItemRepo.getFeedPaged(feedCategory).pagedData
        .mapLatest { it.map(FeedItemEntity::toDomain) }
        .cachedIn(componentScope)
        .safeCatch { setError(it) }
        .toStateFlow(componentScope, PagingData.empty())
}```

Now i'm getting this error that
> java.lang.NullPointerException: Parameter specified as non-null is null: method com.medial.app.ui.screens.screens.home.tabs.feed.pages.FeedTabPageComponent.<init>, parameter onTabReselected

But isnt it a constructor parameter and hence will always be initialized before childPages is created

I think in this case, `childStack` is initialised during `super` (FeedComponent) constructor invocation. This also calls `createChild` and `createComponent` functions. Since the `createComponent` is overriden, it gets called before the extending class is fully initialised.

This is a general concern in Kotlin and Java - this is how things work. My usual advice is to avoid having base classes - prefer composition over inheritance.

Another solution that is likely to help - get rid of all the `abstract` functions and use IoC instead - i.e. pass the factory to the `FeedComponent` via constructor.

A short-term fix that may work - to make your `childStack` property `by lazy`. But I wouldn't recommend this.