Converting Java To Kotlin: Type mismatch. Required: MenuSlidingTabStrip.OnTabSelectedListener? Found: (Nothing, Nothing) → Boolean

Question

I am converting an Android app that was originally written in Java to Kotlin. I am struggling to understand the following error message:

Type mismatch. Required: MenuSlidingTabStrip.OnTabSelectedListener? Found: (Nothing, Nothing) → Boolean

Here is the fragment of code where the error is being signalled (and it was working perfectly fine before the conversion):

private var tabs: MenuSlidingTabStrip? = null //The Kotlinized class

        tabs!!.setOnTabSelectedListner{ tab, category -> /*Type mismatch...*/
            listView!!.post {
            ...
            }
        }

The issue arose after converting this Java code (found in MenuSlidingTabStrip) :

public void setOnTabSelectedListner(OnTabSelectedListener listener) {
    this.listener = listener;
}

public interface OnTabSelectedListener {
    public void OnTabSelected(View tab, MenuCategory category);

}

To Kotlin

  fun setOnTabSelectedListner(listener: OnTabSelectedListener?) {
    this.listener = listener
}

interface OnTabSelectedListener {
    fun onTabSelected(tab: View?, category: MenuCategory?)
}

Can you see the issue? Do you need more code?

Answer 1

I'd suggest you to use the natively supported lambdas as:

// make `this.listener` look like lambda signature as well
fun setOnTabSelectedListner(listener: (tab: View?, category: MenuCategory?) -> Unit) {
    this.listener = listener
}

tabs!!.setOnTabSelectedListner { tab, category ->
    // ...
}

If you still wanna use the SAM conversion with the manually defined interface, then mark the interface with the fun keyword as described in Kotlin 1.4-M1 Release changelog (will only work with Kotlin 1.4):

fun interface OnTabSelectedListener {
    fun onTabSelected(tab: View?, category: MenuCategory?)
}

// call that method like this:
tabs!!.setOnTabSelectedListner(OnTabSelectedListener { tab, category ->
    // ...
})

If you've not switched to Kotlin-1.4 (which is still in beta state), you have to manually instantiate the anonymous object if you are not willing to use the natively provided lambda syntax:

tabs!!.setOnTabSelectedListner(object : OnTabSelectedListener {
    override fun onTabSelected(tab: View?, category: MenuCategory?) {
        // ...
    }
})