"Not enough information to infer parameter T" with Kotlin and Android

Question

I'm trying to replicate the following ListView in my Android app using Kotlin: https://github.com/bidrohi/KotlinListView.

Unfortunately I'm getting an error I'm unable to resolve myself. Here's my code:

MainActivity.kt:

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)

    val listView = findViewById(R.id.list) as ListView
    listView.adapter = ListExampleAdapter(this)
}

private class ListExampleAdapter(context: Context) : BaseAdapter() {
    internal var sList = arrayOf("Eins", "Zwei", "Drei")
    private  val mInflator: LayoutInflater

    init {
        this.mInflator = LayoutInflater.from(context)
    }

    override fun getCount(): Int {
        return sList.size
    }

    override fun getItem(position: Int): Any {
        return sList[position]
    }

    override fun getItemId(position: Int): Long {
        return position.toLong()
    }

    override fun getView(position: Int, convertView: View?, parent: ViewGroup): View? {
        val view: View?
        val vh: ListRowHolder

        if(convertView == null) {
            view = this.mInflator.inflate(R.layout.list_row, parent, false)
            vh = ListRowHolder(view)
            view.tag = vh
        } else {
            view = convertView
            vh = view.tag as ListRowHolder
        }

        vh.label.text = sList[position]
        return view
    }
}

private class ListRowHolder(row: View?) {
    public val label: TextView

    init {
        this.label = row?.findViewById(R.id.label) as TextView
    }
}
}

The layouts are exactly as here: https://github.com/bidrohi/KotlinListView/tree/master/app/src/main/res/layout

The full error message I'm getting is this: Error:(92, 31) Type inference failed: Not enough information to infer parameter T in fun findViewById(p0: Int): T! Please specify it explicitly.

I'd appreciate any help I can get.

Answer 1

You must be using API level 26 (or above). This version has changed the signature of View.findViewById() - see here https://developer.android.com/about/versions/oreo/android-8.0-changes#fvbi-signature

So in your case, where the result of findViewById is ambiguous, you need to supply the type:

1/ Change

val listView = findViewById(R.id.list) as ListView to

val listView = findViewById<ListView>(R.id.list)

2/ Change

this.label = row?.findViewById(R.id.label) as TextView to

this.label = row?.findViewById<TextView>(R.id.label) as TextView

Note that in 2/ the cast is only required because row is nullable. If label was nullable too, or if you made row not nullable, it wouldn't be required.