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Android Studio converts Java to Kotlin error The type cannot be inferred for this parameter. Please state this explicitly

I'm new to Android development (started 6 months ago) and would like to migrate from Java to Kotlin. I converted my project to Kotlin and fixed all problems but one and I can't figure out how to fix it.

I am trying to get a JSONArray (as seen from the JsonManager class) and use the received data in a second DataDisplayPage class via a method call.

I get the following error console, which are found in this line of the second class: jManager.fetch_data{ theJsonArray ->

.

The type cannot be inferred for this parameter. Please state this explicitly.

Type mismatch: inferred type is (???) -> Expected but Expected OnTaskCompleted

First JsonManager class

interface OnTaskCompleted {
    fun onTaskCompleted(theJsonArray: JSONArray)
}

class JsonManager {
    var listener: OnTaskCompleted? = null

    init {
        Log.d("JsonManager", "Instance created")
    }

    fun fetch_data(callback : OnTaskCompleted) {
        listener = callback
         val url ="https://someURL"

        AndroidNetworking.get(url)
            .build()
            .getAsJSONArray(object : JSONArrayRequestListener {
                override fun onResponse(response: JSONArray) {
                    listener?.onTaskCompleted(response)
                }

                override fun onError(anError: ANError) {
                    Log.d("error", anError.toString())
                }
            })

}

Second DataDisplayPage class

class DataDisplayPage : AppCompatActivity()

    fun onStartUp() {

        val jManager = JsonManager()

        jManager.fetch_data{ theJsonArray  ->
            val newData = DepData()
            newData.setCellData(theJsonArray as JSONArray)
        }
    }
}
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1 answer


At the moment, you cannot use SAM conversion for interfaces defined in Kotlin. There are several things you can do to fix your problem.



  • If you define your interface in Java, the SAM transformation will start and your current code will work without any changes.

  • If you want to use an interface as a method parameter fetch_data

    , and you want it to be written in Kotlin, you will need to pass object

    in that implements it, which is a slightly verbose, Java-like solution:

    jmManager.fetch_data(object: OnTaskCompleted {
    override fun onTaskCompleted(theJsonArray: JSONArray) {
        // ...
    }
})

  • If you want a good clean Kotlin solution, just get rid of the interface and the function fetch_data

    will take the function as its parameter instead of the interface (again, your current code in DataDisplayPage

    will work with this):

    fun fetch_data(callback: (JSONArray) -> Unit) { 
    // ...
    listener?.onTaskCompleted(response)
    callback(response)
    // ...
}
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