Android Kotlin: java.lang.IllegalStateException

问题

This is an extension to Kotlin using Gson to deserialize local json file that I posted earlier.

I want NewsFragment.kt to instantiate an adapter but am unable to access to the recyclerview id worldnews. I get "java.lang.IllegalStateException: worldnews must not be null" when the program tries to execute the code below:

activity?.runOnUiThread {
    worldnews.adapter = MainAdapter(homeFeed)
}

NewsFragment.kt:

class NewsFragment : Fragment() {
    var arr = arrayListOf<String>()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        arguments?.let {
            param1 = it.getString(ARG_PARAM1)
            param2 = it.getString(ARG_PARAM2)
        }

        read_json()

    }

    fun read_json(){
        var json : String? = null

        try {
            val inputStream: InputStream = context!!.assets.open("sample.json")

            json = inputStream.bufferedReader().use { it.readText() }

            val gson = GsonBuilder().create()
            val homeFeed = gson.fromJson(json, HomeFeed::class.java)

            activity?.runOnUiThread {
                worldnews.adapter = MainAdapter(homeFeed)
            }

        } catch (e: IOException) {

        }
    }

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        val view = inflater.inflate(R.layout.fragment_news, container, false)

        view.worldnews.layoutManager = LinearLayoutManager(activity)

        return view
    }
}
class HomeFeed(val News: List<News>)

class News(val title: String, val description: String, val time: String, val link: String)

sample.json:

{"News": [{"title": "Intesa expected to approve state-backed loan for FCA -source","description": "Italy's biggest retail bank Intesa Sanpaolo is expected to give conditional approval at a board meeting on Tuesday to a state-guaranteed $6.3 billion euro three-year loan for Fiat Chrysler (FCA), a source close to the matter said.", "time": "9:38am EDT","link": "https://www.reuters.com//article/health-coronavirus-fiat-chrylser-loan/intesa-expected-to-approve-state-backed-loan-for-fca-source-idUSS8N2B200A"}, {"title": "CANADA STOCKS-TSX opens higher on hopes of economic recovery", "description": "Canada's main stock index rose in early trade on Monday as investors looked to an eventual economic recovery from the coronavirus with more countries scaling back lockdown measures.", "time": "9:37am EDT", "link": "https://www.reuters.com//article/canada-stocks/canada-stocks-tsx-opens-higher-on-hopes-of-economic-recovery-idUSL4N2D7257"}, {"title": "Bars, gyms reopen as Iceland exits emergency coronavirus alert", "description": "Iceland eased its national alert against the coronavirus on Monday, allowing for public gatherings of up to 200 people and night clubs and gyms to reopen as the country nears complete recovery from the outbreak.", "time": "9:20am EDT", "link": "https://www.reuters.com//article/health-coronavirus-iceland/bars-gyms-reopen-as-iceland-exits-emergency-coronavirus-alert-idUSL8N2D71YX"}]}

回答1:

You are calling your read_json() function in your onCreate block of your fragment, at that point your view is null.

Move invocation upon read_json from onCreate to onViewCreated.

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
   read_json(view)
}

In addition to that in the read_json function modify worldnews to view.worldnews:

activity?.runOnUiThread {
   view.worldnews.adapter = MainAdapter(homeFeed)
}

回答2:

The view has not been inflated yet, therefore worldnews doesn't exist when you execute read_json().

Move read_json() inside onResume.

    override fun onResume() {
        super.onResume()
        read_json()
    }

Otherwise you can only access it by specifying the inflated view first, like you do already inside your onCreate:

view.worldnews.layoutManager = LinearLayoutManager(activity)

来源：https://stackoverflow.com/questions/62018242/android-kotlin-java-lang-illegalstateexception

标签

android

kotlin

android-recyclerview

illegalstateexception