任何原因 Kotlin API GET REQUEST code (OKHttp library) get this; java.lang.NullPointerException？

Question

MainActivity

class MainActivity : AppCompatActivity() {

lateinit var adapter: MainAdapter

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

    recyclerView.layoutManager = LinearLayoutManager(this)

    fetchJson()
   }

private fun fetchJson() {
    println("Attempting to FetchJson")

    val url = "https://www.noforeignland.com/home/api/v1/places/"

    val request = Request.Builder().url(url).build()

    val client = OkHttpClient()
    client.newCall(request).enqueue(object: okhttp3.Callback {
        override fun onResponse(call: okhttp3.Call, response: okhttp3.Response) {
            val body = response.body?.string()
            println(body)

            val gson = GsonBuilder().create()

            val homeFeed = gson.fromJson(body, HomeFeed::class.java)

            runOnUiThread() {
            recyclerView.adapter = MainAdapter(homeFeed)
            }

        }

        override fun onFailure(call: okhttp3.Call, e: IOException) {
            println("Failed to execute request")
        }
    })

}

} class HomeFeed(val properties: List<Properties>)

致命异常

    E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.odyssey, PID: 21745
    java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
        at com.example.odyssey.adapter.MainAdapter.getItemCount(MainAdapter.kt:19)
        at androidx.recyclerview.widget.RecyclerView.dispatchLayoutStep1(RecyclerView.java:4044)
        at androidx.recyclerview.widget.RecyclerView.dispatchLayout(RecyclerView.java:3849)
        at androidx.recyclerview.widget.RecyclerView.onLayout(RecyclerView.java:4404)
        at android.view.View.layout(View.java:21912)
        at android.view.ViewGroup.layout(ViewGroup.java:6260)
        at androidx.constraintlayout.widget.ConstraintLayout.onLayout(ConstraintLayout.java:1915)
        at android.view.View.layout(View.java:21912)
        at android.view.ViewGroup.layout(ViewGroup.java:6260)
        at android.widget.FrameLayout.layoutChildren(FrameLayout.java:332)
        at android.widget.FrameLayout.onLayout(FrameLayout.java:270)
        at android.view.View.layout(View.java:21912)
        at android.view.ViewGroup.layout(ViewGroup.java:6260)
        at androidx.appcompat.widget.ActionBarOverlayLayout.onLayout(ActionBarOverlayLayout.java:446)
        at android.view.View.layout(View.java:21912)
        at android.view.ViewGroup.layout(ViewGroup.java:6260)
        at android.widget.FrameLayout.layoutChildren(FrameLayout.java:332)
        at android.widget.FrameLayout.onLayout(FrameLayout.java:270)
        at android.view.View.layout(View.java:21912)
        at android.view.ViewGroup.layout(ViewGroup.java:6260)
        at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1829)
        at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1673)
        at android.widget.LinearLayout.onLayout(LinearLayout.java:1582)
        at android.view.View.layout(View.java:21912)
        at android.view.ViewGroup.layout(ViewGroup.java:6260)
        at android.widget.FrameLayout.layoutChildren(FrameLayout.java:332)
        at android.widget.FrameLayout.onLayout(FrameLayout.java:270)
        at com.android.internal.policy.DecorView.onLayout(DecorView.java:779)
        at android.view.View.layout(View.java:21912)
        at android.view.ViewGroup.layout(ViewGroup.java:6260)
        at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:3080)
        at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2590)
        at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1721)
        at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:7598)
        at android.view.Choreographer$CallbackRecord.run(Choreographer.java:966)
        at android.view.Choreographer.doCallbacks(Choreographer.java:790)
        at android.view.Choreographer.doFrame(Choreographer.java:725)
        at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:951)
        at android.os.Handler.handleCallback(Handler.java:883)
        at android.os.Handler.dispatchMessage(Handler.java:100)
        at android.os.Looper.loop(Looper.java:214)
        at android.app.ActivityThread.main(ActivityThread.java:7356)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
I/Process: Sending signal. PID: 21745 SIG:

主适配器

class MainAdapter(val homeFeed: HomeFeed) : RecyclerView.Adapter<CustomViewHolder>() {


override fun getItemCount(): Int {
    return homeFeed.properties.size
}


override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): CustomViewHolder {
    val layoutInflater = LayoutInflater.from(parent?.context)
    val cellForRow = layoutInflater.inflate(R.layout.activity_data, parent, false)
    return CustomViewHolder(cellForRow)
}

override fun onBindViewHolder(holder: CustomViewHolder, position: Int) {
    val propertie = homeFeed.properties.get(position)
    holder?.view?.text_view_result?.text = propertie.name
}

}

class CustomViewHolder(val view: View): RecyclerView.ViewHolder(view) { 
}

model.Properties

public class Properties {
    private String name;
    private String icon;
    private int id;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getIcon() {
        return icon;
    }

    public void setIcon(String icon) {
        this.icon = icon;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }
}

*当我调试代码时，问题似乎是从；

runOnUiThread() {
     recyclerView.adapter = MainAdapter(homeFeed)
                }

*因为我收到类型为 okhttp3.Call 的 "Cannot find local variable "call"，以及 homeFeed.properties = null *

然后致命异常指向

override fun getItemCount(): Int {
    return homeFeed.properties.size
}

这导致了问题，我真的不知道如何解决 有什么建议吗？

Answer 1

HomeFeed-class 与 JSON-表示不匹配。 JSON-表示形式如下：

{"type":"FeatureCollection","features":[{"type":"Feature", "properties:{"name":"John Smith","icon":"Icon":"id":123456}}]}

所以你必须先用一个特征-class 包装属性-class，就像这样):

public class Feature{

  public String type;
  public Properties properties;
  public Feature(String type, Properties properties){
    this.type = type;
    this.properties = properties;
  }

}

HomeFeed-class:

public class HomeFeed{

  public List<Feature> features;
  public String type;
  public HomeFeed(String type, List<Feature> features){
    this.features = features;
    this.type = type;
  }

}

那么应该可以了。

任何原因 Kotlin API GET REQUEST code (OKHttp library) get this; java.lang.NullPointerException？

Any reason why Kotlin API GET REQUEST code (OKHttp library) get this; java.lang.NullPointerException?

api

json

get

kotlin

okhttp