com.google.gson.internal.LinkedTreeMap 无法强制转换

Question

我第一次使用 GSON 改造。它使用静态数据流畅地运行，但在动态数据的情况下显示上述错误。在设置文本值时在适配器中显示错误

MainActivity

public class MainActivity extends AppCompatActivity {


    String Navigation_Url = "http://www.xxxxxxxx.com";
    ListView listView;
    ArrayList<ListPojo> listPojos = new ArrayList<>();
    ListRetrofitAndroid listRetrofitAndroid;


    ArrayList countries = new ArrayList();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        listView = (ListView) findViewById(R.id.listView);
        getRetrofitData();
    }


    void getRetrofitData() {
        Retrofit retrofit = new Retrofit.Builder().baseUrl(Navigation_Url).addConverterFactory(GsonConverterFactory.create()).build();
        CountryArrayAPI api = retrofit.create(CountryArrayAPI.class);
        Call<List> call = api.getCountries();


        call.enqueue(new Callback<List>() {

            @Override
            public void onResponse(Call<List> call, Response<List> response) {
                Gson gson = new Gson();
                try {
                    listPojos = (ArrayList) response.body();
                    System.out.println("the ArrayList is" + listPojos);


//                    ArrayList<ListPojo> merchantOfferModels = gson.fromJson(response, new TypeToken<ListPojo>() {}.getType());









                    listRetrofitAndroid = new ListRetrofitAndroid(getApplicationContext(), listPojos);
                    listView.setAdapter(listRetrofitAndroid);


                } catch (Exception e) {
                    Log.d("onResponse", "There is an error");
                    e.printStackTrace();
                }

            }

            @Override
            public void onFailure(Call<List> call, Throwable t) {
                Log.d("Failure", t.toString());
            }
        });

    }

}

ListRetrofitAndroid（适配器）

public class ListRetrofitAndroid extends BaseAdapter {

    Context mContext;
    ArrayList<ListPojo> arrayList = new ArrayList<>();
    String TAG = "HomeTab_adapter";


    public ListRetrofitAndroid(Context context, ArrayList<ListPojo> listPojos) {
        super();
        this.mContext = context;
        this.arrayList = listPojos;

    }

    @Override
    public int getCount() {


        return arrayList.size();
    }

    @Override
    public Object getItem(int position) {
        return arrayList.get(position);
    }

    @Override
    public long getItemId(int position) {
        return position;
    }

    @Override
    public View getView(final int position, View convertView, ViewGroup parent) {
        final Holder viewHolder;

        if (convertView == null) {
            LayoutInflater inflater = (LayoutInflater) mContext.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
//            convertView = inflater.inflate(R.layout.list, parent, false);
            convertView = inflater.inflate(R.layout.listview_item, parent, false);

            // well set up the ViewHolder
            //  viewHolder = new ClassScheduleStudentAdapter.Holder();
            viewHolder = new Holder();
            viewHolder.textName = convertView.findViewById(R.id.list_item_main);
            convertView.setTag(viewHolder);


        } else {

            viewHolder = (Holder) convertView.getTag();
        }

        viewHolder.textName.setText(arrayList.get(position).getTitle());


        return convertView;
    }

    class Holder {
        TextView textName;
    }


}

模型（ListPojo）

public class ListPojo {

    String title;

    public ListPojo(String title) {
        this.title = title;
    }

    public String getTitle() {
        return title;
    }
}

界面

public interface CountryArrayAPI {
    @GET("/api/popular_destinations")
    public Call<List> getCountries();
}

JSON

[
    {
        "id": 1,
        "title": "Hiranya Varna Mahavihar",
        "location": "Kwalakhu Rd, Patan 44600, Nepal",
        "main_image": "images/popular_places/patan-durbar-square.jpg"
    },
    {
        "id": 2,
        "title": "Yasodhar Mahavihar-Bubahal",
        "location": "Jana Bahal Rd, Patan 44600, Nepal",
        "main_image": "images/popular_places/patan-durbar-square.jpg"
    },
    {
        "id": 3,
        "title": "Krishna Temple Sundhara",
        "location": "Tyagal - Lalitpur Rd, Patan 44600, Nepal",
        "main_image": "images/popular_places/Patan-2.jpg"
    },
    {
        "id": 4,
        "title": "Hiranya Varna Mahavihar",
        "location": "Kwalakhu Rd, Patan 44600, Nepal",
        "main_image": "images/popular_places/asd.jpg"
    }
]

StsckTrace

09-19 05:15:32.475 18949-18949/com.example.user_pc.myapplication E/AndroidRuntime: FATAL EXCEPTION: main
                                                                                   Process: com.example.user_pc.myapplication, PID: 18949
                                                                                   java.lang.ClassCastException: com.google.gson.internal.LinkedTreeMap cannot be cast to com.example.user_pc.myapplication.pojo.ListPojo
                                                                                       at com.example.user_pc.myapplication.adpater.ListRetrofitAndroid.getView(ListRetrofitAndroid.java:71)
                                                                                       at android.widget.AbsListView.obtainView(AbsListView.java:2255)
                                                                                       at android.widget.ListView.measureHeightOfChildren(ListView.java:1263)
                                                                                       at android.widget.ListView.onMeasure(ListView.java:1175)
                                                                                       at android.view.View.measure(View.java:16497)
                                                                                       at android.support.constraint.ConstraintLayout.internalMeasureChildren(ConstraintLayout.java:934)
                                                                                       at android.support.constraint.ConstraintLayout.onMeasure(ConstraintLayout.java:973)
                                                                                       at android.view.View.measure(View.java:16497)
                                                                                       at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125)
                                                                                       at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
                                                                                       at android.support.v7.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:139)
                                                                                       at android.view.View.measure(View.java:16497)
                                                                                       at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125)
                                                                                       at android.support.v7.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:391)
                                                                                       at android.view.View.measure(View.java:16497)
                                                                                       at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125)
                                                                                       at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
                                                                                       at android.view.View.measure(View.java:16497)
                                                                                       at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125)
                                                                                       at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1404)
                                                                                       at android.widget.LinearLayout.measureVertical(LinearLayout.java:695)
                                                                                       at android.widget.LinearLayout.onMeasure(LinearLayout.java:588)
                                                                                       at android.view.View.measure(View.java:16497)
                                                                                       at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125)
                                                                                       at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
                                                                                       at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2291)
                                                                                       at android.view.View.measure(View.java:16497)
                                                                                       at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:1912)
                                                                                       at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1109)
                                                                                       at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1291)
                                                                                       at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:996)
                                                                                       at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5600)
                                                                                       at android.view.Choreographer$CallbackRecord.run(Choreographer.java:761)
                                                                                       at android.view.Choreographer.doCallbacks(Choreographer.java:574)
                                                                                       at android.view.Choreographer.doFrame(Choreographer.java:544)
                                                                                       at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:747)
                                                                                       at android.os.Handler.handleCallback(Handler.java:733)
                                                                                       at android.os.Handler.dispatchMessage(Handler.java:95)
                                                                                       at android.os.Looper.loop(Looper.java:136)
                                                                                       at android.app.ActivityThread.main(ActivityThread.java:5001)
                                                                                       at java.lang.reflect.Method.invokeNative(Native Method)
                                                                                       at java.lang.reflect.Method.invoke(Method.java:515)
                                                                                       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
                                                                                       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
                                                                                       at dalvik.system.NativeStart.main(Native Method)

如何解决这个问题？

Answer 1

您使用的是ArrayList<ListPojo>，因此您需要确保解析应该使用正确的类型List<ListPojo> 而不是没有类型的list

由于缺少类型信息，gson 在内部返回一个 List 创建对象LinkedTreeMap 并且由于您使用raw 类型（应该是未经检查的强制转换的警告）声明为具体类型，这将在适配器实际期望PojoObject 但在内部它是别的东西时产生问题。所以解决办法

Never use raw-types anywhere possible

这样做

public interface CountryArrayAPI {
    @GET("api/popular_destinations")
    public Call<List<ListPojo>> getCountries();
}

并在api 之前删除/ 并且调用会像

call.enqueue(new Callback<List<ListPojo>>() {

            @Override
            public void onResponse(Call<List<ListPojo>> call, Response<List<ListPojo>> response) {
            listPojos = response.body();

并且总是编码到interface 所以

List<ListPojo> listPojos = ...;

Answer 2

我目前完全不知道如何使用List 处理 json。但是你可以像这样使用数组：

public interface CountryArrayAPI {
    @GET("/api/popular_destinations")
    public Call<ListPojo[]> getCountries();
}