java - Is there an exception in the size of the array returned by getItemCount() in RecyclerView.Adapter?

Question

1. Problem description: The size of the array returned by getItemCount() in RecyclerView.Adapter is abnormal. The network uses okhttp, and the database framework is litepal. The function is to obtain JSON data from the network and store them in the database. Then read it from the database and display it on Addcity.

2. Main error message:

FATAL EXCEPTION: main
Process: com.example.huang.easyweather, PID: 8208    
java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
at com.example.huang.easyweather.CityAdapter.getItemCount(CityAdapter.java:68)
at android.support.v7.widget.RecyclerView.dispatchLayoutStep1(RecyclerView.java:3493)
at android.support.v7.widget.RecyclerView.dispatchLayout(RecyclerView.java:3310)
at android.support.v7.widget.RecyclerView.onLayout(RecyclerView.java:3844)
at android.view.View.layout(View.java:17637)
at android.view.ViewGroup.layout(ViewGroup.java:5575)
at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1741)
at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1585)
at android.widget.LinearLayout.onLayout(LinearLayout.java:1494)
at android.view.View.layout(View.java:17637)
at android.view.ViewGroup.layout(ViewGroup.java:5575)
at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323)
at android.widget.FrameLayout.onLayout(FrameLayout.java:261)
at android.view.View.layout(View.java:17637)
at android.view.ViewGroup.layout(ViewGroup.java:5575)
at android.support.v7.widget.ActionBarOverlayLayout.onLayout(ActionBarOverlayLayout.java:437)
at android.view.View.layout(View.java:17637)
at android.view.ViewGroup.layout(ViewGroup.java:5575)
at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323)
at android.widget.FrameLayout.onLayout(FrameLayout.java:261)
at android.view.View.layout(View.java:17637)
at android.view.ViewGroup.layout(ViewGroup.java:5575)
at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1741)

3. Related code:
CityAdapter:

public class CityAdapter extends RecyclerView.Adapter<CityAdapter.ViewHolder> {

    private List<City> mCityList;

    public class ViewHolder extends RecyclerView.ViewHolder{

        TextView listItemCityView;
        public ViewHolder(View itemView){
            super(itemView);
            listItemCityView=(TextView)itemView.findViewById(R.id.tv_item_city);
        }

    }
    public CityAdapter(List<City> cityList){
        mCityList=cityList;
    }
    @Override
    public ViewHolder onCreateViewHolder(ViewGroup viewGroup, int viewType) {
    
        View view=LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.city_list_item,viewGroup,false);
        return new ViewHolder(view);
    }

    @Override
    public void onBindViewHolder(ViewHolder holder, int position) {
        City city=mCityList.get(position);
        holder.listItemCityView.setText(city.getCityZh());
    }

    @Override
    public int getItemCount() {
       // if(null==mCityList) return 0;  //如果有这条语句是不会报错的，但是RecyclerView获取不到数据
        return mCityList.size();
    }

}
                                     

AddActivity:

private List<String> dataList = new ArrayList<>();
private RecyclerView mRecyclerView;
private List<City> cityList; //城市列表

 mRecyclerView=(RecyclerView)findViewById(R.id.recycler_view);
        LinearLayoutManager layoutManager=new LinearLayoutManager(this);
        mRecyclerView.setLayoutManager(layoutManager);//设置布局管理器
        CityAdapter adapter=new CityAdapter(cityList); 
        mRecyclerView.setAdapter(adapter);//设置适配器
        
        
  //处理获取到的JSON数据并把它们存储在数据库中
    public static boolean handleCityResponse(String response){
            try {
                JSONArray allcities=new JSONArray(response);
                for (int i=0;i<allcities.length();i++){
                    JSONObject cityObject=allcities.getJSONObject(i);
                    City city=new City();
                    city.setCityId(cityObject.getString("id")); //城市id
                    city.setCityZh(cityObject.getString("cityZh"));//城市中文名
                    city.setLeaderZh(cityObject.getString("leaderZh"));//所属市级中文名
                    city.setProvinceZh(cityObject.getString("provinceZh"));//所属省级中文名
                    city.setCountryZh(cityObject.getString("countryZh"));//所属国家中文名
                    //储存在数据库中
                    city.save();
                   // Log.d("AddCity","获取的JSON数据存储到数据库成功");
                }
                return true;
            }catch (JSONException e){
                e.printStackTrace();
            }
        return false;

    }

     public void queryCities(String mSearchCity){
        cityList= DataSupport
                .select("cityZh")
                .where("cityZh = ?",mSearchCity)
                .find(City.class);
        Log.d("AddCity","1");
            for(City city:cityList){
                dataList.add(city.getCityZh());
                Log.d("AddCity",city.getCityZh());
            }
    
    
    
        }
 public void queryFromServer(String cityAddressJson) {
        NetworkUtils.sendOkHttpRequest(cityAddressJson, new Callback() {

            @Override
            public void onFailure(Call call, IOException e) {
                Log.d("AddCity","HTTP连接失败");
            }

            @Override
            public void onResponse(Call call, Response response) throws IOException {
                String returnCityName=response.body().string(); //设置希望返回的字符串
                boolean result=false;
                result=handleCityResponse(returnCityName);//处理返回的JSON数据

                if(result){ //如果成功处理,就开始从数据库查询
                    runOnUiThread(new Runnable() {
                       @Override
                       public void run() {
                        queryCities("北京");
                       }
                   });
                }
            }
        });
    }

Answer 1

When you use CityAdapter adapter=new CityAdapter(cityList);, cityList has not been initialized yet. The correct approach is

cityList = new ArrayList();
CityAdapter adapter=new CityAdapter(cityList);
//之后给cityList赋值是不再直接使用 cityList = DataSupport()...
cityList.clear();
cityList.addAll(DataSupport());