java - RecyclerView.Adapter中的getItemCount() 回傳陣列的size是出現異常？

Question

1.問題描述：RecyclerView.Adapter中的getItemCount() 回傳數組的size是出現異常.網路用的是okhttp,資料庫框架是litepal，功能是從網路上取得JSON資料並將它們存入資料庫然後再從資料庫讀取，在Addcity上顯示。

2.主要報錯訊息：

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.相關程式碼：
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

你在CityAdapter adapter=new CityAdapter(cityList);時，cityList還沒有初始化，正確的做法是

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());