java - RecyclerView.Adapter中的getItemCount() 返回数组的size是出现异常？

问题描述

1.问题描述：RecyclerView.Adapter中的getItemCount() 返回数组的size是出现异常.网络用的是okhttp,数据库框架是litepal，功能是从网络上获取JSON数据并把它们存入数据库然后再从数据库读取，在Addcity上显示出来。

2.主要报错信息：

FATAL EXCEPTION: mainProcess: com.example.huang.easyweather, PID: 8208 java.lang.NullPointerException: Attempt to invoke interface method ’int java.util.List.size()’ on a null object referenceat 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.相关代码： 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'); } });} }}); }

问题解答

你在CityAdapter adapter=new CityAdapter(cityList);时，cityList还没有初始化，正确的做法是

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());