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: 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.相关代码：
CityAdapter:

public class CityAdapter extends RecyclerView.Adapter {

    private List 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 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 dataList = new ArrayList<>();
private RecyclerView mRecyclerView;
private List 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