Android：使用 Asynctask 时 Recyclerview 出现问题

问：

我可以在 Asynctask 中使用 Json 从 API 获取数据，但问题是当我使用 Recyclerview 时。我收到空指针异常错误。请谁能帮我？

                              ***MainActivity.java***
private List<Certifications> mCertifications = new ArrayList<>(); 

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

    recyclerView = (RecyclerView) findViewById(R.id.certificationRecyclerview);
    new fetchData().execute();
    recyclerView.setLayoutManager(new LinearLayoutManager(this));

    mAdapter = new GreenAdapter(mCertifications);
    recyclerView.setAdapter(mAdapter);
  }  

}

                          ***GreenAdapter.java***  

public class GreenAdapter extends RecycrView.Adapter<GreenAdapter.ViewHolder>{
private Context mContext;
private List<Certifications> mCertifications;

public GreenAdapter(List<Certifications> certifications) {
    this.mCertifications = certifications;
}
/** Inner class */

// Stores and recycles views as they are scrolled off  screen

public static class ViewHolder extends RecyclerView.ViewHolder {
    private TextView certificationTextview;
    private TextView meaningTextview;
    private TextView orderTextview;
    public ViewHolder(@NonNull View itemView) {
        super(itemView);
        certificationTextview = (TextView) itemView.findViewById(R.id.certificationInfo);
        meaningTextview= (TextView) itemView.findViewById(R.id.meaning);
        orderTextview = (TextView) itemView.findViewById(R.id.order);

    }
}
// Methods
// Usually involves inflating a layout from XML and returning the holder
//Create New Views
@NonNull
@Override

public GreenAdapter.ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
    Context context = parent.getContext();
    LayoutInflater inflater = LayoutInflater.from(context);
    View certificationView =  inflater.inflate(R.layout.certification_list,
            parent, false);
    //Return a new holder instance
    final ViewHolder certificationViewHolder = new ViewHolder(certificationView);
    return certificationViewHolder;
}
//binds the data to the Textview in each row
// Involves populatind data into the item through holder
@Override
public void onBindViewHolder(@NonNull ViewHolder holder, int position) {
    // get the data model based on position
    Certifications certific = mCertifications.get(position);
    //set itemviews based on your views and data model
    holder.certificationTextview.setText(certific.getCertification());
    holder.meaningTextview.setText(certific.getMeaning());
    holder.orderTextview.setText(certific.getOrder());

}
@Override
public int getItemCount() {
    return mCertifications.size();
}

**I copied the following from other post**

public void clear(){
    mCertifications.clear();
    notifyDataSetChanged();
}
public void addAll(List<Certifications> list){
    mCertifications.addAll(list);
    notifyDataSetChanged();
  }
}




                             ***Certifications.java***
public class Certifications{ 
private String mCertification;
private String mMeaning;
private int mOrder; 

public Certifications(String certification,String meaning, int order) {
    mCertification = certification;
    mMeaning = meaning;
    mOrder = order;
}    public String getCertification(){
    return mCertification;
}
public String getMeaning(){
    return mMeaning;
}
public int getOrder(){
    return mOrder;
} 

}

                          ***FetchData.java*** 

Public class fetchData extends AsyncTask<String,Void, List<Certifications>> {

 private GreenAdapter mGreenAdapter;
// Create an empty ArrayList that we can start adding Certifications to    
List<Certifications> mCertifications = new ArrayList<>();;


protected List<Certifications> doInBackground(String... voids) {

   try {

       URL url = new URL("API");
       HttpsURLConnection httpsURLConnection= (HttpsURLConnection) url.openConnection();
       InputStream inputStream=httpsURLConnection.getInputStream();
       BufferedReader bufferedReader= new BufferedReader(new InputStreamReader(inputStream));
       String line = "";
       while (line!=null){
           line = bufferedReader.readLine();
           data = data + line;
       }
         // Create a JASONObject from the JSON_RESPONSE string
        JSONObject root = new JSONObject(data);

       JSONObject jsonObject = root.getJSONObject("certifications");

提取与名为“US”的键关联的 JSONArray 代表美国认证列表

       JSONArray jsonArray= jsonObject.getJSONArray("US");


       for (int i=0; i<jsonArray.length();i++){
           JSONObject jasonObject=(JSONObject) jsonArray.get(i);

           String  cert = jasonObject.getString("certification");
           String meaning =jasonObject.getString("meaning");
           int order = jasonObject.getInt("order");}

           Certifications certification = new Certifications(cert, meaning, order);
           mCertifications.add(certification);
       }


  **Catch code here**

    return mCertifications; **Data**

}

@Override
protected void onPostExecute(final List<Certifications> dataParsed) {

//当我运行调试时，以下代码出现空指针异常

第一次尝试。结果：NullPointerException

    mGreenAdapter.clear();
    mGreenAdapter.addAll(dataParsed);

第二次尝试。结果：NullPointerException

    mGreenAdapter = new GreenAdapter( dataParsed);
    mRecyclerView.setAdapter(mGreenAdapter);

   }// onPostExecute parenthesis
}// fetchData.java