Question

我正在尝试使用JSON从mysql数据库中获取数据并将其显示在RecyclerView中。应用程序使用JSON字符串很好地接收了数据。但是，当我试图将数据片段片段传递到回收器视图时，它给了我一个"Attempt to invoke virtual method 'void android.support.v7.widget.RecyclerView$Adapter.notifyDataSetChanged()' on a null object reference"错误。该错误是由BackgroundTask.java中的adapter.notifyDataSetChanged();方法引起的。谁能帮我。

我的代码如下：

showRequestsFragment.java

package com.bloodconnect.Fragments;


import android.app.Activity;
import android.content.Context;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import com.bloodconnect.Database.SharedPrefManager;
import com.bloodconnect.Model.Request;
import com.bloodconnect.R;

import java.util.ArrayList;


/**
 * A simple {@link Fragment} subclass.
 */
public class ShowRequestsFragment extends Fragment {

    Context ctx;
    Activity activity;
    RecyclerView recyclerView;
    RecyclerView.Adapter adapter;
    RecyclerView.LayoutManager layoutManager;

    ArrayList<Request> arrayList = new ArrayList<>();

    public ShowRequestsFragment() {
        // Required empty public constructor
    }


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {

        // Inflate the layout for this fragment
        View rootView =  inflater.inflate(R.layout.fragment_show_requests, container, false);

        recyclerView = (RecyclerView) rootView.findViewById(R.id.listRecyclerView);
        recyclerView.setLayoutManager(new LinearLayoutManager(getActivity() ));
        recyclerView.setHasFixedSize(true);

        adapter = new RecyclerAdapter(arrayList);
        recyclerView.setAdapter(adapter);

        BackgroundTask backgroundTask = new BackgroundTask(getActivity());
        backgroundTask.execute();

        return rootView;
    }


}

Request.java

package com.bloodconnect.Model;

import java.lang.reflect.Array;
import java.util.Arrays;

public class Request {

    private String name;
    private String blood_group;
    private String date_of_request;
    private String lat_lng;
    private String address;

    public Request(String name, String blood_group, String date_of_request, String lat_lng, String address) {
        this.name = name;
        this.blood_group = blood_group;
        this.date_of_request = date_of_request;
        this.lat_lng = lat_lng;
        this.address = address;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getBlood_group() {
        return blood_group;
    }

    public void setBlood_group(String blood_group) {
        this.blood_group = blood_group;
    }

    public String getDate_of_request() {
        return date_of_request;
    }

    public void setDate_of_request(String date_of_request) {
        this.date_of_request = date_of_request;
    }

    public String getLat_lng() {
        return lat_lng;
    }

    public void setLat_lng(String lat_lng) {
        this.lat_lng = lat_lng;
    }

    public String getAddress() {
        return address;
    }

    public void setAddress(String address) {
        this.address = address;
    }
}

RecyclerAdapter.java

package com.bloodconnect.Fragments;

import android.support.annotation.NonNull;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import com.bloodconnect.Model.Request;
import com.bloodconnect.R;

import java.util.ArrayList;

public class RecyclerAdapter extends RecyclerView.Adapter <RecyclerAdapter.RecyclerViewHolder> {

    ArrayList<Request> arrayList;

    public RecyclerAdapter(ArrayList<Request> arrayList){
        this.arrayList = arrayList;
    }

    @NonNull
    @Override
    public RecyclerViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {

        View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.show_request_template, parent, false);
        RecyclerViewHolder recyclerViewHolder = new RecyclerViewHolder(view);

        return recyclerViewHolder;
    }

    @Override
    public void onBindViewHolder(@NonNull RecyclerViewHolder holder, int position) {

        Request request = arrayList.get(position);
        holder.name.setText(request.getName());
        holder.blood_group.setText(request.getBlood_group());
        holder.date_of_request.setText(request.getDate_of_request());
        holder.lat_lng.setText(request.getLat_lng());
        holder.address.setText(request.getAddress());

    }

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


    public static class RecyclerViewHolder extends RecyclerView.ViewHolder {

        TextView name, blood_group, date_of_request, lat_lng, address;

        public RecyclerViewHolder(View view){
            super(view);

            name = (TextView) view.findViewById(R.id.show_request_name);
            blood_group = (TextView) view.findViewById(R.id.show_request_bloodgroup);
            date_of_request = (TextView) view.findViewById(R.id.show_request_date);
            lat_lng = (TextView) view.findViewById(R.id.show_request_location);
            address = (TextView) view.findViewById(R.id.show_request_address);

        }
    }

}

BackgroundTask.java

package com.bloodconnect.Fragments;

import android.app.Activity;
import android.content.Context;
import android.os.AsyncTask;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

import com.bloodconnect.Model.Request;
import com.bloodconnect.R;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.ArrayList;

public class BackgroundTask extends AsyncTask<Void, Request, Void> {

    Context ctx;
    Activity activity;
    RecyclerView recyclerView;
    RecyclerView.Adapter adapter;
    RecyclerView.LayoutManager layoutManager;
    LayoutInflater inflater;
    ViewGroup container;

    ArrayList<Request> arrayList = new ArrayList<>();

    public BackgroundTask(Context ctx){
        this.ctx = ctx;
        activity = (Activity) ctx;
    }

    String json_url = "http://142.93.216.24/android/api.php?apicall=get_requests";

    @Override
    protected void onPreExecute() {


 recyclerView = (RecyclerView) activity.findViewById(R.id.listRecyclerView);
    layoutManager = new LinearLayoutManager(ctx);
    recyclerView.setLayoutManager(layoutManager);
    recyclerView.setHasFixedSize(true);

    adapter = new RecyclerAdapter(arrayList);
    recyclerView.setAdapter(adapter);
    }

    @Override
    protected Void doInBackground(Void... voids) {
        try {
            URL url = new URL(json_url);
            HttpURLConnection httpURLConnection = (HttpURLConnection) url.openConnection();
            InputStream inputStream = httpURLConnection.getInputStream();
            BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputStream));

            StringBuilder stringBuilder = new StringBuilder();
            String line;

            while ((line = bufferedReader.readLine()) != null){
                stringBuilder.append(line + "\n");
            }

            httpURLConnection.disconnect();

            String json_url = stringBuilder.toString().trim();

            Log.d("JSON STRING", json_url);

            JSONObject jsonObject = new JSONObject(json_url);

            JSONArray jsonArray =  jsonObject.getJSONArray("server_response");
            int count = 0;

            while (count < jsonArray.length()){
                JSONObject JO = jsonArray.getJSONObject(count);
                count++;

                Request request = new Request(JO.getString("name"), JO.getString("blood_group"),
                        JO.getString("date_of_request"), JO.getString("lat_lng"), JO.getString("address"));

                publishProgress(request);
            }


        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } catch (JSONException e) {
            e.printStackTrace();
        }

        return null;
    }

    @Override
    protected void onProgressUpdate(Request... values) {
        arrayList.add(values[0]);
        adapter.notifyDataSetChanged();
    }

    @Override
    protected void onPostExecute(Void aVoid) {
        super.onPostExecute(aVoid);
    }
}

Answer 1

在这种情况下，您在后台类中使用的适配器将始终为空。

启动后台进程时

 BackgroundTask backgroundTask = new BackgroundTask(getActivity(),adapter,arrayList);
    backgroundTask.execute();

，然后在BackgroundTask类中，构造函数将为：

public BackgroundTask(Context ctx, RecyclerView.Adapter adapter, ArrayList<Request> arrayList){
    this.ctx = ctx;
    this.adapter = adapter;
    this.arrayList = arrayList;
    activity = (Activity) ctx;
}

Answer 2

您正在将数据添加到arraylist中，需要在执行后设置适配器，然后使用adapter.notifyDataSetChanged();

@Override
    protected void onPostExecute(Void aVoid) {
        super.onPostExecute(aVoid);
//set your adapter after loads complete and notify your adapter to data set changed
  adapter.notifyDataSetChanged();
    }

在Fragment和MYSQL数据中使用RecyclerView

2 个答案: