我不断收到 com.android.volley.RequestQueue.add 空对象引用错误

Question

我需要显示来自 JSON 文件的产品列表，但我不断收到此错误：

https://i.imgur.com/WEu4DzV.png

我尝试在我的代码中移动所有内容（所以也许我把它弄乱了一点）但我无法让它工作。我是初学者，所以像这样的错误对我来说很难解决。

这是产生错误的片段：

package com.example.app;

import android.content.Context;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import com.android.volley.Request;
import com.android.volley.RequestQueue;
import com.android.volley.Response;
import com.android.volley.VolleyError;
import com.android.volley.toolbox.JsonObjectRequest;
import com.android.volley.toolbox.Volley;

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

import java.util.ArrayList;


public class ListFragment extends Fragment {

private RecyclerView mRecyclerView;
private ListAdapter mListAdapter;
private ArrayList<Item> mList;
private RequestQueue mRequestQueue;
Context context;


@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        return inflater.inflate(R.layout.fragment_list, container, false);
}

@Override
public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);
        mRecyclerView = view.findViewById(R.id.recycler_view);
        mRecyclerView.setHasFixedSize(true);
        mRecyclerView.setLayoutManager(new LinearLayoutManager(getContext()));
        mRecyclerView.setAdapter(mListAdapter);
}

@Override
public void onActivityCreated(@Nullable Bundle savedInstanceState) {
        parseJSON();
        super.onActivityCreated(savedInstanceState);
}

@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mList = new ArrayList<>();
        mListAdapter = new ListAdapter(context, mList);
}


private void parseJSON() {

        String url = "http://my_ip_adress/sestavsisvujsvetweb/api/seznammagnetek";
        JsonObjectRequest request = new JsonObjectRequest(Request.Method.GET, url, null, new Response.Listener<JSONObject>() {

            @Override
            public void onResponse(JSONObject response) {
                int number = 0;
                try {
                    number = response.getInt("monumentid1");
                } catch (JSONException e) {
                    e.printStackTrace();
                }
                String name = null;
                try {
                    name = response.getString("name1");
                } catch (JSONException e) {
                    e.printStackTrace();
                }
                String region = null;
                try {
                    region = response.getString("region1");
                } catch (JSONException e) {
                e.printStackTrace();
                }

                mRequestQueue = Volley.newRequestQueue(context);
                mList.add(new Item(number, name, region));

        }
    }, new Response.ErrorListener() {
        @Override
        public void onErrorResponse(VolleyError error) {
            error.printStackTrace();
        }
    });
        mRequestQueue.add(request); //this causes the problem, why? :(


}

}

Answer 1

出现问题是因为您在知道它已成功检索到您的信息之前尝试将请求添加到 requestQueue 中。首先，检查JsonObjectRequest是否有onSuccess函数，如果没有，则需要添加到onResponse里面的requestQueue。否则，android 是同步的，因此它将在实际检索 Json 之前执行您的行。

所以快速解决方法是这样的：

if(request != null) {
   mRequestQueue.add(request);
}

这将确保如果它为空则不添加。在你的 onCreate 中添加这个：

mRequestQueue = Volley.newRequestQueue(context);