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

Question

我需要显示 JSON 文件中的产品列表，但我一直收到此错误:

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

我尝试移动我代码中的所有内容(所以可能我搞砸了一点)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修复。

这是产生错误的 fragment :

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

出现此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请求队列中。首先检查 JsonObjectRequest 是否有 onSuccess 函数，如果没有，则需要在 onResponse 的 requestQueue 里面添加。否则，android 是同步的，因此它会在实际检索 Json 之前执行您的行。

所以快速修复是这样的:

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

如果它为空，这将确保不添加。将此添加到您的 onCreate 中:

mRequestQueue = Volley.newRequestQueue(context);