我正在尝试使用JSON从MySQL数据库中检索数据，以便在android studio中的片段列表视图中显示，但由于java.lang.NullPointerException，我的应用程序在运行时停止。您可以帮我修复这个错误请。 我的片段代码是：
package com.sawaset.rpl;
import android.os.AsyncTask;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ListView;
import android.widget.SimpleAdapter;
import android.widget.Toast;
import org.apache.http.HttpResponse;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.impl.client.DefaultHttpClient;
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.util.ArrayList;
import java.util.HashMap;
/**
Created by Sibo on 7/25/2017.
**/
public class timetable extends Fragment {
private String jsonResult=null;
public String address = "http://127.0.0.1/projects/soccer/query.php";
ListView listview;
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.table_tab, container, false);
listview = (ListView) rootView.findViewById(android.R.id.list);
/*ArrayAdapter<String> adapter=new ArrayAdapter<String>(getActivity(),android.R.layout.simple_list_item_1,menus);
lv.setAdapter(adapter);*/
accessWebService();
return rootView;
}
//
private class jsonReadTask extends AsyncTask<String, Void, String> {
@Override
protected String doInBackground(String... params) {
HttpClient httpclient = new DefaultHttpClient();
HttpPost httPost = new HttpPost(params[0]);
try {
HttpResponse response = httpclient.execute(httPost);
jsonResult = inputStreamToString(
response.getEntity().getContent()).toString();
} catch (ClientProtocolException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
return null;
}
//
private StringBuilder inputStreamToString(InputStream is) {
String rLine = "";
StringBuilder answer = new StringBuilder();
BufferedReader rd = new BufferedReader(new InputStreamReader(is));
try {
while ((rLine = rd.readLine()) != null) {
answer.append(rLine);
}
} catch (IOException e) {
Toast.makeText(getActivity(), "Error:" + e.toString(), Toast.LENGTH_LONG).show();
}
return answer;
}
@Override
protected void onPostExecute(String result) {
drawer();
}
}
//
public void accessWebService() {
jsonReadTask task = new jsonReadTask();
task.execute(new String[]{address});
}
public void drawer() {
try {
JSONObject jsonResponse = new JSONObject(jsonResult.substring(jsonResult.indexOf("{"), jsonResult.lastIndexOf("}") + 1));
JSONArray jsonMainNode = jsonResponse.getJSONArray("teams");
final ArrayList<HashMap<String, String>> MyarrList = new ArrayList<HashMap<String, String>>();
HashMap<String, String> map;
if (jsonMainNode!= null) {
for (int i = 0; i < jsonMainNode.length(); i++) {
JSONObject c = jsonMainNode.getJSONObject(i);
map = new HashMap<String, String>();
map.put("t_id", c.getString("t_id"));
map.put("t_id", c.getString("t_id"));
map.put("t_id", c.getString("t_id"));
MyarrList.add(map);
SimpleAdapter sAdp = new SimpleAdapter(getActivity(), MyarrList, R.layout.columns, new String[]{"t_id", "t_id", "t_id"}, new int[]{R.id.col1, R.id.col2, R.id.col3});
listview.setAdapter(sAdp);
}
}
} catch (JSONException e) {
Toast.makeText(getActivity(), "Error:" + e.toString(), Toast.LENGTH_LONG).show();
}
}
}
logcat如下所示
HostConnection::get() New Host Connection established 0x9bbdd240, tid 17548
08-13 22:07:20.395 17531-17531/com.sawaset.rpl E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.sawaset.rpl, PID: 17531
java.lang.NullPointerException: Attempt to invoke virtual method 'int java.lang.String.indexOf(java.lang.String)' on a null object reference
at com.sawaset.rpl.timetable.drawer(timetable.java:105)
at com.sawaset.rpl.timetable$jsonReadTask.onPostExecute(timetable.java:92)
at com.sawaset.rpl.timetable$jsonReadTask.onPostExecute(timetable.java:55)
at android.os.AsyncTask.finish(AsyncTask.java:667)
at android.os.AsyncTask.-wrap1(AsyncTask.java)
at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:684)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6119)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
08-13 22:07:21.056 1280-1280/? E/EGL_emulation: tid 1280: eglCreateSyncKHR(1669): error 0x3004 (EGL_BAD_ATTRIBUTE)
08-13 22:07:21.847 2415-17536/com.google.android.googlequicksearchbox:search E/AudioRecord: Could not get audio input for session 2753, record source 1999, sample rate 16000, format 0x1, channel mask 0x10, flags 0
08-13 22:07:21.853 2415-17536/com.google.android.googlequicksearchbox:search E/AudioRecord-JNI: Error creating AudioRecord instance: initialization check failed with status -22.
08-13 22:07:21.853 2415-17536/com.google.android.googlequicksearchbox:search E/android.media.AudioRecord: Error code -20 when initializing native AudioRecord object.
08-13 22:07:21.856 2415-17536/com.google.android.googlequicksearchbox:search E/ActivityThread: Failed to find provider info for com.google.android.apps.gsa.testing.ui.audio.recorded
08-13 22:07:27.002 2415-17536/com.google.android.googlequicksearchbox:search E/AudioRecord: Could not get audio input for session 2761, record source 1999, sample rate 16000, format 0x1, channel mask 0x10, flags 0
08-13 22:07:27.004 2415-17536/com.google.android.googlequicksearchbox:search E/AudioRecord-JNI: Error creating AudioRecord instance: initialization check failed with status -22.
08-13 22:07:27.006 2415-17536/com.google.android.googlequicksearchbox:search E/android.media.AudioRecord: Error code -20 when initializing native AudioRecord object.
08-13 22:07:27.007 2415-17536/com.google.android.googlequicksearchbox:search E/ActivityThread: Failed to find provider info for com.google.android.apps.gsa.testing.ui.audio.recorded
08-13 22:07:32.099 2415-17536/com.google.android.googlequicksearchbox:search E/AudioRecord: Could not get audio input for session 2769, record source 1999, sample rate 16000, format 0x1, channel mask 0x10, flags 0
08-13 22:07:32.100 2415-17536/com.google.android.googlequicksearchbox:search E/AudioRecord-JNI: Error creating AudioRecord instance: initialization check failed with status -22.
任何帮助都非常感谢
答案 0 :(得分：0)
java.lang.NullPointerException: Attempt to invoke virtual method 'int java.lang.String.indexOf(java.lang.String)' on a null object reference
at com.sawaset.rpl.timetable.drawer(timetable.java:105)
这意味着 timetable.drawer 中的第105行存在空对象引用。我相信这是以下代码行：
final ArrayList<HashMap<String, String>> MyarrList = new ArrayList<HashMap<String, String>>();
唯一一次为MyarrList分配任何值的时间都在if (jsonMainNode!= null)的第107行的timetable.drawer检查中。因此，如果jsonMainNode等于null，那么您的MyarrList也将为空。
正如Selvin在下面提到的，解决导致null jsonResult的路径时出错将导致null jsonResponse，这将导致null jsonMainNode并抛出NullPointerException。