java.lang.NullPointerException：尝试在 OnPostExecute() 上的空对象引用错误上调用接口方法 - AsyncTask
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【中文标题】java.lang.NullPointerException：尝试在 OnPostExecute() 上的空对象引用错误上调用接口方法 - AsyncTask【英文标题】：java.lang.NullPointerException: Attempt to invoke interface method on a null object reference error on OnPostExecute() - AsyncTask 【发布时间】：2015-05-04 22:32:53 【问题描述】：我正在尝试从 AsyncTask 获取结果以显示在我的搜索活动中。
我有一个来自以前应用程序的示例，它是在一位同事的帮助下制作的（我是那里的实习生）。
我根据这个应用程序的需要调整了示例，但现在我得到了NullPointerException：
03-04 03:50:23.865: E/androidRuntime(8224): FATAL EXCEPTION: main
03-04 03:50:23.865: E/AndroidRuntime(8224): Process: com.cyberdog.what2watch, PID: 8224
03-04 03:50:23.865: E/AndroidRuntime(8224): java.lang.NullPointerException: Attempt to invoke interface method 'void com.cyberdog.what2watch.JsonHandling$IOnFinish.onGetData(org.json.JSONObject)' on a null object reference
03-04 03:50:23.865: E/AndroidRuntime(8224): at com.cyberdog.what2watch.JsonHandlingTMDBAsync.onPostExecute(JsonHandling.java:321)
03-04 03:50:23.865: E/AndroidRuntime(8224): at com.cyberdog.what2watch.JsonHandlingTMDBAsync.onPostExecute(JsonHandling.java:1)
03-04 03:50:23.865: E/AndroidRuntime(8224): at android.os.AsyncTask.finish(AsyncTask.java:632)
03-04 03:50:23.865: E/AndroidRuntime(8224): at android.os.AsyncTask.access$600(AsyncTask.java:177)
03-04 03:50:23.865: E/AndroidRuntime(8224): at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:645)
03-04 03:50:23.865: E/AndroidRuntime(8224): at android.os.Handler.dispatchMessage(Handler.java:102)
03-04 03:50:23.865: E/AndroidRuntime(8224): at android.os.Looper.loop(Looper.java:135)
03-04 03:50:23.865: E/AndroidRuntime(8224): at android.app.ActivityThread.main(ActivityThread.java:5274)
03-04 03:50:23.865: E/AndroidRuntime(8224): at java.lang.reflect.Method.invoke(Native Method)
03-04 03:50:23.865: E/AndroidRuntime(8224): at java.lang.reflect.Method.invoke(Method.java:372)
03-04 03:50:23.865: E/AndroidRuntime(8224): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:909)
03-04 03:50:23.865: E/AndroidRuntime(8224): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:704)
我还检查了onPostExecute() 上的其他样本，看来我做了正确的事情.. 但仍然出现错误。
我在onPostExecute() 处使用了断点，结果不为空，尽管它确实运行了两次代码。
异步类的代码：
class JsonHandlingTMDBAsync extends AsyncTask<String, Void, JSONObject> // params,
// progress,
// result
private String api_key_tmdb = "2fb98d2bca5895c89a6efaf70903f706";
private String tmdb_multi_url = "http://api.themoviedb.org/3/search/multi?query=";
int Total_pages;
int number_entries_per_page;
private JSONObject resultObject;
private IOnFinish listener;
String Titel="";
String Year ="";
public JsonHandlingTMDBAsync(String titel, String year, IOnFinish listener)
this.listener = listener;
this.Titel=titel;
this.Year=year;
@Override
protected JSONObject doInBackground(String... params)
return loadJSON(params);
private JSONObject loadJSON(String... params)
resultObject = new JSONObject();
StringBuilder builder = new StringBuilder();
HttpClient client = new DefaultHttpClient();
HttpGet httpGet;
if (Titel.trim().length() == 0 && Year.trim().length() != 0)
httpGet = new HttpGet(tmdb_multi_url + Year + "&api_key="
+ api_key_tmdb);
else if (Titel.trim().length() != 0 && Year.trim().length() == 0)
httpGet = new HttpGet(tmdb_multi_url + Titel + "&api_key="
+ api_key_tmdb);
else if (Titel.trim().length() == 0 && Year.trim().length() == 0)
System.out.print("error null titel/year");
httpGet = new HttpGet(tmdb_multi_url + "star+wars&api_key="
+ api_key_tmdb);
else
httpGet = new HttpGet(
"http://www.omdbapi.com/?s=star&y=2008&r=JSON"
+ "&plot=short");
try
HttpResponse response = client.execute(httpGet);
StatusLine statusLine = response.getStatusLine();
int statusCode = statusLine.getStatusCode();
if (statusCode == 200)
HttpEntity entity = response.getEntity();
InputStream content = entity.getContent();
BufferedReader reader = new BufferedReader(
new InputStreamReader(content));
String line;
while ((line = reader.readLine()) != null)
builder.append(line);
else
Log.e(JsonHandling.class.toString(), "Failed to download file");
resultObject = new JSONObject(builder.toString());
catch (ClientProtocolException e)
e.printStackTrace();
catch (IOException e)
e.printStackTrace();
catch (JSONException e)
// TODO Auto-generated catch block
e.printStackTrace();
return resultObject;
@Override
protected void onPostExecute(JSONObject result)
super.onPostExecute(result);
listener.onGetData(result);
活动代码：
package com.cyberdog.what2watch;
import java.util.List;
import org.json.JSONObject;
import com.cyberdog.what2watch.JsonHandling.IOnFinish;
import android.app.Activity;
import android.content.Context;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.inputmethod.InputMethodManager;
import android.widget.AbsListView;
import android.widget.AbsListView.OnScrollListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ListView;
import android.widget.Toast;
public class SearchActivity extends Activity implements OnScrollListener, IOnFinish
/** Called when the activity is first created. */
private static SearchCustomAdapter adapter;
EditText tvTitel = null;
EditText tvYear = null;
ListView lvSearch = null;
JsonHandling jh = new JsonHandling(this);
int pageNumber =0;
int previeusTotal=0;
private static List<Serie> searchResult;
private IOnFinish iof;
private static List<Serie>series;
@Override
public void onCreate(Bundle savedInstanceState)
super.onCreate(savedInstanceState);
setContentView(R.layout.search);
tvTitel = (EditText) findViewById(R.id.etf_search_titel);
tvYear = (EditText) findViewById(R.id.etf_search_year);
lvSearch = (ListView)findViewById(R.id.lvSearch);
Button btnSearch = (Button) findViewById(R.id.btnSearch_search);
btnSearch.setOnClickListener(new OnClickListener()
public void onClick(View v)
InputMethodManager inputManager = (InputMethodManager) getSystemService(Context.INPUT_METHOD_SERVICE);
inputManager.hideSoftInputFromWindow(getCurrentFocus()
.getWindowToken(), InputMethodManager.HIDE_NOT_ALWAYS);
JsonHandling.getInstance().getJSONFromUrl(tvTitel.getText().toString(), tvYear.getText().toString(),iof);
);
public void search(int pageNumber)
// adapter for the custom list
//lvSearch = (ListView) findViewById(R.id.lvSearch);
// SearchCustomAdapter lvAdapt = new SearchCustomAdapter(
// SearchActivity.this, jh.search_db(tvTitel.getText()
// .toString(), tvYear.getText().toString(),
// searchSite, pageNumber));
// lvSearch.setAdapter(lvAdapt);
//lvAdapt.notifyDataSetChanged();
@Override
public void onGetData(JSONObject obj)
if (obj == null)
Toast.makeText(this,
"obj is null",
Toast.LENGTH_LONG).show();
Serie serie = new Serie("not found","");
Serie[] seriesList = new Serie[series.size()];
seriesList[0] = serie;
adapter = new SearchCustomAdapter(this, seriesList);
lvSearch = (ListView) findViewById(R.id.lvSearch);
lvSearch.setAdapter(adapter);
if(obj !=null)
if (JsonHandling.pageNumber == 0)
series =Serie.serieListFromJSON(this, JsonHandling.getInstance().getJSONArraySerieFromUrlTMDB(obj));
Serie[] seriesList =series.toArray(new Serie[series.size()]);
adapter = new SearchCustomAdapter(this, seriesList);
lvSearch = (ListView)findViewById(R.id.lvSearch);
lvSearch.setAdapter(adapter);
//lvSearch.setOnItemClickListener(this);
lvSearch.setOnScrollListener(this);
@Override
public void onScroll(AbsListView view, int firstVisibleItem,
int visibleItemCount, int totalItemCount)
int lastInScreen = firstVisibleItem+visibleItemCount;
totalItemCount = 20+20 * pageNumber;
//int totalSearchResults = 0;
Log.i("total, last in screen, pagenumber outside", totalItemCount+":" + lastInScreen+ ":"+pageNumber);
if((lastInScreen +20*pageNumber == totalItemCount))
Log.i("total, last in screen, pagenumber inside", totalItemCount+":" + lastInScreen+ ":"+pageNumber);
pageNumber++;
search(pageNumber);
else
@Override
public void onScrollStateChanged(AbsListView arg0, int arg1)
// TODO Auto-generated method stub
和json/interface部分代码：
public interface IOnFinish
void onGetData(JSONObject obj);
public void getJSONFromUrl(String titel, String year, IOnFinish listener)
JsonHandlingTMDBAsync task = new JsonHandlingTMDBAsync(titel, year, listener);
task.execute();
在我的活动课程中，我在onGetData() 上有一个断点，但它从未到达，尽管onPostExecute() 有该对象并被调用了两次（不知道为什么）。
【问题讨论】：
【参考方案1】：尝试调用接口方法'void com.cyberdog.what2watch.JsonHandling$IOnFinish.onGetData
因为IOnFinish接口的iof对象是null。
在setContentView 之后初始化iof：
setContentView(R.layout.search);
iof=this;
【讨论】：
我知道我错过了一些东西，而你的回答是一个 D'oh 时刻：s 谢谢你拯救了我的一天 :)【参考方案2】：当你调用listener.onGetData(result);时，最好先检查监听器，比如
if (listener != null)
listener.onGetData(result);
因为无法保证侦听器在您的代码中不为空。
【讨论】：
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