android - java.lang.NullPointerException : Attempt to invoke interface method on a null object reference error on OnPostExecute() - AsyncTask 异常

Question

我正在尝试从 AsyncTask 中获取结果以显示在我的搜索 Activity 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)

我还检查了关于 onPostExecute() 的其他示例，看来我做对了......但仍然出现错误。

我在 onPostExecute() 处使用了一个断点，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);
}}

Activity 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 Activity 类中，我在 onGetData() 上有一个断点，但它从未到达，尽管 onPostExecute() 有对象并被调用了两次(不知道为什么)。

Answer 1

Attempt to invoke interface method 'void com.cyberdog.what2watch.JsonHandling$IOnFinish.onGetData

因为IOnFinish接口(interface)的iof对象是null。

在 setContentView 之后初始化 iof :

setContentView(R.layout.search);
iof=this;