Android ListView 未填充 json 数据

Question

我想用 json 值填充我的列表视图。我能够获取所有 json 值，但是 不知道是什么问题。

在我的 logcat 中，它显示 NullPointerException，不幸的是我的活动停止了。

请给个建议......

我的 java

  public class Certify_Absent_List extends Activity {
    private static ListView listview;
    private static List<String> list;
    private String teachername,date; 

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        super.onCreate(savedInstanceState);
        //setContentView(R.layout.);
        listview=(ListView) findViewById(R.id.listView1); 
        list=new ArrayList<String>();
        teachername=getIntent().getExtras().getString("teachername");
        date=getIntent().getExtras().getString("today");
        new Absent_list_bg_task().execute();
    }
    private class Absent_list_bg_task extends AsyncTask<String,Void,String>
    {

         protected String doInBackground(String... params) {  

             try{ 

                            String link="http://192.168.1.11:81/schoolapp/absent_list.php"; 
                            String data  = URLEncoder.encode("teachername", "UTF-8") 
                            + "=" + URLEncoder.encode("teacher1", "UTF-8");  
                            data += "&" + URLEncoder.encode("date", "UTF-8") 
                            + "=" + URLEncoder.encode("2013-01-01", "UTF-8");
                           /* String data  = URLEncoder.encode("teachername ", "UTF-8") 
                            + "=" + URLEncoder.encode("teacher1", "UTF-8");
                            data += "&" + URLEncoder.encode("date", "UTF-8") 
                            + "=" + URLEncoder.encode("2013-01-01", "UTF-8");*/
                            URL url = new URL(link);
                            URLConnection conn = url.openConnection(); 
                            conn.setDoOutput(true); 
                            OutputStreamWriter wr = new OutputStreamWriter
                            (conn.getOutputStream()); 
                            wr.write( data ); 
                            wr.flush(); 
                            BufferedReader reader = new BufferedReader
                            (new InputStreamReader(conn.getInputStream())); 
                            StringBuilder sb = new StringBuilder();
                            String line = null;
                            // Read Server Response
                            while((line = reader.readLine()) != null)
                            {
                               sb.append(line);
                               break;
                            }
                           return sb.toString(); 
                         }catch(Exception e){
                            return new String("Exception: " + e.getMessage());
                         }
                }
                   @Override
                protected void onPostExecute(String result)
                   {

                       ArrayAdapter<String> adapter = null;
                        JSONArray arr = null;
                        JSONObject obj = null;
                          Toast.makeText(Certify_Absent_List.this,result, Toast.LENGTH_LONG).show(); 

                       if(result.equals("wrong"))  
                       {
                          Toast.makeText(Certify_Absent_List.this,"Invalid username or password.", Toast.LENGTH_LONG).show(); 
                       }

                       else
                       {
                            Toast.makeText(Certify_Absent_List.this,result, Toast.LENGTH_LONG).show();
                            try {
                                arr=new JSONArray(result);
                            } catch (JSONException e) {
                                // TODO Auto-generated catch block
                                e.printStackTrace();
                            }

                            for(int i=0;i<arr.length();i++)
                            {
                                try {
                                    obj=arr.getJSONObject(i);
                                } catch (JSONException e) {
                                    // TODO Auto-generated catch block
                                    e.printStackTrace();
                                } 
                                try {
                                    list.add(obj.getString("ChildName"));
                                } catch (JSONException e) {
                                    // TODO Auto-generated catch block
                                    e.printStackTrace();
                                }
                                Toast.makeText(Certify_Absent_List.this,list.get(i).toString(), Toast.LENGTH_LONG).show();

                            } 


                       }
                       adapter=new ArrayAdapter<String>(Certify_Absent_List.this, android.R.layout.simple_list_item_1, list);
                       //adapter.notifyDataSetChanged();
                       listview.setAdapter(adapter);


    }
    }
    }

Json 输出

[{"ChildName":""},{"ChildName":"childname1"},{"ChildName":"childname7"},{"ChildName":"childname11"},{"ChildName":"childname11"},{"ChildName":"childname11"}]

LogCat：

    03-21 07:35:29.128: E/AndroidRuntime(1443): java.lang.NullPointerException
    03-21 07:35:29.128: E/AndroidRuntime(1443):     at com.example.certify_teacher_login.Certify_Absent_List$Absent_list_bg_task.onPostExecute(Certify_Absent_List.java:121)
03-21 07:35:29.128: E/AndroidRuntime(1443):     at com.example.certify_teacher_login.Certify_Absent_List$Absent_list_bg_task.onPostExecute(Certify_Absent_List.java:1)
03-21 07:35:29.128: E/AndroidRuntime(1443):     at android.os.AsyncTask.finish(AsyncTask.java:632)
03-21 07:35:29.128: E/AndroidRuntime(1443):     at android.os.AsyncTask.access$600(AsyncTask.java:177)
03-21 07:35:29.128: E/AndroidRuntime(1443):     at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:645)
03-21 07:35:29.128: E/AndroidRuntime(1443):     at android.os.Handler.dispatchMessage(Handler.java:102)
03-21 07:35:29.128: E/AndroidRuntime(1443):     at android.os.Looper.loop(Looper.java:137)
03-21 07:35:29.128: E/AndroidRuntime(1443):     at android.app.ActivityThread.main(ActivityThread.java:4998)
03-21 07:35:29.128: E/AndroidRuntime(1443):     at java.lang.reflect.Method.invokeNative(Native Method)
03-21 07:35:29.128: E/AndroidRuntime(1443):     at java.lang.reflect.Method.invoke(Method.java:515)
03-21 07:35:29.128: E/AndroidRuntime(1443):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:777)
03-21 07:35:29.128: E/AndroidRuntime(1443):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:593)
03-21 07:35:29.128: E/AndroidRuntime(1443):     at dalvik.system.NativeStart.main(Native Method)

Answer 1

编辑： 在您的 onCreate 中，您需要取消注释：

//setContentView(R.layout.);

并添加您要加载的布局（listView1 所在的位置）。

setContentView(R.layout.your_layout_name);

之后，加载您的 listView：

listview = (ListView) findViewById(R.id.your_list_view_id);

其他：永远不要这样做private static ListView listview; 这会导致您的应用程序中的内存泄漏，不要保留对您的视图的静态引用，如果您执行多个活动，可能会导致崩溃。

Answer 2

   //setContentView(R.layout.); 

为什么要评论那行？您也不扩展 ListActivity 或使用片段。除非那是打字错误，否则它不会起作用！

Answer 3

尝试使用以下代码。

 JSONArray jarray;


            jarray= new JSONArray(result);//result is your response from webservice



                String []name= new String[jarray.length()];






                for(int f=0;f<jarray.length();f++){
                    JSONObject jobj = jarray.getJSONObject(f);
                    name[f]=    jobj.getString("Childname");


                }

                for(int a=0;a<jarray.length();a++){

                    items.add(new EntryItem(name[a]);//items is your listview and Enrty Item is your POJO class.
                }   
                adapter  = new EntryAdapter(contact.this,items);//Entry adapter is  your adapter class.
                    lv.setAdapter(adapter);