当我将列表视图添加到我的应用程序时出现这些错误

Question

我应该创建一个应用程序，在成功登录后，将有一个为用户发送的请求的列表视图。我的登录工作正常。但是当我开始创建列表视图时，我的问题就开始了。

我正在关注一个 listview 教程，我对其进行了测试，工作正常，因此我将其合并到我的代码中。但是当我这样做时，我的应用程序开始崩溃

这是具有列表视图的活动的代码。

    import java.io.BufferedReader;
    import java.io.InputStreamReader;
    import java.io.OutputStreamWriter;
    import java.net.URL;
    import java.net.URLConnection;
    import java.net.URLEncoder;
    import java.util.ArrayList;

    import org.json.JSONArray;
    import org.json.JSONObject;

    import android.app.Activity;
    import android.content.Context;
    import android.content.SharedPreferences;
    import android.content.SharedPreferences.Editor;
    import android.os.AsyncTask;
    import android.os.Bundle;
    import android.util.Log;
    import android.view.Menu;
    import android.view.View;
    import android.widget.AdapterView;
    import android.widget.AdapterView.OnItemClickListener;
    import android.widget.ListView;
    import android.widget.Toast;

    public class AdminActivity extends Activity {

        ArrayList<Actors> actorsList;

        ActorAdapter adapter;

        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_admin);
            Bundle extras = getIntent().getExtras();
            String usertype = extras.getString("UserType");
            String userid = extras.getString("User_ID");
            actorsList = new ArrayList<Actors>(); // EDITED I FORGOT THIS 
            //PART, IT FIXED THE LOG IN ISSUE BUT STILL THE LIST VIEW DOESNT
            //DISPLAY AND AFTER DIRECTING ME TO THE NEXT PAGE, IT CRASHES AFTER AWHILE
            new GetRequests(this).execute(userid,usertype);
            ListView listview = (ListView)findViewById(R.id.list);
            adapter = new ActorAdapter(getApplicationContext(), R.layout.row, actorsList);

            listview.setAdapter(adapter);
            Log.i("lv", "does it go here");
            listview.setOnItemClickListener(new OnItemClickListener() {
                @Override
                public void onItemClick(AdapterView<?> arg0, View arg1, int position,
                        long id) {
                    Log.i("lvclick", "does it go here");
                    // TODO Auto-generated method stub
                    Toast.makeText(getApplicationContext(), actorsList.get(position).getName(), Toast.LENGTH_LONG).show();              
                }
            });
            Toast.makeText(this.getBaseContext(), userid + usertype, Toast.LENGTH_LONG).show();



        }

        @Override
        public boolean onCreateOptionsMenu(Menu menu) {
            // Inflate the menu; this adds items to the action bar if it is present.
            getMenuInflater().inflate(R.menu.admin, menu);
            return true;
        }

        public class GetRequests extends AsyncTask<String,Void,String>{

            private Context context;
            public GetRequests(Context context) {
                this.context = context;
            }

            protected void onPreExecute(){

            }

              @Override
              protected String doInBackground(String... arg0) {
              try {
                Log.i("fetch", "does it go here");
                       String userid = (String)arg0[0];

                       String usertype = (String)arg0[1];

                       String link="http://192.168.1.37/mobile/get-requests.php";

                       String data  = URLEncoder.encode("userid", "UTF-8") + "=" + URLEncoder.encode(userid, "UTF-8");

                       data += "&" + URLEncoder.encode("usertype", "UTF-8") + "=" + URLEncoder.encode(usertype, "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){

               if(result.length() == 0){
                   Toast.makeText(this.context, "Invalid User Id or Password", Toast.LENGTH_LONG).show();
               }
               else{

                    try {
                        Log.i("got", "does it go here");
                           JSONArray jArr = new JSONArray(result);
                           for(int i = 0; i < jArr.length(); i++){
                               JSONObject jobj = jArr.getJSONObject(i);
                                Actors actor = new Actors();

                                actor.setName(jobj.getString("Request_ID"));
                                actor.setDescription(jobj.getString("User_ID"));
                                actor.setDob(jobj.getString("Date_Needed"));
                                actor.setCountry(jobj.getString("fromTime_Needed"));
                                actor.setHeight(jobj.getString("fromTime_Needed"));
                                actor.setSpouse(jobj.getString("Subject"));
                                actor.setChildren(jobj.getString("Group"));
                                actor.setImage(jobj.getString("Prof_ID"));

                                actorsList.add(actor);

                           }
                           Toast.makeText(this.context, result, Toast.LENGTH_LONG).show();


                       }
                   catch(Exception e) {
                           e.printStackTrace();
                   }


               }   
              }
           }

        public void logout(View view){
              SharedPreferences sharedpreferences = getSharedPreferences
              (MainActivity.MyPREFERENCES, Context.MODE_PRIVATE);
              Editor editor = sharedpreferences.edit();
              editor.clear();
              editor.commit();
              Log.i("logout", "does it go here?");
              moveTaskToBack(true); 
              AdminActivity.this.finish();
           }
       public void exit(View view){
          moveTaskToBack(true); 
          AdminActivity.this.finish();
       }
    }

如果没有这个块，它将首先显示我的登录信息，然后将我引导到我设置意图的位置。

listview.setAdapter(adapter);
            Log.i("lv", "does it go here");
            listview.setOnItemClickListener(new OnItemClickListener() {
                @Override
                public void onItemClick(AdapterView<?> arg0, View arg1, int position,
                        long id) {
                    Log.i("lvclick", "does it go here");
                    // TODO Auto-generated method stub
                    Toast.makeText(getApplicationContext(), actorsList.get(position).getName(), Toast.LENGTH_LONG).show();              
                }
            });

使用上面的代码块，我的应用会在我打开后立即崩溃。

*已编辑：修复了登录问题，但一段时间后仍然崩溃。似乎是什么问题？

Answer 1

您似乎没有初始化actorsList;，这将引发NPE。因此，将您的声明更改为

ArrayList<Actors> actorsList = new ArrayList<Actors>();

您的另一个问题是您尝试使用空列表设置Adapter，因为您在执行AsyncTask 后直接执行此操作。由于它“异步”运行，因此在调用setAdaoter()之前不会完成

您需要移动此代码

ListView listview = (ListView)findViewById(R.id.list);
adapter = new ActorAdapter(getApplicationContext(), R.layout.row, actorsList);

listview.setAdapter(adapter);

到onPostExecute()，这样它就可以在任务完成后加载Adapter。

另外，请不要粘贴堆栈跟踪的屏幕截图...很难阅读。您可以单击保存按钮并将其粘贴到代码块中的帖子中，就像您对代码所做的那样。

Answer 2

actorsList 变量永远不会被初始化。

更改：ArrayList<Actors> actorsList; 至：ArrayList<Actors> actorsList = new ArrayList();

请记住，当您致电 actorsList.add(actor); 时，您必须通知数据发生了变化。

理想情况下，您可以选择使用ArrayAdapter，因为您有一种方便的方法来添加新值。你可以在本地初始化它并覆盖 getView 方法或者你可以让 ActorAdapter 扩展这个类型