android-應用程序崩潰後修改ArrayList <ObjectType>

Question

我正在處理的應用程序將有一個ListView以及Button動態添加項目到ListView。我爲ListView製作了一個自定義適配器。我的想法是，當用戶點擊Button它會顯示一個AlertDialog與TextView和兩個按鈕，「OK」和「取消」。這是我的活動，其中ListView將顯示 -

import java.util.ArrayList; 

import android.os.Bundle; 
import android.app.Activity; 
import android.app.AlertDialog; 
import android.app.AlertDialog.Builder; 
import android.app.Dialog; 
import android.content.DialogInterface; 
import android.graphics.Typeface; 
import android.view.LayoutInflater; 
import android.view.Menu; 
import android.view.View; 
import android.view.animation.Animation; 
import android.view.animation.AnimationUtils; 
import android.widget.Button; 
import android.widget.EditText; 
import android.widget.ListView; 
import android.widget.TextView; 
import android.widget.Toast; 

public class Add_Topics extends Activity { 

AddTopics_MenuAdapter adapter; 
ListView topics_list; 
public ArrayList<topic> the_subjects; 

@Override 
public void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_add__topics); 

    the_subjects = Model.loadTopics(); 

    topics_list = (ListView) findViewById(R.id.topics_list); 

    overridePendingTransition(R.anim.reverse_in_left, R.anim.reverse_out_left); 

    Animation enter = AnimationUtils.loadAnimation(getBaseContext(), R.anim.upcoming_menu); 
    Animation enter_slow = AnimationUtils.loadAnimation(getBaseContext(), R.anim.enter_l2r_slide); 

    TextView des = (TextView)findViewById(R.id.des_at); 
    TextView title = (TextView)findViewById(R.id.title_at); 

    Button add_topic = (Button)findViewById(R.id.add_topic_button); 

    Typeface roboto_lt = Typeface.createFromAsset(getAssets(), "Roboto-Light.ttf"); 

    des.setTypeface(roboto_lt); 
    title.setTypeface(roboto_lt); 
    add_topic.setTypeface(roboto_lt); 

    title.startAnimation(enter); 
    des.startAnimation(enter_slow); 

    adapter = new AddTopics_MenuAdapter(this, the_subjects); 
    topics_list.setAdapter(adapter); 
} 

public void onClickAddTopic(View v) { 
    showDialog(0); 
} 
protected Dialog onCreateDialog(int id) { 
    switch(id) { 
    case 0: 
     LayoutInflater li = LayoutInflater.from(this); 
     View edt_txt_add_tpc = li.inflate(R.layout.add_topics_res, null); 
     edt_txt_add_tpc.requestFocus(); 
     final EditText tpc_nm = (EditText) edt_txt_add_tpc.findViewById(R.id.topic_name); 
     Builder bld = new AlertDialog.Builder(this); 
     bld.setIcon(R.drawable.ic_launcher); 
     bld.setTitle("Add Topic/Subject"); 
     bld.setView(edt_txt_add_tpc); 
     bld.setPositiveButton("OK", new DialogInterface.OnClickListener() { 

      public void onClick(DialogInterface dialog, int whichButon) { 
       String new_topic_name = tpc_nm.getText().toString(); 
       adapter = null; 
       the_subjects.add(new topic(new_topic_name)); 
       adapter = new AddTopics_MenuAdapter(getBaseContext(), the_subjects); 
       adapter.notifyDataSetChanged(); 
      } 

     }); 
     bld.setNegativeButton("Cancel", new DialogInterface.OnClickListener() { 

      public void onClick(DialogInterface dialog, int whichButton) { 

     } 
     }); 
     return bld.create(); 
    } 
    return null; 
} 

@Override 
public void onBackPressed() { 
    super.onBackPressed(); 
    overridePendingTransition(R.anim.slide_in_left, R.anim.slide_out_left); 
} 
@Override 
public boolean onCreateOptionsMenu(Menu menu) { 
    getMenuInflater().inflate(R.menu.activity_add__topics, menu); 
    return true; 
    } 
} 

但是，整個應用程序崩潰時我挖掘「OK」按鈕。如果我省略the_subjects.add(new topic(new_topic_name));，它工作正常。但是，當我重新加載活動時，新添加的項目會消失。

這裏是我的自定義ListView適配器 -

import java.util.ArrayList; 

import android.content.Context; 
import android.graphics.Typeface; 
import android.view.LayoutInflater; 
import android.view.View; 
import android.view.ViewGroup; 
import android.view.animation.Animation; 
import android.view.animation.AnimationUtils; 
import android.widget.ArrayAdapter; 
import android.widget.TextView; 

public class AddTopics_MenuAdapter extends ArrayAdapter<topic> { 

private final Context context; 
final ArrayList<topic> topics; 

public AddTopics_MenuAdapter(Context context, ArrayList<topic> topics) { 
    super(context, R.layout.add_topics_menu, topics); 
    this.context = context; 
    this.topics = topics; 
} 
@Override 
public View getView(int position, View convertView, ViewGroup parent) { 

    LayoutInflater inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE); 

    View topicsView = inflater.inflate(R.layout.add_topics_menu, parent, false); 

    TextView topic_name = (TextView) topicsView.findViewById(R.id.topic_title); 

    Typeface rbt_lt = Typeface.createFromAsset(getContext().getAssets(), "Roboto-Light.ttf"); 

    Animation enter = AnimationUtils.loadAnimation(getContext(), R.anim.upcoming_menu); 

    topic_name.setText(topics.get(position).toString()); 
    topic_name.setTypeface(rbt_lt); 
    topic_name.setAnimation(enter); 

    return topicsView; 

    } 
} 

Model.java -

import java.util.ArrayList; 

import android.util.Log; 
import android.widget.Toast; 

public class Model { 

public static ArrayList<topic> list_of_topics; 
public static String[] subjects; 

public static ArrayList<topic> loadTopics() { 
    list_of_topics = new ArrayList<topic>(); 
    list_of_topics.add(new topic("History")); 
    list_of_topics.add(new topic("Geography")); 
    return list_of_topics; 
} 
} 

topic.java -

公共類話題{

String Topic; 

public topic(String topic) { 
    Topic = topic; 
} 
} 

這裏的羅我的錯誤克 -

03-07 18:13:28.271: E/AndroidRuntime(20349): FATAL EXCEPTION: main 
03-07 18:13:28.271: E/AndroidRuntime(20349): java.lang.NullPointerException 
03-07 18:13:28.271: E/AndroidRuntime(20349): at com.Swap.StudyBuddy.Add_Topics$1.onClick(Add_Topics.java:80) 
03-07 18:13:28.271: E/AndroidRuntime(20349): at com.android.internal.app.AlertController$ButtonHandler.handleMessage(AlertController.java:170) 
03-07 18:13:28.271: E/AndroidRuntime(20349): at android.os.Handler.dispatchMessage(Handler.java:107) 
03-07 18:13:28.271: E/AndroidRuntime(20349): at android.os.Looper.loop(Looper.java:194) 
03-07 18:13:28.271: E/AndroidRuntime(20349): at android.app.ActivityThread.main(ActivityThread.java:5409) 
03-07 18:13:28.271: E/AndroidRuntime(20349): at java.lang.reflect.Method.invokeNative(Native Method) 
03-07 18:13:28.271: E/AndroidRuntime(20349): at java.lang.reflect.Method.invoke(Method.java:525) 
03-07 18:13:28.271: E/AndroidRuntime(20349): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839) 
03-07 18:13:28.271: E/AndroidRuntime(20349): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:606) 
03-07 18:13:28.271: E/AndroidRuntime(20349): at dalvik.system.NativeStart.main(Native Method) 

我無法弄清楚弄清楚如何修改ArrayList<topic>沒有崩潰的應用程序。請幫我解決這個問題。提前致謝！！

Answer 1

您會取代你的代碼...你的代碼是

  String new_topic_name = tpc_nm.getText().toString(); 
      adapter = null; 
      the_subjects.add(new topic(new_topic_name)); 
      adapter = new AddTopics_MenuAdapter(getBaseContext(), the_subjects); 
      adapter.notifyDataSetChanged(); 

你不想爲你的適配器設置爲空值那麼您將使用

  adapter.clear(); 

它會刪除你的整個數據在您的適配器只需更換該單線