Android中的AlertDialog - 空指針異常

Question

我在網上查了很多，現在變得沮喪，因爲我找不到關於以下錯誤的信息。有人可以幫我弄清楚這是怎麼回事？

08-17 13:15:47.800: W/dalvikvm(10356): threadid=1: thread exiting with uncaught exception (group=0x401f0760) 
08-17 13:15:47.800: E/AndroidRuntime(10356): FATAL EXCEPTION: main 
08-17 13:15:47.800: E/AndroidRuntime(10356): java.lang.NullPointerException 
08-17 13:15:47.800: E/AndroidRuntime(10356): at android.app.AlertDialog.resolveDialogTheme(AlertDialog.java:120) 
08-17 13:15:47.800: E/AndroidRuntime(10356): at android.app.AlertDialog$Builder.<init>(AlertDialog.java:337) 
08-17 13:15:47.800: E/AndroidRuntime(10356): at com.work.plan.Login$1.onClick(Login.java:64) 
08-17 13:15:47.800: E/AndroidRuntime(10356): at android.view.View.performClick(View.java:3112) 
08-17 13:15:47.800: E/AndroidRuntime(10356): at android.view.View$PerformClick.run(View.java:11956) 
08-17 13:15:47.800: E/AndroidRuntime(10356): at android.os.Handler.handleCallback(Handler.java:587) 
08-17 13:15:47.800: E/AndroidRuntime(10356): at android.os.Handler.dispatchMessage(Handler.java:92) 
08-17 13:15:47.800: E/AndroidRuntime(10356): at android.os.Looper.loop(Looper.java:132) 
08-17 13:15:47.800: E/AndroidRuntime(10356): at android.app.ActivityThread.main(ActivityThread.java:4025) 
08-17 13:15:47.800: E/AndroidRuntime(10356): at java.lang.reflect.Method.invokeNative(Native Method) 
08-17 13:15:47.800: E/AndroidRuntime(10356): at java.lang.reflect.Method.invoke(Method.java:491) 
08-17 13:15:47.800: E/AndroidRuntime(10356): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:841) 
08-17 13:15:47.800: E/AndroidRuntime(10356): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:599) 
08-17 13:15:47.800: E/AndroidRuntime(10356): at dalvik.system.NativeStart.main(Native Method) 

這裏是導致錯誤的類：

OnClickListener LoginListener = new OnClickListener(){ 

     public void onClick(View arg0) { 
      String checkUserName = userName.getText().toString(); 


      Cursor mCursor = mDbHelper.fetchUserByName(checkUserName); 

      if(mCursor.moveToFirst()){ 
       String checkDatabaseUser = mCursor.getString(mCursor.getColumnIndexOrThrow(LoginDbAdapter.KEY_NAME)); 
       Toast.makeText(getBaseContext(), checkDatabaseUser,Toast.LENGTH_LONG).show(); 
       if(checkDatabaseUser.equals(checkUserName)){ 
        Intent i = new Intent(); 
        i.setClass(getBaseContext(), WorkoutPlanActivity.class); 
        startActivity(i); 
       } 
      }else { 
     **//Error happening here -->** AlertDialog.Builder alert = new AlertDialog.Builder(m_ClassContext); 
        alert.setTitle("Error Logging in"); 
        alert.setMessage("The user name is not in the system."); 
        alert.setPositiveButton("Ok",new DialogInterface.OnClickListener() { 

         public void onClick(DialogInterface dialog, int which) { 
          // TODO Auto-generated method stub 
          dialog.cancel(); 
         } 
        }); 
        alert.show(); 
      } 
     } 

    }; 

我改變了警惕這種

AlertDialog alert = new AlertDialog.Builder(getBaseContext()).create(); 
       alert.setTitle("Error Logging in"); 
       alert.setMessage("The user name is not in the system."); 
       alert.setButton("Ok",new DialogInterface.OnClickListener() { 

        public void onClick(DialogInterface dialog, int which) { 
         // TODO Auto-generated method stub 
         dialog.cancel(); 
        } 
       }); 
       alert.show(); 

，現在我得到以下錯誤：

08-17 13:38:38.460: W/dalvikvm(10513): threadid=1: thread exiting with uncaught exception (group=0x401f0760) 
08-17 13:38:38.460: E/AndroidRuntime(10513): FATAL EXCEPTION: main 
08-17 13:38:38.460: E/AndroidRuntime(10513): android.view.WindowManager$BadTokenException: Unable to add window -- token null is not for an application 
08-17 13:38:38.460: E/AndroidRuntime(10513): at android.view.ViewRoot.setView(ViewRoot.java:450) 
08-17 13:38:38.460: E/AndroidRuntime(10513): at android.view.WindowManagerImpl.addView(WindowManagerImpl.java:181) 
08-17 13:38:38.460: E/AndroidRuntime(10513): at android.view.WindowManagerImpl.addView(WindowManagerImpl.java:95) 
08-17 13:38:38.460: E/AndroidRuntime(10513): at android.app.Dialog.show(Dialog.java:269) 
08-17 13:38:38.460: E/AndroidRuntime(10513): at com.work.plan.Login$1.onClick(Login.java:74) 
08-17 13:38:38.460: E/AndroidRuntime(10513): at android.view.View.performClick(View.java:3112) 
08-17 13:38:38.460: E/AndroidRuntime(10513): at android.view.View$PerformClick.run(View.java:11956) 
08-17 13:38:38.460: E/AndroidRuntime(10513): at android.os.Handler.handleCallback(Handler.java:587) 
08-17 13:38:38.460: E/AndroidRuntime(10513): at android.os.Handler.dispatchMessage(Handler.java:92) 
08-17 13:38:38.460: E/AndroidRuntime(10513): at android.os.Looper.loop(Looper.java:132) 
08-17 13:38:38.460: E/AndroidRuntime(10513): at android.app.ActivityThread.main(ActivityThread.java:4025) 
08-17 13:38:38.460: E/AndroidRuntime(10513): at java.lang.reflect.Method.invokeNative(Native Method) 
08-17 13:38:38.460: E/AndroidRuntime(10513): at java.lang.reflect.Method.invoke(Method.java:491) 
08-17 13:38:38.460: E/AndroidRuntime(10513): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:841) 
08-17 13:38:38.460: E/AndroidRuntime(10513): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:599) 
08-17 13:38:38.460: E/AndroidRuntime(10513): at dalvik.system.NativeStart.main(Native Method) 

這最終成爲該解決方案，謝謝你們

private Context m_ClassContext = this; 

AlertDialog alert = new AlertDialog.Builder(m_ClassContext).create(); 
       alert.setTitle("Error Logging in"); 
       alert.setMessage("The user name is not in the system."); 
       alert.setButton("Ok",new DialogInterface.OnClickListener() { 

        public void onClick(DialogInterface dialog, int which) { 
         // TODO Auto-generated method stub 
         dialog.cancel(); 
        } 
       }); 
       alert.show(); 

Answer 1

而不是使用m_ClassContext使用：

//Please change NameOfYourActivity to what it should be 
AlertDialog.Builder alert = new AlertDialog.Builder(NameOfYourActivity.this);