Android：從列表視圖中刪除SQLite項目。 NullPointerException

Question

我很新的android（和編程），所以我希望這個問題不是太愚蠢。我試圖通過教程來回答我的問題，並通過搜索通過stackoverflow沒有成功。

我想填充一個SQLite數據庫列表：當我點擊列表上的一個項目時，我希望它從列表視圖和數據庫中刪除。通過調試，我認爲問題出在我的ArrayAdapter方法上，因爲即使像「getCount（）」這樣的基本方法似乎也不產生輸出。

對於我的SQLite數據庫處理器類，這裏是獲取數據庫中的所有項目和刪除項目的方法：

  // Getting All breadcrumbs 
    public List<Breadcrumb> getAllBreadcrumbs() { 
    List<Breadcrumb> breadcrumbList = new ArrayList<Breadcrumb>(); 
     // Select All Query 
     String selectQuery = "SELECT * FROM " + TABLE_BREADCRUMBS; 

    SQLiteDatabase db = this.getWritableDatabase(); 
    Cursor cursor = db.rawQuery(selectQuery, null); 

    // looping through all rows and adding to list 
    if (cursor.moveToFirst()) { 
     do { 
      Breadcrumb breadcrumb = new Breadcrumb(); 
      breadcrumb.setId(Integer.parseInt(cursor.getString(0))); 
      breadcrumb.setBreadcrumbLatitude(cursor.getInt(1)); 
      breadcrumb.setBreadcrumbLongitude(cursor.getInt(2)); 
      // Adding location to list 
      breadcrumbList.add(breadcrumb); 
     } while (cursor.moveToNext()); 
    } 

    // Deleting single breadcrumb 
     public void deleteBreadcrumb(Breadcrumb breadcrumb) { 
     SQLiteDatabase db = this.getWritableDatabase(); 
     int id = breadcrumb.getId(); 
     System.out.println("Comment deleted with id: " + id); 
     db.delete(TABLE_BREADCRUMBS, KEY_ID 
       + " = " + id, null); 
     db.close(); 
      } 

的麻煩活動的XML佈局：

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" 
xmlns:tools="http://schemas.android.com/tools" 
android:layout_width="match_parent" 
android:layout_height="match_parent" 
android:paddingBottom="@dimen/activity_vertical_margin" 
android:paddingLeft="@dimen/activity_horizontal_margin" 
android:paddingRight="@dimen/activity_horizontal_margin" 
android:paddingTop="@dimen/activity_vertical_margin" 
tools:context=".CollectedBreadcrumbsActivity" > 

<ListView 
    android:id="@+id/list" 
    android:layout_width="match_parent" 
    android:layout_height="wrap_content" > 
</ListView> 

</LinearLayout> 

這裏是活動：

public class CollectedBreadcrumbsActivity extends Activity implements OnItemClickListener { 

private ListView listview; 
private DatabaseHandler db; 

@Override 
public void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    requestWindowFeature(Window.FEATURE_NO_TITLE); 
    setContentView(R.layout.activity_collected_breadcrumbs); 

    DatabaseHandler db = new DatabaseHandler(this); 
    List<Breadcrumb> breadcrumbs = db.getAllBreadcrumbs(); 

    listview = (ListView) findViewById(R.id.list); 
    ArrayAdapter<Breadcrumb> adapter = new ArrayAdapter<Breadcrumb>(this, 
      android.R.layout.simple_list_item_1, breadcrumbs); 
     listview.setAdapter(adapter); 
    listview.setOnItemClickListener(this); 
} 

public void onItemClick(AdapterView<?> l, View v, int position, long id) { 
    Log.d("HelloListView", "You clicked Item: " + id + " at position:" + position); 
    Log.d("HelloListView", "Number of items in adapter:" + listview.getAdapter().getCount()); 
    @SuppressWarnings("unchecked") 
    ArrayAdapter<Breadcrumb> adapter = (ArrayAdapter<Breadcrumb>) listview.getAdapter(); 
    if (listview.getAdapter().getCount() > 0) { 
     Breadcrumb breadcrumb = (Breadcrumb) listview.getAdapter().getItem(position); 
     db.deleteBreadcrumb(breadcrumb); 
     adapter.remove(breadcrumb); 
     } 
    adapter.notifyDataSetChanged(); 
    } 
} 

該listview顯示出來好嗎，但是wh恩我單擊列表中的項目，應用程序崩潰與以下日誌：

05-07 19:08:40.671: D/HelloListView(29562): You clicked Item: 5 at position:5 
05-07 19:08:40.671: W/dalvikvm(29562): threadid=1: thread exiting with uncaught exception (group=0x40e4c438) 
05-07 19:08:40.671: E/AndroidRuntime(29562): FATAL EXCEPTION: main 
05-07 19:08:40.671: E/AndroidRuntime(29562): java.lang.NullPointerException 
05-07 19:08:40.671: E/AndroidRuntime(29562): at com.detimil.breadcrumbs1.CollectedBreadcrumbsActivity.onItemClick(CollectedBreadcrumbsActivity.java:42) 
05-07 19:08:40.671: E/AndroidRuntime(29562): at android.widget.AdapterView.performItemClick(AdapterView.java:298) 
05-07 19:08:40.671: E/AndroidRuntime(29562): at android.widget.AbsListView.performItemClick(AbsListView.java:1268) 
05-07 19:08:40.671: E/AndroidRuntime(29562): at android.widget.AbsListView$PerformClick.run(AbsListView.java:3059) 
05-07 19:08:40.671: E/AndroidRuntime(29562): at android.widget.AbsListView$1.run(AbsListView.java:3950) 
05-07 19:08:40.671: E/AndroidRuntime(29562): at android.os.Handler.handleCallback(Handler.java:615) 
05-07 19:08:40.671: E/AndroidRuntime(29562): at android.os.Handler.dispatchMessage(Handler.java:92) 
05-07 19:08:40.671: E/AndroidRuntime(29562): at android.os.Looper.loop(Looper.java:137) 
05-07 19:08:40.671: E/AndroidRuntime(29562): at android.app.ActivityThread.main(ActivityThread.java:4950) 
05-07 19:08:40.671: E/AndroidRuntime(29562): at java.lang.reflect.Method.invokeNative(Native Method) 
05-07 19:08:40.671: E/AndroidRuntime(29562): at java.lang.reflect.Method.invoke(Method.java:511) 
05-07 19:08:40.671: E/AndroidRuntime(29562): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1004) 
05-07 19:08:40.671: E/AndroidRuntime(29562): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:771) 
05-07 19:08:40.671: E/AndroidRuntime(29562): at dalvik.system.NativeStart.main(Native Method) 

同樣，我認爲這個問題必須與我的ArrayAdapter，因爲即使該日誌Log.d("HelloListView", "Number of items in adapter:" + listview.getAdapter().getCount());不會產生應用程序崩潰的事情之前。

如果有人對此有任何想法，我會非常感謝幫助。再次，如果這是一個愚蠢的問題，很抱歉。

非常感謝。

Answer 1

你db是null，

因爲

DatabaseHandler db = new DatabaseHandler(this); 

它會創建一個本地實例，但不實例db場 所以要像這樣

db = new DatabaseHandler(this); 

Answer 2

您的數據庫應全球不在本地啓動。

我想補充一點，總體來說使用ArrayAdapter從數據庫加載並不是一個好主意。您應該使用CursorAdapter和CursorLoader來管理數據庫。

這裏的自定義CursorAdapter的一個很好的教程：link

另一個用於CursorLoaders：link