NPE而從數據庫填充列表

Question

級＃1 - 必須從數據庫獲取數據並準備ListAdapter：

public class DataListView extends ListActivity { 

    public ArrayList<String> results = new ArrayList<String>(); 
    public String tableName = DBHelper.tableName; 
    public SQLiteDatabase newDB; 

    public void displayResultList() { 
     setListAdapter(new ArrayAdapter<String>(this, 
       android.R.layout.simple_list_item_1, results)); 
     getListView().setTextFilterEnabled(true); 
    } 

    public void openAndQueryDatabase() { 
     try { 
      DBHelper dbHelper = new DBHelper(this.getApplicationContext()); 
      newDB = dbHelper.getWritableDatabase(); 
      Cursor c = newDB.rawQuery("SELECT FirstName, Age FROM " + 
        tableName, null); 

      if (c != null) { 
       if (c.moveToFirst()) { 
        do { 
         String firstName = c.getString(c.getColumnIndex("FirstName")); 
         int age = c.getInt(c.getColumnIndex("Age")); 
         results.add("Name: " + firstName + ", Age: " + age); 
        }while (c.moveToNext()); 
       } 
      }   
     } catch (SQLiteException se) { 
      Log.e(getClass().getSimpleName(), "Could not create or Open the database"); 
     } finally { 
      if (newDB != null) 
       newDB.execSQL("DELETE FROM " + tableName); 
       newDB.close(); 
     } 

    } 
} 

類＃2 - 具有顯示列表：

public class StartActivity extends ListActivity { 
    @Override 
    public void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 

     DataListView list = new DataListView(); 
     list.openAndQueryDatabase(); 
     list.displayResultList(); 
    } 
} 

但它拋出錯誤：

01-02 18:22:25.584: E/AndroidRuntime(30425): FATAL EXCEPTION: main 01-02 18:22:25.584: E/AndroidRuntime(30425): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.saigmn/com.saigmn.StartActivity}: java.lang.NullPointerException 01-02 18:22:25.584: E/AndroidRuntime(30425): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1956) 01-02 18:22:25.584: E/AndroidRuntime(30425): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1981) 01-02 18:22:25.584: E/AndroidRuntime(30425): at android.app.ActivityThread.access$600(ActivityThread.java:123) 01-02 18:22:25.584: E/AndroidRuntime(30425): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1147) 01-02 18:22:25.584: E/AndroidRuntime(30425): at android.os.Handler.dispatchMessage(Handler.java:99) 01-02 18:22:25.584: E/AndroidRuntime(30425): at android.os.Looper.loop(Looper.java:137) 01-02 18:22:25.584: E/AndroidRuntime(30425): at android.app.ActivityThread.main(ActivityThread.java:4424) 01-02 18:22:25.584: E/AndroidRuntime(30425): at java.lang.reflect.Method.invokeNative(Native Method) 01-02 18:22:25.584: E/AndroidRuntime(30425): at java.lang.reflect.Method.invoke(Method.java:511) 01-02 18:22:25.584: E/AndroidRuntime(30425): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784) 01-02 18:22:25.584: E/AndroidRuntime(30425): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551) 01-02 18:22:25.584: E/AndroidRuntime(30425): at dalvik.system.NativeStart.main(Native Method) 01-02 18:22:25.584: E/AndroidRuntime(30425): Caused by: java.lang.NullPointerException 01-02 18:22:25.584: E/AndroidRuntime(30425): at com.saigmn.DataListView.openAndQueryDatabase(DataListView.java:45) 01-02 18:22:25.584: E/AndroidRuntime(30425): at com.saigmn.StartActivity.onCreate(StartActivity.java:20) 01-02 18:22:25.584: E/AndroidRuntime(30425): at android.app.Activity.performCreate(Activity.java:4492) 01-02 18:22:25.584: E/AndroidRuntime(30425): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1049) 01-02 18:22:25.584: E/AndroidRuntime(30425): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1920)

一些測試後，我發現裏面有openAndQueryDatabase水木清華（）。

將兩個被調用的方法都放入StartActivity類時 - 它工作正常。

請幫我找出這個問題。

Answer 1

if (newDB != null) 
    newDB.execSQL("DELETE FROM " + tableName); 
    newDB.close(); 

這可能可能是源。將這兩個語句放在if塊中，以確保close()調用不是在nullnewDB實例上進行的。嘗試：

if (newDB != null) 
{ 
    newDB.execSQL("DELETE FROM " + tableName); 
    newDB.close(); 
} 

現在我很確定這是您的NullPointer的來源。檢查DataListView.java中的第45行。