在SQLite數據庫上執行查詢時出現NullPointerException

我試圖從我創建的數據庫中讀取數據，但是在嘗試獲取所有記錄時，我總是得到一個NullPointerException。在SQLite數據庫上執行查詢時出現NullPointerException

我幾乎從另一個應用程序中拷貝了代碼，我已經完美運行了，但不知何故，我在這裏做錯了。

的NullPointerException異常是在

return mDb.query(DATABASE_TABLE_LOCALLOGIN, new String[] {LOCALLOGIN_ID, LOCALLOGIN_LOGIN, LOCALLOGIN_PASSWORD}, null, null, null, null, null);

行下面是相關的代碼（不介意字符串數組，其後來添加表： GoingOutDbAdapter.java

public class GoingOutDbAdapter { 
private static final String DATABASE_NAME = "GoingOutData"; 
private static final String DATABASE_TABLE_LOCALLOGIN = "LocalLogin"; 

public static final String LOCALLOGIN_ID = "LocalLogin_id"; 
public static final String LOCALLOGIN_LOGIN = "Login"; 
public static final String LOCALLOGIN_PASSWORD = "Password"; 

private static final String TAG = "Debugstring"; 

private DatabaseHelper mDbHelper; 
private SQLiteDatabase mDb; 

private static final String[] DATABASE_CREATE = { 
    "CREATE Table " + DATABASE_TABLE_LOCALLOGIN + " (" 
    + LOCALLOGIN_ID + " integer PRIMARY KEY Autoincrement, " 
    + LOCALLOGIN_LOGIN + " text NOT NULL, " 
    + LOCALLOGIN_PASSWORD + " text NOT NULL);"}; 

private final Context mCtx; 

private static class DatabaseHelper extends SQLiteOpenHelper { 
    DatabaseHelper(Context context) { 
     super(context, DATABASE_NAME, null, DATABASE_VERSION); 
    } 
    @Override 
    public void onCreate(SQLiteDatabase db) { 
     for(int i = 0; i < DATABASE_CREATE.length; i++){ 
      Log.d(TAG,DATABASE_CREATE[i]); 
      db.execSQL(DATABASE_CREATE[i]); 
     } 
    } 
} 

public GoingOutDbAdapter(Context ctx) { 
    this.mCtx = ctx; 
} 

public GoingOutDbAdapter open() throws SQLException { 
    mDbHelper = new DatabaseHelper(mCtx); 
    mDb = mDbHelper.getWritableDatabase(); 
    return this; 
} 

public void close() { 
    mDbHelper.close(); 
} 

public Cursor fetchAllLocalLogins() { 
    return mDb.query(DATABASE_TABLE_LOCALLOGIN, new String[] {LOCALLOGIN_ID, LOCALLOGIN_LOGIN, LOCALLOGIN_PASSWORD}, null, null, null, null, null); 
} 

}

MyActivity .java，我叫fetchAllLocalLogins

public class MyActivity extends Activity { 
/** Called when the activity is first created. */ 

private GoingOutDbAdapter mDbHelper; 

@Override 
public void onCreate(Bundle savedInstanceState) { 

    super.onCreate(savedInstanceState); 

    mDbHelper = new GoingOutDbAdapter(this); 

    setContentView(R.layout.main); 

Cursor localLogin = mDbHelper.fetchAllLocalLogins(); 

} 
}

來源

2012-04-09 Mats Raemen

在這裏發表您的logcat – 2012-04-09 09:06:53

你會大概要調用的方法open()您進行查詢之前：

//... 
mDbHelper.open(); //whitout this call mdb will be NULL 
Cursor localLogin = mDbHelper.fetchAllLocalLogins();

來源

2012-04-09 09:10:05 Luksprog

這樣做，現在我得到「android.database.CursorIndexOutOfBoundsException：指數-1要求，尺寸爲1」「mLoginField.setText（localLogin.getString（localLogin.getColumnIndexOrThrow（GoingOutDbAdapter.LOCALLOGIN_LOGIN）））;」在我的內容視圖中使用mLoginField作爲EditText – 2012-04-09 09:41:08

@MatsRaemen從查詢返回的'Cursor'位於'-1'處，因此您應該在調用'localLogin.moveToFirst（）'或'localLogin.moveToNext（）'之前你試圖獲取數據。 – Luksprog 2012-04-09 09:43:16

你又救了我，非常感謝！ – 2012-04-09 09:59:30

你應該執行數據庫操作之前打開數據庫。 mDbHelper.open（）;

來源

2012-04-09 09:40:25 Srikanth

在SQLite數據庫上執行查詢時出現NullPointerException

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