Question

我正在尝试删除当前存储在数据库中的所有条目，因此我设置了按钮“bDeleteTimings”，并且我希望在它被用作确认时显示AlertDialog删除。我是SQLite和AlertDialogs的新手，所以请帮助我们。我在“tdb.deleteEntries”行上得到一个NullPointerException。感谢。

这是我按下bDeleteTimings时使用的代码：

case R.id.bDeleteTimings:
        new AlertDialog.Builder(this)
                .setIcon(android.R.drawable.ic_dialog_alert)
                .setTitle("Delete Timings")
                .setMessage("Are you sure you want to delete all currently set timings? (Cannot be undone)")
                .setPositiveButton("Yes", new DialogInterface.OnClickListener() {
                            @Override
                            public void onClick(DialogInterface dialog,int which) {
                                tdb.deleteEntries();
                            }

                        }).setNegativeButton("No", null).show();
        break;
    }//end switch

这是我的数据库代码：

public class TimingsDb {

public static final String KEY_ID = "_id";
public static final String KEY_SUBJECT = "subject_name";
public static final String KEY_WEEK = "week";
public static final String KEY_DAY = "day_name";
public static final String KEY_LESSON = "lesson";

private static final String DATABASE_NAME = "TimingsDB";
private static final String DATABASE_TABLE = "TimingsTable";
private static final int DATABASE_VERSION = 1;

private DbHelper ourHelper;
private final Context ourContext;
private SQLiteDatabase ourDatabase;

private static class DbHelper extends SQLiteOpenHelper {

    public DbHelper(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
        // TODO Auto-generated constructor stub

    }// end Constructor

    @Override
    public void onCreate(SQLiteDatabase db) {
        // TODO Auto-generated method stub
        db.execSQL("CREATE TABLE " + DATABASE_TABLE + " (" + KEY_ID
                + " INTEGER PRIMARY KEY AUTOINCREMENT, " + KEY_SUBJECT
                + " TEXT, " + KEY_WEEK + " INTEGER, " + KEY_DAY + " TEXT, "
                + KEY_LESSON + " INTEGER);");
    }// end onCreate

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        // TODO Auto-generated method stub
        db.execSQL("DROP TABLE IF EXISTS " + DATABASE_TABLE);
        onCreate(db);
    }// end onUpgrade
}// end class

public TimingsDb(Context c) {
    ourContext = c;
}

public TimingsDb open() throws SQLException {
    ourHelper = new DbHelper(ourContext);
    ourDatabase = ourHelper.getWritableDatabase();
    return this;
}// end open

public void close() {
    ourHelper.close();
}// end close

public void deleteEntries(){
    ourDatabase.execSQL("DELETE * FROM "+DATABASE_TABLE+";");
}//end deleteEntries

} //结束TimingsDb

（这个类中显然还有其他方法，但我删除了它们以避免并发症）

（编辑）这是LogCat：

02-27 19:31:30.040: E/AndroidRuntime(8044): FATAL EXCEPTION: main
02-27 19:31:30.040: E/AndroidRuntime(8044): java.lang.NullPointerException
02-27 19:31:30.040: E/AndroidRuntime(8044):     at     com.shearer.schooltimetable.TimingsDb.deleteEntries(TimingsDb.java:86)
02-27 19:31:30.040: E/AndroidRuntime(8044):     at com.shearer.schooltimetable.EditTimetable$1.onClick(EditTimetable.java:55)
02-27 19:31:30.040: E/AndroidRuntime(8044):     at com.android.internal.app.AlertController$ButtonHandler.handleMessage(AlertController.java:166)
02-27 19:31:30.040: E/AndroidRuntime(8044):     at android.os.Handler.dispatchMessage(Handler.java:99)
02-27 19:31:30.040: E/AndroidRuntime(8044):     at android.os.Looper.loop(Looper.java:137)
02-27 19:31:30.040: E/AndroidRuntime(8044):     at android.app.ActivityThread.main(ActivityThread.java:5041)
02-27 19:31:30.040: E/AndroidRuntime(8044):     at java.lang.reflect.Method.invokeNative(Native Method)
02-27 19:31:30.040: E/AndroidRuntime(8044):     at java.lang.reflect.Method.invoke(Method.java:511)
02-27 19:31:30.040: E/AndroidRuntime(8044):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
02-27 19:31:30.040: E/AndroidRuntime(8044):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
02-27 19:31:30.040: E/AndroidRuntime(8044):     at dalvik.system.NativeStart.main(Native Method)

Answer 1

ourDatabase中{p> null为deleteEntries()。由于您在open()初始化它，似乎您还没有打开数据库。您也可以在getWritableDatabase()中拨打deleteEntries()。

此外，还存在SQL语法问题。移除*中的DELETE * FROM...。您只能将*列通配符与SELECT s。

一起使用

Answer 2

你可以简单地放下桌子。

case R.id.bDeleteTimings:
    new AlertDialog.Builder(this)
            .setIcon(android.R.drawable.ic_dialog_alert)
            .setTitle("Delete Timings")
            .setMessage("Are you sure you want to delete all currently set timings? (Cannot be undone)")
            .setPositiveButton("Yes", new DialogInterface.OnClickListener() {
                        @Override
                        public void onClick(DialogInterface dialog,int which) {
                                SQLiteDatabase mDB;
                                    DbHelper dbhelper = new DbHelper(this);
                                    mDB = dbhelper.getWritableDataBase();
                                    mDB.execSQL("DROP TABLE "+DbHelper.DATABASE_TABLE);
                                    dbhelper.onCreate(mDB);
                        }

                    }).setNegativeButton("No", null).show();
    break;
}//end switch

Answer 3

使用此

db.delete(DATABASE_TABLE, null, null);

从SQLite数据库中删除条目＆amp; AlertDialog

3 个答案: