Why the MySQLiteHelper (extends SQLiteOpenHelper) new many times in my Singleton?

I am a newer in Android,now I code something about sqlite. There are some questions,I try many times to solve it,but I fail. I write the MySQLiteHelper class (extends SQLiteOpenHelper)

public class MySQLiteHelper extends SQLiteOpenHelper {


public static final String CLASSIFICATION = "CREATE TABLE IF NOT EXISTS classification(classification_id Integer primary key AUTOINCREMENT,classify VARCHAR)";
public static final String ACCOUNT = "CREATE TABLE IF NOT EXISTS account(account_id Integer primary key AUTOINCREMENT,username VARCHAR,password VARCHAR,classification_id Integer REFERENCES classification(classification_id))";
public MySQLiteHelper(Context context, String name, SQLiteDatabase.CursorFactory factory, int version){
    super(context,name,factory,version);
}

@Override
public void onCreate(SQLiteDatabase db) {
    db.execSQL(CLASSIFICATION);
    db.execSQL(ACCOUNT);
}

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

}

}

And I write the MySQLiteDatabase class to operate the database ( in Singleton)

public class MySQLiteDatabase  {
private static MySQLiteDatabase instance;
private SQLiteDatabase sqLiteDatabase;
private MySQLiteDatabase(Context context){
    MySQLiteHelper helper = new MySQLiteHelper(context,"test.db",null,1);
    sqLiteDatabase = helper.getWritableDatabase();
    sqLiteDatabase.execSQL("INSERT INTO classification(classification_id,classify) VALUES(1,'QQ')");
}
public static MySQLiteDatabase getInstance(Context context){
    if(null == instance){
        instance = new MySQLiteDatabase(context);
    }
    return instance;
}

}

the questions: When I use the follow code to create the tables,it always create tables(but I want to create tables only once,so I make it Singleton)

my = MySQLiteDatabase.getInstance(SampleActivity.this);

why the follow code always execute?

if(null == instance){
        instance = new MySQLiteDatabase(context);
    }

please help me.