未创建Android SQLite数据库 [英] Android SQLite database is not created

问题描述

未创建 SQLite数据库。在过去的几个小时里，我一直在玩这个游戏，但是现在我已经很累了。 Log Cat 没有收到任何错误或警告。这是到目前为止我尝试过的代码。

The SQLite database is not created. I am playing with this for the past few hours but now i got tired a lot. I am not getting any errors or warnings in the Log Cat. Here is my code what I have tried so far. Help will be appreciated.

public class MySQLiteHelper extends SQLiteOpenHelper{
    final static String DB_NAME = "citiesdata.db";
    final static String TABLE_NAME = "allcities";
    final static String ID = "_id";
    final static String CITY_CODE = "city_code";
    final static int DB_VERSION = 1;
    private SQLiteDatabase db;
    Context cont;

    public MySQLiteHelper(Context context) {
        super(context, DB_NAME, null, DB_VERSION);
        cont = context;
        Toast.makeText(cont, "constructor called", Toast.LENGTH_LONG).show();
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        Toast.makeText(cont, "onCreate method called", Toast.LENGTH_LONG).show();
        String createTable = "CREATE TABLE IF NOT EXISTS " + TABLE_NAME + "(" + ID + " INTEGER PRIMARY KEY AUTOINCREMENT, " + CITY_CODE + " TEXT);";
        db.execSQL(createTable);

//      INSERTING RECORDS IN DATABASE
        SQLiteDatabase writableDB = this.getWritableDatabase();
        ContentValues values = new ContentValues();
        values.put(CITY_CODE, "333");
        writableDB.insert(TABLE_NAME, null, values);
        writableDB.close();
        readCity();
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        Toast.makeText(cont, "onUpgrade method called", Toast.LENGTH_LONG).show();
        db.execSQL("DROP TABLE IF EXISTS " + TABLE_NAME);
        onCreate(db);
    }


    public void readCity(){
        Toast.makeText(cont, "readCity method called", Toast.LENGTH_LONG).show();
        String selectQuery = "SELECT  * FROM " + TABLE_NAME;
        SQLiteDatabase db = this.getReadableDatabase();
        Cursor cursor = db.rawQuery(selectQuery, null);
        Toast.makeText(cont, cursor.getString(cursor.getColumnIndex(CITY_CODE)), Toast.LENGTH_LONG).show();
        db.close();
    }
}

在主UI线程上的按钮上调用构造函数

Calling the Constructor in the main UI Thread on a button click like this.

new MySQLiteHelper(MainActivity.this);

权限是...

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
<uses-permission android:name="android.permission.WRITE_INTERNAL_STORAGE"/>

多谢了。

推荐答案

最后，我将问题直接打入了头脑。但是如何？

At last, I shot the problem directly into the head. But How?

SQLiteOpenHelper 的逻辑是它不调用通过仅创建对象来创建onCreate 方法。

The logic of the SQLiteOpenHelper is that it doesn't call the onCreate method by only creating its object.

1. 每当您尝试访问数据库以检索或插入数据时，如果数据库尚未创建/不可用，它将首次调用 onCreate 方法。


2. <$ c $如果数据库可用，但版本低于您提供的版本，则将调用c> onUpgrade 方法。

1. Whenever you try to access the database for retrieving or inserting data, if the database is not already created/available, it will call the onCreate method for the first time.
2. The onUpgrade method will be called if the database is available but the version is lower than the one you provide.

我在我的 MySQLiteHelper 类中创建了一个 fillTable 方法，在该类中，我第一次尝试访问该数据库通过调用 SQLiteOpenHelper 的 getWritableDatabase 方法编写。 fillTable 方法的定义如下所示。

I created a fillTable method in my MySQLiteHelper class where I tried to access the database for the first time for writing by calling getWritableDatabase method of the SQLiteOpenHelper. The definition of the fillTable method looks like this.

public void fillTable(){
    db = this.getWritableDatabase();
    ContentValues values = new ContentValues();
    values.put(CITY_CODE, "333");
    db.insert(TABLE_NAME, null, values);
    db.close();
    readCity();
}

在该类的新实例上调用fillTable方法：

Call the fillTable method on a new instance of the class:

new MySQLiteHelper(MainActivity.this).fillTable();

感谢所有尝试在评论中帮助我的人。

Thanks to all those who tried to help me in comments.

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