如何从自定义数据库助手创建游标

Question

我正在阅读 Griffith 的 Head First：Android 开发 一书，并且某个代码引发了 Cannot resolve Symbol 错误。

我正在按照本书的说明从自定义数据库助手构建游标。这是抛出错误的代码。

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_drink);

        int drinkNo = (Integer)getIntent().getExtras().get(EXTRA_DRINKNO);

        try{
            SQLiteOpenHelper starbuzzDatabaseHelper = new StarbuzzDatabaseHelper(this);
            SQLiteDatabase db = starbuzzDatabaseHelper.getReadableDatabase();

它说 Cannot resolve symbol "StarbuzzDatabaseHelper" 尽管 StarbuzzDatabaseHelper 确实存在于我的项目中。这是书中的代码。不是我的。我假设这是书中的错误。我试过这个：

SQLiteOpenHelper starbuzzDatabaseHelper = new SQLiteOpenHelper(this); 还是不行。

这是我的完整代码：

public class DrinkActivity extends AppCompatActivity {

    public static final String EXTRA_DRINKNO = "drinkNo";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_drink);

        int drinkNo = (Integer)getIntent().getExtras().get(EXTRA_DRINKNO);

        try{
            SQLiteOpenHelper starbuzzDatabaseHelper = new StarbuzzDatabaseHelper(this);
            SQLiteDatabase db = starbuzzDatabaseHelper.getReadableDatabase();
            Cursor cursor = db.query(
                    "DRINK",
                    new String[]{"NAME", "DESCRIPTION", "IMAGE_RESOURCE_ID"},
                    "_id = ?",
                    new String[]{Integer.toString(drinkNo)},
                    null, null, null);
            if(cursor.moveToFirst()){
                String nameText = cursor.getString(0);
                String descriptionText = cursor.getString(1);
                int photoId = cursor.getInt(2);

                TextView name = findViewById(R.id.name);
                name.setText(nameText);

                TextView description = findViewById(R.id.description);
                description.setText(descriptionText);

                ImageView photo = findViewById(R.id.photo);
                photo.setImageResource(photoId);
                photo.setContentDescription(nameText);
            }
            cursor.close();
            db.close();
        }catch(SQLiteException e){
            Toast.makeText(this, "Database Unavailable", Toast.LENGTH_SHORT).show();
        }
    }
}

这是数据库助手 class:

public class StarbuzzDatabaseHelper extends SQLiteOpenHelper {

    public static final String DB_NAME = "starbuzz";
    public static final int DB_VERSION = 2;

    public StarbuzzDatabaseHelper(Context context, String name, SQLiteDatabase.CursorFactory factory, int version) {
        super(context, DB_NAME, factory, DB_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        updateMyDatabase(db, 0, DB_VERSION);

    }

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

    private void updateMyDatabase(SQLiteDatabase db, int oldVersion, int newVersion){
        if(oldVersion <1){

            db.execSQL("CREATE TABLE DRINK ("
                    + "_id INTEGER PRIMARY KEY AUTOINCREMENT, "
                    + "NAME TEXT, "
                    + "DESCRIPTION TEXT, "
                    + "IMAGE_RESOURCE_ID INTEGER);");

            insertDrink(db, "Latte", "Espresso and Steamed Milk", R.drawable.latte);
            insertDrink(db, "Capuccino", "Espresso, hot milk and steamed milk foam", R.drawable.cappuccino);
            insertDrink(db, "filter", "Our best drink coffee", R.drawable.filter);
        }
        else if(oldVersion < 2){
            db.execSQL("ALTER TABLE DRINK ADD COLUMN FAVORITE NUMERIC");
        }
    }

    private static void insertDrink(SQLiteDatabase db, String name, String description, int resourceID){
        ContentValues drinkValues = new ContentValues();
        drinkValues.put("NAME", name);
        drinkValues.put("DESCRIPTION", description);
        drinkValues.put("IMAGE_RESOURCE_D", resourceID);
        db.insert("DRINK", null, drinkValues);
    }
}

Answer 1

而不是

SQLiteOpenHelper starbuzzDatabaseHelper = new StarbuzzDatabaseHelper(this);

尝试

StarbuzzDatabaseHelper starbuzzDatabaseHelper = new StarbuzzDatabaseHelper(this);

编辑：将其添加到您的助手 class

 public StarbuzzDatabaseHelper(Context context) {
    super(context, DB_NAME, null, DB_VERSION);
}

Answer 2

您已经定义了 starbuzzDatabaseHelper class 的构造函数，需要 4 个参数：-

public StarbuzzDatabaseHelper(Context context, String name, SQLiteDatabase.CursorFactory factory, int version) {
    super(context, DB_NAME, factory, DB_VERSION);
}

尝试实例化您正在使用的实例:-

SQLiteOpenHelper starbuzzDatabaseHelper = new StarbuzzDatabaseHelper(this);

当您 could/should 使用 :-

StarbuzzDatabaseHelper starbuzzDatabaseHelper = new StarbuzzDatabaseHelper(
    this,StarbuzzDatabaseHelper.DBNAME,
    your_factory,
    StarbuzzDatabaseHelper.DB_VERSION
);

即传递 4 个参数（见下文）
实例化（创建一个实例）您的数据库助手，而不是受 SQLiteOpenHelper 实例限制的实例。
提供可以解决的方法

我建议您将构造函数更改为：-

public StarbuzzDatabaseHelper(Context context) {
    super(context, DB_NAME, null, DB_VERSION);
}

因此需要 1 个参数（上下文）

然后在您的 activity（活动）中使用以下内容：-

StarbuzzDatabaseHelper starbuzzDatabaseHelper; //<<<< Class Scope for DBHelper

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_drink);

    int drinkNo = (Integer)getIntent().getExtras().get(EXTRA_DRINKNO);

    //<<<< try/catch can be confusing as exceptions may not stop when you very likely should.
    starbuzzDatabaseHelper = new StarbuzzDatabaseHelper(this);
    SQLiteDatabase db = starbuzzDatabaseHelper.getReadableDatabase();

    ...... rest of your code

另一个问题

您还有一个问题，您的 insertDrink 方法由于打字错误而无法插入饮料。

即创建 table 时的列是 IMAGE_RESOURCE_ID INTEGER，但在插入中调用列 IMAGE_RESOURCE_D。我强烈建议对 table 和列名采用单一来源 (CONSTANTS)，并尽可能始终使用它们。

我建议对您的 StarbuzzDatabaseHelper 进行以下更改 class 这应该 remedy/simplify 很重要 :-

public class StarbuzzDatabaseHelper extends SQLiteOpenHelper {

    public static final String DB_NAME = "starbuzz";
    public static final int DB_VERSION = 2;
    public static final String TB_DRINK = "DRINK";
    public static final String COL_DRINK_ID = BaseColumns._ID;
    public static final String COL_DRINK_NAME = "NAME";
    public static final String COL_DRINK_DESCRIPTION = "DESCRIPTION";
    public static final String COL_DRINK_IMAGERESOURCEID = "IMAGE_RESOURCE_ID";
    public static final String COL_DRINK_FAVOURITE = "FAVOURITE";

    public StarbuzzDatabaseHelper(Context context) {
        super(context, DB_NAME, null, DB_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        updateMyDatabase(db, 0, DB_VERSION);

    }

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

    private void updateMyDatabase(SQLiteDatabase db, int oldVersion, int newVersion){
        if(oldVersion <1){

            db.execSQL("CREATE TABLE DRINK ("
                    + COL_DRINK_ID + " INTEGER PRIMARY KEY AUTOINCREMENT, "
                    + COL_DRINK_NAME + " TEXT, "
                    + COL_DRINK_DESCRIPTION + " TEXT, "
                    + COL_DRINK_IMAGERESOURCEID + " INTEGER);");

            insertDrink(db, "Latte", "Espresso and Steamed Milk", 1);
            insertDrink(db, "Capuccino", "Espresso, hot milk and steamed milk foam", 2);
            insertDrink(db, "filter", "Our best drink coffee", 3);
        }
        else if(oldVersion < 2){
            db.execSQL("ALTER TABLE DRINK ADD COLUMN FAVORITE NUMERIC"); //<<< rather then null
        }
    }

    private static void insertDrink(SQLiteDatabase db, String name, String description, int resourceID){
        ContentValues drinkValues = new ContentValues();
        drinkValues.put("NAME", name);
        drinkValues.put("DESCRIPTION", description);
        //drinkValues.put("IMAGE_RESOURCE_D", resourceID); //<<< NO SUCH COLUMN
        drinkValues.put(COL_DRINK_IMAGERESOURCEID,resourceID); // NO SPELLING ISSUES this way
        db.insert("DRINK", null, drinkValues);
    }

    // Alternative insert publicly available so activities can add drinks
    public long insertDrink(String drink_name,
                            String drink_description,
                            int resource_id,
                            double drink_favourite) {
        SQLiteDatabase db = this.getWritableDatabase();
        ContentValues cv = new ContentValues();
        cv.put(COL_DRINK_NAME, drink_name);
        cv.put(COL_DRINK_DESCRIPTION, drink_description);
        cv.put(COL_DRINK_IMAGERESOURCEID,resource_id);
        cv.put(COL_DRINK_FAVOURITE,drink_favourite);
        return db.insert(TB_DRINK,null,cv);
    }

    // method to return a Cursor with a sinhle drink as per the id
    public Cursor getDrinkById(long id) {
        String whereclause = COL_DRINK_ID + "=?";
        String[] whereargs = new String[]{String.valueOf(id)};
        SQLiteDatabase db = this.getWritableDatabase();
        return db.query(TB_DRINK,
                null,
                whereclause,
                whereargs,
                null,
                null,
                null
        );
    }

    // method to return All drinks as a cursor sort according to drink name
    public Cursor getAllDrinks() {
        return this.getWritableDatabase().query(
                TB_DRINK,
                null,
                null,
                null,
                null,
                null,
                COL_DRINK_NAME + " ASC"
        );
    }
}

以下代码已用于测试上述内容，也是如何利用 class 的示例：-

public class DrinkActivity extends AppCompatActivity {

    StarbuzzDatabaseHelper starbuzzDatabaseHelper;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main); //<<<< change to your layout

        starbuzzDatabaseHelper = new StarbuzzDatabaseHelper(this);
        Cursor csr = starbuzzDatabaseHelper.getDrinkById(1); //<<< using 1 for testing
        if (csr.moveToFirst()) {
            Log.d("CURSORINFO","Drink name is " + csr.getString(csr.getColumnIndex(StarbuzzDatabaseHelper.COL_DRINK_NAME)));
            //<<<< replace the line above with your code >>>>
        }
    }
}

备注

由于插入导致您的数据库不太可能有任何数据，因此我建议删除数据库。这可以通过 clearing/delete 应用程序的数据或卸载应用程序来完成。

如何从自定义数据库助手创建游标

How to create a cursor from a custom database helper

android

sqliteopenhelper

android-sqlite

另一个问题

备注