Question

我正在浏览格里菲斯的 Head First：Android开发一书，而某些代码会导致Cannot resolve Symbol错误。

我正在根据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();

它表示Cannot resolve symbol "StarbuzzDatabaseHelper"，即使我的项目中存在StarbuzzDatabaseHelper。这是本书的代码。不是我的。我假设这本书出错了。我试过这个：

SQLiteOpenHelper starbuzzDatabaseHelper = new SQLiteOpenHelper(this);它仍然无效。

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();
        }
    }
}

这是数据库助手类：

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

您已将 starbuzzDatabaseHelper 类的构造函数定义为需要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);

虽然你可以/应该使用： -

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（创建实例）您的数据库帮助程序而不是作为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）

然后在您的活动（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

您还遇到一个问题，即您的insertDrink方法因拼写错误而无法插入饮品。

即。创建表时的columng为IMAGE_RESOURCE_ID INTEGER，但在插入中，您调用列IMAGE_RESOURCE_D。我强烈建议对表名和列名采用单一来源（CONSTANTS），并尽可能使用。

我建议对您的StarbuzzDatabaseHelper类进行以下更改，然后应该补救/简化问题： -

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"
        );
    }
}

以下代码用于测试上述代码，也是如何使用该类的示例： -

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

因为我的数据库不太可能因插入而有任何数据我建议删除数据库。这可以通过清除/删除应用程序的数据或通过卸载应用程序来完成。

Answer 2

而不是

SQLiteOpenHelper starbuzzDatabaseHelper = new StarbuzzDatabaseHelper（this）;

试

StarbuzzDatabaseHelper starbuzzDatabaseHelper = new StarbuzzDatabaseHelper（this）;

编辑：将其添加到助手类

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

如何从自定义数据库帮助程序

2 个答案:

另一个问题

备注