如何从自定义数据库帮助程序创建游标

I'm going through Griffith's Head First: Android Development book and a certain code is throwing a Cannot resolve Symbol error.

我正在浏览Griffith的Head First:Android开发书和某些代码正在抛出一个无法解决Symbol错误。

I am building a cursor from the custom database helper as per the instruction of the book. This is the code that is throwing the error.

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();

It says Cannot resolve symbol "StarbuzzDatabaseHelper" even though StarbuzzDatabaseHelper does exist in my project. This is code from the book. Not mine. I'm assuming that its an error from the book. I tried this:

它说无法解析符号“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); and it still didn't work.

SQLiteOpenHelper starbuzzDatabaseHelper = new SQLiteOpenHelper(this);它仍然无法正常工作。

This is my full code:

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();
        }
    }
}

and this is the database helper class:

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);
    }
}

2 个解决方案

#1

You have defined the constructor for the starbuzzDatabaseHelper class to require 4 parameters as per :-

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);
}

The attempt to instantiate an instance you are using :-

尝试实例化您正在使用的实例: -

SQLiteOpenHelper starbuzzDatabaseHelper = new StarbuzzDatabaseHelper(this);

Whilst you could/should be using :-

虽然你可以/应该使用: -

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

• i.e. passing 4 parameters (see below)

即通过4个参数(见下文)

• Instantiating (creating an instance) your Database Helper rather than an instance limited by being an SQLiteOpenHelper instance.

实例化(创建实例)您的Database Helper而不是作为SQLiteOpenHelper实例限制的实例。

• providing a method that can be resolved.

提供一种可以解决的方法。

I would suggest that you alter the constructor to be :-

我建议你改变构造函数: -

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

Thus requiring the 1 parameter (the context)

因此需要1个参数(上下文)

and then use the following in your activity (activities) :-

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

Another issue

You also have an issue in that the your insertDrink method will not insert drinks because of a typo.

您还有一个问题,即insertDrink方法因插入错误而不会插入饮料。

i.e. the columng when you create the table is IMAGE_RESOURCE_ID INTEGER, yet in the insert you call the column IMAGE_RESOURCE_D. I'd strongly suggest adopting the use of a single source (CONSTANTS) for table and column names and always using those wherever possible.

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

I'd suggest the following changes to your StarbuzzDatabaseHelper class this should then remedy/simplify matters :-

我建议您对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"
        );
    }
}

The following code has been used to test the above and is also an example of how to utilise the class :-

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 >>>>
        }
    }
}

Notes

• as it is unlikely that your database has any data due to the insert I'd suggest delete the database. This can be done by clearing/delete the App's data or by Uninstalling the App.

由于插件我不建议删除数据库,因此您的数据库不太可能有任何数据。这可以通过清除/删除应用程序的数据或卸载应用程序来完成。

#2

Instead of

SQLiteOpenHelper starbuzzDatabaseHelper = new StarbuzzDatabaseHelper(this);

SQLiteOpenHelper starbuzzDatabaseHelper = new StarbuzzDatabaseHelper(this);

try

StarbuzzDatabaseHelper starbuzzDatabaseHelper = new StarbuzzDatabaseHelper(this);

StarbuzzDatabaseHelper starbuzzDatabaseHelper = new StarbuzzDatabaseHelper(this);

edit: Add that to your helper class

编辑:将其添加到助手类

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