Java 从数据库填充listview，不需要'；我什么也没表现出来

我看了这个教程： 但当我看到活动时，似乎没有任何项目列表视图：\

有什么问题吗？我把代码放在下面

主要活动。java

public class Prova2 extends Activity {
int[] imageIDs = {
        R.drawable.spaghettiscoglio,
        R.drawable.abbacchioascottadito,
        R.drawable.agnellocacioeova,
        R.drawable.agnolotti,
        R.drawable.alettedipollo,
        R.drawable.amaretti,
        R.drawable.anatraarancia,
        R.drawable.alberinatale
};
int nextImageIndex = 0;

DBAdapter myDb;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.prova2);

    openDB();
    populateListViewFromDB();
    registerListClickCallback();
}
@Override
protected void onDestroy() {
    super.onDestroy();
    closeDB();
}

private void openDB() {
    myDb = new DBAdapter(this);
    myDb.open();
}
private void closeDB() {
    myDb.close();
}

/*
 * UI Button Callbacks
 */
public void onClick_AddRecord(View v) {
    int imageId = imageIDs[nextImageIndex];
    nextImageIndex = (nextImageIndex + 1) % imageIDs.length;

    // Add it to the DB and re-draw the ListView
    myDb.insertRow("Jenny" + nextImageIndex, imageId, "Green");
    populateListViewFromDB();
}

public void onClick_ClearAll(View v) {
    myDb.deleteAll();
    populateListViewFromDB();
}


private void populateListViewFromDB() {
    Cursor cursor = myDb.getAllRows();

    // Allow activity to manage lifetime of the cursor.
    // DEPRECATED! Runs on the UI thread, OK for small/short queries.
    startManagingCursor(cursor);

    // Setup mapping from cursor to view fields:
    String[] fromFieldNames = new String[]
            {DBAdapter.KEY_NAME, DBAdapter.KEY_STUDENTNUM, DBAdapter.KEY_FAVCOLOUR, DBAdapter.KEY_STUDENTNUM};
    int[] toViewIDs = new int[]
            {R.id.tvFruitPrice,     R.id.ivFruit,           R.id.tvFruitPrice,     R.id.smalltext};

    // Create adapter to may columns of the DB onto elemesnt in the UI.
    SimpleCursorAdapter myCursorAdapter =
            new SimpleCursorAdapter(
                    this,       // Context
                    R.layout.sis,   // Row layout template
                    cursor,                 // cursor (set of DB records to map)
                    fromFieldNames,         // DB Column names
                    toViewIDs               // View IDs to put information in
            );

    // Set the adapter for the list view
    ListView myList = (ListView) findViewById(R.id.listViewFromDB);
    myList.setAdapter(myCursorAdapter);
}

private void registerListClickCallback() {
    ListView myList = (ListView) findViewById(R.id.listViewFromDB);
    myList.setOnItemClickListener(new AdapterView.OnItemClickListener() {
        @Override
        public void onItemClick(AdapterView<?> parent, View viewClicked,
                                int position, long idInDB) {

            updateItemForId(idInDB);
            displayToastForId(idInDB);
        }
    });
}

private void updateItemForId(long idInDB) {
    Cursor cursor = myDb.getRow(idInDB);
    if (cursor.moveToFirst()) {
        long idDB = cursor.getLong(DBAdapter.COL_ROWID);
        String name = cursor.getString(DBAdapter.COL_NAME);
        int studentNum = cursor.getInt(DBAdapter.COL_STUDENTNUM);
        String favColour = cursor.getString(DBAdapter.COL_FAVCOLOUR);

        favColour += "!";
        myDb.updateRow(idInDB, name, studentNum, favColour);
    }
    cursor.close();
    populateListViewFromDB();
}

private void displayToastForId(long idInDB) {
    Cursor cursor = myDb.getRow(idInDB);
    if (cursor.moveToFirst()) {
        long idDB = cursor.getLong(DBAdapter.COL_ROWID);
        String name = cursor.getString(DBAdapter.COL_NAME);
        int studentNum = cursor.getInt(DBAdapter.COL_STUDENTNUM);
        String favColour = cursor.getString(DBAdapter.COL_FAVCOLOUR);

        String message = "ID: " + idDB + "\n"
                + "Name: " + name + "\n"
                + "Std#: " + studentNum + "\n"
                + "FavColour: " + favColour;
        Toast.makeText(Prova2.this, message, Toast.LENGTH_LONG).show();
    }
    cursor.close();
}

public class DBAdapter {

/////////////////////////////////////////////////////////////////////
//  Constants & Data
/////////////////////////////////////////////////////////////////////
// For logging:
private static final String TAG = "DBAdapter";

// DB Fields
public static final String KEY_ROWID = "_id";
public static final int COL_ROWID = 0;
/*
 * CHANGE 1:
 */
// TODO: Setup your fields here:
public static final String KEY_NAME = "name";
public static final String KEY_STUDENTNUM = "studentnum";
public static final String KEY_FAVCOLOUR = "favcolour";

// TODO: Setup your field numbers here (0 = KEY_ROWID, 1=...)
public static final int COL_NAME = 1;
public static final int COL_STUDENTNUM = 2;
public static final int COL_FAVCOLOUR = 3;


public static final String[] ALL_KEYS = new String[] {KEY_ROWID, KEY_NAME, KEY_STUDENTNUM, KEY_FAVCOLOUR};

// DB info: it's name, and the table we are using (just one).
public static final String DATABASE_NAME = "MyDb";
public static final String DATABASE_TABLE = "mainTable";
// Track DB version if a new version of your app changes the format.
public static final int DATABASE_VERSION = 2;

private static final String DATABASE_CREATE_SQL =
        "create table " + DATABASE_TABLE
                + " (" + KEY_ROWID + " integer primary key autoincrement, "

        /*
         * CHANGE 2:
         */
                // TODO: Place your fields here!
                // + KEY_{...} + " {type} not null"
                //  - Key is the column name you created above.
                //  - {type} is one of: text, integer, real, blob
                //      (http://www.sqlite.org/datatype3.html)
                //  - "not null" means it is a required field (must be given a value).
                // NOTE: All must be comma separated (end of line!) Last one must have NO comma!!
                + KEY_NAME + " text not null, "
                + KEY_STUDENTNUM + " integer not null, "
                + KEY_FAVCOLOUR + " string not null"

                // Rest  of creation:
                + ");";

// Context of application who uses us.
private final Context context;

private DatabaseHelper myDBHelper;
private SQLiteDatabase db;

/////////////////////////////////////////////////////////////////////
//  Public methods:
/////////////////////////////////////////////////////////////////////

public DBAdapter(Context ctx) {
    this.context = ctx;
    myDBHelper = new DatabaseHelper(context);
}

// Open the database connection.
public DBAdapter open() {
    db = myDBHelper.getWritableDatabase();
    return this;
}

// Close the database connection.
public void close() {
    myDBHelper.close();
}

// Add a new set of values to the database.
public long insertRow(String name, int studentNum, String favColour) {
    /*
     * CHANGE 3:
     */
    // TODO: Update data in the row with new fields.
    // TODO: Also change the function's arguments to be what you need!
    // Create row's data:
    ContentValues initialValues = new ContentValues();
    initialValues.put(KEY_NAME, name);
    initialValues.put(KEY_STUDENTNUM, studentNum);
    initialValues.put(KEY_FAVCOLOUR, favColour);

    // Insert it into the database.
    return db.insert(DATABASE_TABLE, null, initialValues);
}

// Delete a row from the database, by rowId (primary key)
public boolean deleteRow(long rowId) {
    String where = KEY_ROWID + "=" + rowId;
    return db.delete(DATABASE_TABLE, where, null) != 0;
}

public void deleteAll() {
    Cursor c = getAllRows();
    long rowId = c.getColumnIndexOrThrow(KEY_ROWID);
    if (c.moveToFirst()) {
        do {
            deleteRow(c.getLong((int) rowId));
        } while (c.moveToNext());
    }
    c.close();
}

// Return all data in the database.
public Cursor getAllRows() {
    String where = null;
    Cursor c =  db.query(true, DATABASE_TABLE, ALL_KEYS,
            where, null, null, null, null, null);
    if (c != null) {
        c.moveToFirst();
    }
    return c;
}

// Get a specific row (by rowId)
public Cursor getRow(long rowId) {
    String where = KEY_ROWID + "=" + rowId;
    Cursor c =  db.query(true, DATABASE_TABLE, ALL_KEYS,
            where, null, null, null, null, null);
    if (c != null) {
        c.moveToFirst();
    }
    return c;
}

// Change an existing row to be equal to new data.
public boolean updateRow(long rowId, String name, int studentNum, String favColour) {
    String where = KEY_ROWID + "=" + rowId;

    /*
     * CHANGE 4:
     */
    // TODO: Update data in the row with new fields.
    // TODO: Also change the function's arguments to be what you need!
    // Create row's data:
    ContentValues newValues = new ContentValues();
    newValues.put(KEY_NAME, name);
    newValues.put(KEY_STUDENTNUM, studentNum);
    newValues.put(KEY_FAVCOLOUR, favColour);

    // Insert it into the database.
    return db.update(DATABASE_TABLE, newValues, where, null) != 0;
}



/////////////////////////////////////////////////////////////////////
//  Private Helper Classes:
/////////////////////////////////////////////////////////////////////

/**
 * Private class which handles database creation and upgrading.
 * Used to handle low-level database access.
 */
private static class DatabaseHelper extends SQLiteOpenHelper
{
    DatabaseHelper(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase _db) {
        _db.execSQL(DATABASE_CREATE_SQL);
    }

    @Override
    public void onUpgrade(SQLiteDatabase _db, int oldVersion, int newVersion) {
        Log.w(TAG, "Upgrading application's database from version " + oldVersion
                + " to " + newVersion + ", which will destroy all old data!");

        // Destroy old database:
        _db.execSQL("DROP TABLE IF EXISTS " + DATABASE_TABLE);

        // Recreate new database:
        onCreate(_db);
    }
}

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数据库中的值是否正确更新？是否尝试增加数据库_版本并将一些示例数据放入DatabaseHelper中的数据库中。是否通过insertRow（…）在数据库中正确更新了值？是否尝试增加数据库_版本并将一些示例数据放入DatabaseHelper.onCreate中的数据库中（）通过insertRow（…）