Guide on Database in Android Studio

Spinner categorySpinner, itemSpinner;
TextView priceTextView;
Button addButton, checkoutButton;
int totalCost = 0;

DatabaseHelper dbHelper;
ArrayList categoryList = new ArrayList<>();
HashMap categoryMap = new HashMap<>();

ArrayList itemList = new ArrayList<>();
HashMap itemPriceMap = new HashMap<>();

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

    dbHelper = new DatabaseHelper(this);

    categorySpinner = findViewById(R.id.categorySpinner);
    itemSpinner = findViewById(R.id.itemSpinner);
    priceTextView = findViewById(R.id.priceTextView);
    addButton = findViewById(R.id.addButton);
    checkoutButton = findViewById(R.id.checkoutButton);

    loadCategories();

    categorySpinner.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
        @Override public void onItemSelected(AdapterView> parent, View view, int position, long id) {
            if (position > 0) {
                String category = categoryList.get(position);
                loadItems(categoryMap.get(category));
            }
        }
        @Override public void onNothingSelected(AdapterView> parent) {}
    });

    itemSpinner.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
        @Override public void onItemSelected(AdapterView> parent, View view, int position, long id) {
            if (position >= 0 && position < itemList.size()) {
                String item = itemList.get(position);
                int price = itemPriceMap.get(item);
                priceTextView.setText("Price: $" + price);
            }
        }
        @Override public void onNothingSelected(AdapterView> parent) {}
    });

    addButton.setOnClickListener(v -> {
        String item = (String) itemSpinner.getSelectedItem();
        if (item != null) {
            totalCost += itemPriceMap.get(item);
            Toast.makeText(MainActivity.this, "Item added! Total: $" + totalCost, Toast.LENGTH_SHORT).show();
        }
    });

    checkoutButton.setOnClickListener(v -> {
        Intent intent = new Intent(MainActivity.this, SummaryActivity.class);
        intent.putExtra("total", totalCost);
        startActivity(intent);
    });
}

private void loadCategories() {
    categoryList.clear();
    categoryList.add("Select Category");

    Cursor cursor = dbHelper.getCategories();
    while (cursor.moveToNext()) {
        int id = cursor.getInt(0);
        String name = cursor.getString(1);
        categoryList.add(name);
        categoryMap.put(name, id);
    }
    cursor.close();

    ArrayAdapter adapter = new ArrayAdapter<>(this, android.R.layout.simple_spinner_item, categoryList);
    categorySpinner.setAdapter(adapter);
}

private void loadItems(int categoryId) {
    itemList.clear();
    itemPriceMap.clear();

    Cursor cursor = dbHelper.getItemsByCategoryId(categoryId);
    while (cursor.moveToNext()) {
        String name = cursor.getString(2);
        int price = cursor.getInt(3);
        itemList.add(name);
        itemPriceMap.put(name, price);
    }
    cursor.close();

    ArrayAdapter adapter = new ArrayAdapter<>(this, android.R.layout.simple_spinner_item, itemList);
    itemSpinner.setAdapter(adapter);
}

private static class DatabaseHelper extends SQLiteOpenHelper {

    public static final String DB_NAME = "restaurant.db";
    public static final int DB_VERSION = 1;

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

    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL("CREATE TABLE IF NOT EXISTS categories (id INTEGER PRIMARY KEY, name TEXT)");
        db.execSQL("CREATE TABLE IF NOT EXISTS items (id INTEGER PRIMARY KEY, category_id INTEGER, name TEXT, price INTEGER)");

        db.execSQL("INSERT INTO categories (name) VALUES ('Drinks'), ('Main Course'), ('Dessert')");
        db.execSQL("INSERT INTO items (category_id, name, price) VALUES " +
                "(1, 'Coke', 2), (1, 'Water', 1), (1, 'Juice', 3), " +
                "(2, 'Burger', 5), (2, 'Pizza', 8), (2, 'Pasta', 7), " +
                "(3, 'Ice Cream', 4), (3, 'Cake', 5), (3, 'Pudding', 3)");
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        db.execSQL("DROP TABLE IF EXISTS items");
        db.execSQL("DROP TABLE IF EXISTS categories");
        onCreate(db);
    }

    public Cursor getCategories() {
        return getReadableDatabase().rawQuery("SELECT * FROM categories", null);
    }

    public Cursor getItemsByCategoryId(int categoryId) {
        return getReadableDatabase().rawQuery("SELECT * FROM items WHERE category_id = ?", new String[]{String.valueOf(categoryId)});
    }
}

int total = getIntent().getIntExtra("total", 0);
    TextView summaryText = findViewById(R.id.summaryText);
    summaryText.setText("Total cost: $" + total);
}