我使用 volley 庫并從服務器加載 json 并將其顯示在 recyclerview 中。我想創建一個購物車并計算 recyclerview 項目的總價。我的問題是沒有計算物品的總成本。請幫幫我這是我的代碼：public class checkout extends AppCompatActivity {    String shop = SHOP;    List<Product6> productList6;    ProductsAdapter6 productsAdapter6;    RecyclerView recyclerView;    private ProductsAdapter6 adapter6;    private GridLayoutManager layoutManager;    RecyclerView.LayoutManager RecyclerViewLayoutManager;    private TextView subTotal;    private double mSubTotal = 0;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_checkout);        recyclerView = (RecyclerView) findViewById(R.id.recyclerview);        recyclerView.setHasFixedSize(true);        RecyclerViewLayoutManager = new LinearLayoutManager(getApplicationContext());        recyclerView.setLayoutManager(RecyclerViewLayoutManager);        adschild2();        productList6 = new ArrayList<>();        subTotal = (TextView) findViewById(R.id.sub_total);        mSubTotal = grandTotal(productList6);        subTotal.setText(String.valueOf(mSubTotal));    }    private int grandTotal(List<Product6> items) {        int totalPrice = 0;        for (int i = 0; i < items.size(); i++) {            totalPrice += items.get(i).getBack();        }        return totalPrice;    }    private void adschild2() {        StringRequest stringRequest = new StringRequest(Request.Method.GET, shop, new Response.Listener<String>()