```
import java.util.ArrayList;
import javax.swing.JOptionPane;

public class MainApp {

    public static void main(String[] args) {

        ArrayList<Product> cart = new ArrayList<Product>();

        String s1 = JOptionPane
                .showInputDialog("Please enter the quantity of the product you have selected");

        int r = Integer.parseInt(s1);

        String s2 = JOptionPane
                .showInputDialog("Please enter the name of the product");

        String s3 = JOptionPane
                .showInputDialog("Please enter the price of the product");
        double d1 = Double.parseDouble(s3);

        for (int i = 0; i < r; i++) {

            Product p1 = new Product();
            p1.setName(s2);
            p1.setPrice(d1);

            cart.add(p1);

        }

        double total_price = 0;
        for (int i = 0; i < cart.size(); i++) {
            total_price = total_price + cart.get(i).getPrice();
        }

        JOptionPane.showMessageDialog(null, cart.toString()
                + "The total price of all the products in the cart is : "
                + total_price + " KD", "Shopping Information",
                JOptionPane.INFORMATION_MESSAGE);

        cart.removeAll(cart);

        JOptionPane.showMessageDialog(null, "Thank you for shopping with us! ",
                "Prompt", JOptionPane.INFORMATION_MESSAGE);

        String s4 = JOptionPane
                .showInputDialog("Would you like to add items to your cart");

        while (s4 != "stop") {

            String s5 = JOptionPane
                    .showInputDialog("Please enter the name of the product");

            String s6 = JOptionPane
                    .showInputDialog("Please enter the price of the product");
            double d2 = Double.parseDouble(s6);

            Product p2 = new Product();

            p2.setName(s5);
            p2.setPrice(d2);

            cart.add(p2);

            String s7 = JOptionPane
                    .showInputDialog("Would you like to add more items?");
            if (s7 == "No" || s7 == "no")
                s4 = "stop";

        }

        JOptionPane.showMessageDialog(null, cart.toString(), "Cart Details",
                JOptionPane.INFORMATION_MESSAGE);

    }

}

```

Everything works fine up until the while loop... somehow I cant get it to work for me. When I enter 'No' or 'no' at the end, the loop keeps running and doesn't stop until I enter space for string questions. It just gives me the following error at the end:

```
Exception in thread "main" java.lang.NullPointerException
    at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1008)
    at java.lang.Double.parseDouble(Double.java:540)
    at mainapp.MainApp.main(MainApp.java:66)
Java Result: 1
BUILD SUCCESSFUL (total time: 41 seconds)

```