java - Netbeans 的 Java 代码中的 while 循环

Question

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);

    }

}

一切工作正常，直到 while 循环......不知怎的，我无法让它为我工作。当我在末尾输入“否”或“否”时，循环将继续运行，直到我为字符串问题输入空格为止。它只是在最后给了我以下错误:

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)

Answer 1

尝试:

while(!s4.equals("stop")){ 

String 是一个对象，而不是原始类型，因此您必须使用 .equals 而不是 ==。