netbeans 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);

    }

}

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)

Answer 1

Try:

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

String is an object, not a primitive type so you have to use .equals instead of ==.

Answer 2

Use equals method. Strings shouldn't be compared with == or != operator.

Answer 3

while (!"stop".equalsIgnoreCase(s4))

Try this.

Please refer this SO thread for more understanding : Java String.equals versus ==

Answer 4

I belive the following line produces the exception error:

double d2=Double.parseDouble(s6); 

To clarify this, tell us what line is the line 66 on you main method

this happens because the value of s6 is null . try to make sure this is sorted first.

Also as everyone else said, use equals method to check String equality. <--(this won't produce Exception error but it makes your program go wrong semantically).

Answer 5

Why not use String equals method instead of ==?.For == to work, you would need to call intern() method on s4 which would change the reference to point to String pool.

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

Answer 6

Thank you so much everyone for telling me about the .equals method. I had forgotten about that. I submitted my work today after thinking it over and had everything working before I did so...

package mainapp;

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);



int count=0;
while(count>=0){

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);
count++;

String s8=JOptionPane.showInputDialog("Would you like to add more items to your cart?");
if(s8.contains("Yes")|| s8.contains("yes")){

}

else{    
 String s7=JOptionPane.showInputDialog("Please enter 'Stop' if you are done adding items to your cart");
 if (s7.contains("stop")|| s7.contains("Stop")){
     count=-1;
 } 

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

    }
}

Answer 7

I had tried using the .equals() method, it didn't work for me. I think I wasn't writing the loop using the correct format; with the 'count' and all. The 'for' was becoming a disaster too. So I wrote it differently in a way that made sense to me and it ended up working... thanks again to all those who responded!