java - 编译但未计算总计

Question

我得到的不是计算输出，而是“null”，而没有我正在寻找的实际总打印输出。我认为这是因为 Item 没有正确定义。忽略shop.txt，它对我的​​问题没有影响

package Shopping;
import java.util.ArrayList;
import java.util.Scanner;
import java.io.File;
import java.io.FileNotFoundException;
import java.text.NumberFormat;
public class Shop
{
    public static void main(String[] args)
    {
        //I/O stream

        String fileName = "shop.txt";
        Scanner inputStream = null;
        System.out.println("The file " + fileName +
                "\ncontains the following lines:\n");
        try
        {
            inputStream = new Scanner(new File(fileName));
        }
        catch (FileNotFoundException e)
        {
            System.out.println("This is a shopping list " +
                    fileName);
            System.exit(0);
        }
        while (inputStream.hasNextLine())
        {
            String line = inputStream.nextLine();
            System.out.println(line);
        }
        inputStream.close();


        ArrayList<Item> Cart = new ArrayList<Item>();

        Item item;
        String itemName;
        double itemPrice;
        int quantity;
        double totalPrice = 0.0;
        double sum = 0.0;
        int priority;

        Scanner scan = new Scanner(System.in);
        String continueShopping = "y";
        do
        {
            System.out.print("Enter the name of the item: ");
            itemName = scan.nextLine();
            System.out.print("Enter the unit price: ");
            itemPrice = scan.nextDouble();
            System.out.print("Enter the quantity: ");
            quantity = scan.nextInt();


            //  create a new item and add it to the cart

            item = new Item(itemName, itemPrice, quantity);
            Cart.add(item);

            for (int i = 0; i < Cart.size(); i++)
            {
                Item itm = Cart.get(i);
                System.out.println(itm);
            }
            // Print out the results

            System.out.print("Continue shopping (y/n)? ");
            scan.nextLine();
            continueShopping = scan.nextLine();
        }
        while (continueShopping.equals("y"));
        for (int i = 0; i < Cart.size(); i++)
        {
            Item itm = Cart.get(i);
            System.out.println(itm);
            totalPrice = itm.getQuantity() * itm.getPrice();
            sum += totalPrice;
        }
        NumberFormat type = NumberFormat.getCurrencyInstance();
        System.out.println("The total price is: " + type.format(sum));
    }
}

元素类别

package Shopping;
import java.text.NumberFormat;

public class Item
{

    private String name;
    private double price;
    private int quantity;


    public Item(String itemName, double itemPrice, int quantity2)
    {
    }

    public void Item(String itemName, double itemPrice, int numPurchased)
    {
        name = itemName;
        price = itemPrice;
        quantity = numPurchased;
    }

    //Info about the item

    public String toString()
    {
        NumberFormat type = NumberFormat.getCurrencyInstance();

        return (name + "\t" + type.format(price) + "\t" + quantity + "\t"
                + type.format(price * quantity));
    }

    //Retrieve the item price

    public double getPrice()
    {
        return price;
    }

    //Retrieve item name

    public String getName()
    {
        return name;
    }

    //Retrieve quantity

    public int getQuantity()
    {
        return quantity;
    }
}

Answer 1

您的 Item 类有一个空构造函数

public Item(String itemName, double itemPrice, int quantity2) {
}

和一个名为 Item

的方法

public void Item (String itemName, double itemPrice, int numPurchased)
{
    name = itemName;
    price = itemPrice;
    quantity = numPurchased;
}

删除空构造函数并从方法中删除返回类型 void 以将其转换为构造函数。