在本练习中，您将完成一个将购物车实现为项目数组的类。文件项。java包含名为Item的类的定义，该类对人们将购买的商品进行建模。项目有名称、价格和数量（购买的数量）。文件购物车。java将购物车实现为一个项目对象数组。

>

编写一个模拟购物的程序。该程序应该有一个循环，只要用户想购物就可以继续。每次通过循环读取用户想要添加到购物车的商品的名称、价格和数量。将项目添加到购物车后，应打印购物车内容。循环后打印“请付款…”包含购物车中商品总价的消息。

package Shopping;

import java.text.NumberFormat;

public class Item
{
    private String name;
    private double price;
    private int quantity;
    // ----------------------------------------------------- --
    // Create a new item with the given attributes.
    // ----------------------------------------------------- --
    public Item (String itemName, double itemPrice, int numPurchased)
    {
        name = itemName;
        price = itemPrice;
        quantity = numPurchased;
    }
    // ----------------------------------------------------- --
    // Return a string with the information about the item
    // ----------------------------------------------------- --
    public String toString ()
    {
        NumberFormat fmt = NumberFormat.getCurrencyInstance();
        return (name + "\t" + fmt.format(price) + "\t" + quantity + "\t"
                + fmt.format(price*quantity));
    }
    // -----------------------------------------------
    // Returns the unit price of the item
    // -----------------------------------------------

    public double getPrice()
    {
        return price;
    }
    // -----------------------------------------------
    // Returns the name of the item
    // -----------------------------------------------
    public String getName()
    {
        return name;
    }
    // -----------------------------------------------
    // Returns the quantity of the item
    // -----------------------------------------------
    public int getQuantity()
    {
        return quantity;
    }
}



package Shopping;

import Shopping.Item;

import java.text.NumberFormat;

public class ShoppingCart
{
    private int itemCount; // total number of items in the cart
    private double totalPrice; // total price of items in the cart
    private int capacity; // current cart capacity
    Item[] cart; // declare an instance variable cart for an array of Item


    // ---------------------------------------------------------
    // Creates an empty shopping cart with a capacity of 5 items.
    // ---------------------------------------------------------

    public ShoppingCart()
    {
        capacity = 5;
        itemCount = 0;
        totalPrice = 0.0;
        cart = new Item[capacity];

    }

    // -----------------------------------------------------
    // Adds an item to the shopping cart.
    // -----------------------------------------------------
    public void addToCart(String itemName, double price, int quantity)
    {
        if (itemCount > 5)
        {
            System.out.println("Now the shopping cart is full.");
        }
        else
        {
            addToCart(itemName, price, quantity);
            totalPrice = totalPrice + (price *  quantity);
        }
        itemCount = itemCount+1;
    }


    // -----------------------------------------------------
    // Returns the contents of the cart together with
    // summary information.
    // -----------------------------------------------------

    public String toString()
    {
        NumberFormat fmt = NumberFormat.getCurrencyInstance();
        String contents = "\nShopping Cart\n";
        contents += "\nItem\t\tUnit Price\tQuantity\tTotal\n";
        for (int i = 0; i < itemCount; i++)
            contents += cart[i].toString() + "\n";
        contents += "\nTotal Price: " + fmt.format(totalPrice);
        contents += "\n";
        return contents;
    }
    // -----------------------------------------------------
    // Increases the capacity of the shopping cart by 3
    // -----------------------------------------------------
    private void increaseSize()
    {
        capacity = capacity + 3;
    }
}

线程“main”java中出现异常。郎在购物。购物车。addToCart（ShoppingCart.java:39）是我一直收到的错误