我正在为一个类做一个 Java 应用程序，需要帮助解决运行应用程序后出现的错误。
该应用程序应该允许用户将商品添加到购物车（商品名称、价格、数量）。用户描述的商品会被添加到购物车数组中，添加成功后会打印出来。
购物车最初的最大容量为 5 件商品，但如果购物车大小 > 5，则会通过 increaseSize 方法增加 + 3。
如下所述，我相信 addToCart 方法不会将用户描述的项目添加到购物车中。这就是为什么它在 toString 方法中是空值的原因。
所以，我基本上需要 addToCart 方法的帮助。如果我错了，请纠正我。谢谢你。
运行应用程序的输出如下：
run:
Enter the name of the item: a
Enter the unit price: 2
Enter the quantity: 2
Exception in thread "main" java.lang.NullPointerException
at shop.ShoppingCart.toString(ShoppingCart.java:62)
at java.lang.String.valueOf(String.java:2854)
at java.io.PrintStream.println(PrintStream.java:821)
at shop.Shop.main(Shop.java:49)
Java Result: 1
下面是 ShoppingCart 类。
toString 方法不应该产生任何错误，但我认为问题出在 addToCart 方法中。
// **********************************************************************
// ShoppingCart.java
//
// Represents a shopping cart as an array of items
// **********************************************************************
import java.text.NumberFormat;
public class ShoppingCart
{
private Item[] cart;
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
// -----------------------------------------------------------
// Creates an empty shopping cart with a capacity of 5 items.
// -----------------------------------------------------------
public ShoppingCart()
{
capacity = 5;
cart = new Item[capacity];
itemCount = 0;
totalPrice = 0.0;
}
// -------------------------------------------------------
// Adds an item to the shopping cart.
// -------------------------------------------------------
public void addToCart(String itemName, double price, int quantity)
{
Item temp = new Item(itemName, price, quantity);
totalPrice += (price * quantity);
itemCount += quantity;
cart[itemCount] = temp;
if(itemCount==capacity)
{
increaseSize();
}
}
// -------------------------------------------------------
// 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()
{
Item[] temp = new Item[capacity+3];
for(int i=0; i < capacity; i++)
{
temp[i] = cart[i];
}
cart = temp;
temp = null;
capacity = cart.length;
}
}
下面是店主。
ArrayList 目前未使用，但稍后将在更正 toString 错误后使用。
package shop;
// ***************************************************************
// Shop.java
//
// Uses the Item class to create items and add them to a shopping
// cart stored in an ArrayList.
// ***************************************************************
import java.util.ArrayList;
import java.util.Scanner;
public class Shop
{
public static void main (String[] args)
{
ArrayList<Item> cart = new ArrayList<Item>();
Item item;
String itemName;
double itemPrice;
int quantity;
Scanner scan = new Scanner(System.in);
String keepShopping = "y";
ShoppingCart cart1 = new ShoppingCart();
do
{
System.out.print ("Enter the name of the item: ");
itemName = scan.next();
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
cart1.addToCart(itemName, itemPrice, quantity);
// *** print the contents of the cart object using println
System.out.println(cart1);
System.out.print ("Continue shopping (y/n)? ");
keepShopping = scan.next();
}
while (keepShopping.equals("y"));
}
}
以下是物品类别：
package shop;
// ***************************************************************
// Item.java
//
// Represents an item in a shopping cart.
// ***************************************************************
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;
}
}