删除元素数组会导致NullPointerException
删除元素数组会导致NullPointerException
提问于 2019-05-28 06:49:29
我是爪哇的初学者。我得做购物车。但是我被remove方法卡住了。
请帮帮我!
我尝试创建了remove方法。但是它不能很好的工作..
Customer.java
public class Customer {
private String id;
private String firstName;
private String lastName;
private ShoppingCart shopCart;
public Customer(String id, String firstName, String lastName) {
this.id = id;
this.firstName = firstName;
this.lastName = lastName;
shopCart = new ShoppingCart();
}
public ShoppingCart getShopCart() {
return this.shopCart;
}
@Override
public String toString() {
return ("Customer ID is: " + this.id + "\n" + "Customer's name is: "
+ this.firstName + " " + this.lastName + "\n\n"
+ "Shopping list: \n\n" + shopCart.toString());
}ShoppingCart.java
public class ShoppingCart {
private int itemCount;
private double totalPrice;
private static int capacity;
private Item[] cart;
public ShoppingCart()
{
capacity = 5;
itemCount = 0;
totalPrice = 0.0;
cart = new Item[capacity];
}
public void addBooktoCart(String title, String description, double price, int pageCount) {
if (itemCount < 5) {
Item item = new Book(title, description, price, pageCount);
totalPrice += price;
cart[itemCount] = item;
itemCount += 1;
}
else {
System.out.println("The maximum number that you can input is 5." +
"You cannot add item anymore");
}
}
//This remove method may be a problem.
public void removeItemFromCart(String title) {
boolean found = false;
for (int i = 0; i < cart.length; i++) {
if (cart[i] != null)
{
if (cart[i].getTitle().equals(title)) {
totalPrice -= cart[i].getPrice();
cart[i] = null;
itemCount -= 1;
found = true;
}
}
}
if(found != true) {
System.out.println("Item is not found in cart. Nothing removed");
}
}
@Override
public String toString() {
String contents = "";
for (int i = 0; i < itemCount; i++)
{
contents += cart[i].toString() + "\n\n";
}
String strDouble = String.format("%.2f", totalPrice);
contents += "The total price is: $" + strDouble + "\n\n"
+ "How many items do you have in your shopping cart? \n"
+ "There are " + this.itemCount + " items in my shopping cart. "+ "\n";
return contents;
}
}Test.java
public class Test {
public static void main(String[] args) {
Customer c1 = new Customer("12345", "Hyun Min", "Lim");
c1.getShopCart().addBooktoCart("Harry Potter", "Fantasy Genre", 10.99, 309);
c1.getShopCart().addCDtoCart("Abbey Road", "Pop Genre", 6.50, 18);
c1.getShopCart().addMovietoCart("Forrest Gump", "Drama Genre", 13.23, 141);
c1.getShopCart().removeItemFromCart("Harry Potter");
System.out.println(c1.toString());
}我想删除元素数组。但是输出是
Exception in thread "main" java.lang.NullPointerException。
如果编译器运行得很好，我该怎么办？我认为remove方法是一个问题。
回答 1
Stack Overflow用户
发布于 2019-05-28 07:09:25
NullPointerException可能是由if (cart[i].getTitle().equals(title))这一行引起的，而不是访问cart对象本身，而是在getTitle的返回值上调用equals方法。在比较之前，请检查以确保返回值不为null：
if (cart[i] != null)
{
/* Assign the value to a local variable and then
* check if it's null before accessing it.
*/
String value = cart[i].getTitle();
if (value != null && value.equals(title)) {
totalPrice -= cart[i].getPrice();
cart[i] = null;
itemCount -= 1;
found = true;
}
}在任何情况下，您都应该使用某种调试技术来确定到底是什么导致了您的错误。了解更多关于使用Java here和IntelliJ here进行调试的信息。
页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：
https://stackoverflow.com/questions/56333183
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