这个问题很复杂，所以我会尽力把所有事情都说清楚，如果我需要详细说明或从这篇文章中删除任何不相关的内容，请告诉我。我的程序用于读取 XML 文件并打印包含 2 个产品的数组，这些产品是从 XML 文件中恢复的数据，它 3 次在数组第一次打印后将第三个元素添加到数组中，并在数组打印后将其删除第二次。程序在添加或删除第三个元素时告诉用户这意味着所需的输出是
Product list:
code of the first product name of the first product price of the first product
code of the second product name of the second product price of the second product
XML Tester has been added to the XML document
Product list:
code of the first product name of the first product price of the first product
code of the second product name of the second product price of the second product
code of the XML tester name of the XMl tester price of the XML tester
XML tester has been deleted from the XML document.
Product list:
code of the first product name of the first product price of the first product
code of the second product name of the second product price of the second product
但是，在第一次尝试使用新添加的测试仪第二次打印产品时出现空指针异常错误
这发生在我的代码的第67行如下为了让错误发生的位置更明显并更清楚
p.setDescription(description);
我的调试器说描述此时设置的项目是“Murach 的开始 Java”，我不确定为什么这是一个空指针，因为这个确切的描述在我的程序第一次打印到控制台时起作用
这是我的程序的样子 附有我在调试程序时注意到的事情的注释，以便更容易帮助解决这个问题
import java.util.ArrayList;
import java.io.*;
import javax.xml.stream.*; // StAX API
public class XMLTesterApp
{
private static String productsFilename = "products.xml";
public static void main(String[] args)
{
**This first part the XML file contains two values and it prints both fine*****
System.out.println("Products list:");
ArrayList<Product> products = readProducts();
printProducts(products);
*** This part adds a tester line to the print out and this is the point the
*** null pointer exception error starts to happen This is used as a test to
*** make sure my file writer method is working by an instructor and the program
*** is supposed to fail here if I am doing something wrong but I can't figure
*** out what im doing wrong ************************
Product p1 = new Product("test", "XML Tester", 77.77);
products.add(p1);
writeProducts(products);
System.out.println("XML Tester has been added to the XML document.\n");
System.out.println("Products list:");
products = readProducts();
printProducts(products);
products.remove(2);
writeProducts(products);
System.out.println("XML Tester has been deleted from the XML document.\n");
System.out.println("Products list:");
products = readProducts();
printProducts(products);
}
private static ArrayList<Product> readProducts()
{
ArrayList<Product> products = new ArrayList<>();
Product p = null;
XMLInputFactory inputFactory = XMLInputFactory.newFactory();
try
{
//create a stream reader object
FileReader fileReader = new FileReader("products.xml");
XMLStreamReader reader = inputFactory.createXMLStreamReader(fileReader);
//read XML file
while (reader.hasNext())
{
int eventType = reader.getEventType();
switch (eventType)
{
case XMLStreamConstants.START_ELEMENT :
String elementName = reader.getLocalName();
//get the product and its code
if (elementName.equals("Product"))
{
p = new Product();
String code = reader.getAttributeValue(0);
p.setCode(code);
}
// get the product description
if (elementName.equals("Description"))
{
String description = reader.getElementText();
************Error occurs on the line under this note*************
p.setDescription(description);
}
// get the product price
if (elementName.equals("Price"))
{
String priceS = reader.getElementText();
double price = Double.parseDouble(priceS);
**** As I test I removed the set descirption line and then
this line returned the same error that that line
did*********************************************
p.setPrice(price);
}
break;
case XMLStreamConstants.END_ELEMENT :
elementName = reader.getLocalName();
if(elementName.equals("Product"))
{
products.add(p);
}
break;
}
reader.next();
}
}
catch (IOException | XMLStreamException e)
{
System.out.println(e);
}
return products;
}
private static void writeProducts(ArrayList<Product> products)
{
XMLOutputFactory outputFactory = XMLOutputFactory.newFactory();
try
{
//create a stream reader object
FileWriter fileWriter = new FileWriter("products.xml");
XMLStreamWriter writer = outputFactory.createXMLStreamWriter(fileWriter);
//write to the XML file
writer.writeStartDocument("1.0");
writer.writeStartElement("Products");
for (Product p : products)
{
writer.writeStartElement("Prodcut");
writer.writeAttribute("Code", p.getCode()) ;
writer.writeStartElement("Description");
writer.writeCharacters(p.getDescription());
writer.writeEndElement();
writer.writeStartElement("Price");
double price = p.getPrice();
writer.writeCharacters(Double.toString(price));
writer.writeEndElement();
writer.writeEndElement();
}
writer.writeEndElement();
writer.flush();
writer.close();
}
catch (IOException | XMLStreamException e)
{
System.out.println("e");
}
}
private static void printProducts(ArrayList<Product> products)
{
for (Product p : products)
{
printProduct(p);
}
System.out.println();
}
private static void printProduct(Product p)
{
String productString =
StringUtils.padWithSpaces(p.getCode(), 8) +
StringUtils.padWithSpaces(p.getDescription(), 44) +
p.getFormattedPrice();
System.out.println(productString);
}
}
这是一堂课，我花了几个小时审查我的代码和教科书，但无法弄清楚我的代码做错了什么。