为什么在运行 StAX Parser 时出现 NullPointerException?

问题描述

我正在尝试用 Java 编写 StAX

线程main"中的异常；java.lang.NullPointerException 在org.example.shoesshop.parser.STAXParser.parse

这是 StAX Parser 的一个类:

公共类 STAXParser 扩展 DefaultHandler {私有静态列表<鞋子>parse

这是一个

还有一个用于鞋子的 Java 类
@
我还需要知道如何将接收到的数据写入新的 
 解决方案 
已更新: 对原始答案的评论:
它不起作用，它给出了同样的错误
这意味着问题是因为 shoes 变量是 null，这在 debugger 中很容易看到.使用调试器会为我们节省很多时间，所以请开始使用调试器.
为了使 shoes 为 null，看来代码遇到了一个不是鞋子元素.
要修复代码，添加一个空检查，并在处理 Shoes 元素结束时设置 shoes = null:
} else if (startElement.getName().getLocalPart().equals("title")) {if (shoes != null) {//<===== 添加这个shoes.setTitle(reader.getElementText());//<===== 解决这个问题(见原始答案)}
if (
原始答案
你的代码是:
} else if (startElement.getName().getLocalPart().equals("title")){
问题是代码没有检查如果事件跟在 START_ELEMENT 事件之后的类型.可能是这样的:
元素很可能是空的，即<title/>或<title><title/>，在这种情况下下一个事件是 END_ELEMENT，asCharacters() 返回 null.
元素有注释，例如<title><!-- 没有标题--><title/>，在这种情况下，下一个事件是 COMMENT.p>
元素具有混合内容，例如<title>foo<![CDATA[bar]]><title/>，在这种情况下，下一个事件不是全文.
检索元素的文本内容是如此常见，以至于他们为此添加了一个辅助方法::
读取纯文本元素的内容.前提条件:当前事件是START_ELEMENT.后置条件:当前事件是对应的END_ELEMENT.
抛出:
 - 如果当前事件不是 START_ELEMENT 或者遇到非文本元素
这意味着你的代码应该是:
} else if (startElement.getName().getLocalPart().equals("title")) {shoes.setTitle(reader.getElementText());
I'm trying to write a StAX 
Here's a class for StAX Parser:
public class STAXParser extends DefaultHandler {
    private static List<Shoes> parse
Here's an 
<?
Also here's a Java class for Shoes
@
Also I need to know how to write a received data into a new 
 解决方案 
UPDATED: Comment to original answer:
That means the problem is because the shoes variable is null, as would have easily been seen with a debugger. Using a debugger would have saved us all a lot of time, so please start using one.
In order for shoes to be null, it appears that the code encountered a <title> element that is not a child of a Shoes element.
To fix the code, add a null-check, and also set shoes = null at the end of processing the Shoes element:
} else if (startElement.getName().getLocalPart().equals("title")) {
    if (shoes != null) { // <===== ADD THIS
        shoes.setTitle(reader.getElementText()); // <===== Fix this (see original answer)
    }
if (
ORIGINAL ANSWER
Your code is:
} else if (startElement.getName().getLocalPart().equals("title")){
    
The problem is that the code isn't checking what type if event follows the START_ELEMENT event. It could be that:
Most likely, the element is empty, i.e. <title/> or <title><title/>, in which case the next event is an END_ELEMENT, and asCharacters() returned null.
The element has a comment, e.g. <title><!-- there is no title --><title/>, in which case the next event is a COMMENT.
The element has mixed content, e.g. <title>foo<![CDATA[bar]]><title/>, in which case the next event is not the full text.
Retrieving the text content of an element is such a common thing that they added a helper method for that: :
Which means that your code should be:
} else if (startElement.getName().getLocalPart().equals("title")) {
    shoes.setTitle(reader.getElementText());
                        
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