Java XML 读取器错误

Question

当我运行 XML 阅读器的开头时出现错误:

public static void main(String[] args) 
{
    System.out.println("XML Reader");

    try
    {
        DocumentBuilderFactory docFactory = DocumentBuilderFactory.newInstance();
        DocumentBuilder docBuilder = docFactory.newDocumentBuilder();
        Document doc = docBuilder.parse("database.xml");

        //normalize text
        doc.getDocumentElement().normalize();
        System.out.println("The root of this doc is "+doc.getDocumentElement().getNodeName());

        NodeList listOfAddresses = doc.getElementsByTagName("address");
        int totalAddresses = listOfAddresses.getLength();
        System.out.println(totalAddresses+" addresses in "+ doc.getDocumentElement().getNodeName());

        //main loop
        for(int i = 0; i<listOfAddresses.getLength(); i++)
        {
            Node items = listOfAddresses.item(i);

            if(items.getNodeType() == Node.ELEMENT_NODE)
            {
                System.out.println("Address #"+i);

                Element element = (Element)items;

                NodeList nameList = element.getElementsByTagName("name");
                Element nameElement = (Element)nameList.item(0);
                NodeList nameOutput = nameElement.getChildNodes();


                System.out.println("name: "+nameElement);
            }
        }


    }

    catch(SAXParseException err)
    {
        System.out.println("Sax Parse Exception error on line "+err.getLineNumber());
    }

    catch(SAXException e)
    {
        System.out.println("SAX Exception error");
        Exception x = e.getException();
        ((x == null) ? e : x).printStackTrace();
    }

    catch(Throwable t)
    {
        System.out.println("Trowable error");
        t.printStackTrace();
    }
}

netbeans 给我以下输出:

run:XML ReaderThe root of this doc is database2 addresses in databasejava.lang.NullPointerExceptionAddress #0Trowable error    at xmlreader.XMLreader.main(XMLreader.java:42)

有人可以帮我解决这个问题吗？

Answer 1

您没有检查是否存在元素 0。本质上，如果索引无效，.item() 将返回 null，因此可能不存在索引为 0 的项目返回 null，然后您尝试对空指针调用 getChildNodes()。这就是你的 NPE。

您应该迭代节点列表中的元素。

改变

Element nameElement = (Element)nameList.item(0);

至

for(int x = 0; x < nameList.getLength(); x++) {
  nameElement = nameList.item(x);
  NodeList nameOutput = nameElement.getChildNodes();
  System.out.println("name: "+nameElement);
}