Java XML 读卡器错误

Question

I get an error when i run the beginning of my XML reader:

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 is giving me the following output:

run:
XML Reader
The root of this doc is database
2 addresses in database
java.lang.NullPointerException
Address #0
Trowable error
    at xmlreader.XMLreader.main(XMLreader.java:42)

Could someone help me figure this one out?

Answer 1

You're not checking if there IS an element 0. Essentially, .item() will return null if the index isn't valid, as such there may not be an item with index 0 which returns null and then you try to call getChildNodes() on the null pointer. Hence your NPE.

You should be iterating over the elements in the node list.

change

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

to

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