Operations on Nodes

[Eleeist]

Hi,

I am trying to learn how to read XML files in Java. I find it really difficult to grasp the structure of the DOM and how to operate on Nodes. I have read tons of tutorials but most provide just the code with litte or no explanation what is happening.

Here is the XML file I am trying to read:

<?xml version="1.0"?>
<company>
    <staff>
        <firstname>test</firstname>
        <lastname>test2</lastname>
        <nickname>test3</nickname>
        <salary>test4</salary>
    </staff>
    <staff>
        <firstname>test5</firstname>
        <lastname>test6</lastname>
        <nickname>test7</nickname>
        <salary>test8</salary>
    </staff>
</company>

And here is the code that supposed to output the firstnames of staff:

import javax.xml.parsers.*;
import org.w3c.dom.*;
import java.io.File;

public class Test {

    public static void main(String[] args) throws Exception {

        DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
        DocumentBuilder builder  = factory.newDocumentBuilder();
        Document document = builder.parse(new File("/Users/Eleeist/Desktop/test.xml"));
        
        Element companyElement = document.getDocumentElement();
        
        NodeList elements = companyElement.getElementsByTagName("firstname");
        
        System.out.println(elements.item(0).getNodeValue() + " " + elements.item(1).getNodeValue());
        
    }

}

However, the output is just "null null".

Could somebody please explain to me what is happening and what should I do to make it work? :)

Thanks.

[eRaaaa]

Try to get the childnode first:
System.out.println(elements.item(0).getChildNodes( ).item(0).getNodeValue() + " " + elements.item(1).getChildNodes().item(0).getNodeVa lue());
or try to use getTextContent
System.out.println(elements.item(0).getTextContent () + " " + elements.item(1).getTextContent());

or use XPath :P

[Fubarable]

I think that a problem is that your XML has many more nodes than you realize. Everything is a Node. To see what I mean, recurse through your XML file and have it print out all of its contents (though I didn't display attributes since your simple file doesn't have any:

import javax.xml.parsers.*;
import org.w3c.dom.*;
import java.io.File;

public class Xml001 {
   private static final String XML_FILE = "/Users/Eleeist/Desktop/test.xml";
   // private static final String XML_FILE = "src/yr12/m02/a/Xml001.xml";

   public static void main(String[] args) throws Exception {
      DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
      DocumentBuilder builder = factory.newDocumentBuilder();
      File xmlFile = new File(XML_FILE);
      Document document = builder.parse(xmlFile);

      recurseThru(document, "", 0);

      // Element companyElement = document.getDocumentElement();
      // NodeList elements = companyElement.getElementsByTagName("firstname");
      // System.out.println(elements.item(0).getNodeValue() + " "
      // + elements.item(1).getNodeValue());
   }

   private static void recurseThru(Node node, String tabs, int level) {
      System.out.println(tabs + "level: " + level);
      System.out.println(tabs + "node:  " + node.getNodeName());
      System.out.println(tabs + "value: " + node.getNodeValue());
      System.out.println(tabs + "text:  " + node.getTextContent());

      if (node.hasChildNodes()) {
         tabs = "\t" + tabs;
         level++;
         NodeList nodeList = node.getChildNodes();
         for (int i = 0; i < nodeList.getLength(); i++) {
            Node childNode = nodeList.item(i);
            recurseThru(childNode, tabs, level);
         }
      }
   }

}

[Eleeist]

Indeed, I have not known that there were so many nodes. I tried aRaaaa advice and it works.

How would I go about reading elements attributes? Let's say the new XML file looks like this:

<?xml version="1.0"?>
<company>
    <staff>
        <firstname name="test" />
        <lastname>test2</lastname>
        <nickname>test3</nickname>
        <salary>test4</salary>
    </staff>
    <staff>
        <firstname name="test5" />
        <lastname>test6</lastname>
        <nickname>test7</nickname>
        <salary>test8</salary>
    </staff>
</company>

And here is the updated code:

import javax.xml.parsers.*;
import org.w3c.dom.*;
import java.io.File;
 
public class Test {
 
    public static void main(String[] args) throws Exception {
 
        DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
        DocumentBuilder builder  = factory.newDocumentBuilder();
        Document document = builder.parse(new File("/Users/Eleeist/Desktop/test.xml"));
         
        Element companyElement = document.getDocumentElement();
         
        NodeList elements = companyElement.getElementsByTagName("firstname");
         
        System.out.println(elements.item(0).getTextContent() + " " + elements.item(1).getTextContent());
         
    }
 
}

[Fubarable]

How would I go about reading elements attributes? Let's say the new XML file looks like this:

Look up API for Node and check the methods that are available. With this you should be able to figure it out, I'll bet.

[Eleeist]

Got it:

import javax.xml.parsers.*;
import org.w3c.dom.*;
import java.io.File;
 
public class Test {
 
    public static void main(String[] args) throws Exception {
 
        DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
        DocumentBuilder builder  = factory.newDocumentBuilder();
        Document document = builder.parse(new File("/Users/Eleeist/Desktop/test.xml"));
         
        Element companyElement = document.getDocumentElement();
         
        NodeList elements = companyElement.getElementsByTagName("firstname");
         
        System.out.println(elements.item(0).getAttributes().item(0).getTextContent() + " " + elements.item(1).getAttributes().item(0).getTextContent());
         
    }
 
}

Ok, the last thing: going down the levels.

Let's say I have this:

<?xml version="1.0"?>
<company>
    <staff>
        <deeper>
            <evendeeper>
                <somevalue>Hey!</somevalue>
            </evendeeper>
        </deeper>
    </staff>
</company>

How do I get to my "Hey" text?

I tried something like this:

System.out.println(elements.item(0).getChildNodes().item(0).getChildNodes().item(0)...;

But I don't think it is the right thing to do (and it does not seem to work).

[Fubarable]

The simplest way to get that (that I know of) is to follow eRaaaaaaaaaaaa's (how many a's?) initial suggestion: use XPath. It's really the best answer to solve you problem.


This can use your DOM document so much of the code you have can be used with XPath.

[Eleeist]

I read about XPath and modified one of my classes to use it, and it works :). I will post here if encounter any problems.

Thanks both of your for suggestions :).