Retrieving attributes and values of child Nodes of an XML Element in Java

Question

Please, how can I get the attribute value of the second in the XML course tag name with attribute "AdvancedAlgorithm" in the XML structure below:

<course name="AdvancedAlgorithm">
    <Teacher name="Francis" class="A" />
    <Teacher name="John" class="B" />
    <Teacher name="Philips" class="C" />
    <course name="AlgorithmForBeginners">
        <Teacher name="Simon" class="E" />
        <Teacher name="Joan" class="F" />
    </course>
    <Teacher name="Edward" class="M" />
</course>

I have tried various means to get it but somehow it gives me first a wrong length and a wrong value.. what am I doing wrong in the code below?

 public void getStructure(NodeList list){

    for(int i= 0;i<list.getLength();i++){
        Element element = (Element)list.item(i);
           if(element.getNodeType()==Node.ELEMENT_NODE && element.getAttribute("name").equals("AdvancedAlgorithm"))
         {
                NodeList node = element.getChildNodes(); //get the child elements
                System.out.println(node.getLength());
                for(int k=0; k<node.getLength();k++){
                    Node currentNode = node.item(i);
                    Element e = (Element)currentNode;
                    System.out.println(e.getAttribute("name"));

            }

         }
    }

My analysis: The NodeList list has a length of 2. Is that true given that the XML has two course tag but when I assign to the the NodeList node the ChildNodes of the Element element, and checked the lenght of NodeList node, I found out it was 11 instead of 5 since the node has 5 sub nodes. My concern is, first, I want to know what is the length of this XML structure and secondly, how to retrieve the second . Thanks

Boris the Spider · Accepted Answer · 2013-03-15 12:43:11Z

2

I am not sure what you want to do exactly, but XPath will probably solve the issue. Here is a quick example using the XML snippet you provided:

public static void main(String[] args) throws UnsupportedEncodingException, ParserConfigurationException, SAXException, IOException, XPathExpressionException {
    final String xml = "<course name=\"AdvancedAlgorithm\">\n"
            + "    <Teacher name=\"Francis\" class=\"A\" />\n"
            + "    <Teacher name=\"John\" class=\"B\" />\n"
            + "    <Teacher name=\"Philips\" class=\"C\" />\n"
            + "    <course name=\"AlgorithmForBeginners\">\n"
            + "        <Teacher name=\"Simon\" class=\"E\" />\n"
            + "        <Teacher name=\"Joan\" class=\"F\" />\n"
            + "    </course>\n"
            + "    <Teacher name=\"Edward\" class=\"M\" />\n"
            + "</course>";
    final Document doc = DocumentBuilderFactory.newInstance().newDocumentBuilder().parse(new ByteArrayInputStream(xml.getBytes("UTF-8")));
    final XPath xPath = XPathFactory.newInstance().newXPath();
    final XPathExpression expression = xPath.compile("//course[@name='AdvancedAlgorithm']//Teacher");

    final NodeList nodeList = (NodeList) expression.evaluate(doc, XPathConstants.NODESET);

    for (int i = 0; i < nodeList.getLength(); ++i) {
        System.out.println(((Element)nodeList.item(i)).getAttribute("name"));
    }
}

Output:

Francis
John
Philips
Simon
Joan
Edward

So the Document is built however (from the String in this case) and then we create and compile an XPathExpression. This expression, //course[@name='AdvancedAlgorithm']//Teacher, means find all the "Teacher" elements in the document regardless of where they are as long as they have a course named "AdvancedAlgorithm" somewhere as a parent.

An expression //course[@name='AdvancedAlgorithm']/course will yield a single value, "AlgorithmForBeginners", as this asks for all courses that have a course with name "AdvancedAlgorithm" as a direct parent.

The expression //course[@name='AdvancedAlgorithm']/course[@name='AlgorithmForBeginners']/Teacher will find all courses named "AlgorithmForBeginners" that have a direct parent that is a course named "AdvancedAlgorithm" and list all of the Teacher elements that are direct children.

To find all teachers of any course named "AlgorithmForBeginners" you would use //course[@name='AlgorithmForBeginners']/Teacher for direct children or //course[@name='AlgorithmForBeginners']//Teacher for any child, i.e. direct or indirect.

Is that what you were looking for; I am not sure what "attribute value of the second" means.

edited Mar 15, 2013 at 12:43

answered Mar 14, 2013 at 23:26

Boris the Spider

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3 Comments

helpdesk Over a year ago

Hi @bmorris, while this is good. won't it be cumbersome if you had a large XML? Is there any other way I could still do this without using the XPath? How about if I want to use XPath to find only the Teachers in the class "AlgorithmForBeginners"? While, I do not want to use XPath, it still would be nice to learn it should in case :)

Boris the Spider Over a year ago

Edited to show those queries. The whole point of XPath is to deal with large xml files! It's meant to query the data intelligently rather than just loop over and try and find what you want - instead of your two nested loops XPath has one single expression both describes what it is you are trying to find to the reader of the code and finds it. How do you mean cumbersome?

helpdesk Over a year ago

Thanks. I will try this out and see how it goes. Maybe the only option for me now since I have tried in vain to use DOM.

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