different between "getDocumentElement" and "getFirstChild"

document, java, nodes, xml

Solution

The Node you get using myDoc.getFirstChild() may not be the document root if there are other nodes before the document root node - such as a comment node. Look at the example below:

import org.w3c.dom.*;

public class ReadXML {

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

        DocumentBuilderFactory docFactory = DocumentBuilderFactory.newInstance();
        DocumentBuilder docBuilder = docFactory.newDocumentBuilder();

        // Document elements
        Document doc = docBuilder.parse(new File(args[0]));

        Node firstChild = doc.getFirstChild();
        System.out.println(firstChild.getChildNodes().getLength());
        System.out.println(firstChild.getNodeType());
        System.out.println(firstChild.getNodeName());

        Node root = doc.getDocumentElement();
        System.out.println(root.getChildNodes().getLength());
        System.out.println(root.getNodeType());
        System.out.println(root.getNodeName());

    }
}

When parsing the following XML file:

<?xml version="1.0"?>
<!-- Edited by XMLSpy -->
<catalog>
   <product description="Cardigan Sweater" product_image="cardigan.jpg">
      <catalog_item gender="Men's">
         <item_number>QWZ5671</item_number>
         <price>39.95</price>
         <size description="Medium">
            <color_swatch image="red_cardigan.jpg">Red</color_swatch>
            <color_swatch image="burgundy_cardigan.jpg">Burgundy</color_swatch>
         </size>
         <size description="Large">
            <color_swatch image="red_cardigan.jpg">Red</color_swatch>
            <color_swatch image="burgundy_cardigan.jpg">Burgundy</color_swatch>
         </size>
      </catalog_item>    
   </product>
</catalog>

gives the following result:

0
8
#comment
3
1
catalog

But if I remove the comment, it gives:

3
1
catalog
3
1
catalog

Problem

I have the following `Document` object - `Document myDoc`. `myDoc` holds an `XML` file by... ``` myDoc = DocumentBuilderFactory.newInstance() .newDocumentBuilder().parse(file); ``` Now I want to get the root of the XML file. Is there any difference between ``` Node firstChild = this.myDoc.getFirstChild() ``` and ``` Node firstChild = (Node)myDoc.getDocumentElement() ``` In the first way, `firstChild` holds a node root of an `XML` file but it will not have the depth of `Node`. However, in the second way, `firstChild` will be the root with all of the depth. For example, I have the following XML ``` <inventory> <book num="b1"> </book> <book num="b2"> </book> <book num="b3"> </book> </inventory> ``` and `file` holds it. In the 1st case, `int count = firstChild.getChildNodes()` gives `count = 0`. The 2nd case will give `count = 3`. Am I right?

Original source