not able to get an element using the document.getElementById, returning null

Question

I am trying to get a html node from a file that will later be used to count all of its descendants. I am having issues with retrieving the element from the DOM. here are the steps i have taken so far.

First here is my html code:

<html>
<head>
    <title></title>
</head>
<body>
<div id="container">
    <a></a>
    <div id="header">
        <div id="firstchild">
            <div>
                <img></img>
            </div>
            <a></a>
            <ul>
                <li>
                    <a>Inbox</a>
                </li>
                <li>
                    <a>Logout</a>
                </li>
            </ul>
            <form></form>
        </div>
        <div id="nextsibling"></div>
    </div>
</div>
</body>
</html>

Second I built this function that will return and parse the file into a document.

public static Document buildDocument(String file){
    try {
        DocumentBuilderFactory docFactory = DocumentBuilderFactory.newInstance();
        DocumentBuilder docBuilder = docFactory.newDocumentBuilder();
        Document document = docBuilder.parse(file);
        return document;
    } catch (ParserConfigurationException | SAXException | IOException ex) {
        System.out.println("the exception is: " + ex.toString());
    }
    return null;
}

Next in my main method I tried to set a Node object to a document elemet by way of getElementById like:

public Document doc = buildDocument("myHTMLFile");
org.w3c.dom.Node node = doc.getElementById("header");//the id of an html element

Correct me if I am wrong but this should result in the retreival of the node. However it is returning a null value. I do not understand why it is not returning the correct value. NOTE: that when debugging the code the document does contain all of the correct data as far as I can tell.

Can you add a more generic exception to the `catch`? (am not a Java guy, btw) — Brett Zamir, Jan 25 '14 at 23:15
Also, you are passing "myHTMLFile"? I assume that has an extension or whatever the `parse` method needs to obtain its contents rather than interpret it as a string? — Brett Zamir, Jan 25 '14 at 23:17
I do not think that the error is in the build document function, When I step through the program I am able to veiw the documant and can see all of the elements within it. It seems that when getElementById is called it is unable to find the id but I know it is there, so I am a loss as to why it is not finding it. — Jared Reeves, Jan 25 '14 at 23:18
@brett, yes that is the extension, I stubbed it for the question. — Jared Reeves, Jan 25 '14 at 23:19

user and · Accepted Answer · 2014-01-25T23:43:52.837

11

You do it wrong. Javadoc javadoc of getElementById said:

Returns the Element that has an ID attribute with the given value. If no such element exists, this returns null . ... The DOM implementation is expected to use the attribute Attr.isId to determine if an attribute is of type ID. Note: Attributes with the name "ID" or "id" are not of type ID unless so defined.

In your case the best solution is using XPath (simple query language to XML):

XPath xpath = XPathFactory.newInstance().newXPath();
Node node = (Node) xpath.evaluate("//*[@id='header']", document, XPathConstants.NODE);

Expression //*[@id='header'] - select all nodes in document which has attribute id with 'header' value.

edited Jan 25 '14 at 23:43

answered Jan 25 '14 at 23:38

user and

550
4
9

Thank you that worked great!!! Once i was able to get the element the rest of my code worked perfectly. – Jared Reeves Jan 26 '14 at 00:05
1

Wow, this api is so stupid.. It is obvious that attribute id is what everybody mean as id... – Vistritium Mar 03 '16 at 17:20

score 4 · Answer 2 · edited May 23 '17 at 11:53

It appears you are working with the generic XML DOM. XML expects IDs to be defined as such, so an element with an attribute, even if named "id", won't work unless designated as such.

Try finding an HTML-specific interface or adding a DOCTYPE which defines the id attribute as an ID type. (I wouldn't recommend the latter though because HTML5 has moved away from attempting an XHTML compatible approach even if it technically supports an XHTML serialization.) See Parse Web Site HTML with JAVA for recommendations on HTML-specific parsers.

score -2 · Answer 3 · answered Jan 25 '14 at 23:25

Try doing the following two things:

In your buildDocument() function, add the following line:
```
  Element element = document.getDocumentElement();
```
Change the return type of the function to "Element" and return "element"
Create an "Element" object instead of a "Document" object and call "getElementById()" on it.
Also check the name of the file, if you are missing .html extension

not able to get an element using the document.getElementById, returning null

3 Answers3