Java XML parsing using DOM to get nodevalue

That's because an element in fact has no nodeValue. Instead it has a text node as a child, which has the nodeValue you want.

In short, you'll want to getNodeValue() on the first child of the element node.

Sometimes the element contains multiple text nodes, as they do have a maximum size, in which case you'll need something á la this, from the page linked earlier:

public static String getNodeValue(Node node) {
    StringBuffer buf = new StringBuffer();
    NodeList children = node.getChildNodes();
    for (int i = 0; i < children.getLength(); i++) {
        Node textChild = children.item(i);
        if (textChild.getNodeType() != Node.TEXT_NODE) {
            System.err.println("Mixed content! Skipping child element " + textChild.getNodeName());
            continue;
        }
        buf.append(textChild.getNodeValue());
    }
    return buf.toString();
}

Try extracting it from the Element rather than from the Node:

try {
    String data = "<a><b c='d' e='f'>0.15</b></a>";
    DocumentBuilderFactory documentBuilderFactory = DocumentBuilderFactory
            .newInstance();
    DocumentBuilder documentBuilder = documentBuilderFactory
            .newDocumentBuilder();
    InputSource is = new InputSource();
    is.setCharacterStream(new StringReader(data));
    Document document = documentBuilder.parse(is);

    NodeList nl = document.getElementsByTagName("b");
    Element el = (Element) nl.item(0);
    Text elText = (Text) el.getFirstChild();
    String theValue = elText.getNodeValue();
    System.out.println(theValue);
} catch (Exception e) {
    System.out.println("Exception " + e);
}

System.out.println(n.getFirstChild().getNodeValue());

 private String getTextValue(Element element, String string) {
    String textVal = null;
    NodeList nl = element.getElementsByTagName(string);
    if(nl != null && nl.getLength() > 0) {
        Element el = (Element)nl.item(0);
        textVal = el.getFirstChild().getNodeValue();
    }

    return textVal;

}

If the node has no further nested descendants, than n.getTextContent() works quite well.

You could use jOOX as a wrapper for standard DOM, to simplify your code.

String data = "<a><b c='d' e='f'>0.15</b></a>";
String value = $(data).find("b").text();

You could also have jOOX convert that value to double, for instance:

Double value = $(data).find("b").text(Double.class);