How to I output org.w3c.dom.Element to string format in java?

Assuming you want to stick with the standard API...

You could use a DOMImplementationLS:

Document document = node.getOwnerDocument();
DOMImplementationLS domImplLS = (DOMImplementationLS) document
    .getImplementation();
LSSerializer serializer = domImplLS.createLSSerializer();
String str = serializer.writeToString(node);

If the <?xml version="1.0" encoding="UTF-16"?> declaration bothers you, you could use a transformer instead:

TransformerFactory transFactory = TransformerFactory.newInstance();
Transformer transformer = transFactory.newTransformer();
StringWriter buffer = new StringWriter();
transformer.setOutputProperty(OutputKeys.OMIT_XML_DECLARATION, "yes");
transformer.transform(new DOMSource(node),
      new StreamResult(buffer));
String str = buffer.toString();

Simple 4 lines code to get String without xml-declaration (<?xml version="1.0" encoding="UTF-16"?>) from org.w3c.dom.Element

DOMImplementationLS lsImpl = (DOMImplementationLS)node.getOwnerDocument().getImplementation().getFeature("LS", "3.0");
LSSerializer serializer = lsImpl.createLSSerializer();
serializer.getDomConfig().setParameter("xml-declaration", false); //by default its true, so set it to false to get String without xml-declaration
String str = serializer.writeToString(node);

Try jcabi-xml with one liner:

String xml = new XMLDocument(element).toString();

Not supported in the standard JAXP API, I used the JDom library for this purpose. It has a printer function, formatter options etc. http://www.jdom.org/

If you have the schema of the XML or can otherwise create JAXB bindings for it, you could use the JAXB Marshaller to write to System.out:

import javax.xml.bind.*;
import javax.xml.bind.annotation.*;
import javax.xml.namespace.QName;

@XmlRootElement
public class BoundClass {

    @XmlAttribute
    private String test;

    @XmlElement
    private int x;

    public BoundClass() {}

    public BoundClass(String test) {
        this.test = test;
    }

    public static void main(String[] args) throws Exception {
        JAXBContext jxbc = JAXBContext.newInstance(BoundClass.class);
        Marshaller marshaller = jxbc.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_FRAGMENT, true);
        marshaller.marshal(new JAXBElement(new QName("root"),BoundClass.class,new Main("test")),System.out);
    }
}