Bonsoir,

Names starting with capital letters should be class names. Yet the
verb-noun structure suggests method names.

A lower case initial letter suggests a variable or method name.

You don't actually say, but I guess you might be thinking about
interface org.w3c.dom.Node and it's methods appendChild() and
insertBefore().

That's more than a single node from my perspective.

The limitations are not so short as that, does your XML document have a
DTD that says otherwise?

It's not clear if CLIENT-SERVER-INTERFACE is the "parent" you speak of
or is the node you are adding to something unmentioned.

An SSCCE might make things clearer. Here's one I hastily cobbled together.

--------------------------------------8<------------------------------
import java.io.IOException;

import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;

import org.w3c.dom.DOMImplementation;
import org.w3c.dom.Document;
import org.w3c.dom.Element;

import com.sun.org.apache.xml.internal.serialize.OutputFormat;
import com.sun.org.apache.xml.internal.serialize.XMLSerializer;

public class DomExample {
    public static void main(String[] args) {

        DocumentBuilderFactory factory =
                DocumentBuilderFactory.newInstance();
        DocumentBuilder builder = null;
        try {
            builder = factory.newDocumentBuilder();
        } catch (ParserConfigurationException e) {
            e.printStackTrace();
            System.exit(1);
        }
        DOMImplementation impl = builder.getDOMImplementation();

        // Create document

        Document doc = impl.createDocument(null, null, null);

        Element e0 = doc.createElement("PARENT");
        doc.appendChild(e0);

        Element e1 = doc.createElement("CLIENT-SERVER-INTERFACE");
        e0.appendChild(e1);

        Element e2 = doc.createElement("SHORTNAME");
        e2.appendChild(doc.createTextNode("mySignal_try"));
        e1.appendChild(e2);

        Element e3 = doc.createElement("CLIENT-SERVER-INTERFACE");
        e0.appendChild(e3);

        Element e4 = doc.createElement("SHORTNAME");
        e4.appendChild(doc.createTextNode("mySignal"));
        e3.appendChild(e4);

        // Print XML

        OutputFormat of = new OutputFormat("XML","ISO-8859-1",true);
        of.setIndent(1);
        of.setIndenting(true);
        of.setDoctype(null, "example.dtd");
        XMLSerializer serializer = new XMLSerializer(System.out ,of);
        try {
            serializer.asDOMSerializer();
            serializer.serialize(doc.getDocumentElement() );
        } catch (IOException e) {
            e.printStackTrace();
        }

    }

}
--------------------------------------8<------------------------------

Output as follows

<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE PARENT SYSTEM "example.dtd">
<PARENT>
    <CLIENT-SERVER-INTERFACE>
        <SHORTNAME>mySignal_try</SHORTNAME>
    </CLIENT-SERVER-INTERFACE>
    <CLIENT-SERVER-INTERFACE>
        <SHORTNAME>mySignal</SHORTNAME>
    </CLIENT-SERVER-INTERFACE>
</PARENT>