Java appending XML docs to existing docs

It's a bit tricky, but the following example runs:

public static void main(String[] args) {    DocumentImpl doc1 = new DocumentImpl();    Element root1 = doc1.createElement("root1");    Element node1 = doc1.createElement("node1");    doc1.appendChild(root1);    root1.appendChild(node1);    DocumentImpl doc2 = new DocumentImpl();    Element root2 = doc2.createElement("root2");    Element node2 = doc2.createElement("node2");    doc2.appendChild(root2);    root2.appendChild(node2);    DocumentImpl doc3 = new DocumentImpl();    Element root3 = doc3.createElement("root3");    doc3.appendChild(root3);    // root3.appendChild(root1); // Doesn't work -> DOMException    root3.appendChild(doc3.importNode(root1, true));    // root3.appendChild(root2); // Doesn't work -> DOMException    root3.appendChild(doc3.importNode(root2, true));   }

I know you got the issue solved already, but I still wanted to take a stab at this problem using the XOM library that I'm currently testing out (related to this question), and while doing that, offer a different approach than that of Andreas_D's answer.

(To simplify this example, I put your <alert-set> and <weather-set> into separate files, which I read into nu.xom.Document instances.)

import nu.xom.*;[...]Builder builder = new Builder();Document alertDoc = builder.build(new File("src/xomtest", "alertset.xml"));Document weatherDoc = builder.build(new File("src/xomtest", "weatherset.xml"));Document mainDoc = builder.build("<DataSet><blank/><blank/></DataSet>", "");Element root = mainDoc.getRootElement();root.replaceChild(    root.getFirstChildElement("blank"), alertDoc.getRootElement().copy());root.replaceChild(    root.getFirstChildElement("blank"), weatherDoc.getRootElement().copy());

The key is to make a copy of the elements to be inserted into mainDoc; otherwise you'll get a complain that "child already has a parent".

Outputting mainDoc now gives:

<?xml version="1.0" encoding="UTF-8"?><DataSet>    <alert-set>        <warning>National Weather Service...</warning>        <start-date>5/19/2009</start-date>        <end-date>5/19/2009</end-date>    </alert-set>    <weather-set>        <chance-of-rain type="percent">31</chance-of-rain>        <conditions>Partly Cloudy</conditions>        <temperature type="Fahrenheit">78</temperature>    </weather-set></DataSet>

To my delight, this turned out to be very straight-forward to do with XOM. It only took a few minutes to write this, even though I'm definitely not very experienced with the library yet. (It would have been even easier without the <blank/> elements, i.e., starting with simply <DataSet></DataSet>.)

So, unless you have compelling reasons for using only the standard JDK tools, I warmly recommend trying out XOM as it can make XML handling in Java much more pleasant.