import org.jdom.Document;
import org.jdom.Element;
import org.jdom.input.SAXBuilder;
import org.jdom.output.Format;
import org.jdom.output.XMLOutputter;
public class MainClass {
public static void main(String[] args) throws Exception {
SAXBuilder builder = new SAXBuilder();
Document books = builder.build("books.xml");
Document onebook = builder.build("onebook.xml");
Element bookToAdd = onebook.getRootElement().getChild("book");
books.getRootElement().addContent(bookToAdd);
new XMLOutputter(Format.getPrettyFormat()).output(books, System.out);
}
}