Here's the same program using DOM instead of JDOM. Which is simpler?
import java.math.*;
import java.io.*;
import org.w3c.dom.*;
import org.apache.xerces.dom.*;
import org.apache.xml.serialize.*;
public class FibonacciDOM {
  public static void main(String[] args) {
    try {
      DOMImplementation impl = DOMImplementationImpl.getDOMImplementation();
      Document fibonacci = impl.createDocument(null, "Fibonacci_Numbers", null);
      BigInteger low  = BigInteger.ZERO;
      BigInteger high = BigInteger.ONE;
      Element root = fibonacci.getDocumentElement();
      for (int i = 0; i <= 25; i++) {
        Element number = fibonacci.createElement("fibonacci");
        number.setAttribute("index", Integer.toString(i));
        Text text = fibonacci.createTextNode(low.toString());
        number.appendChild(text);
        root.appendChild(number);
        BigInteger temp = high;
        high = high.add(low);
        low = temp;
      }
      try {
        // Now that the document is created we need to *serialize* it
        FileOutputStream out = new FileOutputStream("fibonacci_dom.xml");
        OutputFormat format = new OutputFormat(fibonacci);
        XMLSerializer serializer = new XMLSerializer(out, format);
        serializer.serialize(root);
        out.flush();
        out.close();
      }
      catch (IOException e) {
        System.err.println(e);
      }
    }
    catch (DOMException e) {
      e.printStackTrace();
    }
  }
}
62 lines vs. 42 lines