exception in creating xml file using xml outputter

[takedown]

here is my simple code:\[code\]try { Element performances = new Element("performances"); Document doc = new Document(performances); doc.setRootElement(performances); performances.setAttribute(new Attribute("date", dateFormat.format(cal.getTime()))); Element uptime = new Element("uptime"); uptime.addContent(new Element("days").setText(new Long(duptime).toString())); uptime.addContent(new Element("hours").setText(new Long(huptime).toString())); uptime.addContent(new Element("minutes").setText(new Long(muptime).toString())); uptime.addContent(new Element("seconds").setText(new Long(suptime).toString())); doc.getRootElement().addContent(uptime); XMLOutputter xmlOutput = new XMLOutputter(); xmlOutput.setFormat(Format.getPrettyFormat()); xmlOutput.output(doc, new FileWriter("/homa/mazzy/Scrivania/perfor_"+dateFormat.format(cal.getTime())+"xml"));\[/code\]I got this exception \[code\]Exception in thread "AWT-EventQueue-0" org.jdom2.IllegalAddException: The element "performances" could not be added as the root of the document: The Content already has an existing parent document\[/code\]but I don't know what does it means.where is the mistake?