小编use*_*458的帖子

我想为现有的xml节点添加一个属性.我不想在我的xml文件中添加新元素(新节点),我只想添加一个新属性.我怎样才能做到这一点？

特别是我试过这行代码:

Element process = doc.getElementsById("id");
    process.setAttribute("modelgroup", "");

TransformerFactory transformerFactory = TransformerFactory.newInstance();
Transformer transformer = transformerFactory.newTransformer();
DOMSource source = new DOMSource(doc);
StreamResult result = new StreamResult(new  File("C:\\Users\\Blerta\\workspaceKEPLER\\XML_to_JSON\\SampleExample.xml"));
transformer.transform(source, result);

但我得到以下异常:

Exception in thread "main" java.lang.NullPointerException
    at Main.appendAttributes(Main.java:172)
    at Main.displayNodes(Main.java:65)
    at Main.displayNodes(Main.java:138)
    at Main.main(Main.java:42)**