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javaxmlw3c

The return value of "org.w3c.dom.Document.getDocumentElement()" is null

I have this code

DocumentBuilderFactory docb = DocumentBuilderFactory.newInstance();
DocumentBuilder dom = docb.newDocumentBuilder();
Document doc = dom.newDocument();
Element raiz = doc.createElement("Alumnos");
//-Problem here 
doc.getDocumentElement().appendChild(raiz);

This is supposed to create an XML file, but when I try to do it, I get an exception:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException: Cannot invoke "org.w3c.dom.Element.appendChild(org.w3c.dom.Node)" because the return value of "org.w3c.dom.Document.getDocumentElement()" is null

I'm not sure why I get this and why the document element is null. Am I forgetting something?

I tried to create an XML file to append elements later, but I got that error.

Solution

  • You've created an empty document, it doesn't yet have a "documentElement", so getDocumentElement() returns null. The method doc.createElement just creates an element, it doesn't add it to the document, you need to do that explicitly:

    DocumentBuilderFactory docb = DocumentBuilderFactory.newInstance();
DocumentBuilder dom = docb.newDocumentBuilder();
Document doc = dom.newDocument();
Element raiz = doc.createElement("Alumnos");
doc.appendChild(raiz);

    In other words, remove the call to getDocumentElement(), but call appendChild(raiz) directly on doc. This will make Alumnos the document element (root element).

    Afterwards, calls to doc.getDocumentElement() will return the same object as referenced by raiz.