public void populate() {
        try {
            // open the file
            File xmlPerson = new File(fileName);
            DocumentBuilderFactory dbFactory = DocumentBuilderFactory.newInstance();
            DocumentBuilder dBuilder = dbFactory.newDocumentBuilder();
            // uses DOM to parse the xml
            Document doc = dBuilder.parse(xmlPerson);
            doc.getDocumentElement().normalize();
            NodeList nList = doc.getElementsByTagName("Person");
            // loop through all nodes named Person
            for (int i = 0; i < nList.getLength(); i++) {
                Node nNode = nList.item(i);
                // get child nodes of Person
                NodeList sublist = nNode.getChildNodes();
                if (nNode.getNodeType() == Node.ELEMENT_NODE) {
                    // the person element
                    Element eElement = (Element) nNode;
                    // add the persons attribute (name)
                     if (sublist.getLength() < 1) {

                    } else {
                    people.add(eElement.getAttribute("name"));
                     }
                    // loop through child nodes (topics)
                   
                        for (int j = 0; j < sublist.getLength(); j++) {
                            Node cNode = (Node) sublist.item(j);
                            if (cNode.getNodeType() == Node.ELEMENT_NODE) {
                                // the topic element
                                Element cElement = (Element) cNode;
                                // add the topic element (subject)
                                if (sublist.getLength() < 1) {

                    } else {
                                topics.add(cElement.getAttribute("subject"));
                                }
                            }
                        }
                    
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }

    }