声明一个实例变量并在main方法中初始化它

Question

在下面的代码中，程序应该从xml文件中读取值。我将双变量声明为实例变量，并在main方法中初始化它们。问题是程序没有看到初始值为a1，a2 .... a7。我收到NullPointerException错误。我的问题是它与静态事物有关，或者它不可能在一个地方和另一个地方声明一个变量。

public class ParseXml extends RoboticsAPIApplication {

    private Controller kuka_Sunrise_Cabinet_1;
    private  LBR lbr_iiwa_14_R820_1;

    private static double a1, a2, a3, a4, a5, a6, a7;

    public void initialize() {
        kuka_Sunrise_Cabinet_1 = getController("KUKA_Sunrise_Cabinet_1");
        lbr_iiwa_14_R820_1 = (LBR) getDevice(kuka_Sunrise_Cabinet_1,
                "LBR_iiwa_14_R820_1");

    }

    public void run() {

        lbr_iiwa_14_R820_1.move(ptp(a1, a2, a3, a4, a5, a6, a7));
    }

    public static void main(String[] args) {

        try {
            // get instance of the class and use it to parse new file xmlfile

            File xmlfile = new File("C:/Users/Acer/Desktop/neues.xml");
            DocumentBuilderFactory factory = DocumentBuilderFactory
                    .newInstance();
            DocumentBuilder db = factory.newDocumentBuilder();
            Document doc = db.parse(xmlfile);

            // Normalize
            doc.getDocumentElement().normalize();

            // get the frame Element
            NodeList nlist = doc.getElementsByTagName("frame");

            // cycle through the Elements
            for (int i = 0; i < nlist.getLength(); i++) {

                Node nnode = nlist.item(i);
                if (nnode.getNodeType() == Node.ELEMENT_NODE) {
                    Element fframe = (Element) nnode;

                    // Initialization
                    a1 = Double.parseDouble(fframe.getElementsByTagName("A1")
                            .item(0).getTextContent());
                    a2 = Double.parseDouble(fframe.getElementsByTagName("A2")
                            .item(0).getTextContent());
                    a3 = Double.parseDouble(fframe.getElementsByTagName("A3")
                            .item(0).getTextContent());
                    a4 = Double.parseDouble(fframe.getElementsByTagName("A4")
                            .item(0).getTextContent());
                    a5 = Double.parseDouble(fframe.getElementsByTagName("A5")
                            .item(0).getTextContent());
                    a6 = Double.parseDouble(fframe.getElementsByTagName("A6")
                            .item(0).getTextContent());
                    a7 = Double.parseDouble(fframe.getElementsByTagName("A7")
                            .item(0).getTextContent());

                }
            }

        } catch (ParserConfigurationException e) {
            // TODO Automatisch generierter Erfassungsblock
            e.printStackTrace();
        } catch (SAXException e) {
            // TODO Automatisch generierter Erfassungsblock
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Automatisch generierter Erfassungsblock
            e.printStackTrace();
        }

        ParseXml app = new ParseXml();
        app.runApplication();

    }
}