Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException at gui.乘客Gui$2.动作执行(乘客Gui.java

Question

与下面提到的代码有问题。Eclipse错误代码如下::

线程" awt-esventqueue-0" java.lang.nullpointerexception中的例外 在gui.passengergui $2。

任何建议将不胜感激吗？我已经把头发拔出了近一个小时，试图弄清楚问题是什么。

private JPanel contentPane;
private JFileChooser fileChooser;
private PassengerController controller;
private PassengerTabelPanel tabelPanel;
private JTextField nameField;
private JTextField cityField;
private JTable passengerTable;
private JRadioButton  standardRadio;
private JRadioButton businessRadio;
private ButtonGroup classGroup;
private PassengerFormListener formListener;

/**
 * Launch the application.
 */
public static void main(String[] args) {
    EventQueue.invokeLater(new Runnable() {
        public void run() {
            try {
                PassengerGui frame = new PassengerGui();
                frame.setVisible(true);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    });
}
/**
 * Create the frame.
 */
public PassengerGui() {
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setBounds(100, 100, 584, 368);
    JMenuBar menuBar = new JMenuBar();
    setJMenuBar(menuBar);
    JMenuItem importDataItem = new JMenuItem("Import Data");
    menuBar.add(importDataItem);
    JMenuItem exportDataItem = new JMenuItem("Export Data");
    menuBar.add(exportDataItem);
    fileChooser = new JFileChooser();
    fileChooser.addChoosableFileFilter(new PassengerFileFilter());
    contentPane = new JPanel();
    contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
    setContentPane(contentPane);
    contentPane.setLayout(null);
    classGroup = new ButtonGroup();
    //set up class radio
            classGroup.add(standardRadio);
            classGroup.add(businessRadio);

    JLabel nameLabel = new JLabel("Name: ");
    nameLabel.setFont(new Font("Tahoma", Font.PLAIN, 14));
    nameLabel.setBounds(12, 11, 69, 31);
    contentPane.add(nameLabel);
    JLabel cityLabel = new JLabel("City: ");
    cityLabel.setFont(new Font("Tahoma", Font.PLAIN, 14));
    cityLabel.setBounds(12, 47, 69, 31);
    contentPane.add(cityLabel);
    JLabel lblNewLabel_2 = new JLabel("Class: ");
    lblNewLabel_2.setFont(new Font("Tahoma", Font.PLAIN, 14));
    lblNewLabel_2.setBounds(12, 89, 69, 31);
    contentPane.add(lblNewLabel_2);
    nameField = new JTextField();
    nameField.setBounds(60, 18, 129, 20);
    contentPane.add(nameField);
    nameField.setColumns(10);
    cityField = new JTextField();
    cityField.setColumns(10);
    cityField.setBounds(60, 53, 129, 20);
    contentPane.add(cityField);
    JRadioButton businessRadio = new JRadioButton("Business");
    businessRadio.setBounds(58, 95, 109, 23);
    contentPane.add(businessRadio);
    JRadioButton standardRadio = new JRadioButton("Standard");
    standardRadio.setBounds(60, 127, 109, 23);
    contentPane.add(standardRadio);
    JButton btnSubmit = new JButton("Submit");
    btnSubmit.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent e) {
            String name = nameField.getText();
            String city = cityField.getText();
            String passengerClass = classGroup.getSelection().getActionCommand();

            PassengerFormEvent ev = new PassengerFormEvent(this, name, passengerClass, city);

Answer 1

您没有实例化ButtonGroup变量classGroup。您在null引用中使用dot(。(操作员。

这样做：

ButtonGroup classGroup = new ButtonGroup();

以及在此实例化之后的代码中的某个地方，执行此操作：

String passengerClass = classGroup.getSelection().getActionCommand(); 

Answer 2

您没有实例化 classGroup。您需要诸如： -

之类的代码

classGroup = new ButtonGroup();

在您可以在classGroup上调用方法。

编辑：在回答第二个问题时，这是因为您添加到对接的按钮也为无效。您需要在添加它们之前创建它们。