java - Arraylist到DefaultComboBoxModel

我试图通过刷新按钮更新JCombobox的内容，并且为此，我尝试使用Defaultcombobox模型。下面有我的代码：

public GUI2() {
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setBounds(100, 100, 450, 300);
    contentPane = new JPanel();
    contentPane.setLayout(new BorderLayout(0, 0));
    contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
    setContentPane(contentPane);
    StickWidget Left = new StickWidget();
    Left.setName("Left Stick");
    contentPane.add(Left.draw(), BorderLayout.WEST);
    StickWidget Right = new StickWidget();
    Right.setName("Right Stick");
    contentPane.add(Right.draw(), BorderLayout.EAST);

    JPanel panel = new JPanel();
    contentPane.add(panel, BorderLayout.CENTER);
    panel.setLayout(new BorderLayout(0, 0));

    DefaultComboBoxModel newModel = new DefaultComboBoxModel(ReadJoystick.read().toArray());

    JPanel panel_1 = new JPanel();
    panel.add(panel_1, BorderLayout.NORTH);
    combobox = new JComboBox(newModel);
    combobox.setPrototypeDisplayValue("XXXXXXX");
    panel_1.add(combobox);
    JButton btnRefresh = new JButton("Refresh");
    btnRefresh.addActionListener(this);
    panel_1.add(btnRefresh);
    newModel = new DefaultComboBoxModel();
}

public void actionPerformed(ActionEvent e) {
    newModel.removeAllElements();

        newModel.addElement(ReadJoystick.read().toArray());

    combobox.setModel(newModel);
}

ReadJoystick.read（）返回一个应该转换为数组并传递到新模型中的arraylist。每当我按下刷新按钮时，java都会引发空指针异常，我也不知道为什么。

为什么我的程序无法按原样运行？

使用NewModel的正确方法是什么？

谢谢！

编辑：错误：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at xxx.xxxxx.xxxxxx.xxx.GUI2.actionPerformed(GUI2.java:78)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
at java.awt.Component.processMouseEvent(Component.java:6516)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3321)
at java.awt.Component.processEvent(Component.java:6281)
at java.awt.Container.processEvent(Container.java:2229)
at java.awt.Component.dispatchEventImpl(Component.java:4872)
at java.awt.Container.dispatchEventImpl(Container.java:2287)
at java.awt.Component.dispatchEvent(Component.java:4698)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
at java.awt.Container.dispatchEventImpl(Container.java:2273)
at java.awt.Window.dispatchEventImpl(Window.java:2719)
at java.awt.Component.dispatchEvent(Component.java:4698)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:747)
at java.awt.EventQueue.access$300(EventQueue.java:103)
at java.awt.EventQueue$3.run(EventQueue.java:706)
at java.awt.EventQueue$3.run(EventQueue.java:704)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
at java.awt.EventQueue$4.run(EventQueue.java:720)
at java.awt.EventQueue$4.run(EventQueue.java:718)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:717)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)

最佳答案

问题出在构造函数中，我认为您有newModel变量作为实例变量，但是您正在创建新变量：

DefaultComboBoxModel newModel = new DefaultComboBoxModel(ReadJoystick.read().toArray());

改写成

newModel = new DefaultComboBoxModel(ReadJoystick.read().toArray());

这个说法：

newModel = new DefaultComboBoxModel();

在构造函数中仍然是局部变量，因此我将删除此行或使用this.newModel代替