Java Swing GUI组合框问题

Question

我的swing GUI出现了一个奇怪的问题。我正在尝试刷新按钮清除组合框列表以刷新组件。它不起作用，我可能只是犯了一些愚蠢的错误。这是我的代码：

public class GUI2 extends JFrame implements ActionListener {

/**
 * 
 */
private JPanel contentPane;
public JComboBox combobox;
public ReadJoystick stick;
private JButton refresh;
/**
 * Launch the application.
 */
public static void main(String[] args) {
    EventQueue.invokeLater(new Runnable() {
        public void run() {
            try {
                GUI2 frame = new GUI2();
                frame.setVisible(true);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    });
}

/**
 * Create the frame.
 */
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));

    JPanel panel_1 = new JPanel();
    panel.add(panel_1, BorderLayout.NORTH);
    ReadJoystick stick = new ReadJoystick();
    JComboBox combobox = new JComboBox<Object>(stick.read().toArray());
    combobox.setPrototypeDisplayValue("XXXXXXX"); 
    panel_1.add(combobox);
    JButton btnRefresh = new JButton("Refresh");
    btnRefresh.addActionListener(this);
    panel_1.add(btnRefresh);
}

public void actionPerformed(ActionEvent e) {
    combobox.removeAll();
   }
}

此代码当前读取计算机可用的操纵杆，并将其显示在组合框列表中。下面是我当前GUI的图片：

每次按下刷新按钮都会抛出此错误：

typeException in thread "AWT-EventQueue-0" java.lang.NullPointerException
at xxxxxxxx.xxxxxx.xxxxxx.GUI2.actionPerformed(GUI2.java:74)
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)

Answer 1

removeAll是一个继承自Container的方法，用于删除容器中包含的所有子组件......实际上并不是您想要做的。

相反，你可以使用combobox.setModel(null)或combobox.setModel(new DefaultComboBoxModel())来改变支持空组合之前的组合框的数据/模型

使用单独的“模型”回摆其中许多组件，这样可以轻松切换视图显示的内容

NullPointerException来自于您在构造函数中隐藏了combobox变量的事实

public GUI2() {
    //...
    JComboBox combobox = new JComboBox<Object>(stick.read().toArray());

这会将类实例字段保留为null，而应该只读取

combobox = new JComboBox<Object>(stick.read().toArray());

有关详细信息，请查看How to Use Combo Boxes