无法从 jcombobox 中删除第一个元素

Question

我无法从 jcombobox 中删除第一个元素。我的代码如下，

JComboBox cBox= cBox= new JComboBox();
...    
while (cBox.getItemCount() > 0)
  cBox.removeItemAt(0);

对于测试运行，我在 cBox 中有 3 个项目。当它到达 removeItemAt(0) 时，调试变得混乱，进入了一些绝对不相关的文件访问代码。这样做两次然后得到以下异常。我尝试了直接得到相同异常的 removeAllItems() 。但是， removeItem(1) 可以正常工作，直到只剩下 1 个元素。该异常不会使应用程序崩溃，之后我在组合框中看不到任何项目，因此它工作了一点。我到底做错了什么。

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at util.Gui$4.actionPerformed(Gui.java:111)
at javax.swing.JComboBox.fireActionEvent(Unknown Source)
at javax.swing.JComboBox.contentsChanged(Unknown Source)
at javax.swing.AbstractListModel.fireContentsChanged(Unknown Source)
at javax.swing.DefaultComboBoxModel.setSelectedItem(Unknown Source)
at javax.swing.DefaultComboBoxModel.removeElementAt(Unknown Source)
at javax.swing.JComboBox.removeItemAt(Unknown Source)
at util.Gui.prepareSubLists(Gui.java:164)
at util.Gui$3.actionPerformed(Gui.java:97)
at javax.swing.JComboBox.fireActionEvent(Unknown Source)
at javax.swing.JComboBox.setSelectedItem(Unknown Source)
at javax.swing.JComboBox.setSelectedIndex(Unknown Source)
at javax.swing.plaf.basic.BasicComboPopup$Handler.mouseReleased(Unknown Source)
at java.awt.AWTEventMulticaster.mouseReleased(Unknown Source)
at java.awt.Component.processMouseEvent(Unknown Source)
at javax.swing.JComponent.processMouseEvent(Unknown Source)
at javax.swing.plaf.basic.BasicComboPopup$1.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)

Answer 1

你的条件语句是不是错了？将while替换为if，这样

if(cBox.getItemCount() > 0){
  cBox.removeItemAt(0);
}

---

这是SSCCE：

public final class JComboBoxDemo {
    public static void main(String[] args){
        SwingUtilities.invokeLater(new Runnable(){
            @Override
            public void run() {
                createAndShowGUI();             
            }
        });
    }

    public static void createAndShowGUI(){
        final JFrame frame = new JFrame("JComboBox Demo");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().setLayout(new FlowLayout());
        frame.getContentPane().add(JComboPane.newInstance());
        frame.setSize(new Dimension(250, 100)); // for demonstration purposes only
        //frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

    private static final class JComboPane extends JPanel{
        private JComboPane(){
            super();
            setLayout(new BoxLayout(this, BoxLayout.Y_AXIS));
            JCenteredComboBox comboBox = JCenteredComboBox.newInstance();
            JCenteredButton button = JCenteredButton.newInstance(comboBox);
            add(comboBox);
            add(button);
        }

        public static final JComboPane newInstance(){
            return new JComboPane();
        }

        private static final class JCenteredComboBox extends JComboBox{
            private JCenteredComboBox(){
                super(new String[]{"Item 1", "Item 2", "Item 3"});
                setAlignmentX(Component.CENTER_ALIGNMENT);
            }

            public static final JCenteredComboBox newInstance(){
                return new JCenteredComboBox();
            }
        }

        private static final class JCenteredButton extends JButton{
            private JCenteredButton(final JComboBox comboBox){
                super("Remove First Item");
                setAlignmentX(Component.CENTER_ALIGNMENT);
                addActionListener(new ActionListener(){
                    @Override
                    public void actionPerformed(ActionEvent e) {
                        if(comboBox.getItemCount() > 0){
                            comboBox.removeItemAt(0); // your logic
                        }
                    }
                });
            }

            public static final JCenteredButton newInstance(final JComboBox comboBox){
                return new JCenteredButton(comboBox);
            }
        }
    }
}

当你运行它时，按下JButton 将删除JComboBox 中的第一项。你可以一直按到它为空。

Answer 2

这个异常可能是因为当一个组合项被移除时触发了一个事件，而在这个事件处理方法中你仍然引用组合框项。

例如，当您使用 combo.removeItemAt(0) 或 removeAllItems() 从组合框中删除代码中的某个位置（除了 actionPeformed() 之外）时，仍然会触发/执行事件 actionPerformed。但通常 actionPerformed() 方法包含对用户操作（用户单击组合框上的某处）做出反应的代码。因此，当最后一个项目被删除时，组合框中将不再有任何项目，并且在 actionPerformed() 中对项目或索引的任何引用都会导致异常。

解决方案是将代码从 actionPerformed 移动到例如mouseClicked() 或其他事件处理程序，具体取决于您要执行的操作。