如何自定义 JComboBox 以便弹出窗口是 JTree（而不是列表）？

import javax.swing.*;
import javax.swing.tree.DefaultMutableTreeNode;
import javax.swing.tree.DefaultTreeModel;
import javax.swing.tree.TreeSelectionModel;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class CustomComboBoxExample {
    public static void main(String[] args) {
        JFrame frame = new JFrame("Custom JComboBox Example");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        // 创建一个自定义的JComboBox
        CustomComboBox customComboBox = new CustomComboBox();

        // 添加一些示例数据
        DefaultMutableTreeNode root = new DefaultMutableTreeNode("Root");
        DefaultMutableTreeNode node1 = new DefaultMutableTreeNode("Node 1");
        DefaultMutableTreeNode node2 = new DefaultMutableTreeNode("Node 2");
        DefaultMutableTreeNode node3 = new DefaultMutableTreeNode("Node 3");
        root.add(node1);
        root.add(node2);
        root.add(node3);
        customComboBox.setTreeModel(new DefaultTreeModel(root));

        // 添加JComboBox到窗口
        frame.getContentPane().add(customComboBox, BorderLayout.NORTH);
        frame.setSize(300, 200);
        frame.setVisible(true);
    }
}

class CustomComboBox extends JComboBox {
    private JTree tree;
    private JScrollPane scrollPane;

    public CustomComboBox() {
        // 创建JTree和JScrollPane
        tree = new JTree();
        tree.getSelectionModel().setSelectionMode(TreeSelectionModel.SINGLE_TREE_SELECTION);
        scrollPane = new JScrollPane(tree);

        // 设置弹出窗口的组件为JScrollPane
        setRenderer(new ComboBoxRenderer());
        setEditor(new ComboBoxEditor());

        // 添加事件监听器，当下拉箭头被点击时显示弹出窗口
        addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                showPopup();
            }
        });
    }

    public void setTreeModel(DefaultTreeModel treeModel) {
        tree.setModel(treeModel);
    }

    private void showPopup() {
        if (!isPopupVisible()) {
            setPopupVisible(true);
            SwingUtilities.invokeLater(new Runnable() {
                @Override
                public void run() {
                    scrollPane.requestFocus();
                }
            });
        }
    }

    class ComboBoxRenderer implements ListCellRenderer {
        private DefaultListCellRenderer defaultRenderer = new DefaultListCellRenderer();

        @Override
        public Component getListCellRendererComponent(JList list, Object value, int index, boolean isSelected, boolean cellHasFocus) {
            return defaultRenderer.getListCellRendererComponent(list, value, index, isSelected, cellHasFocus);
        }
    }

    class ComboBoxEditor implements ComboBoxEditor {
        @Override
        public Component getEditorComponent() {
            return scrollPane;
        }

        @Override
        public void setItem(Object anObject) {
        }

        @Override
        public Object getItem() {
            return null;
        }

        @Override
        public void selectAll() {
        }

        @Override
        public void addActionListener(ActionListener l) {
        }

        @Override
        public void removeActionListener(ActionListener l) {
        }
    }
}