为什么在JTree中不显示下拉箭头？

Question

我有这个

JFileTree

：

public class JFileTree extends JPanel {
    private final FileSystemView fileSystemView;
    private final JTree tree;
    private DefaultTreeModel treeModel;
    public JFileTree() {

        setLayout(new BorderLayout(3, 3));
        setBorder(new EmptyBorder(5, 5, 5, 5));
        fileSystemView = FileSystemView.getFileSystemView();
        DefaultMutableTreeNode root = new DefaultMutableTreeNode();
        treeModel = new DefaultTreeModel(root);
        TreeSelectionListener treeSelectionListener = tse -> {
            DefaultMutableTreeNode node = (DefaultMutableTreeNode) tse.getPath().getLastPathComponent();
            showChildren(node);
            setFileDetails((File) node.getUserObject());
        };
        File[] roots = fileSystemView.getRoots();
        for (File fsr : roots) {
            DefaultMutableTreeNode node = new DefaultMutableTreeNode(fsr, true);
            root.add(node);
            File[] files = fileSystemView.getFiles(fsr, true);
            for (File file : files) {
                if (file.isDirectory()){
                    node.add(new DefaultMutableTreeNode(file, true));
                }

            }
        }
        tree = new JTree(treeModel);
        tree.setRootVisible(false);
        tree.setShowsRootHandles(true);
        tree.addTreeSelectionListener(treeSelectionListener);
        tree.setCellRenderer(new FileTreeCellRenderer());
        tree.expandRow(0);
        showRootFile();
        JScrollPane treeScroll = new JScrollPane(tree);
        treeScroll.setPreferredSize(new Dimension(200, (int) treeScroll.getPreferredSize().getHeight()));
        add(treeScroll, BorderLayout.CENTER);
        try {
            UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
        } catch (ClassNotFoundException | UnsupportedLookAndFeelException | IllegalAccessException |
                 InstantiationException ignored) {}
    }
    public void showRootFile() {
        tree.setSelectionInterval(0, 0);
    }
    private void showChildren(final DefaultMutableTreeNode node) {
        tree.setEnabled(false);
        SwingWorker<Void, File> worker = new SwingWorker<Void, File>() {
            @Override
            public Void doInBackground() {
                File file = (File) node.getUserObject();
                if (file.isDirectory()) {
                    File[] files = fileSystemView.getFiles(file, true); 
                    if (node.isLeaf()) {
                        for (File child : files)
                            if (child.isDirectory())publish(child);
                    }
                }
                return null;
            }
            @Override
            protected void process(List<File> chunks) {
                for (File child : chunks) {
                    node.add(new DefaultMutableTreeNode(child));
                }
            }
            @Override
            protected void done() {
                tree.setEnabled(true);
            }
        };
        worker.execute();
    }
    private void setFileDetails(File file) {
        fileSystemView.getSystemIcon(file);
    }
    public static void main(String[] args) {
        JFrame f = new JFrame("Project view");
        f.setContentPane(new JFileTree());
        f.pack();
        f.setMinimumSize(f.getSize());
        f.setVisible(true);
    }
}
class FileTreeCellRenderer extends DefaultTreeCellRenderer {
    private FileSystemView fileSystemView;
    private JLabel label;
    public FileTreeCellRenderer() {
        label = new JLabel();
        label.setOpaque(true);
        fileSystemView = FileSystemView.getFileSystemView();
    }
    @Override
    public Component getTreeCellRendererComponent(JTree tree, Object value, boolean selected, boolean expanded, boolean leaf, int row, boolean hasFocus) {
        DefaultMutableTreeNode node = (DefaultMutableTreeNode) value;
        File file = (File) node.getUserObject();
        label.setText(fileSystemView.getSystemDisplayName(file));
        label.setToolTipText(file.getPath());
        boolean hasChildren = node.getChildCount() > 0;
        if (hasChildren) {
            if (expanded) {
                label.setIcon(fileSystemView.getSystemIcon(file));
            } else {
                label.setIcon(fileSystemView.getSystemIcon(file.getParentFile()));
            }
        } else {
            label.setIcon(fileSystemView.getSystemIcon(file));
        }

        if (selected) {
            label.setBackground(backgroundSelectionColor);
            label.setForeground(textSelectionColor);
        } else {
            label.setBackground(backgroundNonSelectionColor);
            label.setForeground(textNonSelectionColor);
        }

        return label;
    }
}

信用：@Andrew Thompson （对不起，如果它有点长，它只是 Andrew Thompson 的 FileBro 的精简版）

我的问题是下拉箭头不会出现在每个节点上，除非我单击该节点。我想要的是在应用程序初始化时显示它。

我尝试设置

tree.setShowsRootHandles(true);

，但它只适用于最顶层目录。但它没有用。我请 OpenAI 的 ChatGPT 帮我调试这个，但即使经过几个小时的调试，仍然没有任何进展（除了随机试错）。我究竟做错了什么？我不是一个非常有经验的 swing 开发人员，所以如果这是一个简单的修复，请多多包涵。这是它的样子： 这就是我想要的样子：