public class FileDirectoryFilter extends JFrame{
    FileDirectoryFilter(){
        DefaultMutableTreeNode node = new DefaultMutableTreeNode("Root");
        node.setUserObject("Start directory");
        node.add(new DefaultMutableTreeNode("child1")); //искуственно наполнение. удалить
        JTree tree = new JTree(node);
        //add(tree);
        JScrollPane treeView = new JScrollPane(tree);
        add(treeView);
        setVisible(true);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        pack();
    }
    static ArrayList<String> scan_path_for_files(File path){
        ArrayList<String> files = new ArrayList<>();
        for(File file: path.listFiles(new FileFilter() {
            @Override
            public boolean accept(File pathname) {
                String extension = null;
                int i = path.toString().lastIndexOf('.');
                if(path.isDirectory()){
                    extension = path.toString().substring(i+1);
                    return extension.equals("xml");
                }
                return false;
            }
        }))
            files.add(file.getName());
        return files;
    }
    static String intent ="";
    static void walk_throw_directory(File start_dir){
        intent+= "---|";
        for (File path:start_dir.listFiles())
        if (path.isDirectory()){
            System.out.println(intent + path.getName());
            walk_throw_directory(path);
        }
        for(String file_path: scan_path_for_files(start_dir)){
            System.out.println(intent+ file_path);
            intent = intent.substring(0,intent.length() -4);
        }
    }
    public static void main(String[] args) {
        new FileDirectoryFilter();
        File path = new File("D:/Projects");
    if(!path.isDirectory()){
        System.err.println("Path " + path + " is not a directory");
        System.exit(1);
    }
    walk_throw_directory(path);
    }
}