id   parent   name
1    0        Программирование
2    1        Java
3    0        Базы данных
4    3        MySql

private void categoriesTree(int id, int depth) {

        LinkedHashMap<Integer, HashMap> Tree = new LinkedHashMap();

        try {

            ResultSet rs = stmt.executeQuery("SELECT * FROM `categories` WHERE `parent`=" + id + ";");

            while (rs.next()) {

                HashMap<String, String> content = new HashMap();

                content.put("name", rs.getString("name"));
                content.put("hurl", rs.getString("hurl"));
                content.put("depth", Integer.toString(depth));

                CategoriesTree.put(rs.getInt("id"), content);

                categoriesTree(rs.getInt("id"), depth);

            }

        } catch (SQLException e) {
            System.out.println(e);
        }
}

java.sql.SQLException: Operation not allowed after ResultSet closed

private class Node {

        int id;
        int parentId;
        String name;
        List<Node> children;
    }

    public ArserTest() {

        Map<Integer, Node> category = new LinkedHashMap();

        Node node = new Node();
        node.id = 1;
        node.parentId = 0;
        node.name = "Программирование";
        node.children = new LinkedList();
        category.put(1, node);

        node = new Node();
        node.id = 2;
        node.parentId = 1;
        node.name = "Web";
        node.children = new LinkedList();
        category.put(2, node);

        node = new Node();
        node.id = 3;
        node.parentId = 2;
        node.name = "Java, PHP";
        node.children = new LinkedList();
        category.put(3, node);


        HashSet<Integer> delete = new HashSet();

        for (Map.Entry<Integer, Node> entry : category.entrySet()) {
            if (category.containsKey(entry.getValue().parentId)) {
                category.get(entry.getValue().parentId).children.add(entry.getValue());
                delete.add(entry.getKey());
            }
        }

        for (Integer element : delete) {
            category.remove(element);
        }

        // вывод категорий
        for (Map.Entry<Integer, Node> entry : category.entrySet()) {
            System.out.println(category.get(entry.getKey()).name);
            ListIterator<Node> itr = category.get(entry.getKey()).children.listIterator();

            while (itr.hasNext()) {
                Node n = itr.next();
                System.out.println(n.name);
                
                ListIterator<Node> itr2 = n.children.listIterator();
                while (itr2.hasNext()) {
                    System.out.println(itr2.next().name);
                }
                
            }
        }


        //System.out.println(category.get(1).children.get(0).children.get(0).name);
        //System.out.println(category);

    }