Listing File Hierarchy in console using a Tree structure

Question

first off i'm a college student. i'm not that good at java... we got this CA in class and try as i might i just can't get my head around it
i was wondering if someone who know a bit more about java then i do would point me in the right direction, were i'm going wrong in my code
i have to list out sub-files and sub-directorys of a folder (i.e. C:/test) to console using tree structure
like this
startingdir
dir1 //subfolder of startingdir
dir11 //subfolder of dir1
dir111 //subfolder of dir11
dir12 //subfolder of dir1
file1A // document on dir1
dir2 //subfolder of startingdir
Tree.java
import java.util.Iterator;
import java.util.LinkedList;
import java.util.ListIterator;
import java.util.NoSuchElementException;
import java.util.Deque;
public class Tree<E> {
    // Each Tree object is an unordered tree whose
    // elements are arbitrary objects of type E.
    // This tree is represented by a reference to its root node (root), which
    // is null if the tree is empty. Each tree node contains a link to its
    // parent and a LinkedList of child nodes
    private Node root;
    //////////// Constructor ////////////
    public Tree () {
    // Construct a tree, initially empty.
        root = null;
    //////////// Accessors ////////////
    public boolean isEmpty () {
    // Return true is and only if this tree is empty.
         return (root == null);
    public Node root () {
    // Return the root node of this tree, or null if this tree is empty.
        return root;
    public Node parent (Node node) {
    // Return the parent of node in this tree, or null if node is the root node.
        return node.parent;
    public void makeRoot (E elem) {
    // Make this tree consist of just a root node containing element elem.
        root = new Node(elem);
    public Node addChild (Node node, E elem) {
    // Add a new node containing element elem as a child of node in this
    // tree. The new node has no children of its own. Return the node
    // just added.
        Node newChild = new Node(elem);
        newChild.parent = node;
        node.children.addLast(newChild);
        return newChild;
    public E element (Node node) {
         return node.getElement();
    //////////// Iterators ////////////
    public Iterator childrenIterator (Node node) {
        return node.children.iterator();
    public Iterator nodesPreOrder () {
    // Return an iterator that visits all nodes of this tree, with a pre-order
    // traversal.
        return new Tree.PreOrderIterator();
    //////////// Inner classes ////////////
    public class Node {
        // Each Tree.Node object is a  node of an
        // unordered tree, and contains a single element.
        // This tree node consists of an element (element),
        // a link to its parent
        // and a LinkedList of its children
        private E element;
        private Node parent;
        private LinkedList<Node> children;
        private Node (E elem) {
            // Construct a tree node, containing element elem, that has no
            // children and no parent.
            this.element = elem;
            this.parent = null;
            children = new LinkedList<Node>();
        public E getElement () {
        // Return the element contained in this node.
            return this.element;
        public String toString () {
        // Convert this tree node and all its children to a string.
            String children = "";
            // write code here to add all children
            return element.toString() + children;
        public void setElement (E elem) {
        // Change the element contained in this node to be elem.
            this.element = elem;
    public class PreOrderIterator implements Iterator {
        private Deque<Node> track; //Java recommends using Deque rather
        // than Stack. This is used to store sequence of nomempty subtrees still
        //to be visited
        private PreOrderIterator () {
            track = new LinkedList();
            if (root != null)
                track.addFirst(root);
        public boolean hasNext () {
            return (! track.isEmpty());
        public E next () {
            Node place = track.removeFirst();
            //stack the children in reverse order
            if (!place.children.isEmpty()) {
                int size = place.children.size(); //number of children
                ListIterator<Node> lIter =
                        place.children.listIterator(size); //start iterator at last child
                while (lIter.hasPrevious()) {
                    Node element = lIter.previous();
                    track.addFirst(element);
            return place.element;
        public void remove () {
            throw new UnsupportedOperationException();
    FileHierarchy.java
import java.io.File;
import java.util.Iterator;
public class FileHierarchy {
    // Each FileHierarchy object describes a hierarchical collection of
    // documents and folders, in which a folder may contain any number of
    // documents and other folders. Within a given folder, all documents and
    // folders have different names.
    // This file hierarchy is represented by a tree, fileTree, whose elements
    // are Descriptor objects.
    private Tree fileTree;
    //////////// Constructor ////////////
    public FileHierarchy (String startingDir, int level) {
        // Construct a file hierarchy with level levels, starting at
        // startingDir
        // Can initially ignore level and construct as many levels as exist
        fileTree = new Tree();
        Descriptor descr = new Descriptor(startingDir, true);
        fileTree.makeRoot(descr);
        int currentLevel = 0;
        int maxLevel = level;
        addSubDirs(fileTree.root(), currentLevel, maxLevel);
    //////////// File hierarchy operations ////////////
    private void addSubDirs(Tree.Node currentNode, int currentLevel,
            int maxLevel) {
    // get name of directory in currentNode
    // then find its subdirectories (can add files later)
    // for each subdirectory:
    //    add it to children of currentNode - call addChild method of Tree
    //    call this method recursively on each child node representing a subdir
    // can initially ignore currentLevel and maxLevel   
        Descriptor descr = (Descriptor) currentNode.getElement();
        File f = new File(descr.name);
        File[] list = f.listFiles();
        for (int i = 0; i < list.length; ++i) {
            if (list.isDirectory()) {
File[] listx = null;
fileTree.addChild(currentNode, i);
if (list[i].list().length != 0) {
listx = list[1].listFiles();
addSubDirs(currentNode,i,1);
} else if (list[i].isFile()) {
fileTree.addChild(currentNode, i);
// The following code is sample code to illustrate how File class is
// used to get a list of subdirectories from a starting directory
// list now contains subdirs and files
// contained in dir descr.name
////////// Inner class for document/folder descriptors. //////////
private static class Descriptor {
// Each Descriptor object describes a document or folder.
private String name;
private boolean isFolder;
private Descriptor (String name, boolean isFolder) {
this.name = name;
this.isFolder = isFolder;
FileHierarchyTest.javapublic class FileHierarchyTest {
private static Tree fileTree;
public static void main(String[] args) {
FileHierarchy test = new FileHierarchy ("//test", 1);
System.out.println(test.toString());

Answer 1

Denis,
Do you have [red hair|http://www.dennisthemenace.com/]? ;-)
My advise with the tree structure is pretty short and sweet... make each node remember
1. it's parent
2. it's children
That's how the file system (inode) actually works.
<quote>
The exact reasoning for designating these as "i" nodes is unsure. When asked, Unix pioneer Dennis Ritchie replied:[citation needed]
In truth, I don't know either. It was just a term that we started to use. "Index" is my best guess, because of the
slightly unusual file system structure that stored the access information of files as a flat array on the disk, with all
the hierarchical directory information living aside from this. Thus the i-number is an index in this array, the
i-node is the selected element of the array. (The "i-" notation was used in the 1st edition manual; its hyphen
became gradually dropped).</quote>