Help using ListIterator to traverse a tree

Question

Need help traversing a tree.
I have built the tree and now I am trying to traverse it to print it out. The main tree is a LinkedList, and each object on the list has a LinkedList of "children" which may or may not be empty.
My thought was to have a local ListIterator in each LinkedList and use hasNext() and next() to return the objects. So I build the tree, then I try to reset the local iterator for everyone in the tree. But it's not working. If I initialize the ListIterator in the LinkedList class, I get a "ConcurrentModificationException" when I try to access the list. Or if I don't initialize the ListIterator but try to set it later, I get a "NullPointerException" at the same point.
My questions are these:
Should I be able to keep a local ListIterator in the LinkedList?
And if so, am I doing it right?
Should I be able to "reset" the ListIterator after having created it?
And if so, am I doing that right?
Is there a different way I should be traversing my tree?
And, my burning question about this whole ListIterator thing: It appears from the online docs that a ListIterator is an Interface and not an Object. How can the listIterator method return an Interface?
Here's my code:
// the tree class
public class PartTree
     private Part          rootPart;
     private File          dataModel;          
     private PartList     theTree;          
     private Part          currentPart;
     public PartTree(String dmPath, String rootPartType)
               throws FileNotFoundException, IOException
          rootPart     = new Part();
          dataModel     = new File(dmPath);
          lostKids     = new PartList();
          theTree          = new PartList();
          rootPart.setName(rootPartType);
          rootPart.setChildList(theTree);
          refresh();
     public void refresh()
               throws FileNotFoundException, IOException
          theTree = new PartList();
          String            line;          
          Part           ptr;          
          try
               BufferedReader in =
                     new BufferedReader(new FileReader(dataModel));
               line     = new String(in.readLine());
               line = in.readLine();
               while(null != line)
                     ptr = new Part(line);
                     insertPart(ptr);
                     line = in.readLine();
          catch(FileNotFoundException e)
               throw new FileNotFoundException(e.getMessage());
          theTree.resetTreeIter();
          currentPart = rootPart;
// the linked list class
public class PartList extends LinkedList
     private Iterator listIter;               
     //private Iterator listIter = listIterator(0);     
     public synchronized void resetTreeIter()
          listIter = listIterator(0);
          Part ptr;
          Iterator it = listIterator(0);
          while( it.hasNext() )
               ptr = (Part)it.next();
               if( ptr.hasKids() )
                    ptr.getChildList().resetTreeIter();
     public synchronized Part nextPart()
          if( listIter.hasNext() )
               return (Part)listIter.next();
          return null;
}I know I am getting to the resetTreeIter method because when I replaced all the code there with a new exception, that got displayed.
Thanks for any help,
Connie

Answer 1

Much though I hate the Visitor pattern, this does look like a good use of it. If you define an interface public interface PartVisitor
    public void visit(Part part);
} and add a method to Part public void acceptPreOrder(PartVisitor visitor)
    visitor.visit(this);
    if (hasKids())
        Iterator it = getChildList().iterator();
        while (it.hasNext())
            it.next().acceptPreOrder(visitor);
}Then to print them, you merely say topLevelPart.accept(
    new PartVisitor()
        public void visit(Part part)
            System.out.println(part);
    });/