public class TreeNode implements Position {

  private Object element;   // the data object stored at this position
  private TreeNode leftChild;      // leftmost child; null if no children
  private TreeNode rightSibling;   // next child to the right; null if last
  private TreeNode parent;         // parent; null if the root

  // *** BASIC ACCESSORS FOR THE FIELDS
 
  public Object element() {   // required to implement the Position interface
    return element;
  }

  public TreeNode leftChild() {
    return leftChild;
  }

  public TreeNode rightSibling() {
    return rightSibling;
  }

  public TreeNode parent() {
    return parent;
  }

  // *** CONVENIENCE METHODS

  public boolean isInternal() {
    return (leftChild != null);
  }

  public boolean isRoot() {
    return (parent == null);
  }

  // *** ITERATORS

  /* Iterates over all children from left to right. */
  public TreeChildrenIterator children() {
    return new TreeChildrenIterator(this);
  }

  /* Iterates over all proper ancestors from bottom to top. */
  public TreeAncestorsIterator ancestors() {  
    return new TreeAncestorsIterator(this);
  }

  /* Iterates over this node and all descendants, in prefix order. */
  public TreePreorderIterator descendants_preorder() {   
    return new TreePreorderIterator(this);
  }

  public TreePreorderIterator descendants_proper_preorder() {   
    TreePreorderIterator iter = descendants_preorder();
    iter.next();   // get and throw away first return value, namely "this"
    return iter;   // now the iterator will only return proper descendants
  }


  // *** EXAMPLE
  // Typical use of an iterator is shown by this code, which prints out
  // a node and its descendants in prefix order:
  //
  //   Iterator iter = p.descendants_preorder();
  //   while (i.hasNext()) 
  //      ((String)i.next()).print();

}