public Object retrieve(Comparable obj, BTNode aNode)
   {
      Comparable answer = null;
      
      if(obj.compareTo(aNode.getData()) == 0)
         answer = (Comparable)aNode.getData();
      else if(obj.compareTo(aNode.getData())>0 && !aNode.isLeaf())
         answer = (Comparable)retrieve(obj, aNode.getRight());
      else if(obj.compareTo(aNode.getData())<0 && !aNode.isLeaf())
         answer = (Comparable)retrieve(obj, aNode.getLeft());
      
      return answer; 
   }