Binary Tree Problem, help !!!!!!!

Question

Hi I have some Tree Traversal problem,
my output has this problem,what's going on????
Exception in thread "main" java.lang.NullPointerException
at Tree.inTraversal2(Tree.java:150)
at TreeTest.main(TreeTest.java:33)
My Tree.java:
class TreeNode {
   TreeNode leftNode;          
   TreeNode rightNode;
   int data;
   public TreeNode(int nodeData)
      data = nodeData;             
      leftNode = rightNode = null;
   public void insert(int value)
      if( value < data )
         if( leftNode == null )
            leftNode = new TreeNode(value);
         else
            leftNode.insert(value);
      else if ( value > data )
         if( rightNode == null )
            rightNode = new TreeNode(value);
         else
            rightNode.insert(value);
   }  //end method insert
}  //end class TreeNode
public class Tree {
   TreeNode root;
   Stack s;
   public Tree()
      root = null;
   public void insertNode(int value)
      if( root == null )
         root = new TreeNode(value);
      else
         root.insert(value);
   public void inTraversal2(TreeNode node)
         if( node == null )
         return;
     TreeNode p;
     p = node;
     do
            while(p != null)
                 s.push(p); //problem comes in here
                 p = p.leftNode;
             if(!s.empty())
                 p = s.pop();
                 System.out.print( p.data + " " );
                 p = p.rightNode;
               }while( (p != null) || (!s.empty()));
}  //end class TreeMy Stack.java:
public class Stack {
      TreeNode items[];
     int StackSize;
     int top;
    public Stack(int size)
         StackSize = size;
        items = new TreeNode[size];
        top = -1;
    public boolean empty()
         if(top == -1)
           return true;
        else
          return false;
    public boolean full()
         if(top == StackSize-1)
           return true;
         else
           return false;
    public void push(TreeNode c)
        if(full() == true)
           System.out.println("stack overflow");
        else
            items[++top] = c;
    public TreeNode pop()
         if(empty())
           System.out.println("stack underflow");
        return items[top--];
}

Answer 1

You don't seem to instantiate Stack anywhere.