java - Iterate through an AVL tree -

i have implemented avl tree , wanted write iterator, searches in pre order. have peace of code , nullpointer in "stack.push(current)" , dont know why

@override public iterator<e> iterator() {      return new iterator<e>(){         node current;         int counter;         stack<node> stack;          public void iterator() {             counter++;             stack = new stack<node>();             current = root;             stack.empty();         }         @override         public boolean hasnext() {             if(counter == count(root)) return false;             else return true;         }          @override         public e next() {             stack.push(current);             counter++;             if(current.left.value != null) return current.left.value;             else return current.right.value;         }      }; } 

thanks in beforehand :)

your root object never declared or instantiated. setting current = root; in current context, root nothing.

public void iterator() {         counter++;         stack = new stack<node>();         current = root;         stack.empty();     }