Not much can be said, but welcome to Sun documentation :)

--
Tony Morris
http://tmorris.net/

Java Questions and Answers
http://jqa.tmorris.net/

Did you replace the ... parts with appropriate "null" returns? If i do
that, it compiles fine for me.

Also note that the StackIterator class is not a local class, but an
inner class of Stack. If it where a local class the code would look
like this:

public class Stack {
   private ArrayList<Object> items;

   //code for Stack's methods and constructors
   //not shown

   public Iterator<Object> iterator() {
       class StackIterator implements Iterator {
           int currentItem = items.size() - 1;

           public boolean hasNext() {
                return false; // implement!
           }

           public ArrayList<Object> next() {
                throw new NoSuchElementException // implement!
           }

           public void remove() {
                throw new UnsupportedOperationException();
           }
      }
      return new StackIterator();
   }
}