@Override
public Event next() {
if (!walker.isOpen()) throw new IllegalStateException();
fetchNextIfNeeded();
if (next == null) throw new NoSuchElementException();
Event result = next;
next = null;
return result;
}
@Override
public boolean hasNext() {
if (!walker.isOpen()) throw new IllegalStateException();
fetchNextIfNeeded();
return next != null;
}