private static class OutputProbe
            implements Output<String, String> {

        public List<String> keys = Collections
                .synchronizedList(new ArrayList<String>());

        public List<String> values = Collections
                .synchronizedList(new ArrayList<String>());

        public void write(String key, String value) {
            if (key == null || value == null) {
                keys.size();
            }
            keys.add(key);
            values.add(value);
        }

        public void close() {
            // Do nothing
        }
    }