@Override
    public int size() {
        return topicNames.size();
    }
@Test
    public void testSize() {
        Set<String> topicNames = mkSet("foo", "bar", "baz");
        Set<Uuid> topicIds = new TopicIds(topicNames, TopicsImage.EMPTY);
        assertEquals(topicNames.size(), topicIds.size());
    }