### Question:
QueryIndex { private static <V> QueryIndex<V> empty(Registry registry) { return new QueryIndex<>(registry, null); } private  QueryIndex(Registry registry, String key); static QueryIndex<V> newInstance(Registry registry); QueryIndex<T> add(Query query, T value); boolean remove(T value); boolean isEmpty(); List<T> findMatches(Id id); void forEachMatch(Id id, Consumer<T> consumer); @Override String toString();  }### Answer:
@Test public void empty() { QueryIndex<Integer> idx = QueryIndex.newInstance(registry); Assertions.assertTrue(idx.findMatches(id("a")).isEmpty()); }