@Test
public void testConcurrentMap() throws B2JsonException {

    ConcurrentMap<LocalDate, Integer> map = new ConcurrentHashMap<>();
    map.put(LocalDate.of(2015, 5, 2), 7);
    ConcurrentMapHolder holder = new ConcurrentMapHolder(map);
    String json =
            "{\n" +
            "  \"map\": {\n" +
            "    \"20150502\": 7\n" +
            "  }\n" +
            "}" ;
    assertEquals(json, b2Json.toJson(holder));
}