public void storeData(Data data) {
    dataStore.put(data.getKey(), data);
  }
@Test
  void testStoreData() {
    try {
      shard.storeData(data);
      var field = Shard.class.getDeclaredField("dataStore");
      field.setAccessible(true);
      var dataMap = (Map<Integer, Data>) field.get(shard);
      assertEquals(1, dataMap.size());
      assertEquals(data, dataMap.get(1));
    } catch (NoSuchFieldException | IllegalAccessException e) {
      fail("Fail to modify field access.");
    }

  }