public String hash() {
    try (LockResource r = new LockResource(mLock.readLock())) {
      return mHash.get();
    }
  }
@Test
  public void hashEmpty() {
    PathProperties emptyProperties = new PathProperties();
    String hash = emptyProperties.hash();
    Assert.assertNotNull(hash);
    Assert.assertEquals(hash, emptyProperties.hash());
  }