public static IdGenerator decrementingLongs() {
    AtomicLong longs = new AtomicLong();
    return () -> Long.toString(longs.decrementAndGet());
  }
@Test
  public void decrementing() {
    IdGenerator gen = IdGenerators.decrementingLongs();
    assertThat(gen.getId(), equalTo("-1"));
    assertThat(gen.getId(), equalTo("-2"));
  }