@Override
  public long getRef(String uuid) {
    Long ref = refsByUuid.get(requireNonNull(uuid, "uuid can not be null"));
    checkState(ref != null, "No reference registered in the repository for uuid '%s'", uuid);

    return ref;
  }
@Test
  public void getRef_throws_ISE_if_uuid_not_in_repository() {
    assertThatThrownBy(() -> underTest.getRef(SOME_UUID))
      .isInstanceOf(IllegalStateException.class)
      .hasMessage("No reference registered in the repository for uuid '" + SOME_UUID + "'");
  }