private long tilNextMillis(final long lastTimestamp) {
        long timestamp = timeGen();
        while (timestamp <= lastTimestamp) {
            timestamp = timeGen();
        }
        return timestamp;
    }
@Test
    public void testTilNextMillis() throws Exception {
        Class<?> uUIDUtilsClass = UUIDUtils.getInstance().getClass();
        Class<?>[] p = {long.class};
        Method method = uUIDUtilsClass.getDeclaredMethod("tilNextMillis", p);
        method.setAccessible(true);
        long lastTimestamp = System.currentTimeMillis();
        long result = (long) method.invoke(UUIDUtils.getInstance(), lastTimestamp);
        assertThat(result, greaterThan(lastTimestamp));
    }