@Test
public void testUnique() {

    int num = 20000;
    Set<String> set = new HashSet<>(num);
    int maxlen = 0;
    for(int i=0;i<num;i++) {
        String id = ShortID.randomID();
        maxlen = Math.max(maxlen, id.length());
        set.add(id);
    }
    assertTrue(maxlen < 17);
    assertEquals(num, set.size());
}