@Test
public void testPutIfAbsent() {

   ConcurrentLongHashMap<String> map = new ConcurrentLongHashMap<>();
   assertEquals(map.putIfAbsent(1, "one"), null);
   assertEquals(map.get(1), "one");
   assertEquals(map.putIfAbsent(1, "uno"), "one");
   assertEquals(map.get(1), "one");
}