public V putIfAbsent(long key, V value) {
        requireNonNull(value);
        long h = hash(key);
        return getSection(h).put(key, value, (int) h, true, null);
    }
@Test
    public void testPutIfAbsent() {
        ConcurrentLongHashMap<String> map = ConcurrentLongHashMap.<String>newBuilder()
                .build();
        assertNull(map.putIfAbsent(1, "one"));
        assertEquals(map.get(1), "one");

        assertEquals(map.putIfAbsent(1, "uno"), "one");
        assertEquals(map.get(1), "one");
    }