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

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