code: public static <K, V> V putIfAbsent(ConcurrentMap<K, V> map, K key, V value) {
          final V existingValue = map.putIfAbsent(key, value);
          if (existingValue == null) {
              return value;
          }
  
          return existingValue;
      }
docs: How putIfAbsent should work, returns the one value that actually ends up in the {@link
  ConcurrentMap}
  
  @param map The map
  @param key The key
  @param value The value to put
  @return The value that exists in the Map for this key at the point in time that {@link
  ConcurrentMap#putIfAbsent(Object, Object)} is called