code: @SuppressWarnings("unchecked")
      public static <T extends Config> T add(Object key, T instance) {
          return (T) CACHE.putIfAbsent(key, instance);
      }
docs: Adds a {@link Config} object into the cache.
  
  @param key       the key object to be used to identify the instance in the cache.
  @param instance  the instance of the {@link Config} object to be stored into the cache.
  @param <T>       type of the interface.
  @return          the previous value associated with the specified key, or
  <code>null</code> if there was no mapping for the key.