code: public UserManagedCacheBuilder<K, V, T> withKeySerializer(Serializer<K> keySerializer) {
      if (keySerializer == null) {
        throw new NullPointerException("Null key serializer");
      }
      UserManagedCacheBuilder<K, V, T> otherBuilder = new UserManagedCacheBuilder<>(this);
      otherBuilder.keySerializer = keySerializer;
      return otherBuilder;
    }
docs: Adds a configuration for key {@link Serializer} to the returned builder.
  
  @param keySerializer the key serializer to use
  @return a new builder with the added key serializer configuration