code: public UserManagedCacheBuilder<K, V, T> withKeyCopier(Copier<K> keyCopier) {
      if (keyCopier == null) {
        throw new NullPointerException("Null key copier");
      }
      UserManagedCacheBuilder<K, V, T> otherBuilder = new UserManagedCacheBuilder<>(this);
      otherBuilder.keyCopier = keyCopier;
      otherBuilder.useKeySerializingCopier = false;
      return otherBuilder;
    }
docs: Adds a configuration for key {@link Copier} to the returned builder.
  
  @param keyCopier the key copier to use
  @return a new builder with the added key copier configuration
  
  @see #withKeySerializingCopier()