code: @SuppressWarnings("unchecked")
    public final <K2> Cache2kBuilder<K2, V> keyType(Class<K2> t) {
      Cache2kBuilder<K2, V> me = (Cache2kBuilder<K2, V>) this;
      me.config().setKeyType(t);
      return me;
    }
docs: The used type of the cache key. A suitable cache key must provide a useful
  {@code equals} and {@code hashCode} method. Arrays are not valid for cache keys.
  
  @throws IllegalArgumentException in case the type is illegal
  @see CacheType for a general discussion on types