Geocoding and reverse geocoding

I am using Graphhopper for routing at the moment. That is, I create requests and process responses via

GHRequest req = new GHRequest(startLat, startLon, destLat, destLon);
GHResponse rsp = graphhopper.route(req);

I was wondering if something similar is possible for geocoding and reverse geocoing? I have found GHGeocodingRequest in the graphhopper source but I can’t create a running example with it.

This is only supported through external systems.

can you name an “external system”? Am not sure what that means.

Just search for “open source geocoders”: nominatim, photon, …