Retrofit: How to specify comma-separated parameters in request?

If you are using kotlin and you have var items: List<String> that should go as a query param.

Use this method to form up string:

fun itemsString(items: List<String>) = items.joinToString(separator = ",")

Your url query should be like :

@Query(value = "items", encoded = true) String items

So, as we already found out the problem was in ? -> & typo. But one more thing to be mentioned is that Retrofit can accept complex objects as call parameters. Then String.valueOf(object) will be called to convert object into Query/Path param.

In your case you could define custom class like that:

class LatLng {
  private double lat;
  private double lng;

  ...

  @Override public String toString() {
    return String.format("%.1f,%.1f", lat, lng);
  }
}

And refactor your endpoint method like that:

@GET("/explore?limit=50&radius=25000&v=20140905&venuePhotos=1&oauth_token=xxyyxx")
void getVenues(@Query("ll") LatLng ll, Callback<String> cb);

About parsing the answer:

• Never create Gson objects right in callback. It's simply too heavyweight. Use the one you provide to RestAdapter.
• Why do you mix JsonParser & Gson? They are different tools for basically the same problem.
• Make use of Retrofit's built-in converter mechanism ;)

From words to code:

public class FoursquareResponse<T> {
  private MetaResponse meta;
  private T response;
  // getters
}

Altogether:

@GET("/explore?limit=50&radius=25000&v=20140905&venuePhotos=1&oauth_token=xxyyxx")
void getVenues(@Query("ll") LatLng ll, Callback<FoursquareResponse<VenuesExploreResponse>> cb);

...

foursquare.getVenues(LatLng.valueOf(30.26, -97.74), new Callback<FoursquareResponse<VenuesExploreResponse>>() {
    @Override 
    public void success(FoursquareResponse<VenuesExploreResponse> r, Response response) {
        MetaResponse metaResponse = r.getMeta;
        VenuesExploreResponse myResponse = r.getResponse();

        // Store results from myResponse in List
    }

    @Override
    public void failure(RetrofitError error) {
        Log.d(TAG, "Failures!");
    }
});