public class CoordObject {
    private String lat;
    private String lon;

    public CoordObject(
            String lat,
            String lon({
        this.lat =lat;
        this.lon =lon;
    }

    public String getLat(){
        return lat;
    }
    public String getLon(){
        return lon;
    }
}

public interface OwmInterface {

@GET("/data/2.5/weather")
Call<CoordObject> getCoord(@Query("q") String zipCode,
                           @Query("appid") String appId);
}

final String BASE_URL = "https://api.openweathermap.org/";
    final String API_KEY_OWM = "xxxx";

    Retrofit retrofit = new Retrofit.Builder()
            .baseUrl(BASE_URL)
            .addConverterFactory(GsonConverterFactory.create())
            .build();
    OwmInterface request = retrofit.create(OwmInterface.class);

    Call<CoordObject> call = request.getCoord(
            "37421",
            API_KEY_OWM
    );

    call.enqueue(new Callback<CoordObject>() {
        @Override
        public void onResponse(Call<CoordObject> call, Response<CoordObject> response){
            CoordObject coordObjectResponse = response.body();
            Toast.makeText(MainActivity.this, coordObjectResponse.getLat(), Toast.LENGTH_LONG).show();
            Toast.makeText(MainActivity.this, coordObjectResponse.getLon(), Toast.LENGTH_LONG).show();
        }

        @Override
        public void onFailure(Call<CoordObject> call, Throwable t) {
                Toast.makeText(MainActivity.this, "failed" +t, Toast.LENGTH_SHORT).show();
            }
    });