code: @Override
    public String toJson() {
  
      GsonBuilder gson = new GsonBuilder();
      gson.registerTypeAdapterFactory(GeoJsonAdapterFactory.create());
      gson.registerTypeAdapterFactory(GeometryAdapterFactory.create());
      return gson.create().toJson(this);
    }
docs: This takes the currently defined values found inside this instance and converts it to a GeoJson
  string.
  
  @return a JSON string which represents this Feature Collection
  @since 1.0.0