How to I recreate MultiPolygon with Rings/Holes using org.geojson

Question

I have a few original MultiPolygons that contain holes...I would like to extract the Polygons and recreate them WOTHOUT the holes. how is this done?

Original MultiPolygon. https://gist.github.com/boundaries-io/978eaa4a10df9467638a5eb9259c84e6

when breaking up the Multipolygon

   org.geojson.MultiPolygon reCreateMultiPolygon = new org.geojson.MultiPolygon();
             for (List<List<LngLatAlt>> polygonPoints : multiPolygon.getCoordinates()) {
                for (List<LngLatAlt> point : polygonPoints) {
                    Polygon p = new Polygon(point);
                    reCreateMultiPolygon.add(p);
                }
            }
             
             String fileName2 =   saveToFileSystem(reCreateMultiPolygon);

The this is treating each polygon as its own polygon...I am unclear how to recreate this with the holes intact...not treat them as normal polygons. results:/ Incorrect recreated MultiPolygon https://gist.github.com/boundaries-io/89e51458e9f5509b5457f9033474bc2d

my end goal is to create a convert that does a org.geojson.MultiPolygon to rg.springframework.data.mongodb.core.geo.GeoJsonMultiPolygon to save in Mongodb via spring data mongodb. if this can be done via JTS,geotools,etc. thats fine...

Answer 1

If you just want the individual polygons (including the holes) then something like:

  MultiPolygon mp = (MultiPolygon) g;
  List<Polygon> polys = new ArrayList<>();
  for (int i = 0; i < mp.getNumGeometries(); i++) {
    Polygon poly = (Polygon) mp.getGeometryN(i);
    polys.add(poly);
  }

Will give you a List of Polygons. Note, these are JTS geometries not geojson ones.

Update

For just the holes you need to do something like:

private List<Polygon> getHoles(Geometry mp) {
    GeometryFactory gf = new GeometryFactory();
    List<Polygon> holes = new ArrayList<>();
    if (mp instanceof MultiPolygon || mp instanceof Polygon) {
      for (int i = 0; i < mp.getNumGeometries(); i++) {
        Polygon poly = (Polygon) mp.getGeometryN(i);
        for (int h = 0; h < poly.getNumInteriorRing(); h++) {
          LinearRing hole = poly.getInteriorRingN(h);
          holes.add(gf.createPolygon(hole));
        }
      }
    }
    return holes;
  }