code: public Geometry execute(Geometry geometry) {
  		if (geometry instanceof Polygon) {
  			Polygon polygon = (Polygon) geometry;
  			if (geometry.isEmpty()) {
  				return null;
  			} else {
  				LinearRing[] interiorRings = new LinearRing[polygon.getNumInteriorRing() + 1];
  				int count = 0;
  				for (int n = 0; n < interiorRings.length; n++) {
  					if (n == index) {
  						interiorRings[n] = ring;
  					} else {
  						interiorRings[n] = polygon.getInteriorRingN(count);
  					}
  				}
  				return geometry.getGeometryFactory().createPolygon(polygon.getExteriorRing(), interiorRings);
  			}
  		}
  		return null;
  	}
docs: Execute the operation! When the geometry is not a Polygon, null is returned.