code: private Polygon addRing(Polygon polygon) {
  		LinearRing interiorRing = polygon.getGeometryFactory().createLinearRing(new Coordinate[] {});
  		AddRingOperation op = new AddRingOperation(interiorRing);
  		index.setInteriorRingIndex(polygon.getNumInteriorRing());
  		index.setExteriorRing(false);
  		return (Polygon) op.execute(polygon);
  	}
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