code: public NumberExpression<Integer> numInteriorRing() {
          if (numInteriorRing == null) {
              numInteriorRing = Expressions.numberOperation(Integer.class, SpatialOps.NUM_INTERIOR_RING, mixin);
          }
          return numInteriorRing;
      }
docs: Returns the number of interior rings in this Polygon.
  
  @return number of interior rings