code: public JTSPointExpression<Point> centroid() {
          if (centroid == null) {
              centroid = JTSGeometryExpressions.pointOperation(SpatialOps.CENTROID, mixin);
          }
          return centroid;
      }
docs: The mathematical centroid for this Surface as a Point. The result is not guaranteed to
  be on this Surface.
  
  @return centroid