code: public static MultiPolygon convert(MultiPolygon multiPolygon,int dimension) {
          int nb = multiPolygon.getNumGeometries();
          final Polygon[] pl = new Polygon[nb];
          for (int i = 0; i < nb; i++) {
              pl[i] = GeometryCoordinateDimension.convert((Polygon) multiPolygon.getGeometryN(i),dimension);
          }
          return gf.createMultiPolygon(pl);
      }
docs: Force the dimension of the MultiPolygon and update correctly the coordinate
  dimension
  @param multiPolygon
  @param dimension
  @return