code: public static Polygon makeEnvelope(double xmin, double ymin, double xmax, double ymax) {
          Coordinate[] coordinates = new Coordinate[]{
              new Coordinate(xmin, ymin),
              new Coordinate(xmax, ymin),
              new Coordinate(xmax, ymax),
              new Coordinate(xmin, ymax),
              new Coordinate(xmin, ymin)
          };
          return GF.createPolygon(GF.createLinearRing(coordinates), null);
      }
docs: Creates a rectangular Polygon formed from the minima and maxima by the
  given shell.
  
  @param xmin X min
  @param ymin Y min
  @param xmax X max
  @param ymax Y max
  @return Envelope as a POLYGON