code: public SimpleFeature convertDwgSolid( String typeName, String layerName, DwgSolid solid, int id ) {
          double[] p1 = solid.getCorner1();
          double[] p2 = solid.getCorner2();
          double[] p3 = solid.getCorner3();
          double[] p4 = solid.getCorner4();
          Point2D[] ptos = new Point2D[]{new Point2D.Double(p1[0], p1[1]),
                  new Point2D.Double(p2[0], p2[1]), new Point2D.Double(p3[0], p3[1]),
                  new Point2D.Double(p4[0], p4[1])};
          CoordinateList coordList = new CoordinateList();
          for( int j = 0; j < ptos.length; j++ ) {
              Coordinate coord = new Coordinate(ptos[j].getX(), ptos[j].getY());
              coordList.add(coord);
          }
          coordList.closeRing();
  
          SimpleFeatureTypeBuilder b = new SimpleFeatureTypeBuilder();
          b.setName(typeName);
          b.setCRS(crs);
          b.add(THE_GEOM, Polygon.class);
          b.add(LAYER, String.class);
          SimpleFeatureType type = b.buildFeatureType();
          SimpleFeatureBuilder builder = new SimpleFeatureBuilder(type);
          LinearRing linearRing = gF.createLinearRing(coordList.toCoordinateArray());
          Geometry polygon = gF.createPolygon(linearRing, null);
          Object[] values = new Object[]{polygon, layerName};
          builder.addAll(values);
          return builder.buildFeature(typeName + "." + id);
      }
docs: Builds a polygon feature from a dwg solid.