// get first coordinate
Coordinate first = yourPolygon.getCoordinates()[0];
// get last coordinate (same as first)
Coordinate last = yourPolygon.getCoordinates()[yourPolygon.getCoordinates().length-1];
// get last coordinate before closing
Coordinate lastBeforeClosing = yourPolygon.getCoordinates()[yourPolygon.getCoordinates().length-2];