private void setCommonCoordinates() throws Exception {
		ArrayList<Coordinate> commonCoordinates = new ArrayList<Coordinate>();
		for(Coordinate c1 : v.getCoordinates()){
			for(Coordinate c2 : w.getCoordinates()){
				if(c1.equals(c2)){
					commonCoordinates.add(c1);					
				}	
			}
		}

		if(commonCoordinates.size() != 2){
			System.out.println(commonCoordinates.size());
			throw new Exception("an edge should find two cells that have two coordinates incommon");
		}
		if(commonCoordinates.size() == 2){
			c1 = commonCoordinates.get(0);
			c2 = commonCoordinates.get(1);
		}

	}