synchronized private void addDisplay(String key, PrintWriter out, 
            ParkingSpot spot)
    {
        if(!displayConnections.containsKey(key))
            System.out.println( "Invalid spot number.");
        else 
        {
            displayConnections.put(key, out);
            System.out.println( "Connected display to spot successfully.");
            spot.setPrintWriter(out);
        }
    }