code: public Device getDevice(String id) throws UnknownDeviceException, NullIdException {
  		if ((id == null) || (id.trim().equals(""))) {
  			throw new NullIdException();
  		} else {
  			if (this.devices.containsKey(id)) {
  				return this.devices.get(id);
  			} else {
  				throw new UnknownDeviceException();
  			}
  		}
  	}
docs: Get a device according to his id
  @param id The device id
  @return The device
  @throws UnknownDeviceException
  @throws NullIdException