When the user types /join #channel the client must send JOIN #channel to the server.
If the server replies with a message like :ircuser!~ircuser@example.ucsd.edu JOIN :#channel then the client displays some non-null string indicating the channel is full.
If the server replies with an error message like :cse70.ucsd.edu 471 ircuser #channel :Cannot join channel (+l) then likewise the client displays some non-null string indicating the channel is full.
If the server replies with an error message like :cse70.ucsd.edu 473 ircuser #channel :Cannot join channel (+i) then likewise the client displays some non-null string indicating the channel is invite-only.
If the server replies with an error message like :cse70.ucsd.edu 474 ircuser #channel :Cannot join channel (+b) then likewise the client displays some non-null string indicating the user is banned from the channel.