i got a problem whenever i give the username of less than 4 character in java GUI then tha userlist from python server is regularly updating my UserList textField in JAVA GUI but when the character exceeds more than 3 that UserList textField is not updated rather the user list is being received at the socket from the python server. I can't recognise this is client's problem(in java) or server's problem(python) What can i do?

Have your code check the length of the username before submitting it.
Refuse to except an invalid one.