What is the problems in my code?

How to make a chat room using socket I searched a lot in internet about how to do it, but I don't really understand it. How to match the client with their name and send message to the specific client? How to send message and receive message in the same time for the client part? What is the difference between writeBytes and println? It seems that the client totally cannot connect to the server. How can I do it? https://code.sololearn.com/c8VsAG8swkMw/?ref=app https://code.sololearn.com/c5ZVq4267B8H/?ref=app