How to build chat system in java ???

Answer 1

If you mean to communicate between a server (which manages user connections, message sending, etc.) and clients (users which can send and receive messages), you can use "regular" sockets, though if you want a website UI you might want to consider using WebSockets instead.

I built a very simple TCP chat program a while ago:
https://code.sololearn.com/c7PTwdOXdsWE/?ref=app
https://code.sololearn.com/c072B0Uj5ovg/?ref=app

Answer 2

Hi OMKAR BANDAL
Take a look on socket creation in Java. With them you can code a server-client system that can used as a chat and more.