thanks paul, your help in much appreciated and sorry for the trouble.
when i run the program, and enter String at server console ; the String is not printed in the client console.
When i tried debugging the program, i found that the control is not going beyond line 28 in ChatClient class.
but if i comment out :
lines 44 - 47 and line 50 in ChatServer class
lines 31 - 36 and line 39 in ChatClient class , then i can successfully transmit Strings from server to client.
Why is one-way communication working but two-way communication not working ?