You havnt been able to explain your question well ... but if you are planning to make a messenger like msn or yahoo ... you'll have one server running at all times .... and then when the user signs in ... it'll request the server ... and the server will determine who in his buddy list is active and who is not....

If you are planning a lan messenger .. then its so easy ... study the basic networking concepts of java and you'll be on a flying start on your messenger. If you have any probs ... post them here ... you'll be helped.

yah my messager is something like msn, n i have a server model to do the peocessing part. but i do not know how the server determine who in his buddy list is active and who is not

make a file of online user ids ... when the user gets online ... the file is updated... attach a listener to it ... check it whenever it changes for all the users....

Its tedius but its straight forward.

go to www.seclude.org it's an entire messaging system, like aim or msn or whatnot. It's written in Java, and it's fully open source and free to download. It also implements some very advanced encryption techniques. It's quite possibly the safest way to talk over the internet. Download it and see how they conquered this problem.