I would like to write a java program which can allows me to send a message from my n810 to an other one under the adhoc mode.
how can i do it please.

..."adhoc" means Salut/Bonjour/Zeroconf support?
Did you check whether that's already available in rtcomm (Telepathy stack), or maybe 3rd party software as Pidgin, or do you insist on writing your own application?

adhoc refers to two wifi devices connected to each other, usually without a connection to the Internet. So, a link-local protocol, like Bonjour, would be ideal, because no server is needed.

Really, though... look into other programming languages. Java is difficult to get running on the tablets, and doesn't run terribly well. Try something like Python, maybe?

I can not write it bye some other language because their are lot of part of my project and they r wrote in java, and one part is to send something like bonjour bye the ad hoc mode and send it to the other n810 but i really don t know from where to begin .

I'm still not sure why you need to develop in java. Two different languages can talk to each other over the network, you know...

Anyway, I did a quick google search for java maemo, and the first result was this. Seems to be a good starting point, and cover anything I could tell you. Anything beyond putting Java on your tablet, though (such as help writing the network code, etc.) is really beyond the scope of these forums.

Your best bet would be to install Easy Debian Turbo-Charged Edition.
I believe Java works well in that environment, but is really slow.