Peer to Peer File Transfer

This should be similar to a Bit torrent scenario I want a File transfer in peer to peer networks in java programming language. The interaction in peers should be implemented in different systems. The peers should be able to join and leave the network The peers should communicate through sockets and multi threading scenario only. File should be updated and sent across all the peers, if a new peer joins the network file should be available to that peer too . I need a simple understandable java code with maximum comments explaining what each class or method or function is doing. This should work for transferring word files and the output should show that the file has been published across peers and the user should be able to view it. I do not require any documentation a good understandable with lot of comments for functions and classes will suffice.