Currently there is no syntax problems so the code runs.
Problems what i am having now are
*When client connects and i add it to ActiveSessions, there will be added two instances at the same time(should be only one)
* On client side, pushing button send the object sending and receving works, but the client get's always the same Board object somehow. Like the turns count increases on server side but on client side the counter stays same, always 1 1 1.
As on the server side there's object Board and i share only objects reference to other sessionhandlers this should avoid the problem of making somewhere two different instances.
Two instances at the same time is added, because on client side you create two sockets, on line 23 and on line 79.
Client gets the same Board object because on server side it should be not just send (like you do on line 115), but first cloned. To do this you should implement Cloneable interface in Board class and override clone() method ibid. Then line 115 should look like this: