What is the best way to pass information between threads?

Question

I want to be listening to a server while my program is doing other things, when a message is received from the server I want to interpret it.

I know about threading

Answer 1

If I have a thread listening for the server how can I pass that data to the main thread for interpretation? What is the best way for the main thread to send data to the server?

I'd use a BlockingQueue for this. You define a single BlockingQueue such as the LinkedBlockingQueue. Your listener class then calls queue.take() which will wait for your server to call queue.put(). It leaves all of the synchronization, waits, notifies, etc. to the Java class instead of your own code.

What is the use of the synchronized modifier?

I'd do some reading to understand more about this. This is not the sort of thing that can be answered in a short-ish SO response. The Java concurrency tutorial is a good place to start.