Yes, there is a problem, because these threads could potentially access/modify the data at 'unfortunate' times. For example, thread 1 modifies variable A to 2, then thread 2 immediately modifies A to 3 (as a simple but stupid example). Another possibility being that threads 1 and 2 are both using variable A in a series of operations. So both threads want A to maintain some state while they do those operations, but the other threads modifies A, which interferes.

Yes, it is a standard problem. The solutions is called a "lock". Each thread has to get a lock on the object before it modifies it, then releases the lock when its finished. If a thread has already locked the object the second thread must wait until the lock is released. This guarantees that only one tread can modify the object at any one time. Have a little Google for Java threads locks - there's lots of tutorial stuff at every level of detail.

http://java.sun.com/docs/books/tutor.../locksync.html

Granted I'm fairly new to Java, but could you not declare variables as synchronized to help solve this problem?