In Java, assignment is atomic if the size of the variable is less than or equal to 32 bits but is not if more than 32 bits.
What (volatile/synchronized) would be more efficient to use in case of double or long assignment?
Like,
volatile double x = y;
synchronized is not applicable with primitive argument. How do I use synchronized in this case? Of course I don't want to lock my class, so this should not be used.
If you find locking on the object itself too heavy, then synchronized is the way to go. Prior to Java 1.5 volatile may have been a good choice, but now volatile can have a very large impact by forcing instruction ordering on the method where the assignment happens. Create a separate object (private final Object X_LOCK = new Object();) and synchronize on it when setting or getting the value of that double. This will give you a fine level of control over the locking, which it seems that you need.
In the new concurrency package there are more options, such as AtomicReference which may be a good replacement for volatile if you really need to avoid synchronization.