" The closest thing to guarantee is that if a low priority thread is running when a high priority thread enters runnable the JVM will preempt the running low priority thread & put the high-priority thread in.
Can this fact be used while answering the SCJP1.4 Qs?
Yes. On a practical note however, the lower priority thread may possibly not be preempted immediately by a thread scheduler using time slices. The lower priority thread may be allowed to finish its time slice.