Thread Scheduling

Only one thread can execute at a time.
The Scheduler, which is part of the JVM, determines which thread gets to run.
There is no guarantee which ready thread the scheduler will select, and your code does not have direct control.
There are 2 basic types of schedulers:
1. round robin - used in time-sliced systems. A thread is given a small amount of processing time (called a "quantum"), then the system automatically lets other threads of the same priority run for a small amount of time.
2. preemptive - a running thread completes its execution unless a higher-priority thread becomes ready. For threads with the same priority, one thread must complete before another one can run.