Threads and Concurrent Programming
Threads may be seen as methods that execute at "the same time" as other methods. Normally, we think sequentially when writing a computer program. From this perspective, only one thing executes at a time. However, with today's multi-core processors, it is possible to literally have several things going on at the very same time while sharing the same memory. There are lots of ways that this is done in the real world, and this chapter goes over them in a way that you can apply to your own projects.
14.6 CASE STUDY: Cooperating Threads
The Clerk Class
The Clerk thread should simulate the behavior of serving the next customer in line, so the Clerk thread will repeatedly access
TakeANumber.nextCustomer() and then serve that customer. 
For the sake of this simulation, we’ll just print a message to indicate which customer is being served. Because there’s only one clerk in this simulation, the only variable in its internal state will be a reference to the
TakeANumber object (Fig. 14.22). In addition to the constructor, all we really need to define for this class is the run() method. This leads to the definition of Clerk shown in Figure 14.23. In this case, the sleep() method
is necessary to allow the Customer threads to run. The Clerk will sit in an infinite loop serving the next customer on each iteration.
