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 Customer Class
A Customer thread should model the behavior of taking a number from the TakeANumber gadget. For the sake of this simulation, let’s suppose that after taking a number, the Customer object just prints it out. This will serve as a simple model of “waiting on line.” What about the Customer’s state? To help distinguish one customer from another, let’s give each customer a unique ID number starting at 10001, which will be set in the constructor method. Also, as we noted earlier, each Customer needs a reference to the TakeANumber object, which is passed as a constructor parameter (Fig. 14.20). This leads to the definition of Customer shown in Figure 14.21. Note that before taking a number the customer sleeps for a random interval of up to 1,000 milliseconds. This will introduce a bit of randomness into the simulation.
Another important feature of this definition is the use of the static
variable number to assign each customer a unique ID number. Remember that a static variable belongs to the class itself, not to its instances.
Therefore, each Customer that is created can share this variable. By
incrementing it and assigning its new value as the Customer’s ID, we
guarantee that each customer has a unique ID number.