java pdf 417 reader 9: Threads in Java

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9: Threads
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with threads, you ll probably need to spend some time experimenting. Also, one final disclaimer: This chapter makes no attempt to teach you how to design a good, safe, multithreaded application! You re here to learn what you need to get through the thread questions on the exam. Before you can write decent multithreaded code, however, you really need to study more on the complexities and subtleties of multithreaded code. With that out of the way, let s dive into threads. It s kind of a bad news/good news thing. The bad news is that this is probably the most difficult chapter. The good news is, it s the last chapter in the Programmer s Exam part of the book. So kick back and enjoy the fact that once you ve finished learning what s in this chapter, and you ve nailed the self-test questions, you re probably ready to take and pass the exam.
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A thread in Java begins as an instance of java.lang.Thread. You ll find methods in the Thread class for managing threads including creating, starting, and pausing them. For the exam, you ll need to know, at a minimum, the following methods:
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start() yield() sleep() run()
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The action all starts from the run() method. Think of the code you want to execute in a separate thread as the job to do. In other words, you have some work that needs to be done, say, downloading stock prices in the background while other things are happening in the program, so what you really want is that job to be executed in its own thread. So if the work you want done is the job, the one doing the work (actually executing the job code) is the thread. And the job always starts from a run() method as follows:
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public void run() { // your job code goes here }
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You always write the code that needs to be run in a separate thread in a run() method. The run() method will call other methods, of course, but the thread of execution the new call stack always begins by invoking run(). So where does the run() method go In one of the two classes you can use to define your thread job.
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You can define and instantiate a thread in one of two ways:
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Extend the java.lang.Thread class Implement the Runnable interface
You need to know about both for the exam, although in the real world you re much more likely to implement Runnable than extend Thread. Extending the Thread class is the easiest, but it s usually not a good OO practice. Why Because subclassing should be reserved for classes that extend an existing class, because they re a more specialized version of the more general superclass. So the only time it really makes sense (from an OO perspective) to extend Thread is when you have a more specialized version of a Thread class. In other words, because you have more specialized thread-specific behavior. Chances are, though, that the thread work you want is really just a job to be done by a thread. In that case, you should design a class that implements the Runnable interface, which also leaves your class free to extend from some other class.
Defining a Thread
To define a thread, you need a place to put your run() method, and as we just discussed, you can do that by extending the Thread class or by implementing the Runnable interface. We ll look at both in this section.
Extending java.lang.Thread
The simplest way to define code to run in a separate thread is to
Extend the Thread class. Override the run() method.
It looks like this:
class MyThread extends Thread { public void run() { System.out.println("Important job running in MyThread"); } }
The limitation with this approach (besides being a poor design choice in most cases) is that if you extend Thread, you can t extend anything else. And it s not as if you really
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