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9: Threads
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need that inherited Thread class behavior, because in order to use a thread you ll need to instantiate one anyway. Keep in mind that you re free to overload the run() method in your Thread subclass:
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class MyThread extends Thread { public void run() { System.out.println("Important job running in MyThread"); } public void run(String s) { System.out.println("String in run is " + s); } }
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But know this: the overloaded run(String s) method won t be called unless you call it. It will not be used as the basis of a new call stack.
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Implementing java.lang.Runnable
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Implementing the Runnable interface gives you a way to extend from any class you like, but still define behavior that will be run by a separate thread. It looks like this:
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class MyRunnable implements Runnable { public void run() { System.out.println("Important job running in MyRunnable"); } }
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Regardless of which mechanism you choose, you ve now got yourself some code that can be run by a thread of execution. So now let s take a look at instantiating your thread-capable class, and then we ll figure out how to actually get the thing running.
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Instantiating a Thread
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Remember, every thread of execution begins as an instance of class Thread. Regardless of whether your run() method is in a Thread subclass or a Runnable implementation class, you still need a Thread object to do the work. If you extended the Thread class, instantiation is dead simple:
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MyThread t = new MyThread();
Defining, Instantiating, and Starting Threads (Exam Objective 7.1)
There are some additional overloaded constructors, but we ll look at those in a moment. If you implement Runnable, instantiation is only slightly less simple. To have code run by a separate thread, you still need a Thread instance. But rather than combining both the thread and the job (the code in the run() method) into one class, you ve split it into two classes the Thread class for the thread-specific code and your Runnable implementation class for your job-that-should-be-run-by-a-thread code. First, you instantiate your Runnable class:
MyRunnable r = new MyRunnable();
Next, you get yourself an instance of java.lang.Thread (somebody has to run your job ), and you give it your job!
Thread t = new Thread(r); // Pass your Runnable to the Thread
If you create a thread using the no-arg constructor, the thread will call its own run() method when it s time to start working. That s exactly what you want when you extend Thread, but when you use Runnable, you need to tell the new thread to use your run() method rather than its own. The Runnable you pass to the Thread constructor is called the target or the target Runnable. You can pass a single Runnable instance to multiple Thread objects, so that the same Runnable becomes the target of multiple threads, as follows:
public class TestThreads { public static void main (String [] args) { MyRunnable r = new MyRunnable(); Thread foo = new Thread(r); Thread bar = new Thread(r); Thread bat = new Thread(r); } }
Giving the same target to multiple threads means that several threads of execution will be running the very same job. Besides the no-arg constructor and the constructor that takes a Runnable (the target, the instance with the job to do), there are other overloaded constructors in class Thread. The complete list of constructors is
Thread() Thread(Runnable target)
9: Threads
Thread(Runnable target, String name) Thread(String name) Thread(ThreadGroup group, Runnable target) Thread(ThreadGroup group, Runnable target, String name) Thread(ThreadGroup group, String name)
You need to recognize all of them for the exam! A little later, we ll discuss some of the other constructors in the preceding list. So now you ve made yourself a Thread instance, and it knows which run() method to call. But nothing is happening yet. At this point, all we ve got is a plain old Java object of type Thread. It is not yet a thread of execution. To get an actual thread a new call stack we still have to start the thread. When a thread has been instantiated but not started (in other words, the start() method has not been invoked on the Thread instance), the thread is said to be in the new state. At this stage, the thread is not yet considered to be alive. The aliveness of a thread can be tested by calling the isAlive() method on the Thread instance. In a nutshell, a thread is considered alive at some point after it has been started (you have to give the JVM a little time to get it set up as a thread once start() is called), and it is considered not alive after it becomes dead. The isAlive() method is the best way to determine if a thread has been started but has not yet completed its run() method.
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