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10. B. The code in the HorseTest class is perfectly legal. Line 9 creates an instance of the method-local inner class Horse, using a reference variable declared as type Object. Line 10 casts the Horse object to a Horse reference variable, which allows line 11 to compile. If line 10 were removed, the HorseTest code would not compile, because class Object does not have a name variable. A, C, D, E, and F are incorrect based on the program logic described above. 11. E. This code is identical to the code in question 10, except the casting statement has been removed. If you use a reference variable of type Object, you can access only those members defined in class Object. A, B, C, and D are incorrect based on the program logic described above and in the previous question. 12. A. You can define an inner class as abstract, which means you can instantiate only concrete subclasses of the abstract inner class. The object referenced by the variable t is an instance of an anonymous subclass of AbstractTest, and the anonymous class overrides the getNum() method to return 22. The variable referenced by f is an instance of an anonymous subclass of Bar, and the anonymous Bar subclass also overrides the getNum() method (to return 57). Remember that to instantiate a Bar instance, we need an instance of the enclosing AbstractTest class to tie to the new Bar inner class instance. AbstractTest can t be instantiated because it s abstract, so we created an anonymous subclass (non-abstract) and then used the instance of that anonymous subclass to tie to the new Bar subclass instance. B, C, D, E, and F are incorrect based on the program logic described above.
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Write code to define, instantiate, and start new threads using both java.lang.Thread and java.lang.Runnable. Imagine a stockbroker application with a lot of complex behavior that the user initiates. One of the applications is download last stock option prices, another is check prices for warnings, and a third time-consuming operation is, analyze historical data for company XYZ. In a single-threaded runtime environment, these actions execute one after another. The next action can happen only when the previous one is finished. If a historical analysis takes half an hour, and the user selects to perform a download and check afterward, the warning may come too late to, say, buy or sell stock as a result. We just imagined the sort of application that cries out for multithreading. Ideally, the download should happen in the background (that is, in another thread). That way, other processes could happen at the same time so that, for example, a warning could be communicated instantly. All the while, the user is interacting with other parts of the application. The analysis, too, could happen in a separate thread, so the user can work in the rest of the application while the results are being calculated. So what exactly is a thread In Java, thread means two different things:
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An instance of class java.lang.Thread A thread of execution
An instance of Thread is just an object. Like any other object in Java, it has variables and methods, and lives and dies on the heap. But a thread of execution is an individual process (a lightweight process) that has its own call stack. In Java, there is one thread per call stack or, to think of it in reverse, one call stack per thread. Even if you don t create any new threads in your program, threads are back there running. The main() method that starts the whole ball rolling runs in one thread, called (surprisingly) the main thread. If you looked at the main call stack (and you can,
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anytime you get a stack trace from something that happens after main begins, but not within another thread) you d see that main() is the first method on the stack the method at the bottom. But as soon as you create a new thread, a new stack materializes and methods called from that thread run in a call stack that s separate from the main() call stack. That second new call stack is said to run concurrently with the main thread, but we ll refine that notion as we go through this chapter. You might find it confusing that we re talking about code running concurrently as if in parallel yet you know there s only one CPU on most of the machines running Java. What gives The JVM, which gets its turn at the CPU by whatever scheduling mechanism the underlying OS uses, operates like a mini-OS and schedules its own threads regardless of the underlying operating system. In some JVMs, the java threads are actually mapped to native OS threads, but we won t discuss that here; native threads are not on the exam. Nor is an understanding of how threads behave in different JVM environments required knowledge. In fact, the most important concept to understand from this entire chapter is When it comes to threads, very little is guaranteed. So be very cautious about interpreting the behavior you see on one machine as the way threads work. The exam expects you to know what is and is not guaranteed behavior, so that you can design your program in such a way that it will work regardless of the underlying JVM. That s part of the whole point of Java.
Don t make the mistake of designing your program to be dependent on a particular implementation of the JVM. As you ll learn a little later, different JVMs can run threads in profoundly different ways. For example, one JVM might be sure that all threads get their turn, with a fairly even amount of time allocated for each thread in a nice, happy, round-robin fashion. But in other JVMs, a thread might start running and then just hog the whole show, never stepping out so others can have a turn. If you test your application on the nice turn-taking JVM, and you don t know what is and is not guaranteed in Java, then you might be in for a big shock when you run it under a JVM with a different thread scheduling mechanism.
The thread questions on the exam are among the most difficult. In fact, for most people they are the toughest questions on the exam, and with four objectives for threads you ll be answering a lot of thread questions. If you re not already familiar
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