visual basic barcode printing 2: Declarations and Access Control in Java

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2: Declarations and Access Control
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When you see a question with complex logic, be sure to look at the access modifiers first. That way, if you spot an access violation (for example, a class in package A trying to access a default class in package B), you ll know the code won t compile so you don t have to bother working through the logic. It s not as if, you know, you don t have anything better to do with your time while taking the exam. Just choose the Compilation fails answer and zoom on to the next question.
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Public Access A class declaration with the public keyword gives all classes from all packages access to the public class. In other words, all classes in the Java Universe (JU) (you ll be tested on this acronym) have access to a public class. Don t forget, though, that if a public class you re trying to use is in a different package from the class you re writing, you ll still need to import the public class. (Just kidding about the JU acronym. We just made that up to keep you on your toes.) In the example from the preceding section, we may not want to place the subclass in the same package as the superclass. To make the code work, we need to add the keyword public in front of the superclass (Beverage) declaration, as follows:
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package cert; public class Beverage { }
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This changes the Beverage class so it will be visible to all classes in all packages. The class can now be instantiated from all other classes, and any class is now free to subclass (extend from) it unless, that is, the class is also marked with the nonaccess modifier final. Read on.
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Other (Nonaccess) Class Modifiers
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You can modify a class declaration using the keyword final, abstract, or strictfp. These modifiers are in addition to whatever access control is on the class, so you could, for example, declare a class as both public and final. But you can t always mix nonabstract modifiers. You re free to use strictfp in combination with abstract or final, but you must never, ever, ever mark a class as both final and abstract. You ll see why in the next two sections. You won t need to know how strictfp works, so we re focusing only on modifying a class as final or abstract. For the exam, you need to know only that strictfp is a keyword and can be used to modify a class or a method, but never a variable. Marking a class as strictfp means that any method code in the
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Declarations and Modifiers (Exam Objective 1.2)
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class will conform to the IEEE754 standard rules for floating points. Without that modifier, floating points used in the methods might behave in a platform-dependent way. If you don t declare a class as strictfp, you can still get strictfp behavior on a method-by-method basis, by declaring a method as strictfp. If you don t know the IEEE754 standard, now s not the time to learn it. You have, as we say, bigger fish to fry. Final Classes When used in a class declaration, the final keyword means the class can t be subclassed. In other words, no other class can ever extend (inherit from) a final class, and any attempts to do so will give you a compiler error. So why would you ever mark a class final After all, doesn t that violate the whole OO notion of inheritance You should make a final class only if you need an absolute guarantee that none of the methods in that class will ever be overridden. If you re deeply dependent on the implementations of certain methods, then using final gives you the security that nobody can change the implementation out from under you. You ll notice many classes in the Java core libraries are final. For example, the String class cannot be subclassed. Imagine the havoc if you couldn t guarantee how a String object would work on any given system your application is running on! If programmers were free to extend the String class (and thus substitute their new String subclass instances where java.lang.String instances are expected), civilization as we know it could collapse. So use final for safety, but only when you re certain that your final class has indeed said all that ever needs to be said in its methods. Marking a class final means, in essence, your class can t ever be improved upon, or even specialized, by another programmer. Another benefit of having nonfinal classes is this scenario: imagine you find a problem with a method in a class you re using, but you don t have the source code. So you can t modify the source to improve the method, but you can extend the class and override the method in your new subclass, and substitute the subclass everywhere the original superclass is expected. If the class is final, though, then you re stuck. Let s modify our Beverage example by placing the keyword final in the declaration:
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package cert; public final class Beverage{ public void importantMethod() { } }
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