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

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2: Declarations and Access Control
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Legal and illegal uses of extends and implements
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Look for illegal uses of extends and implements. The following shows examples of legal and illegal class and interface declarations:
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class Foo { } // OK class Bar implements Foo { } // No! Can t implement a class interface Baz { } // OK interface Fi { } // OK interface Fee implements Baz { } // No! Interface can t implement an interface interface Zee implements Foo { } // No! Interface can t implement a class interface Zoo extends Foo { } // No! Interface can t extend a class interface Boo extends Fi { } // OK. Interface can extend an interface class Toon extends Foo, Button { } // No! Class can t extend multiple classes class Zoom implements Fi, Fee { } // OK. class can implement multiple interfaces interface Vroom extends Fi, Fee { } // OK. interface can extend multiple interfaces
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Burn these in, and watch for abuses in the questions you get on the exam. Regardless of what the question appears to be testing, the real problem might be the class or interface declaration. Before you get caught up in, say, tracing a complex threading flow, check to see if the code will even compile. (Just that tip alone may be worth your putting us in your will!) (You ll be impressed by the effort the exam developers put into distracting you from the real problem.) (How did people manage to write anything before parentheses were (was ) invented )
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You now have a good understanding of access control as it relates to classes, methods, and variables. You ve looked at how access modifiers (public, protected, private) define the access control of a class or member. You ve also looked at the other modifiers including static, final, abstract, synchronized, etc. You ve learned how some modifiers can never be combined in a declaration, such as mixing final with abstract or abstract with private. Keep in mind that there are no final objects in Java. A reference variable marked final can never be changed, but the object it refers to can be modified. You ve seen that final applied to methods means a subclass can t override them, and when applied to a class, the final class can t be subclassed.
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2: Declarations and Access Control
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You learned that abstract classes can contain both abstract and nonabstract methods, but that if even a single method is marked abstract, the class must be marked abstract. Don t forget that a concrete (nonabstract) subclass of an abstract class must provide implementations for all the abstract methods of the superclass, but that an abstract class does not have to implement the abstract methods from its superclass. An abstract subclass can pass the buck to the first concrete subclass. Remember what you ve learned about static variables and methods, especially that static members are per-class as opposed to per-instance. Don t forget that a static method can t directly access an instance variable from the class it s in, because it doesn t have an explicit reference to any particular instance of the class. You ve also looked at source code declarations, including the use of package and import statements. Don t forget that you can have a main() method with any legal signature you like, but if it isn t public static void main (String [] args), the JVM won t be able to invoke it to start your program running. Finally, you covered interface implementation, including the requirement to implement public void run() for a class that implements Runnable. You also saw that interfaces can extend another interface (even multiple interfaces), and that any class that implements an interface must implement all methods from all the interfaces in the inheritance tree of the interface the class is implementing. Before you hurl yourself at the practice test, spend some time with the following optimistically named Two-Minute Drill. Come back to this particular drill often, as you work through this book and especially when you re doing that last-minute cramming. Because and here s the advice you wished your mother had given you before you left for college it s not what you know, it s when you know it.
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