how to generate barcode in vb.net 2008 n The access modifiers n Information hiding n Exposing object functionality in Java

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The Access Modifiers
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Java has three access modifiers: private, protected, and public. There is also the default access level, which is known as package-private. Each access level has different restrictions that allow or deny classes access to methods or instance variables. Access modifiers are also used when defining a class. This is beyond the scope of the SCJA, so just assume all classes are public. The Java compiler will produce errors if a restricted method or instance variable is accessed by code that is unauthorized. The private access modifier is the most restrictive and most commonly used access modifier. Any method or instance variable that is marked as private can only be accessed by other methods in the same class. Subclasses cannot access instance variables or methods that are private. The following is an example of the private keyword in use:
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The default access level is the second most restrictive. It is often referred to as package-private. This access level allows access to its methods and instance variables from code that is in the same package. The default access level does not have a keyword to indicate it is in use. A method or instance variable is set to default when an access modifier is omitted. The following is an example of this access level in use:
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The protected access modifier is the third most restrictive. It is the same as the default access level but adds the ability of subclasses outside of the package to access its methods or instance variables. This means the methods that can access this data must either be in the same package (same as default) or be in a subclass of the class that contains the protected data. Remember that a subclass is a class that extends another class. The following is an example of the use of the protected access modifier:
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The final access modifier is public. This is the least restrictive and second most common access modifier. The public access modifier provides no restriction to what can access its methods and instance variables. Any method can access a public method or instance variable regardless of which package it is contained in or which superclass it extends. An item marked as public is accessible to the world. The following is an example of using the public access modifier:
public int streetAddress; public int findZipCode(){ };
Information Hiding
Information hiding is the concept of hiding the implementation details of a class. Information hiding is achieved by using restrictive access modifiers. The advantage of hiding data is so the developer can control how the data is accessed. Instance variables are used to store the state of an object. If outside objects were able to access an object s entire set of instance variables, the risk of introducing bugs would be increased. A developer may create a new class that incorrectly tries to use the internal features of another class. Even if this approach works at first, it requires that the class s internal data structure not change. This concept also applies to methods. Not all methods need to be accessible by external classes. Oftentimes,
Encapsulation
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You need to make an instance variable available only to the class it is declared in. What access modifier would you use You need to make a method only available to other methods in the same package or a subclass of the class it is defined in. What access modifier would you use The private access modifier should be used.
The protected access modifier should be used.
The public access modifier should be used. You need to make a method that is available to any other method in the application. What access modifier would you use You need to make an instance variable that is available Package-private (default) should be used. only to other objects in the same package. What access modifier would you use
a class will be composed of more methods used internally to perform tasks rather than methods designed for external objects. A benefit of hiding data can be seen in this scenario. A class contains an instance variable that must be between a certain range. An outside object may set this variable and disregard the proper range. To prevent this, the variable can be marked private and a public method can be used to set it. This method would contain code that would only change its value if the new value were valid. A general rule is that every method and instance variable should use the most restrictive access modifier possible.
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