showing default package visibility in Java

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There are a couple of other complicating factors with package visibility First, all nonprivate attributes and methods are visible to other classes in the same package This means public, protected, and unlabeled attributes and methods Second, if you don't put a class into a package, it automatically is placed into the default package, which usually consists of all classes in the current file directory The small example classes shown in Listing 3-3 demonstrate this The example consists of two files, PackageVisibilityjava and Foojava, contained in the same file directory Because neither file has been explicitly placed in a package, the default package visibility makes FooprotectedInt visible to the PackageVisibility class Beware Listing 3-3 PackageVisibilityjava, Foojava
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Example showing default package visibility
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/* This example demonstrates "default package visibility" * Foo is defined in the same directory as PackageVisibility * Thus, only the Foo private attribute privateInt is really * private PackageVisibility sees all three other variables */ public class PackageVisibility { public static void main(String args[]) { Foo foo = new Foo(); int fooPublic = foopublicInt; // public int fooPackage = foopackageInt; // package int fooProtected = fooprotectedInt; // default package // Following is not visible, would cause compiler error // int fooPrivate = fooprivateInt; } } public class Foo { // Simple class to demonstrate visibility public int publicInt; protected int protectedInt; int packageInt; private int privateInt; }
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As a rule, you should always use a public, private, or protected specifier in the definition of each attribute and method in a class definition The only exception is if you require package visibility for a library or other Java package, and then the class definition must use a Java package specification The UML provides a notation for specifying visibility of attributes and methods Note in the UML class diagram of Point that the private myX and myY attributes have a "-" at the beginning, which means private visibility, while the methods getY, getX, and setPoint have a "+" to indicate public visibility The UML uses a "#" to indicate protected (and Java package) visibility
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Inheritance is a fundamental feature of object-oriented design and programming While the use of class association, aggregation, and composition are just as important as inheritance when developing real programs, inheritance differs in that implementation support for it must be provided by the object-oriented programming language It is in the language support for inheritance that C++ most differs from C, or Java differs from earlier languages such as Pascal In the earlier Point and Circle example, we noted that having a Color class might be useful, and that it would make sense to generalize the concept of a shape To demonstrate inheritance in Java, we will develop a simple example that uses a Point class, a Color class, and a Shape class that will be used to define Circle and Rectangle classes (We could define other shapes, but two are enough to illustrate how to use the UML and Java to implement inheritance) First, let's design a general Shape class What is common to a Circle and a Rectangle Both will have an origin, and a color Furthermore, all shapes have some other common geometric attributes such as area and perimeter Thus, we will design a parent Shape class that accounts for these common factors by including objects of the Point and Color classes as attributes of Shape Area and perimeter will be handled by defining methods The calculation of area and perimeter will be defined by the subclasses of Shape In our example, we can reuse the Point class from earlier, but will need to add a new Color class We will also need a base Shape class that will be used to derive other specific shapes For our example, we will design new Circle and Rectangle classes that are derived from a Shape class First, here are UML class diagrams for the new Color class and again for Point:
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