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Creating a Package

To create a package, you put a class or an interface in it. To do this, you put a package statement at the top of the source file in which the class or the interface is defined. For example, the following code appears in the source file Circle.java and puts the Circle class in the graphics package:

package graphics;

public class Circle extends Graphic implements Draggable {
    . . .
}

The Circle class is a public member of the graphics package.

You must include a package statement at the top of every source file that defines a class or an interface that is to be a member of the graphics package. So you would also include the statement in Rectangle.java and so on:

package graphics;

public class Rectangle extends Graphic implements Draggable {
    . . .
}

The scope of the package statement is the entire source file, so all classes and interfaces defined in Circle.java and Rectangle.java are also members of the graphics package. If you put multiple classes in a single source file, only one may be public, and it must share the name of the source files base name. Only public package members are accessible from outside the package.

If you do not use a package statement, your class or interface ends up in the default package, which is a package that has no name. Generally speaking, the default package is only for small or temporary applications or when you are just beginning development. Otherwise, classes and interfaces belong in named packages.

Naming a Package

With programmers all over the world writing classes and interfaces using the Java programming language, it is likely that two programmers will use the same name for two different classes. In fact, the previous example does just that: It defines a Rectangle class when there is already a Rectangle class in the java.awt package. Yet the compiler allows both classes to have the same name. Why? Because they are in different packages, and the fully qualified name of each class includes the package name. That is, the fully qualified name of the Rectangle class in the graphics package is graphics.Rectangle, and the fully qualified name of the Rectangle class in the java.awt package is java.awt.Rectangle.

This generally works just fine unless two independent programmers use the same name for their packages. What prevents this problem? Convention.

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By Convention:  Companies use their reversed Internet domain name in their package names, like this: com.company.package. Some companies now choose to drop the first element com. in this example from their package names. Name collisions that occur within a single company need to be handled by convention within that company, perhaps by including the region or the project name after the company name, for example, com.company.region.package.

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